The Fluid Source And Ore-froming Processes Of The Orogenic Gold Mineralization On The North Part Of The Ailaoshan Shear Zone

The Ailaoshan shear zone,SE Tibet is an important Cenozoic orogenic gold belt.The Zhenyuan Au and Jinchang Au-Ni deposits from the north part of the Ailaoshan have been picked to discuss the alteration characteristics,fluid source,ore-forming processes and gold depositional mechanism.In addition,the thermodynamic method has been used to check out the mineral assemblages and alteration range of the orogenic deposits fromed at different temperatureThere are five types of host rocks at Zhenyuan gold deposit occurring broad alteration zonations and forming ore bodies,including carbonaceous slate,meta-sandstone,meta-basalt,quartz porphyry and lamprophyre.They all occur similar alteration assemblages:carbonate(mainly dolomite and ankerite)-sericite-pyrite and minor arsenopyrite.The alteration minerals(carbonate and sericite)formed in various types of host rocks show different contents,indicating that the altered minerals can inherit the component of the host rocks.The results of the LA-ICP-MS analyses on the pyrites from meta-sandstone and basalt show that the leaching of the different content of the host rocks result in higher contents of Cu,Mo,Ag,Sb,Te,Hg,Tl,Pb and Bi in pyrites from meta-sandstone and higher contents of Co and Ni in pyrites from meta-basalt.Pyrites from these two host rocks have same range of Au and As,indicating that they mainly sourced from deep crust or mantle with little upper crust contribution.The ore-stage gold bearing pyrites from various host rocks have small range of ?34S values,peak at 0.Combining with the tectonic evolution,the ore fluids of the Zhenyuan deposit sourced from the metamorphic lithospheric mantle,rather than the host strata.The Sr/Nd isotope analyses on altered lamprophyre and C-O isotope analyses on carbonate show that the crustal components have been added into ore fluids during fluid migration upwards.The Jinchang deposit has characteristic Au-Ni mineral assemblages.The host rocks are dominated by serpentinited ultra-mafic rocks and carbonaceous slate and sandstone.Except for the alteration minerals found in the Zhenyuan deposit,the Jinchang deposit develops the characteristic Cr-rich sericite and chlorite.The analysis on the content of the chlorite show that the gold-bearing ore fluids activate Ni from the ultra-mafic rocks as well as other elements like:Cr,Fe,Mg and Cu etc.,resulting in these characteristic mineral assemblages.The reaction between ore fluids and relatively oxidized ultra-mafic rocks lead to the negative ?34S values relative to Zhenyuan depositThe HCh thermodynamic modeling has been used to model the equilibrium reaction between orogenic ore fluids and meta-sediment as well as Mg/Fe mica.The results show that the alteration zonation are relatively narrow at high temperature.The forming sulfide is pyrrhotite.Multiple infiltration of ore fluids can react the pyrrhotite to pyrite.The alteration zonations are wider at low temperature.The forming sulfide turn to the pyrrhotite.Except for albite,other alteration minerals like quartz,sericite,carbonate and graphite are prone to deposit at low temperature.Another reason for the relatively wider alteration zonation in shallow low-temperature crust is the higher permeability.The water-rock reaction is the main precipitation mechanism for the disseminated orogenic gold deposits.However for the vein-type deposits,the water-rock reaction is concomitant with other mechanism(like decompression)to control gold deposition.