NOTE : A search engine for this site is available at the bottom.

Click here to read about Antimony in Gold Metallurgy
Click here to read about Arsenic in Gold Metallurgy
Click here to read about Pregrobbing in Gold Metallurgy

Click here to return to page listing aspects of gold metallurgy which can be accessed free and in $25 page monographs.

Contact me to learn how to sqeeze another % or so more gold out of your circuit with no capital expenditure

               THE METALLURGICAL MINERALOGY OF GOLD 

                       David Martin Menne 

                              INDEX

1.0 OVERVIEW OF COMPLEX AND REFRACTORY GOLD ORES 

2.0 POORLY EXPOSED GOLD

  2.1 PHYSICAL ENCAPSULATION

    [a] FINE DISPERSION

    [b] RIMMING BY PRECIPITATES

      Iron oxides
      Auric oxide
      Silver chloride
      Silica
      Other

    [c] SURFACE COATING BY ZETA-POSITIVE COLLOIDS

  2.2 CHEMICAL BONDING IN GOLD MINERALS

    [a] SIDEROPHILES [METALS]

    [b] CHALCOPHILES [SULPHIDES]

  2.3 SOLID SOLUTION

    [a] SULPHOSALTS

    [b] SULPHIDES

3.0 REAGENT AND PRODUCT CONSUMERS

  3.1 LIGAND CONSUMERS

    [a] CYANICIDES 

    [b] SULPHOXIDANTS

  3.2 PREGROBBING

    [a] SIMPLE ION EXCHANGE

    [b] GOLD METAL REPRECIPITATION

    [c] GOLD POLYMER FORMATION

    [d] MIXED GOLD CYANIDE PRECIPITATES

    [e] MINERAL REPRECIPITATION

  3.3 OXIDANT CONSUMERS

  3.4 NATURAL ACIDS

4.0 AUTO-PASSIVATION 

5.0 OTHER

  5.1 PYROMETALLURGICAL LIMITATIONS

  5.2 BIO-OXIDATION

    [b] KEY REJECTION FACTORS

      Biocides
      Ore variability
      Excess alkalinity

  5.3 GENERAL

APPENDIX I : GEOLOGY OF GOLD

 I.1.0 GENESIS OF GOLD DEPOSITS                               36

   I.1.1 HYPOTHERMAL DEPOSITS [GOLD OF DEEP VEIN ORIGIN]    45

   I.1.2 MESOTHERMAL DEPOSITS [GOLD OF MID-VEIN ORIGIN]     47

   I.1.3 EPITHERMAL DEPOSITS [GOLD OF SHALLOW VEIN ORIGIN]  47

 I.2.0 IMPORTANT LARGE TONNAGE [GENERALLY] LOW GRADE DEPOSITS 47

   I.2.1 GOLD OF SHALLOW VEIN ORIGIN [EPITHERMAL]           47

   I.2.2 SEDIMENTARY DEPOSITS [CARLIN TYPE]                 48

 I.3.0 OTHER PRIMARY DEPOSITS                                 49

   I.3.1 [ORTHO-] MAGMATIC SEGREGATIONS                     49

   I.3.2 PEGMATITES                                         49

   I.3.3 PYROMETASOMATIC DEPOSITS [SKARNS]                  49

 I.4.0 DEPOSITS DEVELOPED BY ALTERATION AND WEATHERING        50

   I.4.1 OXIDE ZONE                                         50

   I.4.2 SECONDARY [SUPERGENE] ENRICHMENT                   50

   I.4.3 PLACERS                                            55

APPENDIX II : GOLD ASSOCIATIONS CAUSING COMPLEX OR 
              REFRACTORY EXTRACTIVE METALLURGY 

 II.0 REFRACTORY GOLD ASSOCIATIONS 

   II.1 ANTIMONY, ARSENIC AND BISMUTH ASSOCIATIONS

     II.1.1 ANTIMONY

       [a] Mineralogy
       [b] Deposits
       [c] Processing

     II.1.2 ARSENIC ASSOCIATIONS

       [a] Mineralogy
       [b] Deposits
       [c] Processing

     II.1.3 BISMUTH

   II.2 CARBON ASSOCIATIONS     

       [a] Mineralogy
       [b] Deposits
       [c] Processing 
         Introduction
         Chlorination 
         Carbonates

   II.3 CADMIUM, COBALT, COPPER, NICKEL ASSOCIATIONS     

     II.3.1 COPPER

       [a] Mineralogy
       [b] Deposits
       [c] Processing

     II.3.2 NICKEL

   II.4 IRON ASSOCIATIONS

       [a] Mineralogy
       [b] Deposits
       [c] Processing

   II.5 SILVER ASSOCIATIONS     

       [a] Deposits
       [b] Deposits
       [c] Processing

   II.6 TELLURIDE AND SELENIUM ASSOCIATIONS 
     II.6.1 TELLURIUM

       [a] Mineralogy
       [b] Deposits
       [c] Processing

     II.6.2 SELENIUM

   II.7 SILICA

       [a] Mineralogy
       [b] Deposits
       [c] Processing

   II.8 SULPHUR

     II.8.1 SULPHIDES

       [a] Mineralogy
       [b] Deposits
       [c] Processing

     II.8.2 SULPHATES

   II.9 OTHER

     II.9.1 LEAD 

       [a] Mineralogy
       [b] Deposits
       [c] Processing

     II.9.2 MANGANESE

       [a] Mineralogy
       [b] Deposits
       [c] Processing

     II.9.3 MERCURY 

       [a] Mineralogy
       [b] Deposits
       [c] Processing

     II.9.4 PLATINOIDS

       [a] Mineralogy
       [b] Deposits
       [c] Processing

APPENDIX III : GOLD ASSOCIATIONS OF LITTLE CONSEQUENCE 
               EXTRACTIVE METALLURGY 

   III.0 IMPURITIES IN NATIVE GOLD 

   III.1 ALKALI EARTHS : Be, Mg, Ca, Sr AND Ba 

   III.2 ALKALI METALS : Li, Na AND K 

   III.3 ALUMINIUM

   III.4 BORON

   III.5 CHLORIDE

   III.6 GERMANIUM

   III.7 HALIDES

   III.8 PHOSPHATES

   III.9 RARE EARTHS : Ga, In, Sc

   III.10 TIN

   III.11 TITANIUM

   III.12 THORIUM

   III.13 TUNGSTEN

   III.14 URANIUM

   III.15 VANADIUM

   III.16 ZINC

   III.17 ZIRCON



APPENDIX IV : PREGROBBING ORE PROCESSING 

 IV.1.0 OBJECTIVE                                                      
                                   
 IV.2.0 SUMMARY                                                        
                                   
 IV.3.0 INTRODUCTION                                                   
                                   
 IV.4.0 THE CHEMISTRY OF PREGROBBING ORES                              
                                   
  IV.4.1 CARBON/RESIN/WASH REVERSIBLE : TYPE I                         

  IV.4.2 CYANIDE REVERSIBLE : TYPE II                                  

   IV.4.2.1 AUROCYANIDE POLYMER FORMATION : TYPE IIa                   

   IV.4.2.2 REDUCTION TO GOLD METAL : TYPE IIb                         

  IV.4.3 ALKALI REVERSIBLE : TYPE III                                  

  IV.4.4 ACID REVERSIBLE : TYPE IV                                     

   IV.4.4.1 AUROCYANIDE ENCAPSULATED                                   
         BY MINERAL REPRECIPITATION : TYPE IVa                         

   IV.4.4.2 METALLIC GOLD ENCAPSULATED                                 
         BY MINERAL REPRECIPITATION : TYPE IVb                         
                                   
 IV.5.0 PREGROBBING ORE MINERALOGY/GEOLOGY                             
                 
  IV.5.1 CARBONACEOUS ORES                                             

   IV.5.1.1 ORGANIC                                                    

   IV.5.1.2 GRAPHITIC                                                  

   IV.5.1.3 CARBONATE                                                  

   IV.5.1.4 NON-GEOLOGIC PREGROBBING                                   

  IV.5.2 NON-CARBONEOUS ORES                                           

   IV.5.2.1 GOLD ADSORBING                                             

   IV.5.2.2 CYANICIDAL                                                 

   IV.5.2.3 OXYGEN CONSUMING                                           

 IV.6.0 PROCESSING AT DEPOSITS EXHIBITING PREGROBBING                    
                                   
  IV.6.1 AFRICA                                                        

   IV.6.1.1 ASHANTI                                                    

   IV.6.1.2 BIBIANI                                                    

   IV.6.1.3 NEW MACHAVIE                                               

  IV.6.2 AUSTRALASIA                                                   

   IV.6.2.1 MACRAES                                                    

   IV.6.2.2 STAWELL                                                    

  IV.6.3 USA                                                           

   IV.6.3.1 ALLIGATOR RIDGE                                            

   IV.6.3.2 ATLAS GOLD BAR                                             

   IV.6.3.3 BIG SPRING                                                 

   IV.6.3.4 CARLIN                                                     

   IV.6.3.5 JERRITT CANYON                                             

   IV.6.3.6 KERR-ADDISON                                               

   IV.6.3.7 McDERMOTT                                                  

   IV.6.3.8 MERCUR                                                     

   IV.6.3.9 ROYAL MOUNTAIN KING                                        

  IV.6.4 SOUTH AMERICA                                                 

   IV.6.4.1 MORRO DO OURO                                              

   IV.6.4.2 ROSARIO DOMINICANA                                         

   IV.6.4.3 QUEIROZ                                                    

  IV.6.5 OTHER                                                         
                 
 IV.7.0 NON-PROCESS OPTIONS FOR PREGROBBING ORES                       
                                   
 IV.8.0 PROCESS OPTIONS FOR PREGROBBING ORES                           
                                   
  IV.IV.8.1 GRAVITY RECOVERY                                                  

   IV.8.1.1 FEEDSTOCK AND PRODUCTS                                         

   IV.8.1.2 CONCENTRATE TREATMENT                                          

  IV.8.2 CARBON FLOTATION                                                  

   IV.8.2.1 CARBON RECOVERY PRIOR TO CIL                                   

   IV.8.2.2 CARBON RECOVERY SUBSEQUENT TO CIL                              

   IV.8.2.3 CARBON DEPRESSION                                              

  IV.8.3 DISSOLVED GOLD CAPTURE                                            

   IV.8.3.1 SOLIDS DENSITY                                                 

   IV.8.3.2 CARBON MANAGEMENT                                              

   IV.8.3.3 OPTIMIZING PROCESS DESIGN                                  

  IV.8.4 OXIDATION OF PREG-ROBBERS                                         

   IV.8.4.1 CHLORINE OXIDATION                                             

   IV.8.4.2 PRESSURE OXIDATION                                         

   IV.8.4.3 ROASTING                                                   

   IV.8.4.4 BIO-LEACHING                                               

  IV.8.5 DE-ACTIVATION BY ADSORPTION                                   

   IV.8.5.1 USING POLAR ORGANICS AND SALTS                               

   IV.8.5.2 USING NON-POLAR ORGANICS                                     

  IV.8.6 COMPETITIVE ADSORPTION                                        
                 
 IV.9.0 CONCLUSION                                                     



1.0 OVERVIEW OF COMPLEX AND REFRACTORY GOLD ORES 

The refractory nature of gold derives from :

 [a] Encapsulation

    - Unliberated grains of gold

    - Gold in solid solution 

    - Rimming

 [b] Chemically bound or alloyed with other elements

 [c] Associated mineral reactivity with 
   cyanide, oxygen and aurodicyanide
  
 [d] Adsorption capacity of associated minerals 

    - Towards cyanicides

    - Towards oxygen consumers

    - Towards aurodicyanide

 [e] Coarse occurence, preventing complete dissolution in 
   conventional leach time

An ore may be characterized by placing on triaxial coordinates, 
the proportions of the three above-mentioned components, for the 
refractory portion. 

Swash [1986] uses a further, vertical coordinate to distinguish for 
a given deposit, the proportion of free-milling gold to the 
refractory component. 

Swash's practice is not followed in this monograph, in order to 
focus exclusively on the refractory components, and on similarities 
for better classifying these components. 

Furthermore multi-dimensional representations are difficult to 
depict. 

It should be noted that, apart from gold in solid solution, the 
refractory coordinates for a given ore are influenced by 
treatment and pretreatment, such as : 

 [a] Extent of :

    - Comminution

    - Pre-aeration

 [b] Excess of :

    - Reagent [cyanide, oxidant]

    - Competing adsorbent [carbon, resin]

 [c] Acceleration by :

    - Temperature

    - Agitation

    - Activators
 
The coordinate classifications thus generally imply cyanidation 
under typically adequate conditions for cyanidation, ie :

    - 500 ppm NaCN, pH 10.5

    - DO around 6 to 9 ppm

    - Ambient temperature [around 20C]

    - Agitation sufficient to maintain solids suspension 
      [40% solids, d80 = 75m, in slurry], for 24 hours

By this convention, nugget gold is a borderline refractory 
component, the relatively slow leaching rates under standard 
conditions generally causing losses of unleached cores. 

However in a satisfactorilly designed circuit such gold is in 
practice usually recovered by gravity, and does not become subject 
to cyanide recovery. 

Redox potential is the single most important factors in gold 
recovery, and indeed the original mineral deposition and nature 
of mineral associations. 

It is followed by pH. 

Figures 1 and 2 respectively illustrate the major natural processes 
of aqueous near-ambient mineral formation, and gold extraction 
technology. 

Similar concepts can be extended to extreme conditions, even molten 
material [with appropriate wider definition of acidity, [eg as 
electron acceptor - see Ellingham diagramme ?]. 

The basis of such diagrammes in aqueous systems were laid by 
Marcelle Pourbaix, and are bound by : 
 
 [a] The decomposition limits of water

 [b] The limits of proton and hydroxide concentration in water

Major fences which limit zones of formation and applicability of 
processing options within these bounds comprize : 

 [a] For liberation, those defining limits of soluble and insoluble 
   species of minerals holding gold

 [b] For extraction, those defining regions of stability of 
   complexed gold 

 [c] For both cases, the fences set by species necessarilly co-
   existing with the soluble species of minerals and gold.

The limits of these fences are not invariant, but are displaced 
by around 60/E millivolt per decade reagent change, where E is 
the number of electrons transfered in the reaction to cross the 
fence. 

Similarly a decade change in proton concentration displaces the 
fence by 1/d pH units where d is the number of protons transfered 
in the reaction to cross the fence. 

The electrochemical basis of these fences explains how. 

The same basis explains the particular slopes of a fence, which 

 [a] Is vertical if electron transfer is not involved in the 
   reaction to cross the fence

 [b] Is horizontal if proton transfer is not involved in the 
   reaction to cross the fence

 [c] Has in other cases a slope set by the ratio of electrons to 
   protons transfered in the reaction to cross the fence.

It should be noted that the utility of these concepts are 
tempered by two practical factors :

 [a] Correct definition of species and reactions, which may be more 
   complex than anticipated

 [b] Reactions occuring within a practical time frame. The fences 
   are theoretical ie thermodynamic [more correctly thermostatic] 
   limits, which may never be breached except by manipulation of 
   concentrations changing. In practice reactions may occur at a 
   practically negligible rate, eg because :

    - The activation energy of a reaction may be very high

    - In the case of heterogenous reactions [eg those between 
      solid, liquid or gas phases], large diffusion barriers 
      arize from concentration of non-reacting species, sometimes 
      reaction products, or poor local shear.

Marsden and House [1992] demonstrate on figure 3 the typical 
reactions for a metal-water system, and on figure 4 the Eh/pH 
windows applied in industrial gold extraction processes.

Baum [1988] illustrates the typical occurence of refractory 
problems for epithermal gold deposits as a function of depth, and 
so as determined fundamentally by the Eh/pH priflie and 
metamorphic processes involved.

Baum [1988] estimates that the sequence of significance of 
refractory factors determined by mineralogy for bulk mineable 
gold-silver operations in the Western United States [which 
include a number of heap leaching operations], from most to least 
significant, are : 
                   
                   [a] Liberation

                   [b] Ore variability

                   [c] Clay content 

                   [d] Acid-forming minerals

                   [e] Low permeability

                   [f] Cyanicides

                   [g] Oxygen consumers

                   [h] Sulphate formation

                   [i] Toxic elements

                   [j] Particle size distribution

                   [k] Coarse gold

                   [l] Base metal minerals

                   [m] Artificial contaminants
                   

von Michaelis [1992] provides a fairly comprehensive yet succinct 
summary of problems, their features, identification and 
remediation. Three tables below provide this summary : 

von Michaelis [1992] has also collected a number of useful 
guidelines, such as on deciding to toll smelt rather than 
implementing refractory processing at site [Nendick, - ]; and 
when concentration by flotation is, and is not warranted.

FIGURE 5 : TYPICAL REFRACTORY ORE TEST SCHEME
                      .------------------------.
                      | Mill to d80 = 74micron |
                      .--------.---------------.
               Au              |              ORE 
           EXTRACTION          |        CHARACTERIZATION      
               .---------------.----------------.
.--------------.--------------. .---------------.---------------.
|   Conventional cyanidation  | | Mineralogy/diagnostic leaches |
|after milling to d80=74micron| | + assay & cyanide extraction  |
.--------------.-------.------. |  by density fractions and     |
               |       |        |  by grit/sands/slimes sizes : |
               | .-----.-----.  |[over 100micron]    [<26micron]|
    REFRACTORY | |  Develop  |  |  Also ICP of solids, cyanide  |
               | | flowsheet |  |    liquors, and fractions.    |
               | .-----------.  .---------------.---------------.
.--------------.--------------.                 |                 
|    Intensive cyanidation    .---------.-------.------.-----etc 
|    [with LeachWELLTM 60X]   |         |              |          
|after milling to d80=74micron|   .-----.-----.  .-----.-----.    
.--------------.--------------.   |    pH     |  |  Pb(NO3)  |    
    REFRACTORY |                  .-----.-----.  .-----.-----.    
.--------------.--------------.   .-----.-----.  .-----.-----.    
|   Conventional cyanidation  |   |  Develop  |  |  Develop  |    
|after milling to d80<26micron|   | flowsheet |  | flowsheet |    
.--------------.-------.------.   .-----------.  .-----------.    
               | .-----.-----.                                    
    REFRACTORY | |  Develop  |                                   
               | | flowsheet |                                 
               | .-----------.    
               |                    .--------------------------.    
.--------------.--------------. YES |  Consider toll milling   |      
| Can a small rich fraction of.-----.  or boutique processes : |     
|  concentrates be produced ? |     | Arseno, Cashman, NITROX, |    
.--------------.--------------.     | HMC, INTECH [for Cu] etc |    
               |                    .--------------------------.    
            NO |                                                  
               |                                                 
.--------------.--------------.                                   
|  Assess S/As levels in ore  |                                  
|     and in concentrates     .--.                               
.-----------------------------.  |
         .-----------------------.---------------------.
.--------.---------.    .--------.--------.   .--------.-------.
|  High S, low As  |    | High S, high As |   | Low S, high As |
.--------.---------.    .--------.--------.   .--------.-------. 
.--------.---------.    .--------.--------.   .--------.-------.
|      Assess      |    |      Assess     |   |     Assess     |
| roast/acid plant |    |  pressure leach |   |  bio-oxidation |
|     flowsheet    |    |    flowsheet    |   |    flowsheet   |
.------------------.    .-----------------.   .----------------. 

Gasparrini [1982] has suggested the following test scheme for 
determining some more common causes of refractory gold behaviour 

                      .-------------------.
                      | Analyze for Te    |
                      .--------.----------.
                               |            
             FOUND             |             
               .---------------.-------.
.--------------.------------. .--------.----------.
|  Search for Au-tellurides | |   Analyze for Ag  .-------.
.--------------.------------. .--------.----------.       |
               |                       |                  |     
               |                 FOUND |                  |     
               |                       |                  |     
               |          .------------.------------.     | 
               .----------. Search for electrum and |     | 
               |          | secondary Ag2S coatings |     | 
         FOUND |          .------.------------------.     | 
               |                 |                        |
.--------------.----.            |         .--------------.-----.
| Do Te cyanidation |            |         |  Search for very   |
.-------------------.            .---------. fine gold and host |
                                 |         .--------------.-----.
                                 |                        |
                           FOUND |                        |
                                 |                        |  
                             .---.--------------------.   |
                             | Do Ag coating testwork |   |
                             .------------------------.   |
                                                          |       
                                   .----------------------.----. 
                                   | Do Au liberation testwork |
                                   .---------------------------. 
                                
Diagnostic leach methods have been developed by Scott [1988] and 
Hillard ao [1985], and are incorporated into the following more 
extensive scheme : 

Although 24-hour cyanidations are deemed adequate, acid/alkali 
pre-treatments should proceed for at least 120 hours to allow for 
some of the relatively slow alteration rates expected.

                             ACID LIBERATION

                             .-------------.
                             | Cyanidation |
                             .------.------.
                                    |             
                     .--------------.--------------.
                     |                             | 
             LEACHED |                             | pH 3
                     |                             |
            .--------.--------.           .--------.--------.
            | Cyanide-soluble |           | H2SO4/SO2 leach |         
            | precious metals |           .--------.--------.         
            .-----------------.                    |                        
                                                   |                     
                                            .------.------.             
                     .----------------------. Cyanidation |             
                     |                      .------.------.             
                     |                             | 
            LEACHED  |                             | pH 3        
                     |                             |        
.--------------------.--------------------.  .-----.------.             
| Precious metals in acid-labile minerals |  | HNO3 leach |             
.-----------------------------------------.  .-----.------.             
                                                   |                     
                                                   |                     
                                            .------.------.             
                     .----------------------. Cyanidation |
                     |                      .------.------.    
                     |                             | 
            LEACHED  |                             | pH 3        
                     |                             |        
       .-------------.-------------.     .---------.--------. 
       | Precious metals in oxide- |     | LeachWELLTM III+ | 
       |   acid soluble minerals   |     |   [acid] leach   | 
       .---------------------------.     .---------.--------. 
                                                   |                     
                                                   |                     
                                            .------.------.             
                     .----------------------. Cyanidation |
            LEACHED  |                      .------.------.    
                     |                             |             
                     |                             |             
       .-------------.-------------.      .--------.--------.    
       | Precious metals in Fe/Mn  |      | Silicate-bound  |    
       |    [hydr]oxide matrix     |      | precious metals |    
       .---------------------------.      .-----------------.    


                            ALKALI LIBERATION

                              .-------------.
                              | Cyanidation |
                              .------.------.
                                     |             
                     .---------------.---------------.
                     |                               | 
                     |                               | 
             LEACHED |                               | 1 M   
                     |                               |       
            .--------.--------.              .-------.------.  
            | Cyanide-soluble |              | NH4CO3 leach |           
            | precious metals |              .-------.------.           
            .-----------------.                      |                        
                                              .------.------.             
                     .------------------------. Cyanidation |             
                     |                        .------.------.             
                     |                               |                    
            LEACHED  |                               | 1 M                   
                     |                               |                    
.--------------------.----------------------.  .-----.------.             
| Precious metals in ligand-labile minerals |  | NaOH leach |             
.-------------------------------------------.  .-----.------.             
                                                     |                     
                                              .------.------.             
                     .------------------------. Cyanidation |
                     |                        .------.------.    
                     |                               |        
            LEACHED  |                               | pH 11            
                     |                               |             
       .-------------.-----------.         .---------.--------. 
       |   Precious metals in    |         | LeachWELLTM III+ | 
       | alkali-soluble minerals |         |  [alkali] leach  | 
       .-------------------------.         .---------.--------. 
                                                     |          
                                              .------.------.   
                       .----------------------. Cyanidation |   
              LEACHED  |                      .------.------.   
                       |                             |                   
         .-------------.-------------.      .--------.--------.         
         | Precious metals in Ni/Cu  |      |    Refractory   |   
         |        etc matrix         |      | silicate-bound  |   
         .---------------------------.      | precious metals |   
                                            .-----------------.   
                                                                 
The systematic classification of the most significant groups of 
refractory gold used in this study is :

 [a] Fine dispersion

 [b] Chemical rimming

 [c] Surface coating by Zeta-positive colloids 

 [d] Chemical bonding in gold minerals

 [e] Solid solution

 [f] Ligand consumers

 [g] Cyanicides

 [h] Pregrobbing

 [i] Simple ion exchange

 [j] Gold metal reprecipitation

 [k] Gold polymer formation

 [l] Mixed gold cyanide precipitates

 [m] Mineral reprecipitation

 [n] Oxidant consumers

 [o] Auto-passivation

 [p] Pyrometallurgical limitations : eutectics and gas diluents

 [q] Bio-oxidation limitations : biological toxins and/or natural 
   pH buffers 

 [r] High activation energy and/or diffusion barriers

2.0 POORLY EXPOSED GOLD

Auriferrous pyrite has a decreased microhardness, deemed due to 
linear dislocations caused by gold in the lattice. 

Magnetites in some deposits can carry gold, through lattice 
substitution with iron.

Scorodite from auriferous arsenopyrite veins generally contains 
significant levels of gold. 

Most is usually present as finely divided native gold, and some in 
the lattice, probably substituting for iron. 

Jarosites [basic sulphates of ferous iron and other elements] 
frequently contain high gold levels. 

The highest levels have been recorded for plumbojarosites, where 
most of the gold collects as fine particulates, also some lattice 
substitution. 

Where auriferous galena is encountered [not common], tellurium is 
commonly found with the gold.

The Te would be needed in the sulphur positions to provide electron 
compensation if gold replaced some lead; else gold tellurides might 
be finely dispersed in the galena. 

There is a general association of gold [and silver] with tungsten 
in many deposits. 

Wolframite and scheelite may contain some gold, probably as fine 
metal. 

This is generally associated with primary and rapid cooling, 
forming fine [6 micron] acicular arsenopyrite crystals. 

The refractory nature of arsenic is generally due to the 
encapsulation of gold either in very fine native form, or in 
solid solution. 

Gold is often encapsulated as fine inclusions in copper sulphides.

In copper, silver and antimony sulphides and sulphide-arsenides, 
gold occurs both in native form and as a lattice constituent.

This is particularly so in the case of chalcocite, bornite, 
chalcopyrite and enargite, which frequently carry relatively high 
gold content not detectable as metal at the highest magnifications. 

However for stibnite, the evidence is that little gold enters the 
mineral lattice. 

Any gold values found are generally from fine but separate native 
gold, aurostibite or with intimately intergrown gold-bearing 
tetrahedrite [Cu3SbS3, Cu12Sb4S13, or 4Cu2S.Sb2S3]. 

Oxidation is invariably required to render the gold amenable to 
cyanidation. 

The gold does not generally occur in domains or points of high gold 
content, and much evidence shows that the "invisible" fraction is 
bound in the pyrite and arsenopyrite matrix. 

Practically all pyrite and arsenopyrite contains antimony, 
sufficient to bind the gold in these mineral lattices, so 
neutralizing any charge imbalances. 

Tetrahedrite-tennantite are the sulphosalts most generally 
enriched by gold; generally as Au lattice substitution for Cu 
with native gold or other gold-bearing minerals being rare.

Similar enrichment does not occur in the lead analogues : 
boulangerite and jamesonite.

The evidence indicates that early formed high temperature 
arsenopyrite takes up gold largely in solid solution, or as atomic 
layers on the growing faces of the sulphide minerals. 

This explains the frequent occurence of much invisible gold and 
silver in relatively unfractured and unrecrystallized arsenopyrite. 

Similar silver minerals such as argentite, freibergite and 
argyrodite also frequently carry relatively high gold content not 
detectable as metal at the highest magnifications. 

The gold occurs both in native form and as a lattice constituent. 

2.1 PHYSICAL ENCAPSULATION

Apart from gold released by weathering, it is always bound in a 
mineral matrix. 

The degree of liberation of such gold is then a function of the 
size distribution of the gold and of the comminuted matrix. 

For cyanidation, the existence of permeable fault lines, and the 
common occurence of gold preferentially along such fault lines, 
increases the effective liberation of gold. 

The most common encapsulation of gold is in sulphides. 

However occlusion in siliceous gangue also occurs, which can have a 
major impact on the chosen refractory treatment process.

Only roasting has been shown to cause appropriate microfracturing, 
releasing such gold; however high pressure roll crushing might also 
address this problem and allow recovery using other oxidation 
routes. 

Figures 6 to 8 [Marsden & House, 1990] show the sulphide 
association of free or non-refractory gold.  

The hydrothermal breccia type mineralization of gold along cracks 
and fissures, accesible to solution, is a major reason for the 
initial success and rapid evolution of heap leaching technology 
in the Western American [Nevada] gold belt. 

[a] FINE DISPERSION

Figures 9 to 11 show various gold associations with sulphide 
minerals where encapsulation of the gold will prevent simple 
recovery by cyanide, but where finer milling will in general 
liberate such gold, and allow it to be recovered by cyanidation. 

Figure 12 shows the extreme case where the gold is colloidal or in 
solid solution, where finer milling will achieve no significant 
improvement in recovey.

