Fluid inclusions in orogenic gold deposits in the northern Qaidam margin-East Kunlun region

Abstract：More than 20 orogenic gold mineral occurrences have been discovered and explored in the northern Qaidam margin-East Kunlun region. They are the product of the late Caledonian and late Variscan-Indosinian multiple collisional orogeny. Based on a study of fluid inclusions in 12 gold deposits in the region, two kinds of ore-forming fluids are recognized in these orogenic gold deposits: low-salinity H2O-CO2-NaCl-CH4 fluids and low-salinity H2O-CO2-NaCl±CH4 fluids. The former, whose XCH4, XCO2 and XH2O values are 0.14-0.34 (mean 0.24), 0.11-0.59 (mean 0.34) and 0.64-0.31 (mean 0.42) respectively, temperatures range from180 to 270℃ and pressures range from 180 to 560 MPa, are the product of late Caledonian collisional orogeny and mainly flowed along the ductile deformation zone between the base of the upper crust and the upper part of the middle crust below the boundary of the Caledonian collisional orogenic belt and formed extensive gold mineralizations in the region. For the latter, the values of XCH4, XCO2 and XH2O are 0-0.12 (mean 0.06), 0.18-0.25 (mean 0.21) and 0.79-0.69 (mean 0.73), the temperatures range from 280 to 449℃ (mostly 280-360℃) and the pressures range from 80 to 230 MPa. They were mainly related to late Variscan-Indosinian collisional orogeny and influenced by magmatic intrusion in some deposits. The fluids flowed extensively along the upper crust below the boundary of the late Variscan-Indosinian collisional orogenic belt, thus resulting in the final location of orogenic gold deposits in the region. The salinities of the two kinds of fluids are approximately consistent, generally varying from 1.4 to 11.4 wt% NaCl equiv and clustering at 2.7-9.1 wt% NaClequiv. The results of pressure-depth conversion of the ore-forming fluids indicate a process of strong crustal uplift during the late Variscan-Indosinian collisional orogeny in the region, with the maximum uplift magnitude up to 12 km. This shows that the orogenic gold mineralizations in the region took place in a background of crustal uplift.