Geochemical Characteristics And Genesis Of Zhenxu Talc Deposit In The Southeastern Of Youjiang Fold Belt

The Zhenxu talc deposit,located in the secondary syncline of the north-eastern side of the box-shaped compound anticline of Daming mountain in the southeastern margin of Youjiang fold belt in central Guangxi,is a large scale deposit,whose talc reserves reaches21.73 million tons.However,the ore-formation resources and its genesis remain controversial as few researches have been completed.This study mainly focuses on the ore-formation spaces and the chareacteristics of talc ore-bodies in the deposit based on the integrated regional geological background data,our own detailed field geological investigation and comprehensive analysis of prior data of the deposit.The ore-controlling factors and genesis of ore deposit are further discussed by the method of element geochemic analysis of talc.The main achievements of this paper are as following:(1)The Zhenxu talc deposit is located at the Gudeng syncline,and ore-hosting rocks are composed of the thick layer dolomite of the Upper Devonian rongxian formation(D₃r),the thin-middle layer siliceous rock band and chert nodules microcrystalline limestone of the Lower Carboniferous yaoyunling formation(C₁y).The talc mineralization is closely related to the stratigraphic assemblage,and ore-bodies are mainly hosted in the core of the Gudengbao syncline,where crashed small folds and cracks are well developed and the ore bodies behaved as stratified,lenticular and irregular-shaped.The thick ore bodies generally formed near the turning point of the crumpled rock and in the fault cracks.(2)Ore-bodies can be divided into four types in color,including black,pink,white,and yellow.The black talc probably formed at first,composed as the main type of ore-bodies.The latterly formed pink talc usually were hosted in the black talc and surrounding rock fractures.The white talc is mostly found in the edge of ore body or near surface oxidation zone.A small amount of yellow talc can be seen in some fault zones.Ore quality is relatively high,including a few of calcite and a small amount of organic matter particles.(3)The total content of rare earth elements in talc is very low,and the geochemical characteristics of different ore-types talc are relatively similar.The distribution curves of rare earth elements standardized in chondrites are nearly horizontal or slightly to the right,and light rare earth elements are weakly enriched,with a large concave valley of Ce,suggesting a strong negative anomaly of Ce.The value of?Ce ranges from 0.31 to 1.04,with an average value of 0.44.The Y/Ho ratio ranges from 38.17 to 44.4,with an average of 41.82.(4)Comparison and analysis on the regional data show that the talc in the geochemical features of rare earth elements is significantly different with the magmatic rocks,sedimentary-type talc,and modern sea,but the distribution curve,the delta Ce values and the Y/Ho ratio are consistent with the southwest karst area river and dolomite in mining areas,suggesting that the ore-forming fluid should be most likely related to atmospheric precipitation.(5)Comprehensive analysis indicates that both siliceous rocks and magnesium-rich carbonate rocks could provide the silicon and magnesium materials which are essential for the talc mineralization.The weak alkaline hydrothermal solution,formed after the atmospheric precipitation flowing into the carbonate rocks formation through geothermal heating and water-rock interaction,can greatly enhances the dissolution capacity of amorphous silicon,resulting the dissolution of the silicon substances in the siliceous rocks and chert nodules in the carboniferous strata,forming silicon-rich hydrothermal fluids and then the precipitation and mineralization of talc by metasomatism reaction with magnesium in the Devonian dolomite,deposied at favorable spatial aggregation,such as the core of syncline,the fault fracture zone,and crumb cracks.