Marsden and House [1992] note that ilmenite and rutile can also 
contain fine gold difficult to liberate.

[b] RIMMING BY PRECIPITATES

Marsden and House [1992] illustrate the coating of gold grains 
from placer deposits by mercury, silica and iron oxides [figure 
13], and note the occurence of coatings of Ca, Fe and Mg oxides 
and carbonates.

Rimming may be particularly prominent in tailings from previous 
processing, eg at Forrest Hill [Cristovici, 1986].

Iron oxides

Rimming of gold by rust is common in weathered iron sulphides 
[Koen & Feather [1973], Head [1935].

The mechanism and extent of the formation of iron coatings of 
gold is explained by the Enzweiler and Joekes (1991) gold ore 
genesis model. 

Koen and Feather [-] note the important role of iron oxide 
coatings in preventing the cyanidation of liberated gold. Such 
iron derives from the sulphides originally assiciated with the 
gold. 

Their experiments showed it was possible to form such coatings on 
electrum, but not pure gold.

von Michaelis notes an iron oxide rimming problem at the Venice 
Mine, Zimbabwe.

Figures 14 to 15 from Marsden and House [1992] show the occurence 
of gold locked by films of hydrous iron oxides, recovered from 
California, Nevada and Rodalquilar, Spain.

Haematite coatings contribute to the refractory nature of gold at 
McLaughlin, Clear Lake. 

Auric oxide

Flatt ao [1991] also refer to an extremely refractory rimming, by 
Au2O3. 

Tarnished and iridescent films of such gold have been recovered 
from vein cavities have been recovered from the Phillipines, and 
more recently from an Indonesian prospect. 

The origin has been infered to be alteration of AuCl43- to 
Au(OH)3 and then dehydration to Au2O3 at temperatures below 
150C. 

An unusual but highly refractory rimming phenomenon was noted by 
Fander [-]. 

An oxide gold layer [confirmed by microprobe], formed probably by 
chloro-complex decomposition under highly oxidizing conditions, 
proved resistant to all cyanide, aqua regia etc attack. 

Only after toasting at 150C, which transformed this layer to 
elemental gold, could it be leached. 

Silver chloride

Edwards [1954] has noted the occuence of gold coating by 
chlorargyrite [AgCl]. 

Silica

Baum [1988] notes that the formation of complex silica-sulphate 
coatings on minerals including gold is increased in the presence 
of amorphous or poorly crystalline mineral phases, such as :

 [a] Amorphous silica sinters

 [b] Al or Al/Mg silicates

 [c] Partially altered volcanic glass, and glassy volcanic rocks.

Natural Na2SO4, or formed in processing [eg by sodium introduced 
with cyanide or alkali] is believed to greatly increase the 
solubility, and propensity for scaling, of silicates in a 
metallurgical environment.

Mobilization of clay and silica may occur over the long periods 
of heap leaching, leading to precipitation of poorly crystalline 
secondary phases such as illite, gibbsite, and swelling, mixed-
layer clays. 

Kurtz ao [1987] state that this can lead to plugging problems, and 
the erosion of agglomerated fines. 

Baum [1988] notes that most Tertiary and younger pyroclastic 
rocks of calc-alkaline character exhibit porosities up to 30%; 
whereas the more basic volcanics have very low porosities [<6%].

Other

Baum [1988] notes that the formation of complex coatings on 
minerals including gold is also increased in the presence of 
other amorphous or poorly crystalline mineral phases, such as : 

 [a] Hydrous Fe-Mn-Al oxides

 [b] K-Al/Fe sulphates

Baum [1988] notes the following other frequently scale-forming 
minerals found in epithermal gold-silver ores : 
                                                   
 [a] Gypsum                                          
                                                   
 [b] Bassanite                                       
                                                   
 [c] Anhydrite                                       
                                                   
 [d] Calcite                                         
                                                   
 [e] Siderite
                    
 [f] Dolomite         
                    
 [g] Phosphates       
                    
 [h] Iron sulphates   
                    
 [i] Amorphous silica 

Liebenburg [1972] has suggested that Sb, Mn and Pb compounds can 
rim refractory gold.

In my experience, the solutions suspending these minerals become 
saturated in ther salts during milling. 

Precipitation occurs on sutable sites when the slurry thereafter 
cools down the process train, or absorbs atmospheric carbon 
dioxide, or has time to deposit onto active or seeded sites. 

Solution tenors can, in the case of precipitation onto active or 
seeded sites remain practically onstant, as more salts ae brought 
into solution fom the mineral, as these salts are removed by the 
precipitation onto the active sites.

[c] SURFACE COATING BY ZETA-POSITIVE COLLOIDS

Other coatings, as precipitates, more often as colloids of 
opposite [viz positive] charge to the gold surface have been 
noted inter alia by Gasparini [-], and Baum [-]. 

Such coatings include acanthite and cinnabar. 

Epithermal deposits of the adularia-sericite type [generally 
formed over 1000000 years ago], contain silver sulphides such as 
acanthite [Ag2S], proustite [Ag3AsS3] and pyrargyrite [Ag3SbS3] 
which colloidally coat gold, preventing effective cyanidation. 

Gasparini [1980] has noted refractory rimming of gold by 
acanthite. 

Mintek [####] has reported similar rimming by cinnabar. 

Baum [1988] refers to rimming of gold by clay and related gangue 
colloids, noting in particular, illite. 

The mechanism is probably electrostatic, such rimming colloids 
being [unusually] Zeta-positive under alkaline cyanidation 
conditions. 

The sulphide stibnite can be expected to exhibit similar 
behaviour, as well as a range of other oxide minerals which have 
a negative Zeta potential under such conditions, is given in 
table 6. 

TABLE 6 : Minerals exhibiting high Point of Zero Charge. 

   MINERAL                         pH AT 
                             ISOELECTRIC POINT 
                             [point of zero charge]
  Alumina        Al2O3             9.0

  Andalusite     Al2O[SiO4]        7.2

  Antimonite     Sb2S3            over7

  Apatite        Ca[PO4]2          7.5

  Boehmite       AlO.OH           7.8 to 8.8

  Calcite        CaCO3             8.3 to 10.8

  Chromite                           5.6 to 7.0

   MINERAL                         pH AT 
                             ISOELECTRIC POINT 
                             [point of zero charge]

  Chrysotile     Mg6[Si4O10][OH]8   12.4 

  Corundum       Al2O3             6.9 to 9.4

  Fluorite       CaF2              7.0 to 10.2

  Haematite      Fe2O3             4.8 to 9.0

  Hydroxyapatite Ca5[PO4]3OH        7.0

  Kyanite        Al2O[SiO4]        7.9

  Malachite      Cu2CO3[OH]2        9.7

  Magnesia       MgO             12.0

  Mullite        [Al2O3][SiO2]4     8.0

  Oil                             9.0

  Pyrite         FeS              7.0

Baum [1988] notes the following troublesome colloidal minerals :

 [a] Clay

 [b] Sericite

 [c] Pyrophillite

 [d] Chlorite

 [e] Talc

 [f] Diaspore

 [g] Jarosite

 [h] Scorodite

 [i] Carbonates

 [j] Zeolites

 [k] Alunite

 [l] Goethite; Limonite

 [m] Hydrous manganese oxides

In the experience of Baum [1988] over more than a decade, 40% of 
gold ores are overground, leading to substantial losses in the -
500# slimes fraction. 

Gold recoveries have been increased by up to 10% by flash 
regrinding to freshen gold particle surfaces for flotation or 
cyanidation. 
 
2.2 CHEMICAL BONDING IN GOLD MINERALS

[a] SIDEROPHILES [METALS]

Siderophilic associations [native metal and alloy compounds] 
contribute to the refractory nature of gold. 

Being voracious oxygen scavengers, they starve all gold of this 
reagent when intimately or closely associated. 

Gold forms a number of natural alloys and compounds, most of 
which significantly retard cyanidation rates. 

In many cases the retardation is due to the substantial oxidant 
demand to oxidize the less noble component [eg 5 electrons/atom for 
Te, Sb, Bi]. 

Table 7 [Boyle, 1979] lists the minerals formed by gold.

TABLE 7 : GOLD MINERALS
                              Alloys
                 Cuproauride            Au,Cu        
                 Porpezite              Au,Pd        
                 Rhodite                Au,Rh        
                 Iridic gold            Au,IR        
                 Platinum gold          Au,Pt                                   
                 Bismuthian gold        Au,Bi      
                 Aurobismuthinite                         
                 Bismuthaurite                            
                 Cuprian gold           Au,Cu
                 Auriferride            Au,Fe
                 Electrum               Au,Ag : over 20% Ag
                 Kustelite              Ag,Au : over 50% Ag
                  [aurian silver] 
                           Tellurides :
                 Calaverite             AuTe2
                 Krennerite             (Au,Ag)Te2
                 Montbrayite            (Au,Sb)Te3
                 Kostovite              AuCuTe4
                 Nagyagite              Pb5Au(Te,Sb)4S5-8
                                        Au2SeTe
                       Silver/gold minerals
                 Petzite [antamokite]   Ag3AuTe2
                 Muthmannite            (Ag,Au)Te
                 Sylvanite              (Au,Ag)Te4
                 Uytenbogaardite        Ag3AuS2       
                 Fischerite             Ag3AuSe2  
                                        AgAuS
                                        Ag3AuTe
                                        Au4AgTe10
                        Other compounds :
                 Gold amalgam           Au2Hg3 (?) 
                 Maldonite              Au2Bi      
                 Auricupride            AuCu3      
                 Palladium cuproauride  (Cu,Pd)3Au2
                 Auristannide           Au,Sn
                 Aurostibite            AuSb2                      
                 Maldonite              Au2Bi
                 Hessite, Vandiestite 

[b] CHALCOPHILES [SULPHIDES]

A few compound sulphides of gold and other metals exist : 

Uytenbogaardite [Ag3AuS2] 

AgAuS

Nagyagite [Pb5Au(Te,Sb)4S5-8]

2.3 SOLID SOLUTION

Gold occurs in solid solution in sulphides. Most prominent is 
Arsenopyite, particularly in fine [acicular] crystals, where 
rapid formation prevents migration [zone refining] to form larger 
occluded grains. 

Figure 16 [Cathelineau et al, 1988] shows the sort of zonal 
enrichments which may occur due to such refining effects. 

Solid solution gold is also reasonably common in Pyrite, however 
the concentations are generally not as high, as illustrated by 
the frequency distribution determined by electron microprobe 
analysis of numerous pyrite and arseno-pyrite grains of Fairview 
Gold Mine sulphides [Swash, 1986] - figure 17.

[a] SULPHOSALTS

Silver minerals such as argentite, freibergite and argyrodite 
also frequently carry relatively high gold content not detectable 
as metal at the highest magnifications. 

Silver and arsenic is commonly associated with pyrite in the host 
rocks and deposits. 

The associations with arsenopyrite are silver and antimony. 

Gold is not consistently related to arsenic nor antimony levels in 
pyrite, nor to antimony in arsenopyrite. 

The exceptions are where pyrite contains visible gold as blebs or 
in late fractures. 

[b] SULPHIDES

Evidence indicates that early formed high temperature pyrite and 
arsenopyrite take up gold largely in solid solution, or as atomic 
layers on the growing faces of the sulphide minerals. 

This explains the frequent occurence of much invisible gold and 
silver in relatively unfractured and unrecrystallized pyrite and 
arsenopyrite. 

In contrast, gold and silver at lower temperatures migrate to 
nearby low chemical potential sites such as fractures and grain 
boundaries where they crystalize as argentiferous gold 
[presumably because the lower temperatures dictate slower cooling 
rates]. 

Where reworking, recrystallization and fracturing are present, 
the gold is largely present in native form. 

Thermal processing has been shown to promote gold exsolution and 
migration preferentially along fractures and grain boundaries, 
where it crystalizes out. 

Gold generally associates more with arsenopyrite, silver with 
pyrite.

Halides, even of precious metals [chlorargyrite, AgCl] are 
generally practically devoid of gold. 

Gold in some wad-silver ores is amenable to cyanidation. 

The remainder requires release by intensive reduction, as for 
silver. 

The gold and silver may occur both as uniformly distributed, and 
erratically. 

Gold and silver-bearing wads carry considerable quantities of Si, 
Al, Ti, P, As and Sb. 

3.0 REAGENT AND PRODUCT CONSUMERS

This subject is dealt with fairly briefly, to illustrate the 
effects rather than to provide a comprehensive treatise, which 
would comprise a major task.

Although the classification of reactive minerals in cyanidation 
is generally by cyanide and oxygen reactivity, most minerals 
react with both.

Reactions are also often not direct, but with a reaction product 
such as Fe2+, generally deriving from reactive ferrous sulphides. 

The Fe2+ need furthermore not be fresh, but can be maintained eg 
through sorption in sheet silicates for long times before 
reacting in cyanidation. 

One of the major reactions with cyanide is also basically due to 
to the cyanicidal action of the mineral. 

Apart from the reactivity of the mineral to cyanide itself, 
discussed below, the effect of cyanicides is determined to some 
extent by the strength of the complex formed. 

Thus a weak complex such as with zinc will have little effect, 
particularly at higher pH where this complex is broken and the 
bound cyanide released. 

3.1 LIGAND CONSUMERS

[a] CYANICIDES 

Fe, Cu, Ni, and Co, which bind cyanide strongly, can create a 
refractory behaviour through excess consumption of this reagent.

In general, this refractoriness may be reduced by ensuring a 
sufficiency of cyanide. 

Unfortunately most of the data on cyanide complex strength is for 
breaking by acid [which combines with the CN- ion] rather than 
alkali, which alternatively binds the metal, releasing CN-.
 
However the data does still provide an indication of practical 
relative cyanide complex strengths, although anomalies exist such 
as the near impossiblity of breaking up the iron complexes of 
cyanide by acid, compared to the rlative ease by cyanide.

Tables 8 [a] and [b] indicate the typical proportion of cyanide 
complexes dissociable by acid, and active to gold. 

TABLE 8 [a] : PROPORTION OF REACTIVE CYANIDE 
              IN STRONG COMPLEXES 

Complex                Distilled        Active to gold      
                     by acid [APHA]                         
                        [1 hour,                            
                     pH 4.3 buffer]                         
                                                            
Co(CN)63-                  20%             0%               
Au(CN)2-                   49%             0%               
Fe(CN)63-                  83%             0%               
Fe(CN)64-                  89%             0%               
                                                            
TABLE 8 : PROPORTION OF REACTIVE CYANIDE 
          IN WEAK COMPLEXES 

Complex                Distilled        Active to gold      
                     by acid [APHA]                         
                        [1 hour,                            
                     pH 4.3 buffer]                         

HCN                       100%             0%               
Cu(CN)43-                 100%            40%               
Ni(CN)43-                 100%            80%               
Zn(CN)43-                 100%           100%               

Simple estimation by stability constant analysis does not always 
give an accurate reflection of the distributions of cyanides in 
practice. 

As an example, cyanide titration by silver nitrate should be 
barely affected by pH in the range around the pKa of HCN [9.3], 
even though the pKa of the argentodicyanide is far higher, viz 
18.7. 

It appears that CN- bound by H+ reacts very slowly. 

This is confirmed by the study of Scoggins [1970], who found that 
that Ni(CN)2- forms in seconds at pH 9.8, but takes over two hours 
to achieve substantially complete conversion at pH 5.6 [the symbols 
appear reversed figure 18, as shown in his paper]. 

However, where leaching takes place well away from the bulk 
fluid, eg within micaceous blades, local cyanide concentrations 
may remain very deficient, causing serious refractoriness to 
develop. 

Thus mixed gold-metal cyanides, which are practically insoluble 
under normal cyanidation conditions can precipitate within the 
particle-bound fluid region. 

Apart from the consumption of cyanide by ligand formation, losses 
may occur by oxidation, eg by Cu2+ [to cyanate], or by 
sulphoxides [to thiocyanate].

The sources of Fe, Cu, Ni and Co is most commonly from their 
reactive sulphides [see section 5.2.2]. 

However important classes of this type of refractory behaviour 
results from oxide minerals [however generally excluding iron, 
except for the carbonate ankerite], and ionic species adsorbed onto 
minerals particularly micaceous clays near the water table. 

[b] SULPHOXIDANTS

This is noted merely to place on record a mechanism likely to be 
of growing importance in gold recovery, albeit presently in a 
development stage.

Dissolution by thiosulphate is likely to become a gold recovery 
route as the processing of refractory sulphides proceeds. 

It is generated autogenously, and avoids the significant 
liquid/solid separation problems and high cyanide consumptions 
inherent in the convential processing of such materials. 

Because of the ease of reprecipitating gold from the relatively 
weak thiosulphate complex, in-leach adsorptin, chelation or 
precipitation recovery is likely to be required. 

Thiosulphate is however a metastable species, sulphate being the 
final state of sulphur under the appropriate conditions for 
thiosulphate leaching of gold.

Sulphoxidants, which would promote the degradation of the labile 
thiosulphate to sulphate, would prove a problem in this process 
route. 

Being of a future and peripheral nature, further information on 
such promotors have not been included or sought. 

3.2 PREGROBBING

An understanding of the main pregrobbing mechanisms is important 
in managing the problem. 

The simplest pregrobbing is simply adsorption of gold by ion 
exchange. 

Aurodicyanide is a large anion, with naturally good 
attractability to ion exchange sites. 

This reversible [type I] pregrobbing forms the basis of gold 
losses in processes such as Merrill-Crowe where there is no 
competing adsorption eg by carbon or resin. 
                           
It has been found that generally about 5 to 35% of the gold on 
slurry solids is dissolved but held by ion exchange. 

Apart from competing carbon/resin adsorption, such gold can be 
removed by washing, particularly if the wash water is hot and/or an 
eluant, eg saline. 

Type II or "irreversible" pregrobbing is deemed so because a long 
time and/or unusually severe conditions are required to 
redissolve gold pregrobbed by the four mechanisms involved. 

All four type II mechanisms involve precipitation. 

The most common are types IIa [precipitation of the auromonocyanide 
polymer] and type IIb [precipitation of mixed metal cyanides]. 

Type IIc and IId pregrobbing are not discussed here as they are 
less universal, being restricted to Carlin carbonaceous ores.
 
Furthermore their behaviour with respect to assay-robbing, a 
primary matter of this monograph, is unknown. 

One chemical reaction which has been noted, is decomposition of a 
bicarbonate at pH about 11.6, followed by precipitation of 
carbonates while pH is pulled down autogenously through this 
precipitation, towards the bicarbonate/carbonate buffer point 
around 8.3. 

The CaCO3 precipitate appears to cover and lock underlying gold. 

Residual iron, left after oxidation of iron sulphides also 
affects cyanidation, generally exhibiting a pregrobbing 
phenomenon.

Iron Ores exhibiting combinations of low/high acidity and 
low/high oxygen demand reflect the extent of Fe oxidation and 
washing out of hydrolysis products. 

Thus FeII causes greater oxygen demand as well as acidity, while 
FeIII only causes greater acidity. 

Residual FeII is often present in the oxide ore zones in arid 
regions, insufficient washing having occured to remove it. 

In wet regions, acid clays below the water table will have residual 
FeII deleterious to gold recovery. 

Figure 19 [Mann, 1984] explains the origin and ocurrence of Fe++ 
in the deeper portions of the lateritic weathering profiles of 
greenstones, source of most gold deposits. 

Edwards ao [1965] have drawn attention to the close association 
of divalent cations and clay particles, as they are closely held 
in the Stern layer and not readily subject to ion exchange with 
simple monovalent cations such as sodium. 

Bivalent cations such as calcium is required to promote their 
diffusion, and only at local pH levels not too far from the that 
of ferrous hydroxide And within clay layers, due to dynamic 
diffusion effects. 

The pH within the layers of precipitation [around pH 6]. 

It should be noted that bulk pH levels do not accurately reflect 
local pH levels at the particle surface sour waterlogged clays 
can be as low as pH 4. 

Considerable quantities of hydroxide must diffuse in to increase 
this to over pH 6. 

Furthermore twice as much hydroxide is required to precipitate 
the iron, compared to the stoichiometric amount of Ca++ 
displacing Fe++]. 

Further background of relevance in this very important area of 
clay-associated ions affecting cyanidation is provided by Wayman 
[1967] and Hem [1967]. 

The introduction of CIP promoted the resorption of dissolved gold 
held by slurry solids at many operations, including :

 [a] Gold Quarry, Nevada

 [b] Cortez Gold Mines, 1800 tpd since 1981

 [c] Mercur, Utah [Barrick], 3600 tpd since 1981

 [d] Renco, Zimbabwe, 600 tpd since 1980

In the earlier process of filtration, such gold would have been 
lost.

Even irreversible losses of dissolved gold, can be reduced, as 
demonstrated at 1.5 tpd pilot scale at Salsigne, Orleans, France.

USA patent 1519396/1922 claimed the application of organic acids 
to coat pregrobbing sites.

Simple bottle roll or BLEG determinations will not reveal any gold 
lost by pregrobbing. 

Even CIL bottle rolls reveal only gold subject to type I [carbon-
reversible] pregrobbing. 

[a] SIMPLE ION EXCHANGE

Sheet silicates, some forms of natural carbon and carbonaceous 
mineralization have significant adsorbtive capacity. 

In the absence of sufficient competition by carbon or resin, this 
may lead to binding and loss of gold. 

A less recognized but relatively widesprad problem is the holding 
by such minerals, of cyanicides and oxygen consumers such as 
divalent iron. 

Competition for cyanide within the sheet structure can lead to 
local precipitation of gold and pregrobbing. 

The interpretation of adsorption behaviour can be complcated by 
the non-uniform nature of particle surface charge. 

In general, particles are charged in solution, positively at low 
pH, negatively at high pH. 

Cations rather than anions tend to form the inner, tightly held 
layer of ions at high pH cyanidation conditions, with a more 
loosely held counter layer of anions. 

At low pH the situation reverses, with a neutral point called the 
Point [pH] of Zero Charge [Isoelectric Point]. 

The isoelectric point is unique to each mineral. 

However some complex minerals such as clays have a different point 
of Zero charge for the edges and platy portions of the particle. 

The edges are furthermore not homogenous, the layered structure 
leading to alternatively more negative and positive planes. 

[b] GOLD METAL REPRECIPITATION

Electrochemical reprecipitation involves electron transfer, re-
plating gold. 

This is the dissolution of gold by weak complexes such as 
chlorides, is not possible in the presence of reductants. 

Electrochemical reprecipitation even occurs with the strongest 
gold complexant known - cyanide. 

This may be caused by highly reactive reductive sulphides [see 
section 5.2.2]; and also by elemental sulphur which may form during 
sulphide oxidation, particularly when very low pH conditions 
develop. 

[c] GOLD POLYMER FORMATION

Less serious is precipitation in this region of auromonocyanide 
itself, caused by capture of one of the two cyanides associated 
with aurodicyanide from the local cyanide deficiencies caused by 
complexation with Fe, Cu, Ni and Co. 

This leads to the formation of an auromonocyanide polymer, which 
redissolves very slowly, certainly not effectively within the 
residence times generally provided in agitation leaching. 

Type IIa gold pregrobbing results from cyanicidal action, which 
strips the aurodicyanide of one cyanide radical, forming the 
uncharged aurononocyanide radical. 

The auromonocyanide radical, with one free electron forms. 

This readily combines with others to form long strings of the brown 
polymer [AuCN]n. 

Type IIa pregrobbing can occur when bulk cyanide concentrations 
are sufficient to satisfy bulk cyanicide demands.

The reason is that on a local level, within the hydrodynamic 
boundary layer surrounding particles, local generation of 
cyanicides might exceed the capacity of cyanide diffusing in, to 
satisfy their demands. 

Cyanide is stripped off aurodicyanide within the boundary layer, 
leading to its polymerization as aromonocyanide. 

On further cyanide diffusion satisfying the demands of the 
cyanicides, sufficient concentrations of cyanide can be restored to 
allow redissolution of the auromonocyanide polymer. 

However, only the tips of the polymer strings are attacked, and 
the rate is very slow - see figure 20. 

It is that gold which is robbed by the type IIa mechanism which 
contributes to the 10 to 20% of gold found dissolved in reclaimed 
tailings - residual cyanide discharged with tailings continues 
dissolvong primary liberated gold, but also reverses the 
polymerization. 

Often immediate re-leaching of the tailings will reveal 
negligible primary liberated gold; whereas a dissolution allowed 
to extend over months will allow depolymerization of a 
significant quantity of gold.

For the residence time of around 24 hours for a typical gold 
recovery circuit, gold robbed by the type IIa mechanism can be 
considered [practically] to be robbed irreversibly. 

[d] MIXED GOLD CYANIDE PRECIPITATES

Type IIb pregrobbing is irreversible for even longer periods, 
such as the lengthy periods applied in heap leaching. 

Gold robbed by the type IIb mechanism, is co-precipitated with 
metals, typically the hexacyanides. 

The pregrobbing action of both iron and copper are well-
demonstrated. Cobalt and nickel also appear to be able to act as 
type IIb pregrobbers. 

[e] MINERAL REPRECIPITATION

The mechanism is not satisfactorilly known or explained, but is 
believed to involve immediate re-capture into a precipitating 
matrix being formed simultaneously with cyanidation.

The evidence, that high pH [around 11.6] generally makes the 
bound gold accessible, and of subsequent tendency of fresh 
filtrates to form a calcareous precipitate buffering at about pH 
8.3, points to the release of gold from a bicarbonate, and 
recapture into a precipitating carbonate.

Carbonate is itself very often a refractory constituent for gold 
recovery : many systems require matrix destruction to gain access 
to the gold. 

3.3 OXIDANT CONSUMERS

Siderophile associations [see section 5.1.2] contribute to the 
refractory nature of gold. Being voracious oxygen scavengers, 
they starve all gold of this reagent when intimately or closely 
associated. 

Reactive sulphides are the most common consumers of oxyen [the 
sulphur often further forming sulphoxides which consume cyanide 
to form thiocyanate]. 

Invariably, the reactive sulphides owe their reactivity to alkaline 
hydrolysis, the most reactive being those decomposing at the lowest 
pH. 

3.4 NATURAL ACIDS

Cyanidation requires alkaline conditions, but many partially 
weathered ores are acid, formong slurries of aound pH 4 to 6.

The additional lime required to neutralize these slurries 
increases the propensity for scale formation, particularly of 
gypsum, as the natural acid, generally derived from oxidizing 
sulphides, is generally the sulphate.

4.0 AUTO-PASSIVATION

Filmer [-] demonstrated how gold in intimate contact with large 
surfaces of sulphides creating the electron sink needed for gold 
dissolution, can promote such a high current density in the 
oxidation of the gold, that it passivates.

Similar high local oxidation intensities is believed to 
contribute to the rimming of gold by a very resistant layer, as 
noted in section 5.1.1 [a].

5.0 OTHER

5.1 PYROMETALLURGICAL LIMITATIONS

In the roasting of gold ores, the presence of eutectic and glass 
formers [eg potassium, sodium, and antimony], can melts which 
flow and encapsulate gold that would otherwize be released by 
this process.

In roasting, the presence of substantial quantities of carbonate 
can reduce the fugacity and availability of oxygen during 
roasting, introducing a kinetic restraint. 

5.2 BIO-OXIDATION

Naturally occuring toxins, including poor water quality, can make 
gold ores refractory to bio-oxidation. 

Substantial quantities of sulphides, devoid of gold but more 
amenable than the gold-bearing minerals, can also make specific 
ores refractory [in terms of excess oxygen requirements and acid 
generation; also gold lock-up inherent in associated jarosite 
formation]].

Natural buffers above the precipitation pH of ferric hydoxide or 
excess carbonates and alkali's might make acid pressure oxidation 
of sulphides impractical, due to unacceptably high acid 
consumption. 

[b] KEY REJECTION FACTORS

Biocides

The presence of biocides, eg high Mo levels. Hg is also 
mentioned, as well as Ag; however high Ag ores [over 6 kg Ag/t] 
have been succesfully treated. 

Tolerance to dissolved species is however often surmountable by 
developing tolerance in the microbes, Thus microbes have been 
developed to the folowing tolerance limits : 

 [a] 120000 ppm Zn                                                  
 [b]  72000 ppm Ni                                                  
 [c]  55000 ppm Cu                                                  
 [d]  50000 ppm Fe                                                  
 [e]  30000 ppm Co                                                  
 [f]  20000 ppm As                                                  
 [g]  12000 ppm Uranyl                                              
 [h]   6000 ppm Al                                                  
 [i]   5000 ppm Co                                                  
 [j]    700 ppm Ag                                                  
 [k]    300 ppm Sb                         
 [l]    250 ppm Cl                         
 [m]    200 ppm Mg                         
 [n]    200 ppm Ca                         
 [o]    100 ppm Cu                         
 [p]     80 ppm Se                         
 [q]     20 ppm Pb                         
 [r]     20 ppm Tl [phosphate-free medium] 
 [s]      0.01 ppm Ag                      
 [t]      0.005 ppm Au                     

These limits, and metabolite and catabolite repression might be 
controlled by intertage bleeds, reducing their concentration and 
thus effects on reaction rates in subsequent stages.

