A GIS DATA VIEWER FOR PHYSICAL AND GEOCHEMICAL CHARACTERIZATION OF ABANDONED MINE LANDS: AN EXAMPLE FROM THE ANIMAS RIVER WATERSHED, SILVERTON, COLORADO

A U.S. Geological Survey investigation implemented between 1996 and 2000 has yielded an extensive GIS database. The database includes hundreds of geochemical analyses of waters, stream sediments, rocks and mine waste samples. Samples were collected from a hydrothermally altered Oligocene caldera complex that was intensively mined for base and precious metals from the 1870’s to the early 1990’s. GIS coverages were compiled at scales between 1:24,000 to 1:6,000 and include: 1) bedrock and surficial geology, 2) hydrothermal alteration derived from AVIRIS and field data, 3) ferricrete (iron oxyhydroxide-cemented deposits) and iron bogs, 4) linear features derived from Thematic Mapper and high resolution digital elevation data, 5) airborne geophysics (magnetic, EM, and radiometric), 6) mines and mills, 7) hydrologic features, 8) extensive surface water and spring chemistry, and 9) mine waste leachate data. Commercial GIS products provide flexible tools for viewing and querying data. Queried results permits the identification of the key physical factors (e.g., lithology, drainage density, climate, etc.) and the testing of hypotheses that relate how components of the system might impact and control mining versus non-mining related metal loads to surface water and ground waters. For example, raster imagery of geophysical data can be rendered transparent in order to view underlying terrain images. Geologic vector data can be subsequently displayed and queried on the combined geophysical-terrain image to relate lithology to geophysical and topographic data. Derivative coverages can be used to evaluate small watershed characteristics such as total area relative to the area of specific alteration assemblages versus the total area affected by mining. These factors can then be compared with a variety of geochemical "signals", such as surface water pH or specific element concentrations, that might be associated with a particular alteration assemblage, mined area or combinations of the two. In general, spatial correlations between physical factors and geochemical or other "signals" can be used to identify, characterize and compare baseline and anomalous conditions in different geologic, topographic, and climatic settings, as well as ultimately better understand the physical processes that control metal loading.