Fluorite deposits in the Zhejiang Province, southeast China: The possible role of extension during the late stages in the subduction of the Paleo-Pacific oceanic plate, as indicated by the Gudongkeng fluorite deposit

Southeast China with its widespread occurrences of Mesozoic igneous rocks, is one of the largest magmatic provinces worldwide, containing a large variety of mineral types, including hypothermal W-Sn, porphyry Cu, epithermal Au deposits. The Gudongkeng deposit is an example consisting of fluorite mineralisation trending SE over a distance of about 6 km. The major minerals in the deposit include fluorite and calcite of a hydrothermal origin and interpreted as a vein-type deposit associated with the Linghou Granite that yields a zircon U–Pb date of 100 ± 1 Ma, but intriguingly the fluorite yields an Sm–Nd isochron age of 73 ± 3 Ma, placing the genesis of the fluorite as a post-magmatic event. Fluid inclusions in the fluorite have low homogenisation temperatures of 104–195° C, salinities of 0.5 to 2.24 wt% NaCl eqv., and densities of 0.67–0.94 g/cm³. The inclusions contain NaCl–H₂O–CaF₂ with δD_V-SMOW values between −64.3 and-50.1‰ and δ¹⁸O_V-SMOW values between −7.6 and −2‰. The initial ⁸⁷Sr/⁸⁶Sr ratios of the fluorite range from 0.709816 to 0.710165 are characteristic of a crustal contribution. The ore-forming fluids can be mainly attributed to a NaCl–H₂O–CaF₂ system of moderate to low temperature, low salinity, and low density. The ore-forming fluid has a meteoric water source, with the reaction between the fluid and wallrocks being the main mechanism for fluorite precipitation at relatively low temperatures. Gudongkeng is a low temperature epithermal deposit which was formed in an extensional setting within the Cathaysia Block during the Late Cretaceous.