A Study On The Mineralizationof Iron Ores In The Shikebutai Iron Deposit In Western Tianshan,China

Shikebutai Fe deposit is an important marine volcanic-related sedimentary iron-rich deposit within the Awulale Fe metallogenic belt in the west of Tianshan moutains.With a few research results about basic geology and stable isotope,there are also some arguments among the origin of deposit,mineralization geological background and minerogenic substances.This paper will figure out the petrography characteristics of volcanic rocks and their formation ages,analyse their petrogenesis and the environment of formation,and discuss the mineralization geological background through the research of geochemistry,petrology and chronology.We will also discuss the minerogenic substances and the mineralization process by the studing of O,Fe isotope composition of hematite,S isotope composition of pyrite and barite,LA-ICP-MS analysis of hematite.At the same time,the discussion of the relationship between the ore body and altered wall rock will be taken up after the Ar-Ar chronology research of sericite within the altered country rock.The Shikebutai iron deposit,which islocated in the Yili rift in western Tianshan,China,is hosted in Upper Carboniferous intermediate-acidic volcaniclastic rocks and low-grade metamorphic schist and phyllite.The ore-bodies are stratiform,stratoid orlenticular.Ore minerals are predominantly hematite and specularite,with a small amount of pyrite and siderite.Gangue minerals are dominated by jasper,barite and quartz,with a small amount of calcite.The ores display banded,lamellar and massive structures.Ore minerals usually show cryptocrystalline,filling or hypidiomorphic textures.Furthermore,Ore textures and mineral assemblages indicate the following general paragenetic sequence: pyrite-hematite-jasper-barite stage,siderite-pyrolusite stage,quartz-specularite stage and oxide minerals stage.The lamellar ore structure and stratabound characteristics indicate the Shikebutai iron deposit is of sedimentary origin.The results of electron probe component analysis on ore minerals show that:?1?Hematite from massive ore have variable Al₂O₃,Na₂ O,Mgo and SiO₂ contents indicating that massive hematite ore are the products of rapid sediment and crystallization,and the rapid extravasation of iron-rich fluid;?2?Hematite from lamellar ore and banded ore have concentrated Al₂O₃,Na₂ O,Mgo and SiO₂ contents,which shows that they formed in calm sedimentary environment with slow extravasation of iron-rich fluid;?3?The extravasation rate of mineral-rich fluid and the sedimentary environment changed continuously during metallogenic process;?4?The high Co-Ni ratio of pyrite shows that it was derived from volcanism;?5?A negative correlativity between FeoT and MnO+MgO is found in siderite which is divided into two groups corresponding with two different types of siderite under microscope respectively,which implies siderite experienced a process of differentiation during late metallogenic stage.The results of LA-ICP-MS in situ trace element analysis of magnetite show a similar trace element compositon to that of BIF iron deposit,which supports that the formation of this deposit are related to sedimentary exhalative processes associated with submarine volcanism.The ?34S values of pyrite range from-7‰ to 7‰ and that of barite is between 12‰ to 19‰,which indicate that the sulfur was magmatic-derived,and meantime it had fractionated between sulfide and sulfate.The ?¹⁸O values for hematite from the Shikebutai vary from-1.7‰ to 5.7‰,which implies that the ore–forming materials are related to the volcanism and magmatic evolution.LA–ICP–MS U–Pb dating of igneous zircons from andesitic porphyrites represent the age of mineralization?320 318 Ma?.Ar-Ar dating of sericite from schist yielded metamorphic ages from 261.3Ma to 247.8Ma which is later than the metallogenic age,demonstrating the iron mineralization and the metamorphism belong to two different geological events.Theresults of rock geochemical analysis show a negative correlativity between SiO2 and major elements,which indicates the volcanic host rocks of Shikebutai display a compositional continuum.Host rocks are enriched in LREEs,and depleted in HFSEs?e.g.,Ta,Nb,Sr,P,Ti?and HREEs,supporting a volcanic arc rocks.In summary,Shikebutai iron deposit is considered to be a submarine volcanic exhalative-sedimentary high-grade iron deposit.