Genesis And Geological Significance Of Ilmenite (Magnetite)-rich Intermediate-mafic Complex In Ruze-Zhaigun Area,Northern Guangxi

The Ruze-Zhaigun area is located in the southeast margin of the Yangtze Plate and the southwest segment of the Jiangnan orogenic belt,where the Cathaysia Plate and the Yangtze Plate meet.It is an important part of the Jiuwandashan metallogenic concentration area in the western segment of the Nanling metallogenic belt.In recent years,several intermediate-basic complexes rich in Fe-Ti oxides(Titanium,magnetite)have been discovered in Ruze-Zhaigun area,northern Guangxi,which indicates that this area has good prospecting potential for titanite(magnetite)ore.In this thesis,the newly discovered Ruze and other titanium-rich(magnetite)iron ore intermediate-basic complexes have been systematically studied in petrology,petrogeochemistry,mineral chemistry,and isotope chronology.The main achievements and understanding are as follows:(1)The intermediate-basic complex of titanium-rich(magnetic)iron ore is often produced in"rhythmic layer"on the field outcrops,and the bottom of the rhythmic layer is mainly composed of metamorphic(gabbro)diabase and metamorphic(feldspar)hornblende pyroxenite and other rocks,which often contain rich Fe-Ti oxide(titanium,magnetite).The top of the rhythmic bed is mainly composed of metamorphic(gabbro)diabase and metamorphic diabase porphyrite(basalt porphyrite),and the content of Fe-Ti oxide(titanium,magnetite)is less.(2)The results of geochemical study show that the Ti O₂content in the bottom of rhythmic layer(1.58%?1.90%)of the titanium-rich(magnet)iron ore medium-basic complex is significantly higher than that in the top of rhythmic layer(0.5%?1.03%),which is consistent with the characteristics that the rocks at the bottom of rhythmic layer are more enriched in Fe-Ti oxide(titanium,magnetite).The geochemical characteristics of the rocks are similar to those of the island arc(active continental margin)type magmatic rocks.(3)LA-ICP-MS zircon U-Pb dating shows that the weighted mean ages of(gabbro)diabase and diorite porphyrite in the intermediate-basic complex of titanium-rich(magnetic)iron ore are(842.9�3.9)Ma(MSWD=0.22)and(850.0�25)Ma(MSWD=2.9),respectively,which are both formed in the Neoproterozoic.(4)The chemical composition of amphibole and biotite shows that the primary magma of amphibole and biotite originated from mantle magma,and crust-mantle magma mixing occurred during the process of upwelling and crystallization differentiation.The physical and chemical conditions show that the three minerals,clinopyroxene,amphibole and biotite,were formed in high temperature and high oxygen fugacity environment.(5)According to the above research results,it is concluded that the magma evolution of the intermediate-basic complex in Ruze-Zhaiguan titanium-rich(magnetic)iron deposit experienced two stages of continuous emplacement:(1)early magma evolution caused by subduction,collision orogeny,mantle-derived magma to rise to the earth's crust deep parts(20?25 km)occurs when the cooling crystallization differentiation,under the condition of low oxygen fugacity,ilmenite separation and crystallization in great quantities,and monoclinic pyroxene hornblende and early together out of the dark mineral crystal precipitation accumulation formation at the bottom of the"rhythm"is rich of ore-bearing bed;(2)when the magma rising faster to shallow crust(1?5 km),as the magma evolution of monoclinic pyroxene,hornblende and biotite minerals such as start bulk crystallization,consumes magmatic melt of Fe²⁺,raising the oxygen fugacity in magmatic melts gradually,is beneficial to the crystallization of magnetite,eventually more magnetite is formed in the rock.