Abstract

A detailed geochemical analysis of the well-exposed, multistage, Pine Creek, California, tungsten deposit has constrained the roles of magmatic, metamorphic, and meteoric fluids in the development of the zoned scheelite ore and calc-silicate skarn.Metamorphic conditions of P < 2 kb, T < 600 degrees C, and X (sub CO 2 ) < 0.25 are recorded in the metasedimentary rocks of the septum. Metamorphism yielded calcite + diopside + graphite and calcite + dolomite + periclase + brucite + forsterite assemblages in the variably dolomitic, siliceous, carbonate rocks of the Pine Creek septurn. Following intrusion of the Morgan Creek quartz monzonite, metasomatism developed the zoned endoskarn-exoskarn at the contact. The massive garnet (andradite (sub 20-50) ) + pyroxene (hedenbergite (sub 20-80) ) + or - quartz skarn assemblage carries most of the scheelite. Calculated T-X (sub CO 2 ) equilibria, corrected for solid solution effects, restrict skarn formation to X (sub CO 2 ) ) < 0.2 at T = 500 degrees + or - 50 degrees C. Calculated f (sub O 2 ) - f (sub F 2 ) ) - f (sub S 2 ) ) diagrams indicate f (sub O 2 ) values near quartz-fayalite-magnetite and f (sub S 2 ) values well below pyrrhotite-pyrite during skarn formation. Slightly falling temperatures were accompanied by increased f (sub S 2 ) ) and hydration of the skarn assemblages.Stable isotope data for calcites show a mixing trend from unaltered marble (delta 18 O = 24ppm, delta 13 C = 3.5 degrees ) to values in equilibrium with magmatic fluids (delta 18 O = 8ppm, delta 13 C = -6ppm). Biotite-quartz thermometry for the quartz monzonite yields temperatures of 410 degrees to 460 degrees C, well below the solidus and suggesting later reequilibration. Pyroxene-quartz analyses suggest that these minerals equilibrated at temperatures of 460 degrees to 560 degrees C. The delta 18 O values measured for garnets suggest disequilibrium for the garnet-quartz and garnet-pyroxene pairs. D/H results for epidote and biotite suggest that meteoric water influx became more important with time. Sulfur isotope data are consistent with a reduced magmatic source.

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