Fluid inclusion evidence for the magmatic-hydrothermal evolution of closely linked porphyry Au, porphyry Mo, and barren systems, East Qinling, China

Abstract

The Xiong’ershan district in central China hosts broadly coeval porphyry Au (Qiyugou deposit), porphyry Mo (Leimengou deposit), and barren (Huashan pluton) systems. The key controls on the ore potential and different mineralization styles in these systems are not well understood, with first-order differences in fluid chemistry and melt sources being the main alternatives. The fluid inclusion characteristics of all three porphyry systems have been studied using an integrated approach that combines field geology, petrography, microthermometry, and laser ablation−inductively coupled plasma−mass spectrometry analysis of single fluid inclusions. The results permit a reconstruction of the magmatic-hydrothermal evolution of the ore-forming fluids, and to elucidate whether specialized hydrothermal fluids strongly enriched in ore metals (i.e., Mo, Au, Cu) were essential to form the economically significant deposits. The fluid compositions across the three hydrothermal stages from the Qiyugou Au deposit remain approximately the same over time, suggesting that progressive magma fractionation, fluid-rock reaction along fluid path, and mineral precipitation had a limited effect on fluid composition. The syn-ore stage fluids of the Leimengou Mo deposit are characterized by higher Cs/Na, Sr/Na, and B/Na, but lower K/Na and Cl/Na ratios, and also have salinities and homogenization temperatures distinct from the earlier fluids. This demonstrates that Mo mineralization was caused by a second pulse of fluid input from a highly fractionated felsic magma subsequent to the pre-ore stage. At the Huashan barren pluton, fluids from phase II have higher Cs/Na, B/Na, Li/Na, and Rb/Na ratios with lower homogenization temperatures than fluids occurring in porphyritic rocks of phase III, reflecting a higher degree of magma fractionation of this plutonic complex. The Huashan pluton does not host economic mineralization which is likely caused by the low ore metal tenor, inefficient fluid extraction from the melt, or the flat-roof geometry preventing accumulation of a large volume of fluid in the apical part. The Au tenor of the Qiyugou deposit was most likely contributed by mantle-derived material of higher Mg/Na, Fe/Na, Pb/Na, and Zn/Na ratios. Taken together, the metal charged magmatic-hydrothermal fluids, steeply dipping geometry, and small volume of the porphyry stocks all suggest that a much larger magma chamber feeding the porphyry systems should be present at deeper levels with good potential for Mo mineralization below the current level of exposure at Qiyugou deposit.

Geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of the Late Jurassic granitoids from the Shuangyuangou deposit, South Qinling, China: Implications for petrogenesis and porphyry Cu metallogeny

“…The formation of these deposits are mostly correlated to convergent margins settings (e.g., Cao et al, 2023; Santosh & Groves, 2022), where magmatic arcs (island arcs and continental arcs) provide the significant environment for the formation of giant porphyry Cu deposits (PCDs) (Cooke et al, 2005; Hou et al, 2020; Richards, 2003; Sillitoe, 2019). However, continental collision zones and intra‐continental settings are also important settings for porphyry mineralization (Hou et al, 2020; Hou & Yang, 2009; Richards, 2009; Wang, Liu, et al, 2021; Wang, Luo, et al, 2021; Yang, Lu, et al, 2015), such as large PCDs produced in non‐arc settings in Gangdese belt and East Qinling metallogenic belt (Hou et al, 2020; Tang et al, 2021; Tang, Wagner, et al, 2022). The PCDs in island arc setting are generally formed by magmas originated from partial melting of the mantle wedge that was metasomatized by dehydration of the subducted oceanic crust (Cooke et al, 2005; Richards, 2003; Sillitoe, 2010), whereas those in the non‐arc settings are mainly related to the remelting of the juvenile lower crust caused by the upwelling of the mantle magma (Chen et al, 2023; Hou et al, 2013, 2020; Xiong et al, 2019).…”

“…and intra-continental settings are also important settings for porphyry mineralization (Hou et al, 2020;Hou & Yang, 2009;Richards, 2009;Wang, Luo, et al, 2021;Yang, Lu, et al, 2015), such as large PCDs produced in non-arc settings in Gangdese belt and East Qinling metallogenic belt (Hou et al, 2020;Tang et al, 2021;Tang, Wagner, et al, 2022). The PCDs in island arc setting are generally formed by magmas originated from partial melting of the mantle wedge that was metasomatized by dehydration of the subducted oceanic crust (Cooke et al, 2005;Richards, 2003;Sillitoe, 2010), whereas those in the non-arc settings are mainly related to the remelting of the juvenile lower crust caused by the upwelling of the mantle magma (Chen et al, 2023;Hou et al, 2013Hou et al, , 2020Xiong et al, 2019).…”