Geology, geochemistry and genesis of the Hongshijing gold deposit in Ruoqiang, Xinjiang

Abstract

The Hongshijing gold deposit, which occurs in the Middle and Late Carboniferous volcanic and pyroclastic rocks, is of the brittle-ductile shear zone type controlled by a rift belt. The Hongshijing gold deposit is one controlled by a brittle shear zone located in the Late Paleozoic rift zone. The altered-rock type and quartz type orebodies are contained in the gold-bearing formation, which consists of basalt and tuffaceous sandstone. The major mineralizing stage is at 267 - 261 Ma and reiteration mineralizing stage at 220 - 209 Ma. The ore minerals include pyrite, magnetite, copper, bornite, ferrohydrite, native gold, and the gangue minerals include quartz, sercite, calcite, Fe-dolomite, leucoxene, anorthose, biotite, baria, cajuelite, and agustite. The wall rock alteration associated with gold mineralization comprises silicification, carbonization, pyritization, sericitization and chloritization. The contents of gold are 2.4 × 10−9 in the gold-bearing formation, 5.7 × 10−9 in the tuffaceous sandstone and 1.4–1.5 × 10−9 in the basalt. Au is associated with Te, Se, Ni, Cu. Au = 74.331 × Te +0.0335 × Ni−0.0211 × Cu-2.650. Geochemical investigations revealed that the mineralizing materials came from the basalt and tuffaceous sandstone in the gold-bearing formation. Under the action of ductile brittle shear structure, the gold-bearing formation was metamorphosed and altered, and the ore-forming materials activated, migrated and mineralized. The mineralizing fluid shows three mineralizing stages, with the characteristics of middle to low temperature (381-115°C) and middle to low mass fractions of NaCl (2.18%-16.77%). The fluid is of the Ca2+ -Mg2+ -Na+ -Cl− type and H2O-NaCl system. According to the data from hydrogen and oxygen isotopic composition (δD = −114.6%= −68.8%, δ18OH 2O = −2.47%−5.91%), the conclusion can be drawn that the mineralizing fluid of the Hongshijing gold deposit was a kind of mixed hydrothermal solution composed mainly of meteoric water, magmatic water and formation water in basalt and tuffaceous-sandstone or metamorphic water. The results show the mineralizing fluid and mineralizing materials came from the basalt and suffaceous-sandstone in the gold-bearing formation in Late Variscan orogeny. Under the action of brittle shearing, the gold-bearing formation was metamorphosed and altered, and the ore-forming material activated, migrated and mineralized withT andP going down and changes in pH, Eh,f O 2 andf S 2 under the earth’s surface from 1.57 to 2.25 km.