Paper No. 2
Presentation Time: 8:30 AM
GEOLOGY AND GEOCHEMISTRY OF SRHDG DEPOSITS IN THE YUNNAN-GUIZHOU-GUANGXI AREA, P.R. CHINA
Yunnan, Guizhou and Guangxi provinces contain > 290 t Au in > 20 SRHDG deposits with average grades of 4-16 g/t Au. They occur in an area that also contains coeval(?) epithermal Hg, Sb, As, Tl, and U deposits and older MVT Pb-Zn deposits, bedded barite deposits, and metallifeous black shales. The Yangtze craton consists of Proterozoic metamorphic rocks overlain by a thick Cambrian-Triassic carbonate and shale sequence. In the Devonian, the SW margin of the craton underwent extensional faulting, deposition of a thick sequence of Devonian-Triassic marine sedimentary rocks, and in a few places eruption of basalts. These rocks were uplifted and folded during the Early Jurassic to Latest Cretaceous (190-65 Ma) Yanshanian Orogeny which formed in response to subduction of the Pacific Plate under the Eurasian plate. Igneous rocks associated with the Yanshanian Orogeny are mainly found in eastern China, distant to the Carlin-type gold deposit area. Intrusive rocks are generally absent in the deposits, but a few contain small mafic or felsic dikes emplaced during the Yanshanian Orogeny. The SRHDG deposits consist of submicron-sized gold in micron-sized arsenian pyrite disseminated in fractured Cambrian through Triassic carbonaceous shale and carbonate rocks, and are associated with anomalous Hg, Sb, As, U and Tl. Alteration typically includes silicification, argillization and sulfidation. Aqueous fluid inclusions contain CO2, have relatively low temperatures of homogenization (250-150°C) and salinities (8-2 eq. wt. % NaCl) that typically decrease from early to later stages. The indicated pressures of 105-330 x105 Pascals correspond to depths in excess of 1.0 to 3.0 km, assuming hydrostatic conditions. The dD values of fluid inclusions (-87 to -47) and the calculated d18O values for water in ore fluids (-2.1 to 13.5) reflect interactions between meteoric water and sedimentary rocks. The d13C values of calcite (-9 to 2) and d 34S values of sulfides (-24 to 17) suggest that C and S were derived from marine carbonate (and organic carbon) and diagenetic sulfides (and organic sulfur) in the vicinity of the deposits. Although the source of heat is unclear, it appears the deposits formed by cocnvecting meteoric water during uplift and extension near the end of the Yanshanian orogeny (140 to 75 Ma).