Isotopic characteristics of gold deposits in the Yangshan Gold Belt, West Qinling, central China: Implications for fluid and metal sources and ore genesis

摘要

The six main Triassic gold deposits of the Yangshan gold belt are hosted by Devonian metasedimentary rocks and Triassic granitic dikes. It has been suggested by previous workers that they are best classified as either Carlin-like or orogenic gold deposit types. Sulfide minerals spatially associated with disseminated and quartz vein-hosted gold mineralization at Yangshan were deposited in five stages that include syn-metamorphic, early ore, main ore, late ore, and post-ore stages. Based on the relationships defined by ore-mineral assemblages and paragenesis, H, O, C, S, Pb, and Sr isotopic data were obtained from sulfides, quartz, and ore host rocks. The delta O-18 values of quartz from gold-bearing veins range from 15.9 parts per thousand to 21.5 parts per thousand, and the calculated delta O-18(H2O) values of ore-forming fluids vary from 6.4 parts per thousand to 11.8 parts per thousand, which are consistent with those from typical orogenic gold deposits. The delta D-H2O values of fluid-inclusions extracted from the quartz vary from -82 parts per thousand to -56 parts per thousand, which are in accord with the delta D-H2O values of most orogenic gold deposits. Interpretation of the delta C-13(CO2) values (-4 parts per thousand to -2.5 parts per thousand) in fluid inclusion, shows that marine carbonate is the dominant source for the carbon in the ore-forming fluids, with a minor contribution frommagmatic sources. The delta S-34 values of hydrothermal pyrite, arsenopyrite, and stibnite range from -6.6 parts per thousand to 3 parts per thousand, which suggests sulfur is derived from a deep source. The Pb isotope values for hydrothermal sulfides from both the granitic dike-hosted ores and phyllite-hosted ores overlap the fields for their respective wall rocks, which suggests the source of lead is either the local host rocks, or ore-related fluids that may have pervasively penetrated the host rocks. The age-corrected (Sr-87/Sr-86)(i) values of pyrite are 0.70627 to 0.71304, and values for arsenopyrite are 0.71258 to 0.71294, showing that the strontium in the ore fluid could have been derived from the granitic dikes and the regionally extensive Mesoproterozoic to Neoproterozoic Bikou Group. The concordance of delta S-34, Pb and (Sr-87/Sr-86) i values between those for hydrothermal sulfides and country rocks suggests that these crustal rocks contributed the gold that was subsequently concentrated in the ore deposits. The syn-metamorphic stage and early ore stage fluids were likely derived from the country rocks during prograde metamorphism at depth. The oxygen isotope compositions indicate relatively widespread interaction of such fluids with delta O-18-rich country rocks during the main ore stage. Compared with most orogenic gold deposits and Carlin-type gold deposits, the gold deposits in the Yangshan gold belt exhibit isotopic characteristics that are most like those of orogenic gold deposits. (C) 2016 Elsevier B.V. All rights reserved.