A magmatic-hydrothermal indium-bearing polymetallic vein mineralization belt in the western Jiangnan Orogen: Evidence from zinc and cadmium isotopes of sphalerite

Abstract

Polymetallic veins of Zn-Pb (Cu) deposits containing elevated levels of indium are hosted within the upper part of the Banxi Group in the western Jiangnan Orogen. The indium in the polymetallic vein deposits is hosted within sphalerite and chalcopyrite. Cu++In3+↔Zn2++Fe2+ is the most plausible indium substitution mechanism for ZnS in these deposits. Hydrothermal fluids were enriched in Cu, In, Co, Mn and Fe in the later mineralization stage. Ores, sphalerite and dolomite minerals displayed analogous Ce and Eu anomalies, and Zn/Cd ratios, while being noticeably different from its host rocks, that provided its mineralized metals were not derived from its host rock. Despite the Zn and Cd isotopic variations in sphalerite being relatively narrow (δ66ZnJMC = +0.01‰–+0.28‰; δ114CdSpex = −0.14‰–+0.11‰), they display increase during mineralization and are negatively correlated to Cd concentrations; the Rayleigh distillation might be the fractionation mechanism. Zn/Cd ratios and Zn-Cd isotopic variations suggest the magmatic-hydrothermal origin of the polymetallic vein-type deposits. These hydrothermal fluids might derive from the fractionation of Zn, Cd, Cu, In and Sn in the granitic magma. This is triggered by crust thickening and the detachment of the lower crust along the Jiangnan Orogen during the intracontinental orogenic events of the Guangxi orogeny. This period of granitic magmatism might be another important indium mineralization episode in South China.