Characteristics,Genesis And Mineralization Indication Of The Xiejialong Intrusion In The Tongling Area

The Mesozoic magmatic rocks in the Tongling area are widely distributed,and most of them are closely related to copper and gold deposits.However,there are also a few barren intrusions in this area.A detailed study on the barren intrusions would give a clue to better understand the genesis of the magmatic rocks and associated ore deposits,and provide new evidence for the theory of mieralization diversity of magmatic rocks.Therefore,a detailed petrology,mineralogy and geochemistry study has been carried out on the Xiejialong pluton,one of the largest barren plutons in the Tongling area,with focus on an analysis of petrogenesis and the reasons that lead to mieralization diversity of magmatic rocks.Rocks of the Xiejialong pluton are medium-to fine-grained granodiorite,and consist of quartz?20–25%?,plagioclase?40–45%?,K-feldspar?15–20%?,hornblende?8–10%?,and locally biotite?1–2%?.Electron probe analysis shows that the plagioclases are oligoclase and andesine,and hornblendes are magnesiohornblende,tremolite,and edenite.Petrographic observation shows that the order of mineral crystallization in the Xiejialong granodiorite is amphibole,hornblende+plagioclase,K-feldspar,quartz,displaying a normal cystallization sequence of the first crystallization of mafic minerals,followed by felsic minerals.There are a large number of equilibrium textures in the Xiejialong pluton,such as short prismatic apatite,and normal zoning of plagioclase,indicating the pluton is from a single magma source area.The Xiejialong pluton show relatively high Sr?886.00-937.00ppmppm?content,but low Y?17.30-19.20ppm?and Yb1.61-1.77?ppm?contents,indicative of apparent adakitic characteristics.On the hornblende Al₂O₃-TiO₂ diagram,most spots of the Xiejialong pluton lie in the area of crust-derived magma.These features,together with low concentrations of MgO,Cr,Co,and Ni of the Xiejialong pluton,indicate that the Xiejialong pluton was formed directly from partial melting of lower crust.Crystallization depths and temperatures of hornblende and plagioclase in the Xiejialong pluton are 1.16-2.90km and 709-819?,respectively,based on hornblende geobarometer and amphibole–plagioclase geothermometer.The oxygen fugacity?logf?O₂??and water content of the Xiejialong pluton are-13.6^-12.1 and 1.7³.1%,respectively.The comparative study indicate that there are significant differences in magma source,ore-forming element content and water content between the intrusions associated with copper-gold deposits and barren intrusions in the Tongling area,which may be the key factors controlling the mineralization of magmatic rocks in the Tongling area.