Origin of the Xitieshan Pb-Zn deposit, Qinghai, China: Evidence from petrography and S-C-O-Sr isotope geochemistry

Abstract

The Xitieshan Pb-Zn deposit in the North Qaidam orogenic belt, contains a geologic resource of 64 million tons grading 4.86 percent Zn and 4.16 percent Pb. The mineralized zones are hosted by the Tanjianshan Group, a metamorphosed succession of volcanic-sedimentary rocks in a Middle to Late Ordovician back-arc basin. Lower greenschist to middle amphibolite facies metamorphism and deformation of the original Xitieshan Pb-Zn ores and host rocks resulted in zones hosted in marble and schist. Sulfides in the marble consist of pyrite, sphalerite, galena, and marcasite with accessory chalcopyrite with undeformed textures that reflect the original carbonate replacement mineralization. Pyrrhotite, pyrite, sphalerite, and galena are the major sulfides in the schist-hosted ores, which occur as laminated sulfide layers that pinch, swell, and conform to the foliation of the schist. The schist-hosted mineralization occurs as a remobilized paragenetic assemblage that reflects the recrystallization and physical migration of the original sulfides across the brittle-ductile boundaries of these minerals during dynamic metamorphism. The sulfur isotopic compositions of ore-stage sulfides cluster around values of −1‰ to +5‰, consistent with sulfur derived from leaching volcanic rocks in the host sequence, or magmatic-related fluids. Carbon, oxygen, and strontium isotope patterns of the altered marble (δ13C = −3.8 to −1.5‰, δ18O = 8.7 to 12.2‰, and 87Sr/86Sr = 0.713092 to 0.713503) indicate extensive isotopic exchange with hydrothermal fluids. Gangue minerals, including ankerite and siderite, intergrown with sulfides at Xitieshan have C, O, and Sr isotopic compositions (δ13C = −1 to +1.7‰, δ18O = 2.3 to 7.8‰, and 87Sr/86Sr = 0.715707 to 0.722918) and textures indicative of replacement reactions between an originally marine carbonate rock and magmatic-related hydrothermal fluids. Considering the Xitieshan ore fluids replaced carbonate in a volcanoclastic sequence together with the similarities of host rocks, mineralogy, alterations, and sulfur isotope composition, the most likely genetic model for the deposit is an intrusion-related carbonate replacement deposit that underwent subsequent metamorphism and deformation.