ELEMENTS, VOL . 11, PP. 337–342 OCTOBER 2015 337 1811-5209/15/0011-0337$2.50 DOI: 10.2113/gselements.11.5.337 The Geomicrobiology of Supergene Metal Deposits INTRODUCTION Microorganisms interact with minerals in the Earth’s crust by catalysing weathering processes. This means that minerals are often subjected to reduction–oxidation (redox) reactions that alter the chemical and physical state of the metals they contain (Ehrlich and Newman 2008). To promote cellular growth and sustain metabolic activity, all microorganisms require major and trace metal nutri- ents, which can be acquired from metal-bearing minerals. Essential metals for metabolism include magnesium, sodium, potassium, iron, cobalt, copper, molybdenum, nickel and zinc (Ehrlich and Newman 2008). Metals such as iron, arsenic, magnesium, vanadium, selenium and uranium can also be used by microorganisms to generate energy (Ehrlich and Newman 2008). Some metals have no known function within a cell’s metabolic pathways and just accumulate over time, leading to the biomineralisation of the cell membrane during fossilisation (Silver 2003). From the perspective of an individual microorganism, biogeochemical reactions occur on spatial and temporal scales of micrometres and seconds, respectively. However, when microbial populations ‘work’ over geological time scales, biogeochemical processes can have profound local to global effects, such as the oxygenation of Earth’s atmos- phere which promotes weathering and the formation of supergene metal deposits. We must never underestimate the importance of microorganisms. The oxidation or reduction of metals by microorganisms can occur via direct and/or indirect mechanisms – direct mecha- nisms involve microbial enzymes, whereas indirect mechanisms involve by-products of microbial metabolism (e.g. acid, oxidising agents and ligands). Rapid devel- opments in molecular biology techniques have advanced our understanding of the role of micro- organisms in metal solubility, mobility and precipitation. Here, we discuss the impact of the biosphere on the evolution of supergene copper, uranium, gold and iron deposits, highlighting the often over looked role that microorganisms play in the formation of such deposits. THE ROLE OF MICROORGANISMS IN SUPERGENE COPPER DEPOSITS An example of a supergene copper deposit that has been influenced/developed by microorganisms is that of the Morenci copper mine (Arizona, USA). Here, oxidation and enrichment processes have produced a classic example of a supergene profile of an original copper porphyry system (FIG. 1). This profi le is characterised by an increase in copper grade with depth (Enders et al. 2006). The profile contains a limonitic leached cap containing hematite that overlies an enriched ‘blanket’ of high-grade chalcocite with occasional covellite as replacements and coatings on primary pyrite and chalcopyrite. Weathering and alteration cycles of copper enrichment correspond to the lowering of the local water table, which resulted in copper being leached from the cap, transported downward and finally concentrated at depth (Enders et al. 2006). The surfaces of metal sulphide minerals are ideal substrates for the attachment of iron- and/or sulphur-oxidising micro- organisms that catalyse biogeochemical processes and that have contributed to the formation of the Morenci copper deposit. Acidithiobacillus ferrooxidans is a microor- ganism capable of oxidising both iron and sulphur and is associated with bioleaching operations and acid mine drainage environments. A microorganism closely related to A. ferrooxidans was recovered from samples of sulphide minerals at Morenci under near-neutral environmental pH conditions. This microorganism forms microenvironments on sulphide mineral surfaces (FIG. 2) and oxidises the iron and/or sulphur in the rock, which results in acidification (Mielke et al. 2003; Dockrey et al. 2014). M icrobe-catalyzed redistribution of metals in the Earth’s crust can produce remarkable, and often economic, metal enrichments. These catalytic processes rely on redox transformations to produce secondary-mineral assemblages. Classic supergene systems relate to copper, where weathering is driven by microbial activity. Roll-front uranium deposits represent a similar, albeit lateral, evolution from aerobic weathering to anaerobic enrichment. Gold is generally resistant to oxidation but a remark- able biogeochemical cycle can produce secondary gold. Finally, banded iron formations, which are microbially catalysed sedimentary deposits, can be further weathered to form high-grade ore. Metals are as important to enzyme catalysts as these catalysts are to metal enrichment. KEYWORDS: prokaryotes, mineral dissolution, mineral precipitation, supergene processes, metal deposits Carla M. Zammit 1 , Jeremiah P. Shuster 2 , Emma J. Gagen 3 , and Gordon Southam 4 1 School of Earth Sciences, The University of Queensland St Lucia, QLD 4072, Australia E-mail: c.zammit1@uq.edu.au 2 E-mail: j.shuster@uq.edu.au 3 E-mail: e.gagen@uq.edu.au 4 E-mail: g.southam@uq.edu.au