CONTROLS OF SURFACE AND GROUND-WATER CHEMISTRY IN MINERALIZED BUT UNMINED AREAS, SOUTHERN ROCKY MOUNTAINS

The USGS has completed multidisciplinary studies in mineralized, but unmined alpine catchments in the southern Rocky Mountains to determine baseline conditions and processes controlling surface and ground-water chemistry. Study areas include the upper Animas River watershed, Lake City, Mt. Emmons, and Montezuma in Colorado and Questa in New Mexico. Although host-rock lithologies range from Precambrian gneisses to Cretaceous sedimentary units to Tertiary volcanic complexes, mineralization is Tertiary in age and associated with the intermediate to felsic composition plutons. Pyrite is ubiquitous. Metal variability in waters is caused by two main factors-mineralogy and hydrology. Parameters that potentially affect water quality include: host-rock lithology, intensity of hydrothermal alteration, sulfide mineralogy and chemistry, gangue mineralogy, length of flow path, evaporation, and redox conditions. Springs and headwater streams have pH values as low as 2.5, sulfate up to 3,700 mg/L and high dissolved metal concentrations (for example: Al up to 170; Fe up to 250; Cu up to 3.5 and Zn up to 14 mg/L). With the exception of evaporative waters, the lowest pH values and highest Fe and Al concentrations occur in waters draining the most intense hydrothermally altered areas consisting of the mineral assemblage quartz-sericite-pyrite. Stream beds tend to be coated with iron floc, and some reaches are underlain by ferricrete. These areas may or may not have elevated Cu and Zn concentrations, depending on sulfide mineralogy and mineral chemistry. In the Animas River watershed, the highest dissolved Zn concentrations occur in waters draining areas containing base-metal veins, some of which are surrounded by propylitically-altered lithologies. Dissolved concentrations of other trace elements, including As, Be, Cd, Cr, Co, F, Li, Ni, and Sr, are quite variable and, in general, are controlled by mineralogy and pH. Of all the study areas, water draining the hydrothermally altered areas at Questa has the highest Cu and sulfate concentrations. This study area is the only one that is proximal to an active mine. Mass-balance calculations are being utilized to integrate mineralogical and mineral chemistry data with ground and surface-water chemistry to constrain weathering scenarios and water chemistry evolution.