A HYDROGEOCHEMICAL INVESTIGATION OF THE LEADVILLE MINING DISTRICT IN CLOSE PROXIMITY TO THE CANTERBURY AND LEADVILLE MINE DRAINAGE TUNNELS, LEADVILLE, COLORADO

The Leadville mining district is one of the most extensively mined regions in the world, producing gold, silver, lead, zinc, copper, and manganese from the 1860s through the 1990s. The region is intensively fractured with primary fault orientations trending northeast and northwest, and secondary orientations trending north and west. Most of the faults are steeply dipping between 80-90º and extend hundreds of meters below the land surface. Fault movement is generally complex as evidenced by normal and reverse vertical displacement that occurred with lateral shifting and sequential offsetting during fault reactivation. Paleozoic dolomite and limestone, Pennsylvanian sandstone and shale, and Cretaceous-Tertiary porphyry are present throughout the region at depth, and Quaternary glacial sediment is interspersed at various thicknesses across the land surface. A multi-component analysis was performed to better understand the hydrogeologic characteristics of the region. Of relevance to evaluating groundwater quality and quantity in the region are two large-scale drainage tunnels that were constructed originally to remove water from the surrounding mines. It is hypothesized there exists a structural blockage within the upper lying Canterbury Tunnel that is contributing to a rise in water levels within the Leadville mine drainage tunnel that could affect local water quality. Water-chemistry samples were collected to examine geochemical signatures that provide evidence for the existence or absence of fluid connectivity between the tunnels. A geophysical survey was implemented to examine the subsurface structure and potential effects of fault offsets in controlling fluid pathways. The data from these and other sources were used to construct a geologically detailed groundwater flow model to estimate flow dynamics and to evaluate recharge areas for the drainage tunnels. The data analysis and model results were then examined collectively to ascertain the potential for a significant hydraulic connection between the tunnels and to gain an improved understanding of hydraulic controls within the bedrock aquifer. The Leadville mining district exhibits diverse geologic and hydrologic characteristics, which provide a scientifically interesting setting to evaluate fluid controls in fractured environments.