Sources of fluids and metals and evolution models of skarn deposits in the Qimantagh metallogenic belt: A case study from the Weibao deposit, East Kunlun Mountains, northern Tibetan Plateau

Abstract

The Weibao Cu-Pb-Zn deposit is typical of skarn deposits in the Qimantagh metallogenic belt (QMB), East Kunlun Mountains, northern Tibetan Plateau. It comprises three ore blocks from west to east, known as Weixi, Main and Weidong. Mineralization in the Weibao deposit is intimately related to Late Triassic intrusions occurring at Weixi and Weidong, and orebodies are predominantly hosted by the Langyashan Formation (marine carbonate rocks), and to a lesser extent the Devonian volcanic rocks. Skarns from Weixi and Weidong are characterized by a high garnet/pyroxene ratio and diopside- and andradite-rich composition of pyroxenes and garnets, indicating a proximal, oxidized type. In contrast, skarn mineralogy of Main indicates a slightly reduced condition, typical of Pb-Zn skarn deposits. At least five hydrothermal mineralization stages can be identified and the microthermometric study indicates a general trend of cooling and dilution of the magmatic-hydrothermal fluids. Significant precipitation of Cu-Fe sulfides commenced from the fluid with the temperature of 340–448 °C and the salinity of 2.1–15.0 wt% NaCl equiv. Pb-Zn sulfides however mainly precipitated when the temperatures declined to <370 °C and the salinity declined to <7.6 wt% NaCl equiv. Carbon, hydrogen and oxygen isotopic composition of the fluids suggests a predominantly igneous source for the initial fluids, which were modified outward by fluid-rock interaction and mixing with meteoric waters. Both sulfur and lead isotope composition of sulfides supports a mixed sulfur and metal reservoir consisting of Triassic intrusive rocks and wall rocks. Compared to early skarn-forming and late quartz-carbonate stages, two ore-forming stages show clear evidence of fluid boiling and fluid-rock interaction. Combined with microthermometric data of ore-forming stages, it can be deduced that fluid cooling, boiling and fluid-rock interaction were responsible for the significant metal precipitation. The Weibao deposit shows many similarities with skarn deposits in the QMB, and its genetic model therefore can be extrapolated to other skarn deposits in this region.