Magmatic evolution and porphyry–epithermal mineralization in the Taftan volcanic complex, southeastern Iran

Abstract

The Taftan volcanic complex is located above the Makran subduction zone in Sistan and Baluchestan province, southeastern Iran. The earliest volcanic activity at Taftan started in the late Miocene (~8 Ma) with eruption of andesitic to dacitic lava onto a Cretaceous to Eocene volcanic and sedimentary paleosurface ~20 km to the northwest of the current volcanic edifice. Later Plio-Pleistocene volcanism consisted of voluminous andesitic and dacitic lavas and pyroclastic flows (~3.1 to 0.4 Ma). Taftan, and the neighbouring Bazman volcano, are the southeasternmost and youngest manifestations of arc-related volcanism in Iran, which began with the Eocene–Miocene Urumieh-Dokhtar Magmatic Arc (UDMA) in northwest and central Iran, and extends into western Pakistan (Chagai Arc). Major porphyry Cu±Mo±Au deposits are associated with Miocene to Pliocene intrusive rocks in the Kerman section of the Eocene–Neogene Urumieh-Dokhtar Magmatic Arc in southeastern Iran (e.g., Sar Cheshmeh, Meiduk), and at Reko Diq and Saindak in the Late Cretaceous–Quaternary Chagai Arc in western Pakistan. In a gap of ~300 km between these two belts, several porphyry and epithermal prospects are exposed in the Miocene–Pliocene sections of the Taftan volcanic complex, including the Kharestan (6.10 ± 0.80 Ma) and Bidester porphyry-epithermal Cu-Au deposits (~4.3 Ma), and the Siah Jangal epithermal Au deposit (late Miocene). In addition, several argillic to advanced argillic and fumarolic alteration zones occur in Plio-Pleistocene volcanic rocks around the current volcanic edifice. These deposits have received limited exploration. Electron microprobe analyses of amphibole (magnesio-hastingsite) phenocrysts and magnetite–ilmenite mineral pairs from the Taftan and Bazman volcanic rocks indicate a change of crystallization temperature and magmatic oxidation state from ~1000°C and FMQ ≈ +1 in andesitic rocks, to ~900°C and FMQ ≈ +2 in dacitic rocks. Magmatic water content was >4 wt.%, as indicated by the ubiquitous presence of amphibole phenocrysts. Major and trace element compositions of the Taftan and Bazman volcanic rocks show calc-alkaline to high-K calc-alkaline affinity, with relative depletions of Nb, Ta, and Ti and enrichments of large-ion lithophile elements (LILE), Th, and U on normalized diagrams. These trace element patterns, including listric-shaped normalized rare earth element profiles and relatively high Sr/Y and La/Yb ratios, are similar to those of fertile Miocene igneous rocks from the Kerman Belt and Chagai Arc, and suggest that the Taftan suite in particular should be prospective for porphyry Cu ore formation. Regionally, there is no clear geochemical difference between the Neogene Kerman Belt rocks, which are thought to post-date the onset of collision between the Afro-Arabian and Eurasian plates (late Oligocene to earliest Miocene), and the subduction-related Bazman– Taftan and Chagai Belt magmas. The porphyry deposits formed in these distinct settings are also virtually indistinguishable. This suggests that most of the processes affecting the geochemistry and metallogeny of the magmas in both settings take place in the lithosphere, albeit that the ultimate source of the magmas is in the supra-subduction zone asthenospheric mantle wedge. In collisional environments, subduction-related material previously crystallized in the deep lithosphere is simply being remobilized.