Mineralogical and isotopic constraints on the genesis of the Jingchong Co–Cu polymetallic ore deposit in northeastern Hunan Province, South China

Abstract

The Jingchong Co–Cu polymetallic deposit and a few other Co (–polymetallic) ore deposits are located in northeastern Hunan Province, in the central segment of the Jiangnan Orogen, South China. These deposits occur along the NE-trending Changping (Changsha–Pingjiang) deep fault zone and show a spatial association with the Late Jurassic–Early Cretaceous Lianyunshan granitoids, which intruded the Archean to Paleoproterozoic Lianyunshan Group comprising metaclastic-sedimentary rocks interlayered with mafic-intermediate igneous rocks. The Jingchong deposit is hosted within an altered fracture zone at the footwall of the Changping fault zone, and comprises an upper Pb–Zn ore zone and a lower Co–Cu ore zone. Wall rock alteration is dominated by silicification and chloritic alteration. Three types of ores were identified, i.e., quartz–sulfide veins, altered structural breccias and altered fractured rocks. Integrated field work and microscopy observations reveal a three-stage hydrothermal process related to mineralization: 1) quartz+chlorite (ChlI)+pyrite, 2) quartz+chlorite (ChlII)+pyrite+chalcopyrite+cobaltite, and 3) quartz+carbonate+galena+sphalerite±pyrite±chalcopyrite. LA-ICPMS analyses reveal that the Co contents are 1315–6473ppm, 76–3866ppm, and 5–435ppm for pyrite from the first-, the second-, and the third stages, respectively. EPMA analyses of chlorite show that ChlI and ChlII have Fe/(Fe+Mg) ratios of 0.65–0.80 and 0.76–0.92, and AlIV contents of 1.30–1.65 apfu (atom per formula unit) and 1.18–1.48 apfu, respectively. Empirical geothermometry of chlorite yielded temperatures of 256–318°C (av. 281°C) and 239–290°C (av. 256°C) for first- and second-stage chlorite, respectively. The δ34SVCDT values of sulfides (−4.9‰ to 0.2‰, av. −1.6‰) suggest a magmatic source for the sulfur, and the 3He/4He (0.017–0.30Ra) and 40Ar/36Ar (285–306) ratios of fluid inclusions in pyrite suggest that the ore-forming fluids were mainly crust-derived magmatic hydrothermal fluids with possible meteoric water inputs. The 206Pb/204Pb (18.042–18.746), 207Pb/204Pb (15.592–15.741) and 208Pb/204Pb (38.237–39.150) ratios of ore sulfides are similar to those of the Lianyunshan Group, suggesting that the metals may have been derived from the latter. Integrating the geological, mineralogical and geochemical data documented in this study, we propose that the Jingchong Co–Cu polymetallic deposit formed from a hydrothermal system related to the Lianyunshan pluton, which was derived from crustal anataxis of the Lianyunshan Group in a back-arc extensional setting in the Late Mesozoic.