Implications for the contribution of Pacific plate subduction to fluorite mineralization in southeast China: Evidence from Nanzhou large fluorite deposit, Fujian province

Abstract

In SE China, there are numerous Mesozoic igneous rocks and a tectonically active area known as the Wuyi Fluorite Mineralization Belt. The Nanzhou fluorite deposit can effectively represent most hydrothermally filled fluorite deposits strictly controlled by the fault zones. These were mainly trending NE, NNE, followed by NW, and nearly EW. It is the epitome of fluorite mineralization in the Wuyi fluorite metallogenic belt, which can provide clues for the subduction of the Mesozoic Pacific plate to the Eurasian plate. As the surrounding rocks of the Nanzhou fluorite ore bodies, the zircon U-Pb isotopic age of the Anjiang syenogranite is 147±1 Ma, while the biotite plagioclase granulite of the Xiafeng Formation in the Wanquan group is 817±3 Ma and 811±3 Ma. However, fluorite is of hydrothermal origin. It yields an Sm-Nd isochron age of 103.8±5.0 Ma and an Rb-Sr isochron age of 106±15.0 Ma, more than c.a. 40 Ma younger than the Anjiang Syenogranite, indicating fluorite is a product of late hydrothermal mineralization. The right-leaning REE distribution curves of most fluorite samples are similar to those of Yanshanian Anjiang syenogranite and biotite plagioclase granulite of the Xiafeng Formation, which indicates that fluorite is closely related to the surrounding rocks. Most fluid inclusions in fluorite have salinities of 0.5–2.24 wt% NaCl eqv and low homogenization temperatures of 140–200 °C. The inclusions include a NaCl-H2O system with δDV-SMOW between −79.7 to −77.6‰ and δ18OV-SMOW values range from 0.4 to 2.1‰. The (87Sr/86Sr)i ratios of the fluorite, which are between 0.717427 and 0.719482, are characteristic of a crustal contribution. Based on the above description, the mineralized fluid of Nanzhou fluorite deposit belongs to the NaCl-H2O system with medium-low temperature and low salinity, which is obviously different from the magmatic hydrothermal fluid. In addition, the age of fluorite mineralization is younger than that of igneous rocks by ca. 40 Ma, so they are not correlated. It is proposed that the ore-forming fluid may be involved in atmospheric precipitation and circulate within the northeastward fault with increasing geothermal gradient to form Nanzhou-fluorite deposit. The Nanzhou-fluorite deposit was formed in 103.8±5.0 Ma and 106±15.0 Ma, which corresponds to the plate subduction and rollback period of the Late Cretaceous (in ca. 110–70 Ma). The formation of the Nanzhou deposit has important implications for the subduction of the Pacific plate to the Eurasian plate.