Spectral Characteristics, Rare Earth Elements, and Ore-Forming Fluid Constrains on the Origin of Fluorite Deposit in Nanlishu, Jilin Province, China

Abstract

Nanlishu fluorite has had an annual output of nearly 10,000 tons since its discovery and mining in 1987. At present, few studies have been undertaken on the genesis of the Nanlishu fluorite deposit. Microscopy, Fourier transform infrared (FTIR) spectroscopy, Raman microprobe spectroscopy, and ultraviolet–visible (UV–Vis) spectroscopy were used to study the mineral composition, mineral structure, and spectral characteristics, and fluid inclusion and rare earth element (REE) geochemistry studies were conducted to discuss the characteristics and evolution of the ore-forming fluids and the mineralization process. The Nanlishu fluorite ore rock is composed mainly of fluorite, with minor amounts of quartz, pyrite, and apatite, and trace amounts of calcite. The purple in the fluorite is caused by the Ca colloids. The homogenization temperatures of fluid inclusions in Nanlishu fluorite vary from 116 to 179 °C, while the salinity ranges from 0.35 to 4.03 wt% NaCl equiv, and the density varies from 0.90 to 1.00 g/cm3. Ore-forming fluids were derived from heated meteoric water characterized by low temperature, low salinity, and low density. The total rare earth element (∑REE, excluding Y) content of the ore ranged from 8.072 to 27.230 ppm. The light/heavy REE ratio (LREE/HREE) varies from 0.971 to 1.513. The δEu value is 1.010–1.074 and the δCe value is 0.893–1.048. Compared with other deposits, the Nanlishu fluorite has lower ∑REE content, no obvious enrichment of LREEs or HREEs, and no anomalies in δEu and δCe. Water/rock reaction is the main mechanism for Nanlishu fluorite precipitation. The Nanlishu fluorite is a fault-controlled, low-temperature hydrothermal, fracture-filling deposit.