Some neutralization during oxidation is required, as few microbes 
can tolerate under pH 0.7.

Other substances, however for which no concentrations are 
available include : 

 [a] Oxidants eg NO3-, NO2-, Cl2, H2O2 
 [b] Cd 

Ore variability

Highly variable ore, putting a strain on the capacity of the 
microbes to adapt to changing matrices. This prevented 
consideration of bio-oxidation at the Getchell gold operation, 
Nevada. 

Excess alkalinity

Excess alkalinity, preventing achievement of the low pH 
[around 1.1 to 1.5] required. 

This single factor has caused failures of bio-oxidation at 
Tuscarora [1960's], and at the Giant Bay trial at Texasgulf's 
Cripple Creek operation. 

Low alkalinity also prevented consideration of the bio-
oxidation option at Barrick's Mercur and Freeport's Jerritt 
Canyon operations.

5.3 GENERAL

As mentioned at the start of section 5, kinetic restraints might 
effectively make a gold ore refractory to recovery.

In practice reactions may occur at a practically negligible rate, 
eg because : 

    - The activation energy of a reaction may be very high

    - In the case of heterogenous reactions [eg those between 
      solid, liquid or gas phases], large diffusion barriers 
      arize from concentration of non-reacting species, sometimes 
      reaction products, or poor local shear.

A remarkably large fraction of gold losses have been traced to 
incomplete dissolution of larger gold particles. 

Most modern plants collect these as a gravity concentrate; which 
is often most effectively further beneficiated by high intensity 
cyanidation, eg at Welkom, South Africa, where a 3.5 t batch of 
concentrates containing 3500 g Au/t has been treated daily since 
1977.

Coarse gold is usually captured by gravity, cleaned for direct 
smelting [the high grade tails and middlings being returned to 
the mill], or intensively cyanided.

                    APPENDIX I : GEOLOGY Of GOLD 



                               INDEX


 I.1.0 GENESIS OF GOLD DEPOSITS                                 I.2

   I.1.1 HYPOTHERMAL DEPOSITS [GOLD OF DEEP VEIN ORIGIN]       I.11

   I.1.2 MESOTHERMAL DEPOSITS [GOLD OF MID-VEIN ORIGIN]        I.13

   I.1.3 EPITHERMAL DEPOSITS [GOLD OF SHALLOW VEIN ORIGIN]     I.13



 I.2.0 IMPORTANT LARGE TONNAGE [GENERALLY] LOW GRADE DEPOSITS  I.13

   I.2.1 GOLD OF SHALLOW VEIN ORIGIN [EPITHERMAL]              I.13

   I.2.2 SEDIMENTARY DEPOSITS [CARLIN TYPE]                    I.14



 I.3.0 OTHER PRIMARY DEPOSITS                                  I.15

   I.3.1 [ORTHO-] MAGMATIC SEGREGATIONS                        I.15

   I.3.2 PEGMATITES                                            I.15

   I.3.3 PYROMETASOMATIC DEPOSITS [SKARNS]                     I.15



 I.4.0 DEPOSITS DEVELOPED BY ALTERATION AND WEATHERING         I.16

   I.4.1 OXIDE ZONE                                            I.16

   I.4.2 SECONDARY [SUPERGENE] ENRICHMENT                      I.16

   I.4.3 PLACERS                                               I.21

                    APPENDIX I : GEOLOGY Of GOLD

This monograph provides background to the THE METALLURGICAL 
MINERALOGY OF GOLD, and summarises :

 [a] The genesis of gold deposits

    - Deep vein [hypothermal]

    - Mid vein [mesothermal

    - Upper vein [epithermal]

 [b] The important large tonnage [generally low grade] deposits

    - Gold of shallow vein origin [epithermal]

       * Back arc volcanogenic

       * Island arc

    - Sedimentary deposits [Carlin type]

 [c] Other primary deposits

    - [Ortho-] Magmatic segregations

    - Pegmatites

    - Pyrometasomatic deposits [Skarns]

 [e] Deposits developed by weathering

    - Supergene [near-surface]

    - Oxidized gold ores

    - Placers

Gold minerals and mineral associations; refractory gold 
by major associated element] are described in Appendices II and 
III.

I.1.0 GENESIS OF GOLD DEPOSITS 

It has proved necessary to draw upon geoscience sources up to 50 
years old. 

Some data and interpretations may very well be dated; and the 
appropriate sections will be updated [but not within this study] 
through consultation with associates with particular skills in this 
area. 

Figures I.1 [a] to [e] summarize the structure and genesis of the 
geological processes which form gold [and other] deposits. 

These figures also provides a good overview summary of the 
elemental associations within Cordilleran deposits. 

Many elements and their minerals act homologously with gold. 

In the compilation below, comments on the behaviour of gold in 
the presence of the minerals of a given element are extended or 
cross-noted where appropriate, where similar behaviour in 
homologous cases exists. 

These extended comments are placed again under each element 
category, so that the information does not require scouring of 
all the text to reveal it, should the associations of a specific 
element [and potentially it's homologues] be sought.

Systems of gold deposit classification are numerous, and those 
currently popular, based on the method hosted, are generally 
fairly complex. Geologically, gold deposits have been classified as 

 [a] Placer :

    - Paleoplacer [Wits quartz pebble conglomerates]
      [Source of 60% of world gold]

    - Recent [Source of 15% of world gold]

 [b] Epithermal gold-silver

    - Intrusive-hosted

    - Volcanic-hosted

    - Sediment-hosted [Carlin-type]

       - Native Au

       - Pyrite/Arsenopyrite

       - Stibnite, realgar, orpiment, cinnabar

       - Kaolinite, dolomite, calcite

       - Host rock : carbonaceous, calcareous, 
                     variously silicified 

    - Acid sulphate type [enargite gold deposits]

       - Pyrite

       - Native gold

       - Luzonite, enargite, covellite

       - Alunite, kaolinite, pyrophyllite

    - Adularia-sericite type

       - Galena, chalcopyrite, sphalerie

       - Pyrite

       - Pyrargyrite, proustite, acanthite

       - Native gold, electrum

       - Quartz, K-feldspar, calcite

     - Vein

     - Disseminated

 [c] Archaen greenstone 

    - Gold-quartz veins

    - Iron-formation hosted [incl Lower Proterozoic],
      which generally exhibit weatering profiles, viz 
  
     - Alluvial deposits

       - Surficial lateritic zone

       - Supergene enrichment related to 
         historic or recent water tables. 

 [d] Turbidite-hosted [Slate belt, Shale-hosted]

 [e] Skarn gold [Igneous intrusion into carbonate-rich rocks]

 [f] Stratabound [iron formation-hosted]

This study applies the classification of Emmons [1937], which 
based on the formation processes. 

This approach is deemed one of the most understandable, and is also 
the most meaningful for correlation of metallurgical behaviour, 
including the roles of oxidants : 

 [a] Magmatic segregations

 [b] Pegmatites

 [c] Pyrometasomatic deposits

 [d] Hypothermal deposits

 [e] Mesothermal deposits

 [f] Epithermal deposits

 [g] Cryothermic deposits

 [h] Sedimentary deposits

Gold deposits formed by hypo-, mezo- and epithermal liquor 
transport and deposition comprize of the most important types of 
gold deposit.

In nature gold may be solubilized as halogen, sulphide, 
polysulphide, antimony-arsenic-sulphur, telluride, telluride-
sulphide, cyanide, thiosulphate, thiocyanate, organometallic and 
other complexes. 

Under restricted conditions gold may also be rendered mobile as a 
colloid. 

Boule [1979] notes numerous mechanisms of gold transport by 
aqueous media, and believes that decomposition of gold complexes 
[under hypogene hydrothermal conditions] to be the most important 
mode of consequent precipitation. 

This might be caused by : 

 [a] Eh [oxidation precipitates gold from sulphides, arsenides and 
   antimonides]

 [b] pH, eg contact with alkali carbonates or acid bog waters, 
   which respectively decompose acid [eg AuCl4-] or alkali [eg 
   Au(S2O3)23- or AuS- complexes. 

 [c] pH changes by indirect mechanisms such as 

    - Dilution with neutral groundwater

    - Withdrawal of alkali and alkali earth metals [the balancing 
      cations of the complexes] eg by sericitization, 
      albitization etc which increases pH

 [d] Withdrawal of sulphide ion, eg by reaction with iron in the 
   host rocks [to form pyrite, pyrrhotite, arsenopyrite and 
   gudmundite etc] decomposes Au(S2O3)23- or AuS- complexes. 

 [e] Reduction in concentration of the complex ligand, eg Cl- 
   or S-, leading to complex decomposition.

Boyle [1979] notes that adsorption and/or coprecipitation of gold 
[under supergene conditions] is the second most important mode of 
gold concentration in natural settings. 

Coagulation and/or coprecipitation of gold colloids can occur by 
interaction with a great many natural substances. 

A discussion of the coagulation and/or coprecipitation of gold 
colloids is beyond the scope of this study, being very complex.

Numerous substances can affect and even reverse the relative 
charges on the colloids, and the coagulating and/or coprecipitating 
solids; some solids even having anisotropic charge distribution. 

An example is kaolinitic clays, which carry a net negative charge 
down to pH 3.6, but which which retain a negative charge on the 
001 face always greater than a positive charge on the edge faces.

These positive sites account for the commonly observed 
concentration of gold in such clays, however repelled if there 
is a sufficiency of highly adsorptive anions such as phosphate to 
neutralize these sites.

One should note though that the generally negative charge of 
gold, and positive charge of ferric oxides below pH about 7.5 could 
explain the common enrichment of gold in many limonitic gossans. 

Boyle [1979] also notes a number of other mechanisms, including : 

[a] Vapour processes, eg
 
    - Decrease of in ligand in vapour form, eg from wall 
      reactions

    - Sublimation/condensation

[b] Death and sedimentation of gold-gathering biota

The deposition of gold is dictated by magmatic, sedimentary and 
metamorphic cycles. 

Geologic activity can cause molten gold-bearing magma to intrude 
into overlying rocks. 

During the fractional crystallization of such a magma, gold 
associates with other metals [eg PGM's] which are partitioned in 
the sulphide melt, and does not crystalize with early-formed 
silicate minerals. 

Later, hot aqeous fluids might pass through fracture zones formed 
in the sulphides by further geologic activity. 

In such so-called hydrothermal fluids associated with these 
igneous and metamorphic processes, gold is transported mainly as 
AuHS-, but as AuCl2-above 400C in chloride-rich, sulphur poor 
solutions under acidic and oxiding conditions. 

Deposition occurs as these fluids migrate upwards through 
fracture systems and other permeable zones, as lower temperatures 
and pressures are encountered, or when mixing with other fluids 
or chemical interaction with rock walls occur which :

 [a] Reduce the ligand [HS-, Cl-] activities, or 

 [b] Cause fluid composition [pH and/or sulphur or oxygen fugacity] 
   less favourable to hold gold and associated mineralization in 
   solution. 

Apart from the local chemistry, the depth at which deposition 
occures generally dictates the temperature, consequently the 
stable mineral species, and thus the associated mineralization. 

A deposit may be subjected to a series of geological processes, 
which involve further intrusion by magma, and/or hydrothermal 
fluid transport and deposition, and/or weathering and/or physical 
concentration in placers.

Because of the numerous sequences and combinations of sequences 
which can dictate the deposition of gold and associated 
mineralization, the characteristics of gold deposits are 
extremely diverse.

They do not lend themselves to providing an encompassing basis for 
classifying gold occurences and metallurgical behaviour. 

However, some broad classification is possible considering the 
effects of weathering once a sulphide deposit has been formed.

Sulphide weathering can cause near-surficial gold to be brought 
into solution by thiosulphate; and is generally deposited by 
reduction in manganese-rich rock. 

It is generally enriched from electrum to native gold. 

Under such weathered caps, generally above and near the water 
table, a zone of enrichment occurs; below which lies primary ore.

Fully weathered ore leaves less soluble oxides and silicates as 
gangue, and causes supergene [near-surface] enrichment.

This process is fairly important, as it forms deposits generally 
richer than the primary ore, and more accessible [open pitable].

The presence of calcite has significant impact on these 
processes, mainly through effecting pH buffering, and the 
precipitation of ferrous carbonates which can very effectively 
passivate pyritic surfaces against oxidation. 

However a complete discussion of these aspects is beyond the 
scope of this study. 

Distinctions can be made between :

 [a] Weathering in a high or low rainfall region, and between 

 [b] The weathering of deposits rich in 

    - Alkali [high melting point, relatively chemically resistant 
      typically Al-Si rich] and

    - Acid [low melting point, less chemically resistant typically 
      alkali metal rich] gangue. 

Weathered gangue in low rainfall regions tends to be ferruginous, 
that in high rainfall regions, kaolinitic.

[c] Metamorphic processes

With more extended weathering and transport of rock, the high 
density of gold causes physical concentration in placers. 

Possibly the most meaningfull examination of the associations of 
gold is derived from the primary origins in vein systems. The 
most useable, by element. 

The treatment below is to use the one as an introduction to the 
other. 

In a reconstructed vein system, from the surface downwards, 16 
characteristic associations of gold exist [Emmons, 1937]. 

Gold occurs in two ranges, a shallow and a deep level. 

The primary associations of gold are those minerals which form 
and are stable under the corresponding low and high 
temperature/pressure conditions occuring at these depths. 

The later hypogene and supergene gold concentration processes can 
then be considered.

Sinkankas [-] provides a most complete list of common 
hydrothermal mineral associations, for deep, middle and upper 
vein portions of deposits, which correspond respectively to high, 
medium and low pressure/temperature conditions during 
formation/alteration.

Table I.1 shows the important constituents and accessory 
constituents found in various hypo-[deep], meso-[midvein] and 
epithermal [shallow vein] gold deposits. 

TABLE I.1 : HYDROTHERMAL GOLD ASSOCIATIONS

Deep vein minerals   Mid vein portions    Upper vein portions

  Amphibole
                                              Allemontite
                        Andorite
                                              Anhydrite
                        Ankerite
                                              Antimony
  Apatite
                                              Apophyllite
  Arsenopyrite          Arsenopyrite          Arsenopyrite        
                                              Argentite
                                              Augelite
  Axinite               Axinite
                        Barite                Barite
                        Bismuth               Bismuth
  Bismuthinite                                Bismuthinite
                        Boulangerite          Boulangerite
  Breithauptite
                                              Brucite
                        Bournotite
                        Calaverite            Calaverite
                        Calcite               Calcite
  Cassiterite
                                              Cervantite
                                              Chalcocite
  Chalcopyrite          Chalcopyrite
  Childrenite
                        Chloanthite
  Chlorite                                    Chlorite
                                              Cinnabar
  Cobaltite             Cobaltite

  Cubanite
                        Dolomite              Dolomite
                        Enargite
  Epidote               Epidote
  Fluorite              Fluorite              Fluorite
  Frankckeite           Frankckeite                 
  Galena                Galena                Galena
                                              Gibbsite
                                              Goethite
  Gold                  Gold                  Gold
                                              Greenockite
  Gummite               Gummite
  Hausmannite
  Haematite
                                              Hessite
  Huebnerite
  Ilmenite
  Jamesonite            Jamesonite            Jamesonite
                                              Jarosite
                                              Krennerite
  Loellingite           Loellingite
                                              Ludlamite

TABLE I.1 : HYDROTHERMAL GOLD ASSOCIATIONS [continued]

Deep vein minerals   Mid vein portions    Upper vein portions

                                              Magnesite
                                              Manganite
  Magnetite
                                              Marcasite
                                              Mirargyrite
                                              Millerite
  Molybdenite
  Muscovite
  Niccolite             Niccolite
                                              Opal
                                              Orpiment
  Orthoclase
  Pentlandite
                                              Petzite
                                              Phosphophyllite
                        Polybasite            Polybasite
                                              Proustite
                                              Pyrargyrite
  Pyrite                Pyrite
  Pyrrhotite
  Quartz                Quartz                Quartz
                        Rammelsbergite
                                              Realgar
                        Rhodocrosite          Rhodocrosite
  Rutile
  Scheelite             Scheelite
                        Siderite              Siderite
                        Silver                Silver
                        Skutterudite
  Sphalerite            Sphalerite            Sphalerite
  Stannite

                                              Stephanite
                                              Stibiconite
                                              Stibnite
                                              Strontianite
                                              Sylvanite
                                              Tellurium
                        Tennantite
  Tetradymite           Tetradymite
                        Tetrehedrite          Tetrehedrite
  Topaz
  Tourmaline
  Uraninite             Uraninite
                                              Wavellite
                                              Witherite
  Wolframite            Wolframite
  Wurtzite              Wurtzite 
                                              Zeolites
  Zinnwaldite

These associations are discussed below in Appendices II and III. 

I.1.1 HYPOTHERMAL DEPOSITS [GOLD OF DEEP VEIN ORIGIN] 

These deposits are formed by thermal fuids at considerable 
depths, ie temperatures and pressures; eg at :

 [a] Porcupine, Ontario

 [b] Homestake, South Dakota

 [c] Morro Velho, Brazil

 [d] Kalgoorlie, Western Australia

Chalcopyrite veins, mostly with pyrite, many with pyrrhotite 
overly the deep vein gold. 

The gangue is quartz, and at some places carbonates and feldspar. 
Orthoclase and sodic plagioclase is not rare, high calcium 
plagioclase is rare. 

Both generally carry some precious metals. Uranium, probably the 
main horizon of uraninite occurs in this layer. 

Within the deep vein itself, gold is deposited with pyrite, 
commonly arsenopyrite. 

Tellurides are not uncommon, and abundant in places. Gangue 
comprizes quartz, carbonates and some feldspar, sometimes with 
tourmaline. 

Arsenopyrite with chalcopyrite etc generally underlies the deep 
vein gold.

The following associations are recorded in hypogene [endogene] 
auriferrous deposits : 

 [a] Most common mineral associations [in order of frequency] :

    - Quartz

    - Carbonates

    - Pyrite

    - Arsenopyrite

    - Sphalerite

    - Galena

    - Pyrrhotite

    - Chalcopyrite

 [b] Fairly common mineral associations [in order of frequency] :

    - Stibnite 

    - Sulphosalts

    - Tellurides

    - Molybdenite

    - Scheelite

    - Wolframite

    - Tourmaline

    - Haematite

 [c] Least common mineral associations [in order of frequency] :

    - Fluorite

    - Barite

    - Alunite

    - Magnetite

    - Niccolite

    - Cobaltite

    - Argentite [acanthite]

    - Selenides

    - Cinnabar

    - Uraninite [pitchblende]

    - Brannerite

Elements associated with gold in hypogene [endogene] auriferous 
deposits include :  S, Se, Te, As, Sb, Bi, Cu, Ag, Zn, Cd, Hg, 
Sn, Pb, Mo, W, Fe, Pt, Pd, Co, Ni. 

In these hypogene deposits, the following gangue constituents 
are generally enriched : Si, Ca, Mg, Mn, Al, Ba, B, U, Th, V, Cr, 
F and P. 

I.1.2 MESOTHERMAL DEPOSITS [GOLD OF MID-VEIN ORIGIN] 

These deposits are formed by thermal fluids at intermediate 
depths, ie at intermediate temperatures and pressures; eg at :

 [a] Sierra Nevada, Canada

 [b] Nova Scotia, Canada

 [c] Victoria, Australia

 [d] Charters Towers, Queensland

I.1.3 EPITHERMAL DEPOSITS [GOLD OF SHALLOW VEIN ORIGIN] 

Figure I.2 provides a broad brush view of mineralization and 
associations in epithermal gold deposits. 

The associations are discussed in greater detail in the next 
section [I.2.1]. 

I.2.0 IMPORTANT LARGE TONNAGE [GENERALLY] LOW GRADE DEPOSITS 

I.2.1 GOLD OF SHALLOW VEIN ORIGIN [EPITHERMAL] 

Epithermal deposits are formed by thermal fluids at shallow 
depths, ie at relatively low temperatures and pressures; eg at : 

 [a] Comstock Lode, Nevada

 [b] Goldfield, Nevada

 [c] Cripple Creek, Colorado

 [d] El Oro, Mexico

 [e] Brad, Roumania

 [f] Waihi, New Zealand

One can distinquish two types of epithermal formations :

 [a] Native gold & electrum with chalcopyrite in adularia-sericite 
   type [generally formed over 1000000 years ago], which contain :
    - Silver sulphides such as acanthite [Ag2S], proustite 
      [Ag3AsS3] and pyrargyrite [Ag3SbS3] which colloidally coat 
      gold, preventing effective cyanidation;

    - K-feldspar that acts as a eutectic former in 
      pyrometallurgical treatment, and 

    - calcite which consumes acid in pressure oxidation
 
    Adaluria sericite mineralization also includes argentite, 
    tennantite, tetrahedrite, native silver and selenides. 

 [b] Native gold with covellite [CuS], enargite [Cu3AsS4], luzonite 
   [Cu3[As,Sb]S4] in acid sulphate [enargite] deposits [usually 
   formed in the last 500000 years], which contain :

    - Alunite which can act as buffer and cyanicide, and also a 
      eutectic-former in smelting, and

    - Kaolinite and pyrophyllite which limits cyanidation mass 
      transfer through viscosity effects

   In the acid-sulphate type, enargite, pyrite and electrum is 
   the characteristic mineralization, and advanced argillic 
   alteration characterized by kaolinite dominates the immediate 
   wall rocks. 

Island arcs form one of the most important classes of shallow 
vein gold deposits, and figure I.3 gives a schematic description of 
such deposits. 

In the Island Arc system, overlying the shallow vein gold is 
stibnite, passing downwards to galena with antimonates. 

Bonanza gold and gold-silver deposits occur in the shallow gold 
level; with the following mineral associations : 

 [a] Common : Argentite, arsenic and antimony minerals.

 [b] At some places :  Tellurides, selenides

 [c] Relatively small amounts : galena, sphalerite, chalcopyrite

In the shallow gold vein, gangue includes quartz, adularia, 
alunite, with calcite, rhodochrosite, and other carbonates. 

Below the shallow gold level is a generally consistent barren 
zone, representing the bottoms of many tertiary precious metals 
veins. 

Small amounts of pyrite, chalcopyrite, sphalerite and galena 
occur in quartz, carbonates etc. 

I.2.2 SEDIMENTARY DEPOSITS [CARLIN TYPE] 

Figure I.4 describes, schematically, the formation of Carlin Type 
Disseminated gold deposits. 

These deposits are formed by processes of aggradation, and the 
constitute the source of widespread gold placers.

Where conditions are unfavourable for the dissolution of gold, 
much of the gold remains practically in place as the land surface 
is worn away.

Some minerals, such as carbonates, are removed by dissolution - 
atmospheric carbon dioxide in rainwater being sufficient. 

The main products of rock weathering are fine feldspar and clays, 
readily carried away by suspension. 

Some of the heavier products of weathering, such as biotite, 
chlorite, specularite and molybdenite [also flaky gold], are 
washed away because of their flaky shape.

The gold, highly maleable, is easily mashed or rounded but not 
readily divided while more brittle minerals are broken. 

It accumulates with hard, resistant minerals such as quartz along 
with the pebbles of various rocks from which the gravel was 
derived. 

Other characteristics of placers are discussed in section I.4.3.

I.3.0 OTHER PRIMARY DEPOSITS 

I.3.1 [ORTHO-] MAGMATIC SEGREGATIONS 

These deposits are formed by consolidation of molten magma; eg at 

 [a] Waaikraal, Transvaal

 [b] Golden Curry, Montana

I.3.2 PEGMATITES 

These deposits are formed by "aquo-igneous" solutions resulting 
from the differentiation of magmas; eg at : 

 [a] Gold Hill, Utah 

 [b] Natasmine, South West Africa 

I.3.3 PYROMETASOMATIC DEPOSITS [SKARNS] 

These deposits are formed by the high temperature and pressure 
fluids emanating from the invading rocks, near the contacts of 
such igneous intrusives; eg at :

 [a] Cable, Montana

 [b] Dolcoath, Montana

Over 99.9% of gold production is from gold occuring in invaded 
and intruded rocks within a mile of contact. 

Many deposits occur in invaded rocks. 

The greatly concentrated deposits occur where small stocks of 
intruded rocks occur, with invaded rocks near them. 

A few valuable deposits occur within a mile of invaded rocks; 
with practically no deposits further away - some of the few 
deposits believed to have no igneous associations are in northern 
Illinois, and at Otago, New Zealand. 

I.4.0 DEPOSITS DEVELOPED BY ALTERATION AND WEATHERING 

These deposits are formed by cold [near-ambient temperature and 
pressures], from atmospherically derived ground waters which 
dissolve metals from the rocks traversed, and reprecipitates 
these where openings are provided.

In general, these deposits are not workable for gold directly as 
formed, but where further enrichment by supergene processes, or 
placer formation occur.

Outcrops of gold deposits that are long exposed to weathering at 
the surface of the earth are generally enriched by surface waters 
which remove material other than gold from the ore, thus 
concentrating the gold.

Sulphuric acid will dissolve gold if in the presence of chlorides 
and manganese dioxide. The following components may occur : 

 [a] Sulphuric acid results from the weathering of sulphides

 [b] Chlorides are generally present in desert regions, where there 
   has been insufficient rain to wash out the soluble chlorides

 [c] Manganese dioxide forms in the high oxide zone of 
   manganiferous lodes

The acidity and redox potential of such gold-rich solutions is 
reduced on encountering many minerals, such as carbonates, 
pyrrhotite and chalcocite. 

The ease and rapidity by which gold is precipitated from the 
relatively weak complexes which mobilize it, ensure that gold is 
not carried far below the water table [in cases where there are 
significant quantities of the acid/oxidant consuming minerals].

I.4.1 OXIDE ZONE 

The oxide zone is associated mainly iron oxides, also manganese 
oxides/hydroxides. These may rim the gold. 

The gangue contains much fines, because of clays originating from 
the weathering. 

This can create permeability and diffusion problems, causing 
Bingham plastic slurries and blinding activated carbon. 

Carbonate minerals are common and affect pH control. 

I.4.2 SECONDARY [SUPERGENE] ENRICHMENT 

The gold forms rich secondary deposits near the water table, but 
not far below it; generally as native gold of great purity.

Examples of Archean supergene enriched gold ores are : 

 [a] Mt Martin, Western Australia [AUR]

 [b] Boddington Deep, Western Australia 

The following associations are recorded in supergene auriferrous 
deposits : 

 [a] Most common mineral associations [in order of frequency] :

    - Limonite

    - Wad

 [b] Fairly common mineral associations [in order of frequency] :

    - Jarosites

    - Scorodite

    - Azurite

    - Malachite

    - Anglesite

 [c] Least common mineral associations [in order of frequency] :

    - Bindheimite

    - Beudantite

    - Antimony ochres

    - Gypsum

    - Tellurites/tellurates

    - Clay minerals

    - Calcite

    - Opaline quartz

    - Uranium minerals eg autonite, carnotite, zeunerite 

The important role of thiosulphate in concentrating gold in wet 
tropical areas has recently been recognized. 

Thiosulphate is a labile intermediate sulphoxide which has a high 
affinity for gold, and is stable at relatively moderate 
conditions of pH around 8 and Eh around 400 mV.

Because of the role in manganese dioxide providing the redox 
potential to stabilize mobile gold complexes, outcrops enriched 
by the weathering of the matrix, leaving the gold are generally 
non-manganiferous.

The role of acidity is also important. Thus gold might accumulate 
at the very outcrop in the case of manganiferous calcite, which 
neutralizes the acid. 

This very surficial accumulation process provides the gold for 
some placer deposits. 

There are two major bases for gold associations, derived from 
hypogene and supergene processes respectively.

Gold mobilized in the upper zone during sulphide weathering [eg 
as thiosulphate] migrates downwards, and is reprecipitated in the 
more reducing environment in the region of the water table.

This supergene enriched zone often carries sulphides with high 
metal:sulphur ratios, and which are relatively reactive, with 
increased cyanide and oxygen demand which reduces leaching 
efficiency and may reprecipitate cyanide-leached gold.

Figure I.5 illustrates the above profiles.

Practically all in-situ primary ore deposits are overlain at the 
surface by an oxidized zone, which itself overlies a zone of 
secondary enrichment over the primary deposit itself. 

Lewis [1982] has summarized the alteration assemblages of 
hydrothermal gold deposits, see tables I.2 [a] to [c].

