Abstract

Shallow, sub-ore-grade molybdenite mineralization has been discovered in the active, high-temperature geothermal system penetrated by Continental Scientific Drilling Program corehole VC-2A at Sulphur Springs, in the western ring-fracture zone of the Valles caldera, New Mexico. This mineralization is hosted by fractured, quartz-sericitized, intracaldera ash-flow tuffs younger than 1.12 Ma. The molybdenite is an unusual, poorly crystalline variety that occurs in vuggy veinlets and breccia cements also containing quartz, sericite (illite), pyirite, and fluorite, as well as local sphalerite, rhodochrosite, and chalcopyrite. Fluid-inclusion data suggest that this assemblage was deposited from very dilute solutions at temperatures near 200 °C. Geochemical modeling indicates that under restricted pH and fO2 conditions at 200 °C, the molybdenite and associated phases would be in equilibrium with hydrothermal fluids now circulating in the deep subsurface. The shallow molybdenite zone intersected in VC-2A may be the near-surface expression of deep, Climax-type stock work molybdenum mineralization.

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