THE DETERMINATION OF SUBSURFACE HYDROGEOLOGY IN A NATURALLY ACIDIC RIVER USING SURFACE DATA AND MODELING TOOLS, SUMMIT COUNTY, COLORADO

The alteration of disseminated pyrite related to hydrothermal cells and alteration zones surrounding small rhyolitic to granitic intrusions has produced acidic waters (pH 3.5 to 5) in the upper Snake River, located in Summit County, Colorado. The Montezuma shear zone transects the area and may provide a conduit through which mineralized surface and ground water is allowed to travel. Surface water samples were collected from river reaches impacted by historical mining activities as well as reaches with no observed upgradient mining activities. Water samples contain elevated dissolved concentrations of aluminum (up to 22 ppm), fluoride (up to 3.0 ppm), silica (up to 32 ppm), sulfate (up to 400 ppm), zinc (up to 3.8 ppm), manganese (up to 9.8 ppm), and iron (up to 18 ppm). In many cases, the highest concentrations of these elements were found in samples that were not associated with mining activities. Additionally, large ferricrete deposits, which have been used to identify areas containing altered and mineralized rock, have been mapped in the upper Snake River drainage, which has not been impacted by mining activities. A strong correlation was observed between the location of the Montezuma shear zone, iron-rich ferricrete deposits, and low-pH waters with high metal concentrations.

The metal content of recharge water depends on the amount of time that the ground or surface water is in contact with altered rocks or weathered surfaces, which is largely controlled by the flow path. To formulate and calibrate a comprehensive model of the processes leading to formation of ferricrete in the upper Snake River, the geologic, hydrologic, and geochemical processes are characterized. Conceptual models of the flow system are currently being defined using the MODFLOW code, and the mass fluxes from the flow model are used to constrain PHREEQC reaction and transport models, which simulate pyrite oxidation for the conceptual flow models. The investigation of surface water and authigenic mineral chemistry with hydrologic and computer modeling will aid in the estimation of the subsurface volume of altered rock and lead to a better understanding of pre-mining water chemistry in the basin.