Metallogenesis Of The Bijiashan Antimony Orefield In Weishan County,Western Yunnan Province

The Bijiashan Sb orefield in the Weishan County,Yunnan Province,is located in the middle section of the Lanping-Simao basin in Sanjiang Orogeny,which belongs to the Weishan-Yongping polymetallic ore belt.The ore bodies are lamellated,lamellated-like and lentoid and formed in the interformational fracture between the limestone of Triassic Sanhedong Formation and shale of Triassic Waluba Formation,which are strictly controlled by the strata occurrence.Ore-controlling structures are mainly Heihuijiang anticline and NW orientation faults.Ore is divided into primary ore and oxidized ore.The hydrothermal ore-forming process can be divided into three stages:stage?is characterized by the fluorite-stibnite,with minor fluorite,pyrite and galena;stage?is characterized by the stibnite-fluorite-quartz,with minor calcite,the first two stage is the main stages of ore-forming;stage?is characterized by the fluorite-quartz-carbonates,with no mineralization.Romeite and cerussite are widely formed in supergene process.Wallrocks?shale and mudstone?,ores,siliceous rocks have?REE respectively of197.5²28.5ppm,10.1²2.7ppm?avg16.8ppm?,1.4³3.0ppm?avg12.1ppm?.Fresh wallrocks have the amount of Sb up to 64ppm and 165ppm,which are far higher than the average amount in crust?0.2ppm?and the average amount in shale and mudstone?1.5-2ppm?.This means the sedimentary rocks provide Sb and REE to the deposit.REE distribution patterns of fluorites show that they contains more HREE than LREE.White fluorite's and purple fluorite's the average amount of?REE respectively is 7.16ppm and 14.20ppm,LREE/HREE respectively is 0.33 and 0.34,?La/Yb?_N is0.12 and 0.14,?La/Ho?_N is 0.06 and 0.08,Eu anomaly is 0.76 and 0.71,Ce anomaly is0.75 and 0.67,Y anomaly is 2.14 and 2.27,thus,white fluorite is richer in HREE,and have less Eu_N and Ce_N than purple fluorite.The La/Ho ratio decreases while the Y/Ho ratio stays the same,along with the widely-formed secondary fluid inclusions in fluorite,indicate the fluorite went through reactivation process.Fluid inclusions in gangue minerals from stage?,?,?display homogenization temperatures concentrated respectively at 260³00?,180²30?,130¹60?,with the salinities of 0.3-11.7%,0.2-10.1%,0.2⁵.5%NaCleqv.First melt of ice occurred at-32.0^-10.7?in stage?.Fluid inclusions in stibnite which intergrown with fluorite from stage?and?have homogenization temperatures of 250²90?,200²40?,with the salinities of 3.2²1.0%and 0.2⁹.1%NaCleqv.Stage?'s and?'s first melt of ice are-71.7^-15.5?,-15.4?.Stibnite and intergrown fluorite from stage?and?are dominated by NaCl-H₂O solution,and they display similar homogenization temperatures,which means they are probably from the same source.The minimum trapping pressure of the three stages respectively are 0¹60×10⁵Pa,8²0×10⁵Pa,1⁸×10⁵Pa,with the minimum depth of mineralization of 1600m,72m.The pH values are estimated by the concentration of H⁺in H₂O-NaCl solution,result in 5.6⁶.3,5.2⁵.5 and 4.7⁵.0 respectively.With the decrease of temperature and pressure,stibnite precipitation due to boiling process.pH values and low temperature indicate that Sb is transported and distributed in the form of Sb?OH?₃.?³⁴S values of stibnite distribute from-20.2^-11.9‰with average value of-14.7‰.?³⁴S values of native sulphur are-12.3‰and-11.3‰.?³⁴S values of galena are-1.3‰and-1.0‰.?³⁴S value of sphalerite is?2.8‰.Meanwhile,Raman results show that the fluid is consist of C₄H₆ and CH₄,which means the S and fluid all come from a reducibility environment.?D values of the fluid in fluorite and calcite distribute between-169‰and-111‰.?¹³C_V-PDB value of fluorite is-21.7‰and?¹³C_V-PDB of calcite are between 1.4².8‰.?D and?¹⁸O indicate ore-forming fluid may come from basin brine and mixed with meteoric water.?¹³C indicate the CO₂ come from marine carbonates.With the collision of India-Europe plates,this area developed intense tectonic and magmatic activities,which provided Bijiashan Sb orefield of drive and thermal energy.The ore-forming fluid was middle-low temperature,middle-low salinty basin brine with the mixture of meteoric waters,which leach ore-forming matters from sedimentary rocks,and have undergone with several times of reactiviation,thus,the deposit is sedimentary-reformation type.