Study On The Characteristics Of Volcanic Rocks And Its Ore-bearing Potentiality In Manghuai Formation,Shuangjiang County,Western Yunnan

The study area is located in the southern part of the Sanjiang area in Western Yunnan,and it is an important combining site between the Eurasian plate and the Indian plate.Frequent magmatic activity and complex tectonic setting have formed relatively large scale volcanic rocks,especially the volcanic rocks of the Triassic Manghuai formation.Predecessors have done a lot of research work on the volcanic rocks of the Manghuai formation,however,the rock assemblage characteristics of volcanic rocks have not been accurately determined and there is still much controversy over diagenetic tectonic settings in the Manghuai formation,at the same time,no ore-bearing potentiality analysis has been carried out on the volcanic rocks of the Manghuai formation.Combined with previous research results,this paper studies the rock assemblage characteristics,acid volcanic geochemistry and chronological characteristics of Manghuai formation through field geological survey and profile measurement in the study area,and aims to reveal the magmatic evolution,petrogenesis,diagenesis,tectonic environment analysis and ore-bearing potentiality characteristics of acid volcanic rocks in the study area.The rock assemblages of the Manghuai formation are mainly clastic>65%,pyroclastic and volcanic lavas<35%.Among them,volcanic clastic rocks are mainly volcanic breccia and Anshan tuff,and clastic rocks are mainly composed of clastic sandstone and clastic mudstone,and volcanic lava is mainly basalt,Ayama Xuantakeiwa,andesite,angenite and rhyolite assemblage,lava with porphyritic texture,with phenocrysts of plagioclase phenocrysts and matrix with cryptocrystalline structure.Major elements analysis shows that the content of Si O₂ in rhyolite is between72.13%and 74.4%,73.3%on average shows obvious acidic rock characteristics,and the content of Na₂O+K₂O is from 5.3%to 7.95%,showing the characteristics of alkaline magmatic rocks;the content of Al₂O₃ is from 13.95%to 14.57%,the mean value is 14.31%,and the aluminum saturation index A/CNK is 1.2 to 1.69.Strong peraluminous characteristics;the RET man index is between 0.9 and 2.1,the mean value is 1.52,and the mean of alkalinity rate AR is 2.74,it shows that calc alkaline characteristics;the Fe₂O₃ contents is low,from 1.59%to 2.7%,Ti O₂ is 0.2%to 0.23%,the mean value is 0.21%,and the content is low as the whole.The acid volcanics0.5%below the island arc region it shows that the magma experienced the separation and crystallization of the hornblende and plagioclase during the evolution process,and the Mg O content is from 0.22%to 0.55%,the mean value was 0.41%,the Mg^#index was lower,from 9.54 to 14.85,and the mean value was 12.29,which showed the characteristics of late magma differentiation.The results of trace and rare earth elements show that the trace elements of the fluvial rock have the characteristics of enriching large ionic lithophile elements and losing high field strength elements.Among them,Ba,P,Ti and Sr showed strong loss characteristics,and Rb,Th,U and K showed strong enrichment characteristics,suggesting that magma could be derived from the melting of the crust.The rare earth elements have the strong differentiation characteristics,which represent the lower melting degree of the magma source region and the higher degree of magmatic differentiation.In the La-La/Sm diagram,it is shown that rhyolite is formed by partial melting of upper crust material.Zr saturation temperature is 833?,indicating that the magma originated from the melting under the condition of high geothermal gradient,suggest that suffered during the process of magmatic origin of mantle-derived magma invasion heating.Dating results of zircon LA-ICP-MS U-Th-Pb is 238±1.7Ma,indicating that the diagenetic age of the rocks is middle Triassic.In the Y+Nb-Rb and Y-Nb tectonic environment discrimination diagrams,the sample points were cast in the post-collision granite area and the volcanic arc granite region respectively.The sample points in the Si O₂-Al₂O₃ tectonic environment were scattered in the post-orogenic granite region.The sample points are scattered in the volcanic rock area of orogenic belt in the structure discrimination diagram of the log?-log?.Comprehensive regional geological data,this paper considers that the diagenetic tectonic environment of this set of volcanic rocks is the collision environment after the orogenic belt,which is formed in the transition stage of extrusion to extension.The rhyolite rocks show the alteration and silicification characteristics of weak kaolin,and the calcite strip is visible in some volcanic rocks,and pyrite sulfide is visible in the Anshan tuff.These alteration features are scattered in scattered spots on the whole,and weak rock alteration shows the characteristics of porphyry copper deposits formed;Rocks in diagenetic age are conducive to the search for copper polymetallic ore;The characteristics of main trace elements show that the ore-bearing potentiality is poor.On the whole,the volcanic rocks of the Manghuai formation in the study area has poor ore-bearing potentiality.