Genesis Of High-grade Fe Skarns

The mineralogy,petrology and genesis of two typical skarn-type Fe deposits,namely,Zibo Jinlinnig and Laiwu Zhangjiwa were studied.The Jinling complex is spatially and temporally associated with the Jinling skarn-type iron deposit.The complex is composed of biotite diorite,hornblende diorite,monzonite and quartz diorite.U–Pb dating of zircons from the biotite diorite and monzonite using laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)yields ages of 126 ± 1.9 Ma and 128 ± 1.4 Ma,respectively.The unaltered rocks in the complex are characterized by variable contents of SiO2(54.6–65.3 wt.%),MgO(2.7–9.2 wt.%),total FeO(3.5–8.8 wt.%),Na2 O + K2O(5.2–8.9 wt.%),high Mg# values(73–88),Cr(103–452 ppm)and Ni(49–212 ppm)contents.The initial(87Sr/86Sr)t ranges from 0.70450 to 0.70555 and eNd(t)shows a range of 3.0 to 8.0.The primary magma of this complex is proposed to be the product of the reaction between the delaminated lower crust and the mantle peridotite.The reaction produced the orthopyroxene,which accounts for the high MgO content of primary magma.The primary magma is characterized by a high oxygen fugacity and underwent a crustal contamination process during the evolution,which emplaced multiply and formed the present Jinling complex.The relatively oxidized signature of the primary magma is favorable for the enrichment of Fe in the exsolved fluid.Detailed mineralogical analyses of the three types of amphiboles from the altered dioritic complex in the Zhangjiawa iron deposit in Laiwu were carried out.Among them,two types of amphiboles are of magmatic origin and have significant difference in FeO content.Partially,these two amphiboles are shaped with concentric circle structure.The third amphibole is actinolite and of hydrothermal origin.The mechanism of formation of the actinolite is the dissolving of the two types already formed magmatic amphiboles and reprecipitation from the fluid.The compositional difference between the two types of magmatic amphibole is due to the exsolution of Fe-rich and Cl-rich fluids from evolved magma.The Ferich fluids provided the necessary Fe budget for the formation of the deposit.In addition,the formation of the actinolite indicates a further migration of Fe from the altered complex to the fluids.This study also carried out detailed mineral geochemistry,mineral fluid inclusions and single-mineral oxygen isotopes analyses of the skarn minerals in Jinling deposit.The results show the decrease of the salinity of ore-forming fluids coupled with the decreased temperature.During the crystallization of the garnet and diopside,the ore-forming fluid underwent phase separation.The main ore-forming stage,namely,the precipitation of massive magnetite took place after the fluid phase separation.According to the field observation,the precipitation stage of the magnetite is further subdivided into the pre-precipitation stage and the main precipitation stage.Oxygen isotopic results of single minerals indicate that during the fluid evolution,there were other sources of fluids involved.Based on the geological facts and the above research results,this study proposes a mineralization model for skarn type iron deposits,which contains 8 steps:(1)the formation of magma with high oxygen fugacity,(2)exsolution of Fe-rich and Cl-rich ore-forming fluids,(3)migration of Fe from solid complex to fluids,(4)the precipitation of Ca-Si-Fe-O minerals from the ore-forming fluid,(5)the phase separation of the ore-forming fluid,(6)the pre-precipitation of magnetite,(7)the main precipitation of magnetite and(8)formation of late-stage skarn minerals and escape of the fluid.