ACIDIC DRAINAGE AND METAL TRANSPORT FROM THE WEATHERING OF THE DINERO MINE-WASTE PILE, LAKE FORK WATERSHED, LEADVILLE, COLORADO

Two trenches were dug into the south Dinero mine-waste pile near Leadville, Colorado, to study the weathering of rock fragments and the mineralogic sources of metal contaminants in the surrounding wetland and Lake Fork Watershed. Water seeping from the base of the south Dinero waste rock pile was pH 2.9 with a specific conductance of 1030 microSiemens. The mine-waste pile was mostly devoid of vegetation, open to infiltration of precipitation, and saturated at the base because of placement in the wetland. The south mine-waste pile is composed of poorly sorted material, ranging from boulder-sized rocks to a fine-grained matrix. The trenches showed both matrix-supported and clast-supported zones, with faint horizontal color banding, suggesting zonation of Fe oxides. However, secondary minerals such as jarosite and gypsum occurred throughout the 1-2 m depth of the trenches. Infiltration of water and transport of dissolved material through the pile is evidenced by optically continuous secondary mineral deposits that fill and (or) line voids. Iron-sulfate material exhibits microlaminations with shrinkage cracking and preferential dissolution of microlayers that evidence drying and wetting events. In addition to fluids, submicron-sized to very fine-grained particles, such as jarosite, are transported through channel ways in the pile. Rock fragments are coated with a mixture of clay, jarosite, and manganese oxides. Dissolution of minerals is a primary source of metals. Skeletal remnants of grains, outlined by Fe-oxide minerals, are common. Potassium jarosite is the most abundant jarosite phase, but Pb- and Ag-bearing jarosite are common. Grain-sized clusters of jarosite suggest that entire sulfide grains were replaced by very fine-grained jarosite crystals. The mine-waste piles were removed from the wetland and reclaimed upslope. A series of entrapment ponds, lined with limestone rip rap, were created where the mine waste was once situated. A flooded adit discharges low-pH metal-bearing waters into the ponds. Despite this remediation effort to interrupt the transport path of the metal-bearing fluids, a white Zn-sulfate precipitate was observed in 2003 around the edges of the most distal pond. This material is soluble, and it goes into solution during wetting.