Study On Magnetite From Mesozoic Intrusive Rocks In Tongling Area,Anhui Province

The Tongling area of Anhui Province is located in the middle of the copper,gold,iron and polymetallic metallogenic belt in the Middle-Lower Yangtze River Valley,where Mesozoic intrusive rocks are closely related to copper-gold mineralization.However,some problems concerning the origin and evolution of magma of the intrusive rocks and their relationship with copper-gold mineralization still need to be studied in depth.Magnetite is a common accessory mineral in magmatic rocks,and its microstructure and chemical composition are of great significance in tracing the origin,evolution and physicochemical properties of magma.In this article,three typical intrusive rocks,Hucun granodiorite(Hucun pluton),Tongguanshan quartz diorite(Tongguanshan pluton)and Baimangshan pyroxene diorite(Baimangshan pluton),which are closely related to copper-gold mineralization in Tongling area,Anhui Province,are selected as the research objects to carry out the genetic mineralogy of magnetite in the intrusive rocks,and the following understandings were obtained.There are obvious differences in the microstructure of magnetite from the three types of intrusive rocks in Tongling area.The microstructure of magnetite in Hucun pluton and Tongguanshan pluton is mostly homogeneous,and only a few grains have porous structure composed of magnetite and hematite.According to the composition of minerals and morphology,the microstructure of magnetite in Baimangshan pluton can be divided into two types: homogeneous structure with magnetite as main phase and the exsolution structure of ilmenite.The latter can be further divided into three types: banded exsolution structure of ilmenite,trellis exsolution structure of ilmenite and the combination structure of banded exsolution and trellis exsolution of ilmenite,among which the trellis exsolution structure is the most developed.Generally,the banded exsolution width of ilmenite is usually larger than that of trellis exsolution,indicating that the former has higher formation temperature.The microstructure characteristics of magnetite record that the formation temperature of magnetite in Baimangshan pluton is higher than that in Hucun pluton and Tongguanshan pluton,and it has experienced the evolution process from high temperature to low temperature.Mineral inclusions are common in magnetite.The mineral inclusions of magnetite in Hucun pluton and Tongguanshan pluton are mainly composed of zircon and apatite,while those in Baimangshan pluton are mainly apatite and sphene,and no zircon inclusions are found.The mineral inclusion assemblage of magnetite indicates that the formation temperature of magnetite in Baimangshan pluton may be higher than that in Tongguanshan pluton and Hucun pluton.Among the three types of intrusive rocks,the content of trace elements such as Ti,Al,Mg,Mn,Zn and Sn of magnetite in Baimangshan pluton is the highest,followed by Tongguanshan pluton and the lowest in Hucun pluton,indicating that the formation temperature of magnetite from pyroxene diorite to quartz diorite and then to granodiorite gradually decreases.The calculation results of Fe-Ti oxide assemblage in Baimangshan pluton show that the temperature range is 570.57?589.71?,and the oxygen fugacity value is-18.64?-18.05,which indicates that magnetite was formed in the magmatic system with high oxygen fugacity.The difference of V content of magnetite suggests that the primitive magma of Hucun pluton and Tongguanshan pluton has higher oxygen fugacity than that of Baimangshan pluton.In general,the three types of intrusive rocks in Tongling area are characterized by high oxygen fugacity,which is benefit to the stable migration and evolution of copper and gold in the magma.The chemical composition of magnetite also shows that the material sources of the three types of intrusive rocks all have the characteristics of crust-mantle mixed sources,but the mixing ratio of crustmantle materials is different,and it can be rule out that they gradually evolved from basictype magma through fractional crystallization.