New Mexico Mineral Symposium — Abstracts

The Questa Mine, Taos County, New Mexico, is a porphyry molybdenum deposit. Porphyry molybdenum deposits are large, low-grade deposits that contain disseminated and stockwork veinlets of molybdenum sulfides and are associated with porphyritic intrusions. Climax and Henderson are examples of other large porphyry molybdenum deposits that have yielded numerous mineral specimens. Recent development of the underground Questa orebody and ongoing studies of the mine rock piles, open pit, and nearby alteration scars have lead to the recognition of minerals not previously associated with the deposit. This presentation will describe these new discoveries.

The deposit was first famous for collectable specimens of molybdenite and ferrimolybdite that were produced from old underground workings and the open pit. Questa's molybdenite occurs in three associations: "dotty" 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 coarse breccia matrix and is crosscut by later vein molybdenite. Paint molybdenite cuts both vein and dotty moly. Minerals typically associated with the Questa molybdenite mineralization include: fluorite, calcite, rhodochrosite (generally pale), anhydrite, galena, and dolomite. As is common with stockwork molybdenum deposits, these minerals were typically found in veins with limited amounts of open space and highly collectable minerals specimens were the exception. Galena is found intergrown with the molybdenite. The presence of fluorite as disseminations in feldspar at Questa is consistent with the pegmatitic high-fluorine nature of the deposit, which suggests the Questa deposit is similar to the Henderson, Colorado molybdenite deposit.

New minerals identified in the recent phase of mining breccia and vein type deposits in the operational underground workings include: beryl (aquamarine and emerald), celestine, and topaz. The greater abundance of open space in these deposits has lead to the discovery of a number of collectable specimens. In addition, exploration drilling has revealed some tungsten minerals, especially wolframite.

Together these data suggest that the Questa molybdenite deposit was formed by highly-fractionated, complex magmatic fluids similar to those that formed the Henderson and Climax deposits.