Synthesis on productive, sub-productive and barren intrusions in the Urumieh-Dokhtar magmatic arc, Iran, constraints on geochronology and geochemistry

Abstract

The Cenozoic Urumieh-Dokhtar magmatic arc (UDMA) is considered as one of the major Cu-bearing regions of the world, with high potential for the occurrence of porphyry Cu ± Mo ± Au systems. This study conducts a comprehensive synthesis of geological, geochronological, and geochemical data of intrusive igneous rocks in selected productive, sub-productive, and barren intrusions in the UDMA as a means to identify efficient criteria of geochemical discrimination. U-Pb dating of zircon grains provides emplacement ages of 19.3 ± 0.4 to 21.1 ± 0.2 Ma for the Ghahan diorite, and tightly clustered ages of 14.5 ± 0.2 Ma (Kahang tonalite), 14.9 ± 0.1 Ma (Kahang quartz diorite), 12.2 ± 0.1 Ma (Meiduk granodiorite), and 27.3 ± 0.4 to 29.2 ± 0.3 Ma (Zaftak granite to quartz diorite) porphyry copper deposit systems (PCDs). According to geochemical characteristics, the Zaftak and Ghahan samples display sub-productive signatures, whereas the productive Kahang and Meiduk samples are characterized by adakitic signatures. The lowest average Sr/Y ratios are observed in the Zaftak and Ghahan barren and sub-productive intrusive rocks (Eocene: 15.9, Oligocene: 13.5 and Miocene: 20.2), in contrast to productive Kahang and Meiduk (Sr/Y = 70.7 and 63.9).This study suggests two major mineralization episodes for UDMA porphyry copper deposits: 1) Oligocene to Early Miocene non-adakitic deposits, and 2) Early Miocene to Late Miocene deposits having adakite-like geochemical signatures. Type 1) deposits developed mostly during final closure of the Neo-Tethys Ocean before plate subduction had initiated; Type 2) developed in a post-collisional setting. In contrast to barren and sub-productive intrusions, productive PCDs (especially of Middle to Late Miocene age) are characterized by the enrichment of LREEs relative to HREEs. This transition was associated with compressional stress and tectonic shortening during an Eocene-Oligocene (~30–35 km crustal thickness) to mid-late Miocene (~45–55 km thick or 12–15 kbar). Our data support a more nuanced interpretation of a previously accepted progressive decrease of PCD mineralization in the UDMA. For example, ca. 19 to 21 Ma mineralization ages in the central UDMA (Ghahan region) can no longer be considered to be a single stage of Early Miocene mineralization. According to adakite evidence, the formation of high-grade high-tonnage porphyry deposits in the UDMA remains limited to the time interval after collision. Before the collision, other ore deposits such as Zaftak and Ghahan had formed with sub-productive signatures. In a progression through the Eocene- Oligocene, and especially into the Miocene, the magma arc had matured, with increasing probability of productive mineralization.