The mineral resources of the Comstock district represented one of the largest precious metal deposits developed in the world, with more than 11 million US short tons of ore mined, producing more than US$ 396 million in silver and gold (Bonham, 1969; Smith, 1943; Smith and Tingley, 1998).
The Comstock mineralisation is hosted in Miocene age hydrothermal systems developed within a series of Mesozoic metasedimentary and igneous rocks that are overlain by Oligocene to Miocene ash-flow tuffs (Hudson, 2003). Twelve hydrothermal alteration assemblages are recognised in the Comstock district, which are assigned to deep low sulfidation alteration or intermediate-depth high sulfidation alteration (Vikre, 1989; Hudson, 2003). The Comstock Lode occurs along an ancient north trending normal fault within a thick sequence of volcanic rocks. Deposits of high-grade gold and silver ore occur in the fault trace and in the volcanic rock immediately above the normal faults. At structural traps or points of physical or chemical changes in the system, mineral deposits were formed in the faults and within the overlying alluvium. Due to the dynamic nature of the environment and the pressure of the circulating fluids much of the mineralization cemented shattered rock forming mineralized breccias.
Recent mapping during exploration and environmental studies have identified key structural and geochemical characteristics particularly of higher-grade ore zones. In terms of the structural controls the presence of shallow angle northeast-striking faults as important controls on higher grade mineralization has been identified as well as the emplacement of barren quartz porphyries in the system that appear to act as barriers for mineralizing fluids. In addition, later supergene alteration appears to follow the same controls. Post mineralization, further movement created the “soft broken ground” appearance of the mineralized areas. Fluid flow through these zones redissolved selective portions of the deposit and re-deposited the metals elsewhere.
Although in hydrothermal systems Ag and Au show similar behaviour, in supergene systems they are quite different. Silver occurred primarily as silver sulfides and as silver in galena in the Comstock. On oxidation and release some migration occurred and secondary forms as Ag-bearing manganese oxide, native silver alloys and supergene sulfides such as acanthite were formed possibly in response to redox processes or depletion in oxygen in the system. Gold, by comparison released from trace amounts in sulfides, chiefly pyrite, and from Au-Ag alloy shows two distinct forms in the weathered material. Coarser gold that is residual from the primary Au-Ag alloys, often characterised by Ag-depleted rims and very fine-grained euhedral crystals (on a scale of <1-30 µm in size. This reflects two mechanisms, one mechanical separation and refining of the rims of exposed gold and second the dissolution and transport of Au in supergene fluids. Numerical calculations focused on speciation and physical chemical controls of supergene processes using modern water chemistry as an analogue supports the mechanisms and further points to the importance of thiosulfate complexing of silver and not chloride. For gold, thiosulfate is a possibly transient complex that may explain the mobility but more stable in this environment are gold (I) complexes of gold with ammonia and this may also infer the importance of organic acids as well.
