Sulfur and lead isotope geochemistry of hypogene mineralization at the Barite Hill Gold Deposit, Carolina Slate Belt, southeastern United States: a window into and through regional metamorphism

摘要

The Barite Hill gold deposit, at the southwestern end of the Carolina slate belt in the southeastern United States, is one of four gold deposits in the region that have a combined yield of 110 metric tons of gold over the past 10 years. At Barite Hill, production has dominantly come from oxidized ores. Sulfur isotope data from hypogene portions of the Barite Hill gold deposit vary systematically with pyrite-barite associations and provide insights into both the pre-metamorphic Late Proterozoic hydrothermal and the Paleozoic regional metamorphic histories of the deposit. The #delta#~(34)S values of massive barite cluster tightly between 25.0 and 28.0 per thousand, which closely match the published values for Late Proterozoic seawater and thus support a seafloor hydrothermal origin. The #delta#~(34)S values of massive sulfide range from 1.0 to 5.3 per thousand and fall within the range of values observed for modern and ancient seafloor hydrothermal sulfide deposits. In contrast, #delta#~(34)S values for finer-grained, intergrown pyrite (5.1-6.8 per thousand) and barite (21.0-23.9 per thousand) are higher and lower than massive counterparts, respectively. Calculated sulfur isotope temperatures for the latter barite-pyrite pairs (triangle open = 15.9-17.1 per thousand) range from 332-355 deg C and probably reflect post-depositional equilibration at greenschist-facies regional metamorphic conditions. Thus, pyrite and barite occurring separately from one another provide pre-metamorphic information about the hydrothermal origin of the deposit, whereas pyrite and barite occurring together equilibrated to record the metamorphic conditions. Preliminary fluid inclusion data from sphalerite are consistent with a modified seawater source for the mineralizing fluids, but data from quartz and barite may reflect later metamorphic and (or) more recent meteoric water input. Lead isotope values from pyrites range for ~(206)Pb/~(204)Pb from 18.005-18.294, for ~(207)Pb/~(204)Pb from 15.567-15.645, and for ~(208)Pb/~(204)Pb from 37.555-38.015. The data indicate derivation of the ore leads from the country rocks, which themselves show evidence for contributions from relatively unradiogenic, mantle-like lead, and more evolved or crustal lead. Geological relationships, and stable and radiogenic isotopic data, suggest that the Barite Hill gold deposit formed on the Late Proterozoic seafloor through exhalative hydrothermal processes similar to those that were responsible for the massive sulfide deposits of the Kuroko district, Japan. On the basis of similarities with other gold-rich massive sulfide deposits and modern seafloor hydrothermal systems, the gold at Barite Hill was probably introduced as an integral part of the formation of the massive sulfide deposit.