Fertilizing porphyry Cu deposits through deep crustal hot zone melting

摘要

Porphyry copper deposits are the world's largest source of copper, but their ultimate source and deep process are disputed.We compare the Sr/Y ratios and Nd isotopes of coeval ore-bearing and ore-barren intermediate rocks from collisional and subduction settings to unravel the sourced depth and nature. During the Miocene mineralization epoch in the Iranian plateau, themedian Sr/Y ratios and themedian ε_(Nd)(T) values of ore-bearing rocks (56 ± 38, 1.1 ±1.5) are slightly higher than those of the ore-barren rocks (30± 31,-0.3± 2.4), respectively. Such patterns also exist at mineralization epochs during the Miocene collision in the eastern Gangdese, and the early Cretaceous subduction in Southeast China. These results indicate that all ore-bearing and ore-barren rocks are from the thickened (N1.2 Gpa) juvenile dominated (N70% mantle contribution) lower crust, and the orebearing rocks are sourced from slightly deeper and more juvenile sources compared to the coeval ore-barren rocks. The subtle geochemical difference but contrasting ore-potentials indicate heterogeneous origins, and the ore-bearing rocks may be related to ore-enriched sources at the lower crust. A deep crustal hot zonewith melting at different depths with heterogeneous sources can account for such phenomena. Delamination of a thickened orogenic root induces asthenosphere upwelling and generates abnormal heat flow in the lower crust. The porphyry mineralization is an indication of the balancing between themelting of thickened lower crust and mantle upwelling, which explains the rarity of mineralization compared to the prolonged orogenic magmatism.