Study On The Characteristics Of Ore-forming Fluids And Genesis Of Fluorite Deposits In Wujianfang-Xiabianqiang Area,Linxi,Inner Mongolia

The Linxi area,Inner Mongolia located in the polymetallic metallogenic belt of the southern Great Xing'an Range,is one of the important fluorite-concentrating areas in Northeast China.In this paper,three typical fluorite deposits?spots?in the Wujianfang-Xiabianqiang district of??Linxi are chosed as research objects.According to the systematic field geological investigation,the characteristics of fluid inclusions in different stages and types of ores in each deposit?spot?are studied by means of petrography,inclusion micro-thermometry and Raman spectroscopy for fluid inclusion analysis.This research aims to elaborately deciphering the nature and evolution of ore-forming fluids of regional fluorite deposits,and providing scientific insights into further discussion of the genesis of these deposits.The new findings in this study are include as follows:1)Three typical fluorite deposits?spots?in the Wujianfang-Xiabianqiang district mainly occur gas-liquid two-phase inclusions,with a small amount of pure gas-phase and pure liquid-phase inclusions,and the Wujianfang fluorite deposit have three-phase inclusions,CO₂-rich inclusions are not found.The results of the laser Raman test show that the inclusions are mainly H₂0,and the‘daughter mineral'of the three-phase inclusions of Wujianfang fluorite is calcite,which is inferred to be captured.2)The average homogenization temperature of the ore-forming fluids in the main ore-forming stage of the Wujianfang,the Xiabianqiang,and the Xingda fluorite deposits is 265?,190?,162?,respectively.The average salinity is 4.20%,0.74%,0.69%NaCl_eqv,and the average fluid density is 0.80 g/cm³,0.88 g/cm³,0.92 g/cm³,respectively.In general,the ore-forming fluids of fluorite deposits in the study area has obvious stage evolution trend,and the main ore-forming period is the medium-low temperature hydrothermal period.The ore-forming fluids are classified into the NaCl-H₂O fluid system with medium-low temperature,low-salinity,and low density.Based on the geological characteristics,ore characteristics,and homogenization temperatures and salinity characteristics of the area,it is believed that the type of fluorite deposits in this area belongs to the medium-low temperature hydrothermal vein-like fissure filling type.3)The results of the U-Pb age of the disseminated-fluorite-bearing fine-grained alkali feldspar granite show that it was emplaced at 140¹21 Ma,indicating the Wujianfang fluorite deposit was formed in the early Cretaceous.4)The widely distributed fluorite deposits in the southern Great Xing'an Range are genetically related to the tin-lead-zinc polymetallic deposits in this area.This paper concludes that:first,they are coeval in metallogenic ages;second,they have no strict specificity for the ore-hosting wall rocks;third,they have the following metallogenic zoning regularities:tin?tungsten?-copper?lead-zinc?-Lead-zinc?silver?-fluorite;fourth,their ore-forming materials are at least partly derived from highly differentiated granitoids.5)Under the background of extension-thinning in late Mesozoic,frequent and intense tectonic-magmatic events occurred in the Linxi area.In the late stage of magmatic crystallization,the residual magma was rich in components such as F and Si.Later,due to the immiscibility of the magmatic-hydrothermal fluids,the sepatred F-rich hydrothermal fluids continued to migrate upward along the fault fractures to the shallow crust,at the same time,atmospheric precipitation is continuously added.To the shallow surface,the F-rich fluid reacted with the surrounding rock,leading to massive precipitation of fluorite,forming fluorite deposits?spots?in this area.