Fluid inclusion geochemistry and magmatic oxygen fugacity of the Wenquan Triassic molybdenum deposit in the Western Qinling Orogen, China

Abstract

The Wenquan Mo deposit in Gansu Province, China, is one of the large porphyry Mo deposits in the Western Qinling Orogen that associated with the Triassic continental collision between the North China Block and South China Block. In this study, detailed fluid inclusions (FIs) analyses were performed on early stage quartz–biotite–K-feldspar veins, middle stage quartz–polymetallic sulfide veins and late stage sulfide–calcite veins. Four compositional types of FIs are observed, including pure CO2 (PC-type), CO2-H2O (C-type), H2O-NaCl (W-type), and daughter mineral-bearing polyphase (S-type, with no halite daughter mineral). The early stage quartz contains all the four types, which are mainly homogenized between 285 °C and 495 °C, with salinities ranging from 4.2 to 17.6 wt% NaCl eqv. However, in the middle stage, PC-type FIs are not observed, and S-, W- and C-types FIs yield homogeneous temperatures (Th) of 200–397 °C and salinities of 2.4–13.1 wt% NaCl eqv. Late stage quartz contains only W-type FIs with Th of 102–246 °C and salinities of 1.2–10.1 wt% NaCl eqv. Results indicate that the fluid-system evolved from an initial CO2-rich magmatic-hydrothermal fluid to a late CO2-poor meteoric fluid. In the early and middle stages, it is possible that heterogeneous FIs associations may have been trapped from a fluid immiscibility system. The dramatically decreasing percentage of PC- and C-types FIs in the ore stage indicates that CO2 escaped, which is possibly the result of earlier fluid immiscibility and phase separation. In addition, there are double peaks of Th in this stage that are obviously lower than in the early stage, showing that meteoric water flowed into and mixed with magmatic fluids, thereby cooling the hydrothermal system. These results suggest that metal precipitation resulted from fluid immiscibility, CO2 escape, and inflow of meteoric water occurred in the middle stage. The initial fluids of the Wenquan deposit are characterized by relatively high temperature, high salinity, high fO2, and are CO2-rich and NaCl-poor, indicating that the Wenquan deposit is a collisional-type porphyry Mo deposit.The oxygen fugacity (fO2) constrained by trace element compositions of zircons from the Wenquan Mo-bearing porphyry is uneven with ΔFMQ −10.3–+4.68 (average of −4.32), which could result from the input of mafic magma and its inhomogeneous mixing with felsic magma. Such values are still much higher than those of the adjacent contemporaneous Mo-barren Lujing, Baijiazhuang, Jiaochangba, Luchuba, and Zhongchuan granitic plutons, which confirms the affinity between Mo mineralization and the oxidized melt system and implies that the Wenquan porphyry has relatively high fO2 characteristics for its tectonic location and epoch. Fluid inclusion, initial mineralization depth (∼8.6 km), magma fO2, and the zircon high U/Yb ratio of the Wenquan Mo deposit strongly suggest that Mo mineralization was related to continental collision, rather than oceanic crust subduction. This study therefore proposes that the Wenquan Mo deposit was formed in a rapid crust uplifting setting caused by syn-orogenic crustal thickening followed by post-orogenic delamination of the lithospheric root during the Late Triassic.