Metallogenesis And Prospecting Of The Jiaoxi Tungsten Deposit In The Western Part Of The Bangong-Nujiang Metallogenic Belt,Tibet

Jiaoxi deposit is the first discovered Miocene quartz vein-type deposit in the western part of the Bangong-Nujiang metallogenic belt,which is dominated by Cu-Au mineralization.As the first tungsten deposit developed in the ophiolitic melange belt,Jiaoxi deposit is quite different from other tungsten deposit in South China and show its significance in ore deposit study and prospecting.In this contribution,we present the zircon U-Pb dating,whole-rock major,trace and Sr-Nd-Pb,fluid inclusion and S-Pb-H-O-He-Ar stable isotope data of the Jiaoxi deposit.We will reveal the geological characteristics and ore genesis of Jiaoxi deposit,make appraisal on metallogenic potentials and carry out ore-prospecting.The main conclusions are as follows:(1)The W mineralization in Jiaoxi deposit occurs in wolframite-bearing hydrothermal quartz veins and is associated with Miocene granites.Orebodies mainly host in the upper shales and granites in the deep and display "Five floor" model vertically.Alteration in this deposit include the greisenization,silification and carbonate alteration.The formation of this deposit can be divided into an early quartz-oxide stage succeeded by a sulfide stage,and a final fluorite-carbonate stage.Geochronological data constrained the age of W mineralization at 13.6?13.1 Ma.(2)Magmatic intrusions at the deposit include the biotite monzogranite porphyry,biotite monzogranite,granite porphyry,and muscovite granite at depth and show crystallization ages ranging between 14.5 and 13.7 Ma.The highly evolved muscovite granite is the source for W mineralization.Regional geology,geochronology and geochemistry,indicate that the Miocene post-collisional peraluminous granites and associated W mineralization were triggered by the roll-back and gradual break-off of the northward-subducting Indian continental lithosphere slab.The ancient basement beneath the central Lhasa subterrane,the up-welling mantle melts and the Shiquan River-Nam Tso Melange Zone provided the important heat source,ore-forming materials and the magma channel,for the Miocene granite-related tungsten mineralization.Fluid inclusion and H,O,S and Pb stable isotope investigation results reveal that the wolframite precipitated from a high temperature(340?3 80?)low salinity(2.1 to 7.5 wt.%NaCl equivalent)hydrothermal fluid at a pressure>0.7 kbar.Fluid composition analysis and thermodynamic calculations reveal that the Cl concentration of the initial felsic magmatic fluid(W>60 ppm,Cl=ca.7 wt.%)dropped from 7.5 wt.%to 1.5 wt.%during fluid mixing,promoting the breakdown of Fe(Mn)-Cl complexes and causing the W saturation to decrease from 70 to 15 ppm,leading to wolframite precipitation.The input of cold meteoric water induced a decrease in temperature,significantly reducing the solubility of wolframite and also causing tungsten deposition.The widespread greisen alteration in the deep-seated granite indicates that the increase of pH in the ore-forming fluid through fluid-rock interaction was also an important factor that controlled wolframite precipitation.Based on the ore-forming controlling factors and the geology characteristics of the Jiaoxi deposit,we established a "trinity" prospecting model.Taking into account the regional geochemical,induced polarization sounding,remote sensing grade analysis results,this paper made ore-prospecting in the deeper part and periphery of the Jiaoxi deposit.