PART I PROJECT DEVELOPMENT
1. GOLD MINERALOGY Gold minerals and alloys
leaching method (Tumily,et al., 1987), which is used more routinely as a first line of control and enables the relatively rapid turnaround of process-based leachability information.
The process mineralogy of gold is not limited to the gold minerals, submi- croscopic gold and their carriers, but also includes the study of gangue min- erals and other species that may affect gold processing, such as cyanicides.
1. GOLD MINERALOGY
Solid-solution gold tends to concentrate preferentially in arsenopyrite, with a maximum reported concentration of 1.7% (m/m) (Cook and Chryssoulis, 1990). In the larger (450mm) arsenopyrite crystals it is usually inhomoge- neously distributed, concentrating along growth bands, giving undisputed evidence for incorporation of gold in the arsenopyrite structure during crystal growth (Chryssoulis and Cabri, 1990; Oberthu¨r et al., 1997). More impor- tantly, solid-solution gold is typically strongly enriched in the finer grained sulfides (o20mm) and is therefore harder to liberate from arsenopyrite (Cook and Chryssoulis, 1990;Novgodorava, 1993).
Table 1 Gold minerals
Native elements, alloys and metallic compounds
Native gold (o20 mol% Ag) Au
Electrum (20–80 mol% Ag) (Au,Ag)
Palladian gold (porpezite) (Au,Pd)
Rhodian gold (rhodite) (Au,Rh)
Iridic gold (Au,Ir)
Platinum gold (Au,Pt)
Goldamalgam (Au,Ag)Hg
Weishanite (Au,Ag)3Hg2
Maldonite Au2Bi
Auricupride Cu3Au
Tetra-auricupride AuCu
Hunchinite Au2Pb
Bogdanovite Au5(Cu,Fe)3(Te,Pb)2
Bezsmertnovite Au4Cu(Te,Pb)
Sulfide/selenite
Uytenbogaardite Ag3AuS2
Fischesserite Ag3AuSe2
Petrovskaite AuAg(S,Se)
Tellurides
Calaverite AuTe2
Krennerite (Au,Ag)Te2
Muthmannite (Au,Ag)Te
Petzite Ag3AuTe2
Sylvanite (Au,Ag)2Te4
Kostovite CuAuTe4
Montbrayite (AuSb)2Te3
Nagyagite [Pb(Pb,Sb)S2] [Au,Te]
Silicates/Other
As chloritea (Mg,Al,Fe)12[(Si,Al)8O20](OH)16
Auroantimonate AuSbO3
aKucha and Plimer (2001).
Mineralogical investigation of gold ores 23
Pyrite is the most common of the sulfide minerals and may also incorporate significant amounts of gold in its crystal structure, to the point where solid- solution gold becomes the principal form of gold in the ore and pyrite its chief carrier (Thomas, 1997). Good examples are the disseminated sulfidic gold ores of the Carlin and Cortez Trends in Nevada (Wells and Mullens, 1973), the trend stretching across Northwestern China into Kirghizstan and Kazakhstan, and the trends along the Northwest Sichuan depression and the Youjiang basin in Southern China (Rui-Zhong et al., 2002). The morpho- logical types of pyrite pertaining to gold are illustrated inFig. 3.
In ores with more than one morphological types of pyrite, solid-solution gold is usually confined to one or two types only, which is not surprising given their different origin (Arehart et al., 1993a, b; Simon et al., 1999). In me- sothermal greenstone belt gold deposits solid-solution gold is hosted mostly by the coarser (450mm) grained euhedral to subhedral pyrites. In epithermal deposits the finer (o10mm) grained and in particular microcrystalline
Sylvanite
Calaverite
Stib.
Petzite
Pyrr.
Silver
Gold
Oxides Metals
Safflorite Auricupride
Bezmertnovite
Reflectance (% at 589nm)
Electrum
Bism
Maldonite
Montbrayite
Nagyagite
Criddleite Kostovite
Sulfides
Krennerite Aurostibite
Fig. 1. Gold minerals highlighted in diagram of reflectance in air at 589 nm vs. VHN microhardness for ore minerals.
S.L. Chryssoulis and J. McMullen 24
(o2mm) pyrite is strongly enriched in submicroscopic gold. This effect is illustrated inFig. 4.
The positive correlation between arsenic and gold concentrations in pyrite was recognized early in the course of routine determinations of solid-solution gold in pyrite by secondary-ion mass spectrometry (SIMS) (Chryssoulis and Cabri, 1990). The maximum solubility of gold as a function of arsenic con- tent in pyrite is given by
CAu ¼0:2CAs (1)
whereCis the concentration of As, Au is in mol%. This is based on an Au vs.
