A new hypothesis on parameters controlling the formation and size of porphyry copper deposits: Implications on thermal gradient of subducted oceanic slab, depth of dehydration and partial melting along the Kerman copper belt in Iran

摘要

Porphyry copper deposits in the Tethyan metallogenic belt broadly occur in early Mesozoic, late Mesozoic, Paleogene, and Neogene within a wide range of tectonic setting. These deposits in Iran often occur within the Cenozoic Urumieh-Dokhtar Magmatic Arc (UDMA), mostly in the Kerman porphyry copper belt (KPCB) located in the southeast of this arc. These deposits along the UDMA are mainly associated with the granitoids of Miocene age (18.82-9.20 Ma). In this research, geochemical characteristics, Pb and Sr isotopes, tectonic environment, oxidizing conditions and source of magmas for the most important porphyry copper deposits of the UDMA have been compared and discussed. On the basis of trace element discrimination diagrams, the Kerman belt granitoids formed in a volcanic arc-type setting, whereas the Jebal Barez granitoids seem to have formed in a different tectonic setting. The Eu/Eu-star of granitoids from the Kerman belt are greater than 1.2, representing the oxidizing conditions for the crystallization of the granitoid magma, whereas Jebal Barez granitoids were formed under reducing conditions. The Sr/Y ratio for Kerman granitoids is greater than 60, indicating adakitic affinities and (La/Yb)n for these granitoid rocks is between 17 and 35, suggesting that their parental magma was generated from a deep garnet-bearing source. Based on (Sr-87/Sr-86)i values for Kerman belt granitoids of 0.704-0.705, it is suggested that parental magmas originated from a subducted oceanic slab. In the Sarcheshmeh porphyry copper deposit, the thermal gradient of the oceanic slab at depth was less than 9 degrees C/Kin (warm-cold slab). The oceanic slab was highly hydrated and more than 80% of the water within the oceanic slab was released in the deep zone of the sub-arc region at a depth of 110-130 Km. Under saturated conditions, the oceanic slab partially melted (20-25%). A large volume of magma and magmatic water formed Sarcheshmeh as a giant porphyry copper deposit. In the Iju deposit, a small porphyry copper deposit, the oceanic slab was slightly hydrated and minor water was released at the depth of 110-130 Km. A small volume of magma was formed due to water under saturated melting conditions. Finally, a new hypothesis is introduced regarding the roles of down-dip thermal gradient of the oceanic slab, water content, depth of dehydration and degree of partial melting of oceanic slab which are important controls on the formation and size of the porphyry copper deposits within continental volcanic arcs.