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Toxic Substance Availability to the Regional Aquifer System from Mineralized Ground and Mining Activity in the Northern Patagonia Mountains, Arizona

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W. Ian Ridley, Byron R Berger, and Frederick H Lichte, USGS, Denver, CO 80225.

Extensive and intensive hydrothermal activity has altered and mineralized rocks within the Sonoita Creek drainage system, south-central Arizona, a part of the regional Santa Cruz River aquifer system. Sonoita Creek transects and is fed by a network draining many square kilometers of altered -including acid sulfate- rock, Cu-Pb-Zn-Ag-Au mineralizations, and mine dumps and tailings in the Patagonia Mountains. Annual precipitation in this region is 44-46 cm. The regional flow patterns in the Patagonia Mountains are complex with a southerly flow at the water table toward Mexico and a westerly flow into the Santa Cruz River system. The ground-water budget is dominated by runoff and stream-base flow (90%) and 10% underflow. However, the chemistry of all these components must be affected by altered, mineralized ground, because of its areal extent, and the common location of mining operations directly in drainages. We consider the underflow system to be principally controlled by the regional faults and fractures which trend predominantly NW and NE. Regional and mine-scale mapping also demonstrates that these structures have also determined the flow of hydrothermal mineralizing solutions and the locations of highly altered ground. Mines that clearly have environmental impact include: World's Fair, Three R, and a variety of excavations in the vicinity of Red Mountain. The Three R mine is particularly important, being an arid-climate analogy of Summitville, Colorado, and showing many of the same environmental impacts. In order to better understand the potential for contamination of the aquifer system, we are currently compiling alteration zoning information for each deposit, combined with a detailed chemical study of secondary minerals that are potential sources of toxic trace elements.

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