Study Of Geologic Characteristic And Genesis Of The Matou Mo(Cu) Deposit In Chizhou City, Anhui Province

Porphyry type molybdenum (copper) deposit in the Matou aera is in the vicinity of Meicun Village Guichi District Chizhou City, Anhui Province, Which is located at the joint part of the Lower Yangtze fault depression belt and the Caledonian orogenic belt of Wannan. Based on thorough geological field work, we conducted systematical study on granodioritic porphyry, Matou molybdenum (copper) deposit by means of petrochemistry, isotope geochemistry and fluid geochemistry. Such as the zircon LA-ICP-MS U-Pb dating, the microthermometry data from fluid inclusions and estimation of pressure and depth, the sulfur isotope and oxygen isotope of ore, the Re-Os dating of molybdenite, the joint structure statistics of mining area. Finally, we discuss the mechanism of formation of molybdenum (copper) deposit in the Matou aera.The granodioritic porphyry, which is associated with the Matou molybdenum (copper) deposit, universally suffers from hydrothermal alteration. The SiO2ranges from54.62%to61.61%. The K2O ranges from2.54%to5.48%. The K2O+Na2O ranges from4.70%to5.72%. It belongs to high-K calc-alkaline granite and is characteristics of potassium-rich. The∑REE ranges from69.36×10-6to148.65×10-6. The (La/Yb)N ranges from12.48to24.12. They are all enriched in LILE (Ba) and LREE, depleted in HFSE (Nb, Ta, Ti) and Sr and HREE. They also have significant fractionation and slight δEu negative anomalies (δ(Eu)=0.71-0.94). Zircon LA-ICP-MS U-Pb dating yields146.7±2.3Ma(MSWD=3.0, N=14), which suggests that it is produced from late Jurassic to early Cretaceous.There are three types of fluid inclusions in the quartz veins:the two-phase inclusions with liquid-rich, the two-phase inclusions with vapor-rich and the three-phase inclusions containing daughter minerals, the number of which decrease in turn. The homogenization temperatures of primary fluid inclusions with no mineralization range from117℃to362℃with an average value of223℃,while their salinity range from0.2wt%NaCl eq to46.8wt%NaCl eq with an average value of9.98wt%NaCl eq. The homogenization temperatures of primary fluid inclusions in the mineralized quartz veins range from120℃to438℃with an average value of235℃, while their salinity range from0.2wt%NaCl eq to47.2wt%NaCl eq with an average value of9.90wt%NaCl eq. Metallogenic pressure is about10MPa and metallogenic depth is approximatelylkm. The research results proved that it should be similar to a kind of hypabyssal and medium-high temperature porphyry type molybdenum (copper) deposit.The value of δ34S of ore-minerals ranges from1.2‰to9.0‰with an average value of4.51‰. The value of δ34S of pyrite ranges from1.2‰to6.4‰with an average value of3.54‰. The value of δ34S of molybdenite ranges from5.0‰to9.0‰with an average value of6.82‰and all change little. The results deviate from the δ34S∑S values of meteorite(0±2‰), which reflects the contamination of sulfur from crust. The δ18O values of quartz ranges from10.03‰to11.18‰with an average value of10.75‰and change little, while δ18OH2O values range from0.59‰to1.74‰with an average of1.31‰. All the values above indicate that the ore-forming fluid was mixed with meteoric water in the late stage. The weighted mean age of146±1Ma is interpreted as the mineralization age of molybdenite, which is the same as zircon U-Pb age of rock of Xiachong and Re-Os age of copper (gold) metallogenic belt of middle and Lower Yangtze river reaches. The results show that the mineralization age of Matou molybdenum (copper) deposit is from late Jurassic to early Cretaceous.From late Jurassic to early Cretaceous, the tectonic regime of Guichi aera is in extension stage so that mantle occurrs partial melting. Mantle magma goes up to crust-mantle boundary or lower crust and changes crustal thermal state, which results in crust partial melting. Meanwhile, crust-derived magma intrusives along the deep fault and forms the granodioritic porphyry. Ore-forming fluid is formed at magma cooling crystallization late stage. Shortly afterwards, due to mixing action of meteoric water and tectonic decompression, ore forming materials of fluid deposits in the appropriate structural parts. Finally, hypabyssal and medium-high temperature porphyry type molybdenum (copper) deposit is formed in the Matou aera.