BIOGEOCHEMISTRY AND ORE DEPOSITION

Bacterial metabolism, involving redox reactions with carbon, sulfur, and metals appears to have been important since the dawn of life on Earth. In the Archean, anaerobic bacteria thrived before the Proterozoic oxidation of the atmosphere and the oceans and these organisms continue to prosper in niches removed from molecular oxygen. Both aerobes and anaerobes have profound effects on the geochemistry of dissolved metals and metal-bearing minerals. Aerobes can oxidize dissolved metals and reduced sulfur, as well as sulfur and metals in sulfide minerals, can contribute to the supergene enrichment of sulfide ores, and catalyze the formation of acid mine drainage. Heterotrophic anaerobes, which require organic carbon for their metabolism, catalyze a number of thermodynamically favorable reactions such as Fe-Mn oxyhydroxide reductive dissolution (and the release of sorbed metals to solution) and sulfate reduction. Bacterial sulfate reduction to H₂S can be very rapid if reactive organic carbon is present, and can lead to precipitation of metal-sulfides and perhaps increase the solubility of elements such as silver, gold and arsenic that form stable Me-H₂S aqueous complexes. Similarly, the bacterial degradation of complex organic compounds such as cellulose and hemicellulose to simpler molecules such as acetate, oxalate and citrate, can enhance metal solubility by forming Me-organic complexes and cause dissolution of silicate minerals. Bacterially-induced mineralization is being used for the bioremediation of metal contaminated environments. Through similar processes, bacteria may have been important contributors in some sedimentary ore-forming environments, and could be important along the low-temperature edges of high temperature systems such as those that form volcanogenic massive sulfides.