Geochronology of mineralization and tectonic setting of Baishidong skarn-type iron deposit in Helong area of Yanbian,Jilin Province,NE China

The Baishidong iron deposit is the only skarn-type iron deposit discovered in the study area. According to mineral assemblage and paragenesis, the mineralization can be divided into four metallogenic stages: early garnet-diopside skarn stage, late magnetite-tremolite-epidote skarn stage, early quartz-pyrite-chalcopyrite stage and late quartz-calcite-pyrite stage. Through LA-ICP-MS zircon U-Pb dating of diorite, which is closely related to mineralization, the results show that the weighted average age is 164.6±1.4 Ma, which limits the mineralization time of Baishidong iron deposit during or slightly later than Middle Jurassic. The diorite rocks are rich in sodium(Na_2O/K_2O=1.24--1.76), aluminium(Al_2O_3=17.41%--18.76%), LREE and large-ion lithophile elements(Ba, K and Sr), depleted in HREE and high-field-strength elements(Y, Nb, Ta, P and Ti), and show strongly fractionated patterns(LREE/HREE=6.58--9.93), no apparent Eu anomalies(δEu=0.91--1.13), which shows similar characteristics to island arc or active continental margin arc magma. The zircon εHf(t) values range from-22.6 to 5.9, and the age of the two-stage model(t_(DM2)) is 836--2 633 Ma. Above data combined with the geochemical characteristics, it is indicated that the magma was a mixture of multiple sources, composed of ancient materials and newly formed crust. During the process of evolution and ascending of magma, separation and crystallization occurred, and a large amount of continental crust material was mixed at the same time. Combined with regional tectonic evolution, the formation of this deposit may be related to the westward subduction of the Paleo-Pacific Plate to the Eurasian Plate.