Abstract:

It faces much difficulty to find the porphyry copper deposit in western Tianshan, Xinjiang, and it is essential to research on the CuMo mineralization. The 3571 Cu deposit was recently found next to the Lailisigaoer Mo deposit, and they constitute the Lailisigaoer3571 CuMo orefield. It is suggested that the CuMo orefield occurred in the Early Carboniferous continental margin arc which was formed by the Junggar oceanic crust subduction under the KazakhstanYili Plate. The Siluric siltmud hornstones were intruded by several small granodiorite porphyry bodies in the orefield, and CuMo mineralization may have happened during the structure movement, magma activity and hydrothermal processes on the continental margin arc setting. The CuMo sulfide orebody in shape of veins, tabular and lens occurred in the top or edge parts of the granodiorite porphyry, contact belts and the wallrock nearby. The hydrothermal alterations changed from Ksilication to sericitization, and then propylitization, and the mineralization shows the change in the structure of ores from dissemination to veindissemination, and then veins. The major ore minerals include molybdenite, chalcopyrite, pyrrhotine, bornite, and pyrite. The mineralization is characterized by a porphyry CuMo type. The granodiorite porphyry is rich in Si, Al, K, Na, Rb, Ba, Th, and poor in Mg, Ta, Hf, Zr, Sm, Sr, Y, and Yb, and the trace element character is similar to that of continental margin granites. The average ∑REE of the porphyry is 133.84×10-6 and lower than that of Stype granites. ISr is from 0.7079 to 0.7103, εNd(t) is from -0.61 to -3.71, and the magma was from the mantlecrust mixing or the lower crust. The ore δ34SVCDT is from 2.7‰ to 4.3‰, 206Pb/204Pb from 18.0997 to 18.2588, 207Pb/204Pb from 15.6052 to 156242, 208Pb/204Pb from 38.1245 to 38.3078, and the metal was from the upper mantle and lower crust directely related to the continental margin arc magma. The δ18OH2O VSMOW of the late stage oreforming fluid is from -1.33‰ to 5.46‰,  δDH2O VSMOW from -81‰ to -101‰, and the water in the oreforming fluid is mainly the magma water with meteoric water mixed. The porphyry was dated as 346±1.2 Ma in the Lailisigaoer Mo oredistrict and 350±0.65 Ma in the 3571 Cu oredistrict by LAICPMS UPb method. The CuMo mineralization occurred in the Early Carboniferous Period. The Lailisigaoer3571 CuMo orefield should have been in the same metallogenic series with the Axi Au orefield, and this is very important for the exploration in western Tianshan, Xinjiang.

Key words:  porphyry CuMo ore deposit, magma, ore geology, S, Pb, O and Hisotopes, UPb dating, Lailisigaoer3571, western Tianshan