Genesis of the Shangxu orogenic gold deposit, Bangong-Nujiang suture belt, central Tibet, China: Constraints from H, O, C, Si, He and Ar isotopes

Abstract

Shangxu is an orogenic gold deposit in the Bangong-Nujiang suture zone, central Tibet, China, formed in the Early Cretaceous orogene, related to convergence and collision between the Qiangtang and Lhasa terranes. The mineralization at Shangxu is hosted by Jurassic turbidite sedimentary rocks of the Mugagangri Group, and is associated with a regional fault system. Hydrothermal minerals develop muscovite, carbonate, sulfides and chlorite. Hydrothermal fluids record three main hydrothermal stages based on mineral paragenesis. The earliest is barren quartz stage (H1), which is pre-ore. During the early mineralization quartz-pyrite stage (H2a), defined by massive quartz veins with minor euhedral pyrite and gold, hydrothermal fluids had a δ18Ofluid of 6.1–6.4‰, δD of −74 to −116‰, δ13CCO2 of −5.4 to −7.6‰, and δ30Si of −0.1‰. In the quartz-pyrite-sulfides stage (H2b), characterized by abundant quartz, granular pyrite, muscovite with minor chalcopyrite, galena, sphalerite and gold, fluids had a δ18Ofluid of 7–8.2‰, δD of −109 to −120‰, δ13CCO2 of −9.6‰ and δ30Si of −0.1 to −0.2‰. During the ankerite-sulfide stage (H3a), distinguished by abundant ankerite, muscovite, sulfides with minor quartz and chlorite, hydrothermal system had a fluid with δ18Ofluid of 4.9–5.3‰, δD of −125.3‰, δ30Si of −0.1‰, δ13CCO2 of −12.4‰ in quartz inclusion fluid and δ13CCO2 of −2.7‰ in ankerite. The calcite-sulfide stage (H3b) is characterized by calcite, sulfides, with minor quartz and chlorite. Quartz formed earlier than calcite from a fluid having a δ18Ofluid of 6.4‰, δD of −112.6‰, δ30Si of −0.1‰, and δ13CCO2 of −7.4‰, after which calcite precipitated from a hydrothermal fluid with δ18Ofluid of 5.5–9‰, δ13CCO2 of −0.5 to −2.8‰. Hydrothermal fluids in H2b pyrite have 3He/4He ratios of 0.27–0.42Ra and 40Ar/36Ar ratios of 313–372.The stable isotope composition of hydrothermal fluids from the Shangxu gold deposit is similar to that of typical orogenic gold deposits. The early stage (H1), methane-bearing fluids were probably sourced from the basin sediments, leading to precipitation of early barren quartz veins and siderite alteration. From the quartz-sulfide stage (H2) to carbonate-sulfide stage (H3), hydrothermal fluids were likely derived from dissolution of the marine carbonate cement from sedimentary host rocks during metamorphism at depth. The decreasing δD and δ13C of the ore-forming fluids from early to late suggests mixing with δ13C-depleted oxidized graphite in sedimentary rocks, meteoric waters or reation between δD-depleted organic matters. Generally, these fluids were likely generated at depth through prograde metamorphic devolatilization of hydrous minerals in the deeper equivalents of the sedimentary rocks of the Mugagangri Group. Sulfur and gold, by inference, likely originated from sedimentary/diagenetic pyrite or the metasedimentary rocks between the greenschist and amphibolite facies, and migrated with the metamorphic fluids. During transportation to the site of deposition, gold-bearing fluids variably reacted with country rocks.