The Mineralization Of The Xianglushan Skarn W Deposit,jiangxi Province

The Xianglushan W deposit,in the Jiangnan porphyry-skarn W polymetallic belt,is a large tonnage skarn type deposit with a resource of 2200 Mt at about 0.641%WO3.The deposit comprises lenticular and stratiform-like orebodies,mainly along the contact between argillaceous limestone of the Cambrian Yangliugang Formation and a biotite granite pluton.The deposits in the Jiangnan porphyry-skarn W belt mainly formed at Late Jurassic,and contain W-Mo or W-Cu mineralization,but the Xianglushan W deposit formed at Early Cretaceous,and only contain W mineralization without Mo and Cu mineralization.The study of the Xianglushan W deposit will promote not only the researching of mineralization in the Jiangnan porphyry-skarn W belt but also the paragenesis regularity of W,Mo and Cu.Based on previous work,systematic research work about the ore deposit geology,alteration zoning,skarn mineralogy,properties and magma source of ore-forming rock were done,the diagenetic and metallogenic ages were determined.Then the metallogenic dynamic background,the ore-forming processes,source and evolution of ore-forming fluids are discussed,and the metallogenic model of the deposit is established.The similarities and differences of metallogenesis in the Jiangnan porphyry-skarn W polymetallic belt in different periods and their relations are discussed through comparison of Late Jurassic W-Mo,W-Cu deposits and Early Cretaceous W,Sn deposits.Specific improvement has been made as follows:(1)The mineralization stages of Xiangglushan W deposit can be divided into several stages including skarn stage,retrograde stage,quartz-sulfide-scheelite stage,and late stage calcite veins and fluorite veins.Major mineral assemblages of each stages are:garnet+pyroxene+scheelite+quartz+muscovite+pyrrhotite,scheelite+quartz+sericite+fluorite+pyrite,quartz+scheelite+mica+ sphalerite+pyrrhotite,calcite+fluorite.Electronic probe analyze of skarn mineral show that,pyroxenes in the deposit belong to diopside-hedenbergite series(Hd41.03%?88.72%Di1.71%?44.05%Jo1.38%?16.57%).There are three sages of garnet.Garnet in the pyroxene-garnet skarnare mainly grossular(Grs40.26%?80.63%Alm14.36%?37.38%Sps0.04%?7.47%),the middle stage garnets which were formed through hydrothermal alteration of early satge garnets are almandine-grossular-spessartite(Grs20.74%?27.63%Alm28.34%?42.85%Sps19.45%?41.11%),the late stage garnets which are in the late stage barren garnet skarn are mainly grossular(Grs66.48%?70.04%Alm11.43%?17.72%Sps2.69%?6.91%).Micas are mainly muscovite and phlogopite,composition of micas are different for different stages.Changes in the compositins of garnet and mica indicate that Fe and Mn contents in the fliuids increase first then decrease qwith the evolution of the fluids.(2)The granitic pluton and an aplitic dyke in the mining area contain zircon grains with U-Pb ages of 123.8±0.8 Ma and 117.3±1.7 Ma,respectively.Six molybdenite samples collected from skarn ores yielded a Re-Os weighted mean age of 125.5±0.7 Ma,and muscovite separates from greisen ores yielded a 40Ar/39Ar plateau age of 122.8±0.78 Ma.The molybdenite Re-Os and muscovite Ar-Ar ages are consistent with the zircon U-Pb age of the hosting granite,magmatism and mineralization in the Xianglushan deposit occurred in the Early Cretaceous.A compilation of precise ages for magmatism and mineralization in the Jiangnan porphyry-skarn W polymetallic belt shows that there are two stages of mineralization in the Jiangnan porphyry-skarn W polymetallic belt,they are W-Cu and W-Mo mineralization at 150-135 Ma and W,Sn,Mo mineralization at 130-120 Ma,respectively.(3)Biotite granite from the deposit is peraluminous high-K calc-alkali granite,and the genetic type is ? type,and it was formed in an reduced condition,crystallization fractionation played an important role in the formation of the pluton.?Nd(t)values of biotite granite range from-6.3 to-6.6,corresponding to two-stage model ages range from 1429Ma to 1455Ma.Pb isotope composition of the biotite granite is(206Pb/204Pb)i=18.084?18.259,(207Pb/204Pb)i=15.613?15.619,(208Pb/204Pb)i=38.244?38.535.Zrcons from biotite granite have ?Hf(t)range mainly from-3.1 to 5.7,corresponding to two-stage model ages mainly range from 1218 Ma to 1363 Ma.Conclusion can be drawn according to the data above,that the magma source of biotite granite is crustal,and the magma derived mainly from the partial melting of the Shuangqiaoshan group.(4)LA-ICP-MS in-situ micro analyses of trace and rare earth elements of scheelite from different mineralization stages show that REE characteristics of scheelite in greisen veins and retrograde rocks inherited some characteristics of the REE characteristics of the biotite granite,the ore-forming fluids may mainly from the magmatic hydrothermal.The major substitution mechanism of REE3+ for Ca2+ in scheelite is 3Ca2+=2REE3++?Ca.CL images of scheelites show that some scheelites have growth zoning,and variations of REE concentrations is one of the major reasons that caused the growth zoning of scheelites.Mineralogy and geochemical characteristics indicate that ore-forming fluids of the Xianglushan deposit is reduced,and fluids in the retrograde stage was more reduce than fluids in other stages.(5)The mineralization process of the Xianglushan W deposit was:subduction of the Pacific plate caused upwelling of the asthernosphere,and thus induced the interaction of the crust and the mantle,made the crust heated,and induced the partial melting of the upper crust material,then formed the fertile magma.The magma underwent crystallization fractionation in the process of emplacement,and the W-rich hydrothermal fluid was separated.Metasomatism and alteration between the hydrothermal fluids and impure limestone of Yangliugang Formation formed the Xianglushan W deposit.(6)Granite associated with the W,Sn,W-Mo and W-Cu deposits in the Jiangnan porphyry-skarn W polymetallic belt are belong to metaluminous-peraluminous high-k calc-alkaline 1 type or S type granite.Magma source of these granites are mainly Shuangqiaoshan Group or Shangxi Group.Contrast to the granite associated with the W-Mo or W-Cu deposits which may be contaminated by matter from mantle,the granite in Xianglushan W deposit hardly have been contaminated by matter from mantle and have a higher degree of differentiation.Crystal fractionation is unlikely the reason which caused the different Early Cretaceous ore metals of the W deposit and Sn deposit in northern Jiangxi Province.However,the Early Cretaceous different ore metals of Sn and W deposits may be caused by the difference of the magma source(different source rocks participate into the partial melting,different degrees of partial melting and partial melting temperatures)and ore-forming conditions(for example the oxygen fugacity et al.)which affected the partition and transportation of Sn in the magma-hydrothermal system.(7)Late Jurassic mineralization and Early Cretaceous mineralization in the Jiangnan porphyry-skarn W polymetallic belt were both related to the subduction of the Pacific plate,but the subduction mode was different.In the Late Jurassic,oblique subdction of the Pacific plate induced the W-Mo and W-Cu polymetallic mineralization at 150?135Ma.The subduction angle changed from oblique to near-paralle subdction(strike-slip)at about 135Ma,the regional tectonic environment changed from extrusion to extension,induced the Early Cretaceous(130-120Ma)W,Sn,Mo mineralization.