A Quantitative Method for Measuring Temperature-time Histories in Hydrothermal Magmatic Ore Systems

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In 2010, the United States produced over $800 million of molybdenum oxide, and the demand for molybdenum as an alloying agent in high-grade steel will likely increase as global economies recover. The majority of the world's molybdenum reserves are contained within porphyry copper (±molybdenum) and porphyry molybdenum (±copper) deposits. Although porphyry molybdenum deposits are often volumetrically smaller than porphyry copper systems, they are still a significant source of molybdenum owing to higher ore grades. Understanding porphyry molybdenum deposition is important for accurate predictions of spatial and temporal ore distribution during exploration. The timing and duration of molybdenum mineralization relative to associated magmatism will be investigated. The study will lend insight into the complex variations of ore grade, alteration geochemistry, and magmatic-hydrothermal textures observed within porphyry molybdenum ore systems.Detailed uranium/lead zircon (±titanite) and argon/argon biotite (±hornblende) dating of plutonic rocks associated with porphyry molybdenum deposits in the western US will be obtained. In addition, rhenium/osmium molybdenite dates, and fluid trapping temperatures for molybdenite in quartz veins from economically important deposits will be determined. These data will be used to establish precise temperature-time histories of the deposits and the timing of mineralization within the thermal maturation of the magma system. This information should be useful in enhancing exploration for molybdenum - an increasingly valuable international commodity. The results of this work should 1) enable better understanding of porphyry mineralization in light of new hypotheses regarding the incremental assembly of homogeneous plutons, and 2) further development of combined rhenium/osmium geochronology and fluid inclusion thermometry as a broadly applicable thermochronologic tool.