TABLE I.2 [a] : ALTERATION ASSEMBLAGES OF DEEP-SEATED HYDROTHERMAL 
              GOLD DEPOSITS 

TYPE      ALTERATION Au/Ag  OCCURENCE   ASSOCIATION    EXAMPLES

Chlorite/  Quartz    over Native gold,  High :        Yellowknife
carbonate  carbonate  1   some tellur-  Arsenopyrite  Kalgoorlie
           sericite       ides, occas-  Common :     Kolar, India 
           chlorite       ionally       Sb, Mo, W
                          aurostibite   pyrite, base
                          in meta-      metal sulphides,
                          morphized     sulphosalts
                          mafics        [tetrahedrite/
                                         tennantite]


Silicate   Quartz   over  Commonly      High :        Juneau, 
           albite    1    free Au,      Arsenopyrite  Rossland
           amphibole      some tell-    Common :      Alaska;        
           pyroxene       urides, dis-  Mo, W, Bi,    Kirkland
           biotite        seminated     pyrrhotite    Canada
                          Au/sulphides  tourmaline
                          in meta-         
                          sediments

TABLE I.2 [a] : ALTERATION ASSEMBLAGES OF DEEP-SEATED HYDROTHERMAL 
              GOLD DEPOSITS - [Continued] 

TYPE      ALTERATION Au/Ag  OCCURENCE   ASSOCIATION    EXAMPLES

Iron       Epigenetic :   Ankeritic     Common :      Homestake
formation  Caronate-      chert         Arsenopyrite  Archean :  
           facies iron    passing       pyrrhotite    Morro Velho
           formation      into a        chlorite      Barberton  
                          cumming-                    Carshaw,   
                          tonite/                     Malga,     
                          grunerite                              
                          schist                                 
                          above garnet                           
                          isograd                                

           Syngenetic :  Higher Au                    Geralton   
           Precipitated  with quartz                  [Ontario]  
           with iron     veins cut-                   Lupin      
           oxides and    ting iron                    [NW Territ-
           silica in a   formation,                   ory,Canada]
           marine sed-   Auriferous                   Water Tank 
           imentary      sulphide                     Hill [W    
           environment   replace-                     Australia] 
                         ment adj-                    Vubachikwe 
                         acent to                     [Zimbabwe] 
                         to quartz  

TABLE I.2 [b] : EPITHERMAL ALTERATION ASSEMBLAGES OF HYDROTHERMAL 
              GOLD DEPOSITS 

TYPE      ALTERATION Au/Ag  OCCURENCE   ASSOCIATION    EXAMPLES

Au/Te      sericitic- about             Common :      Cripple
vein       carbonate    1               K-spar        Creek,
                                                      Fiji,
                                                      Roumania

Alunitic   Quartz   about 1
vein       alunite        As, Hg, [Te]                Goldfield
           kaolinite                                Chinquasshih,
           grading out                                 Taiwan
           to argillic                              Kasuga, Japan
                                                  El Indio, Chile

Normal     Silici-    about As, Hg        Common :     Oatman,Balel
Au vein    fication,    1                Adaluria     USSR, Bagulo
           argillic                                  Philippines


Carlin     Silici-   over Calcareous    High :      Carlin
dis-       fication,  1   carbonaceous  As, Hg,    Jerritt Canyon 
seminated  carbonate      sediment      Sb, Tl, F   Cortez
           dissolution                              Getchell

TABLE I.2 [b] : EPITHERMAL ALTERATION ASSEMBLAGES OF HYDROTHERMAL 
              GOLD DEPOSITS - [Continued]

TYPE      ALTERATION Au/Ag  OCCURENCE   ASSOCIATION    EXAMPLES

Au disem-  Silici-   about  Sinter,       High :       Round
inated in  fication,   1    lakes         As, Hg,     Mountain,
volcanics  extreme          nearby        Sb, Tl, F   Borsalis
           leaching              
           sercicite

Silver,    Sericitic- about Intermediate Common :     Mn rich :
base       argillic   0.01   volcanics   adaluria     San Juan,
metal      with[out]                     calcite      Tonopah
           K-spar                        quartz       Mn-poor :
                                         Sb,Se with    Pachuca
                                         some Pb,Zn,Cu Comstock
                                                       Guana-
                                                       juato

Dis-       Argillic        Stockworks   Mn-rich :     Candelaria
seminated  to seri-        in inter-    Mn-poor :     Delamar,
silver     citic,          mediate                    Rochester
           silici-         volcanics                 
           fication                     Diseminated   Waterloo
                                        Au in clastic Creede
                                        sediments     Hardshell
                                        [lake beds] :
                                        Silica, barite
                                        Mn-oxide, Pb,
                                        Zn.

Boyle [1979] distinquishes an additional group, Au veins in 
faulted/folded sedimentary rocks :

TABLE I.2 [c] : ALTERATION ASSEMBLAGES OF HYDROTHERMAL GOLD 
              DEPOSITS IN FAULTED/FOLDED SEDIMENTARY ROCKS 

TYPE      ALTERATION Au/Ag  OCCURENCE   ASSOCIATION    EXAMPLES

Faulted/   Metamorph- well              Common:        Salsigne,
folded     osed seq-  over              quartz,        France;
sedimen-   uences of    1               feldspar,      Pilgrims
tary rocks shale,                       mica, chlorite Rest, 
           sandstone,                   arsenopyrite,  Transvaal;
           greywacke,                   pyrite         Bendigo,
           often of                     Sometimes :    Australia;
           marine                       Sphalerite,    Muruntau,
           origin                       galena,        Uzbekistan 
                                        chalcopyrite, 
                                        pyrrhotite.
                                        Rare :
                                        Au tellurides,
                                        auriferous
                                        sulphides,
                                        aurostibite

I.4.3 PLACERS 

Placers form when weathering releases gold from outcropping 
auriferous veins.

Table I.3 lists minerals that are stable in placer deposits.

TABLE I.3 : MINERALS STABLE IN PLACER DEPOSITS 

      Metals/nonmetals                      Oxides          
                                                            
      Gold/electrum                Cassiterite      Pyroxene
                                                            
      Platinoids                   Monazite         Titanite
                                                            
      Diamond                      Magnetite        Pyroxene
                                                            
      Sulphides                    Haematite         Apatite 
                                                            
      Cinnabar                     Ilmenite         Quartz  
                                                            
      Pyrite                       Xenotime         Feldspar
                                                            
                                   Zircon           Amphibole
                                                            
                                   Corundum         Apatite 
                                                            
                                   Spinel           Quartz  
                                                            
                                   Garnet           Feldspar
                                                                     
With continuing erosion, the gold-bearing mantle rock of a 
deposit will gradually settle downhill, constituting an eluvial 
deposit.

With continued erosion, gold is carried to form placer deposits, 
by lodgement in streams together with sand and gravel.

Coarser gold generally remains in creeks and gulches near its 
source, with finer gold being carried to rivers, accululating in 
the gravels, or even to sea wher it may be concentrated as beach 
or marine placers as at Nome, Alaska.

Subsequent uplifting of the region, and erosion of a stream as it 
cuts below its flood plain can leave high, widely spread gold 
deposits as terraces.

Ancient beds might be filled by lava flows; new streams forming 
courses which cut the ancient channels, exposing outcrops in 
hillsides. Such deposits have yielded much of Californian gold.

Placers also form by eolian processes, where wind in arid 
regions blows away lighter particles, leaving heavy material.

Glacial deposits involve the transport of mantle rock, gravel and 
loose material in ice; any contained gold being incorporated in 
drift formed when the ice melts.

The physical distribution of goldbearing and other terraces, the 
fineness of the gold and dissolution and reprecipitation in 
placers is not dealt with here; as this has only indirect relevance 
to the chemistry of hydrometallurgical gold recovery. 

The classifications of young placers are essentially according to 
distance from the source rock; finer gold usually being 
transported further than coarse : 

 [a] Eluvial [or residual] placers are located at and around the 
   parent deposit. These generally are the lowest grade as 
   prolonged mechanical erosion by water has not occured. In 
   tropical regions, they are commonly lateritized [ie the 
   weathering of the host rock has formed hydrated iron and 
   aluminium oxides with silica. 

 [b] Colluvial [or deluvial deposits are generally on the slopes 
    surrounding outcropping source rocks, where they have been 
   transported some distance but not so far as to be located in 
   an established stream system. 

 [c] Proluvial deposits form in terraces on flatter lowlands in 
   valleys

 [d] Fluvial/alluvial deposits occur in stream/river systems

 [e] Marine placers are formed by the natural sorting by wave 
   motion of a beach environment.

Paleoplacers are very old [generally pre-cambrian, over 570 000 000 
years] fossilized placers. 

They comprize lithified conglomerates of rounded quartz in a 
matrix of pyrite, fine quartz, macaceous material and small 
quantities of heavy and resistant minerals such as magnetite, 
uraninite, PGM's, titanium and gold. 

                            APPENDIX II : 

                     GOLD ASSOCIATIONS CAUSING 
            COMPLEX OR REFRACTORY EXTRACTIVE METALLURGY 


II.1.0 INTRODUCTION 

The mineral associations of gold [including their alteration 
products], which have been demonstrated to be cause complex or 
refractory gold extraction are discussed below. 

The scheme adopted for discussion [non-exhaustively] of each 
refractory grouping comprizes : 

 [a] Mineralogical features

 [b] Examples of deposits [which may include 
   mineralogical/processing features]

 [c] Processing characteristics

II.2.0 ANTIMONY, ARSENIC AND BISMUTH ASSOCIATIONS 

Pourbaix diagrammes for these elements, which assist in 
understanding their chemical behaviour, are given in figures II.1 
to II.3. [Figure II.2 : Bhakta and Lei, 1989]. 

Gold is not consistently related to arsenic nor antimony levels 
in pyrite, nor to antimony in arsenopyrite. 

The exceptions are where pyrite contains visible gold as blebs or 
in late fractures. 

In arsenides and antimonides, gold probably substitutes for the 
platinoids [eg in sperrylite], copper [eg in domeykite], and 
possibly Co, Ni and Fe. 

Gold tends to concentrate in supergene arsenates and antimonates, 
indicating a coherence with these elements. 

This coherrence is well-known in hypogene mineral assemblages. 

The gold and silver may occur both as uniformly distributed, and 
erratically. 

Gold and silver-bearing wads carry considerable quantities of Si, 
Al, Ti, P, As and Sb. 

Gold generally associates more with arsenopyrite, silver with 
pyrite.

As for antimony, arsenic supergene sulphides are also relatively 
unstable, and can hydrolyze and consume oxygen which will starve 
the solution of oxygen at moderate to high pH values, and 
potentially reprecipitate gold. 

Practically all arsenopyrite contains antimony, sufficient to 
bind the gold in these mineral lattices, so neutralizing any 
charge imbalances. 

The evidence indicates that early formed high temperature 
arsenopyrite takes up gold largely in solid solution, or as 
atomic layers on the growing faces of the sulphide minerals. 

Gold and silver-bearing wads carry considerable quantities of Si, 
Al, Ti, P, As and Sb. 

The gold and silver may occur both as uniformly distributed, and 
erratically. 

Table II.1 [Hedley and Tabatchnick, 1958] shows the reaction of 
arsenian and antimonial minerals with cyanide. 

TABLE II.1 : DISSOLUTION OF ANTIMONY/ARSENIC MINERALS BY CYANIDE 

Arsenopyrite    FeAsS                            0.9%
                                        
Realgar         AsS                              9.4%

Stibnite        Sb2S3                           21.1%

Orpiment        As2S3                           73.0%

II.2.1 ANTIMONY 

[a] Mineralogy

Stibnite occurs overlying shallow vein gold, passing downwards to 
galena with antimonates. 

Details of the stratigraphy and other mineral associations of 
gold in shallow vein systems is given in a companion monograph on 
mineralogical aspects of complex and refractory gold ores. 

The gold minerals formed in this environment by antimony include : 

Aurostibite [AuSb2] 

Montbrayite [(Au,Sb)2Te3]] 

Nagyagite [Pb5Au(Te,Sb)4S5-8]

Trace Sb may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold ores. 

For stibnite, the evidence is that little gold enters the mineral 
lattice. 

Any gold values found are generally from fine but separate native 
gold, aurostibite or with intimately intergrown gold-bearing 
tetrahedrite [Cu3SbS3, Cu12Sb4S13, or 4Cu2S.Sb2S3]. 

[b] Deposits

Antimony occurs in gold ores at Blue Spec [Pilbara] and 
Consolidated Murchison [Tranvaal], and New England Antimony 
Mines, New South Wales. 

Paradise Peak, Gabbs, Nevada, has significant levels of bismuth-
bearing stibnite.

Locking within silver-antimony sulphosalts [myargyrite, AgSbS2; 
pyrargyrite, Ag3SbS3; freibergite, [Cu,Ag,Fe,Zn]12Sb4S13; 
polybasite, [Ag,Cu]16Sb2S11] contributes to the refractory nature 
of gold at McLaughlin, Clear Lake. 

[c] Processing

Antimony contributes to the refractory nature of gold, by being a 
voracious oxygen scavenger, starving all gold of this reagent 
when intimately or closely associated. 

Stibnite is the most readily hydrolized sulphide, and most 
seriously retards cyanidation at elevated pH due to oxygen 
scavenging by both the Sb2+ and S2-; and cyanide scavenging of 
the latter.

Aurostibite has a low solubility in cyanide, probably due to the 
enormous oxidant requirement of Sb, which must generally be 
satisfied before the oxidation step of gold in cyanidation 
proceeds. 

Antimony reporting to roaster off-gases present significant 
environmental problems, and requirements to scrub the gasses, 
particularly where high grade sulphuric acid must be produced.

Antimony is said to contribute to gold losses following roasting, 
by forming ferrites where good temperature control is not 
excercised [Udupa ao, 1990]. 

Nagy ao [1966] record that ambient pressure and temperature pre-
leaching with sulphuric acid or ammonia can remove antimony prior 
to cyanidation. 

Processing methods to overcome the problems include :

 [a] High oxidant [peroxide] cyanidation at Blue Spec, Western 
   Australia

 [b] Low alkalinity leaching at Golden Giant [Hemlo], Canada

 [c] High pressure/low alkalinity cyanidation at 
   Consolidated Murchison, N Eastern Transvaal [Davis and 
   Peterson, 1986]

 [d] Chloride dissolution at Consolidated Murchison [originally], 
   and more recently, high oxidant/low pH cyanidation in a pipe 
   reactor 

 [e] Thiourea at New England Antimony Mines

Calder and Henderson [1986] claim that the Calmet raised
temperature/pressure cyanidation could be optimized for ores 
containing antimony with tellurium [and tin].

II.2.2 ARSENIC ASSOCIATIONS 

[a] Mineralogy

Cathelineau ao [1988] shows that gold in arsenopyrite occurs as a 
lattice-bound species, not as native gold. 

Co-precipitation occurs in fairly reducing conditions, around 
that fixed by the Ni/NiO Eh buffer, at low pH and at temperature 
around 170 to 250C. 

The distribution of Au within the arsenopyrite is heterogenous, 
with enriched growth zones generally defining the crystal 
development, as illustrated in figure II.4. 

Gold is a common microconstituent of sulphide-arsenides such 
arsenopyrite. 

Trace As may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold ores.  

This explains the frequent occurence of much invisible gold and 
silver in relatively unfractured and unrecrystallized 
arsenopyrite. 

In arsenides gold possibly substitutes for Fe.

In copper, silver and antimony sulphide-arsenides, gold occurs 
both in native form and as a lattice constituent.

This is particularly so in the case of chalcocite, bornite, 
chalcopyrite and enargite, which frequently carry relatively high 
gold content not detectable as metal at the highest magnifications. 

[b] Deposits

According to Ramsden & Creelman [1985], Gold at Hellyer occurs in 
solid solution in arsenopyrite, [possibly some pyrite]. 

Paradise Peak, Gabbs, Nevada, has significant levels of orpiment 
and realgar. 

[c] Processing

Arsenic reporting to roaster off-gases present significant 
environmental problems, and requirements to scrub the gasses, 
particularly where high grade sulphuric acid must be produced.

Recently, processes aimed to achieve satisfactory environmental 
management when treating refractory arsenopyritic ores have been 
applied at Getchell in the Western USA, at Harbour Lights, 
Western Australia and METBA in Greece. 

Because significant quantities of refractory gold [around 5% of 
total gold production, probably 2 to 3 times more as reserves] is 
associated with arsenopyrite, much effort has been aimed to 
understand the nature of its occurence in this mineral. 

Simple arsenic sulphides, in contrast to most other sulphides, 
have a relatively low dissociation pH above which gold extraction 
is significantly retarded. 

It decreases from pH 10.5 for Realgar [4As4S4] to pH 9.5 for 
Orpiment 4[As2S3]. 

Gold in coarser arsenopyrite formed by secondary recrystalization 
[generally at lower temperatures], or subjected to moderate heat 
promoting gold diffusion and exsolution, is generally not 
refractory. 

In contrast, gold and silver at lower temperatures migrate to 
nearby low chemical potential sites such as fractures and grain 
boundaries where they crystalize as argentiferous gold.

This occurs presumably because the lower temperatures dictate 
slower cooling rates. 

Where reworking, recrystallization and fracturing are present, 
the gold is largely present in native form. 

Thermal processing has been shown to promote gold exsolution and 
migration preferentially along fractures and grain boundaries, 
where it crystalizes out. 

Scorodite from auriferous arsenopyrite veins generally contains 
significant levels of gold. 

Most is usually present as finely divided native gold, and some 
in the lattice, probably substituting for iron. 

The refractory nature of arsenic is generally due to the 
encapsulation of gold either in very fine native form, or in 
solid solution, through ionic substitution.

This is generally associated with primary and rapid cooling, 
forming fine [6 micron] acicular arsenopyrite crystals.

Oxidation is invariably required to render the gold amenable to 
cyanidation. 

The gold does not generally occur in domains or points of high 
gold content, and much evidence shows that the "invisible" 
fraction is bound in the arsenopyrite matrix. 

Leaver and Woolf [1928] note the benefits of sweetening 
arseopyrite with pyrite during roasting.

von Michaelis [1987] reported that Ferber Mining Co had piloted 
an unspecified process for recovery of As and S - probably by 
pyrometallurgy.

II.2.3 BISMUTH 

The minerals formed by gold which include bismuth are :

Aurobismuthinite 

Bismuthaurite 

Bismuthian gold [Au,Bi] 

Maldonite [Au2Bi] 

Trace Bi may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold ores.  

Bi commonly replaces Te [see the companion monograph on 
mineralogical aspects of complex and refractory gold ores]. 

Gold generally does not concentrate in sulphates, eg anhydrite. 
However gold has concentrated in supergene sulphates of copper 
and iron, with Te, and some concentration of Bi.                         

At the Renco Mine, Zimbabwe, Hg2Cl2 was added to 600 tpd ore 
since 1980 to catalyze the dissolution of gold from maldonite. 

The requirement proved quite variable; the average dose 
ultimately being reduced from around 200 g Hg/t to 20 g Hg/t as 
experience was gained in the application of this toxic reagent.

Aeration prior to addition is necessary, and is achieved in froth 
flotation cells using 0.6 kW/m3 for about 1.2 hours. 

II.3.0 CARBON ASSOCIATIONS     

[a] Mineralogy

Gold generally does not concentrate in sulphates, eg anhydrite. 

However gold has concentrated in supergene sulphates of copper, 
with Te, and some concentration of Mn, Bi, Co & Ni [potential 
cynicides] and Zn. 
    
In shallow gold veins, the gangue includes quartz, adularia, 
alunite, with calcite, rhodochrosite, and other carbonates. 

Table II.2, which includes diamond [strictly a valuable ore or 
tracer rather than gangue], lists the heavier minerals which also 
resist weathering, and are thus found in placer deposits. 

Vein carbonate associations in gold-quartz deposits generally 
involve a paragenetic sequence of carbonates beginning with 
ankerite, giving way to more calcic carbonates, ending with 
calcite. 

Presulphide carbonate is generally mixed and complex, whereas the 
late carbonate is commonly calcite. 

The early carbonates are commonly replaced by sulphides, whereas 
the late calcite fills minute fracturesand only rarely replaces 
the sulphides. 

Native gold occurs commonly with ankerite, and along cleavage 
planes of calcite and other carbonates.

Common carbonates have very low gold contents, due to the low 
gold-carbonate bond strength. 

However Au substitution for Cu allow these minerals to 
accommodate small quantities of gold. 

The association of gold with carbonaceous matter in Witwatersrand 
ores is believed to be due to biological redistribution.

Carbonaceous pregrobbing can arize from non-geological sources, 
such as decaying mine timbers, and particularly in tailings 
retreatment, burnt plant growth and boiler ash discarded on 
dumps.

[b] Deposits

Carbonaceous ores occur at Ashanti and Prestia [Ghana], Carlin 
and Jerrit Canyon [Nevada], and Natalinsk and Bakyrchik [USSR]. 

Carbonaceous ores are an important source of gold in the Nevadan 
Carlin-type gold deposits, and have also been recorded at 
McIntyre, Canada.

Finely disseminated gold occurs in the thoro/uranic hydrocarbon 
thucolite [Th,U,C,H,O], on the Witwatersrand.

Kidston has carbonates, in addition to copper in its 
mineralization.

Ore at Chimney Creek, Winnemucca is high in carbonates.

[c] Processing

Introduction 

Early processes relied in blinding the preg-robbing carbon with 
poorly soluble organics.

[Diesel] oil-based methods [flotation/blinding of carbonaceous 
material] were applied at New Machavie, Witwatersrand, with poor 
results. 

At Bibiani, Ghana, flotation reagents [xanthate, pine oil] 
overcame pregrobbing during subsequent grinding in cyanide. 

NaCO3 [and/or NaOH], steam and air pretreatment to oxidize 
reactive sulphides at Atlas Gold Bar was abandoned as it 
liberated organics which fouled the carbon.

Processes to treat refractory carbonaceous sulphidic ores have 
been recently applied at :

 [a] Queen Charlotte Island, British Columbia, 

 [b] The Carlin belt in Nevada [eg the Enfield Bell deposit 
   of Freeport], 

 [c] The Barbrook Mine in the Eastern Transvaal, 

 [d] Morro Velho, Brazil and 

 [e] Pueblo Viejo in the Dominican Republic. 

Gold at Mercur, Utah, seems to be associated with carbon rather 
than sulphides. 

Ore is upgraded from about 1% carbon to 6% carbon at Ashanti, and 
roasted prior to cyanidation.

CIL reduces the loss of gold by pregrobbing, and is specifically 
applied to achieve this :

 [a] At Mercur after pressure oxidation of sulphides [which does 
   not, at the temperatures under 200C], deactivate the carbon, 
   and 

 [b] At Carlin

Refractory sulphidic ore at Mercur typically contains about 0.4% 
carbon, 1.2% S2- and 16% carbonate. 

Mercur increased carbon concentrations in CIL from 15 to 40 g/l, 
in order to minimize carbonaceous pregrobbing. 

Carbon per se can render gold refractory, by being active to gold 
adsorption, causing pregrobbing.

Simple pregrobbing [type I, ie by ion exchange] is not serious, 
as subsequent CIP contacting, the present standard in gold 
processing, reverses this.

There is another form of pregrobbing [type IV] associated with 
carbonaceous ore, which is serious as it is practically 
irreversible.

Hydrocarbons are commonly present in sedimentary rocks. 

They are reducing agents and probably reduced solutions that 
carried gold, the reduction precipitating the gold. 

Graphite may be associated with gold because it is a residual of 
the hydrocarbon that reduced it. 

Where the carbonate itself encapsulates gold, acid which 
restricts cyanidation is required. 

The consumption of acid to achieve the required liberation of 
gold may also be excessive. 

Nice [1971] reported on a number of options for dealing with 
carbonaceous ores.

Pregrobbing is generally associated with carbonaceous sediments 
and humic acids, rather than graphitic carbon.

Roasting can be effective in reducing carbonaceous pregrobbing 
from such ores; pressure oxidation generally not. 

Operations which have practiced mild chlorination on carbonaceous 
pregrobbing ores include : 

 [a] Carlin, Nevada, oxidizing framboidal pyrite, lignitic carbon, 
   humates and fumates on 450 tpd ore containing 8.6 g Au/t, 
   0.5%S, 1%C in a carbonate system at 0.9 atm; after a pre-
   aeration with steam and air at 84C, pH 10; since 1972

 [b] Jerrit Canyon, Nevada, oxidizing framboidal pyrite, lignitic 
   carbon, humates and fumates on 1800 tpd ore containing 7 g 
   Au/t, 1%S, 1%C at 0.9 atm; 40C, pH 10; since 1981

 [c] Big Springs, USA - late 1980's

Kerosene has been used to blind pregrobbing behaviour at Kerr-
Addison, Canada [McQuiston and Shoemaker, 1975].

Other process applied to mildly pregrobbing ores include 
weathering of the ore, carbon or resin adsorption, oil blinding, 
and double oxidation [air/chlorine].

Chlorination 

For more highly pregrobbing ore [generally with carbon over 1%], 
pre-oxidation by chlorine or roasting is applied. 

Chlorination has been applied with great success, where the 
quantity of oxidisable material is low, and chlorine costs are 
low [typically where the primary market for a near-by chlor-
alkali manufacturer is for caustic]. 

At USA operations applying chlorine, consumptions under 40 kg 
Cl2, at costs of about $80/t chlorine have applied. 

With deeper ores, as more mineral components become chlorine-
consuming [eg sulphides], and where the cost of chlorine is high,
chlorination becomes impractical.

Where consumptions at USA operations have climbed to 100 kg Cl2/t 
and over, the process is deemed unsuitable.

The aim of double oxidation is to use cheaper air to oxidize the 
more reactive components, saving the more costly chlorine for the 
less reactve components whih require it.

Carbonates

Carbonate can also impose an excessive acid consumption on ores 
requiring acid conditions for the destruction of other matrices 
which encapsulate gold, such as on occasion, pyrite.

Epithermal deposits of the adularia-sericite type [generally 
formed over 1000000 years ago], contains calcite which consumes 
acid in pressure oxidation 

Gangue associated with deep vein gold comprizes quartz, 
carbonates and some feldspar, sometimes with tourmaline. 

Carbonate affects pH control, especially in oxidative 
pretreatment processes. 

Associated magnesium [eg in dolomite], can severly affect slurry 
viscosity as pH is increased over the precipitation limit. 

The ferrous iron associated with siderite can severely rob 
cyanide, scavenge oxygen and contribute to gold pregrobbing.

In pressure oxidation, carbon dioxide released on acidulation can 
leave less capacity to maintain oxygen partial pressure. 

This occured at Sao Bento, which containd 8% carbonate in the 
flotation concentrate, which is now disengaged by prior bacterial 
oxidation which also reduces the oxidative load and increases the 
throughput capacity of the pressure oxidation reactors. 

In roasting, carbon dioxide being released from carbonates can 
reduce the effectiveness of oxygen reaching the sulphides. 

II.4.0 CADMIUM, COBALT, COPPER, NICKEL ASSOCIATIONS     

Trace Cd, Co, Cu and Ni may occur in native gold, with other 
elements as set out in the companion monograph on mineralogical 
aspects of complex and refractory gold ores. 

These metals have significant affinity for cyanide, figures II.5 to 
II.7 clearly demonstrate [by the relative size of the regions of 
stability of the complexed form at similar stoichiometries and 
molar concentations, that the sequence of stability is : 

                          Co over Ni over Cu. 

However figure II.8 [a] shows that Cu over Ni [marginally], with 
respect to breakdown by protonation of the cyanide ligand. Such 
protonation reduces the concentration of [metal]-associable 
cyanide, until the higher complexes are no longer stable. 

From stability constant data, one can demonstrate that :

                             Cu over Cd

Co, Ni, Cu and Cd can contribute to the problems associated with 
tellurium, as they commonly replaces Te [see the companion 
monograph on mineralogical aspects of complex and refractory gold 
ores. 

In arsenides and antimonides, gold probably substitutes for Co 
and Ni. Ni minerals can act analogously to copper, such as the 
reactive Ni sulphide, Violarite, which can consume both cyanide 
and oxygen. 

Gold generally does not concentrate in sulphates, eg anhydrite. 

However gold has concentrated in supergene sulphates of copper 
and iron, with Te, and some concentration of Ni. 

It should be appreciated that a number of dissolved metal species 
exist for Cd, Cu etc; depending on un-complexed cyanide 
concentrations; as shown in figures II.8 [b] and II.8 [c]. 

II.4.1 COPPER 

[a] Mineralogy

Gold is a common microconstituent of copper sulphides and 
sulphide-arsenides. 

Tetrahedrite-tennantite are the sulphosalts most generally 
enriched by gold; generally as Au lattice substitution for Cu 
with native gold or other gold-bearing minerals being rare.

In copper, silver and antimony sulphides and sulphide-arsenides, 
gold occurs both in native form and as a lattice constituent.

This is particularly so in the case of chalcocite, bornite, 
chalcopyrite and enargite, which frequently carry relatively high 
gold content not detectable as metal at the highest magnifications. 

Tetrahedrite-tennantite are the sulphosalts most generally 
enriched by gold; generally as Au lattice substitution for Cu 
with native gold or other gold-bearing minerals being rare.

Similar enrichment does not occur in the lead analogues : 
boulangerite and jamesonite.

Native gold commonly occurs with covellite [CuS], enargite 
[Cu3AsS4] and luzonite [Cu3[As,Sb]S4].

