Origin of the recently-discovered Zhongpai Co-rich pyrite deposit in the Lanping-Simao Basin, southwest China: Constraints of trace elements and S isotopes of pyrite and U-Pb ages of monazite

Abstract

Hydrothermal deposits are important sources of metal Co, but their origins are poorly understood. Although some hydrothermal Cu deposits, such as the Baiyangping, Changjie and Jinman deposits, in the Lanping-Simao Basin, Yunnan Province, SW China, are known to have considerable amounts of Co, the concentrations of Co in these deposits are generally low (usually < 0.1 wt%) and studies of Co enrichment are few in literature. An exception is the recently discovered Zhongpai pyrite deposit which contains high-grade Co ores with up to 1 wt% Co (average Co ore grade of 0.46 wt%). The Co-rich pyrite ores are structurally controlled and are stratiform or lenticular bodies in mudstone and sandstone that are intercalated with limestone of the Middle Jurassic Huakaizuo Formation. In this paper, we present in-situ trace elements and S isotopes of pyrite and U-Pb ages of monazite, to constrain the ore-forming process and geodynamic setting of this deposit. The ores are mainly composed of pyrite (∼90 vol%), with lesser amounts of chalcopyrite and other minerals. The mineral paragenesis can be divided into three generations: pyrite (Py1)-monazite-quartz-bitumen, pyrite (Py2)-chalcopyrite-tetrahedrite-arsenopyrite, and pyrite (Py3)-calcite-ankerite-quartz. The U-Pb age of hydrothermal monazite is 12.7 ± 0.4 Ma. Py1 contains Co of up to 63600 ppm with an average of 19200 ppm. No Co minerals are identified in all three generations and Co is mainly in pyrite due to the isomorphic substitution Co²⁺ ↔ Fe²⁺. Pyrite of three generations has Co/Ni ratios from 0.4 to 22.4 consistent with the hydrothermal origin. In addition, the Co/Ni ratios and Se contents of Py1 are higher than those of Py2 and Py3, whereas the trends of the As contents are the opposite. This implies that the early Co-rich fluids formed under high temperatures. The δ³⁴S values of pyrite from all three generations range from + 2.6 ‰ to + 9.6 ‰, indicating sulfur was derived mainly from evaporitic sulfate within the ore-bearing strata. In summary, the Co-rich pyrite ores formed during the late middle Miocene and constrain the timing of mineralization in the Sanjiang metallogenic belt, SW China. The Co enrichment was jointly controlled by geological setting and ore formation physicochemical condition.