The Evolution Of Granitic Magma And Its Relationship With The Donggebi Porphyry Molybdenum Deposit In The Xinjiang Autonomous Region

According to the typical porphyry molybdenum deposit model,the ore-forming granitic porphyry derives from partial melting of relatively old continental crust,which posesses high abundance of Mo.However,a series of newly discovered porphry molybdenum deposits in Central Asian Orogenic Best show positive initial ?Hf value,such as Donggebi porphyry molybdenum deposit,suggesting that they are likely derived from a young continental crust and inconsistent with the typical model.In order to elaborate the scientific issue,this study focuses on the petrology,petrogenesis and process of molybdenum concentration in Donggebi porphyry molybdenum deposit in East Tianshan area Xinjiang Uygur autonomous region.The ore-forming granitic porphyry in Donggebi porphyry imolybdenum deposit is biotite monzogranite and consists mainly of albite,potassic feldspar,quartz and biotite and the accessory minerals include apatite,rutile,xenotime,erenute and zircon.The biotite monzogranite displays high content of SiO2,Na2O and K2O and low content of TiO2,MgO and CaO.Trace elements show obvious negatively anomalies in Eu,Ba,Sr,P and Ti.Many characteristics suggest that the biotite monzogranite belong to I-type granite,including high Mg/Fe ratios of biotite,low abundance of Zr+Nb+Ce+Y of bulk rock,high(Na2O+K2O)/CaO ratios,obvious positive correlation between P2O5 and SiO2,slightly positive correlation between La and Rb and no correlation between Y and Rb.The existance of mafic rocks near the monzogranite,high SiO2 content of monzogranite,low Zr/Hf Nb/Ta and FeO+MgO+TiO2/SiO2 raito and high Na2O+K2O/CaO ratio suggest that the monzogranite is highly fractionated.This is consistent with the high MnO content and high Mn/Na ratio of apatite.Based on a series of thermobarometers?it is calculated that the crystallization pressure is between 1.2-1.8 kbar,temperature between 645-695?.According to the SO3 content of apatite,we are able to calculate the concentration of SO3 of equilibrated melt which is between 8-68 ppm and further calculation imply that the volume of magma chamber is very large(25-215 km3).A combination of Ti diffusion model and Ti-in-quartz thermobarometer indicate that the magma chamber may last a long time(more than 5000 years).This study suggests that a relatively large magma chamber and high degree of fractional crystallization are the the key factors in controlling the formation of porphyry molybdenum deposit rather than the high abundance of Mo in parental magma.