GEOCHEMICAL DOMAINS AND ORE DEPOSITS IN THE LA PLATA MOUNTAINS, COLORADO

The La Plata mining district is located in southwestern Colorado and contains numerous gold, silver, and copper deposits. Geologic and structural data for the district was obtained from digitizing historic geologic maps. Stream sediment samples were collected approximately every 5 km in a 200 km2 area, and analyzed using an ICP-OES for metal concentrations. GIS-based hydrologic modeling tools were used to delineate upstream watersheds for each sediment sample location. A geodatabase has been created consisting of geologic formations, structures, veins, rock ages, historic mines, and geochemical anomalies associated with over forty watersheds within the La Plata mountain range. Spatial and geostatistical analysis tools in the GIS software are being used to analyze the data to aid in visualization and quantification of the relationships among geology, structure, and geochemistry.

Preliminary interpretation of the geochemical data indicates that vein density within each basin does not correlate to anomalous metal concentrations. There is a strong base metal signature (As, Cu, Mo) centered on a known copper porphyry deposit (the Allard Stock). Also there is a weak epithermal signature (Ag, Hg, Sb) developed in the Upper Junction Creek and Columbus Basin areas. Anomalous base metal mineralization is associated with mapped syenite intrusive complexes.

Spatial correlation exists among copper (> 110 ppm), arsenic (> 31 ppm) and molybdenum (> 11 ppm) anomalies in the study area. Copper has a weak positive correlation with thallium and cobalt, and molybdenum has a weak correlation with arsenic, thallium and vanadium, and strong correlation with iron and sulfur. All gold values were less than the 2 ppm detection limits of the ICP-OES. Analyses of the preliminary results are providing a greater insight into regional geochemical domains of mineralization and in the modeling of spatial associations of known ore deposits within the mining district.