The Geological Characteristics And Genesis Research Of Nuri Cu-Mo-W Deposit In Shannan, Tibet

The strongly granitic magma instrusion activities has widely distributed along the north yarlung zangbo river suture zone in Gangdese volcanic–arc belt since the Mesozoic,benefited from the collision of the India and Eurasia plates.Many large?super-large skarn or porphyry deposits has been founded in this belt,which has a unique and enormous mineral resource potential,it is becoming the world-class copper,iron,molybdenum,lead and zinc polymetallic metallogenic belt,and most famoused large scale deposits with high grade mainly distributed in middle-eastern of Gangdise,in which the largest Nuri Cu-Mo-W skarn deposit in south Tibet has founded in recent years is the most important molymetallic base,it also the first large scheelite deposit in Tibet,the results of unique mineralization combination have important reference values in studying the collision mineralization process.The deposit mainly composed of Bima strata of early-cretaceous?K₁b?and later-Cretaceous magma,and the ore-body hosted in bending of the“M”or decollement structure,which mainly appears in bedded,near bedded garnets skarns,a minority of quartz vein and sparse disseminated mineralization,the three minerogenetic stages include skarn stage,quartz-sulfide stage,supergene stage,and the first one divided to skarn and oxide,the middle one divided to quartz-sulfide and quartz-calcite;many oxide and aqueous salts formed in the third stage.But the molybdenite and copper pyrite occur in quartz-sulfide stage,and the scheelite occur in later oxide stage.The ore-body has characteristics of zoning distribution obviously in the Nuri dep osit,which is divided into three sections:south,central and north sections.In the plan e,overall,it is the Cu-W mineralization at the south section,the central section is give n priority to with molybdenum mineralization,and there is W-Mo mineralization at th e north section;Spatially,in the shallow part,the mineralization is mainly skarn type with copper-tungsten mineralization,meanwhile,the mineralization also has independ ent ore body.With the increasing depth,the scheelite mineralization reduce significant ly and molybdenum mineralization gradually increase,there is independent molybden um ore body,With the continued increasing depth,the mineralization abate,there is q uartz vein type and sparse disseminated mineralization.Ore-body has good spatial con-tinuity,the attitude of the host layer is gently inclined to NNW.Combined with wild geological phenomenon and indoor work results showed that:the large area skarnization and partial quartz-vein interpenetrating mainly influenced by water-rock reaction between multiple stage magma hydrothermalism and surrounding rock.There are three types of fluid inclusion:two-phase with gas-liquid,three-phase with gas-liquid-solid,one-phase type,overall,the ore-forming fluid have the laws for the temperature from early-skarn stage to quartz-calcite stage:in the skarn-oxide stage,the homogenization temperature of liquid inclusion is ranged167⁵64?same to the other hydrothermal metasomatic garnet forming temperature,and the salinity range is 2.07⁶5.33wt_?NaCl?/%,corresponding to the fluid density range is 0.37⁰.96 g.cm^-3;in the quartz-sulfide and quartz-calcite stage,the homogenization temperature of liquid inclusion is ranged 143³91.2?,and the salinity range is 1.4⁴1.3wt_?NaCl?/%,corresponding to the fluid density range is0.74⁶.41 g.cm^-3.the homogenization,salinity of liquid inclusion is showed decrease trend,but the liquid density is rising from early to later stage,and estimated metallogenic depth is 1.07⁶.41km,combine with the H,O isotope of liquid inclusion host in quartz and garnet,the results showed that ore-forming solution mainly resourced from the hypomagma and mixed with meteoric water,the?³⁴S of sulfide from the Nuri deposit range is-2.9‰¹.8‰,which have a high degree homogenization implied that the single original region.The geochemical characteristics of trace and rare earth elements compositions from scheelite and molybdenite in order to trace the source of ore-forming mineral.The test results showed that Hf/Sm,Nb/La?Th/La ratio is far less than 1,meanwhile the molybdenite is low with negative cerium and europium anomalies that is characterized by enrichment in light REE.Indicating that the feature of original rich-W-Mo ore-forming fluid source from the deep crust magmatic crystallization and differentiation,And mix with a large number of meteoric water in the process of migration,Which is an riched-CL hydrothermal.As the water-rock reaction,the CO₂and Ca content level increased,the initial balance system was breaked,Eventually lead to formed the mass scheelite due to Ca²⁺and WO₄^2-combinatived.The reason of negative Ce anomalies might be found Ore–forming fluid with high temperature and high oxygen fugacity has an oxidation to Ce³⁺from the system.With the water-rock reaction and mixed with meteoric water,Oxidizing gradually reduce,reducing gradually enhance.The characteristics of chronology of zircon U-Pb and Sr-Nd-Pb isotope compositon from magma in Nuri deposit showed that:the magmatic evolutionary timed series is quartz-diorite porphyrite?93.42±0.76 Ma??andesite?92.04±0.71 Ma??biotite granite?50.46±0.56 Ma??granite diorite porphyry?24.83±0.21 Ma?.The magma belongs to high-k calc-alkaline series or calc-alkaline series and aluminum rock,in which the intrusive rock have I-type granite characteristics.The geochemical compositon and Sr-Nd-Pb-Hf isotope of magma in Nuri deposit showed that the quartz-diorite porphyrite formed by the melt react with the mantle wedge in rising process have adakite characteristics derived from the neo-tethys subduction partial melting in late-Jurassic to early-Cretaceous;the andesite formed by the mantle wedge partial melting,which mixed with few crust material in late-Jurassic to early-Cretaceous when the neo-tethys north subduction in low angle,as the subduction continue,the sedimentary on the surface of slabs dehydration which caused the mantle wedge partial melting have the common arc magma characteristics.the biotite granite have the characteristics of arc-magma derived from the newborn basaltic lower crust partial melting,in the India-Asian plate collision stage,the lower crust of southern margin of the thickening Lhasa block slab breakoff,which caused the asthenpsphere material upwelling and the basement dalamination.The granite diorite porphyry have the characteristics of Adakitic derived from the lower crust melting of subducted India-continent,and the melt interaction with depleted mantle in the upload process.The Re-Os isochron age of molybdenite is 14.27Ma,and the result is quite same with the age of granite diorite porphyry have close relationship in Nuri deposit,meanwhile,the geochemical characteristics of ore-bearing porphyry in many porphyry deposits samed with the granite diorite porphyry,imply which have a larger metallogenic potential and obtain the high attention in Nuri deposit.Combined with the geological characteristics,mineralogy,petrology and geochemical characteristics,the author thinks there exist a porphyry-skarn metallogenic system,so establish a Cu-Mo-W metallogenic model.