Hydrothermal rutile as a pathfinder for discriminating overprinted magmatic-hydrothermal gold and molybdenum mineralization: The giant Laowan Au deposit, Tongbai orogen, Central China

The Laowan deposit is a recently discovered large lode-gold deposit with a Au reserve >208 t in the Tongbai Orogen of Central China. This Au deposit is characterized by the co-occurrence of stockwork-type Mo mineralization. Hydrothermal rutiles are present both in the altered rock-type gold ores and in the alteration halos around the fluorite-K-feldspar-quartz Mo veins. A detailed investigation of the mineralogical, textural, and chemical analyses (EPMA and LA–ICP–MS) of the Au- and Mo-related rutiles was carried out in this study. Au-related rutiles formed during silicification and sericitization, intergrown with a hydrothermal mineral assemblage of pyrite + chalcopyrite + electrum + sericite + quartz + calcite + REEPO₄ minerals, whereas Mo-related rutiles formed during sericitization and silicification closely associated with sericite + quartz + apatite + calcite + molybdenite + other Fe-Cu sulfides. Both rutiles formed via hydrothermal breakdown of metamorphic titanites in the host rocks of garnet-epidot-amphoble–quartz schist. The formation mechanism involves coupled dissolution-reprecipitation of CaTiSiO₅ (titanite) + CO₂-rich fluid = TiO₂ (rutile) + CaCO₃ (calcite) + SiO₂. The textural features show that most of the Au-related rutiles nucleate and grow at the original textural position of the precursor titanites, suggesting a mechanism of coupled dissolution and in situ precipitation due to relatively low mobility of Ti. The Mo-related rutiles grow in a microenvironment locally enriched in Ti due to the small-scale mobilization of Ti released during titanite dissolution, suggesting a relatively high solubility of Ti. These results indicate that the Mo-mineralized fluid system probably contains more halogens (particularly F) than the Au-mineralized fluid system does because TiO₂ solubility and mobilization increase with increasing halogen concentration in the fluid system. The Au-related rutiles are characterized by enrichment of W, Nb, and Ta, whereas the Mo-related rutiles are characterized by enrichment of Sn, W, and Nb. The mass balance estimation indicated that the Au-related rutiles retained most of the trace elements released from the breakdown of titanites and significantly incorporated W and, to a lesser extent, Nb and Ta from the hydrothermal fluid during rutile precipitation. The Mo-related rutiles incorporated significant amounts of Sn, W, Nb, and, to a lesser extent, Th and Pb from the hydrothermal fluid and Sc, V and Cr from other mafic minerals, such as amphibole, during hydrothermal alteration. The compositional fingerprints of hydrothermal rutiles indicate that the Mo-mineralized fluid contained Sn, W and Nb and more halogen (particularly F), which are distinguished from the Au ore-forming fluid, which contained significant amounts of W but less Nb and Ta and lower amounts of halogen. The Au- and Mo-related rutiles formed from two different fluid systems during different periods (∼140 Ma and ∼130 Ma), recording the overprinting of two separate magmatic-hydrothermal fluid events in the same deposit.