Environmental Effects of Hydrothermal Alteration and Historical Mining on Water and Sediment Quality in Central Colorado

The U.S. Geological Survey conducted an environmental assessment of 198 catchments in a 54,000-km area of central Colorado, much of which is on Federal land. The Colorado Mineral Belt, a northeast-trending zone of historical baseand precious-metal mining, cuts diagonally across the study area. The investigation was intended to test the hypothesis that degraded water and sediment quality are restricted to catchments in which historical mining has occurred. Water, streambed sediment, and aquatic insects were collected from (1) catchments underlain by single lithogeochemical units, some of which were hydrothermally altered, that had not been prospected or mined; (2) catchments that contained evidence of prospecting, most of which contain hydrothermally altered rock, but no historical mining; and (3) catchments, all of which contain hydrothermally altered rock, where historical but now inactive mines occur. Geochemical data determined from catchments that did not contain hydrothermal alteration or historical mines met water quality criteria and sediment quality guidelines. Base-metal concentrations from these types of catchments showed small geochemical variations that reflect host lithology. Hydrothermal alteration and Church is a senior research geochemist specializing in acid mine drainage issues (http://amli.usgs.gov/), Fey is a physical scientist, Klein is an economic geologist, Schmidt is an ecotoxicologist and Mendenhall Fellow, Wanty is an aqueous geochemist, DeWitt is a research geologist, Rockwell is a geologist specializing in remote sensing, and San Juan is a geographic systems expert. All at the U.S. Geological Survey, P.O Box 25046, MS 973, Denver, CO 80225. Email: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]. mineralization typically are associated with igneous rocks that have intruded older bedrock in a catchment. This alteration was regionally mapped and characterized primarily through the analysis of remote sensing data acquired by the ASTER satellite sensor. Base-metal concentrations among unaltered rock types showed small geochemical variations that reflect host lithology. Base-metal concentrations were elevated in sediment from catchments underlain by hydrothermally altered rock. Classification of catchments on the basis of mineral deposit types proved to be an efficient and accurate method for discriminating catchments that have degraded water and sediment quality. Only about 4.5 percent of the study area has been affected by historical mining, whereas a larger part of the study area is underlain by hydrothermally altered rock that has weathered to produce water and sediment with naturally elevated geochemical baselines.