Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong

摘要

Barite Hill is a stratiform gold-silver deposit associated with base-metal sulfides and barite located in the Carolina slate belt part of the Piedmont physiographic province in McCormick County, southwestern South Carolina. The Barite Hill Mine, from which approximately 1,835,000 grams of gold were produced between 1990 and 1994, is one of several known volcanogenic gold-silver and base-metal deposits in the Lincolnton-McCormick district of Georgia and South Carolina. Host rocks are Late Proterozoic, sericitically-altered, felsic metavolcanics near a contact between pyroclastic units within the metavolcanic and metasedimentary rocks of the Persimmon Fork Formation. A suite of Au-Ag-Te minerals is associated with pyrite-quartz alteration in the matrix of volcanic breccia and in veins cutting the breccia clasts in the Main Pit area of the Barite Hill Mine, and with fine-grained, silicified rocks and chert in the Rainsford Pit. The stratigraphically uppermost ore zone in the Main Pit is overlain by a zone with base-metal and barite enrichment in metasedimentary and metavolcanic rocks of the upper pyroclastic unit. The first stage in development of the Barite Hill deposit was submarine volcanism in a primitive to mature island arc setting located away from the North American craton. Hydrothermal fluids moved through, altered, and mineralized the volcanic pile and were exhaled onto the sea floor to form stratiform barite and pyrite-rich base-metal sulfides, A second stage, probably during waning phases of the hydrothermal activity, is indicated by the occurrence of precious metals and base-metal sulfides along with a telluride-selenide-bismuth suite of minerals, which suggests eptthermal conditions of deposition. A third stage is recorded by deformation of pyrite-quartz zones and barite-rich layers during greenschist-facies metamorphism and development of folds with near-vertical axial-plane cleavage, which is probably related to Ordovician compression and collision of the Carolina terrane with the North American plate during the Taconic orogeny. A fourth stage is indicated by late- or post-tectonic concentration of gold in late-stage quartz veins that cut cleavage and by coarsely crystalline barite veins that cut cleavage and probably postdate the gold-bearing quartz veins. A fifth stage is offset of the orebodies along high-angle faults, A sixth stage is weathering and oxidation in which supergene barite formed as a result of dissolution of barite from sulfide-rich lenses by strongly acidic solutions followed by reprecipitntion ns euhedral crystals lining cavities in ferric-oxide-hydroxide gossans.