Abstract

Volcanogenic massive sulfide deposits (pyrite + or - chalcopyrite + or - sphalerite in the West Shasta Cu-Zn district are hosted by the Devonian Balaklala Rhyolite (dacite) and underlain by the Copley Greenstone (basaltic andesitc). The deposits occur both near to and at distance from stockwork hydrothermal vent areas. Whole-rock delta 18 O values for greenstone and metadacite vary from 3.65 to 11.0 per mil. Whole-rock delta D values for both rock types fall into a narrow range of values from -45 to -60 per mil which also compare well with the range for sea-floor hydrothermally altered greenstones. The isotopic compositions of the Balaklala Rhyolite and Copley Greenstone do not appear to vary systematically away from the mineralized part of the mining district. Their mineralogic as well as oxygen and hydrogen isotope compositions can largely be explained by reaction at 200 degrees to 300 degrees C with seawater which circulated through the rocks.The delta 34 S values of pyrite from massive sulfide bodies and from stockwork mineralization in the central part of the mining district vary from 4.1 to 5.3 per mil. Three pyrite-chalcopyrite mineral pairs indicate temperatures of equilibration of 340 degrees to 365 degrees C while the variation in delta 18 O values of vein quartz from the stockwork (7.8 to 11.0ppm) is consistent with a temperature range for mineralization of 215 degrees to 320 degrees C. Reduced sulfur predominated in the hydrothermal fluid and sulfur in the ore fluid had a delta 34 S value of about 4.6 per mil. Sulfur in the ore fluid could have originated from either marine sulfate or magmatic sulfide under certain conditions.

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