2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 11
Presentation Time: 8:00 AM-12:00 PM


BREESE, Michael R., New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, MCLEMORE, Virginia T., New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Pl, Socorro, NM 87801 and WALKER, Bruce M., Molycorp Inc, P.O. Box 469, Questa, NM 87556, mrb@nmt.edu

The Questa molybdenite deposit at Red River, Taos County, New Mexico is known for it's molybdenite and ferrimolybdate occurrences, but to date little has been mentioned about the locale's other minerals, including anhydrite, fluorite, galena, paragonite, and thorite(?). Questa's molybdenite occurs in three associations; "clotty" molybdenite, vein molybdenite, and thin fracture fillings of "paint" molybdenite, each of which reflect different stages of mineralization. Clotty molybdenite occurs as a component of a magmatic hydrothermal matrix and is cross-cut by later vein molybdenite, while paint molydenite cuts both vein and clotty moly. Galena is found interfoliate with the molybdenite. The presence of fluorite and possible thorite, as disseminations in breccia matrix feldspar, at Questa are both consistent with the pegmatitic high-fluorine nature of the deposit, which suggests the Questa deposit is similar to the Henderson, Colorado molybdenite deposit. Notably, the Henderson deposit contains some 3R molybdenite polytype in open-space veins, suggesting there could be small amounts of of 3R polytype in Questa's molybdenite. However, X-ray diffraction analyses of all three associations indicate that Questa's molybdenite polytype is 2H1. X-ray diffraction also has determined compositional and/or alteration differences between so-called "sooty" molybdenite and Questa's paint, vein, and clotty molybdenite. The presence of anhydrite and paragonite are still being investigated, but are almost certainly related to one or more stages of hydrothermal mineralization. Together, these data suggest that the Questa molybdenite deposit was formed by highly-fractionated, complex magamatic fluids similar to those that formed the Henderson deposit.