Abstract

Molybdenum mineralization and coextensive alteration in the Questa, Hudson Bay Mountain, and Endako stockwork molybdenum deposits occur in single and composite veinlets which exhibit an ordered sequence of emplacement. Fluid inclusions in the deposits can be classified as liquid-rich types A (liquid + vapor), C (liquid + vapor + halite), D (liquid + vapor + halite + sylvite), and E (H 2 O-rich liquid + CO 2 -rich liquid + CO 2 vapor) and as vapor-rich type B. Study of these inclusions in the evolving fracture-controlled alteration-mineralization shows that type D inclusions are found with potassic alteration assemblages and type C inclusions occur with phyllic alteration assemblages.Salinities are divided into three compositionally distinct groups. Early fluorine-rich, biotitestable alteration at Questa and Hudson Bay Mountain is associated with hypersaline (40-70% NaCl + KCl) type D inclusions which homogenize most frequently by halite dissolution. Homogenization temperatures uncorrected for pressure range from 320 degrees to > 600 degrees C. Type D inclusions from both deposits fall along similar linear compositional trends in the NaCl-KCl-H 2 O system regardless of temperature or behavior of homogenization. The bulk of the molybdenite deposition coincides with quartz-sericite-pyrite alteration and with either moderate to high salinity (30-60% NaCl equivalent) type C inclusions, which homogenize by halite dissolution or vapor disappearance, or with low to moderate salinity (5-15% NaCl equivalent) type A inclusions. Evidence for local, intermittent boiling is present throughout the Questa and Hudson Bay Mountain, but not the Endako, hydrothermal systems. Homogenization temperatures for type A and C inclusions vary from 300 degrees to > 600 degrees C with definite modes at about 390 degrees C.The inclusion data are accepted as evidence for evolution from magmatic to meteoric hydrothermal conditions. Hypersaline (type D) solutions were precursors to the actual oreforming fluids and evolved directly from granitic source intrusions. Fluid reequilibration with the granitic intrusions and/or dissolution of NaCl precipitated by earlier hypersaline solutions along the halite trend are possible origins of the saline type C inclusion fluids. Salinity enhancement by boiling or mixing of hypersaline with meteoric fluids is not supported by the data. Pressures appear to have fluctuated between lithostatic and hydrostatic load, and intermittent overpressures with adiabatic decompression are suggested.

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