Differentiated enrichment of magnetite in the Jurassic W–Sn and Cu skarn deposits in the Nanling Range (South China) and their ore-forming processes: An example from the Huangshaping deposit

Abstract

Abundant Jurassic magnetite-rich W–Sn and magnetite-poor Cu skarn deposits were discovered in the Nanling Range of South China. However, their ore-forming processes and the mechanism of differentiated magnetite enrichment are still poorly constrained. The Huangshaping represents a typical skarn deposit in the Nanling Range, which developed both W–Mo–Sn and Cu mineralization systems, providing a good window to address this issue. In the Huangshaping W–Mo–Sn system, abundant magnetite was formed in three generations (Mag-1, Mag-2, and Mag-3). Mag-1 shows core (Mag-1c)-mantle (Mag-1 m)-rim (Mag-1r) texture, with abundant nano-inclusions trapped in the oscillatory-zoned Mag-1 m. Porous Mag-2 replaced Mag-1 along sharp and irregular boundaries, and both Mag-1 and Mag-2 were replaced by cassiterite. Mag-3 occurred along with quartz-bismuthinite veins and also replaced Mag-1. Magnetite-4 (Mag-4) has a homogeneous texture and occurs locally in the Huangshaping Cu system. The mineral assemblage, texture, and geochemistry suggest a hydrothermal origin for all these magnetite types and possible magmatic-hydrothermal pulses in the W-Mo-Sn system. The first magmatic-hydrothermal pulse was likely associated with granite porphyry and formed Mag-1 in a closed system with increasing oxygen fugacity (fO2) and decreasing concentrations of trace elements. The abundant nano-inclusions in Mag-1 m suggest probable local supersaturation of trace elements in the magnetite-fluid interface. The irregular contact, porosities, and lower trace element concentrations (e.g., Mg, Al, Si, Ca, and Mn) of Mag-2 reveal that it may have formed via the dissolution and reprecipitation process (DRP) of Mag-1. This process was possibly controlled by decreasing fO2. The second magmatic-hydrothermal pulse derived from the granite porphyry may have formed Mag-3, evidenced by the increasing temperature, decreasing fO2, and high trace element concentrations (e.g., Mg, Al, Si, Ca, and Mn). In the Cu system, the quartz porphyry-derived ore fluid may have formed Mag-4 under lower temperature and fO2 than in the W–Mo–Sn system. Besides, we suggested that the paucity of magnetite in these Jurassic Cu skarn deposits of the Nanling Range was likely caused by the low Fe concentration in the ore-related magma, genetically associated with the amphibole fractionation during the magma formation. This may respond to the extensional formation environment of Jurassic Cu and W–Sn skarn deposits in the Nanling Range, caused by the asthenosphere upwelling linked to the Paleo-Pacific subduction.