Petrogenesis And Mineralization Of Granodiorite Porphyries In Guichi Region,Anhui Province

Guichi region of Anhui Province is one of the important metallogenic provinces in Middle and Lower Yangtze metallogenic belt where the formation of Cu,Au,Mo and polymetallic deposits has closely genetic relations with Late Mesozioc granodiorite porphyries.Although certain results on rock-and ore-forming have been abtained,petrogenesis of granodiorite porphyries has been debated at present.In this study,petrology,mineralogy,element geochemistry,zircon chronology and Hf isotope,and in-situ element compositions of minerals from granodiorite porphyries were systematically investigated in the Guichi region,respectively.Combined with previous research results,we obtain the main conclusions as follow.1.Magma mixing was likely to the formation of Guichi granodiorite porphyries,on the basis of mineral geochemistry.Whereas,contiental crust composition plays major role.Its magma crystallization temperature is 600⁸00°C.2.The Guichi granodiorite porphyries?SiO₂ = 63.3 64.8 %?are a high calc-alkaline I-type granite,and have low Cr contents(30×10^-6 50×10^-6),low Nb/U?3.40 7.23?and Nb/Ta?11.6 13.5?ratios,higher Mg#,enrichment of Rb and K,relatively depletion of Ba and Sr,and weak Eu negative anomalies show that the mixing of mantle-derived and crust-derived magmas might have been the main mechanism of the Guichi granodiorite porphyries,followed by fractional crystallization of plagioclase,K-feldspar and accessory minerals.The proterozoic metamorphic basement material is the main source.The magmatic activity is closely related to subduction of the Palaeo-Pacific plate.3.The results of high precise zircon U-Pb isotopic ages show the formation of the Guichi granodiorite porphyries is late Jurassic,is the product of first-stage magma activity in the Middle and Lower Yangtze metallogenic belt.4.The in-situ geochemistry and Lu-Hf isotopic characteristics of zircon(?_Hf?t?=-12.7 ^-3.0,t_DM2=1.4².0Ga),and Ree characteristics of apatite further support that the Guichi granodiorite porphyries was crust-mantle mixed product.Paleoproterozoic-Mesoproterozoic lower crust material is dominant.5.The Cu,Au and Mo mineralization might be closely related with high oxygen fugacity of the magma in the Guichi region.