Ore Genesis And Metallogenic Regularities Of Iron Deposits In Helong Area,Jilin Province

Helong area,one of the iron ore clusters in Northeast China,is located at the joint part of the North China Craton and the Xing'an–Mongolia Orogenic Belt.Many iron deposits?ore spots?have been found in this area,such as three medium iron deposits?Dakaihe,Guandi and Jinan?,and some small iron deposits?ore spots?,for example,Quanshuidong,Jiashancun,Dongnancun,Baligou as well as Tushanzi.In order to improve the theoretical research and provide basis for the ore prospecting in Helong area,metallogenic conditions,mineralization characteristics,ore genesis,formation age of the Guandi and Jinan iron deposits have been researched.Moreover,the regional iron metallogenic regularities have been concluded.The iron orebodies of Guandi iron deposit,exhibiting stratoid and lenticular shapes,are mainly hosted in the Jiashan Formation,Anshan Group.The ore types include banded iron formation?BIF?and massive magnet amphibolite.The ore-bearing rocks mainly include plagioclase gneiss,amphibole plagioclase gneiss,amphibolite and so on,which experienced greenschist to low amphibolite facies regional metamorphism.The iron orebodies of Jinan iron deposit,exhibiting stratoid and lenticular shapes,are mainly hosted in the Jinan Formation,Anshan Group.The ore types are banded iron formation?BIF?and massive magnet amphibolite.The ore-bearing rocks mainly include biotite plagioclase gneiss,amphibole biotite plagioclase gneiss,biotite amphibole plagioclase gneiss,and amphibolite,which experienced amphibolite facies regional metamorphism.According to the petroligical and geochemical characteristics of plagioclase gneiss,amphibole plagioclase gneiss,amphibolite in Guandi iron deposit,and amphibolite in Jinan iron deposit,the protoliths of the ore-hosting metamorphic rocks in Helong area are a series of basic-intermediate acidic volcanic sedimentary rocks,including rhyolitic dacitic-dacitic pyroclastic rock,andesite,basalt,and subalkaline basalt?tholeiite?.The results of LA-ICP-MS zircon U–Pb dating indicated that the zircons in the ore-bearing rocks of Guandi iron deposit consist of the magmatic cores,which have clear magmatic band,and the bright zoneless metamorphic rides.The age of the magmatic cores of the zicrons in the three samples are consistent within a margin of error,2493±13 Ma,2508±28Ma,2483±25 Ma,respectively,and should represent the formation age of the protoliths of the metamorphic rocks in Guandi iron deposit.The age of the metamorphic rides of the zicrons in the three samples are consistent within a margin of error,2459±13 Ma,2467±45Ma,2472±10 Ma,respectively,and should represent the peak metamorphic age of the protoliths of the metamorphic rocks in Guamdi iron deposit.The LA-ICP-MS zircon U–Pb dating of the amphibolite from Jinan iron deposit indicated that the age of metamorphic zircons could be classified into three groups.The first group contains two old metamorphic zircons,which ages were 2468±15 Ma and 2469±9 Ma and consistent with the peak metamorphic age of Guandi iron deposit.The second group contains twenty-six metamorphic zircons,with a weighted average age of 2275±25 Ma and consistent with the youngest metamorphic age of Guamdi iron deposit,possibly representing the retrograde metamorphic age in Helong area.The last group included the youngest metamorphic zircon,which age was 1795±18 Ma and corresponding to the Paleoproterozoic tectonic metamorphic event in the North China Craton.The?_Hf?t?values of zircon grains from amphibole plagiogneiss in Guandi iron deposit ranged from-5 to+4.2,which may be the melting result of the new material and ancient crustal material,and reflected the characteristics of the island arc magma,and that the initial magma was mainly derived from the partial melting of the Mesoarchean crust.The amphibolite of Guandi iron deposit and Jinan iron deposit are similar with island-arc tholeiite or back-arc basin basalt in the trace element content and relative ratio.Combined with the tectonic setting discrimination diagram,the Helong area was under the environment of back-arc basin in Neoarchean period.Comprehensively compared with the typical BIFs both at home and abroad,Guandi and Jinan iron deposit in Helong area were consistent with the Algoma-type iron deposits in the geological conditions,mineralogenetic epoch and tectonic setting.Combined with the research results in this paper and the previous data,the iron metallogenic processes in Helong area could be divided into the earlier volcanic sedimentation process and the later metamorphism process.The basic-intermediate acidic volcanic event formed in Helong area before 2.5 Ga,resulted in the formation of the iron-bearing volcano-sedimentary rocks,which experienced greenschist to amphibolite facies regional metamorphism and resulted in the metamorphic rocks of Anshan Group at early Palaeoproterozoic?².46 Ga?.During the metamorphic process,the iron in Jiashan Formation and Jinan Formation was enriched and the Banded Iron Formation?BIF?formed.Whereafter,the metamorphic rocks of Anshan Group underwent retrograde metamorphism at².32 Ga.The iron deposits in Helong Area,a part of the Iron Metallogenic Belt in the North of Liaoning-Jilin-Korea Orogenic Belt,were hosted in Anshan Group,formed at Archaean?².50 Ga?,and distributed along the core and the NE limb of the greatest Helong syncline fold belt.Their iron bodies were damaged by the post-metallogenic fracture and stacked by the activation enrichment of iron-bearing material in the favorable space and hydrothermal channel,which were provided by the fold structure.