Formation Age And Ore-Forming Fluid Characteristics Of Felsic Vein Type Tungsten Deposit In Nanwenhe,Southeastern Yunnan

Southeast Yunnan is located at the combination of Yangtze plate,Indosinian plate and Cathaysian plate,which has experienced multiple magmatic-tectonic activities since Paleoproterozoic,resulting in complex ore-forming history and various ore-forming conditions in this area.many scholars have disputed about the ore-forming age,ore-forming material source and deposit formation mechanism in this area.On the basis of previous studies,this paper carries out a detailed comprehensive geological survey of petrology,mineral deposit science and structural geology in the Nanwenhe tungsten mining area,distinguishes the deposit stages by interspersing-emplacement relationship,and makes LA-ICP-MS U-Pb dating and in-situ trace element analysis of the accessory mineral sphene in felsic vein type and skarn type tungsten deposits,as well as in-situ trace element and Sr isotope characteristics of scheelite.The ore-forming age,ore-forming fluid characteristics and ore-forming material sources of felsic vein type tungsten deposits in this ore area are also discussed.The main conclusions are as follows:(1)There are two stages of tungsten metallogenic events in Nanwenhe tungsten mining area,that is,the early layered skarn-tourmaline quartzite regional metamorphic metallogenic event and the late felsic vein tungsten hydrothermal metallogenic event near Emure W direction.Feldspar vein type tungsten deposit is different from previous quartz vein type tungsten deposit.Its mineral assemblage is mainly feldspar,containing a small amount of quartz,and a large number of sphene accessory minerals and coarse crystal scheelite are developed.(2)LA-ICP-MS U-Pb dating of sphene in feldspathic vein type and skarn type tungsten deposits shows that sphene in skarn type tungsten deposits is round in shape and the Th/U ratio is 0.02-0.08,indicating that sphene is hydrothermal genesis.The weighted average U-Pb age is 225.5 ±2.7Ma.The sphene in feldspathic vein type tungsten deposit is self-shaped and curved vein.Both types of sphene develop dissolution pores and contain zircon mineral particles in SEM images,and the lower Tu/U ratio is 0.02-0.29,indicating that sphene is of hydrothermal origin.The weighted average U-Pb age of curved vein sphincter was 141.5 ±4.7Ma.Therefore,this paper holds that the skarn tungsten deposit in the Nanwenhe tungsten mining area was formed in the Middle Triassic,while the felsic vein tungsten deposit was formed in the Yanshanian early Cretaceous.(3)LA-ICP-MS trace element analysis of sphene and scheelite shows that there are great differences in the contents of Sr,Mo,Nb,Y and REE distribution curves between skarn and felsic vein scheelite,indicating that there may be differences in the properties and sources of ore-forming fluids.The REE distribution curves of accessory mineral sphene in feldspathic vein type and skarn type tungsten deposits both show a left-leaning pattern of light rare earth(LREEs)relative heavy rare earth(HREEs)depletion,but skarn type sphene shows weak negative Ce anomaly(0.80 ±1.03)and weak or no positive Eu anomaly,while felsic vein type sphene shows positive Ce anomaly(1.0-1.31)and negative Eu anomaly(0.71-1.64).It shows that the formation condition of sphene changed from the environment with higher oxygen fugacity in the early stage to the environment with higher oxygen fugacity in the later stage.The Eu anomalies of skarn type and felsic vein type scheelite also confirm the transformation of the ore-forming environment.(4)LA-MC-ICP-MS Sr isotope study of scheelite shows that the 87Sr/ 86 s value of felsic vein type scheelite(0.71266-0.72247)is higher than that of skarn type samples(0.71574-0.71929),indicating that skarn scheelite may be related to metamorphic hydrothermal metasomatism,while felsic vein type scheelite may come from hydrothermal end members with higher Sr ratio.The skarn scheelite formed in the early stage is superimposed and reformed.(5)Indo-China block and South China plate N-S trending nappe caused deep melting in the lower crust during the 225 Ma,resulting in regional metasomatism of Neoproterozoic-Cambrian carbonate-mudstone strata and the formation of skarn orebodies.Then the fluid continues to cool,and the resulting pressure causes vein fissures to occur in skarn and its fragile periphery.Then the southeast Yunnan area gradually transformed from the Tethys tectonic domain to the near Nmurs extension of the Pacific tectonic domain to form felsic fluids and poured into the fractures to form felsic vein type tungsten deposits interspersed with skarn orebodies around 150 Ma.