Geochronology and fluid inclusion studies of the Lailisigaoer and Lamasu porphyry–skarn Cu–Mo deposits in Northwestern Tianshan, China

Abstract

The Lailisigaoer porphyry Mo (Cu) and Lamasu porphyry–skarn Cu deposits are located in the Northwestern Tianshan Orogenic Belt, which is in the northwestern China. According to Re–Os analysis of molybdenite, the time of mineralization of the Lailisigaoer deposit is 377Ma, and SHRIMP zircon U–Pb analysis dates the ore-hosted intrusion of the Lamasu deposit to 378Ma. Five types of fluid inclusions (FIs) have been identified in both deposits: H2O-rich inclusions, CO2-bearing inclusions, halite-bearing inclusions, methane-rich inclusions and pure CO2 inclusions. In the Lailisigaoer deposit, the early-stage fluid of the quartz phenocryst belongs to a NaCl–H2O system with total homogenization temperatures (Th) ranging from 300 to 395°C with a mean salinity of 9.0wt.% NaCl. In the ore-forming stage, the fluid develops into a H2O–NaCl–CO2 system due to boiling and decarbonation. The Th values and salinities cluster between 220 and 370°C and between 0 and 33.0wt.% NaCl, respectively. The hydrothermal characteristics of the late quartz–calcite stage are similar to those of the meteoric fluid. In the Lamasu deposit, the early-stage fluid of the quartz phenocryst belongs to a NaCl–H2O system with Th values ranging from 245 to 459°C and a mean salinity of 6.0wt.% NaCl. In the ore-forming stage, due to the contact metamorphism between the hydrothermal fluid and wall rock, the fluid evolves to a NaCl–H2O–CH4 system. The Th values and salinities cluster between 220 and 360°C and between 0 and 17.3wt.% NaCl, respectively. The fluid of the late quartz–sulfide stage coincides with the meteoric fluid. In the mineralizing fluid of the Lailisigaoer deposit, the captured temperatures and pressures are between 210 and 350°C and between 170 and 600 bars, respectively, which are consistent with fluctuation between hydrostatic and lithostatic pressure corresponding to the estimated paleodepths of 1.7–2.2km. Boiling is the dominant mineralizing mechanism in the Lailisigaoer deposit, whereas the contact metamorphism between the magmatic hydrothermal fluid and country rocks is the dominant mechanism in the Lamasu deposit. Both deposits are formed in the continental arc setting related to the southward subduction of the North Tianshan Oceanic slab beneath the Yili-Central Tianshan block.