Discriminating productive and barren porphyry copper deposits in the southeastern part of the central Iranian volcano-plutonic belt, Kerman region, Iran: A review

Abstract

The Kerman Cenozoic magmatic arc (KCMA), located on the southeast of the Central Iranian volcano-plutonic belt, hosts some world class porphyry copper deposits. Temporally, the deposits overlap with the Alpine–Himalayan collision, which has some key implications for the existence or lack of copper mineralization during orogenic arc system development. Transition from normal calc-alkaline arc magmatism in the Eocene-Oligocene (Jebal Barez-type) to adakite-like calc-alkaline magmatism (Kuh Panj-type) in the mid-late Miocene-Pliocene reflects the onset of collision between the Afro-Arabian and Eurasian plates in the Kerman Cenozoic arc segment. The aim of this review is to determine the role of Cenozoic magmatic events in the development of economic to sub-economic porphyry copper mineralization in the southeastern parts of the Central Iranian volcano-plutonic belt. In order to discriminate between the various magmatic systems (KCMA) involved in that development, a geochemical investigation is carried out using samples collected from all important deposits in the region (this study) and previous published data by earlier researchers in this region. It is evident from these data that the collisional Neogene Kuh Panj porphyry suite is distinctly more evolved than the pre-collisional Eocene-Oligocene Jebal Barez granitoids, with relative enrichments in incompatible elements, Sr/Y (>55), and La/Yb (>20), slightly positive Eu anomalies (Eun/Eu*≈1), and depletions in HFSE, with [La/Sm]n≈4.6–6.6 and [Dy/Yb]n≈1.0–2.0, and relatively non-radiogenic Sr isotope signatures (87Sr/86Sr=0.7042–0.7047). In contrast, Jebal Barez granitoids exhibit low Sr/Y (<21) and La/Yb (<9) ratios, negative Eu anomalies (Eun/Eu*≈0.5), and enrichment in HFSE and radiogenic Sr isotope signatures (87Sr/86Sr=0.7053–0.7075). The temporal along with lithogeochemical and isotopic changes, reflect a progressive transfer of the melting zone from the juvenile mafic lower crust (garnet–free amphibolite) into garnet–amphibolite. This transfer is the result of compressional stress along with tectonic shortening during Eocene-Oligocene (~30–35km crustal thickness) to mid-late Miocene (~45–55km thick or 12–15kbar). The absence of volcanism, under prevailing compressional conditions (mid-late Miocene-Pliocene), prevented the escape of SO2 from the adakite-like, sulfur-rich, highly oxidized magmas (“closed porphyry systems”), which allowed formation of several world-class to giant mineral deposits. Volcanic activity during formation of the subvolcanic Eocene-Oligocene porphyries allowed development of “open porphyry systems”, which, in turn to partial outgassing of volatiles, and therefore, far less significant mineral deposits.