Microstructure And Trace Element Composition Of Minerals And Their Indication For Ore-forming Process In The Panzhihua Layered Intrusion,SW China

The layered intrusions associated with the mantle plume activity in the large igneous province are the essential window for studying the magmatic evolution process.Meanwhile,the layered intrusion with large-scale Fe-Ti-V oxide ores is also an ideal research target to reveal the enrichment process of Fe-Ti-V in magma differentiation.Based on the geological field survey,we collected a set of geological samples in the Lanjiahuoshan deposit,representing the rocks with ores in the lower part of the Panzhihua layered intrusion.On the observation basis of optical microscope,mainly by scanning electron microscope,combined with electron probe micro analyzer,focused ion beam-scanning electron microscope and high-resolution transmission electron microscope,the micro-to nano-scale microstructure of minerals was observed,the in situ composition was obtained,and various geothermobarometris constrained the physical and chemical conditions for the exsolution structures.Based on the mineral composition data from the electron probe micro analyzer,the in situ micro-area trace element composition data of minerals were acquired by the laser ablation inductively coupled plasma mass spectrometer.Taking the microstructural characteristics and trace element characteristics of major minerals as a breakthrough,combined with the theories of crystallography,mineralogy,petrology,and geochemistry,the action mode of thermal events in the magmatic evolution of the study area was analyzed and discussed.We obtained the following understandings.(1)The widely occurring exsolution structures include diopside host exsoluting both clinoenstatite lamella and Fe-Ti-oxide lamella,granular titanomagnetite exsoluting both ilmenite and spinel,and ilmenite in the rich-ore exsoluting hematite,indicating both the crystallization differentiation and the subsolid phase re-equilibrium process in the magmatic evolution.Besides,the pyrrhotite lamella in troilite host may represent the solid phase transformation process of sulfides.(2)According to the exact phase boundary theory,the exsolution pressure of the clinoenstatite lamella in the diopside host is calculated to be-2 GPa.According to two geothermometers,the exsolution temperature of this structure is 1030-1100?.Based on the obtained pressure and temperature conditions,the exsolution structure of diopside and clinoenstatite is a residue inherited from peridotite in the lower crust magma chamber.Then,diopside host exsolute Fe-Ti-oxide lamella in the same magma chamber<1100?.In the Fe-Ti-oxide lamella,the subsequent subsolidus ilmenite+hercynite exsolution may form at<450? in the upper crust magma chamber.(3)The micro structural features of minerals record the superimposed transformation of pre-crystallized minerals due to thermal events initiated by magmatic batch injection.There are characteristics of solid phase modification on the mineral boundaries between each two of silicates,oxides,and sulfides.In the secondaty melt trails,clinopyroxene forms symplectites with titanomagnetite or plagioclase,respectively;and fine olivine has the higher Fo value;both are the crystallized products by thermal events.(4)The hydrothermal fluid activity after the magmatic stage in the layered intrusion was recorded,including the occurrence of hydrated minerals,potassium-bearing minerals,Ti-free magnetite,needle-like and fringe-like sulfides,and also structures between silicates and oxides which include metasomatic pseudomorph structures,worm-like structures and porous.(5)The distribution of trace elements in the silicate-oxide-sulfide mineral phases can be obtained from the trace element data by point-analysis of minerals normalized by the trace element data of whole-rock analysis,and also the trace element mapping of mineral assemblages.Trace element composition also indicates the process of magma evolution and the effects of superposition.The strong positive correlation between high-field strength elements in titanomagnetite indicates that the magma originated from the same deep magma chamber.However,different sub-trends are formed between transition metal elements in clinopyroxene,olivine,and titanomagnetite,indicating that the intrusion experienced multiple magmatic replenishments.As the magmatic replenishments trigger a strong rebalancing process in minerals,resulting in transition metal elements with different sub-trends in the ilmenite.In summary,from the research results of microstructure and trace elements of minerals,the magma that forms the ore-bearing rocks in the lower part of the Panzhihua layered intrusion originates from the same deep magma chamber,and experienced multiple magma replenishment processes in the shallow magma chamber.Magma replenishment and post-magmatic hydrothermal fluid activities modify the microstructure and trace element characteristics of the primary minerals,but have little effect on ore grade.From the perspective of mineral microstructure and trace element characteristics,this study further deepens the understanding of the lithogenic process of the layered intrusion,enriches research content of mineralogy in the deep magmatic evolution process,and refines the model of the mineralization process of the Panzhihua layered intrusion.Meanwhile,the obtained microstructure characteristics and trace element data of minerals also have significance reference value in the research and application of the process mineralogy of titanomagnetite.