New evidence from the Wuliji'Oboo fluorite deposit for the role of the paleo-Pacific Plate and Mongol–Okhotsk suture in creating extensive fluorite mineralization in the Great Xing'an Range, NE China

The Wuliji'Oboo fluorite deposit is a newly discovered deposit hosted by volcanic rocks of the Early Cretaceous Baiyin'gaolao Formation, which are located in the Great Xing'an Range. Fluorite orebodies within this area are controlled by the NNE-trending faults. The ΣREE of fluorite samples ranges from 19.18 to 361.22 ppm, with an average of 131.14 ppm. The homogenization temperatures are concentrated in the ranges of 100–150°C and 230–350°C, while the salinity is concentrated in 0.3–1.0 and 2.0–4.5 wt.% NaCl eqv., and the density has ranges of 0.92–0.96 and 0.75–0.8 g/cm³, respectively. The δD_V-SMOW values of fluid inclusions within the fluorite range between −122.3‰ and −105.6‰ with a mean of −115.36‰. The δ¹⁸O_V-SMOW of fluorite ranges from −12.0 to −8.5‰, average is −9.9‰. Sm–Nd isotopic data on the fluorite ore yield an isochron age of 122 ± 15 Ma and have ɛ_Nd(t) values ranging from +3.15 to +4.16 and (⁸⁷Sr/⁸⁶Sr)_i values ranging from 0.704907 to 0.705765. In this contribution, field, petrologic, and geochemical data lead us to the following salient findings: (a) The ore consists of fluorite, quartz, and calcite, with host rock alteration that includes silicification, calcitization, and sericitization. The major element data reveal that the host rocks are a series of highK calc-alkaline rhyolite. The REEs suggest that F-rich fluids in this region might have experienced a long-distance migration. The Tb/Ca-Tb/La diagram confirms a hydrothermal origin for the deposit. (b) Petrographic evidence and fluid inclusions in the fluorites indicate that this deposit might have two ore-forming stages. The ore-forming fluids can be attributed to a NaCl-H₂O-F system with moderate-low temperature, low salinity, and low density and have a meteoric water source, with water/rock reaction as the main mechanism for fluorite precipitation. (c) The deposit was formed in an anorogenic within-plate extensional tectonic setting related to the lithospheric thinning occurring after the Mongolian–Okhotsk Ocean closure.