The Formation And Evolution Of Zonation Pattern In Magnetite From A Magmatic-hydrothermal System

Located in the northern Qinghai-Tibet Plateau,the East Kunlun orogenic belt contains numerous metal mineral resources?e.g.,Fe,Cu,Pb,Zn,Ni,Au?.Skarn iron deposit is an important constituent part of the East Kunlun metallogenic belt,and many large-medium deposits related to this type have been proved in the past decade,such as Yemaquan,Galinge,Kendekeke,Baishiya and so on.Previous research has done detailedwork through geological survey,distribution of mineralization and alteration,as well as geochronology,and petrogenesis of ore-related magmatic rocks.However,geochemicalfingerprintsinferredfrommagmaticevolutionand magmatic-hydrothermal transition remain poorly understood,which has hampered our knowledge of skarn systems in the East Kunlun metallogenic belt to a certain extent.More importantly,the last five years have seen the progress and development of in-situ analysis,which has made it possible to determine in situ textural and compositional complexities in magnetite.It has soon become a hot topic to depict a refined magmatic-hydrothermal evolution history by using this method,especially on magnetite with zonation pattern from skarn systems.However,there is increasing evidence to show that this in-situ analysis on micron scales cannot always reflect the overall evolution history of hydrothermal fluids.In addition,an enrichment of incompatible elements in magnetite?e.g.,Si,Al,Ca?cannot be explained by traditional isomorphism theory,either.In this study,two skarn iron deposits in the East Kunlun orogenic belt have been picked up as our target.The geochronology,petrogenesis,hydrothermal event,high field strength elements?HFSEs?of granitoids from the Yemaquan orefield have been investigated to reconstruct magmatic evolution history and subsequently trace the geochemical fingerprints by magmatic-hydrothermal transition.Then,based on a detailed paragenetic sequence of skarn assemblage,in-situ textures and trace elements analysis of magnetite has been conducted,which is reinforced to probe the possibility of mineral nanoparticles by using transmission electron microscopy?TEM?,aiming to explore the formation and evolution of zonation pattern in hydrothermal magnetite from skarn systems,as well as geological implications.The Yemaquan orefield has recorded a series of magmatic rocks from mafic to felsic,including diorite,granodiorite,porphyritic granodiorite,granite and syenogranite.Zircon U-Pb geochronology suggest that they formed in late Triassic?228²21 Ma?,and the²25Ma porphyritic rocks is consistent with hydrothermal phlogopite ⁴⁰Ar-³⁹Ar age,indicating a mineralization potential of porphyritic rocks.Other intrusions are relatively a barren suite.The relatively homogenous Sr-Nd-Pb-Hf isotopic compositions of these rocks suggest that they are originated from a same magma source.Starting with the most mafic diorite as a parent magma,the porphyritic rocks have gone through amphibole crystallization for high water contents,whereas biotite is dominant in the formation of barren granite and syenogranite due to a continuous crust contamination,both of which are inferred from trace elements signatures.Compared with barren suites,mineralized porphyritic rocks have distinctly lower Nb/Ta ratios,and they have jumped out of the magmatic evolution in the diagram of Nb/Ta vs.Zr/Hf ratios.According to previous experimental data in an equilibrium model,amphibole crystallization alone cannot account for the lower Nb/Ta ratios in the porphyritic rocks.It is time to call for a crucial role by subsolidus hydrothermal alteration,which is further supported by a preferentially mobility of Nb over Ta in F-bearing fluid from previous research.Based on the variation of skarn mineral assemblages,magnetite mineralization in the Baishiya orefield could be divided into four stages:first stage?S1?,euhedral magnetite coexisted with phlogopite and hedenbergite,and displays homogenous core and chemically oscillatory rim;second stage?S2?,anhedral magnetite is associated with garnet,hedenbergite and minor calcite,and characterized by multiple generations with irregular boundaries;third stage?S3?,there are well-developed porosities in the zoned magnetite with siderite and calcite;fourth stage?S4?,quartz and pyrite could be found in magnetite ores,and there is a narrow rim in the anhedral magnetite crystal.The zonation pattern in magnetite could be distinguished into two types:one type is oscillatory zonation induced by growth,the boundaries are parallel to each other and crystallographic face,another one is caused by dissolution-reprecipitation reactions and metasomatism of hydrothermal fluids.The zonation patterns in magnetite are associated with chemical compositions?e.g.,Si,Al,Ca?,and exhibit systematic variation of trace elements contents.Some trace elements,such as V+Ti,Ni,V,and Ga,could be indicators of the formation temperature of magnetite,which are in accordance with skarn assemblages,while the concentrations of Pb and W could be used to trace the precipitation of sulfide.The evolution history of hydrothermal fluids in the Baishiya orefield is thus divided into two stages:one is a gradual decrease of temperature from S1 to S3,leading to the variation of mineral assemblages and subsequently local fluid compositions,as well as dissolution-precipitation reactions characterized by chemical zonation and porosity;and another one is a steep decrease of temperature with sulfide formation from S3 to S4,marking the end of the magnetite mineralization.However,it is intriguing that the dark zones?back electron scatter image?in magnetite from S1 and S2 have shown a much higher Nb/Ta and Zr/Hf ratios.This chemical signature could not be explained by variation of HFSEs contents in fluid phase or physicochemical parameters.As discussed above with respect to HFSEs mobility in hydrothermal fluid,this study contends that phlogopite crystallization in local scales has resulted in a depletion of F and Cl contents,leading to an instability of HFSEs complex whilst an enrichment of Nb and Zr in local fluid-magnetite interface.The chemical oscillations in magnetite from S1 has further been investigated by TEM to explore this local reaction mechanism at fluid-mineral interfaces.It is the first time to report a new discovery of mineral nanoparticles in hydrothermal magnetite,which could be classified into three types.The nanoscale Al-Mg-rich lamellae formed by exsolution from host magnetite due to cooling in late stage,whereas the zinc spinel nanoparticles were precipitated directly from the interfacial fluid and enclosed by host magnetite.Besides,some Ti-rich magnetite nanoparticles have also been observed.Large numbers of mineral nanoparticles with layered distribution could account for chemical oscillations in micron scales,which has completely reformed our knowledge.Firstly,traditional analytical methods in micron scales have obviously overestimated trace elements contents that have been incorporated into magnetite structure.Furthermore,microstructure in minerals are generally corresponding to trace elements distribution,while the related geological process cannot always be revealed because they may be“invisible”unless nanoscale methods are utilized.Finally,although Zn concentrations in bulk fluid may be less than 100 ppm,zinc spinel phase could still reach supersaturation and precipitation in local fluid-magnetite interfaces,and they were well preserved by the overgrown magnetite,which has protected nanoparticles from re-equilibrated processes in late stage?e.g.,dissolution-reprecipitation reactions?.This reaction mechanism could not only explain the abnormally geochemical behaviors of some trace elements in hydrothermal fluid,but also provide a local kinetic effect caused by interfacial fluid that are suitable for minerals with oscillatory zonation in geological processes,especial for non-equilibrated fluid-driven systems.