The Sources Of Ore-forming Material And Evolution Of Ore-forming Fluid In The Bada Cu-Au Deposit,Eastern Tibet

The Bada Cu-Au deposit is located in the southern part of the alkali-rich porphyry belt in Eastern Tibet,which is a newly discovered large Cu-Au deposit.In this paper,the geological characteristics of the Bada copper-gold deposit have been dissected carefully.The hydrothermal alteration and mineralization characteristics of the deposit were identified,and the mineralization stage of the deposit was determined.Fluid inclusion petrography,micro-thermometry,laser Raman spectroscopic analysis,C-O isotopes and in situ S isotope testing are carried out for veins developed in different metallogenic stages.The process of fluid evolution has been reconstructed,the mechanism of ore-forming material precipitation has been revealed,and the genetic types of Bada Cu-Au deposit have been preliminarily discussed.The main research results are as follows:(1)Geological characteristics of the deposit: The Bada deposit was produced within the quartz monzonite porphyry intrusion and the surrounding sandstones.It is characterized by alteration zoning with potassic as the center,phyllic and propylitization on the outer side,and kaolinization on the top.The Cu-Au orebody mainly occurs in the potassic and phyllic alteration zone.The ore minerals are mainly composed of pyrite,chalcopyrite,galena and electrum,with a small amount of tetrandrite,chalcopyrite,sphalerite,arsenopyrite,hematite,jamesonite.Cu mineralization is mainly produced as chalcopyrite,while the gold is mainly produced as fine Ag-Au grains in dolomite ± quartz vein and fine-grained pyrite,the occurrence forms include intergranular gold and inclusion gold.(2)Nature of the ore-forming fluids: L-,V-,C-and S-types fluid inclusions are developed in the Bada Cu-Au deposit.The homogeneous temperature,salinities and density of the fluid inclusions concentrate on the range of 210-330 ?,4.3-23.1 wt% and 29.7-43.4 wt%,0.90-1.18 g/cm3,respectively.Morever,magnetite and hematite are developed.The ore-forming fluids are characterized by medium-high temperature,medium-high salinity,medium-low density and relatively oxidized.The gas phases in the inclusions are composed of H2 O,CO2,N2 and CH4,while the liquid phase composition is H2 O,and the ore-forming fluid belongs to the H2O-Na Cl ±CO2±CH4 system.(3)Sources of ore-forming fluids and materials: C-O isotopic datas indicate that the ore-forming fluids of Bada Cu-Au deposit is mainly from magmatic hydrothermal in early satge,and ore-forming fluid affected by low temperature alteration(sericitization,carbonation,etc.)and a small amount of meteoric water in main ore stage,while the ore-forming fluid in late stage is contaminated by a small amount of sedimentary strata.Sulfur isotopic compositions(?34S values ranging from 1.43 to 6.61 ‰)suggest that ore-forming metals were primarily originated from the magma.(4)Migration form and precipitation mechanism of ore-forming materials: We consider that the metallogenic metals(Cu,Au)in the Bada Cu-Au deposit are mainly migrate in the form of chlorine complexes.The fluid boiling,fluid-wall rock reaction and fluid mixing leads to the precipitation of Cu and Au.Combine with the emplacement age of ore-bearing porphyry(~35Ma)and regional geological background,we believe that the Bada Cu-Au deposit belongs to the porphyry Cu-Au deposit formed by late collision orogeny on the eastern margin of the Qinghai-Tibet Plateau.