The Petrogenetic Mechanism And Metallogenetic Potentiality Of Bengge Alkaline Igneous Complex,Northwest Yunnan

There is a suit of alkaline igneous complex that exposed in the Bengge area of Shangri-La,northwest Yunnan,and consist of syenite,orthophyre and lamprophyre,and which with gold mineralization.The instrusion is clustered distribution with dozens of small stocks or apophyse,and different types of rocks are disordered and mixed outcrop,lithofacies zonation is not obvious.In addition,it also appears some special rocks such as breccias vein,cryptoexplosive breccias dike,potassium feldspar pegmatite vein,quartz vein and altered monzonite piece,and they are deep origin fluids products of heavy activities.The syenite and orthophyre are mainly mineral composition of potassium feldspar+quartz+amphibole+biotite±pyroxene?potassium of phenocryst?,and the lamprophyre is mainly mineral composition of potassium feldspar+pyroxene+biotite+amphibole?pyroxene and biotite of phenpcryst?.In addition to the altered monzonite piece with a small amount of plagioclases,the all of others rocks are not containplagioclases.Whilethepotassiumfeldsparpegmatitevein is composed of potassium feldspar megacryst+quartz?form with filing?+pyrite+galena,entirely without dark minerals.The minerals of the complex are medium to high of automorphic extent,and with melting corrosion texture,reaction rim texture and metasomatic texture.On the other hand,the orthophyre is also containing amphibololite,lamprophyre,and biotite granulite xenoliths that shows mantle-derived magma mixing with crust-derived magma.The study of mineral chemistry by biotite,feldspar,pyroxene and amphibole of the complex and the xenoliths,also shows a characteristic of crust-mantle mixing.On the petrogeochemistry,Bengge alkaline igneous complex is characterized by higher potassium contents and alkali richment,varying widely K₂O/Na₂O ratio values with from 0.38¹59.67,average of 25.06,so that it belongs to shoshonite and ultra-potassic series.The syenite and orthophyre have a wide range of SiO₂ content ranging from 50.65 wt%to 68.38 wt%,while the lamprophyre has the higher SiO₂content ranging from 53.94 wt%to 58.20 wt%.The K₂O/Na₂O ratio values of lamprophyre do not change with SiO₂ and MgO content,which indicates some melts/fluids from potassium and silicon enrichmengt resource partial melting involved and mixed in the process of petrogenesis.Except the potassium feldspar pegmatite vein and the strongly mineralized sample?synite?,the others rocks of Bengge complex have a higher total content of REE which ranging of 150×10-6²50×10-6.The light rare earth elements and heavy rare earth elements fractionation of all samples except the potassium feldspar pegmatite vein is obvious?LREE/HREE and?La/Yb?_N values are 8.2³0.4 and 4.46⁴0.89 respectively?.The intrusive complex has a large ranges of Eu anomaly,which with from negative anomaly to unconspicuous and to heavy positive anomaly??Eu=0.53¹.92?.The most of altered and mineralized samples have generally obvious Eu positive anomaly??Eu=1.1¹.92?and Ce positive anomaly??Ce=1.2¹.6?,while relatively fresh samples have the weak Eu and Ce negative anomaly??Eu=0.53⁰.9,?Ce=0.85⁰.9?.This features show that Bengge alkaline igneous complex is characterized by both inherited Ce anomaly of upper mantle source region,and had experienced the fluid metasomatism longly and slowly with a lower oxygen fugacity condition.The complex enriched in LILE,depleted in HFSE relatively and in compatible element significantly such as Ni,Cr.It has Rb,Ba,Th positive anomaly,Nb,Ta,Sr,Ti negative anomaly,and a different level negative and positive anomaly respectively with Zr and Hf.By deeply analysis and research for geochemical characteristics of Bengge alkaline igneous complex,the rocks have the similar source region and petrogenetic dynamic mechanism to Indosinian neutral-acid intrusion exposed in the Yidun island arc belt and Cenozoic alkali-rich intrusive rocks in the western Yunnan.So it is not the product of simple crystallization differentiation of single original magma,but the source of both melts derived from thickened amphibole phase lower crust partial melting,and from phlogopite-bearing spinel phase enrichment mantle source partial melting,and also mixed melts from subducted oceanic crust partial melting and subduction fluid metasomatism of enrichment mantle wedge partial melting.These melts from different source evolved respectively in different stages.It appears the crystallization fractionation