Geological And Geochemical Characteristics And Genesis Of The Cihai Iron Deposit In Beishan

The Beishan orogenic belt is one of the important prospective areas for iron ore exploration in China.It is located in the south of the eastern Tianshan orogenic belt and experienced complex tectonic-magmatic-metallogenic events.The Cihai deposit is an economically important large iron deposit in the Beishan tectono-metallogenic belt and the most significant iron prospecting achievements.Based on previous research achievements,field geological research and laboratory observation,testing,analysis,we have conducted the systematic studies on the Beishan Fe deposits,and discussed the genetic mechanism and the associated magmatic process and source,which will provide new information for prospecting and exploration of magmatic Fe deposits in Beishan and eastern Tianshan.The main achievements of this study are as follows:?1?The LA–ICP–MS zircon U–Pb datas in this study indicates that the mafic intrusion in the Cihai occurred at 306.9±3.2 Ma;The ore bodies in the Cihai deposit mainly exsit in diabase intrusion,and the boundary line with the surrounding rock is obvious and clear,thus concludes that its metallogenic age is slightly later than the diagenetic age.?2?The petrogeochemical data show that all Cihai diabase samples are characterized by high sodium,low potassium and high magnesia-iron,and belonging to alkaline-subalkaline series,showing the characteristics of mantle source.Moreover,the samples were enriched with large ion lithophile elements?LILE;e.g.K,Pb,Rb,Sr?and depleted with high field strength elements?HFSEs,e.g.Nb,Ta and Ti?,accompanied by slight Eu abnormalities?Eu/Eu*=0.90¹.11?.The fractionation rare earth elements is not obvious and LREE is slightly enriched.These characteristics indicate that the mantle source had affected by early subduction materials.?3?The Sr-Nd isotope dates show of the diabases have higher values for ⁸⁷Sr/⁸⁶Sr and positive values for?_Hf?t??+0.9⁺6.2?;The?_Hf?t?values of in-suit Hf isotope of zircon range between+13.0⁺18.1.Combined with the geochemical characteristics,its indicated that the parent magma of diabases is formed by the interaction of the depleted asthenospheric mantle and the enriched lithospheric mantle affected by the early plate material.?4?The Fe isotope datas show that the?⁵⁷Fe values of the diabases and ore are-0.29‰⁰.36‰and-0.01‰⁰.43‰,respectively,and reflect that the ore-forming materials are derived from magma.The O isotope composition of magnetite and the S isotope of pyrite in the ore district are similar to that of mantle-derived mafic rocks,suggesting that the ore-forming material originated from deeper mantle source.?5?The Beishan and the adjacent eastern Tianshan region were in post-collision extension stage from Late Carboniferous to Permian.And the enriched lithospheric mantle disintegration,triggered the asthenospheric mantle upwelled and interacted with the dismantled lithospheric mantle to form mafic magma.After the emplacement of mafic magma,the iron-bearing ore produced by the influence of crystallization differentiation penetrated into the fracture of the mafic rock and mineralized.Then,the iron-rich fluid generated by magmatism develops hydrothermal metamorphic mineralization in the mining area.