Ore-Forming Fluids Studies On Typical Copper-polymetallic Deposits In Gangdese, Tibet

The systematic petrographical study,microthermometry,Raman probe,and SR-XRF analysis were performed for fluid inclusions from 3 typical deposits.These deposits are Xiongcun copper-gold deposit which is porphyrylike-epithermai low-temperature multiplex deposit formed in main collisional phase of Gangdese ore-forming belt,Jiama skarn copper-polymetallic deposit and Nanmu porphyry Cu-Mo deposit both formed in post-collisional phase.Then get some understandings:(1) The phenomenon of boil or phase separation occur with ore-forming fluids evolvement in 3 deposits.This phenomenon relate to ore-forming system's pressure continually release.Settling of 3 deposits' ore-boy is controlled by tectonic cracked belt.And this make the condition for ore-forming system's pressure release.(2) Ore-forming deepness of the 3 deposits is shallow,and about 1.5km in Jiama,about 1km in Nanmu,and Xiongcun is most shallowest.This accord with magma fluid's participation of the 3 deposits,Jiama has magma fluid in most proportion,and Xiongcun has magma fluid in least proportion.(3) Nanmu's content of Cu in fluid inclusions is highest,but the ore-forming scale is least. This relate to the lowest cool speed of ore-forming system in Nanmu.(4) There is a great deal of gas in ore-forming fluid system in the 3 deposits.Nanmu differ from Xiongcun and Jiama.The component of the gas in Nanmu is CO₂,and there are hematite, magnetite and goethite daughtermineral in inclusions.These indicate ore-forming fluid's f_o2 is highest.(5) Forming of the 3 deposits relate to acidic alteration,as sericitization and chloritization. Acidic alteration consume H⁺,and make the fluid close to litmusless or alkalescence.This is very important to keep high content of HS^- in ore-forming fluid,and ensure sulfid's deposition and ore-forming.(6) By the research to 3 typical deposits in Gangdese ore-forming belt,ore-forming geochemical process of Cu can be concluded:deep-seated origin fluid supply enough matter for copper deposit's forming;tectonic cracked belt make for pressure release,phase separation and Cu enrich in high salinity fluid,and cracked belt prepare the room for the setting of copper deposit; relative fast cool speed is a important reason to make Cu's deposition and ore-forming,and fluid with lower f_o2,close to litmusless or alkalescence is propitious to Cu's largely deposition as sulfide.Ore-forming process is a process with many factor's action above,and is a dissipative process with entropy decrease.So there need continual steady heat driving and relative open ore-forming condition.(7) The matter's origin of Nanmu and Jiama deposit are associated with related crust-mantle mix origin stocks.Ore-forming fluid and ore-forming porphyry magma possibly have same deep-seated origin,both parallel evolvements.And there is possibly action each other between fluid and magma in the evolvement.There are both mantle-origin and crust-origin matter to take part in ore-forming process in Xiongcun,Jiama and Nanmu deposit.And the proportion of crust-origin in of Nanmu is highest.And the 3 deposits both are produce of crust-mantle action which medium is fluid.(8) Xiongcun form in the transformation phases of intermittence flab(52～42Ma) in Himalayan-Tibetan orogenic belt terminal phases of main-collision(65～41Ma) to late-collision crust extrusion-up(40～38Ma).Jiama and Nanmu form in Himalayan-Tibetan orogenic belt post-collision phases(25～0Ma),relating to east-west extension of Gangdese in Miocene epoch (20～14Ma).Orogenic belt main-collision phase's higher extrusive stress may result deep-seated ore-forming fluid fast up-intrude.In post-collision extended phases,deep-seated ore-forming fluid can keep in crust with longer time.Difference of stress result the difference at ore-forming element in ore-forming effect of two periods fluid,and extended phases have more crust-origin matter.(9) Xiongcun deposit form in Tibetan intensive uplifting phases,40～35Ma.Uplifting and erosion result the form of porphyry-like mineralization and vein mineralization.Jiama and Nanmu deposit form in terminal phases of Tibetan intensive uplifting phases,20～14Ma.There are widely sericitization alter albite and bytown in Nanmu deposit,and do this related to uplifting? The research needs to be performed farther.