A Study On The Geological Features And Genesis Of Beizhan Iron Deposit In Western Tianshan, China

Beizhan iron deposit is the largest iron deposit in Awulale Iron Belt,which is closely related to volcanic rocks and subvolcanic rocks.It is a good object to study the relationship between volcanism and mineralization.The main ore body of Beizhan deposit is hosted by dacite porphyry,and the morphology is changed from the single thick pulse to the branch,divergence,typically controlled by volcanic rupture.In addition,the orebody is close to the contact zone between the dacite porphyry and the limestone strata,where the skarnization is extensively developed.Associated alteration assemblage is mainly characterized by Na-Ca alteration and K-Ca alteration.It is composed of four stages: skarn stage;magnetite stage;sulfide stage;chlorite-carbonate stage.The in-situ trace element analysis of LA-ICP-MS showed that magnetite is not a direct product of magma,but is formed by metasomatism of iron-rich magmatic-hydrothermal fluids.The oxygen isotopic composition of the magnetite shows that the ore-forming fluid is dominated by magmatic hydrothermal fluids.Iron isotope data shows that the iron source is mainly related to the role of magma.The carbon and oxygen isotopic composition of the calcite calcite indicates that seawater is added during the mineralization process.Pyrite,with an average of 2.5 ‰,showes magmatic sulfur characteristics.The above isotopic characteristics indicate that the ore-forming material is dominated by magma,and the ore-forming fluids are mainly from acidic magma,and seawater may be added.The Re-Os isotopic weighted average age of pyrite in Beizhan iron deposit is 303.7Ma and the Ar-Ar age of the muscovite varies from 304.5 to 308.1Ma,so the age of the iron ore is most likely to be between 302 and 308 Ma.Whole rock geochemistry shows that subvolcanic rocks and intrusive rocks enrich in LILEs(e.g.,Rb,K)and LREEs,and depleted in HFSEs(e.g.,Nb,Ta,Ti)and HREEs,similar to the geochemical characteristic of island arc volcanic rocks.It is concluded that the subvolcanic rocks and intrusive rocks are formed in the active volcanic arc tectonic environment,and it is speculated that the volcanic magmatic magma in the mining area may be partially melted by the dewatering metamorphic mantle wedge.Combined with the precious age,the magmatic activity in the mining area can be divided into two stages at least.The early volcanic rocks are formed at about 315 Ma and the late are formed in 300 Ma which are closely related to the mineralization.The analysis shows that the Beizhan iron deposit is a typical skarn-type iron deposit associated with subvolcanic rocks in the western Tianshan Mountains,which is related to the magmatic-hydrothermal activity in the later stage of the Late Carboniferous collision orogeny.The subvolcanic rocks associated with mineralization are formed by partial melting of the mantle wedges formed by melting the oceanic sediments,which may be contaminated by the lower crust during the invasion process.Driven by the volcanic rocks,and the ore-forming fluids are interbedded along the volcanic rocks and volcanic mechanism fracture through filling-confinement of the surrounding rock.