Geological Characteristics And Geochronological Study Of The Yuanjiacun Iron Deposit In Shanxi

Yuanjiacun iron deposit is located in Liangquanzhuang country,southwest of Lanxian,in the Luliangshan area,west of the Central Orogenic Belt of North China Craton,and is a well-known Superior-type iron deposit in China.The ore bodies mainly occur in the strata of the Yuanjiacun Formation of the Paleoproterozoic Luliang Group.They occur in the iron-bearing rock series composed of metamorphic quartzite,Muscovite quartz schist,chlorite schist and sericite schist,and are subjected to low-grade metamorphism of greenschist facies as a whole.The magmatic rocks exposed in the mining area are mainly gneissic granite and metadiabase.The complex tectonic conditions in the mining area are mainly manifested in the superposition of multi-stage folds.The deposit consists of three ore belts and 20 orebodies with different sizes,among which,the orebodies No.10 and No.1 have the highest reserves.Ore types can be divided into quartz magnetite ore,quartz hematite ore,amphibole quartz magnetite ore,amphibole quartz hematite ore and conglomerate type iron ore.Ore minerals are mainly magnetite,hematite,and a small amount of pyrite,goethite,trace fibrinite,marcasite,as well as sporadic visible chalcopyrite,pyrrhotite,arsenopyrite,azurite.Gangue minerals are mainly quartz,followed by chlorite,cummingtonite,grunerite,calcite,dolomite,a small amount of sericite,actinolite and so on.The ore is mainly banded structure with wrinkled structure and massive structure,and the ore texture is mainly metasomatic texture with hypidiomorphic-granual texture,poikilitic texture.The EPMA data of the main iron-bearing minerals in the ore show that the composition of hematite and magnetite is close to the ideal value and the substitution degree of isomorphism is low.Only Co and Al may have substituted a small amount of isomorphism for Fe.The metamorphic magnetite is poorer than the sedimentary magnetite in SiO₂.The sedimentary hematite has higher SiO₂,Al₂O₃ and P₂O₅ than the late-formed hematite.The results show that the slight hybridization of terrigenous clastic materials,metamorphism and hydrothermal processes have a certain degree of"desilication"on hematite composition.According to the geochemical genetic diagrams,the main body falls within the scope of typical BIF iron deposits.Two sets of age data were obtained by ICP-MS zircon U-Pb isotopic dating of magnetite quartzite.The age of magmatic zircon with oscillatory zoning is 2280±30 Ma,indicating that the Yuanjiacun iron formation was formed during the rift activity of2300-2100 Ma in Luliangshan.The other group of zircons with metamorphic origin is1916±88 Ma,which represents the occurrence of metamorphic events during the iron formation period and corresponds to the metamorphic peak period of 1800-1900 Ma in the Central Orogenic Belt of the North China Craton.In addition,Sporadic age after metamorphism indicates frequent hydrothermal events after mineralization.According to mineral association and zircon age characteristics,the metallogenic period of the deposit can be divided into four periods:sedimentary mineralization period,metamorphic mineralization period,hydrothermal mineralization period and weathering mineralization period.The hydrothermal mineralization period can be further divided into two stages:quartz-sulfide stage and quartz-oxide stage.The characteristics of Fe isotopes,Si-O isotopes and rare earth elements in different types of BIF deposits in different regions indicate that all the mineralizing materials of BIF deposits originate from deep-sea hydrothermal activities after the eruption of submarine volcanoes.The S isotope study shows that the difference between Algoma-type iron deposits and Superior-type iron deposits lies in the distance from the crater.Comparing Yuanjiacun iron deposit with Anshan-type iron deposit,the fundamental difference between them is the geological structure background,which leads to a series of differences in characteristics.Combined with the geochemical data of predecessors,the comprehensive study shows that Yuanjiacun Iron Mine is a Superior type iron ore,which differs greatly from Anshan-type iron ore in terms of ore characteristics,iron formation type,metamorphic degree,metallogenic age and sedimentary tectonic setting.