As plot (Fig. 5) of over 1000 microprobe analyses of pyrites of all morpho- logical types from epithermal and mesothermal ore deposits (Reich et al., 2004). They found that the maximum solubility of gold in pyrite from me- sothermal deposits dropped by an order of magnitude (or one gold atom for every 50 arsenic atoms), as per the following equation:
CAu ¼0:02CAs (2)
The chemical state and location of gold in the pyrite structure are still openly debated, but the consensus appears to be for aurous gold (Au+) occupying iron sites, and As3+providing the charge balance. It has been proposed that Fig. 2. Gold minerals (clockwise): calaverite [AuTe2], aurostibite [AuSb2] with aurosti- bate [AuSbO3] coating, maldonite and maldonite with bismuth hydroxide [(Bi(OH)3] coating.
Mineralogical investigation of gold ores 25
the noted association of gold with arsenian pyrite could be the result of arsenic incorporation in the pyrite structure having created more p-type sur- faces (Prokhorov, 1971), which would favour sorption of the electronegative gold radicals (Mironov et al., 1981). Solid-solution gold in most cases is incorporated in the crystal structure of the host mineral during crystal growth; however, in one case (Ravenswood, NSW, Australia) it was diffused into the pre-existing pyrite structure along healed fractures.
Other minerals that may contain significant concentrations of solid- solution gold are loellingite (Pirila, Finland; Lupin, NWT, Canada), enargite (Chuquicamata and Pascua, Chile; Yanacocha, Peru´) and tennantite (El Indio, Chile).Table 2summarizes measured ranges of gold in various sulfide and arsenide minerals.
1.3. Colloidal gold
The termcolloidal goldwas introduced to describe discrete submicron gold inclusions in sulfide minerals, invisible by optical or conventional scanning electron microscopy (SEM), but detectable by SIMS in-depth concentration profiling (Chryssoulis, 1987) and also imaged and analysed by high-resolution transmission electron microscopy (HR-TEM) (Bakkenet al., 1989). Colloidal Fig. 3. Submicroscopic gold carriers: morphological types of pyrite (left to right) coarse, porous, fine-grained and microcrystalline. Porous pyrite can be host to colloidal gold, while maximum solid-solution gold concentrations are reported from microcrystalline pyrite.
S.L. Chryssoulis and J. McMullen 26
Fig. 4. Submicroscopic gold carriers: coarse and fine-grained arsenopyrite. A difference of 100–500 times on average (solid solution) gold content exists between the two mor- phological types, with the maximum reported gold content coming from the fine-grained arsenopyrite.
0.1 1 10 100 1000 10000
0.000 0.000 0.001 0.010 0.100 1.000 10.000 100.000
As (wt%)
Au (ppm)
Pyrite
Collo id al
Solid Solution Colloidal
Fig. 5. Gold vs. arsenic plot for pyrite. Solid-solution gold (J) and colloidal size gold ().
Mineralogical investigation of gold ores 27
gold ‘‘bridges’’ solid-solution gold and optical-microscope visible gold inclusions, thus demonstrating the continuity between these two forms of gold (Novgodorava, 1993).
Colloidal gold can be the product of exsolution of solid-solution gold or nucleation of adsorbed surface gold (Simonet al., 1999). It also forms along the ‘‘reaction front’’ where a gold-barren sulfide is replacing a sulfide or arsenide-containing solid-solution gold (Chryssoulis and Weisener, 1994;
Mumin et al., 1994). Colloidal gold ranges in size between 5 and 500 nm, which is also the size range of insols in liquid colloidal solutions (Thiele, 1950). Colloidal gold is mostly spherical and is not necessarily confined to the sulfide matrix, as it has also been observed in the surrounding clay minerals (Bakken et al., 1989). Colloidal gold also forms by coagulation during roasting where pyrite is oxidized to form magnetite, maghemite and hematite (Stephens et al., 1990), and may also form during pressure oxidation and bioleaching.
The preferential host of colloidal gold is pyrite and to a lesser extent ar- senopyrite. Unlike solid-solution gold there is no requirement for pyrite to be Table 2
Solid-solution gold concentrations in sulfides and sulfosalts
Mineral Au (ppm)
Iron sulfides
Pyrite o0.1–8,800
Marcasite o0.1–31
Pyrrhotite (roaster) o0.1–5 (300)a
Arsenic minerals
Arsenopyrite o0.2–17,000
Loellingite
Tennantite o0.2–72
Enargite-Luzonite 0.3–62
Gersdoffite o0.1–5
Realgar o0.1–4
Orpiment o0.1–3
Copper sulfides
Chalcopyrite (synthetic) o0.1–7 (44)
Bornite (synthetic) o0.1–14 (360)
Covellite o0.1–74
Chalcocite o0.1–44
Antimony sulfides
Tetrahedrite o0.2–59
Stibnite o0.1
aSynthetic.
