Contact metasomatism and ore deposition: Tem Piute, Nevada

Ore at Tem Piute occurs in a thermally zoned, contact-metasomatic garnet-pyroxene skarn zone separating barren limestone and hornfels from unmineralized quartz monozonite. The scheelite is more abundant in garnet areas and the sulfides in the diopsidic bands. Two unusual features are the number of different ore minerals and the thermally controlled mineralogical zoning across the aureole. Pyrrhotite (and associated marcasite), magnetite and, locally, molybdenite and ilvaite are concentrated close to the quartz monzonite in the "inner" portion of the contact aureole. Sphalerite together with lesser amounts of galena, bismuthinite, cosalite, galenobismutite and native bismuth, occur close to the limestone contact in the "outer" portion of the skarn aureole, tungsten-copper-silver deposit. The ore minerals1 occur in a banded Here fluorite is the characteristic gangue. This partitioning of the minerals is in response to thermal differences across the aureole. Certain minerals such as pyrite, chalcopyrite, and scheelite, are uniformly distributed throughout the aureole. All available pyrrhotite is at least in part monoclinic and thus not directly suitable for geothermometry; there are indications, however, that some annealed monoclinic pyrrhotites may yield valid crystallization temperatures. Annealed samples indicate an apparent temperature of deposition in the "inner" part of the aureole of between approximately 455° C and 510° C. Such temperatures are reasonable but, in view of the possibility of compositional changes having occurred during the initial change from the hexagonal state, the values must be viewed with caution. Some of the sulfo-salts in the "outer" portion of the aureole, based on thermochemical calculations, formed below 235° ± 25° C. Thus, the indication is that the ore minerals were deposited at rather low temperatures for a contact-metasomatic ore deposit and that there was a surprisingly high-temperature gradient across this narrow contact zone.