CONTRASTING GROUND-WATER GEOCHEMISTRY BETWEEN ACIDIC DEBRIS-FLOW WATERS AND NEUTRAL-PH BEDROCK WATERS IN MINERALIZED VOLCANICS, RED RIVER VALLEY, NEW MEXICO

The U.S. Geological Survey, in cooperation with the New Mexico Environment Department, is investigating the pre-mining ground-water quality at Molycorp’s Questa molybdenum mine. The primary strategy is to determine the processes controlling ground-water chemistry at an unmined and off-site, proximal analog. The Straight Creek catchment, chosen for this purpose, consists of quartz-sericite-pyrite altered andesitic and rhyolitic Tertiary volcanics similar to the mine site. Exposure of pyritized rock in the upper portions of the catchment produces acid rock drainage (pH 2.7-3.2) which then infiltrates a colluvial debris flow with waters of similarly acid pH (3.4-4.0) and bedrock with circumneutral pH values (5.8-7.6). Most of the dissolved solutes in the debris flow wells can be accounted for by the influent composition of Straight Creek surface water. Both bedrock and debris-flow ground waters are of the Ca-SO4 type, often at or near gypsum saturation because of the abundant and ubiquitous occurrence of secondary gypsum developed from co-existing pyrite (abundant) and calcite (common but less abundant) gangue mineralization. Concentrations of Mg, Fe, and F also are high in the debris-flow waters but more variable in the bedrock waters. As ground water travels down the debris flow the dominant trend is dilution, as demonstrated by plots of element concentration against sulfate concentration and decreasing sulfate concentration in the downgradient direction. Aluminum is initially leached from kaolinite, chlorite, and albite under acid conditions but thereafter its concentration is a function of pH and decreases rapidly as the pH increases above pH 5, as predicted by its first hydrolysis constant, pK1=5. Evidence for precipitation of both aluminum and silica is apparent at pH=4. Trace elements are weathering from specific mineral sources: Zn and Cd from sphalerite, Pb from galena, Co and Ni from pyrite and pyrrhotite, Cu from chalcopyrite, F from fluorite, and Mn from rhodocrosite (Plumlee et al., this meeting). With these results an extensive mass balance calculation has been made for the development and evolution of ground-water chemistry in this mineralized and rapidly weathering mountainous catchment.