Hydrothermal muscovite geochemistry unravelling the ore-forming process of the Jinduicheng porphyry Mo deposit, East Qinling, China

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• Crystallisation temperature of hydrothermal muscovite is similar to ore-forming temperature at Jinduicheng. • High halogen fugacities promoted Mo transportation in the Jinduicheng deposit. • Magmatic and hydrothermal fluid differentiated is the main responsible for Mo precipitation. Molybdenum (Mo) is an energy metal that plays a crucial role in numerous sectors of the national economy. China has been the world’s largest supplier of Mo, with most hosted by porphyry deposits. The Jinduicheng porphyry deposit is an important porphyry Mo deposit in the East Qinling Orogen of central China, with a proven reserve of 1.03 Mt Mo. Although early studies related to the genesis of the deposit, the details of the hydrothermal ore-forming processes remain unclear. Muscovite, a typical rock-forming mineral, has been widely used to trace ore-forming physio-chemical conditions and hydrothermal evolution. With this in mind, we carried out in-situ major and trace element analysis of muscovite from the mineralised granite porphyry and andesite porphyry in the Jinduicheng Mo deposit to reveal the changes in ore-forming physico-chemical conditions and hydrothermal mineralising processes. The studied muscovite samples are hydrothermal (or secondary) type with crystallisation temperatures ∼ 152–364 °C for the altered granite porphyry and ∼ 182–246 °C for the altered andesite porphyry, in response to the ore-forming temperature 150–360 °C of the Jinduicheng deposit. The IV(F), IV(Cl), and IV(F/Cl) values of hydrothermal muscovite at the deposit range from 1.73 to 2.24, −4.02 to −2.12, and 3.67 to 5.88 for the altered granite porphyry, and from 1.58 to 1.78, −3.49 to −2.41, and 4.13 to 5.03 for the altered andesite porphyry, which indicate high F fugacity. During ore-forming processes, high oxygen and halogen fugacities promoted the formation and transportation of stable Cl - and hexavalent Mo complexes, which result in the enrichment of Mo. The Mo precipitation processes at Jinduicheng involve the progressive enrichment of Mo as magmatic and hydrothermal fluid differentiated. Higher oxygen and halogen fugacity favors the hexavalent state of Mo (H 2 MoO 4 , HMoO- 4, or MoO2- 4), which facilitated the formation of stable complexes with hexavalent Mo. Medium to high-temperature fluids, enriched in CO 2 , also contributed to Mo transport through complex anions such as CO2– 3 and HCO– 3. Subsequently, fluid-rock interactions resulted in the formation of K-feldspar-quartz-sulfide and quartz-sulfide veins, with tectonic changes affecting fluid equilibrium and promoting Mo-sulfide precipitation. The late stage of Mo mineralization, the mixing of hydrothermal fluids with meteoric water were added in the deposit, which significantly altered the mineralizing system’s physicochemical conditions, destabilized the fluids and facilitated Mo precipitation. This study also indicates that hydrothermal muscovite geochemistry is useful in clarifying the ore-forming process within hydrothermal systems.