Abstract

Geological and microbiological examination of water and mineral samples collected primarily along the 5200 bench of the Metcalf pit within the Morenci copper deposit revealed that acidophilic iron oxidizing bacteria (thiobacilli) and sulfate-reducing bacteria have contributed to leaching and, in part, to the enrichment of copper in the supergene environment, respectively. The 5200 bench traverses a classic, but tilted, enrichment profile consisting of a 200-m-thick zone of leached capping (0.06% Cu, 0.06% S) that overlies a partially leached, 180-m-thick enriched blanket (0.42% Cu, 0.39% S). The modern climate in southwestern Arizona is semiarid and exhibits a biannual wet-dry cycle where active weathering zones, associated with fracture-flow regimes along the 5200 bench, provided a natural laboratory to study supergene processes. Samples from these weathering zones contained viable thiobacilli that thrived during wet periods of the year reaching populations >107 bacteria/ml within an environment where the pH was diluted to near-neutral conditions. The population of thiobacilli decreased during the dry periods, presumably due to low water activity. During dry periods, evaporative concentration of sulfuric acid also promoted sulfide mineral dissolution and the formation of sulfate salts (e.g., chalcanthite). During the subsequent wet phase, these salts dissolved, contributing to the soluble, mobile fraction of copper important to supergene enrichment. Acidification of the sulfide zone was also promoted by the formation of authigenic iron hydroxides such as goethite and jarosite, similar to acid mine drainage systems. Order of magnitude estimates of contemporary bacterial iron oxidation of a 100-m3 block in the active weathering environment reveals that an oxidized cap could be produced in as little as 9 × 102 to 5 × 103 yr. While it is interesting to think that leaching at Morenci could form on a time scale of several thousand years, it is incorrect to assume that an entire 100-m3 block could be active at one time (i.e., leaching occurs in discrete fractures). The most active fracture in the Metcalf pit, Morenci, which produces a few kilograms of bacteria/yr, could leach between 0.14 and 0.87 t of Cu annually. Small populations of viable sulfate-reducing bacteria (~103/g) occurred within this partially leached zone and at the ground-water interface (i.e., at the redox boundary). Even though the enriched blanket possesses viable sulfate-reducing bacteria, inorganic geochemical processes dominate supergene enrichment in this system. Comparing the annual carbon fixation occurring during biooxidation versus leaching, bacterial sulfate reduction could only fix between 0.2 and 1.1 percent of the copper being leached.

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