Mineralogical and metasomatic evolution of the Jurassic Baoshan scheelite skarn deposit, Nanling, South China

Abstract

The Baoshan scheelite deposit, located in the Nanling region, South China, is a skarn-type tungsten ore deposit. Orebodies occur in the contact zone between Carboniferous limestone and Baoshan granitic pluton. The pluton consists mostly of coarse-grained, porphyritic, and fine-grained biotite granites, which have zircon U-Pb ages of 172.4 ± 0.3 Ma, 166.6 ± 0.3 Ma, and 156.6 ± 1.4 Ma, respectively. These rocks have similar geochemical characteristics with high SiO2, total alkalis, high differentiation indexes and high Rb/Sr ratios of S-type granites formed by partial melting of upper continental crust. The porphyritic granites display the highest degree of fractionation, and are most closely associated with the formation of skarn ores.The skarns are zoned mineralogically outward from garnet, through clinopyroxene, to wollastonite, and overprinted by retrograde minerals. Three stages of skarnization are recognized: Early prograde, late prograde and retrograde. The early prograde stage is mainly composed of Al-rich garnet (Grs60–76) and Mg-rich clinopyroxene (Di41–62) with subordinate Mo-rich scheelite (Sch59–86), all of which are typically overgrown by late prograde assemblages of Fe-rich garnet (And67–95), Fe-rich clinopyroxene (Hd81–87), and Mo-depleted scheelite (Sch97–98). The retrograde stage is characterized by Al-rich garnet (Grs40–66) and pure scheelite (Sch100), accompanied by hydrous and sulfide minerals. Minor subcalcic garnet with magnetite sometimes occurs in the limited endoskarns. Fluid inclusions of the early prograde minerals homogenize at 390.5 to >550 °C, and one inclusion in wollastonite has a salinity of 13.0 equiv. wt% NaCl. Inclusions in minerals of the late prograde and retrograde stages have homogenization temperatures of 196.9–377.4 °C and 160.4–291.4 °C, respectively; their salinities 2.4–19.0 equiv. wt% NaCl and 1.4–11.6 equiv. wt% NaCl, respectively. Accordingly, the high-temperature and moderate- to high-salinity fluids in the early skarn stages are typical of magmatic fluids originating granites, whereas the lower temperature and variable salinity in the later stages suggest mixing with meteoric water.