S.L. Chryssoulis and J. McMullen 28
arsenian (arsenic-bearing typically with more than 0.5% As by mass). This is illustrated inFig. 5; when pyrites with colloidal gold are depicted on an Au vs. As scatterplot they may plot above (Kirazli, Turkey) or below (KCGM) the line of maximum solubility of solid-solution gold in pyrite and there is no correlation with arsenic content (Screamer, NV, USA). Maximum colloidal gold concentrations for some minerals are given inTable 3. Colloidal gold is much more reactive than coarser grained gold because of its much higher specific area (surface/volume). This is evidenced by the rapid dissolution of gold at very low cyanide concentrations from autoclave discharge samples.
1.4. Surface gold
Surface gold refers to gold detected on the surface of mineral particles, the most classic example being gold adsorbed onto carbonaceous particles. Sur- face gold is the result of sorption, reductive deposition (plating), precipitation and possibly ion-exchange deposition from gold-bearing solutions. In roaster off-gas scrubbers it could be the product of volatile gold compound (AuCl2, AuCl(CO)) condensation (Puddephatt et al., 1989). The best examples of sorbed gold are gold preg-robbed by carbonaceous matter or loaded onto activated carbon (Adams and Burger 1998a, b). Dissolved gold is reductively deposited onto pyrite both in nature and during processing (Chryssoulis, 1997). Surface gold has been measured on carbonaceous matter from Cortez (NV, USA) in concentrations up to 15 ppm in the 12 nm surface layer an- alysed, corresponding to 1.7 g/t Au bulk concentration. Reductive adsorption of gold onto copper minerals was demonstrated byAdams et al. (1996).
In nature this preg-robbed surface gold occurs as three species, Au1, AuCl2– and Au(SCN)2–
and this could be the mechanism by which gold becomes in- corporated in the arsenian pyrite lattice as solid-solution gold or in arsenic-poor Table 3
Maximum colloidal gold concentration in minerals
Mineral Au (ppm) Ore
Pyrite 15,500 Porgera, Papua New Guinea
Tetrahedrite 520 Lara, BC, Canada
Enargite 170 Pascua, Chile
Covellite 125 Chuquicamata, Chile
Chalcocite 29 Skouries, Greece
Bornite 33 Chuquicamata, Chile
Maghemitea 1,950 Goldstrike, USA
Hematitea 1,130 Goldstrike, USA
Goethitea 54 Goldstrike, USA
aSecondary formed by roasting gold-rich pyrite.
Mineralogical investigation of gold ores 29
pyrite as gold micro-inclusions (Simon et al., 1999). In the former, the re- duction of gold is in all likelihood partial (Au3+reduction to Au+), while in the latter the reduction is complete to Au1. Precipitation of gold salts usually occurs to some extent in heap leaching operations. Finally, gold deposition by ion-exchange could be an alternative mechanism for incorporation of gold in the arsenian pyrite structure to that proposed byReich,et al. (2004), who suggested that gold might be filling vacant iron sites that were created by the arsenic substitution. This ion-exchange mechanism for gold incorporation into pyrite is very hard to prove given the low gold concentrations involved.
The only indirect evidence is replacement of As/Au-poor pyrite by As/Au- rich pyrite along fractures (Chryssoulis and Grammatikopoulos, 2003).
1.5. Forms and carriers of gold
The terms form and carriersof gold were introduced several years ago to better describe in a more collective manner the response of gold-bearing minerals to flotation. Thus,formof gold refers to the exact locus and chemical state of gold such as gold minerals as well as solid-solution, colloidal and surface-bound gold, while the termcarrierof gold refers to the particles which host one or more forms of gold, thereby controlling response to flotation. An example of the latter is free gold vs. gold with pyrite which could also contain solid-solution/colloidal gold vs. gold in middling particles (Chryssoulis and Grammatikopoulos, 2003). This terminology has been found particularly useful in describing gold deportment in flotation tailings with a focus on recovery. In the case of leach tails, the term carrier loses its relevancy and what becomes more pertinent are terms like: exposed, enclosed and refractory.
2. PROCESS MINERALOGY OF GOLD