Geology and in-situ sulfur and lead isotope analyses of the Jinlongshan Carlin-type gold deposit in the Southern Qinling Orogen, China: Implications for metal sources and ore genesis

Abstract

The Jinlongshan gold deposit, Shaanxi Province, central China, is a large deposit (>150 t Au) in the Southern Qinling Orogen. Gold mineralization is hosted in Upper Devonian to Lower Carboniferous carbonaceous clastic-carbonate sequences. Four stages of pyrite and paragenetic sulfides have been identified based on morphology and paragenesis. Laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) analyses were conducted to determine the chemical and S–Pb isotopic composition of the sulfides. Pre-ore stage pyrites (Py1 and Py2) contain relatively high concentrations of Mn, Co, Ni, Cu, Zn, Sb, and Pb exhibiting a wide range of δ34S values, ranging from –29.9‰ to +48.4‰. These two pyrites were formed by bacterial and thermochemical sulfate reduction of Upper Devonian marine sulfate, respectively. Main-ore stage pyrite (Py3) and arsenopyrite (Apy) contain high concentrations of Au, As, Cu, Sb, Hg, and Tl with restricted ranges of δ34S values from 9.6‰ to 15.3‰, which is interpreted as resulting from the underlying low-grade metamorphic Paleozoic sedimentary rocks by the decomposition of pyrite and/or inorganic reduction of sulfate. Late- or post-ore stage pyrite (Py4) is characterized by low concentrations of Au, Ag, Sn, Hg, and Bi and exhibited variable δ34S values, ranging from –4.4‰ to +27.7‰, that may reflect the mixing of metamorphic fluids with meteoric water. Similarly, the Pb isotope composition of ore-stage sulfides resembles those of underlying low-grade metamorphic Paleozoic sedimentary rocks whereas differs from Precambrian metamorphic basement rocks and granitoids around the basin, indicating that the metals of the Jinlongshan gold deposit were sourced from the underlying Paleozoic sedimentary rocks.Detailed fieldwork revealed that the deposit has mainly undergone four deformation events. The ore bodies developed in NE-trending dextral oblique-slip reverse faults of D3 that crosscut WNW-trending shear zones of D2. The timing and corresponding stress regimes of the D3 event suggest that it formed in a local extensional environment of Yanshanian intracontinental orogeny of Qinling orogenic belt. Gold metallogenesis of the Jinlongshan gold deposit occurred during the transition of tectonic setting from compression to extension, which reactivated the shear zones and migrated deep ore-bearing fluids to NE-trending brittle faults. The geological and geochemical features of the Jinlongshan gold deposit are more analogous to the Carlin-type deposits than other orogenic gold deposits worldwide; therefore, we propose it is a typical Carlin-type gold deposit in South Qinling Orogen.