Chronology And Fluid Geochemistry Of The Nanyangtian Tungsten Deposit In Southeastern Yunnan

The Nanyangtian tungsten deposit is located in the Laojunshan Sn-W polymetallic metallogenic province in Southeastern Yunnan Province.Tectonically it is located at the combination of the Cathaysia Block and Indochina Block.This area has experienced multistage tectonic movement since the Proterozoic,resulting in large-scale production of intermediate-acid magmatic rocks,which provides abundant fluid,material and heat source for the formation of a series of large and super-large-scale tungsten-tin polymetallic deposits in this area.Previous studies on the inclusions of Nanyangtian tungsten deposit are not systematic,and the test results are deviated.There are also great differences in metallogenic age,which greatly restricts the understanding of the metallogenic regularity of the deposits and even the area.Thus,we conducted a detailed geological field investigation of the Nanyangtian tungsten deposit.New microthermometric and Raman spectroscopic data for the fluid inclusions within scheelite and gangue minerals(garnet,pyroxene,quartz,tourmaline,and calcite)from various metallogenic stages were presented.Based on this and the carbon,oxygen,and hydrogen isotopic data,the nature and origin of the ore-forming fluids,the mechanism of scheelite precipitation,and the ore genesis of the Nanyangtian tungsten deposit are discussed.In addition,we present new ⁴⁰Ar-³⁹Ar dating for biotite coexistent with scheelite,and zircon LA-ICP-MS U-Pb dating for granite vein.This study provides precise ages to better constrain the genetic correlation between Laojunshan granitic magmatism and W mineralization.It also provides new information and basis for further study of the digenetic and metallogenic regularity of the ore-concentrated area and even in the Sn-W polymetallic metallogenic area in Southeastern Yunnan.The understanding obtained are as follws:(1)Three metallogenic stages have been identified based on mineral assemblages and on the intercalated relationships between the veins:(1)the skarn-scheelite stage,(2)the quartz-scheelite vein stage,and(3)the carbonate-scheelite vein stage,which correspond to the formation of skarn type,quartz-vein type,and carbonate-vein type ores.The main metallic mineral is scheelite with associated chalcopyrite,sphalerite,pyrite,arsenopyrite,and pyrrhotite.The gangue minerals include garnet,pyroxene,tremolite,calcite,quartz and tourmaline.(2)Detailed fluid inclusion petrography in different type of scheelite sample revealed four types of fluid inclusions:liquid-rich two phase inclusions(type I),gas-rich two phase inclusions(type II),daughter mineral-bearing three phase inclusions(type III)and pure vapor phase inclusions(type IV).Homogenization temperatures of fluid inclusions in scheelite from skarn type ore vary from 176? to 311?,with salinities from 0.2 wt%NaCleq to 10.9 wt%NaCleq.The homogenization temperatures of fluid inclusions in scheelite from quartz-vein type ore are from 200?to 395?,and salinities are from 0.4wt%NaCleq to11.3wt%NaCleq.The homogenization temperatures of fluid inclusions in scheelite from carbonate type ore are from 201?to 395?,and salinities are from 2.6wt%NaCleq to 11.3%wt%NaCleq.Fluid inclusion observations and microthermometric results show that the ore-forming fluids are medium-high temperature and medium-low salinity fluid.Laser Raman microprobe analysis of the fluid inclusion show that H₂O is the dominant component,with a small amount of CH₄.(3)The oxygen and hydrogen isotopic composition of fluid in skarn-scheelite type ore are 6.8‰and-80.2‰to-78.3‰,respectively,and in quartz-vein type ore are6.5‰to 7.1‰and-93.7‰to-79.0‰,respectively.This isotopic characteristic indicates that the ore-forming fluid mainly derived from the magmatic water,and mixing with meteoric water.Meanwhile,homogenization temperatures and salinities of fluid inclusions have positively relationship also show that there is mixing of two different properties of fluids.Combined isotopic and microthermometry results,we believe that scheelite precipitation is caused by the mixing of magmatic and meteoric water.(4)In this paper we also present hydrothermal biotite ⁴⁰Ar/³⁹Ar dating from the scheelite-bearing quartz veins to constrain the timing and duration of mineralization.Two hydrothermal biotite ⁴⁰Ar/³⁹Ar dating yielded pleatu ages of 97.49±0.97 Ma and99.48±0.97 Ma respectively,and isochronal ages of 97.01±1.39 Ma and 99.58±0.98Ma respectively,representing the metallogenic age of the Nanyangtian tungsten deposit,which is well consistent with the emplacement age of the first phase of Laojunshan granite(96?117Ma),indicating a temporal link between the emplacement of the Laojunshan granite and the W mineralization.LA-ICP-MS zircon U-Pb dating of the granite vein in the ore field yield a weighted mean ²⁰⁶Pb/²³⁸U age of89.88±0.53Ma(MSWD=0.49),which is interpreted as the emplacement age after W mineralization.Integrated with high precise diagenetic and metallogenic geochronology data obtained previously,it is suggested that the Southeastern Yunnan region mainly experienced two significant Sn-W metallogenic events during Mesozoic.The first event took place from 97 Ma to 124 Ma and associated with W deposits.The second event,which occurred from 75 Ma to 95 Ma,mainly produced Sn-Cu-Ag-Zn deposits.