Supergene Metal Deposits

Microbe-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 remarkable 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.

Ever since humans discovered how to separate metal from its ore mineral, preserving its metallic luster has been a driving force in the advancement of materials science. In modern times, developing materials that will contain and isolate nuclear waste has pushed corrosion science to new limits. We must now predict corrosion rates over geologic time scales, upwards of a million years. This article reviews the electrochemical basics that underpin metal and mineral corrosion and uses that to understand the case study of copper corrosion in nuclear-waste containers. Electrochemistry can also explain electron-transfer processes on mineral surfaces and so offer insight into weathering and environmentally relevant natural redox processes, such as those forming supergene metal deposits.

A compilation of copper isotopic compositions (δ65Cu) from supergene systems suggests distinct differences in the mean δ65Cu of Cu in leach cap (δ65Cu = −1.2 ± 3.5‰), enrichment zone (mean δ65Cu = +1.2 ± 4.2‰), and fl uids (mean δ65Cu = +0.9 ± 1.3‰) relative to the high-temperature sulfi des that comprise the primary ore (δ65Cu = +0.1 ± 0.6‰). These isotopic differences can be explained by the oxidative dissolution of primary ore minerals, such as chalcopyrite, and the subsequent precipitation of oxides in the near-surface system and of sulfi des at depth. A dynamic mass balance model predicts the observed Cu isotopic compositions of the Cu reservoirs in nature and constrains the temporal isotopic evolution of supergene systems. From the model, these systems isotopically evolve to substantial extents over 500 ka to 5 Ma time scales. In relatively closed systems, percent-level loss of Cu from the solid (with δ65Cu values >>0‰) is possible, suggesting that supergene systems are important components of the global Cu cycle.

Supergene metal deposits host a comprehensive record of climate-driven geochemical reactions that may span the entire Cenozoic. Products of these reactions can be dated by a variety of radiogenic isotopic methods, such as 40Ar/39Ar, (U–Th)/He, U–Pb, and U-series. The frequency of mineral precipitation, determined by dating a representative number of samples of a particular mineral collected from distinct parts of the supergene ore body, refl ects times in the geological past when weathering conditions were conducive to water–rock interaction. The frequency of mineral precipitation through time permits identifying periods in the geological past when climatic conditions were most conducive to chemical weathering and supergene ore genesis.

Supergene minerals form under near-ambient conditions on the Earth’s surface. Supergene mineralization is controlled by the parent rock composition, climatic conditions, geomorphological environment, and chemical compounds added during mineral processing. They appear in alteration zones called “orecretes.” Bronze Age miners exploited these easily accessible high-grade soft ores for Fe, Cu, Pb, and Ag. Some supergene minerals can also grow in poorly ventilated mining galleries and shafts, coat metal mining artifacts and smelting residues, and form from disastrous blasts and fi res in ancient mining settlements. Supergene deposits bridge the gap between humans and metal resources at the interface between rock, soil, air, water, and living organisms. These deposits provide essential clues to geological, environmental, and archeological studies

Supergene metal deposits form when common rock types or deeply buried primary ore bodies are exposed at or near the Earth’s surface and undergo oxidation, dissolution and reconcentration of the metals. Supergene metal deposits are economically interesting because of their accessibility for extraction and increased grades. Scientifically they are attractive because of their mineralogical diversity and what they reveal about surficial history. Apart from supplying mankind’s need for metals, supergene metal deposits provide clues about our past climate and offer an unparalleled opportunity to explore the long-term corrosion behavior of natural and man-made materials and their environmental impact.