2000 GSA Annual Meeting -- Reno, Nevada
Author(s): WANTY, Richard B., BERGER, Byron R., SHANKS, Wayne C. III, RIDLEY, W. Ian, and LICHTE, Fred, U.S. Geological Survey, M.S. 973, Denver Federal Center, Denver, CO 80225 (firstname.lastname@example.org)
Mines in the Patagonia Mountains of south-central Arizona produced Cu, Pb, and Zn, with minor Au and Ag from epizonal vein and replacement deposits. The mines are now inactive; some produce perennial acidic, metal-rich drainage waters from their portals and from discharge of ground water that has passed through mineralized ground. The World's Fair epithermal base and precious metal mine is an example. Drainage from the portal has a pH of 3.1, and is rich in Fe, Mn, Co, and most trace metals. At the base of the dumps, the mine water enters Alum Gulch, which was dry when the World's Fair effluent was sampled. Based on the chemical data, ground-water discharge, which is structurally controlled by a series of sheeted fractures, enters the drainage at the base of the dumps. Evidence for this discharge includes changes in surface-water temperature, increased concentrations of Al, Cd, Cu, Ni, Pb, Sr, and Zn, lower F/Cl ratios, and heavier water-oxygen and sulfate-sulfur isotopes. These fractures also controlled the emplacement of the vein-hosted mineralization. The World's Fair veins are localized in a series of N-S extensional faults between NW-striking right-lateral faults. Because of their orientation with respect to the present-day maximum principal stress (NNW to N), the N-S veins are expected to be hydraulically conductive. The hydraulic head in the Patagonias might drive some flow along the observed ENE fracture system, but the NW- and N-trending fractures should be more conductive. The use of geochemical tools, combined with a detailed interpretation of geologic and tectonic setting, leads to a more thorough understanding of the loci of ground-water discharges. The high dissolved elemental concentrations found near ore deposits offers the opportunity to study ground water ? surface water interactions using the weathering ore deposits as though they are sources of 'tracers.'
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