The Evolution Of Paleozoic Granitic Magma And Its Relation To The Copper Polymetallic Mineralization In The Northern Margin Of The Yili Block,Western Tianshan,Xinjiang

The study area which is located between Junggar block and Yili-central Tianshan block contains a large number of granitic intrusions and Cu-Fe-Mo polymetallic deposits.For these granitic intrusions and Cu polymetallic deposits,the petrogenesis,diagenetic age and tectonic setting of these intrusions and the genetic types of the Cu polymetallic deposits are comprehensively studied.Based on this,the relationship between the granitic intrusions and the Cu polymetallic deposits has been discussed.The main results and conclusions are as follows:1.The Paleozoic granitoids show high Si O2,K2 O,alkali-rich and peraluminous to weakly peraluminous,which indicate that the rocks belong to calc-alkaline to high K calc-alkaline series and I-type granite.The main magmatic source is the Neoproterozoic mafic lower crust with a little mantle-derived material.The Late Devonian granitoids formed during the subduction of Junggar oceanic crust,and the Late Carboniferous granitoids formed under collisional orogenic setting.2.In this area,the geological survey shows that the copper polymetallic deposits could be divided into two types: Lailisigao'er-3571 Cu-Mo deposit belongs to porphyry deposit,Kendenggao'er Cu-Mo deposit,Lamasu Cu deposit and Kekesala-Aimusidaiyi Fe-Cu deposit belong to skarn deposit.The ore-forming fluids and materials of these deposits are characterized of magmatic source.3.Combined with the previous and this study,it can be distinguished that the Paleozoic magmatism can be divided into 3 phase,which occurred in the Late Ordovician,Late Devonian to Early Carboniferous and Late Carboniferous to Early Permian.The metallogenesis can be divided into 2 phases,which occurred in the Late Devonian and Late Carboniferous to Early Permian.The size of the deposits formed in the second stage is much greater than that of the third stage.4.According to the mineralized outcrop of different lithofacies of the each granitic intrusion,the all granitoids can be divided into Cu-Mo mineralized granite,Fe-Cu mineralized granite and barren granite respectively.Based on this,the ore bearing property of granite has been evaluated from several aspects such as petrography,geochemistry and mineral chemistry.Firstly,the difference between the fertilized granite and the barren granite is obvious in the rock type,the degree of differentiation,the temperature and pressure of crystallization.Secondly,for fertilized granite,there are obvious differences in oxygen fugacity,degree of differentiation and Cl/F value between Fe-Cu metallogenic granite and Cu-Mo metallogenic granite.5.After a comprehensive comparative study,it is found that MMEs are similar to the host rocks in mineral composition,mineral geochemistry,whole rock geochemistry and Lu-Hf isotopic geochemistry.In addition,the petrological characteristics such as feldspar with oscillatory zones,needle-like apatite and disequilibrium feldspar xenocrysts have been discovered in MMEs,indicating that the MMEs formed from the mixing of felsic magma and mafic magma.This mechanism makes the mineralized elements within mantle continuously enrich into the felsic magmatic chamber in the lower crust,which is very favorable for the Cu polymetallic mineralization.