FAULT VEINS AND THEIR INFLUENCE on MOUNTAIN WATERSHED HYDROGEOLOGY, THE STANDARD MINE OF ELK BASIN, COLORADO

Polymetallic fault veins are structures that can control mine drainage and associated metal contamination of surface and groundwaters common in the intermountain west. The Standard Mine is a Superfund Site where a network of fault veins host precious and base metal sulfide deposits and hydrothermal alteration. Characterization of these structures was performed to assist the U.S. Environmental Protection Agency in evaluating their impacts on the groundwater flow system and remediation options for the site.

Key observations include: (1) distinctive fault zones collocated with mine workings cut polymetallic quartz veins and have clay-rich fault cores; (2) sulfides are concentrated in fault veins compared with disseminated pyrite in the surrounding aquifer also containing pervasive iron-oxide stained joints; (3) discrete groundwater flow in the mine has asymmetrically deposited flowstone on the footwall; (4) preferential leaching of pyrite at the surface occurs more commonly in the footwall (Upper Cretaceous Ohio Creek Member, Koc) compared with the hanging wall (Tertiary Wasatch Formation, Tw); and (5) permeametry indicates relatively high intergranular porosity and permeability (k) in Koc versus clay-rich gouge and Tw.

A hydrogeological conceptual model includes: (1) the Standard fault vein as the primary source of solutes; (2) relatively high k joints versus low intergranular k controls groundwater flow to the mine from the greater watershed; (3) juxtaposition of possibly lower k Tw against Koc, combined with low k clay-rich fault gouge, likely controls groundwater flow near and in the mine workings; and (4) given the character of joint networks, fault rocks and juxtaposition, watershed geomorphology, and potential variation in surface versus subsurface hydraulic gradients relative to the orientation of the fault vein, it appears the fault vein does not act as a simple conduit allowing near-surface groundwater to drain directly downward into the mine workings. Rather, the fault vein appears to act as an asymmetric, combined conduit-barrier to groundwater flow, where flow likely occurs from the surface to the mine workings within joint networks from the greater volume of the bedrock aquifer. Once in the workings, water freely flows from higher to lower levels and ultimately discharges at the mine portals.