Isotope Geochemistry of the Shimensi Deposit, Northern Jiangxi and Its Analysis

Abstract

The Shimensi deposit is located in the Lower Yangtze metallogenic province. It is a recently-discovered world-class tungsten-polymetallic deposit, accompanied by Cu and Mo. The orebodies are thick with gentle dipping, mainly distributed along the contact zone between the Cretaceous porphyritic biotite granite and the Neoproterozoic biotite granodiorite. The mineralization was closely linked with the zoned hydrothermal alteration, including silicification, greisenization, chloritization, carbonatization and K-feldsparization. Based on ore geology and C, H, O isotope, this paper has studied the origin and evolution of the ore-forming fluids. The δ13CV-PDB values of calcite range from -10.26‰ to -3.26‰, with an average of -7.3‰, indicating that hydrothermal fluids from which calcites precipitated were derived from the granitic magma. The H and O isotopic compositions of the quartz veins (δ18DSMOW=- 106‰ to -86‰, δ18 OSMOW(H2O)=2.2‰ to 4.5‰) suggest that the ore-forming fluids mainly had a magmatic-hydrothermal origin, but to some extents, mixed with meteoric water. The subduction of the Paleo-Pacific Plate beneath the Eurasian Plate resulted in lithospheric thickening and subsequent delamination, and the main N-S stress field in East China changed progressively to an E-W orientation during 170-135Ma. Maximal lithospheric thinning happened in the Late Mesozoic, which led to the direct contact between the crust and asthenospheric mantle. Consequently the heating from the asthenosphere on the crust trigged and widespread development of extensional structures, the intensive magmatism and mineralization in eastern China in Mesozoic. The isotopic chemistry indicates that the ore mineralization in the Shimensi deposit was closely linked with Early Cretaceous magmatism in East China.