The U.S. Geological Survey, in cooperation with the New Mexico Environment Department, is investigating the pre-mining ground-water chemistry at the Molycorp molybdenum mine in the Red River Valley, New Mexico. The primary approach is to determine the processes controlling ground-water chemistry at an unmined, off-site but proximal analog. The Straight Creek catchment, chosen for this purpose, consists of the same Tertiary-age quartz-sericite-pyrite altered andesite and rhyolitic volcanics as the mine site. Straight Creek is about 5 kilometers east of the eastern boundary of the mine site. Both Straight Creek and the mine site are at approximately the same altitude, face south, and have the same climatic conditions.
Thirteen wells in the proximal analog drainage catchment were sampled for ground-water chemistry. Eleven wells were installed for this study and two existing wells at the Advanced Waste-Water Treatment (AWWT) facility were included in this study. Eight wells were sampled outside the Straight Creek catchment: one each in the Hansen, Hottentot, and La Bobita debris fans, four in a well cluster in upper Capulin Canyon (three in alluvial deposits and one in bedrock), and an existing well at the U.S. Forest Service Questa Ranger Station in Red River alluvial deposits. Two surface waters from the Hansen Creek catchment and two from the Hottentot drainage catchment also were sampled for comparison to ground-water compositions. In this report, these samples are evaluated to determine if the geochemical interpretations from the Straight Creek ground-water geochemistry could be extended to other ground waters in the Red River Valley , including the mine site.
Total-recoverable major cations and trace metals and dissolved major cations, selected trace metals, anions, alkalinity; and iron-redox species were determined for all surface- and ground-water samples. Rare-earth elements and low-level As, Bi, Mo, Rb, Re, Sb, Se, Te, Th, U, Tl, V, W, Y, and Zr were determined on selected samples. Dissolved organic carbon (DOC), mercury, sulfate stable isotope composition (d34S and d18O of sulfate), stable isotope composition of water (d2H and d18O of water) were measured for selected samples. Chlorofluorocarbons (CFC) and 3He and 3H were measured for age dating on selected samples.
Linear regressions from the Straight Creek ground-water data were used to compare ground-water chemistry trends in non-Straight Creek ground waters with Straight Creek alluvial ground-water chemistry dilution trends. Most of the solute trends for the ground waters are similar to those for Straight Creek but there are some notable exceptions. In lithologies that contain substantial pyrite mineralization, acid waters form with similar chemistries to those in Straight Creek and all the waters tend to be calcium-sulfate type. Hottentot ground waters contain substantially lower calcium concentrations relative to those in Straight Creek. This anomaly results from the exposure of rhyolite porphyry in the Hottentot scar and weathering zone. The rhyolite contains less calcium than the altered andesites and tuffs in the Straight Creek catchment and probably does not have the abundant gypsum and calcite. The Hansen ground waters have reached gypsum saturation and have similar calcium, magnesium, and beryllium concentrations as Straight Creek ground waters but have lower concentrations of fluoride, manganese, zinc, cobalt, nickel, copper, and lithium. Lower concentrations of elements related to mineralization at Hansen likely reflect the more distal location of Hansen with respect to intrusive centers that provided the heat source for hydrothermal alteration.
The other ground water with water chemistry trends that are outside the Straight Creek trends was from an alluvial well from Capulin Canyon (CC2A). Although it had pH values near 6.0 and most major ions similar to the other Capulin Canyon ground waters, it contained high concentrations