The natural solubility of gold has considerable economic importance. Supergene gold solubility processes not only control dispersion and enrichment patterns in the regolith, but also provide potential mechanisms for efficient and environmentally friendly gold extraction methods. Pristine crystals and other secondary gold growths within weathered regolith are unequivocal evidence that gold is chemically reworked under supergene conditions. The composition and location of these gold forms are indicative of their mechanisms of formation.
Until recently, gold transport in weathered regolith was explained almost entirely by inorganic mechanisms. However, weathering processes are driven by key redox reactions, predominantly involving sulfur and iron. As a result, the regolith develops into an intricately balanced and stratified materials processing and equilibration zone, in which microorganisms play a vital role in catalyzing reactions and in the development of specific environments.Supergene gold enrichment and refinement is a by-product of these weathering and biocatalyzation processes.
Case studies show that supergene gold preserved under oxidizing conditions in weathered bedrock and detrital sediments largely behaves as a resistate mineral and remains chemically unaltered. This gold generally shows grain morphology consistent with the mode of chemical or physical deposition, and preservation of assimilated mineral inclusions and alloy compositions consistent with the style of the original primary mineralization and host-rock geology. These findings are in keeping with the known inert nature of gold metal under ambient oxidizing conditions.
Partial chemical reworking of supergene gold occurs as secondary authigenic overgrowths. These over growths are usually of higher purity than the original gold core. They generally consist of a semidetached porous mass of vermiform and crystalline high-fineness gold and, less commonly, overgrowth rims of electrum and other gold alloys. These chemically reworked grains characteristically occur as rounded detrital grains and nuggets within poorly drained near-surface regolith, mainly alluvial and lateritic materials, and along fractures within weathered bedrock. The formation of authigenic overgrowths is thought to be due to electrorefining of the gold under changing redox conditions in response to seasonal variations in drainage.
More thorough gold dissolution and reprecipitation is associated with permanently water saturated saline environments in which zones of low porosity and permeability locally restricted water flux. These environments favor abundant microbial growth, resulting in anoxic conditions. Bacterial mediated organoreduction processes, including the reduction of sulfate and the dissolution of iron oxides, set up a strong Eh gradient within the regolith with a well-defined upper iron redox front. Under reducing conditions below the redox front, Fe2+ is stable. These low Eh environments are also commonly anomalous in other metal cations (e.g., Ag, Cd, Co, Cu, Hg, Ni, Pb, and Zn) and elements that form soluble hydrous oxyanions (e.g., As, Bi, Mo, P, S, Sb, Se, U, V, and W). These ionic species may form locally identifiable secondary authigenic sulfide, sulfate, phosphate, vanadate, and silicate mineral precipitates. Secondary gold is precipitated below the redox front as an electrum. At the redox front, Fe2+ is oxidized to Fe3+, resulting in the formation of stable iron oxides above the redox front. These redox conditions are formed as a normal consequence of bedrock weathering, but may also form perched environments in drainage sediments that extend into residual weathered bedrock.
Gold exposed at the redox front is deposited as pristine crystals of very high purity just above the iron redox front. With the exception of local gold dispersion to depth along fractures, this secondary enrichment zone defines a subhorizontal supergene blanket in which the aqueous solubility of gold is clearly limited to a narrow stability field of Eh and pH and solution chemistry. Gold accretion under these ideal conditions produces large crystals and growth aggregates owing to the high natural affinity of dissolved gold for gold metal, and results in the characteristic “nugget effect” in gold distribution. Seasonal fluctuations in fluid chemistry probably account for the coexistence of multigenerations of gold crystals and the stepwise lateral extension of the enriched zone in the direction of water flux.
Thus, depending on regolith setting, detrital or residual supergene gold can be depleted from the upper profile to form laterally extensive and vertically enriched supergene deposits just above the iron redox front. These supergene gold deposits provide greatly enlarged exploration targets compared to their precursor mineralized host units, and commonly represent highly economic, readily recoverable economic resources.
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Supergene Environments, Processes, and Products
At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.