PARAGENESIS AND GEOCHEMISTRY OF THE CALVERT TUNGSTEN SKARN DEPOSIT, PIONEER MOUNTAINS, MONTANA

摘要

The Calvert tungsten skarn deposit is located in the northern extent of the West Pioneer Mountains, approximately 40 miles southwest of Butte, Montana. The orebody was a significant tungsten producer from 1956-1957 and 1960-1962 during which time the mine produced a total of approximately 113,000 tons of ore averaging 1.10% WO3. The mine has a measured reserve of 128,000 tons at 1.10% WO3. The scheelite-bearing deposit occurs within skarn developed in the Pennsylvanian Amsden Formation near or in contact with a satellite pluton of the Late Cretaceous Pioneer Batholith.Mineral paragenetic studies reveal at least three stages of skarn and ore formation. The first stage represents early prograde metasomatism and is characterized by diopside, forsterite, and trace grossular garnet. The second stage represents main-stage prograde metasomatism and is characterized by grossular, diopside(?), epidote, calcite, and quartz together with scheelite, hematite, zircon, magnetite, apatite and sphene. Retrograde alteration caused local replacement of early mineral assemblages by actinolite, calcite, and quartz together with scheelite, hematite, magnetite, phlogopite, chlorite, and muscovite.Fluid inclusion microthermometric measurements of the Calvert skarn minerals show that the main prograde mineral assemblage was formed at temperatures between 400°-450°C and pressures between 2-2.5 kbar which correspond to a depth of 7-8.75 km. The salinity of these fluid inclusions increases from 3.2 wt.% to 11.2 wt.% NaCl eq., suggesting either a mixing between a high salinity magmatic fluid and a dilute fluid of meteoric origin, or an increase in salinity due to loss of water to the formation of hydrous skarn minerals.The protolith marble has δ13C and δ18O values that average 0.6‰ (VPDB) and 25.2‰ (VSMOW), respectfully. These isotopic values are depleted relative to unmetamorphosed marine limestone values of 0.0‰ (VPDB) and 30.6‰ (VSMOW). The δ13C and δ18O values of the skarn calcite range from -5.0‰ to -9.4‰ for carbon and from 7.7‰ to 14.0‰ for oxygen. The observed depletion towards isotopic magmatic values indicates an interaction between the marble and metasomatic fluids derived from the source igneous body. Mass-balance calculations indicate that the isotopic values of the skarn calcites could have been produced by the interaction between wallrock (δ13Ci = 0.0‰, δ18Oi = 30.6‰) and magmatic fluid (δ13Ci = -5.5‰, δ18Oi = 7.5‰, XCO2 = 0.01) at a minimum fluid/rock ratio of u3e 40.Mineral stability relationships combined with fluid inclusion data suggest that during formation of the metasomatic skarn the mole fraction CO2 was approximately 0.01 and fugacity of oxygen was 10-21 bars. Scheelite precipitation most likely resulted from cooling coupled with an increase in the activity of calcium due to fluid/rock interaction with the calcite-marble host rock. During retrograde alteration, temperatures dropped below 400°C but the fugacity of oxygen remained high (~ 10-23 bars) allowing minimal sulfides to be precipitated.Mineral paragenesis, mineral chemistry, fluid inclusion and stable isotope studies are consistent with the hypothesis that the Calvert deposit is a distal skarn. The source of the hydrothermal fluids is not known but could include the Foolhen Mountain Tonalite or the Bryant Creek Granite which outcrop in the study area. An alternative possible source is a buried pluton at greater depth. This could account for the large amounts of water involved with the formation of the Calvert skarn.