Mineral paragenesis, fluid inclusions, H–O isotopes and ore-forming processes of the giant Dahutang W–Cu–Mo deposit, South China
Abstract The newly discovered giant Dahutang W–Cu–Mo deposit in the central part of the Jiangnan Orogen hosts an estimated resource of 1.1 million tons (Mt) of WO3, plus a proven resource of 0.65 Mt Cu and 0.08 Mt Mo. The Dahutang deposit is temporally and spatially associated with the Late Mesozoic S-type granites emplaced into the Neoproterozoic Jiuling granodiorite batholith and the Shuangqiaoshan Group. Based on petrographic observations, eight hydrothermal alteration/mineralization stages have been recognized, comprising pegmatitic (Stage I), potassic alteration (Stage II), albite alteration (Stage III), greisenization and main mineralization (Stage IV), polymetallic sulfide mineralization (Stage V), late scheelite mineralization (Stage VI), carbonate alteration (Stage VII) and supergene alteration (Stage VIII). Stage IV can be further divided into the early greisenization (Stage IV-A), the main wolframite mineralization (Stage IV-B) and the main scheelite mineralization (Stage IV-C). Fluid inclusion and H–O isotope analyses on nine Stage IV to VII ore/gangue minerals (wolframite, cassiterite, scheelite, sphalerite, fluorapatite, fluorite, quartz, calcite and chlorite) suggest the presence of an early W and a late Cu–Mo ore-forming fluid system. The early ore-forming fluids belong to a medium to high temperature (200°–420 °C), low salinity H2O–NaCl–CaCl2 system. This primary magmatic-hydrothermal fluid (δ18Ofluid = 5.4 to 8.8‰ and δD = −102 to −75‰) was likely exsolved from the Late Jurassic granites at Dahutang. Wolframite and scheelite coexist closely with fluorapatite and fluorite, respectively, indicating that the volatile-rich fluids at Dahutang were also rich in tungsten, fluorine and phosphorous. Tungsten was likely transported as tungstate species (e.g., WO42− and HWO4−), oxygen fluorine complexes (e.g., [WO2F4]2−, [WO3F2]2−) and phosphorous heteropolytungstate (e.g., [P(W12O40)]3−) in the fluids. Fluorapatite and wolframite may have precipitated first when the temperature dropped from 400° to 320 °C (along with pH increase) at an estimated depth of 7.8 km. This was likely followed by the extensive scheelite mineralization (with fluorite precipitation) that formed huge disseminated/veinlet-type W orebodies when the temperature further dropped to 200 °C (along with pH increase). After the W mineralization, extensive Cu–Mo polymetallic mineralization and the associated sericite-chlorite alterations may have formed by the granite porphyry and/or muscovite granite emplacement. The late ore-forming fluids belong to a medium to high temperature (200°–360 °C), low salinity H2O–NaCl–CaCl2 system. Molybdenum was likely transported mainly as Mo complexes (e.g., H2MoO4/MoO42−), and copper as Cu–Cl complexes. Molybdenite and chalcopyrite may have begun to precipitate when the fluids ascended and the temperature dropped to 330 °C. Meteoric water incursion may have then occurred (δ18Ofluid = 1.9–6.9‰ and δD = −99 to −68‰) and further cooled the fluid system to 250 °C, forming large Cu–Mo orebodies.