|Abstract:||The U.S. Geological Survey, in cooperation with the New Mexico Environment Department and supported by Molycorp, Inc (currently Chevron Minerals), has completed a 5-year investigation (2001-2006) to determine the pre-mining ground-water quality at Molycorp‘s Questa molybdenum mine in northern New Mexico. Current mine-site ground waters are often contaminated with mine-waste leachates and no data exists on premining ground-water quality so that pre-mining conditions must be inferred. Ground-water quality undisturbed by mining is often worse than New Mexico standards and data are needed to help establish closure requirements. The key to determining pre-mining conditions was to study the hydrogeochemistry of a proximal natural analog site, the Straight Creek catchment. Main rock types exposed to weathering include a Tertiary andesite and the Tertiary Amalia tuff (rhyolitic composition), both hydrothermally altered to various degrees. Two types of ground water are common in mineralized areas, acidic ground waters in alluvial debris fans with pH 3-4 and bedrock ground waters with pH 6-8. Siderite, ferrihydrite, rhodochrosite, amorphous to microcrystalline Al(OH)3, calcite, gypsum, barite, and amorphous silica mineral solubilities control concentrations of Fe(II), Fe(III), Mn(II), Al, Ca, Ba, and SiO2, depending on pH and solution composition. Concentrations at low pH are governed by element abundance and mineral weathering rates. Concentrations of Zn and Cd range from detection up to about 10 and 0.05 mg/L, respectively, and are derived primarily from sphalerite dissolution. Concentrations of Ni and Co range from detection up to 1 and 0.4 mg/L, respectively, and are derived primarily from pyrite dissolution. Concentrations of Ca and SO4 are derived from secondary gypsum dissolution and weathering of calcite and pyrite. Metal:sulfate concentration ratios are relatively constant for acidic waters, suggesting consistent weathering rates, independent of catchment. These trends, combined with lithology, mineralogy, and mineral solubility controls, provide useful constraints on pre-mining ground-water quality for the mine site where the lithology is known.