Microorganisms may have played two important roles in the formation of supergene deposits: weathering and enrichment. The bacteria and archaea, which catalyze the oxidation of sulfide minerals, likely had some function in mobilizing metals associated with primary sulfide minerals. Ferric iron (Fe3+), a product of microbial ferrous iron oxidation, is a potent oxidant for sulfide minerals, and sulfuric acid, a product of microbial sulfur oxidation, is a lixiviant for metal ion transport. Extremely thermophilic archaea—primitive, single-celled microbes—are particularly adept at leaching chalcopyrite. Oxidation of disulfide minerals (e.g., pyrite) by Fe3+ produces intermediary thiosulfate (S2O32−), which complexes with gold.
In the enrichment zone of the supergene environment, a diversity of microbes may have served to form secondary copper sulfide minerals, framboidal sphalerite or, possibly, gold nanoparticles. The metabolic strategies employed by these subsurface microbes include the following: sulfate-reduction with subsequent precipitation of metal sulfides, adsorption of metals to cell walls and absorption of metals into cells with subsequent metal reduction, and dissimilatory metal reduction involving microbes that oxidize organic compounds or H2 and use soluble metal ions (e.g., Fe3+) or solid minerals such hematite or goethite as terminal electron acceptors. The actions of these microbes can transform metals on a geologic scale.
Bacteria and archaea that facilitate the oxidation of sulfide minerals are employed in engineered biomining processes to commercially exploit the supergene deposits that these same organisms may have had a role in forming. Copper is leached from marginal-grade, sulfidic ores in dump leach operations and from highergrade, crushed ores on engineered pads in a heap bioleach process. Bacteria are used in aerated, stirred-tank reactors to enhance the extraction of gold that is occluded in sulfide mineral concentrates (sulfidic-refractory gold). Ores of these same sulfidic-refractory precious metals are crushed, inoculated with bacteria and hyperthermophilic archaea, and leached in engineered heaps. The extremophilic archaea are adept at leaching chalcopyrite, which is particularly refractory to ambient temperature leaching with Fe3+. Heap bioleaching of lowgrade, coarsely crushed chalcopyrite ore using thermophilic bacteria and archaea is under development. Lessons learned from decades of biomining have provided perspectives on the role microbes may have played in the supergene environment.
Figures & Tables
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.