Copper minerals typically rich in gold include Bornite, 
Chalcocite, Chalcopyrite and Covellite.

The minerals formed by gold which include copper are relatively 
rare. 

They include : 

Auricupride [AuCu3] 

Cuproauride [AuCu]

Cuprian gold [Au,Cu]

Kostovite [AuCuTe4]

Palladium cuproauride [(Cu,Pd)3Au2]

[b] Deposits

von Michealis [1993] discusses :

 [a] In the USA
 
   - 28 occurences of copper with gold

    - 12 gold mines producing copper as a byproduct,

    - 12 gold mines where Cu can be a gold recovery problem and a 
      further :

    - 6 projects which could proceed; he also notes that 

    - 74 gold-copper properties with copper with potentially gold 
      as a byproduct have been identified in the USA by Metals 
      Economics Group 

 [b] In Canada

    - 13 projects where copper is associated with gold, as 
      compared to 

    - 75 properties with copper with potentially gold as 
      byproduct have been identified by Metals Economics Group

 [c] Elsewhere :

    - 26 occurences of copper with gold in Australia

    - 3 occurences of copper with gold in the Philippines

    - 2 occurences of copper with gold in Indonesia

    - 2 occurences of copper with gold in Mexico

    - 1 occurence of copper with gold each in Bolivia, 
      Mauritania and Papua New Guinea

Gold associations with bornite are relatively rare, but occur at 
Bougainville [Papua New Guinea] and Olympic Dam [South 
Australia]. 

[c] Processing

A total treatise on this subject is well beyond the scope of 
this study. 

With emphasis on situations where gold is the primary product, 
generally beneficiated on site by cyanide, copper associated with 
gold may be refractory in a number of ways : 

 [a] Cu scavenges cyanide

 [b] CuII also scavenges oxygen

 [c] Gold is often encapsulated as fine inclusions in copper 
   sulphides or as ionic substitutions, as for pyrite, 
   arsenopyrite and stibnite. 

Copper is said to contribute to gold losses following roasting, 
by forming ferrites where good temperature control is not 
excercised [Udupa ao, 1990]. 

Nagy ao [1966] record that ambient pressure and temperature pre-
leaching with sulphuric acid or ammonia can remove copper prior 
to cyanidation. 

Copper minerals are one of the most important groups causing 
refractory behaviour due to mineral reactivity. 

Tables II.3 and II.4 [Leaver and Woolf, 1931] summarize some of 
their most important cyanidation properties. 

The main processes applied or applicable to high copper ores are 

 [a] Acid pre-leach

 [b] Ammonia/cyanide leach

 [c] High cyanide concentration leach

 [d] Selective elution of adsorbed copper cyanides

 [e] Cutech and related electrolytic processes

 [f] Non-cyanide lixiviants

TABLE II.3 : CYANIDE REACTIONS OF COPPER MINERALS IN PRACTICE

   Cu-species    NaCN consumed            Products
                     t / t Cu
                 
       CuI            0.14              CuCN complexes

   CuII[oxide]        0.48          CNO- & CuCN complexes

 CuII[sulphide]       0.50          CNS- & CuCN complexes

CuII[Fe-sulphide]     0.76     CNS-, Fe(CN)64- & CuCN complexes 

TABLE II.4 : DISSOLUTION OF COPPER MINERALS WITH CYANIDE 

                                              23C           45C

Tetrahedrite   [Cu12Sb4S13][4 Cu2S.Sb2S3]

Chalcopyrite    CuFeS2          [CuI]         5.6%           8.2%
                                        
Chrysocolla     CuSiO3.5H2O     [CuII]        11.8%         15.7%

Enargite        Cu3AsS4         [CuII]        65.8%         75.1%
             [3Cu2.As2S5]                                        
Bornite         Cu5FeS4         [CuI]         70.0%        100.0%

Cuprite         Cu2O            [CuI]         85.5%        100.0%

Native Copper   Cu              [Cu0]         90.0%        100.0%

Chalcocite      Cu2S            [CuI]         90.2%        100.0%

Malachite       CuCO3.Cu(OH)2   [CuII]        90.2%        100.0%

Covellite       CuS             [CuII]      90% to 95%     100.0%

Azurite         2CuCO3.Cu(OH)2  [CuII]        94.5%        100.0%
                [2CuO.Cu(OH)2]

II.4.2 NICKEL 

Nickel is said to contribute to gold losses following roasting, 
by forming ferrites where good temperature control is not 
excercised [Udupa ao, 1990]. 

II.5.0 IRON ASSOCIATIONS 

[a] Mineralogy

Native gold generally contains some iron. 

Fe commonly replaces Te [see the companion monograph on 
mineralogical aspects of complex and refractory gold ores. 

Gold is a common microconstituent of some sulphides and sulphide-
arsenides such as pyrite. 

It is present either as fine inclusions or ionic substitutions, 
as for arsenopyrite, stibnite and chalcopyrite. 

In contrast, gold and silver at lower temperatures migrate to 
nearby low chemical potential sites such as fractures and grain 
boundaries where they crystalize as argentiferous gold.

This occurs presumably because the lower temperatures dictate 
slower cooling rates. 

Where reworking, recrystallization and fracturing are present, 
the gold is largely present in native form. 

Limonite and wad tend to be the most common supergene oxide 
carriers of gold in or near gold deposits.

In arsenides and antimonides, gold possibly substitutes for Fe.

The gold does not generally occur in domains or points of high 
gold content, and much evidence shows that the "invisible" 
fraction is bound in the pyrite matrix. 

Practically all pyrite and arsenopyrite contains antimony, 
sufficient to bind the gold in these mineral lattices, so 
neutralizing any charge imbalances. 

The evidence indicates that early formed high temperature pyrite 
takes up gold largely in solid solution, or as atomic layers on 
the growing faces of the sulphide minerals. 

This explains the frequent occurence of much invisible gold and 
silver in relatively unfractured and unrecrystallized pyrite. 

Auriferous pyrite has a decreased microhardness, deemed due to 
linear dislocations caused by gold in the lattice. 

Gold generally does not concentrate in sulphates, eg anhydrite. 

However gold has concentrated in supergene sulphates of and iron, 
with Te, also with some concentration of Mn, Bi, Co, Ni and Zn. 

Jarosites [basic sulphates of ferous iron and other elements] 
frequently contain high gold levels. 

The highest levels have been recorded for plumbojarosites, where 
most of the gold is present in fine particulate form, although 
some lattice substitution occurs. 

Gold is not readily recovered from such jarosites. 

Magnetites in some deposits can carry gold, through lattice 
substitution with iron.

Scorodite from auriferous arsenopyrite veins generally contains 
significant levels of gold. 

Most is usually present as finely divided native gold, and some 
in the lattice, probably substituting for iron. 

[b] Deposits

The problems of pyrrhotite at Morro Velho, and the solution : low 
alkalinity pre-aeration, and sodium plumbite addition are fairly 
well-known.

Recent challenges in refractory gold processing have been 
presented by the pyrite encapsulated gold of Porgera, Lihir 
Island, and other deposits in Papua New Guinea.

Ramsden & Creelman [1985] note the occurence of encapsulated gold 
in pyrite and galena at Que River, a volcanogenic base metal 
sulphide deposit. 

D Nicholson, Cardiff University, notes the same type of finely 
dispersed gold occurence [with pyrite and sphalerite] in the 
volcanogenic base metal sulphide deposits of Que River.

[c] Processing

Figure II.9 provides a basis for understanding the cyanidation 
behaviour of iron. 

Gold is not consistently related to arsenic nor antimony levels 
in pyrite, nor to antimony in arsenopyrite.

Iron oxides are frequently associated with gold.

One, relatively rare mineral of gold and iron exists :

Auriferride [Au,Fe] 

von Michaelis [-] distinguishes two modes of refractory behaviour 
of iron sulphides : 

 [a] Sulphides soluble in cyanide

 [b] Sulphides not soluble in cyanide

Iron sulphides of greater metal:sulphur ratio are most prone to 
hydrolysis [ie reactive, and soluble in cyanide]. 

Pyrite [cubic FeS2] is generally unreactive [an exception is the 
framboidal variety, as found in Nevada carbonaceous ores. 

Marcasite [the orthorhombic form] is only marginally more 
reactive than Pyrite, whereas Troilite [FeS, generally a 
synthetic product of heat decomposition of Pyrite] is very 
reactive in alkaline solutions. 

The reactivity of the intermediate iron sulphides Fe[X-1]SX 
generally increases with increasing X from 3 [gamma sulphide], to 
4 [Greigite], 5.5 [Smythite] and the Pyrrhotites [up to 12]. 

Mackinawite which is sulphur-deficient [FeXS[X-1] is extremely 
reactive in alkaline solution. 

Pre-aeration is commonly applied to ores containing reactive 
sulphides to rid the slurry of elemental sulphur and thionates 
prior to cyanidation. 

This conversion is always not simple, with numerous sulphur 
oxidation states and products possible, as illustrated below out 
of Menne and Muhtadi [1988] : 

  2 S2- + 3 O2 + 2 H2O = S0 + 4 OH-..............(1)

  2 S2- + 2 O2 + H2O = S2O32- + 2 OH-............(2)

  5 S2- + 2 O2 + 4 H2O =  S5O62- + 8 OH-..........(3)

  8 S2- + 9 O2 + 6 H2O = 2 S4062- + 12 OH-.......(4)

  4 S2- + 5 O2 + 2 H2O = 2 S2O42- + 4 OH-........(5)

  3 S2- + 4 O2 + 2 H2O = S3O62- + 4 OH-..........(6)

  2 S2- + 3 O2  = 2 SO32-........................(7)

  2 S2- + 3 O2 + H2O = S2O52- + 2 OH-............(8)

  4 S2- + 7 O2 + 2 H2O = 2 S2O62- + 4 OH-........(9)

  S2- + 2 O2 + = SO42-..........................(10)

  4 S2- + 8 O2 + 2 H2O = 2 S2O72- + 4 OH-.......(11)

  4 S2- + 9 O2 + 2 H2O = 2 S2O82- + 4 OH-.......(12)
                                                          
  2 S2- + 5 O2 = 2 SO52-........................(13)
                                                          
In practice many of these reactions are sequential and many of 
the products are stable only in limited ranges of redox potential 
and pH, and when buffered by related species. 

The two species above SO42- in Redox potential are persulphates, 
and not at all stable in water. 

However the remainder are are generally avid consumers of oxygen 
as they oxidize up the series. 

They also almost all consume cyanide forming thiocyanate ie SCN-.  
   
Furthermore there are activation energy barriers for a number of 
the interconversions of the sulphur oxides, which might be 
variously catalyzed eg by autogenous Pb levels.

Note that the extent of refractory behaviour is not absolute, 
but is determined by mineralogical habit and process conditions. 

Thus hydrolysis can be greatly reduced by decreasing the 
operating pH and temperature. 

As an example, pyrrhotite is not very unstable at near-neutral 
conditions, but reacts rapidly at typical cyanidation pH levels 
[around pH 10.5], and vigorously at higher levels [eg pH 11+].

Such high pH levels are often applied in enclosed cold climate 
mills [Canada] to minimize the hazard of HCN vapourization. 

Crystalline pyrite is fairly inert, while the framboidal form is 
oxidized under relatively mild conditions.

Changes in the relative molar volume of the sulphide and the 
hydrolysis products largely determines reactivity : a shrinkage 
leads to cracks promoting further attack. 

Moderate increases in volumes develop a better seal, protecting 
the mineral surface against further attack. 

Because most highly refractory gold is associated with pyrite 
[over 10% total production], much effort has been aimed to 
understand the nature of its occurence in this mineral.

Oxidation is invariably required to render the gold amenable to 
cyanidation. 

This would be expected to make the gold relatively refractory; 
the iron-gold phase formed when some gravity concentrates or 
poorly pre-treated cathodes are smelted are extremely refractory 
to gold recovery.

Thermal processing has been shown to promote gold exsolution and 
migration preferentially along fractures and grain boundaries, 
where it crystalizes out. 

Aeration of pyrrhotite in alkaline solution is said to passivate 
pyrrhotite breakdown, reducing the deleterious effect on 
cyanidation.

Great Victoria Gold Mine, Western Australi, abandoned one of 
their deposits which was rich in pyrrhotite and marcasite, 
resulting in very high cyanide consumptions.

King ao [1947] established that the most effective method of 
treating highly pyrrhotitic ore at Sub Nigel, Witwatersrand, was 
low pH pre-aeration.

Occasional high Oxygen demand at Pamour Purcupine was addressed 
by addition of KMnO4, to restore gold dissolution.

Battle Mountain, Nevada, pre-aerates to ameliorize reactive 
sulphides.

Although low pH aeration is almost universally applied, 
particularly where the quantities of reactive sulphides is high, 
Bellevue Gold, Western Australia found that by adding lime and 1 
kg/t Pb(NO)3 to the mill feed, the relatively small but 
troublesome quantity of reactive sulphides could be oxidized 
before cyanidation. 

II.6 SILVER ASSOCIATIONS    

[a] Mineralogy

von Michaelis [-] distinguishes gold associated with :

 [a] Native silver, 

 [b] Complex [As/Sb] silver ores such as tennantite and 
   tetrahedrite

 [c] Silver chloride

 [d] Oxidized silver ores

 [e] Refractory silver ores

Gold generally associates more with arsenopyrite, silver with 
pyrite.

Bonanza gold and gold-silver deposits occur in the shallow gold 
level; with the following mineral associations : 

 [a] Common : Argentite, arsenic and antimony minerals.

 [b] At some places :  Tellurides, selenides

 [c] Relatively small amounts : galena, sphalerite, chalcopyrite

Details of the stratigraphy and other mineral associations of 
gold in shallow vein systems is given in the companion monograph on 
mineralogical aspects of complex and refractory gold ores. 

The minerals formed by gold which include silver are :

Electrum [Au,Ag : over 20% Ag]

Kustelite [aurian silver : Ag,Au over 50% Ag]

Krennerite [(Au,Ag)Te2] 

Muthmannite [(Ag,Au)Te] 

Petzite [Antamokite] [Ag3AuTe2] 

Sylvanite [(Au,Ag)Te4] 

Uytenbogaardite [Ag3AuS2] 

AgAuS

Fischesserite [Ag3AuSe2]

Ag3AuTe

Au4AgTe10

D Nicolson, Cardiff Uni, notes the occurence of the unusual 
Boliden alloys Au69Ag20Hg11 - Au10Ag70Hg20, derived from 
volcanogenic base metal sulphide deposits.

Native gold generally contains some silver.

Up to 4.2% lattice-bound gold occurs in the silver telluride 
Hessite [Ag2Te]. 

Silver minerals such as argentite, freibergite and argyrodite 
frequently carry relatively high gold content not detectable as 
metal at the highest magnifications. 

The gold occurs both in native form and as a lattice constituent.

Gold is a common microconstituent of some sulphides and sulphide-
arsenides; particularly of copper, silver, antimony; and in 
pyrite and arsenopyrite.

In copper, silver and antimony sulphides and sulphide-arsenides, 
gold occurs both in native form and as a lattice constituent.

This is particularly so in the case of chalcocite, bornite, 
chalcopyrite and enargite, which frequently carry relatively high 
gold content not detectable as metal at the highest magnifications. 

Similar silver minerals such as argentite, freibergite and 
argyrodite also frequently carry relatively high gold content not 
detectable as metal at the highest magnifications. 

Gold is commonly found in auriferous pyrite and arsenopyrite. 

Silver and arsenic is commonly associated with pyrite in the host 
rocks and deposits. 

The associations with arsenopyrite are silver and antimony. 

Halides, even of precious metals [chlorargyrite, AgCl] are 
generally practically devoid of gold. 

The gold and silver may occur both as uniformly distributed, and 
erratically. 

Gold and silver-bearing wads carry considerable quantities of Si, 
Al, Ti, P, As and Sb. 

[b] Deposits

Locking within silver-antimony sulphosalts [myargyrite, AgSbS2; 
pyrargyrite, Ag3SbS3; freibergite, [Cu,Ag,Fe,Zn]12Sb4S13; 
polybasite, [Ag,Cu]16Sb2S11] contributes to the refractory nature 
of gold at McLaughlin, Clear Lake. 

[c] Processing

Figure II.10 aids in the understanding of the influence of silver 
in gold recovery. 

Oxidation [and in some cases reduction] is invariably required to 
render the gold amenable to cyanidation. 

Epithermal deposits of the adularia-sericite type [generally 
formed over 1000000 years ago], contain silver sulphides such as 
acanthite [Ag2S], proustite [Ag3AsS3] and pyrargyrite [Ag3SbS3] 
which colloidally coat gold, preventing effective cyanidation. 

Gold is recovered from Silver deposits :

 [a] Cordilleran vein-type

 [b] Ag,Co,Ni-arsenide veins

 [c] Sediment-hosted Cu,Ag[,Co]
 
 [e] Vein silver

 [f] Epithermal Ag-Au

    - Intrusive-hosted

    - Volcanic-hosted

Gold is also often associated with other silver sulphides such 
as Enargite, Myargyrite, Pyrargyrite, Tennantite, Tetrahedrite 
and Polybasite.

Gold in some wad-silver ores is amenable to cyanidation. 

The remainder requires release by intensive reduction, as for 
silver. 

Gold in some wad-silver ores is amenable to cyanidation. 

The remainder requires release by intensive reduction, as for 
silver. 

von Michaelis [1992] notes that Steensma found that leaching at 
pH 5 was effective on refractory manganiferous silver ores.

II.7.0 TELLURIDE AND SELENIUM ASSOCIATIONS 

II.7.1 TELLURIUM 

[a] Mineralogy

Bonanza gold and gold-silver deposits occur in the shallow gold 
level; with the following mineral associations : 

 [a] Common : Argentite, arsenic and antimony minerals.

 [b] At some places :  Tellurides, selenides

 [c] Relatively small amounts : galena, sphalerite, chalcopyrite

Details of the stratigraphy and other mineral associations of 
gold in shallow vein systems is given in the companion monograph on 
mineralogical aspects of complex and refractory gold ores. 

The minerals formed by gold which include tellurium are :

Calaverite [AuTe2] 

Hessite [Ag2Te] - up to 4.2% Au

Krennerite [(Au,Ag)Te2] 

Kostovite AuCuTe4], 

Montbrayite [(Au,Sb)2Te3]] 

Muthmannite [(Ag,Au)Te] 

Nagyagite [Pb5Au(Te,Sb)4S5-8]

Petzite [Antamokite] [Ag3AuTe2] 

Sylvanite [(Au,Ag)Te4] 

Vandiestite 

Au4AgTe10

Au2SeTe

Ag3AuTe

The tellurides of gold are essentially all hypogene, and 
calaverite is generally the dominant species - figure II.11

Figure II.12 shows the similarity in behaviour of Se. 

Trace Te may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold ores. 

Trace to minor amounts [10000's ppm] of S, Se, Sb and As 
commonly replace Te. 

More rare are Cu, Fe, Ni, Co, Pb, Sn, Hg, W, Mo and Bi. 

No platinoids have been found. 

Up to 4.2% lattice-bound gold occurs in the silver telluride 
Hessite [Ag2Te]. 

Where auriferous galena is encountered [not common], tellurium is 
commonly found with the gold; the Te would be needed in the 
sulphur positions to provide electron compensation if gold 
replaced some lead; else gold tellurides might be finely 
dispersed in the galena. 

[b] Deposits

Markham [1960] identifies two geological environments of Au-Te-Ag 
mineralization :

 [a] Veins and fissures of Tertiary volcanic rocks [eg Emperor, 
   Fiji, Cripple Creek and Carpathian Mountains] :

    - Vuggy veins composed of quartz and carbonate minerals

    - Wallrock alteration adjacent to veins is intense, with 
      introduction of large amounts of water, carbon dioxide and 
      sulphur

    - Native Te is abundant, native Au rare

    - Krennerite dominates over calaverite, tellurides of metals 
      other than gold are subordinate in quantity.

 [b] Precambrian rocks, typicall greenstones or metamorphosed 
   volcanic lavas and flows [eg Kalgoorlie] :

    - Wallrock shows lowgrade retrograde metamorphism, with 
      intense structural deformation

    - Native Au is abundant, native Te rare

    - Calaverite dominates over Krennerite, tellurides of metals 
      other than gold [Hg, Pb, Cu, Bi] are generally present.

Tellurides have also been found at Jamestown, Colorado.

[c] Processing

Figures II.13 and II.14 show that in the cyanide system, gold-
selenium and gold-tellurium systems act fairly analogously. 

Note though that gold selenides and tellurides are rarely found 
together; as Se commonly replaces Te. 

Figure II.15 illustrates best the major basis for the refractory 
behaviour of gold tellurides : the enormous oxidant requirement of 
Te, which must generally be satisfied before the oxidation step of 
gold in cyanidation proceeds. 

Historically this demand was satisfied :

 [a] Using bromo-cyanide; also 

 [b] Extended pre-aeration or cyanidation at high pH, which would 
   promote decomposition of the tellurides. This is promoted by 
   addition of lead [Jackman and Sarbutt, 1991].

More recently the process options have also included :

 [a] Chlorination [particularly where sulphidic chlorine demand is 
   low [<1%S2-], or can be passivated by pre-oxidation], 

 [b] Roasting, particularly where the quantities of associated 
   sulphides are substantial, and lock gold [Kalgoorlie, Western 
   Australia; and Kirkland Lake, Canada], and

 [c] Sulphide leaching/sulphite precipitation [Emperor Gold Mine, 
   Vatukoula]

 [d] For static leaching [in-situ, dump, heap, vat], pre-rinsing - 
   even water was effective [Jackman and Sarbutt,1991; Hertel, 
   1992], and what was in effect this process was applied on 
   Ontario ores in the 1950's [Mitchell, 1950]

von Michaelis [1993] notes that Echo Bay and Lakefield Research 
determined that CIL overcomes telluride refractoriness.

Gold generally does not concentrate in sulphates, eg anhydrite. 

However gold has concentrated in supergene sulphates of copper 
and iron, with Te, and some concentration of Mn, Bi, Co, Ni and 
Zn.                                   

All can add to the refractory nature of the telluride-associated 
gold.

Gold-silver tellurides dissolve more slowly than native gold, 
requiring extended cyanidation times. 

High oxidant and pH levels would probably address this. 

Roasting has been used to treat Tellurides both at Emperor and 
Kalgoorlie.

Mild chlorination in a caustic system [1 atm, pH 13.6, ambient 
temperature] has been applied at Emperor Gold Mines, Vatukoula to 
oxidize tellurides on 0.4 tpd telluride concentrates since 1974

Calder and Henderson [1986] claim that the Calmet raised
temperature/pressure cyanidation could be optimized for ores 
containing tellurium and antimony [with tin].

II.7.2 SELENIUM 

The minerals formed by gold which include selenium are :

Au2SeTe

Fischesserite [Ag3AuSe2]

Trace Se may occur in native gold, with other elements as set out 
the companion monograph on mineralogical aspects of complex and 
refractory gold ores. 

II.8.0 SILICA 

[a] Mineralogy

Silicates, although very widely associated with gold, in general 
contribute very negligible to the gold content of an ore [Henley, 
1975].

White [1943] provided a landmark paper on the role of cataclasis 
on the quartz, the most important gangue in epigenetic gold 
deposits.

In shallow gold veins, the gangue includes quartz, adularia, 
alunite, with calcite, rhodochrosite, and other carbonates. 

Gangue associated with deep vein gold comprizes quartz, 
carbonates and some feldspar, sometimes with tourmaline. 

Table II.5, which includes silicon, lists minerals that are stable 
in placer deposits. 

Trace Si and Ge may occur in native gold, with other elements as 
set out in the companion monograph on mineralogical aspects of 
complex and refractory gold ores. 

Acid sulphate [enargite] deposits [usually formed in the last 
500000 years], contain kaolinite and pyrophyllite which limits 
cyanidation mass transfer through viscosity effects.

[b] Deposits

Gold may be deposited in so-called boiling zones of epithermal 
deposits, and characterized by severe inhibition to cyanidation 
by encapsulation in silica. 

Examples include McLaughlin, California and Porgera, Papua New 
Guinea. 

However the gold associated with silica, even if very fine, is 
generally fairly accessible, ocurring aling fracture pains; eg 
the dark silica-rich jaseroids at Alligator Ridge and elsewhere 
in Nevada. 

Kidston has significant muscovite and chlorite; and up to 25% of 
the Jerritt Canyon ore is clay.

[c] Processing

Baum [1988] notes a number of heap leach problems related to 
slimes, which generally orginate from weathered silicates :

 [a] Difficulty in blending ore and reagents, poor consistency of 
   agglomerates 
 
 [b] Poor agglomerate quality

 [c] Fines migration and segregation, causing channeling and 
   blinding

 [d] Impermeability or slow perculation, ponding of lixiviant

 [e] Poor wetting

 [f] Coating of gold particles

 [g] Poor oxygen availability

 [h] Preg robbing

Baum also notes that :

 [a] In excess of 10% clays will in general cause plugging problems 
   in crushing due to fines buildup and compaction; 

 [b] That slimes from propylitic alteration frequently contain 
   chlorite-group minerals, which can act as significant 
   cyanicides

 [c] Sheet silicates [clay and clay-like minerals] cause 
   significant filtration problems, particularly if more than 5% 
   slimes is present

 [d] Pyrophyllite and other Mg-rich phyllosilicates, or a 
   combination of these with hydrous iron/manganese oxides, have 
   a potential for pregrobbing similar to carbonaceous ores. He 
   has observed in Wrsten US and Latin American epithermal gold-
   silver deposits, significant sorption of gold where there is 
   freshly generated slimes of : 

    - The layer silicates : vermiculite, montmorillonite, illite, 
      kaolinite, especially if associated with

    - Iron/manganese hydroxides

Such sorption has been studied by Krendelev ao [1978].

Gold occluded in siliceous gangue [such as at McRae's Flat, NZ], 
will not be released, which favours processes such as fine 
milling or roasting, which can create stress fractures which 
liberates the gold.

Clay coatings contribute to the refractory nature of gold at 
McLaughlin, Clear Lake. 

Clays such as pyrophyllite [Al2si4O10(OH)2], talc 
[Mg3Si4O10(OH)2], kaolinite [Al2Si4O10(OH)8] and montmorillonite 
[Al4Si8O20(OH)4] can contribute significantly to refractory 
behaviour, by : 

 [a] Decreasing bed permeability in static leaching

 [b] Increasing viscosity and reducing mass transfer in agitated 
   cyanidation, and 

 [c] Blinding carbon in CIP.

Murray andvan Aswegen [1989] reported that hot caustic treatment 
released 95% of gold in siliceous roaster tappings, as compared 
to 30% by all other methods including very fine grinding. 

von Michaelis [1993] provides process details. 

II.9.0 SULPHUR 

Sulphides are in general dealt with at a greater level of detail, 
under the specific metals associated with them.

II.9.1 SULPHIDES 

[a] Mineralogy

Schwartz [1944] reviewed the association of sulphides with native 
gold in 115 deposits, and records :
 
 [a] 42% associated with pyrite

 [b] 40% associated with arsenoyrite

 [c] 26% associated with galena

 [d] 23% associated with sphalerite

 [e] over 15% associated with chalcopyrite.

Other significant associations occured with :

 [a] Pyrrhotite

 [b] Tetrahedrite/tennantite

 [c] Quartz

 [d] Carbonates

 [e] Chlorite

 [f] Carbonaceous material

 [g] Tourmaline

Haycock [1937] determined that submicroscopic gold represented 
25% of 50 Canadian ores examined.

Much gold is recovered from sulphide ores [which produce 66% of 
the world's silver] :

 [a] Porphyry Cu

 [b] Volcanogenic Massive base metal deposits
 
  - Zn[,Pb,Ag] : Au appears within massive and layered ores at 
                  stratigraphic top

   - Cu : Au appears within massive and stringer ores towards the 
          base of the deposit

 [c] Base metal sulphides

 [d] Ni,Cu [Sudbury]

 [e] Intrusion-hosted Pt

 [f] Stratiform massive Cu,Pb,Zn

 [g] Base metal vein deposits 

[b] Deposits

The original occurence of sulphides with gold eposits is 
practically obiquitous.

Gold deposits without sulphides are often associated with the 
weathered products of the sulphides.

[c] Processing

Sulphur reporting to roaster off-gases present significant 
environmental problems, and requirements to scrub the gases, 
especially where no sulphuric acid can be produced.

The various sulphides are discussed under the specific metal they 
have formed from.

II.9.2 SULPHATES 

In shallow gold veins, the gangue includes quartz, adularia, 
alunite, with calcite, rhodochrosite, and other carbonates. 

Baum [1988] has shown the occurence of ultrafine gold in alunite, 
unaccesible to cyanide.

Gold generally does not concentrate in sulphates, eg anhydrite. 

However gold has concentrated in supergene sulphates of copper 
and iron, with Te, and some concentration of Mn, Bi, Co, Ni and 
Zn. 

Jarosite coatings contribute to the refractory nature of gold at 
McLaughlin, Clear Lake. 

