ENVIRONMENTAL GEOLOGY OF EROSIONAL SCAR AREAS DEVELOPED ABOVE CLIMAX-TYPE MOLYBDENITE DEPOSITS IN THE RED RIVER WATERSHED, NEW MEXICO

The Red River valley between Red River and Questa hosts a series of granitic intrusions with associated Climax-type molybdenite deposits, including those mined at Molycorp’s Questa mine. A series of natural erosional scars has developed on altered rocks around the intrusions, and these contribute acidic waters and mineralized alluvial material to the Red River. Many similarities in intrusive, volcanic, and subvolcanic wallrock types, and alteration and mineralization styles occur between the scar areas. Important differences are in the depth to which erosion has cut into zoned mineralization/alteration around the intrusions. The Sulphur Gulch scar at the Questa mine exposes quartz-sericite-pyrite (QSP)-altered volcanics in its upper portions grading downward into high grade, molybdenite-mineralized, potassically-altered granites in its lower portions. Other scar areas cut primarily through QSP and more distal quartz-sericite, propylitic, and argillic alteration, but not into the underlying intrusions. The scars form by coupled chemical weathering (primarily pyrite oxidation and resulting acid-sulfate weathering), physical breakdown (freeze-thaw action and volume expansion from secondary gypsum formation), and subsequent erosion of originally QSP-altered, stockwork-veined volcanic rocks. Reactive minerals (listed from more to less readily weathered) include: pyrite, pyrrhotite, calcite, local Mn-, Mg-, and REE-rich carbonates, sphalerite, galena; fluorite, chalcopyrite; chlorite; illite, albite. A thin (3 to 30 m) weathered veneer (sand- to pebble-size fragments of unweathered rock in a matrix of gypsum, clays, and jarosite) mantles the scars’ denuded slopes, but unoxidized QSP outcrops in gully bottoms. Scars grow along steep headwalls by slope failure. Acidic (pH 2.6 to 4.4), metal-rich waters develop in the near-surface weathered veneer and shallow bedrock of the scars, and in alluvial fans fed by the scars. There are enough carbonates in the QSP-altered bedrocks to neutralize acid in deeper ground waters, which tend to have near-neutral pH (5.7 to 7.6) and lower metal contents. This environmental geology information is used as part of a broader U.S. Geological Survey study constraining pre-mining ground-water quality at the Questa mine site (Nordstrom et al., this meeting).