Rare earth element signatures of economic and sub-economic porphyry copper systems in Urumieh–Dokhtar Magmatic Arc (UDMA), Iran

A wide variety of porphyry copper deposits have been reported along the Cenozoic Urumieh–Dokhtar magmatic arc in Iran. The formation of these deposits is attributed to closure of Neo-Tethys and the subsequent collisional tectonic regime during the Tertiary. This study presents whole-rock rare earth element (REE) data for the giant to small (Sarcheshmeh, Meiduk, Sungun, Darreh-Zerreshk, Dalli, Iju, Parkam and Ali-Abad) with sub-economic (Daraloo and Reagan) porphyry copper systems. The data indicate a temporal and geochemical evolution from sub-economic porphyry systems to economic deposits. All the economic and sub-economic porphyry copper systems exhibit LREE enrichment relative to HREE, whereas the economic deposits exhibit a relatively steep downward LREE to HREE profile, reflecting higher LREE abundance. The Eu anomalies vary from distinct negative in sub-economic deposits (Eu/Eu* = 0.28–0.70) to either markedly less negative or positive anomalies (Eu/Eu* = 0.45–1.67) in economic deposit. The economic porphyry deposits are characterized by relatively high La/Sm and Sm/Yb values, representing high crustal assimilation in a relatively thickened crust and provide insight into fractionation of hornblende with minor garnet in deep crustal parts (MASH zone). Compared with sub-economic deposits, the steep downward LREE to MREE and flatter to slightly upward MREE to HREE in economic deposits indicate hornblende involvement (magma evolution toward more volatile content). It seems that in an ongoing process of closure of Neo-Tethys, during compression and crustal shortening, the early Eocene–Oligocene sub-economic intrusions are followed by adakite-like hydrous Miocene (and younger) economic Cu-bearing intrusions.