Geology, geochronology, and geochemistry of the Gaojiabang tungsten-molybdenum deposit, Anhui Province, Southeast China

Abstract

The Jiangnan Mo-W-Pb-Zn metallogenic belt hosts more than fifteen W-Mo and Mo deposits and is located at the margin between the Middle-Lower Yangtze metallogenic belt and the South China Continent. The recently discovered Gaojiabang W-Mo deposit, Chizhou district, Anhui Province, is the one of the largest W-Mo deposits in the belt (25.4 Mt at a grade of 0.28% WO3, 0.11 g/t Mo, with about 10,000 t of that total at an averarge grade of 6.6 g/t Au). The deposit is associated with Jurassic granite and monzonite porphyries that intruded sedimentary rocks of the Cambrian Huangboling Formation. Alteration displays a zonation from granite porphyry to wall rock. Five distinct alteration zones are recognized: deep-seated K-feldspar-biotite-quartz alteration, a quartz-sericite alteration zone, garnet-diopside-epidote alteration zone, calcite-quartz alteration zone, and zone of thermal metamorphism (hornfels). These features are comparable with porphyry Cu-Au-Mo systems. The first three zones of alteration display a close spatial relationship with W-Mo ores. Three distinct styles of W-Mo mineralization are recognized: minor vein-type W-Mo mineralization within sedimentary units; skarn W-Mo mineralization at the contact between sedimentary rocks and granite porphyry; and disseminated Mo(±W) mineralization hosted within both porphyries. The main ore minerals are scheelite, molybdenite, pyrite, pyrrhotite, and minor chalcopyrite, and display a zoned distribution in which most molybdenite occurs inside deep porphyries and the majority of the scheelite is observed inside shallow skarns and hornfels. Based on the presence of redox-indicative mineral assemblages such as pyrrhotite, molybdenite and pyrite in the absence of sulfates, we consider Gaojiabang to be a relatively reduced W-Mo porphyry-skarn system. Our new U-Pb zircon ages confirm Late Jurassic ages for granite (150.4±1.1 Ma) and monzonite porphyries (149.4±1.6 Ma). A molybdenite Re-Os age of 149.5±1.4 Ma indicates that W-Mo mineralization is closely associated with the two porphyries. This new data makes Gaojiabang the oldest deposit recognised so far in the Middle-Lower Yangtze River area, in turn inferring that the westward subduction of the Pacific Plate commenced at ∼150 Ma. Geochemical data indicate that mineralization in the Gaojiabang deposit derives from multiple sources, dominantly from magmatic rocks and basement strata via mantle-crust interaction, with the single greatest involvment of crustal materials among all deposits in the Middle-Lower Yangtze River area. Sulfides, including pyrite, pyrrhotite and molybdenite, display a wide range of δ34S between +3.8‰ to +12.1‰. The gradual increase of δ34S from inner porphyry to outer wall rock indicates contamination from crustal marine sedimentary facies as the ore-forming fluids evolved. Following the W-Mo mineralization event at Gaojiabang, emplacement of Qingyang-Jiuhua Complex at 144–127 Ma resulted in a previously unrecognized ductile deformation of porphyries, sedimentary rocks, and ores.