II.10.0 OTHER 

II.10.1 LEAD 

[a] Mineralogy

Lead sulphides and sulphosalts are not normally enriched by gold; 
reporting mainly as native specks and blebs within these minerals 
when occuring with these species. 

A mineral formed by gold which includes lead is :

Nagyagite [Pb5Au(Te,Sb)4S5-8]

Trace Pb may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold ores. 

Pb commonly replaces Te [see the companion monograph on 
mineralogical aspects of complex and refractory gold ores. 

Gold is not concentrated in anglesite per se, but might occur due 
to association with admixed scorodite, bindheimite and 
beudantite.

Jarosites [basic sulphates of ferous iron and other elements] 
frequently contain high gold levels. 

The highest levels have been recorded for plumbojarosites, where 
most of the gold is present in fine particulate form, although 
some lattice substitution occurs. 

Where auriferous galena is encountered, tellurium is commonly 
found with the gold; the Te would be needed in the sulphur 
positions to provide electron compensation if gold replaced some 
lead; else gold tellurides might be finely dispersed in the 
galena.

Enrichment of gold in lead sulphosalts [boulangerite and 
jamesonite] generally do not occur, in contrast to the enrichment 
common to the copper analogues tetrahedrite-tennantite, where 
lattice substitution of Au for Cu is fairly common.

Beudantite and bindheimite invariably contain gold, some of which 
is lattice-bound by substitution with lead.

[b] Deposits

Ramsden & Creelman [1985] note the occurence of encapsulated gold 
in pyrite and galena at Que River, a volcanogenic base metal 
sulphide deposit. 

[c] Processing

Lead forms sulphides generally too stable to interfere with 
cyanidation; the cyanide complexes of Pb are practically 
insoluble [down to 3ppm in the presence of sulphate], and have 
little effect of cyanide availability. 

Lead is said to contribute to gold losses following roasting, by 
forming ferrites where good temperature control is not excercised 
[Udupa ao, 1990]. 

Trace [soluble] lead, in the order of 30 ppm Pb, can act as a 
cyanidation catalyst.

II.10.2 MANGANESE 

[a] Mineralogy

In shallow gold veins, the gangue includes quartz, adularia, 
alunite, with calcite, rhodochrosite, and other carbonates. 

Limonite and wad tend to be the most common supergene oxide 
carriers of gold in or near gold deposits.

The gold and silver may occur both as uniformly distributed, and 
erratically. 

Gold and silver-bearing wads carry considerable quantities of Si, 
Al, Ti, P, As and Sb. 

[b] Deposits

Manganese, as it effects refractory gold behaviour, is generally 
associated with manganiferous silver ores.

[c] Processing

Gold in some wad-silver ores is amenable to cyanidation. 

The remainder requires release by intensive reduction, as for 
silver. 

von Michaelis [1992] notes that Steensma found that leaching at 
pH 5 was effective on refractory manganiferous silver ores.

II.10.3 MERCURY 

[a] Mineralogy

Other minerals formed by gold which include mercury are :

Gold amalgam [Au2Hg3 ?]

Trace Hg may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold ores.  

Hg commonly replaces Te [see the companion monograph on 
mineralogical aspects of complex and refractory gold ores. 

Table II.5, which includes mercury, lists minerals that are stable 
in placer deposits. 

[b] Deposits

Paradise Peak, Gabbs, Nevada, has significant levels of cinnabar.

Minor quantities of cinnabar occur at Mercur.

Jerritt Canyon ore contains about 25 ppm Hg as cinnabar.

[c] Processing

Figure II.16 assists in understanding of the influence of mercury 
in gold processing by cyanidation, and carbon adsorption. The 
higher complexes og Hg, which occur at higher free cyanide 
concentrations, do not adsorb as well onto activated carbon. 

Complex gold mineralization with mercury is one cause of poor 
extraction, eg the unusual Boliden alloys Au69Ag20Hg11 - 
Au10Ag70Hg20, which occur in this volcanogenic base metal 
sulphide deposit [D Nicolson, Cardiff Uni] 

Mercury reporting to roaster off-gases present significant 
environmental problems, and requirements to scrub the gases, 
especially where no sulphuric acid can be produced. 

Cyanide mobilizes mercury, and in CIP/CIL processing can lead to 
emmisions dangerous to health in the goldroom.

Starvation doses of sulphide [particularly oldhamite] is used to 
limit the dissolution of mercury by cyanide. 

II.10.4 PLATINOIDS 

Table II.5 which includes platinoids, lists minerals that are 
stable in placer deposits. 

The minerals formed by gold which include platinoids are :

Rhodite [Au,Rh] 

Palladium cuproauride [(Cu,Pd)3Au2] 

Porpezite [Au,Pd] 

Aurosmirid, Iridic gold [Au,Ir] 

Platinum gold [Au,Pt] 

Trace platinoids [Pt, Rh, Pd, IR] may occur in native gold, with 
other elements as set out in the companion monograph on 
mineralogical aspects of complex and refractory gold ores.  

In arsenides and antimonides, gold probably substitutes for the 
platinoids [eg in sperrylite]. 

Little is recorded on the mineralogy, deposits or processing 
applied to these ores. 

However the platinoids in general and platinum in particular is
known to dissolve in cyanide with relative difficulty, generally 
requiring extremes of cyanide concentration, redox potential and 
temperature.

Thus it might be expected that the platinum compounds and alloys of 
gold would also exhibit complex/refractory behaviour.

                           APPENDIX III : 

                        GOLD ASSOCIATIONS OF
            LITTLE CONSEQUENCE IN EXTRACTIVE METALLURGY 

III.1 INTRODUCTION 

The hypogene mineralogy of gold is very simple : the only gold 
mineral with universal occurence [excluding the comparatively 
rare tellurides], is native gold.

Native gold generally contains some silver, copper and iron. Table 
III.1 lists other elements which may be present as traces : 

TABLE III.1 : ELEMENTS OCCURING AS TRACES IN GOLD

                        Li, Na, K
                        Be, Mg, Ca, Sr, Ba
                        Zn, Cd, Hg
                        B, Al, 
                        Ga, In, Sc
                        Si, Ge
                        Sn, Pb
                        Ti, Zr
                        As, Sb, Bi
                        V
                        Se, Te
                        Cr, Mo
                        W
                        Co, Ni
                        Rh, Pd, Ir, Pt
                        U, Th
                        Rare earths
 
Native gold of high purity is generally of secondary origin, ie 
where silver, which is generally associated [see the companion 
monograph on mineralogical aspects of complex and refractory gold 
ores], has been parted by geological processes. 

A Pourbaix diagramme of the gold-cyanide system is included, 
which assists in understanding features of its recovery.

III.2 ALKALI EARTHS : Be, Mg, Ca, Sr AND Ba 

Trace  Be, Mg, Ca, Sr, Ba may occur in native gold, with other 
elements as set out in the companion monograph on mineralogical 
aspects of complex and refractory gold. 

In shallow gold veins, the gangue includes quartz, adularia, 
alunite, with calcite, rhodochrosite, and other carbonates. 

The gangue overlying deep vein gold is quartz, and at some places 
carbonates and feldspar. 

Orthoclase and sodic plagioclase is not rare, high calcium 
plagioclase is rare. 

Both generally carry some precious metals. 

Details of the stratigraphy and other mineral associations of 
gold in deep vein systems is given in the companion monograph on 
mineralogical aspects of complex and refractory gold. 

III.3 ALKALI METALS : Li, Na AND K 
                      
Trace Li, Na and K may occur in native gold, with other elements 
as set out in the companion monograph on mineralogical aspects of 
complex and refractory gold. 

Epithermal deposits of the adularia-sericite type [generally 
formed over 1000000 years ago], contain K-feldspar that acts as a 
eutectic former in pyrometallurgical treatment.

Acid sulphate [enargite] deposits [usually formed in the last 
500000 years], contain Alunite which can act as buffer and 
cyanicide, and also a eutectic-former in roasting. 

III.4 ALUMINIUM 

Table III.2, which includes aluminium, lists minerals that are 
stable in placer deposits. 

In shallow gold veins, the gangue includes quartz, adularia, 
alunite, with calcite, rhodochrosite, and other carbonates. 

Gangue associated with deep vein gold comprizes quartz, 
carbonates and some feldspar, sometimes with tourmaline. 

Gold and silver-bearing wads carry considerable quantities of Si, 
Al, Ti, P, As and Sb. 

The gold and silver may occur both as uniformly distributed, and 
erratically. 

III.5 BORON 

Trace B may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

III.6 CHLORIDE 

See HALIDES

III.7 GERMANIUM 

Trace Si and Ge may occur in native gold, with other elements as 
set out in the companion monograph on mineralogical aspects of 
complex and refractory gold. 

III.8 HALIDES 

Halides, even of precious metals [chlorargyrite, AgCl] are 
generally practically devoid of gold. 

III.9 PHOSPHATES 

Minor gold association, however of no practical interest for 
extraction, has been recorded. 

Gold in some wad-silver ores is amenable to cyanidation. The 
remainder requires release by intensive reduction, as for silver. 

The gold and silver may occur both as uniformly distributed, and 
erratically. Gold and silver-bearing wads carry considerable 
quantities of Si, Al, Ti, P, As and Sb.

Table III.2, which includes phosphorus, lists minerals that are 
stable in placer deposits. 

III.10 RARE EARTHS : Ga, In. Sc 

Trace rare earths such as Ga, In, Sc may occur in native gold, 
with other elements as set out in the companion monograph on 
mineralogical aspects of complex and refractory gold. 

III.11 TIN 

The minerals formed by gold which include tin are :

Auristannide [Au,Sn] 

Trace Sn may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

Gold has been found in association with Sphalerite, but this is 
considered rare. 

D Nicholson, Cardiff University, notes a finely dispersed gold 
occurence [with pyrite and sphalerite] in the volcanogenic base 
metal sulphide deposits of Que River. 

Table III.2, which includes tin, lists minerals that are stable in 
placer deposits. 

III.12 TITANIUM 

Trace Ti may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

Gold and silver-bearing wads carry considerable quantities of Si, 
Al, Ti, P, As and Sb. 

The gold and silver may occur both as uniformly distributed, and 
erratically. 

Table III.2, which includes titanium, lists minerals that are 
stable in placer deposits. 

Gold in some wad-silver ores is amenable to cyanidation. 

The remainder requires release by intensive reduction, as for 
silver. 

III.13 THORIUM 

Trace Th may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

III.14 TUNGSTEN 

There is a general association of gold [and silver] with tungsten 
in many deposits. Wolframite and scheelite may contain some 
gold, probably as fine metal.

Trace W may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

W commonly replaces Te [see the companion monograph on 
mineralogical aspects of complex and refractory gold]. 

III.15 URANIUM 

Uranium, probably the main horizon of uraninite occurs in the 
layer overlying deep vein gold. 

Trace U may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

Details of the stratigraphy and other mineral associations of 
gold in deep vein systems is given in the companion monograph on 
mineralogical aspects of complex and refractory gold. 

III.16 VANADIUM 

Trace V may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

III.17 ZINC 

Trace Zn may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

Zinc sulphides and sulphosalts are not normally enriched by gold; 
reporting mainly as native specks and blebs within these minerals 
when occuring with these species. 

Gold generally does not concentrate in sulphates, eg anhydrite. 

However gold has concentrated in supergene sulphates of copper 
and iron, with Te, also with some concentration of Zn. 

A number of zinc minerals are dissolved by cyanide, viz [Leaver 
and Woolf, 1931] :

TABLE III.3 : DISSOLUTION OF ZINC MINERALS BY CYANIDE

                                                 45C

Willemite       Zn2SiO4                         13.1%
                                        
Hemimorphite    H2Zn2SiO5                       13.4%

Sphalerite      ZnS                             18.4%

Franklinite     (Fe,Mn,Zn)O-(Fe,Mn,Zn)2O3       20.2%

Hydrozincite    2ZnCO3.3Zn(OH)2                 35.1%

Zincite         ZnO                             35.2%

Smithsonite     ZnCO3                           40.2%

Zinc forms sulphides generally to stable to interfere with 
cyanidation; the cyanide complexs of Zn derived from other salts 
are relatively weak, and do not significantly interfere with 
cyanidation undr normal conditions.
                                   
III.18 ZIRCON 

Trace Zr may occur in native gold, with other elements as set out 
in the companion monograph on mineralogical aspects of complex and 
refractory gold. 

Table III.2, which includes zircon, lists minerals that are stable 
in placer deposits. 


             APPENDIX IV : PREGROBBING ORE PROCESSING 



                              INDEX
 IV.1.0 OBJECTIVE                                                 4
                  
 IV.2.0 SUMMARY                                                   5
                  
 IV.3.0 INTRODUCTION                                              6
                  
 IV.4.0 THE CHEMISTRY OF PREGROBBING ORES                         7
                  
  IV.4.1 CARBON/RESIN/WASH REVERSIBLE : TYPE I                    8
  IV.4.2 CYANIDE REVERSIBLE : TYPE II                             9 
   IV.4.2.1 AUROCYANIDE POLYMER FORMATION : TYPE IIa             10   
   IV.4.2.2 REDUCTION TO GOLD METAL : TYPE IIb                   11
  IV.4.3 ALKALI REVERSIBLE : TYPE III                            12
  IV.4.4 ACID REVERSIBLE : TYPE IV                               12
   IV.4.4.1 AUROCYANIDE ENCAPSULATED                   
         BY MINERAL REPRECIPITATION : TYPE IVa                12 
   IV.4.4.2 METALLIC GOLD ENCAPSULATED                   
         BY MINERAL REPRECIPITATION : TYPE IVb                13
                  
 IV.5.0 PREGROBBING ORE MINERALOGY/GEOLOGY                       15

  IV.5.1 CARBONACEOUS ORES                                       15
   IV.5.1.1 ORGANIC                                              15
   IV.5.1.2 GRAPHITIC                                            16
   IV.5.1.3 CARBONATE                                            17
   IV.5.1.4 NON-GEOLOGIC PREGROBBING                             17
  IV.5.2 NON-CARBONEOUS ORES                                     18
   IV.5.2.1 GOLD ADSORBING                                       18
   IV.5.2.2 CYANICIDAL                                           18
   IV.5.2.3 OXYGEN CONSUMING                                     18
 IV.6.0 PROCESSING AT DEPOSITS EXHIBITING PREGROBBING            19      
                  
  IV.6.1 AFRICA                                                  20
   IV.6.1.1 ASHANTI                                              20
   IV.6.1.2 BIBIANI                                              20
   IV.6.1.3 NEW MACHAVIE                                         21
  IV.6.2 AUSTRALASIA                                             21
   IV.6.2.1 MACRAES                                              21
   IV.6.2.2 STAWELL                                              21
  IV.6.3 USA                                                     22
   IV.6.3.1 ALLIGATOR RIDGE                                      22
   IV.6.3.2 ATLAS GOLD BAR                                       22
   IV.6.3.3 BIG SPRING                                           22
   IV.6.3.4 CARLIN                                               22
   IV.6.3.5 JERRITT CANYON                                       22
   IV.6.3.6 KERR-ADDISON                                         22
   IV.6.3.7 McDERMOTT                                            23
   IV.6.3.8 MERCUR                                               23
   IV.6.3.9 ROYAL MOUNTAIN KING                                  23
  IV.6.4 SOUTH AMERICA                                           23
   IV.6.4.1 MORRO DO OURO                                        23
   IV.6.4.2 ROSARIO DOMINICANA                                   24
   IV.6.4.3 QUEIROZ                                              24
  IV.6.5 OTHER                                                   24

 IV.7.0 NON-PROCESS OPTIONS FOR PREGROBBING ORES                 25
                  
 IV.8.0 PROCESS OPTIONS FOR PREGROBBING ORES                     26
                  
  IV.IV.8.1 GRAVITY RECOVERY                                        27        
   IV.8.1.1 FEEDSTOCK AND PRODUCTS                               27        
   IV.8.1.2 CONCENTRATE TREATMENT                                27        
  IV.8.2 CARBON FLOTATION                                        28        
   IV.8.2.1 CARBON RECOVERY PRIOR TO CIL                         29        
   IV.8.2.2 CARBON RECOVERY SUBSEQUENT TO CIL                    29        
   IV.8.2.3 CARBON DEPRESSION                                    29        
  IV.8.3 DISSOLVED GOLD CAPTURE                                  30        
   IV.8.3.1 SOLIDS DENSITY                                       30        
   IV.8.3.2 CARBON MANAGEMENT [CIL]                              30        
   IV.8.3.3 OPTIMIZING PROCESS DESIGN                            31
  IV.8.4 OXIDATION OF PREG-ROBBERS                               31        
   IV.8.4.1 CHLORINE OXIDATION                                   32        
   IV.8.4.2 PRESSURE OXIDATION                                   33
   IV.8.4.3 ROASTING                                             34
   IV.8.4.4 BIO-LEACHING                                         35
  IV.8.5 DE-ACTIVATION BY ADSORPTION                             37
   IV.8.5.1 USING POLAR ORGANICS AND SALTS                       37      
   IV.8.5.2 USING NON-POLAR ORGANICS                             37      
  IV.8.6 COMPETITIVE ADSORPTION                                  38

 IV.9.0 CONCLUSION                                               39





                             FIGURES



FIGURE IV.1 : RE-DISSOLUTION OF POLYMERIZED AUROCYANIDE          43

FIGURE IV.2 : THE ORIGIN AND OCURRENCE OF Fe++ IN GOLD DEPOSITS  44

FIGURE IV.3 : THE CAPACITY OF VITROKELETM RESIN                  
           TO STRIP GOLD OFF CARBON                           45

FIGURE IV.4 : STABILITY REGION FOR GOLD CHLORIDE                 46

IV.1.0 OBJECTIVE

This report offers an overview of the behaviour of preg-robbing 
species towards various treatment processes.

IV.2.0 SUMMARY

Pregrobbing occurs at all operations; however generally being 
relatively insignificant and/or reversed by desorption when gold 
is recovered by resin or activated carbon. 

It is believed that few sites containing primary gold do not 
exhibit significant and practically irreversible pregrobbing on 
at least some portions of their ores.

Significant and practically irreversible pregrobbing of 
significant portions of ore has been reported at a number of 
sites.

Without changing to a resin adsorbent, which could prove very 
effective, the use of reagents to blind off pregrobbing sites is 
not considered a good option. 

The evidence suggests that most conventional pre-oxidation 
processes could be considered particularly if further gold 
liberation is significant.

This is because in general, any residual or additional apparent 
activation of pregrobbing from such oxidation has been shown either 
to be : 

 [a] Trivial [type I, reversible on carbon contacting], or 

 [b] Manageable [residual unoxidized pregrobbing species may be
   destroyed by minor addition of chlorine]. 

The information at hand suggests that oxidative pre-treatment can 
be made generally effective, but will be dictated by the cost of 
oxidant. 

At most locations this will preclude chlorine except as a polishing 
step. 

Pre-oxidation could in many cases be justified by further 
liberation of refractory gold. 

The choice of an optimized approach is dependant upon an adequate 
understanding of the pregrobbing chemistry and mineralogy.

Consideration should be given to processing applied at Morro do 
Ouro for highly pregrobbing ore parcels : In adition to gravity 
and CIL with enhanced carbon movement, flotation of cyanide tails 
to scavenge unrecovered gold for recovery by intense cyanidation. 

There are in addition a number of process design and operations 
management options noted in earlier reports however beyond the 
scope of detail discussion in this document, for improving 
dissolved gold capture. 

IV.3.0 INTRODUCTION

Cowes [1911] found that carbon in the tailings treated at the 
Waihi-Paeroa Mine were responsible for losses [J Chem  Met Soc SA, 
May 1911, page 575].

This report discusses the behaviour of preg-robbing species 
towards various treatment processes in four parts :

 [a] The chemistry of pregrobbing : 

    - Simple ion exchange : Type I
    - Aurocyanide polymer formation : Type IIa
    - Reduction to gold metal : Type IIb
    - Mixed gold hexacyanide precipitates : Type III
    - Metallic gold encapsulated by mineral reprecipitation : 
      Type IVa 
    - Aurocyanide encapsulated by mineral reprecipitation : 
      Type IVb 

 [b] The mineralogy of carbonaceous ores, the most important 
   classes of preg-robbers being :

    - Organic
    - Graphitic
    - Carbonate
    - Non-geologic

 [c] Deposits exhibiting pregrobbing :

    - Africa 
    - Australasia
    - North America
    - South America
    - Elsewhere

 [d] Process options applied to address pregrobbing :

     - Gravity recovery                                       
     - Carbon flotation                                       
     - Dissolved gold capture                                 
     - Passivation of preg-robbers                            

IV.4.0 THE CHEMISTRY OF PREGROBBING ORES

Understanding the chemistry driving pregrobbing, provides the 
basis of addressing the phenomena. 

The mechanisms given below are believed to be most likely to 
cause the behaviours observed to date, but have not been 
confirmed yet by rigorous research. 

The complexities of real slurry systems apart from mineral 
chemistry [eg mass transfer control of unmeasurable conditions 
within the hydrodynamic boundary layer] makes this a major task 
which has not yet been attempted nor completed. 

A key to the chemistry lies in the apparent greater resistance to 
oxidation of the pregrobbing components as compared to other 
reducing species in ores which merely contribute to oxidant 
consumption.

An understanding of the slurry chemistry could allow one to 
reduce the normally uneconomic oxidant demands to treat such ores 
by such processes, through selective chemical attack on the 
pregrobbing species alone.

However there is various evidence that six mechanisms of 
pregrobbing exist :

 [a] [Rapid] carbon-reversible : Type I
       Simple adsorption of aurodicyanide by ion exchange. 

 [b] [Slowly] cyanide-reversible : Type II

    - Without oxidant : Type IIa
       Mono-aurocyanide polymers formed by cyanicides.
    - With oxidant : Type IIb
       Reduction to gold metal.

 [c] [Rapidly] Alkali-reversible : Type III
     - Ionic gold chemically co-precipitated with metals, 
       typically the hexacyanides. 

 [d] Rapidly acid-reversible : Type IV
    - Recyanided without oxidation : Type IVa
       Dissolved aurodicyanide gold adsorbed encapsulated by 
       precipitates formed during treatment. 
    - Requires oxidation for recyanidation : Type IVb
       Undissolved or reduced metallic gold chemically co-
       precipitated with metals, typically the hexacyanides. 

The above type-classification is updated from that used 
previously; and mixtures of pregrobbing types can occur 
simultaneously for particularly complex pregrobbing ores. 

It should be noted that in simplistic test sets, confusion can 
arize on liberation of primary unliberated gold and that lost by 
pregrobbing.

IV.4.1 CARBON/RESIN/WASH REVERSIBLE : TYPE I

Type I pregrobbing is practically universal.

The mechanism of gold pregrobbed by this process is simple ion 
exchange. 

Simple bottle roll or BLEG determinations will not reveal any 
gold lost by pregrobbing. 

Aurodicyanide is a large anion, thus with naturally good 
attractability to ion exchange sites. 

This reversible [type I] pregrobbing forms the basis of gold 
losses in processes such as Merrill-Crowe where there is no 
competing adsorption eg by carbon or resin. 
                           
Simple pregrobbing [type I, ie by ion exchange] is not serious 
where slurries are contacted with carbon, as subsequent CIP 
contacting, the present standard in gold processing, reverses 
this. 

It has been found that where there is no competitive carbon or 
resin adsorption, generally at least 5 to 35% of the gold on 
tailings solids is dissolved but held by ion exchange. 

However apart from competing carbon/resin adsorption, such gold 
can be removed by washing [elution], particularly if the wash 
water is hot and/or an eluant, eg saline. 

Even CIL bottle rolls reveal only gold subject to type I [carbon-
reversible] pregrobbing. 

Sheet silicates, some forms of natural carbon and carbonaceous 
mineralization have significant adsorbtive capacity.

In the absence of sufficient competition by carbon or resin, this 
may lead to binding and loss of gold. 

The interpretation of adsorption behaviour can be complicated by 
the non-uniform nature of particle surface charge. 

In general, particles are charged in solution, positively at low 
pH, negatively at high pH. 

Cations rather than anions tend to form the inner, tightly held 
layer of ions at high pH cyanidation conditions, with a more 
loosely held counter layer of anions. 

At low pH the situation reverses, with a neutral point called the 
Point [pH] of Zero Charge [Isoelectric Point]. 

The isoelectric point is unique to each mineral. 

However some complex minerals such as clays have a different 
point of Zero charge for the edges and platy portions of the 
particle. 

The edges are furthermore not homogenous, the layered structure 
leading to alternatively more negative and positive planes. 

In heaps and vats, ion exchange onto solids can hold dissolved 
gold very effectively as there is no direct in-situ competitive 
scavenging by activated carbon or resin. 

Further background of relevance in this very important area of 
clay-associated ions affecting cyanidation is provided by Wayman 
[1967] and Hem [1967]. 

The introduction of CIP promoted the resorption of dissolved gold 
held by slurry solids at many operations, including :

 [a] Gold Quarry, Nevada

 [b] Cortez Gold Mines, 1800 tpd since 1981

 [c] Mercur, Utah [Barrick], 3600 tpd since 1981

 [d] Renco, Zimbabwe, 600 tpd since 1980

In the earlier process of filtration, such gold would have been 
lost.

A 1.5 tpd pilot scale test at Salsigne, Orleans, France 
demonstrated that Type II pregrobbing of dissolved gold can be 
reduced more by resin [VitrokeleTM V912] of higher activity than 
carbon [Pica G210 AS] applied under similar conditions.

Simple bottle roll or BLEG determinations will not reveal any 
gold lost by pregrobbing. 

IV.4.2 CYANIDE REVERSIBLE : TYPE II

Types IIa and IIB pregrobbing is apparently rather than truly 
so. It is deemed "irreversible" because a long time and/or 
unusually severe conditions are required to redissolve gold 
pregrobbed by the four mechanisms involved. 

Furthermore, releaching is generally done in the absence of a 
carbon or resin adsorbent; thus gold which is leached might be 
held out of solution by the mild and reversible type I 
pregrobbing. 

IV.4.2.1 AUROCYANIDE POLYMER FORMATION : TYPE IIa

This pregrobbing mechanism is serious only where process time is 
limited, as in slurry processing. 

It involves formation of auromonocyanide, caused by capture of 
one of the two cyanides associated with aurodicyanide from the 
local cyanide deficiencies caused by complexation with Fe, Cu, Ni 
and Co. 

The auromonocyanide radical, with one free electron forms. This 
readily combines with others to form long strings of the brown 
polymer [AuCN]n. 

This polymer, which redissolves very slowly, certainly not 
effectively within the residence times generally provided in 
agitation leaching. 

The reason for the slow redissolution s that replenishment of 
cyanide leads to attack only on the terminal aureous cyanide 
moeties of each polymer chain. 

The longer the auromonocyanide polymer chain; the less end 
groups and the slower the redissolution.

Chain length will be determined by local dissolved gold 
concentrations; which will be a maximum for finely grained gold 
which can dissolve faster than it can diffuse into bulk solution. 

Type IIa pregrobbing can occur even when bulk cyanide 
concentrations are sufficient to satisfy bulk cyanicide demands.

This is because on a local level, within the hydrodynamic boundary 
layer surrounding particles, local generation of cyanicides might 
exceed the capacity of cyanide diffusing in, to satisfy their 
demands. 

A less recognized but relatively widespread problem is the 
holding by such minerals, of cyanicides and oxygen consumers such 
as divalent iron. Competition for cyanide within the sheet 
structure can lead to local precipitation of gold and pregrobbing. 

Other reducing minerals can also be involved in type IIa 
pregrobbing, eg the ferrous iron associated with siderite can 
severely rob cyanide, scavenge oxygen and contribute to gold 
pregrobbing by polymerization. 

Figure 1 provides some data on the rates of re-dissolution, and 
demonstrates that major gold losses are inevitable in the limited 
residence time provided (and relatively low cyanide 
concentrations) in CIP/CIL. 

It is that gold which is robbed by the type IIa mechanism which 
contributes to the 10 to 20% of gold found dissolved in reclaimed 
tailings - residual cyanide discharged with tailings continues 
dissolvong primary liberated gold, but also reverses the 
polymerization. 

Often re-leaching of the tailings shortly after placement will 
reveal negligible primary liberated gold; whereas an in-situ 
dissolution allowed to extend over months will allow 
depolymerization of a significant quantity of gold. 

For the residence time of around 24 hours for a typical gold 
recovery circuit, gold robbed by the type IIa mechanism can be 
considered [practically] to be robbed irreversibly. 

IV.4.2.2 REDUCTION TO GOLD METAL : TYPE IIb

Electrochemical reprecipitation involves electron transfer, re-
plating gold. 

This is most common in the dissolution of gold by weak complexes 
such as chlorides, and is promoted in the presence of reductants. 

The presence of such reductants, and/or more severe conditions 
combined low cyanide and redox potential, sufficient to 
precipitate gold as metal is what distinguishes type IIb from 
type IIa pregrobbing. 

