Adakite, A Possible Host Rock for Porphyry Copper Deposits: Case Studies of Porphyry Copper Belts in Tibetan Plateau and in Northern Chile
HOU ZengQian1, MO XuanXue2, GAO YongFeng3, QU XiaoMing1, MENG XiangJin1 (1 Institute of Mineral Resources, CAGS, Beijing 100037, China; 2 China University of Geosciences, Beijing 100083, China; 3 Shijiazhuang College of Economy, Shijiazhuang 050031, Hebei, China)
Porphyry-type copper deposits are the most important type of copper deposits. According to the traditional genetic model, porphyry copper deposits were formed in island or continent-marginal arc environments, the magmas of ore-bearing porphyries were derived from partial melting of the mantle wedge or the crust-mantle transitional zone and, through crystal fractionation and /or crustal contamination, these magmas evolved into calc-alkaline felsic rocks typical of island arc. However, based on a comprehensive study and comparative analysis of three important porphyry copper belts, the authors hold that, instead of being a typical arc magmatic rock, the most potential ore-bearing porphyry is a kind of rock rich in SiO 2(56%), Al 2O 3(15%), Sr(mostly400×10 -6), and poor in Y(mostly 16×10 -6). It has geochemical characteristics of adakite and shows magmatic affinity of this rock. Rather than being derived from the mantle wedge or the crust-mantle transition zone, such a kind of ore-bearing felsic magma must have originated from direct melting of subducted oceanic crust slabs. The subducted slabs was metamorphosed to water-bearing eclogite before being melted. In the Andean arc orogenic belt oceanic plates were subducted rapidly at a low degree and in an oblique direction. Direct melting produced adakitic magmas which then experienced differentiation and closed evolution and finally formed Andean Miocene-Pliocene giant porphyry copper system. In Tibetan collision-orogenic belts, old oceanic crustal rocks were subducted and accumulated in mantle lithosphere, where metamophism and detachment caused their partial melting under the condition of eclogite facies and resulted in the generation of adakitic magmas. Such magmas were mixed with mantle melts and finally produced the Gangdese and the Yulong porphyry copper systems.