trend of olivine+amphibole+clinopyroxene+garnet.The later period melts were action on the products of earlier melts continuously,and the both melts superimposing and mingling with each other rised and emplaced eventually.In this process,it also is accompanied by a certain degree of mantle fluid metasomatism and crust-mantle contamination.The potassium feldspar pegmatite vein may represent a result of the fluids which wer volatile enrichment in the latest stage cooling crystallization directly.While the altered monzonite piece may represent residue of earlier magmatic body after fluids metasomtism and transformation.The study of isotope chronology has shown that Bengge alkaline igneous complex takes the information of genesis ages from late Triassic to the Oligocene?220²00Ma,192Ma,167⁸7Ma,89Ma,35Ma,28.2Ma?.Although these age data is acquired by different testing methods,but it actually reflects the characteristic with multi-period and multi-stage evolution of Bengge alkaline igneous complex.Combining petrology,petrogeochemistry of Bengge intrusion and the space-time distribution of other intrusive rocks with the similar features on the area,it will be considered that Bengg alkaline igneous complex is actually superimposing and composite intrusion which formed from late Triassic and end of the Cenozoic.So,the complex had experienced three period with petrogenetic process:?1?The initial magma period?220²00Ma?:The mixing melts which melt of Ganze-Litang subduction oceanic plate partial melting mixed with melt ofmetasomatic enriched mantle partial melting are made up a initial magmatic intrusion in the deep.?2?The gradual superposing period?200⁶5Ma?:The melts of thickening lower crust partial melting met the initial magmatic intrusion in the rising process,andsuperimposed and contaminated with each other,then rised and emplaced synchronously.?3?The alkali-rich magma and fluids effect period?40³0Ma?:Influenced by India-Eurasian plate strong collision,the alkali-rich magma and fluids that derived from subduction paleo-plate metasomatic enriched mantle partial melting rised rapidly under the background of strike-slip and extension,and met the superimposing and mixing initial intrusion that formed from Indosinian to Yanshanian at Bengge area,and then generated fluid metasomtism and exchange of materials with it.At the same time,there is some melts of thickening lower crust partial melting involved and mixed in this process and went with mantle fluid metasomtism and crust-mantle contamination,emplaced and formed the Bengge alkaline igneous complex eventually.The synite of complex may represent a product of initial magma period.By the fluid inclusion research,the quartz that related to gold mineralization has two types of fluid inclusion which is NaCl-H₂O and NaCl-H₂O-CO₂.Their homogenization temperature is 124.1²84.5?and 197.6²96.9?respectively,and salinity is 0.18%¹0.98%and 1.43%⁷.97%respectively.They belong to low-salinity and low temperature fluid.The wider variation range of salinity and temperature shows there have different sources of fluids to join and mix.The two-phase inclusions H₂O?V?+H₂O?L?andCO₂?V?+H₂O?L?coexisting withthethree-phase CO₂?V?+CO₂?L?+H₂O?L?inclusions represents the immiscible system of fluid,and shows the mineralization of Bengge gold deposit may relate to fluid immiscible.According all above research,it would be considered that the gold mineralization of Bengge gold ore is nothing to do with the syenite.The part of ore is sub-extensional structure crushed zone.The mineralization mainly subject to the multi-period magma interacting with ore-bearing mantle fluid and crust-mantle materials superimposing contamination caused by the former.The ore-bearing mantle fluid with the host magma is actually a kind of trans-magmatic fluid.The fluid in the form of immiscible and independent phase involved in the alkali-rich magma wtith the crystallization and petrogenetic process in the way of metasomatism,and inevitably caused crust-derived materials contamination on a certain degree,and at the same time promoted the mineral enrichment in the deep of the intrusion or adjacent region.Considering the characteristics of the gold deposits that related to the alkali-rich intrusive rocks around the area,it could be predicted that the ore shoot of Bengge may locate in the deep that the region of different types rocks of Bengge Bengge alkaline igneous complex contact with each other or the intrusion contact with the wall rocks especially the carbonatite.