Fluid inclusion and He-Ar-Pb isotope studies of the Dabie-type Leimengou giant porphyry Mo deposit, Qinling Orogen

Abstract

The Leimengou deposit is a Dabie-type porphyry Mo system in the Leimengou-Qiyugou orefield of Qinling Orogen. Orebodies host within the porphyry and the metasedimentary Taihua Supergroup. However, the characteristics and sources of ore-forming fluid, metal precipitation mechanism, and the sources of mineralizing substances remain unclear. In this contribution, by a comprehensive study comprising an investigation of deposit geology, fluid inclusion microthermometry, and He-Ar-Pb isotope, we discuss the source and evolution of ore-forming fluids and the molybdenum precipitation mechanism of the Leimengou deposit. The mineralizing process is divided into the Stage I quartz ± K-feldspar vein in potassic altered zone, Stage IIa molybdenite ± quartz vein, and Stage IIb molybdenite + quartz + pyrite ± purple fluorite vein in silicification-phyllic altered zone, Stage III quartz + pyrite + sphalerite + galena ± chalcopyrite ± green fluorite ± pink calcite in prophylitic altered zone and Stage IV quartz ± calcite vein in carbonatization zone based on cross-relationship. The Stage IIa and IIb veins characterize the main molybdenum mineralization stages. By microthermometry, CO2-bearing (C-), Water (WV- and WL-), and daughter minerals-bearing (S-) types of inclusions were recognized. The Stage I quartz contains C-type fluid inclusion assemblages (FIAs) with a salinity of 6.3 ± 1.4 wt% NaCl equiv., and a total homogenization temperature (Th, total) of 473 ± 3 °C. This suggests that the early fluids show characteristics of high temperature, CO2-rich, and moderate salinity, distinguishing them from those of Endako-type and Climax-type deposits. C-type and coexisting W-type FIAs from Stage IIa show similar salinity, ranging from 1.9 to 13.0 wt% NaCl equiv. and 1.9 to 14.1 wt% NaCl equiv., with Th, total of 349–376 °C and 327–392 °C, respectively. Moreover, C-type FIAs homogenize to distinct phases and also show similar homogenization temperatures, indicating fluid immiscibility. The FIAs in Stage IIb show an average salinity of 8.3 ± 7.4 wt% NaCl equiv. and a Th, total of 302 ± 14 °C. WL-type and WV-type FIAs coexist, with similar temperatures varying from 308 to 310 °C during this stage, indicating significant fluid boiling. These observations indicate that the decrease in temperature, fluid immiscibility, and decompression boiling collectively led to the precipitation of molybdenite. Trapping pressures are estimated to be 82–231 MPa for Stage IIa and 43–115 MPa for Stage IIb, corresponding to ore-forming depths of approximately 8.4 km and 4.3 km, respectively. Additionally, the He-Ar isotopes obtained from pyrite indicate a clear mixing trend of crustal fluids with mantle fluids at Stage IIb, with mantle-derived fluid components increasing during Stage III. This coincides with the tectonic shift from regional compression to extension, leading to the formation of many Dabie-type Mo deposits, alongside crustal thinning and the upwelling of asthenospheric mantle materials. Futhermore, Pb isotopes from the ore, ore-hosted gneiss, and ore-causative granitoids demonstrate that the Taihua Supergroup gneiss supplied ore-forming materials, suggesting a crustal origin for the Leimengou Mo deposit.