Genesis of the Jinji gold deposit in the Jiangnan Orogen, South China: Constraints from geology, chlorite geochemistry, age and H-O-S-Pb isotopes

Abstract

The Jinji gold deposit (northeastern Hunan province) is located in the central Jiangnan Orogenic Belt, South China. The auriferous sulfide-calcite-quartz vein orebodies, controlled by near NNW-trending faults, occur in the Lengjiaxi Group metamorphic rocks spatially associated with the Banshanpu granitic pluton. Metallic minerals include mainly pyrite, arsenopyrite, and pyrrhotite, with minor sphalerite, chalcopyrite, galena, and native gold. The main alteration types include K-feldspar, silicic, sericite, carbonate, and chlorite. The gold mineralization comprises three stages: (1) quartz–K-feldspar–chlorite–carbonate–arsenopyrite–pyrite, (2) quartz–carbonate–chlorite–polymetallic, and (3) quartz–carbonate–fluorite–pyrite. Geothermometry of chlorite yielded formation temperatures of 261–332 °C (stage 1) and 262–290 °C (stage 2), whereas the calculated oxygen fugacity of chlorite is −45 to −40 (stage 1) and −42 to −39 (stage 2). The δ18OH2O and δD values are 8.8–10.5 ‰ and −73.3 to −72.8‰ (stage 1), 3.2–4.8 ‰ and −83.6 to −80.7‰ (stage 2), and −1.4 to −0.4 ‰ and −82.2 to −78.6‰ (stage 3), respectively. These results indicate that the primary ore fluids (stages 1 and 2) were derived from a granitic magma and were mixed with meteoric water in stage 3. The gold-bearing sulfides (pyrite, pyrrhotite and galena) have δ34S values of −4.8 to +5.8‰, whilst their 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios are of 35.975–36.204, 15.160–15.231, and 16.100–16.176, respectively. The sulfide S-Pb isotope compositions indicate that the metals were likely derived from the Banshanpu granitic magma, which was produced by partial melting caused by Early Paleozoic crustal thickening. The Jinji deposit exhibits many features of orogenic gold mineralization, such as the ore-controlling structures and mineralization styles (quartz-calcite-sulfide veins). The Jinji deposit is the first reported gold deposit in the central Jiangnan orogen with a genetic link to early Paleozoic granitic magma.