Regional stable isotope systematics of hydrothermal alteration and massive sulfide deposition in the West Shasta District, California

Abstract

The West Shasta Cu-Zn district is located in California at the southern end of the Klamath Mountains and contains numerous pyrite-dominant, volcanogenic massive sulfide deposits. The deposits are hosted in a bimodal suite of intrusive and extrusive basalt to andesite (Copley Greenstone) and rhyolite (Balaklala Rhyolite) of Devonian age. Regional variations in oxygen, hydrogen, and sulfur isotope ratios in the Copley Greenstone, Balaklala Rhyolite, and Mule Mountain trondhjemite record hydrothermal alteration which took place over a large range in temperature, from 18 O values vary for the Copley Greenstone from 3.5 to 11.6 per mil, and for the Balaklala Rhyolite from 7.6 to 13.3 per mil; delta D values of the Copley range from -71 to -42 per mil, and for most samples of the Balaklala from -73 to -46 per mil. The isotopic variation reflects changing mineralogy in addition to a range of alteration temperatures. With progressive chloritization, the water content of the Copley Greenstone increases from 1.51 to 8.34 wt percent H 2 O, whereas the delta 18 O values decrease. Water/rock mass ratios calculated from the oxygen isotope data are 1.8 to 2.8 for high-temperature alteration.Regional variations in delta D and delta 18 O values of the Copley Greenstone indicate higher temperatures of alteration and greater water/rock interaction adjacent to the Mule Mountain stock, beneath the area of the Iron Mountain mine (largest of the massive sulfide deposits). This suggests that the stock is the heat source responsible for the Iron Mountain deposit. The alteration pattern associated with the stock and the low-temperature isotopic alteration of the Capping porphyry (upper unit of the Balaklala) indicate that the Mule Mountain stock was coeval with the middle unit of the Balaklala.Estimates of the oxygen isotope compositions of the Balaklala and Mule Mountain magmas (delta 18 O approximately 6.5ppm) from analyses of quartz phenocrysts suggest that the two magmas (or magma types) are consanguineous and were derived from the same source as the Copley Greenstone. Sulfur isotope data support this interpretation. There is no clear evidence for temporal variations in the isotopic composition of the magmas.Mass balance considerations and data on the isotopic composition and content of sulfur in the Copley Greenstone (most 95 percent of the sulfur in the ore deposits could have been leached from the Copley Greenstone during hydrothermal alteration. The variation in delta 34 S values of whole-rock samples is larger for the Copley (-4.0 to +22.4ppm; most 0.0 to +6.0ppm) than for either the Balaklala (3.3 to 4.9ppm) or the massive sulfide deposits (0.5 to 6.0ppm, but most 3.0 to 5.0ppm). No correlation is found between the abundance and isotopic composition of sulfur in the Copley Greenstone. Isotopic fractionation effects during transport and local reprecipitation of leached sulfur can explain the sulfur isotope variability of the Copley Greenstone. Sulfur isotope data also support degassing of the Mule Mountain or related magmas as a sulfur source.