Evolution Of Fluids In Deep Processes:Petrogenesis And Metallogenesis Of Cenozoic Alkaline-rich Magma In Western Yunnan Province

Alkali-rich intrusive rocks,first identified by Tu Guangchi,consist of a Si-undersaturated series and another series of Si-oversaturated alkaline granites.Outcrops of both series are commonly found close to one another in special geologic settings and are genetically related to metal ore deposits.Intrusions of alkaline-rich rocks are typical in extension regime of lithosphere,including intracontinental rifts,hotspots and transition zones of orogeny to non-orogeny.Studies on rare earth elements,trace lithophile elements and Sr-Nd-Pb isotopic systematics showed that sources of alkaline-rich intrusions likely are metasomatised enriched mantle.Therefore,such mantle-derived alkaline melts probably record its interaction with the crust intruded and provide independent window into geodynamics of deep mantle.Triggered by India-Eurasia subduction and collision Cenozoic mantle-derived rocks formed in the northwest Yunnan,especially along the Jinshajiang-Ailaoshan Fault where ore-bearing alkaline-rich porphyries occurred.On the basis of previous studies,this thesis focuses on the micro-solid components and fluid and glass inclusions in these rocks and ores from this area,the potential of mantle fluid for providing extra ore-elements were investigated in detail,particularly Si-undersaturated alkaline-rich rocks from Liuhe,Jianchuan and Xiaoqiaotou,and ore-bearing porphyritic granites,which are Si-oversaturated,from Machangqing.The Laowangzhai Gold Deposit and Jinding Lead-Zinc Deposit were also taken in this thesis in order to reach a comprehensive understanding of the relationship of mantle fluids and ore deposits.Important progresses include;(1)Na-rich and Fe-rich micro glasses and ferrous melt xenolith represent direct signature of mantle,indicating a volatile-rich melt.The ferrous melt xenolith is immiscibly hosted and shows clear sharp contact zone.These micro glasses possibly formed via fast cooling of supercritical fluids derived from enriched mantle.Volatile losses due to violent changes of physiochemical environments further lead to the formation of embryonic crystals and amorphous solids.These macro glasses as well as ferrous melt xenolith might be the direct evidence for the existence of mantle fluid which,during fluid-rock interaction,caused the ubiquitous silicification,carbonation and formation of secondary K-and Na-rich minerals.The mantle fluid could also be the source and ore-forming elements and provide the required heat for recruiting extra materials from the wall-rocks.(2)As the upwell of alkaline-rich magmas,the once coupled mantle fluid starts to be decoupled from alkaline-rich magmas and independently altered the host rocks,xenoliths as well as wall-rocks.The expression of mantle fluid thus vary with the change of physicochemical environments and oxygen fugacity.In Liuhe syenite poyphyry and xenoliths there may typomorphic mineral stand for mantle fluid.Micro metals and Fe-rich glasses,titanium-silicates in Machangqin Deposit appear to be related to mantle fluid.As more crustal materials involved the manifestation of mantle fluid evolved to have Au-bearing sulfides,carbonates,silicates in Laowangzhai Deposit and even Pb-Zn sulfides and sulfates in Jinding Deposit.(3)Ferrous melt xenolith,CO2-and H2O-bearing high potassium glasses in quartz peritecticum,quartz-calcite peritecticum,and Na-feldspar pegmatite xenolith appear to represent the four end-members of the mantle fluid in Si-undersaturated alkaline intrusions.The respective four end members are Fe-rich represented by ferrous melt xenolith,K-rich by high potassium glasses,Na-rich by Na-feldspar pegmatite xenolith and CO2-rich by fluid inclusions.In essence the immisciblity of fluids and changes of oxygen fugacity(caused by progressive contributions of crustal materials?)are the main reason for the formation of the four end members.(4)Geochronological data revealed that the formation of metasomatised enriched mantle source and the derivation of alkaline-rich melts are in distinct stages.Ar-Ar ages of metasomatised hornblende in mantle xenoliths are clustered at 102.87±1.19M and 116 Ma,while the age of Liuhe granites is 42.8±1.6 Ma which is approximately comparable to the age of syenite porphyry of 38.63 ± 0.52 Ma.It is clear the mantle xenoliths had been modified by alkaline-rich agents in the mantle.The age of alkaline-rich intrusions in Machangqing is 38.51 ± 0.52 Ma,comparable to the Re-Os model age(35.80 ± 0.20 Ma)of molybdenite.Data on Ti-thermometer,REE and Ce+4/Ce+3 indicate the immixture of magmas.Therefore,we suggest that the prevailing alkaline agents lead to the formation of metasomatised enriched mantle,Si-undersaturated magma upwelled with immiscible mantle fluids caused crustal anataxis via magma-fluid diapirism.The resultant felsic melts from crust was immiscible with the up-transporting alkaline magmas and formed Si-undersaturated alkaline-rich intrusions with granite xenoliths.While if felsic melts were homogenized in the up-transporting alkaline magmas,mantle fluids start metasomatic alteration to the new hybrid magma and the Si-oversaturated alkaline-rich intrusions formed.(5)Mantle-crust interaction does not necessarily lead to the formation of ore deposits,which are likely controlled by the ability of activation and unload of ore-forming elements in mantle fluids.Mantle fluids and variable forms of mantle fluids dominate the formation and distribution of ore deposits in the northwest Yunnan.These results combined with metallogenic theory on trans-magma show that in the process of mantle fluid-alkaline intrusion interactions,high-temperature porphyry type deposits?contact metasomatic type deposits?low-temperature hydrothermal deposits formed systematically.This study is of great significance for understanding of the relationship among alkaline intrusions,mantle fluids and ore formation.