Such electrochemical gold metal reprecipitation even occurs with 
the strongest gold complexant known - cyanide. 

This may be caused by highly reactive reductive sulphides; and 
also by elemental sulphur which may form during sulphide 
oxidation, particularly when very low pH conditions develop. 

The existance of this mechanism was discovered by observing a 
dark precipitate on the LeachMETERTM cathode, coinciding with 
negative readings indicating gold deposition rather than 
dissolution; and confirmation of the precipitate as gold by 
microprobe.

The most common easily hydrolyzed sulphides involved are :

 [a] Members of the Mackinawite/Pyrrhotite family

 [b] Framboidal Pyrite

 [c] The submetals, Orpiment and/or Realgar

Other sulphides the analogues to the above exist. For example the 
base metal analogues of Mackinawite/Pyrrhotite which are low or 
in sub-stoichiometry in sulphur include :

 [a] Ni : Violarite

 [b] Cu : Covellite

Boules have been recognized as contributing significantly to 
pregrobbing by the type IIB mechanism. 

Boules comprize large [cubic metres] semi-cohesive aggregations of 
slurry, arising from the Bingham plasticity of slurries, which only 
shear in regions where the stress is great enough, viz over about 
10 Pa. 

The interior of such boules are replenished of cyanide and oxygen 
with difficulty, leading to regional conditions where gold can be 
reprecipitated as metal. 

Type IIb pregrobbing is very likely to have a significant 
quantity of Type IIa pregrobbing occur with it, in process zones 
where the conditions are less severe.

IV.4.3 ALKALI REVERSIBLE : TYPE III

In the absence of extreme chemical conditions, type III 
pregrobbing is irreversible for even longer periods than for 
type II pregrobbing - eg as only achieved in heap leaching. 

Gold robbed by the type III mechanism, is co-precipitated with 
metals, typically as planar hexacyanides. However a relatively 
stable gold-copper linear cyanide is also known. 

These are extremely stable, particularly in the acid regime; but 
destroyed by high alkalinity - which is of interest as treatment 
of pregrobbing ores at pH 11.6 has proven effective in some 
cases.

The pregrobbing action of both iron and copper are well-
demonstrated. 

Cobalt and nickel also appear to be able to act as type III 
pregrobbers. 

IV.4.4 ACID REVERSIBLE : TYPE IV

Precipitates which bind gold other than mixed cyanides, are 
generally hydroxides and thus acid soluble. 

Some may require fairly severe conditions, such as ferric hydroxide 
[pH<3], and this may be applied as a diagnostic to identify the 
precipitates. 

Because of the relatively severe conditions which may be applied 
in assessing acid-reversible pregrobbing, simplistic test sets 
can lead particularly to confusion between primary liberation of 
gold and recovery of that lost by pregrobbing. 

IV.4.4.1 AUROCYANIDE ENCAPSULATED BY MINERAL REPRECIPITATION : 
      TYPE IVa 

One chemical reaction which has been noted in carbonaceous 
pregrobbing ores, is decomposition of a bicarbonate at pH about 
11.6, followed by precipitation of carbonates while pH is pulled 
down autogenously through this precipitation, towards the 
bicarbonate/carbonate buffer point around 8.3. 

The CaCO3 precipitate appears to cover and lock underlying gold. 

The evidence is :

 [a] That high pH [around 11.6] generally makes the bound gold 
   accessible albeit re-locked if not rapidly scavenged eg by 
   activated carbon or resin, and of 

 [b] Subsequent tendency of fresh filtrates to form a calcareous 
   precipitate buffering at about pH 8.3, points to the release of 
   gold from a bicarbonate, and recapture into a precipitating 
   carbonate. 

Testwork where this occured was unfortunately suspended before 
further investigation eg the effect of acid washing, was done to 
better define the mechanism. 

Residual iron, left after oxidation of iron sulphides also 
affects cyanidation, generally exhibiting a pregrobbing 
phenomenon. 

Iron Ores exhibiting combinations of low/high acidity and 
low/high oxygen demand reflect the extent of Fe oxidation and 
washing out of hydrolysis products. 

thus FeII causes greater oxygen demand as well as acidity, while 
FeIII only causes greater acidity. 

Residual FeII is often present in the oxide ore zones in arid 
regions, insufficient washing having occured to remove it. 

In wet regions, acid clays below the water table will have residual 
FeII deleterious to gold recovery. 

Figure 2 [Mann, 1984] explains the origin and ocurrence of Fe++ 
in the deeper portions of the lateritic weathering profiles of 
greenstones, source of most gold deposits. 

Edwards ao [1965] have drawn attention to the close association 
of divalent cations and clay particles, as they are closely held 
in the Stern layer and not readily subject to ion exchange with 
simple monovalent cations such as sodium. 

Bivalent cations such as calcium is required to promote their 
diffusion, and only at local pH levels not too far from the that 
of ferrous hydroxide And within clay layers, due to dynamic 
diffusion effects. 

The pH within the layers of precipitation [around pH 6]. 

It should be noted that bulk pH levels do not accurately reflect 
local pH levels at the particle surface sour waterlogged clays 
can be as low as pH 4. 

Considerable quantities of hydroxide must diffuse in to increase 
this to over pH 6. 

Furthermore twice as much hydroxide is required to precipitate 
the iron, compared to the stoichiometric amount of Ca++ 
displacing Fe++]. 

IV.4.4.2 METALLIC GOLD ENCAPSULATED BY MINERAL REPRECIPITATION : 
      TYPE IVb 

Under more severe conditions of low cyanide concentrations and 
oxidation potential, dissolved aurocyanide can be metalized. 

The dissolution might occur in the shear zone between boules, and 
the reprecipitation as metal within the anoxic, cyanide-deficient 
interiors of boules. 

Re-encapsulation of metallic gold is generally by formation of a 
protective skin.

Gold inactivated by formation of a protective gold oxide skin is 
known, but the evidence is that this forms in certain geological 
environments rather than during processing with cyanide.

Re-encapsulation of gold is more commonly due to the development 
of surface coatings formed during milling than during susequent 
cyanidation. 

Coatings which have been identified include ferric [hydr]oxides 
but also Zeta-positive colloidal sulphides such as Acanthite 
[silver] and Cinnabar [mercury], as well as lime itself. 

When gold coating occurs during cyanidation, the mechanism is not 
satisfactorilly known or explained.

However it is believed to involve immediate re-capture of ionic 
aurocyanide into a precipitating matrix being formed simultaneously 
due to the pH conditions, during cyanidation. 

The increase in pH encountered during cyanidation could for 
example provide precipitating Magnesium hydroxide nuclei, which 
are Zeta-positive and thus be attracted to metallic gold.

Carbonate is itself very often a refractory constituent for gold 
recovery : many systems require matrix destruction to gain access 
to the gold. 

IV.5.0 PREGROBBING ORE MINERALOGY/GEOLOGY

IV.5.1 CARBONACEOUS ORES

Carbonaceous ores generally have very fine gold, which is 
rapidly leached.

Scheiner ao [1971] determined that the gold in Nevadan 
carbonaceous ores was under 200 nanometres in size, which under 
normal cyanidation circumstances would dissolve in about 4 
minutes; with almost 90% dissolution in half that time. 

There are generally components in carbonaceous ores that provide 
in addition to adsorption by ion exchange [which is reversible], 
an irreversible adsorption which could be due to chemical 
reaction. 

The chemistry has to date not been well correlated with geology, 
according to which four components exist :

 [a] Organic carbon

 [b] Graphitic carbon

 [c] Carbonate carbon

 [d] Non-geologic carbon

IV.5.1.1 ORGANIC

Pregrobbing is generally associated with carbonaceous sediments 
and humic acids, rather than graphitic carbon.

Hydrocarbons are commonly present in sedimentary rocks. 

Finely disseminated gold occurs in the thoro/uranic hydrocarbon 
thucolite [Thorium, Uranium, Carbon, Hydrogen, Oxygen-containing 
mineral], on the Witwatersrand. 

The association of gold with carbonaceous matter in Witwatersrand 
ores is believed to be due to biological redistribution.

However there are convincing theories relating to the origin of 
carbonaceous matter from methane ender severe conditions of 
pressure and temperature.

Organic extracts have been demonstrated to have pregrobbing 
characteristics generally believed to rob gold by chelation. 
However polymerization of aurocyanide is also a possible route.

Such polymerization could be initiated by cyanide abstraction eg 
through the cyanohydrin reaction on reducing end-groups as set 
out below. Organics with reducing end-groups could result from 
decomposition of roots and other plant residues. 

Step 1 : Formation of cyanohydrin intermediate
                                   _               _
                                  C=N             C=N        
                                  |               |          
           H-C-O                H-C-O-H         H-C-O-H      
             |                    |               |     
           H-C-O-H              H-C-O-H  =    H-C-O-H      
             |     + NaCN  =      |               |     + NaOH
           H-C-O-H + H2O        H-C-O-H         H-C-O-H   
             |                    |               |    
             R                    R               R

     Reducing end-group              Cyanohydrin

Step 2 : Hydrolysis to lactone
      _             _        
     C=N           C=N           .---C-O       .-----C-O     
     |             |             |    |        |     |       
   H-C-O-H       H-C-O-H         |  H-C-O-H    |   H-C-O-H   
     |             |             |    |        |     |       
   H-C-O-H = H-O-C-H   + H2O =   |  H-C-O-H =  | H-O-C-H   + NH3   
     |             |             |    |        |     |       
 H-O-C-H       H-O-C-H           .--O-C-H      .---O-C-H     
     |              |                 |              |    
     R              R                 R              R

       Cyanohydrin                        Lactone

Step 3 : Alkaline decomposition to aldonate

 .----C-O       .-----C-O              Na+-O-C-O     Na+-O-C-O
 |    |         |     |                      |             |    
 |  H-C-O-H     |   H-C-O-H                H-C-O-H       H-C-O-H
 |    |         |     |                      |             |    
 |  H-C-O-H  =  | H-O-C-H   + NaOH  =      H-C-O-H  =  H-O-C-H  
 |    |         |     |                      |             |    
 .--O-C-H       .---O-C-H                H-O-C-H       H-O-C-H  
      |               |                      |             |    
      R               R                      R             R

          Lactone                          Aldonate end group

Organic pregrobbing compounds found with gold are generally 
reducing agents, and this reduction could precipitate the gold 
[as metal rather than as polymer]. 

IV.5.1.2 GRAPHITIC

Pregrobbing is generally associated with carbonaceous sediments 
and humic acids, rather than graphitic carbon.

Graphite may be associated with gold because it is a residual of 
the hydrocarbon that reduced it. 

IV.5.1.3 CARBONATE

Where the carbonate itself encapsulates gold, acid which 
restricts cyanidation is required. 

The consumption of acid to achieve the required liberation of 
gold may also be excessive. 

The early carbonates are commonly replaced by sulphides, whereas 
the late calcite fills minute fractures and only rarely replaces 
the sulphides; ie presulphide carbonate is generally mixed and 
complex, whereas the late carbonate is commonly calcite. 

Vein carbonate associations in gold-quartz deposits generally 
involve a paragenetic sequence of carbonates beginning with 
ankerite, giving way to more calcic carbonates, ending with 
calcite. 

Native gold occurs commonly with the ankerite, and along cleavage 
planes of calcite and other carbonates. 

Common carbonates have very low gold contents, due to the low 
gold-carbonate bond strength. 

However Au substitution for Cu allow these minerals to 
accommodate small quantities of gold. 

Kidston has carbonates, in addition to copper in its 
mineralization.

Ore at Chimney Creek, Winnemucca is high in carbonates.

In carbonaceous pregrobbing ores, decomposition of what was 
infered to be a bicarbonate mineral [Cancrinite ?] at pH about 11.6 
has been noted. 

This was followed by precipitation of carbonates while pH is pulled 
down autogenously through this precipitation, towards the 
bicarbonate/carbonate buffer point around 8.3. 

The CaCO3 precipitate appeared to cover and lock underlying gold. 

IV.5.1.4 NON-GEOLOGIC CARBONACEOUS PREGROBBING

Carbonaceous pregrobbing can arize from non-geological sources, 
such as decaying mine timbers, and particularly in tailings 
retreatment, burnt plant growth and boiler ash discarded on 
dumps.

Note that carbon fines generated in the circuit can also cause 
pregrobbing. 

IV.5.2 NON-CARBON ORES

IV.5.2.1 ADSORBING

Baum [1988] notes that [adsorptive] pregrobbing can be a 
significant problem in heap leach problem related to slimes, 
which generally orginate from weathered silicates. 

Baum also notes that pyrophyllite and other Mg-rich 
phyllosilicates, or a combination of these with hydrous 
iron/manganese oxides, have a potential for pregrobbing similar 
to carbonaceous ores. 

He has observed in Western US and Latin American epithermal gold-
silver deposits, significant sorption of gold where there is 
freshly generated slimes of : 

 [a] The layer silicates : vermiculite, montmorillonite, illite, 
   kaolinite, especially if associated with

 [b] Iron/manganese hydroxides.

Such sorption has been studied by Krendelev ao [1978].

IV.5.2.2 CYANICIDAL

The contribution of cyanicides to types IIa, IIb, III, IVa and 
IVb pregrobbing, has been noted earlier [section 4]. 

IV.5.2.3 OXYGEN CONSUMING

The contribution of oxygen consumers to types IIb and IVb 
pregrobbing, has been noted earlier [section 4]. 

IV.6.0 PROCESSING AT DEPOSITS EXHIBITING PREGROBBING

Pregrobbing, at least by the type I pregrobbing mechanism, is 
practically obiquitous. 

The associated type I pregrobbing losses by Merrill-Crowe 
processing have been practically eliminated by the general 
adaption of CIP and later, CIL processing.

In cases where the extent of residual pregrobbing has remained 
significant, further action has been initiated. 

This section provides a record of such operations. 

Minesites which have recorded pregrobbing problems include :

 [a] AFRICA :

    - Agnes
    - Athens [Uvuma, Zimbabwe]
    - Ashanti
    - Barbrook Mine
    - Bibiani, Ghana
    - Bogosu, Ghana
    - Crown Sands
    - Kinross [East Rand]
    - New Consort
    - New Machavie, Witwatersrand
    - President Brand calcine
    - Prestia, Ghana 
    - Randfontein Cooke
    - Renco, Zimbabwe
    - Sheba
    - Twangiza Uganda
    - West Rand Black Reef
    - Worcester, Barberton

 [b] AUSTRALASIA

    - Cosmo Howley
    - Fortnum
    - Hedges
    - Kelian
    - Macraes
    - Moline
    - Stawell
    - Tanami
    - Temora
    - Waihi/Paeroa [New Zealand]

 [c] NORTH AMERICA

    - Alligator Ridge [Amselco]
    - Atlas Gold Bar
    - Big Springs
    - Carlin
    - Cortez Gold Mines
    - Enfield Bell
    - Getchell
    - Gold Acres
    - Gold Quarry, Nevada
    - Jerrit Canyon [Nevada], 
    - Kerr-Addison
    - McDermott
    - McIntyre Porcupine, Canada
    - McLaughlin
    - Maggie Creek
    - Mercur, Utah
    - Mother Lode, Canada
    - Ochali [Columbia, Canada]
    - Queen Charlotte Island, British Columbia
    - Royal Mountain King
    - Zortman

 [d] SOUTH AMERICA

    - Morro do Ouro
    - Morro Velho
    - Queiroz Mine
    - Rosario Dominicana, Dominican Republic
    - Pueblo Viejo, Dominican Republic

 [e] ELSEWHERE :

     - Bakyrchik [USSR]
     - Natalinsk [USSR]
     - Salsigne, Orleans, France.

A variety of data has been recorded for specific operations :

IV.6.1 AFRICA

IV.6.1.1 ASHANTI

Ore is upgraded by flotation from about 1% carbon to 6% carbon at 
Ashanti, and roasted prior to cyanidation. 

IV.6.1.2 BIBIANI

At Bibiani, much of the graphite was recovered in the flotation 
concentrate, and the reagents [xanthate and pine oil] passivated 
the preg-robbers in this pre-carbon [viz Merrill-Crowe] circuit. 

IV.6.1.3 NEW MACHAVIE

At the New Machavie Mine, Witwatersrand, the nonpolar hydrocarbons 
diesel, light oil or kerosene proved most suitable, more so than 
xanthates and pine oil. However even so, these methods [flotation-
blinding of carbonaceous material] gave relatively poor results. 

IV.6.2 AUSTRALASIA

IV.6.2.1 MACRAES

The behaviour of the Macraes ore suggests that : 

 - Most of the gold is micro-fine, leaching so fast even under 
   low oxygen conditions [minutes ?], that changes with oxygen 
   level are small, where rate sampling commences only after 1 
   hour has elapsed. 

 - Apart from the reduced leach rate of some coarser or embedded 
   gold, much of the slower appearance of dissolved gold in the 
   solution phase could be due to retarded diffusion of dissolved 
   gold from within the hydrodynamic boundary layer surrounding 
   particles, into bulk solution. 

   Such retardation could be ruled by colloidal filming of 
   particles, as well as reduced diffusion gradients should the 
   solution tenors not be kept at a minimum, eg by CIL, and high 
   carbon concentrations and/or activity. 

The major chemical mechanisms involved in the preg-robbing remain 
obscure, requiring more detailed testwork and analysis.

The gross features are that apart from oxygen not demonstrating a 
significant effect :

 - Severe [OCl-] oxidation does not conclusively or significantly 
   affect DO levels, nor cyanide consumption. This might be due 
   to competing effects tending to mask one another. 
 
 - The only significant effect of severe [OCl-] oxidation is to 
   increase lime consumption. A possible reaction is :

             C + 2 OCl- + H2O = 2 Cl- + 2 H+ + CO32-

   This could be checked by the about pH 8.3 HCO3-/CO32- buffer 
   expected in an alkali system, and carbonate assays. 
   The simple law of Mass Action [Guldberg and Waage] would 
   indicate that high alkalinity would promote this reaction, 
   which probably explains the improvements observed at high pH 
   [viz over pH 11.5]. 

IV.6.2.2 STAWELL

Although rapid carbon movement is applied, the emphasis is on 
selective mining to avoid losing gold which is unassociated with 
severe pregrobbers to this process.

IV.6.3 USA

IV.6.3.1 ALLIGATOR RIDGE

The roles of :

 [a] An unusual reprecipitating bicarbonate in the locking of gold in 
   the carbonaceous ore treated, and 

 [b] The significant and essential benefits of CIL at high in-tank 
   concentrations and low gold loadings, 

was discovered through testwork aimed at improving gold recovery 
from this Amselco ore.

IV.6.3.2 ATLAS GOLD BAR

The Hazen Research process hot caustic/soda ash proved a disaster 
at Atlas Gold Bar [where oil shale cooked form very effective 
fouling agents for activated carbon]. 

On failure of the pretreatment at full scale, this operation 
reverted to simple CIL, which gave poor performance yet still 
better than using the hot caustic/soda ash pretreatment. 

IV.6.3.3 BIG SPRINGS

Mild chlorination on carbonaceous pregrobbing ores was commenced 
at Big Springs, USA during the late 1980's.

IV.6.3.4 CARLIN

Carlin has practiced mild chlorination on carbonaceous 
pregrobbing ores since 1972, to oxidize framboidal pyrite, 
lignitic carbon, humates and fumates on 450 tpd ore containing 
8.6 g Au/t, 0.5%S and 1%C.

This was executed in a carbonate system at 0.9 atm; after a pre-
aeration with steam and air at 84C, pH 10; since 1972. 

Froth flotation concentrates prepared from Carlin ore, low Eh, 
Philips aromatic oil as collector, desliming and dispersion using 
sodium silicate gave best results. 

CIL reduces the loss of gold by pregrobbing, and is specifically 
applied to achieve this at Carlin.

IV.6.3.5 JERRIT CANYON

Another operation which practiced mild chlorination on 
carbonaceous pregrobbing ores include Jerrit Canyon, Nevada, 
oxidizing framboidal pyrite, lignitic carbon, humates and fumates 
on 1800 tpd ore containing 7 g Au/t, 1%S, 1%C at 0.9 atm.

This process, operating at 40C, pH 10 was practiced since 1981. 

IV.6.3.6 KERR-ADDISON

Kerosene has been used to blind pregrobbing behaviour at Kerr-
Addison, Canada [McQuiston and Shoemaker, 1975].

IV.6.3.7 McDERMOTT

McDermot applied processing similar to that defined in USA patent 
1519396/22 [which might be refered to in the 1992 Canadian 
Proceedings], viz carbon deactivation using soaps and soap 
byproducts. 

IV.6.3.8 MERCUR

Refractory sulphidic ore at Mercur typically contains about 0.4% 
carbon, 1.2% S2- and 16% carbonate. 

The gold seems to be associated with carbon rather than 
sulphides. 

At Mercur autoclaving at higher temperatures was not practical. 
Pressure oxidation of sulphides, at the temperatures under 200C 
does not deactivate the carbon. 

Jha [1984] furthermore demonstrated that by oxidising a gold ore 
containing a high organic content below 200C, actually causes 
pregrobbing. 

A feature of this operation, is that fullscale gold recoveries 
were around 78%, significantly lower than the values around 92% 
indicated in the laboratory. 

CIL reduces the loss of gold by pregrobbing, and is specifically 
applied to achieve this at Mercur.

Carbon concentrations in CIL have been increased from 15 to 40 
g/l, in order to minimize carbonaceous pregrobbing. 

IV.6.3.9 ROYAL MOUNTAIN KING

Royal Mountain King used kerosene alone to selectively prefloat 
carbon and free gold [the remaining gold in sulphides being 
predominantly refractory due to poor liberation, requiring 
release to achieve good recoveries]. 

Caustic as dispersant promoted the greatest recovery of free gold 
to the carbon concentrate. 75% carbon recovery was achieved. 

What could be considered impractically high carbon recycle rates 
and low loadings [in the order of 100 g Au/t] were required to 
achieve reasonable adsorption in competition with pregrobbers.

This was probably due to the kerosene fouling the carbon sites as 
well as those of the preg-robbers. 

IV.6.4 SOUTH AMERICA

IV.6.4.1 MORRO DO OURO

At Morro do Ouro, Knelson concentrators recover sufficient gold 
to reduce the role of the carbon circuit to a scavenging rather 
than a primary recovery role.

Cyanamid reagent 637 was stated to have eliminated 77% of carbon 
from flotation concentrates through depression.

When pregrobbing was severe, Morro do Ouro floated up to 70% of 
the residual gold in the CIP tail, recovering this by an 
intensive leach.

IV.6.4.2 ROSARIO DOMINICANA

Testwork demonstrated that applying CIL while minimizing the 
carbon loading [ie maximizing gold adsorption activity], improved 
gold extraction by over 20% from the refractory carbonaceous ore. 

IV.6.4.3 QUEIROZ

This mine floats 88% of the carbon into 3% mass, containing 47% 
of the gold, in flotation cells of 24 minutes total residence 
time. 

Preconditioning reagents comprize a mix of 3:1 Methyl isobutyl 
carbinol: Aerofroth, plus CuSO4 as activator.

IV.6.5 OTHER 

USSR application of p-nitro benzol azo salicylic acid is quoted 
to be an effective passivator of preg-robbers.

A 1.5 tpd pilot scale test at Salsigne, Orleans, France 
demonstrated that Type II pregrobbing of dissolved gold can be 
reduced more by resin [VitrokeleTM V912] of higher activity than 
carbon [Pica G210 AS] applied under similar conditions.

A number of other operations are reported to apply processes to 
treat refractory carbonaceous sulphidic ores but have not been 
followed up for this report. These operations include : 

 [a] Queen Charlotte Island, British Columbia.

 [b] Enfield Bell, Nevada [Freeport].

 [c] Barbrook, Eastern Transvaal.

 [d] Morro Velho, Brazil.

 [e] Pueblo Viejo, Dominican Republic. 

IV.7.0 NON-PROCESS OPTIONS FOR PREGROBBING ORES

Selective mining should be looked at first, before or in addition 
to examining process options. 

Furthermore, should none of the process options set out in this 
report meet the required financial criteria, toll processing of 
the ore or a concentrate can be considered. 

IV.8.0 PROCESS OPTIONS FOR PREGROBBING ORES

The information presented in section 6.0 under specific deposits 
and operations is for convenience again listed under the 
processes discussed below. 

Other process applied to mildly pregrobbing ores include weathering 
of the ore, carbon or resin adsorption, oil blinding, and double 
oxidation [air/chlorine]. 

Nice also [1971] reported on a number of options for dealing with 
carbonaceous ores. 

There are a number of demonstrated or potential remedial actions 
for preg-robbing ores :

 [a] Gravity recovery
    - Feedstock and products
    - Concentrate treatment

 [b] Carbon flotation
    - Prior to CIL
    - Subsequent to CIL

 [c] Dissolved gold capture
    - Solids density
    - Carbon management
    - Optimizing process design

 [d] Deactivation of preg-robbers
    - Oxidation
    - Organics adsorption

Concentration if possible, is a generally very effective way to 
deal with problem ores. 

The higher grades of concentrates allow fairly intensive processing 
that overcomes pregrobbing to be applied, at an economical cost per 
ounce of gold recovered. 

In the case of pregrobbing, concentration of only a partial 
quantity of feedstock may be warranted, as losses by pregrobbing 
from that portion of gold can be eliminated. 

Furthermore appropriate oxidation could also allow greater gold 
recovery through further liberation of encapsulated gold. 

However one must be aware of the disasterous experience of the 
Hazen Research process at Atlas Gold Bar [where oil shale cooked by 
hot caustic/soda ash formed very effective fouling agents for 
activated carbon] and the potential need for techniques to address 
this. 

An SO2/air would be expected to destroy pregrobbers.

Another approach could be to oxidize some but not all cons, 
possibly a most refractory cut or recycle stream. 

IV.8.1 GRAVITY RECOVERY

Gravity recovery of gold, prior to exposure to cyanide and the 
effects of preg-robbing, will be single most effective action to 
reduce gold losses.

At Morro do Ouro, Knelson concentrators recover sufficient gold 
to reduce the role of the carbon circuit to a scavenging rather 
than a primary recovery role.

Early removal of gold by gravity is also applied at Stawell to 
minimize gold losses by pregrobbing.

IV.8.1.1 FEEDSTOCK AND PRODUCTS

Because of the very substantial and direct removal of the threat 
of losses by pregrobbing, gravity recovery should be applied to 
all the streams where significant recovery could be expected.

This is so even if at a given time the recovery demonstrated might 
not be substantial, it is likely that in the fairly variable 
oredody, there will be ore parcels where recoveries will be 
significant. 

The major gravity recovery point should be as early as 
possible/feasible in the circuit, viz the unit cell concentrate 
stream.

A regrind mill will liberate more gold, at fineness which will 
not necessarilly escape a Knelson concentrator, which is claimed 
to recover micron-sized gold. 

Consequently gravity recovery can executed on a regrind mill 
cyclone underflow, or a portion thereof [as might be dictated by 
the water balance]. 

Options for the secondary treatment of the concentrates is 
discussed in the next section.

IV.8.1.2 CONCENTRATE TREATMENT

Concentrates could be treated directly on a Gemeni table to 
produce around 20% Au ot higher grade concentrates, which are 
readily smelted. 

Where heavy magnetic material [eg iron ex ball charge wear] 
contaminates the gold concentrates, a Gemeni operation could be 
modified with an overhead magnet to promote discard of magnetics 
to Gemeni tailings at this stage. 

The tailings should then be subjected to intensive cyanidation, 
eg with a LeachWELLTM 30X cyanidation catalyst developed for this 
purpose. 

The generousness of the primary cut would be dictated by the gold 
content of these Gemeni tailings, and the amenability of the
tailings to intensive cyanidation. 

Testwork would show any benefits of further liberation of this 
very small tonnage and comparatively high-grade stream of Gemeni 
tailings. 

In the event of poor liberation, oxidative fine milling to 
achieve chemical in addition to physical liberation, should be 
assessed. 

One could consider promotion of high gold dissolution from this 
type of material using only the thiosulphate which may be 
generated autogenously during the refractory ore oxidation.

This may be achieved under relatively mild conditions [below the 
boiling point of water] if ultrafine milling is applied. 

Recently the solution to a key problem in this area - the 
difficulty of loading carbon so leached - has been solved, albeit 
in fairly clean solutions. 

This approach might be modified to address expected problems in 
in-leach adsorption. 

IV.8.2 CARBON FLOTATION

Bibiani

At Bibiani, much of the graphite was recovered in the flotation 
concentrate, and the reagents [xanthate and pine oil passivated 
the preg-robbers in this pre-carbon [viz Merrill-Crowe] circuit. 

Carlin 

On Carlin ore, low Eh, Philips aromatic oil as collector, 
desliming and dispersion using sodium silicate gave best results. 

As noted below, conventional problems of slimes and dispersion 
may be readily addressed by oil agglomeration, with which you are 
very familiar. 

