GEOCHEMICAL INVESTIGATION OF DRAINAGE PONDS FOR THE CRIPPLE CREEK MINING DISTRICT AT THE CARLTON TUNNEL EXIT NEAR VICTOR, COLORADO

In this study we conducted aqueous geochemical measurements of interconnected settling ponds that drain the Carlton Tunnel of the Cripple Creek Mining District, Colorado. The aim was to investigate the geochemical processes that affect mine discharge waters in the settling ponds before the water is released into the natural environment. The mining district is located within a Tertiary age alkaline volcanic/diatreme complex surrounded by Precambrian granites and metamorphic units. In 1941, the Carlton Tunnel was completed and used as the primary groundwater drainage for the Cripple Creek gold mine; a mine that began as an underground operation which required the removal of water by tunnels. The Carlton Tunnel is located about 900 – 950 m below the surface and is still the primary drainage tunnel for the mine today. As the mine discharge waters exit the tunnel, it moves through a series of interconnected settling ponds. These settling ponds served as the location of this study. The results of sample analyses showed that the pH of the water increases whereas, dissolved inorganic carbon (DIC), Ca, Fe, and Mn concentrations decrease along the 10 sampling sites with increased distance from the tunnel exit. Speciation calculations using PHREEQC showed that the water samples were supersaturated with respect to calcite, ferrihydrite and goethite for all 10 sampling sites. This would indicate that the decreasing concentrations of Ca, Fe and Mn over distance is because these metals have precipitated as either calcite or Fe-oxides, or have been adsorbed onto the mineral surface. Partial pressure of CO₂ (pCO₂) calculations using PHREEQC indicate that the ponds have higher pCO₂ than atmospheric. The decreasing DIC concentrations over distance across the 10 sampling sites is attributed to CO₂ outgassing to the atmosphere due to pCO₂ differences between the aqueous samples and atmospheric CO_2(g). As CO₂ is lost to the atmosphere, the pH of the water samples increases over distance. Also, calcite precipitation, as suggested by PHREEQC calculations, would result in the decreased DIC concentrations across the ponds. The study results suggest that mineral precipitation removes metals from solution and CO₂ outgassing along with calcite precipitation drives the water to equilibrium conditions with atmospheric CO_2(g).