Characteristics And Genesis Of Magnetite In BIF Rich Iron Ore

Liaoning Gongchangling iron ore mine is located in the southeastern part of Liaoyang City,and is an important Precambrian Anshan-type sedimentary metamorphic iron ore deposit in China.Ore bodies occur in the Cigou Formation of Anshan Group.The directly hosted enclosing rocks include magnetite quartzite,amphibolite,biotite schist and hornblende schist.The structural conditions of the mining area are complex,and the faults and folds are well developed.The magmatic rocks in the mine area are mainly mixed granite and Maayu granite.The main ore minerals in the mine are magnetite and hematite,with a small amount of chalcopyrite,pyrite,sphalerite,galena and pyrrhotite.The lean ore is dominated by strip structure,while the rich ore is dominated by massive structure.The ore structure mainly consists of crystalline structure and metasomatic structure.The magnetite in the rich ore can be divided into three types:(1)The homogenized magnetite with fine particle size,similar to the characteristics of middle-fine magnetite in metamorphic stage;(2)Magnetite showing abnormal"heterogeneity";(3)Magnetite with ring structure is developed.The particle size of the latter two types of magnetite increases significantly.Through X-ray diffraction analysis,Si and Fe in the"heterogeneous"magnetite have similar image,and the crystal structure changes,leading to the abnormal heterogeneity of magnetite.The relative molecular weight of"heterogeneous"magnetite is 231.13,cell parameter:a0=8.3994(?),chemical formula:Fe²⁺(Fe₁³.₉⁺Si_0.05Fe₀².⁺₀₅)O₄.By electron probe and LA-ICP-MS trace element analysis,the"heterogeneous"magnetite is determined to be a high-silicon magnetite,and the average Si content is 1.82%.The higher Si content,the stronger the heterogeneity.The content of Si and Fe in"heterogeneous"magnetite is negatively correlated,and there is a good linear relationship between Si and Ca element.Therefore,it is considered that Si replaces Fe³⁺in tetrahedral position in"heterogeneous"magnetite crystal.The zonal structure of magnetite is mainly caused by the change of Si content.From the inner zonal structure of magnetite to the outer zonal structure,Si content gradually increases,and the contents of Al,Ca,S and Cl have a trend of gradual increase,and the contents of Si,S and Cl are higher in the dark zonal area than in the light zonal area.Magnetite rich ore and magnetite lean ore have similar distribution characteristics of trace elements,and there is a certain degree of inheritance between them.Through the analysis of microhardness test,the microhardness of"heterogeneous"magnetite is mainly related to the content of similar elements and the size of mineral crystal grain,Si is the main factor causing the increase of microhardness,and the higher the content of Si and Ca,the greater the microhardness.The larger the mineral crystal grain,the greater the hardness.The formation of"heterogeneous"magnetite and zonal magnetite is related to hydrothermal fluids.The magnetite rich ore in Gongchangling?mining area is formed from magnetite lean ore which was originally deposited by later hydrothermal reformation.