Tectonic Setting And Formation Mechanism Of Reduced Porphyry Copper-molybdenum Deposit,Southern West Junggar

Reduced porphyry copper deposits(RPCDs),that was first defined by Rowins(2000),are characterized by abundant hypogene pyrrhotite(especially formed in magmatic stage)and substantial CH4-rich ore-forming fluids,and genetically related to ilmenite-series granitoids with a small scale of mineralization and alteration.Tectonic setting and formation mechanism of RPCDs had become hot topics when more and more RPCDs were discovered around the world as well as in the West Junggar region.Recent researches show that Baogutu porphyry Cu deposit and Hongyuan porphyry Mo-Cu deposit in the southern West Junggar have some typical characteristics of RPCDs.However,tectonic setting and formation mechanism of these two RPCDs are still controversy and indebate.In this thesis,tectonic evolution of the the southern West Junggar and metallogenesis of the Baogutu and Hongyuan porphyry deposits were selected as two key objectives.On the basis of a great quantity of field survey and technological analysis,combining with previous published data,research progresses of this study were concluded as follows: 1)the West Junggar were divided into northern and southern two parts by Xiemisitai fault,and into six tectonic units,including the Saur oceanic island arc and Xiemisitai arc in northern West Junggar,and Barluk arc,oceanic crust-subduction accretionary complex,Baogutu arc and Laba terrane in the southern West Junggar;2)the Barluk arc was an early Devonian to late Carboniferous continental arc,the Laba arc was a late Ordovician to middle Silurian continental arc and the Baogutu arc was a Carboniferous continental island arc,the terminate time of subduction related to these arcs were 302 Ma,410Ma and 321 Ma,respectively;3)identification of copper-rich gabbro in the Baogutu deposit,implying contribution of mafic magmas to formation of this deposit;4)occurrence of pyrrhotite both in alteration and mineralization zones,and chalcopyrite-pyrrhotite-bearing magmatic-fluid pockets indicate that the Hongyuan deposit is a new reduced porphyry molybdenite – copper deposit(RPMCD),mineralization occurred at late magmatic-hydrothermal stage(440-400 ?)with an abruptly changed redox environment and hydrothermal stage(200-160 ?)with a rapidly decreasing temperature-pressure;and 5)the Continental lower crustal delamination(CLCD)and the asthenosphere mantle upwelling was the key factor for the formation of RPCDs.