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), EastKunlun 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 oc-curring at Weixi and Weidong, and orebodies are predominantly hosted by the Langyashan Formation (marinecarbonate rocks), and to a lesser extent the Devonian volcanic rocks. Skarns from Weixi and Weidong arecharacterized by a high garnet/pyroxene ratio and diopside- and andradite-rich composition of pyroxenes andgarnets, indicating a proximal, oxidized type. In contrast, skarn mineralogy of Main indicates a slightly reducedcondition, typical of Pb-Zn skarn deposits. At least five hydrothermal mineralization stages can be identified andthe microthermometric study indicates a general trend of cooling and dilution of the magmatic-hydrothermalfluids. Significant precipitation of Cu-Fe sulfides commenced from the fluid with the temperature of 340–448 °Cand the salinity of 2.1–15.0 wt% NaCl equiv. Pb-Zn sulfides however mainly precipitated when the temperaturesdeclined to < 370 °C and the salinity declined to < 7.6 wt% NaCl equiv. Carbon, hydrogen and oxygen isotopiccomposition of the fluids suggests a predominantly igneous source for the initial fluids, which were modifiedoutward by fluid-rock interaction and mixing with meteoric waters. Both sulfur and lead isotope composition ofsulfides 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 evidenceof fluid boiling and fluid-rock interaction. Combined with microthermometric data of ore-forming stages, it canbe deduced that fluid cooling, boiling and fluid-rock interaction were responsible for the significant metalprecipitation. The Weibao deposit shows many similarities with skarn deposits in the QMB, and its genetic modeltherefore can be extrapolated to other skarn deposits in this region.