McIntyre

McIntyre cyanides concentrates, refloats tails which are recycled 
to the cyanidation circuit

Morro do Ouro

When pregrobbing was severe, Morro do Ouro floated up to 70% of 
the residual gold in the CIP tail, recovering this by an 
intensive leach.

Queiroz

The Queiroz Mine floats 88% of the carbon into 3% mass, 
containing 47% of the gold, in flotation cells of 24 minutes 
total residence time. 

Preconditioning reagents comprize a mix of 3:1 Methyl isobutyl 
carbinol: Aerofroth, plus CuSO4 as activator 

Royal Mountain King

Royal Mountain King used kerosene alone to selectively prefloat 
carbon and free gold [the remaining gold in sulphides being 
predominantly refractory due to poor liberation, requiring 
release to achieve good recoveries]. 

Caustic as dispersant promoted the greatest recovery of free gold 
to the carbon concentrate. 75% carbon recovery was achieved. 

What could be considered impractically high carbon recycle rates 
and low loadings [in the order of 100 g Au/t] were required to 
achieve reasonable adsorption in competition with pregrobbers.
This is probably due to the hydrocarbons fouling these sites as 
well as that of the preg-robbers. 

Oil agglomeration

In many cases the fine-grained nature of the most problematical 
[highly sheared] ore, poor liberation of the organic carbon or 
other pregrobbing species is likely. This is likely to limit the 
scope of this flotation. 

Conventional flotation problems of slimes and dispersion may be 
readily addressed by oil agglomeration. 

Concentration by oil agglomeration would also allow one to grind 
finer, to improve liberation, and provide potentially greater 
scope for benefit by improving the flotability of the relatively 
fine and sparce species being sought. 

IV.8.2.1 CARBON DEPRESSION

Cyanamid reagent 637 has been stated to eliminate 77% of carbon 
from flotation concentrates, through depression thereof. 

IV.8.2.2 CARBON RECOVERY PRIOR TO CIL

Pregrobbing losses can be minimized by aiming for gold recovery 
early in the circuit, followed by intensive pre-leaching of 
concentrates and enriched streams such as flotation middlings 
and recycle streams.

One might even split off the first cut where necessary, to obtain 
grades payable under intensive pre-leaching conditions. 

IV.8.2.3 CARBON RECOVERY SUBSEQUENT TO CIL

An approach of using the problem of preg-robbing to advantage, ie 
to let the species rob gold, and capture it after CIL [Allen, 
1993] is inherently attractive. Residual cyanide would hopefully 
have de-activated the sulphidic material but not the [mainly 
carbonaceous ?] preg-robbers. 

When pregrobbing was severe, Morro do Ouro floated up to 70% of 
the residual gold in the CIP tail, recovering this by an 
intensive leach.

See also T G Chapman : CIL, carbon flotation [then ashing/smelting] 
: A cyanide process based on the simultaneous dissolution and 
adsorption of gold. Rocky Mountain Fund volume on milling methods. 
AIME 1939. 

IV.8.3 DISSOLVED GOLD CAPTURE

On failure of the pretreatment at full scale, this operation 
reverted to simple CIL, which gave poor performance yet still 
better than using the hot caustic/soda ash pretreatment. 

Despite the potential for using pregrobbing as a mechanism to 
achieve the concentration and subsequent capture of gold, it is 
believed that maximizing the capture of dissolved gold by 
activated carbon or resin, at acceptable loadings, will offer the 
most benefit. 

Apart from greater carbon adsorbent concentrations, the three 
major process options, which might be used in tandem, are : 

 [a] Solids density 

 [b] Carbon management

 [c] Optimizing process design

Testwork that demonstrated that VitrokeleR resin could strip 75% 
of the gold loaded on carbon off in one contacting, and 
concentrate it 10-fold [see figure 3], indicating a potential to 
compete better with pregrobbers than by using carbon. 

IV.8.3.1 SOLIDS DENSITY

Testwork has invariably demonstrated the recovery of dissolved 
gold is increased by reducing the quantity of preg-robbing sites 
relative to those provided by activated carbon.

Maintaing carbon concentration, this can be achieved using lower 
slurry solids content.

The associated lower bulk solution tenors also promote the 
migration of dissolved gold from the particle side of the 
hydrodynamic boundary layer, into the bulk solution where it is 
accessible by carbon. 

Normally, the lower solution tenors in the bulk solution will 
excacerbate the low loading of carbon, and put pressure upon the 
gold room.

However means to side-step this problem through boosting the 
activity of carbon through application of High Loading [HL] 
techniques, are available. 

IV.8.3.2 CARBON MANAGEMENT

Given that the activities and concentrations of pregrobbing and 
carbon active sites can differ, also the associated kinetics and 
financial consequences of gold adsorption; there is significant 
scope for optimization.

In operations, carbon management offers one of the most direct 
ways to achieve this.

Beatrix Mine exhibited some pregrobbing losses whwn a partial CIL 
was used; however the carbon in a full CIL outperformed the 
pregrobbing shales resulting in a significant decrease in gold 
loss.

IV.8.3.3 OPTIMIZING PROCESS DESIGN

Process design options to minimize the deleterious effects of 
pregrobbing include :

 [a] Avoiding the mixing of active pregrobbers with higher grade 
   solution tenors, and 

 [b] Avoiding the bypass of semi-leached gold to tanks further down 
   the train where lower carbon concentrations compete less 
   effectively against pregrobbing.

 [c] Using a gravity circuit to reduce the quantity of the gold 
   lost by preg-robbing in subsequent leaching of bulk slurry. 

 [d] Increasing carbon activity by intermittent chemical contacting 
   during adsorption, or using more active resins.

 [e] Using more active and more fouling-resistant resins, such as 
   VitrokeleTM V912, with polar blinding agents for 
   pregrobbing.

A fully optimized design is likely to be based upon a set of 
batch tanks operating in carousel and using carbon whose activity 
is boosted by an appropriate High-Loading [HL] technique. 

Various HL techniques have been suggested in the past, all 
relying on cyanicidal action, which oligomerizes aurodicyanide on 
the carbon to species which are nearly irreversibly loaded [at 
any event within the contact times involved.

Such treatment practically fully restores carbon activity, even 
when partially loaded with gold.

Although the published techniques all carry fairly significant 
disadvantages, viz :

 [a] Risk of Prussic acid [HCN] release [RSA patent 50191/79]

 [b] Severe dore contamination [USA patent 55690/96]

 [c] Poor efficiency and high cost [Australian patent 29918/77-
   510175/80]; Australian patent 81367/91];

new reagents and operating designs can reduce these problems.

An operating model to dictate control of carbon inventory and 
flow would also assist in optimizing plant operation to minimize 
pregrobbing losses.

IV.8.4 OXIDATION OF PREG-ROBBERS

Oxidation would seem to usually only offer an interim solution; 
oxidant consumptions for deeper ores becoming prohibitive. 

IV.8.4.1 CHLORINE OXIDATION

For more highly pregrobbing ore [generally with organic 
carbon over 1%], pre-oxidation by chlorine or roasting is applied. 

Chlorination has been applied with great success, where the 
quantity of oxidisable material is low, and chlorine costs are 
low [typically where the primary market for a near-by chlor-
alkali manufacturer is for caustic]. 

At USA operations applying chlorine, consumptions under 40 kg 
Cl2, at costs of about $80/t chlorine have applied. However 
chlorine pre-oxidation is being phased out as deeper ores start 
requiring increasing amounts of chlorine. Chlorination is generally 
restricted to ores containing under around 0.5% sulphide sulphur. 
It is not an economic method of treating most deeper-lying [more 
sulphidic] ores due to excessive chlorine consumption, eg : 

 [a] 82 kg Cl2/t to achieve the oxidation of 1% S occuring as 
   pyrite

 [b] 100 kg Cl2/t to achieve the oxidation of 1% S occuring as 
   pyrrhotite

With deeper ores, as more mineral components become chlorine-
consuming [eg sulphides], and where the cost of chlorine is high,
chlorination becomes impractical. 

Where consumptions at USA operations have climbed to 100 kg Cl2/t 
[ie $8/t] and over, the process is deemed unsuitable. Even for 
shallow ores where there is little sulphide, the cost of 
chlorination is reduced as far as possible by double oxidation : 

 [a] The first step involves aeration of heated slurry [about80C], 
   with pH balanced to about6 with sodium carbonate. This forms 
   soluble sulphates and oxides from the most reactive sulphides. 

 [b] After heat extraction for recycle, the slurry at about 50C is 
   agitated with Cl2 to oxidize the more refractory, carbonaceous 
   material, prior to liming and cyanidation. 

The aim of double oxidation is to use cheaper air to oxidize the 
more reactive components, saving the more costly chlorine for the 
less reactive pregrobbing components which require it.

Operations which have practiced mild chlorination on carbonaceous 
pregrobbing ores include : 

 [a] Carlin, Nevada, oxidizing framboidal pyrite, lignitic carbon, 
   humates and fumates on 450 tpd ore containing 8.6 g Au/t, 
   0.5%S, 1%C in a carbonate system at 0.9 atm; after a pre-
   aeration with steam and air at 84C, pH 10; since 1972

 [b] Jerrit Canyon, Nevada, oxidizing framboidal pyrite, lignitic 
   carbon, humates and fumates on 1800 tpd ore containing 7 g 
   Au/t, 1%S, 1%C at 0.9 atm; 40C, pH 10; since 1981

 [c] Big Springs, USA - late 1980's

NaCO3 [and/or NaOH], steam and air pretreatment to oxidize 
reactive sulphides at Atlas Gold Bar was abandoned as it 
liberated organics which fouled the carbon.

At Mercur, it was demonstrated that gold ore containing a high 
organic content when oxidised below 200C, results in 
pregrobbing [Jha, 1984]. 

Autoclaving at higher temperatures was not practical. 

A feature of this operation, is that fullscale gold recoveries 
were around 78%, significantly lower than the values around 92% 
indicated in the laboratory. 

IV.8.4.2 PRESSURE OXIDATION

Pressure oxidation is effective in liberating the larger 
quantities of locked gold associated with these deeper zones.

Furthermore the originally perceived inability of pressure 
oxidation to treat carbonaceous ores is addressed by using CIL. 

Carbonate, common in carbonaceous ores, can also impose an 
excessive acid consumption on ores requiring acid conditions.

Acid may be required for example for the destruction of other 
matrices which encapsulate gold, such as on occasion, pyrite. 

Carbonate affects pH control, especially in oxidative 
pretreatment processes. 

Associated magnesium [eg in dolomite], can severly affect slurry 
viscosity as pH is increased over the precipitation limit. 

In pressure oxidation, carbon dioxide released on acidulation can 
leave less capacity to maintain oxygen partial pressure. 

This occured at Sao Bento, which containd 8% carbonate in the 
flotation concentrate.

The carbonate is now disengaged [to CO2] by prior acid bacterial 
oxidation which also reduces the oxidative load and increases the 
throughput capacity of the pressure oxidation reactors. 

Acid pressure oxidation can activate organic carbon to be more 
pregrobbing, addressed by CIL. 

Marsden and House [1992] have gathered a summary of commercial 
pressure oxidation plants, given below in table III.4

TABLE III.4 : COMMERCIAL PRESSURE OXIDATION PLANTS

Plant        Mercur   Goldstrike  McLaughlin  Getchell      Sao    
                                                           Bento,  
Country       USA        USA         USA        USA        Brazil  
                                                                   
Media        alkali      acid        acid       acid        acid   
                                                                   
Feed          ore         ore        ore        ore         cons   
                                                                   
Rate          720        1360       2450       2500         240    
tpd                                                                
                                                                   
Size           77%                   80%        80%         90%    
           <75micron             <75micron  <75micron  <44 micron  
                                                                   
S2-            0.95       17          3        2 to 4        18    
%                                                                  
                                                                   
CO3-           16         3.5                 1.5 to 7.5      8    
                                                                   
C              0.3        0.75                   0.4               
                                                                   
MPa            3.2      2.5 to 3   1.7 to 2.2    3.2         1.6   
                                                                   
O2 kPa         380         340     140 to 280    700               
            [160 kPa ?]                                            
                                                                   
Temperature    220         225        180        210         190
C                                                                 
                                                                   
Residence      1.9         1.2        1.5        1.5          2    
time, h                                                            
                                                                   
Sulphide    over 70       86 to 97   over 85                           
conversion,%         

Ores [or concentrates] with over 4% S2- may be autogenously 
leached, ie without external heat addition, in acid pressure 
autoclaves. 

IV.8.4.3 ROASTING

In roasting, carbon dioxide being released from carbonates can 
reduce the effectiveness of oxygen reaching the sulphides. 

Roasting can be effective in reducing carbonaceous pregrobbing 
from such ores; pressure oxidation generally not. 

IV.8.4.4 BIO-LEACHING 

The most important factors to consider are the limitations on the 
application of bio-leaching to refractory gold ores. 

These limitations include : 

 [a] Poor water quality/biocides. 

 [b] Substantial quantities of sulphides, devoid of gold. 

 [c] Excess alkalinity

 [d] Ore variability

Poor water quality/biocides 

The presence of biocides, eg high Mo levels. Hg is also 
mentioned, as well as Ag; however high Ag ores [over 6 kg Ag/t] 
have been succesfully treated. 

Tolerance to dissolved species is however often surmountable by 
developing tolerance in the microbes, thus microbes have been 
developed to the folowing tolerance limits : 

 [a] 120000 ppm Zn                                            
 [b]  72000 ppm Ni                                                             
 [c]  55000 ppm Cu                                                             
 [d]  50000 ppm Fe                                              
 [e]  30000 ppm Co       
 [f]  20000 ppm As       
 [g]  12000 ppm Uranyl                                
 [h]  10000 ppm Cl [as Na]
 [i]   6000 ppm Al  
 [j]   5000 ppm Co  
 [k]    700 ppm Ag  
 [l]    300 ppm Sb  
 [m]    200 ppm Mg  
 [n]    200 ppm Ca  
 [o]    100 ppm Cu       
 [p]     80 ppm Se       
 [q]     20 ppm Pb       
 [r]     20 ppm Tl [phosphate-free medium]
 [s]      0.01 ppm Ag    
 [t]      0.005 ppm Au   










Other substances, however for which no limiting concentration 
data are available include NO3-, NO2-, Cl2, H2O2, Cd. 

Substantial quantities of sulphides, devoid of gold 

These are particularly a problem if more amenable to oxidation 
than the gold-bearing minerals, can also make specific ores 
refractory [in terms of excess oxygen requirements and acid 
generation]. 

Some neutralization during oxidation is required, as few microbes 
can tolerate under pH 0.7. 

The high acidity may also increase gold lock-up inherent in 
associated jarosite formation. 

Excess alkalinity

Natural buffers above the precipitation pH of ferric hydoxide or 
excess carbonates and alkali's might make acid oxidation of 
sulphides impractical, due to unacceptably high acid consumption. 

Excess alkalinity, preventing achievement of the low pH 
[around 1.1 to 1.5] required. This single factor has caused 
failures of bio-oxidation at Tuscarora [1960's], and at 
the Giant Bay trial at Texasgulf's Cripple Creek operation. 

Low alkalinity also prevented consideration of the bio-
oxidation option at Barrick's Mercur and Freeport's Jerritt 
Canyon operations.

Ore variability

Highly variable ore puts a strain on the capacity of the microbes 
to adapt to changing matrices. This prevented consideration of 
bio-oxidation at the Getchell gold operation, Nevada. 

IV.8.5 DE-ACTIVATION BY ADSORPTION

IV.8.5.1 USING POLAR ORGANICS AND SALTS 

Early processes relied in blinding the preg-robbing carbon with 
poorly soluble organics.

USA patent 1519396/1922 claimed the application of organic acids 
to coat pregrobbing sites.

At Bibiani, Ghana, flotation reagents [xanthate, pine oil] 
overcame pregrobbing during subsequent grinding in cyanide. 

Much of the graphite was recovered in the flotation concentrate, 
and the xanthate and pine oil passivated the preg-robbers in this 
pre-carbon [viz Merrill-Crowe] circuit. 

At the New Machavie Mine, Witwatersrand, the nonpolar 
hydrocarbons diesel, light oil or kerosene proved most suitable, 
more so than xanthates and pine oil. 

Diesel was used to address pregrobbing at the Geduld Mine from the 
1930's, and at Leslie Gold Mine from 1964.

However even these [Diesel] oil-based methods [flotation/blinding 
of carbonaceous material] gave relatively poor results. 

USA patent 1519396/22 [Darrow] teaches art similar to that applied 
at McDermott. Apart from lauric salts, other soaps and soap 
byproducts such as myristic, stearic and palmitic acid salts and 
derivatives are claimed. 

USSR application of p-nitro benzol azo salicylic acid is quoted 
to be an effective passivator of preg-robbers. 

Such procedures originally developed for Merrill-Crowe circuits 
could work for carbon, as my experience is that polar organics in 
general do not foul activated carbon, at least not generally so in 
slurries; in fact usually the converse. 

IV.8.5.2 USING NON-POLAR ORGANICS

Fuel oil, kerosene or a combination of the two prior to cyanidation 
[Silver-[Andre] Dorfman process, see US patents 1461807, 1441326], 
also refered to in Taggart, has been used succesfully at several 
plants [Randol II], including Kinross, RSA; Timmins Ochali Mining, 
Columbia 

Kerosene has also been used to blind pregrobbing behaviour at Kerr-
Addison, Canada [McQuiston and Shoemaker, 1975]. A role of kerosene 
might be to dissolve organics containing a reducing end, avoiding 
gold polymer formation. 

Using hydrocarbons for deactivation of pregrobbers has in the 
past demonstrated some utility in Merrill-Crowe circuits. 

As noted earlier, at New Machavie Mine, Witwatersrand, the 
nonpolar hydrocarbons diesel, light oil or kerosene proved most 
suitable, more so than xanthates and pine oil. 

However the same benefits are not there in carbon circuits, where 
these organic reagents will blind the carbon active sites aimed 
to recover the dissolved gold, as well as the preg-robbing sites. 

At best, some partial and difficult to control benefits might 
arise by appropriate pre-conditioning and heavy relance on the 
rates of reversal [desorption] of blinding agents etc. 

Future scope exists for the application of non-polar blinding 
agents is provided by resins, such as VitrokeleTM V912 which is 
resistant to fouling by organics as well as exhibiting 
significantly greater activity than activated carbon. 

IV.8.6 COMPETITIVE ADSORPTION

Refractory sulphidic ore at Mercur typically contains about 0.4% 
carbon, 1.2% S2- and 16% carbonate. 

Mercur increased carbon concentrations in CIL from 15 to 40 g/l, 
in order to minimize carbonaceous pregrobbing. 

Mercur increased carbon concentrations in CIL from 15 to 40 g/l, 
in order to minimize carbonaceous pregrobbing. 

CIL reduces the loss of gold by pregrobbing, and is specifically 
applied to achieve this at Mercur after pressure oxidation of 
sulphides [which does not, at the temperatures under 200C], 
deactivate the pregrobbing carbon].

There are resins, such as VitrokeleTM V912 which have extremely 
high activity as compared to activated carbon - see figure 3.

IV.8.7 ELUTION

Feldtmann found that at the Prestia Gold Mine [Ghana], gold 
precipitated from cyanide solution by charcoal could be redissolved 
[eluted] by a weak solution of sodium sulphide applied after 
cyanidation then a water wash [Randol IV, p 4486]. 

 [a] W R Feldtmann found that adding sulphide after cyanidation 
   reversed pregrobbing in carbonaceous ores

 [b] F Wartenweiler found that the benefit was obtained as well, if 
   the sulphide were present during cyanidation

* See : The Precipitating Action of Carbon in Contact with 
  Auriferous Cyanide Solutions. Volume XXIV, 24th Session 1914-1915 
  [eds S Herbert Cox, William Gowland & C McDermid], pages 329-371. 

Sulphide is of course itself a pretty good gold eluter - Ray 
Davidson had an early patent [ZA 26834/77] which removed about 97% 
Au in about 4 bedvolumes using warm water [about 65C], and Mike 
Adams [who was also at the Forum] published a substantial paper on 
the room temperature elution of gold using sulphide solutions 
[JSAIMM, Au 1944 pages 187-198]                   

The above suggests that the pregrobbing of gold as occurs in 
carbonaceous ores could be reduced by applying before, during 
and/or after cyanidation, a combination of moeties eg sulphide 
which can tie up aurophores eg ferrous ions which act through 
stripping cyanide off aurodicyanide to precipitate the 
auromonocyanide polymer, plus substances known to deactivate gold 
adsorption such as hydrazine [the Elf Atochem ACTIRED process, 
patent application PCT/E94/03091], benzoate [Ashchem I P, Inc 
patent AU-A-84699/91], acetonitrile and other organics [D M Muir, W 
Hinchliffe, N Tsuchida & M Ruane [1985], Solvent elution of gold 
from CIP carbon, Hydrometallurgy, vol 14, pages 47-65], and invert 
sugar - particularly with ethanol [D M Menne patent application AU 
PK7750]. 
           
C G Fink and G L Putnam [1950]. "The action of sulphide ion and 
of metal salts on the solution of gold in cyanide solutions". 
Mining Eng. Sept Trans AIME, vol 187. 

See also : Chase [190] 

Charles William Dowsett [1932]. USA patent 1,952,976. 

Haden [1941]. 

IV.9.0 CONCLUSION

Pregrobbing occurs at all operations; however generally being 
relatively insignificant and/or reversed by desorption when gold 
is recovered by resin or activated carbon. 

It is believed that few sites containing primary gold do not 
exhibit significant and practically irreversible pregrobbing on 
some of their ores.

Significant and practically irreversible pregrobbing of 
significant portions of ore has been reported a number of sites, 
including : 

 [a] AFRICA :                                                   
                                                                
    - Agnes                                                     
    - Athens                                                    
    - Ashanti                                                   
    - Barbrook Mine                                             
    - Beatrix                                                   
    - Bibiani                                                   
    - Black Reef                                                
    - Bogosu                                                    
    - Crown Sands                                               
    - First Quantum                                             
    - Geduld                                                    
    - Kinross                                                   
    - Konongo                                                   
    - Leslie                                                    
    - New Consort                                               
    - New Machavie                                                     
    - President Brand                                           
    - Prestia                                                   
    - Randfontein Cooke                                         
    - Renco                                                     
    - Sheba                                                     
    - Twangiza                                                  
    - Worcester                                                 
                                                                
 [b] AUSTRALASIA                                                  
                                                                
    - Awak Mas                                                  
    - Cosmo Howley                                              
    - Fortnum                                                   
    - Hedges                                                    
    - Kelian                                                    
    - Macraes                                                   
    - Moline                                                    
    - Monkland                                                  
    - Stawell                                                   
    - Tanami                                                    
    - Temora                                                    
    - Waihi/Paeroa                                           
                                                             
 [c] NORTH AMERICA            
                            
  - Alligator Ridge         
  - Atlas Gold Bar          
  - Big Springs             
  - Carlin                  
  - Cortez Gold Mines       
  - Enfield Bell            
  - Eskay Creek             
  - Getchell                
  - Gold Acres              
  - Gold Quarry             
  - Gordon Lake             
  - Jerrit Canyon           
  - Kerr-Addison            
  - Lone Star Exploration   
  - McDermott               
  - McIntyre Porcupine      
  - McLaughlin              
  - Maggie Creek            
  - Mercur, Utah            
  - Mother Lode             
  - Ochali                  
  - Queen Charlotte Island  
  - Royal Mountain King     
  - Zortman                 
                            
[d] SOUTH AMERICA             
                            
  - Morro do Ouro           
  - Morro Velho             
  - Queiroz Mine            
  - Rosario Dominicana      
  - Pueblo Viejo            
                            
[e] ELSEWHERE :               
  - Bakyrchik               
  - Natalinsk               
  - Salsigne                
                           
Processes which have been applied to address such pregrobbing 
include : 

Historically, adsorption of non-polar and polar substances to 
blind off pregrobbing sites, eg : 

 [a] Diesel [New Machavie Mine, Kerr-Addison] 

 [b] Soaps [McDermot]

 [c] p-nitro benzol azo salicylic acid [USSR]

Success was limited, and due to blinding of activated carbon even 
less effective for this process than fr Merrill-Crowe processing. 

Subsequently, intense treatment of [generally flotation] 
concentrates was applied, including :

 [a] Roasting at Ashanti

 [b] Cyanidation at Bibiani and Quiroz.

Almost uniquely limited to operations within SW USA [Nevada etc] 
chlorine oxidation is applied to ores exhibiting relatively low 
oxidant requirements, eg : 

 [a] Big Springs

 [b] Carlin

 [c] Jerrit Canyon

This approach has only been sustained in that region because of 
very low local chlorine costs [$80/t]. Elsewhere there is scope 
for on-site production of alternative high activity 
[permonosulphonate] oxidants from air, to match such costs. 

Increasing oxygen demands encountered in deeper ores have led to 
developments such as double oxidation, where hydrometallurgical 
pre-oxidation using air reduces chlorine requirements. 

The most recent and widespread practice is to apply CIL, enhanced 
by fast carbon movement [Alligator Ridge, Atlas Gold Bar, Rosario 
Dominicana]. 

Mercur also uses this approach, but with prior pressure oxidation; 
Royal Mountain King with flotation, and Stawell with gravity 
concentration to minimize gold exposure to pregrobbers. 

Morro do Ouro appears to have some of the most interesting non-
oxidative processing, which includes gravity concentration to 
minimize gold exposure to pregrobbers and flotation of cyanide 
tails to scavenge unrecovered gold for recovery by intense 
cyanidation.

The evidence suggests that most conventional pre-oxidation 
processes could be considered particularly if further gold 
liberation is significant.

This is because in general, residual or additional apparent 
activation of pregrobbing from such oxidation has been shown either 
to be : 

 [a] Trivial [type I, reversible on carbon contacting], or 

 [b] Manageable [residual unoxidized pregrobbing species may be
   destroyed by minor addition of chlorine]. 

There are in addition a number of process design and operations 
management options however beyond the scope of detail discussion 
in this document, for improving dissolved gold capture.

Scope also exists for the application of non-polar blinding 
agents with resins such as VitrokeleTM V912 which is resistant to 
fouling by organics as well as exhibiting significantly greater 
activity than activated carbon. 

To identify and correctly apply the correct pregrobbing management 
option one requires an understanding of the chemistry of 
pregrobbing : 

 [a] Simple ion exchange : Type I

 [b] Aurocyanide polymer formation : Type IIa

 [c] Reduction to gold metal : Type IIb

 [d] Mixed gold hexacyanide precipitates : Type III

 [e] Metallic gold encapsulated by mineral reprecipitation : 
   Type IVa 

 [f] Aurocyanide encapsulated by mineral reprecipitation : 
   Type IVb 

This chemistry should be correlated to the mineralogy of the most 
important carbon occurences in the ore, viz : 

 [a] Organic

 [b] Graphitic

 [c] Carbonate

 [d] Non-geologic

Understanding the chemistry driving pregrobbing, provides the 
basis of addressing the phenomena. Thus one key to improved 
operation lies in the apparent greater resistance to oxidation of 
the pregrobbing components as compared to other reducing species in 
ores which merely contribute to oxidant consumption. 

An understanding of the slurry chemistry could allow one to 
reduce the normally uneconomic oxidant demands to treat such ores 
by such processes, through selective chemical attack on the 
pregrobbing species alone.

Alternatively the most cost-effective oxidation process for gold 
liberation might be adapted to provide as well, effective control 
of pregrobbing. 

FIGURE 1 : RE-DISSOLUTION OF POLYMERIZED AUROCYANIDE

FIGURE 2 : THE ORIGIN AND OCURRENCE OF Fe++ IN GOLD DEPOSITS

FIGURE 3 : THE CAPACITY OF VITROKELETM RESIN 
           TO STRIP GOLD OFF CARBON 

FIGURE 4 : STABILITY REGION FOR GOLD CHLORIDE  


   Search this site or the web        powered by FreeFind
 
 
In the FreeFind search you can use :

1.+ and - qualifier : Examples : +always -never
If you prepend a word with + that word is required to be on the page.
If you prepend a word with - that word is required to not be on the page.

2.* wildcard : Examples : *owned or gift*
If a query word starts or ends with a * all words on a page which end or startthe same way
as that query word will match.

3.? wildcard : Examples : a?sorb, a?sor?tion or gift?
If a query word contains a ? any character will match that position.

All of these techniques can be combined: +alway* -ne??r*

CONTACT :

David Martin Menne
International Consultant : Challenges in Gold Extraction

BUSINESS OFFICE :

33 BISHOP STREET
JOLIMONT, PERTH
WESTERN AUSTRALIA
6014
Tel : 0500 54 65 32
Mob : 0418 953 691
Home tel : +61 8 9389 5648
Fax : +61 8 9389 5647
email contact : menne@iinet.net.au

POSTAL AND RESIDENTIAL INFORMATION :

P O Box 629
NEDLANDS, PERTH
WESTERN AUSTRALIA
6009
7 Allenby Road
Dalkeith
Perth
Western Australia 6909