Relationships between arc maturity and Cu–Mo–Au porphyry and related epithermal mineralization at the Cenozoic Arasbaran magmatic belt

Abstract

The actual study deals with the temporal and geochemical evolution of the arc magmatism and the Cu–Mo–Au mineralization in the Neo-Tethyan belt of the Arasbaran region in northwestern Iran. The magmatic activity in this section of the Neo-Tethyan belt periodically occurred from Upper Cretaceous to Quaternary. The Cretaceous magmatic rocks are calc-alkaline to tholeiitic in nature without any mineralization in the Arasbaran region. Cenozoic magmatism begins with alkaline (shoshonitic) volcanism at an early stage and is followed by alkaline to calc-alkaline plutonism. No significant mineralization is associated with the early stage alkaline magmatism and the most porphyry and epithermal mineralizations are related to the late stage adakite-like, calc-alkaline magmatism. The subsequent, post-collisional alkaline magmatism follows in the Plio-Quaternary. Magmatism and the related mineralization decrease in age from the north to the south, and the mineralization changes in time and space from a Cu–Mo to a Cu–Au association.Mineralization stages at the Arasbaran region consist of (a) Upper Oligocene to Lower Miocene (~20Ma) Cu–Mo porphyry and intrusion mineralization, and (b) Upper Miocene (~10Ma), Cu–Au porphyry and epithermal Cu–Au mineralization. Although the exact timing of the porphyry mineralization relative to the final collision is unclear, the presence of most porphyry and epithermal mineralizations in the Upper Oligocene to Upper Miocene, post-collisional, adakite-like intrusions, is related to crustal thickening and delamination of lower crust suggesting that they formed at a late stage of the Arasbaran magmatic arc evolution following a major volcanic stage in the Eocene. Mineralogical and geochemical characteristics of the mineralized porphyry bodies in Arasbaran region indicate that significant amounts of hornblende was fractionated prior to plagioclase crystallization as a consequence of high magmatic water contents. In contrast, the earlier, barren plagioclase-phyric magmas were relatively dry. Fertile plutonic suites in Arasbaran region may be recognized by their relatively late timing with respect to the voluminous Eocene volcanism. Widespread presence of hornblende phenocrysts, and major and trace element compositions with relatively high Sr/Y and La/Yb ratios, high Sr (>400ppm) and Nd (≥20ppm) indicate a long-lived mature arc setting.