Metallogenesis And Metallogenic Model Of Submarine Volcanic Iron Ore In The Aqishan-Yamansu Metallogenic Belt,East Tianshan

As one of the main types of iron deposits in China,volcanic rock-hosted iron deposit is commonly large in scale and high in ore grade.Continental subvolcanic iron deposit is concentrated in the Luzong and Ningwu districts of the Middle and Lower Yangtze River metallogenic belt while submarine volcanic iron deposit is mainly located in the West Tianshan,East Tianshan and Altay of Xinjiang.In recent years,breakthroughs have been made in the exploration of submarine volcanic iron deposit in the Awulale metallogenic belt of the West Tianshan.Three typical metallogenic types of iron deposits have been established: magmatic iron deposit(Taertage),hydrothermal iron deposit(e.g.Zhibo,Chagangnuoer,Songhu and Beizhan)and hydrothermal-sedimentary iron deposit(Shikebutai).In the Aqishan-Yamansu metallogenic belt of the East Tianshan,several submarine volcanic iron deposits such as the Yamansu,Shaquanzi,Heijianshan,Hongyuntan,Chilongfeng were found or enlarged similarly.However,these iron deposits have insufficiently studied on metallogenic process and regional minerogenetic regularity,which hampered further exploration.In this dissertation,three typical deposits in the Aqishan-Yamansu metallogenic belt are chosen for study,which are the Heijianshan,Yamansu and Chilongfeng.These three iron deposits are classified as ore magma iron deposit,magmatic-hydrothermal metasomatic-filling iron deposit and hydrothermalsedimentary iron deposit.Each type of iron deposit is described respectively about orebearing texture,ore body and ore texture,mineral assemblage and wall rock alteration characteristics.A comprehensive and systematic summary of the submarine volcanic iron deposits in the Aqishan-Yamansu metallogenic belt of the East Tianshan about oreforming process,tectonic background,ore-hosting wall rocks,alteration types,mineral assemblages,ore body characteristics,ore texture,geochemical characteristics of major ore minerals,in an order to discuss the metallogenic mechanism and establish the regional metallogenic model.On the basis of mineral assemblages,the iron-rich agglomerates of the Heijianshan iron deposit are subdivided into five types,i.e.,the albite-magnetite type,the albite-Kfeldsparmagnetite type,the K-feldspar-magnetite type,the epidote-magnetite type and the quartz-magnetite type.Electron probe microanalysis on the iron-rich agglomerates indicates that the albite-magnetite type represents the earliest crystallization product from a residual iron-rich melt,the albite-K-feldspar-magnetite and K-feldspar-magnetite types display features of magmatic-hydrothermal transition,whereas the epidote-magnetite and quartz-magnetite types represent products of hydrothermal alteration.The composition characteristics of epidote-magnetite and quartz-magnetite types are similar to the magnetite in ore minerals,suggesting that the epidote-magnetite and quartz-magnetite types represent the crystallization of residual iron-rich ore magma and the product of complete hydrothermal metasomatism.In addition,the genesis of the iron-rich agglomerates of Heijianshan iron deposit can be ascribed to formation by the crystallization of the hydrous and oxidized iron-rich ore magma in the cracks of andesite lava and the release of gas,forming the bubble-agglomerates and amygdaloidalagglomerates.Bulk-rock geochemistry and Sr-Nd isotope show that the Yamansu basalt originates from arc magma,and the basalt is metasomatized by the ocean crust during the formation,suggesting that the Yamansu basalt is formed in a back-arc setting.Combined with Fe,O isotopes and in-situ geochemistry of magnetite,three ore types are identified on the basis of proportions of ore minerals relatively to gangue minerals of magnetite.The iron isotope fractionation characteristics of different types of ores are different,indicating that there are two metallogenic environments: magmatic-hydrothermal environment and hydrothermal environment.Therefore,the magmatic-hydrothermal metasomatic-filling iron deposits in the Aqishan-Yamansu submarine volcanic iron deposit belt have a continuous metallogenic model from magmatic to hydrothermal process.Fe,O isotopes and in-situ geochemistry of hematite and S isotopes of barite of the Chilongfeng iron deposit indicate that the provenance and environment of barite is sea water.Hematite should have the same provenance and environment as barite,which is marine environment.There is no relationship between the enrichment of ore-forming material and hydrothermal alteration,but the main components silicon and iron are precipitates in the chemical sediments with the characteristics of ore-forming fluid.Therefore,silicon and iron are derived from magmatic-hydrothermal fluids that related to submarine volcanism.Three types of submarine volcanic iron deposits in the Aqishan-Yamansu metallogenic belt of the East Tianshan reflect a relatively complete volcanism and metallogenic process: 1)iron-rich magma generation(formation of parent magma);2)fractional crystallization of ron rich ore magma separation(formation of iron-rich agglomerates in Heijianshan deposit);3)magmatic hydrothermal fluid mineralization(formation of most deposits in the metallogenic belt similar to Yamansu deposit);4)hydrothermal effusive-eruptive sedimentary mineralization(formation of Chilongfeng iron deposit).Different mineralization types in the Aqishan-Yamansu metallogenic belt of the East Tianshan result from temporal variations(time of volcanism,different stage of magma evolution)and spatial disparities(different locations of volcanic apparatus),which constitute a continuous magmatic-hydrothermal metallogenic process.