Characteristics of ore-forming fluids and ore-forming model of the Longping fluorite deposit in southern Jiangxi Province

Abstract

The Longping large-scale fluorite deposit in Xingguo county, southern Jiangxi Province, is located in the Xingguo-Ningdu fluorite metallogenic belt on the west side of Dayu-Nancheng deep fault. Fluorite ore bodie are primarily in the silicified fracture zone of the outer contact zone of the Yongfeng complex in the form of plate, stratoid and vein. This paper divides the ore-forming stages of the deposit, and discusses the nature, source, evolution and ore-forming model of the oreforming fluid of the deposit through the study of fluid inclusions and H-O isotopes. According to the interpenetrating relationship of minerals and the characteristics of mineralogy, the Longping fluorite deposit is divided into early ore-forming stage (fluorite-quartz stage), main ore-forming stage (quartz-fluorite stage) and late ore-forming stage (fluoritecalcite stage). The types of inclusions are mainly liquid rich two-phase inclusions. Fluid inclusions in the fluorite and quartz show homogenization temperatures (111~374℃), salinities (0.53%~3.55% NaCleq), and densities (0.58~1.02 g/cm3). Raman spectroscopy showed that the fluid phase is dominated by wate. Generally, the ore-forming fluid belongs to NaCl-H2O fluid system, which is characterized by medium-low temperature, medium-low salinity and medium-low density. However, there exists fluid end-elements with high temperature and relatively high salinity in the early stage of mineralization (temperature higher than 300℃, salinity 2.0%~2.7%NaCleq). It is inferred that the magmatic hydrothermal fluids in the late stage of the Yongfeng complex participate in the early ore-forming stage and provides the heat and the source of F. The δDV-SMOW values of the fluid inclusions in the Longping fluorite ranged between -66.8% and -53.1% (average value is -59.5%), while the δ18OV-SMOW values ranged from 5.9% to -3.3% (average value is -4.6%). Collectively, the ore-forming fluid is dominated by meteoric water and geothermal water. Both the mixing magmatic hydrothermal fluid with meteoric water and water-rock reaction are the likely mechanisms of fluorite precipitation in early ore-forming stage, while fluid cooling is the main mechanism of fluorite precipitation in the main and late ore-forming stage. Therefore, it can be inferred that the Longping fluorite deposit is a fault-controlled, medium-low-temperature hydrothermal, fracture-filling type deposit.