Composition and evolution of ore-forming fluids in the Sansheng porphyry W-Mo deposit, Inner Mongolia, NE China: Evidence from LA-ICP-MS analysis of fluid inclusions

Abstract

Unveiling the composition and evolution of ore-forming fluids in porphyry W-Mo mineralization systems is key to understanding their formation processes, but detailed quantitative studies are still limited. The Sansheng porphyry W-Mo deposit (17,285 t WO3 @ 0.569 % and 24,361 t Mo @ 0.226 %), located in southern Great Xing’an Range W belt, NE China, is characterized by quartz vein- and veinlet-disseminated-type W-Mo ore bodies mainly hosted in the cupolas of Early Cretaceous granitic intrusion. In this study, we provide an elaborate study that integrates detailed petrographic, laser Raman, microthermometric, and LA-ICP-MS analyses of fluid inclusions (FIs) to reconstruct the fluid evolution history of the Sansheng W-Mo mineralization system and to bridge the aforementioned knowledge gap. Four stages of hydrothermal veins are identified: stage 1 is the dominant stage of W mineralization, and characterized by the development of wolframite, quartz, K-feldspar, muscovite, and minor amounts of molybdenite; stage 2 is the main stage of Mo mineralization, and featured with the mineral assemblages of molybdenite, quartz, sericite together with minor amounts of pyrite; stage 3 involves Cu-Pb-Zn mineralization, and is characterized by the occurrence of polymetallic sulfides, quartz, and sericite; and stage 4 is dominated by barren carbonate-quartz veins. In wolframite and quartz, the FIs are classified as liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and pure vapor mono-phase FIs (PV-type) in types. Microthermometric results reveal decreasing homogenization temperatures in primary FIs from stages 1 to 4. Stage 1 fluids are characterized by high homogenization temperatures (320–420 ℃), moderate salinities (7.9–14.8 wt% NaCl equiv.), highly variable W contents (17–7251 ppm) and constant Mo contents (22–288 ppm). FIs in wolframite and coexisting quartz share similar homogenization temperatures and salinities, suggesting that they were formed concurrently. Stage 2 fluids have similar salinities (7.0–13.9 wt% NaCl equiv.) to stage 1 fluids but lower homogenization temperatures (250–340 ℃), and contain highly variable Mo contents (21–11176 ppm) and low W contents (5–23 ppm). Stage 3 fluids display moderate homogenization temperatures (220–300 ℃) and low salinities (3.1–10.1 wt% NaCl equiv.). The contents of W and Mo in stage 3 fluids decrease dramatically, marking the termination of W-Mo mineralization. Stage 4 fluids are characterized by moderate homogenization temperatures (200–270 ℃), low salinities (0.2–5.0 wt% NaCl equiv.), and the lowest contents of W (1–4 ppm) and Mo (8–16 ppm). All stage fluids contain high concentrations of B and Mn, and high ratios of Li/Na, K/Na, Rb/Na, Ba/Na, Pb/Na, and Zn/Na, similar to magmatic-hydrothermal fluids in composition and against typical basinal brines. In addition, they also have high Rb/Sr and low K/Rb ratios, which are equivalent to those of the Sansheng ore-bearing granite. In combination with the constant Cs/(Na + K) and Cs/Rb ratios of all stage FIs, we propose that the multistage hydrothermal fluids originated from a common source–the highly evolved and geochemically uniform Sansheng granitic magma, and that the derivation of ore-forming fluids was a continuous fluid exsolution process. Fluid boiling and fluid-rock interaction are collectively responsible for the deposition of wolframite and molybdenite, while the role of fluid cooling may be subordinate. The meteoric water began to enter the hydrothermal system in stage 3, and fluid cooling and dilution due to the fluid mixing process played a critical role in the deposition of Cu-Pb-Zn sulfides. Our findings highlight the significance of in-situ FI analyses in revealing the composition and evolution of multistage fluids in porphyry W-Mo deposits, which is critical for elucidating detailed ore-forming processes of intrusion-related W-Mo mineralization systems in NE China and globally.