The Mesozoic Magmatism At The Xinqiao Mine,Tongling,Anhui Province And Its Relation To Mineralization

Located in the Tongling ore deposit concentrated area,the Xinqiao Cu-S-Fe deposit is a typical skarn-type deposit,with a close genetic relation to the Jitou pluton.Based on previous studies on the Xinqiao deposit,a comprehensive field investigation has been carried out on the Xinqiao mine,and a detailed petrographic observation with chronological,mineralogical and geochemical analyses completed on the Jitou intrusions,with a focus on the discussion on the Mesozoic magmatism and its metallogenic significance in the Xinqiao mine.The Jitou pluton is a composite pluton mainly composed of quartz diorite and diorite porphyry,in which there are dioritic to gabbroic and rich-mica xenoliths.The Jitou quartz diorite and diorite porphyry intrusions are dated at(141.6±1.1)Ma and(129.9±0.6)Ma using the LA-ICP-MS zircon U-Pb method and represents the Early Cretaceous product of two periods of magmatism.A series of disequilibrium magma crystallization textures in phenocrysts are observed under microscopy,including unregular zone,rounded margin,sieve,and poikilitic textures.The Jitou diorite intrusions are high-K calc-alkaline series.The trace and rare earth elements geochemical and mineralogical analyses combined with the microscopic observations indicate occurrence of crystalline fractionation,assimilation and contamination,and magma mixing.The Jitou intrusions belong to the magmatic rocks with a mixing source of mantle with crust.The geological and geochemical data are consistent with the formation of quartz dioritic and dioritic magmas through the crystalline fractionation,assimilation and contamination,and magma mixing of high temperature evolved gabbroic dioritic magma during its moving up to middle crust at the depth of 13 to 15 km in the background of lithospheric extension and thinning.Cooling and crystallization of two periods of the quartz dioritic and dioritic magmas emplacing up to upper crust at the depth of 7 to 8 km in the condition of tectonic disturbance would be responsible for the formation of the Jitou pluton.Continuous accumulation of a lot of ore-forming materials resulted from magmatic hydrothermal activity would lead to the final formation of the Xinqiao deposite through contact replacement and interstratified hydrothermal replacement.