Evolution Of Ore-forming Fluids In Shimensi Tungsten Polymetallic Deposit Of Northern Jiangxi:Constraints From In Situ Trace Element Analysis Of Mineral

Located in the eastern of the Jiangnan Orogenic Belt on the southeast margin of the Yangtze Plate,the world-class Shimensi tungsten polymetallic deposit is in the northern part of the Dahutang orefield,northern Jiangxi.The main mineralization type of the Shimensi deposit is veinlet disseminated scheelite,followed by cryptoexplosive breccia type tungsten(copper and molybdenum),quartz vein type worframite,and locally vulcanite type.Predecessors have reached a relatively consistent understanding of the geological characteristics,diagenesis and mineralization age of the deposit,but there are still controversies in the evolution process of ore-forming fluids.Therefore,this paper takes quartz vein type ore bodies as the research object,and discusses the source,nature and evolution process of ore-forming fluids of quartz vein type ore bodies through field geological exploration,scheelite,wolframite and quartz micro-area analysis,and fluid inclusion research.The main results are as following:1.Quartz vein type mineralization has early and late phases of mineralization.Quartz veinlets and middle-fine veins formed earlier,among which scheelite and wolframite,biotite and quartz coexisted,while the quartz thick veins formed relatively later,and scheelite only coexisted with quartz.2.The characteristics of fluid inclusions and the analysis of quartz micro-area show that the temperature of ore-forming fluids in quartz veinlets(179.7?349.5?),quartz middle-fine veins(178.5?368.5?)to quartz thick veins(122.6?308.7?)gradually decreases.It belongs to the Na Cl-H₂O-CH₄±(N₂-CO₂)system with low salinity(1.40?9.08%)and low density(0.56?0.95 g/cm³).The changes in the physicochemical conditions of the ore-forming fluids in the quartz veins at different stages are an important reason for the enrichment and precipitation of tungsten.3.During the evolution of ore-forming fluids,the quartz veinlets and middle-fine veins with low-p H,reductive ore-forming fluids evolved into high-p H and oxidizing fluids in quartz thick veins.It is acco MPanied by the decrease of salinity,pressure,REE,Mo,Sn,Nb,Ta and F contents of ore-forming fluid,and the increase of Sr and HCO₃^-content.The decrease of F during fluid evolution implies the mineralization and precipitation of WO₄^2-.4.Scheelite micro-area analysis and previous studies on the isotopic composition suggest that the ore-forming fluid originated from granite magma,and a small amount of atmospheric precipitation,biotite granodiorite-derived fluids were added in the late stage of fluid evolution.After the precipitation of tungsten minerals in quartz veinlets and middle-fine veins,the crystallization of fluorite,molybdenite and the reaction of magmatic hydrothermal fluid with surrounding rocks have an important influence on the composition of the late ore-forming fluids.