EFFECTS OF HYDROTHERMAL ALTERATION AND HISTORICAL MINES ON THE DISTRIBUTION OF PH VALUES AND DISSOLVED TRACE-METAL CONCENTRATIONS IN STREAMS OF THE UPPER ANIMAS RIVER WATERSHED, SOUTHWESTERN COLORADO

Dissolved trace-metal concentrations and pH values in streams throughout the upper Animas River watershed are highly variable and depend on factors such as: hydrothermal alteration, acid-neutralizing capacity of the rocks, and mixing of different waters. Hydrothermal alteration and mineral assemblages had a primary influence on distribution of trace-metal concentrations. Many of the historical mines are located in the altered and mineralized areas. Numerous mines, prospect pits, and mining-related features are in the study area, and many of the features do not affect water-quality conditions; therefore, a subset of mines was selected representing draining mines, mine waste rock piles, and permitted mine and mill sites that may affect the environment. Spatial distribution maps are presented for low-flow and high-flow conditions, and the maps show color ranges for different pH values and dissolved trace-metal concentrations in study area streams. The distribution maps also display areas of hydrothermal alteration and locations of selected historical mine sites. Because of the combined effects of hydrothermal alteration and historical mines, it is difficult to attribute low pH values and high trace-metal concentrations to either source; however, several historical mines clearly affect the study area streams. Several background iron and manganese springs cause low pH values and high trace-metal concentrations, for example in Mineral Creek near Mount Moly, Cement Creek, and California Gulch.