SURFICIAL GEOLOGIC MAPPING AND SEDIMENT GEOCHEMISTRY USED TO DOCUMENT PRE- AND POST-MINING CONDITIONS, ANIMAS RIVER VALLEY, COLORADO

Quaternary deposits were mapped along the Animas River and its major headwater tributaries in the western San Juan Mountains between 1997 and 2000 (USGS Digital Data Series DDS-71). These deposits preserve a Pleistocene to Holocene geologic record that documents both natural geologic events as well as a record of historical mining activity. Glacial moraines and high valley-floor deposits blocked the upper tributary drainages of Mineral Creek and Middle Fork of Mineral Creek during the last glacial maximum at 18 Ka, preventing sediment transport until these deposits were breached by erosion. Local changes in stream gradient are associated with the development of opposing alluvial fans and debris cone deposits along upper Cement Creek and the upper Animas River north of Howardsville. Comparisons of the concentrations of major- and trace-elements in fluvial sediment profiles from valley, terrace, and oxbow deposits representing pre-mining (1870) deposition clearly indicate that milling in the Animas River watershed upstream from Silverton elevated the concentrations of deposit-related trace elements (Cu, Pb, Zn, Cd, Ag) in sediment in the floodplain. Deposit-related trace elements were enriched in the sediment deposited in historical meanders near Durango. High manganese concentrations (>20,000 ppm) from the meander profile near Durango implicates active release of mill tailings that can be attributed to milling of the manganese-rich ore processed at the Sunnyside Flotation Mill near Eureka between 1917 and 1930. Mining activity has locally increased the metal concentrations especially for Zn and Pb, which are elevated between five to ten times the pre-mining geochemical baseline when sampled down stream from old mine workings. A terrace profile from Elk Park south of Silverton, has elevated deposit-related trace element concentrations that can be correlated with mill tailings deposits from the Animas River immediately south of Eureka and with the profile in the meander near Durango.