Metallogeny of the Neo-Tethys arc in central Iran

J.P. Richards Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada

Jeremy.Richards@UAlberta.CA

Keywords: Tethys, Iran, metallogeny, porphyry copper deposits, epithermal gold deposits

ABSTRACT: Cenozoic closure of the Neo-Tethys oceans between Arabia and Eurasia resulted in the colli-sional juxtaposition of several small microcontinental blocks separated by ophiolite-bearing suture zones incentral Iran. Pre-collisional Eocene–Miocene subduction-related magmatism is best developed in the Uru-mieh-Dokhtar belt along the northern margin of Neo-Tethys II, and several large porphyry copper deposits arelocated in this belt. Syn- or post-collisional alkalic magmatism scattered throughout the orogen offers poten-tial for epithermal gold mineralization.

1 INTRODUCTION

Iran features several major volcano-plutonic belts re-lated to closure of oceanic basins between the Eura-sian and Afro-Arabian continental masses in the Mesozoic and Cenozoic. Several large porphyry copper deposits (Sar Cheshmeh, Meiduk, Sungun) are associated with these magmatic belts, and the po-tential for epithermal gold mineralization is high. Geologically, the region resembles the southwest Pacific more than the Andean arc, because it appears that many of the plate interactions involve elimina-tion of small ocean basins between continental blocks and microplates, rather than protracted sub-duction beneath a cratonic margin. Closure is largely complete in Iran, although to the southeast in the Gulf of Oman oceanic crust is still subducting be-neath the Makran. Collision between Eurasia and Arabia is at an early stage, reflected by the Zagros fold-and-thrust belt and active tectonism in central and northern Iran. Uplift and erosion associated with this tectonism has exposed epizonal and mesozonal (porphyry) systems at the surface in central Iran, making this a prospective area for exploration.

2 GEOLOGY OF CENTRAL IRAN

The Neo-Tethys ocean was formed in response to the breakup of Gondwana, and effectively separated Africa from Eurasia. In Iran, the breakup is believed to have featured at least two oceanic sub-basins (Neo-Tethys I and Neo-Tethys II) separated by

smaller continental microplates (e.g., the Lut Block, and Sanandaj-Sirjan Zone; Fig. 1; Hooper et al., 1994; Glennie, 2000). These ocean basins have been destroyed by convergence since the late Mesozoic, and evidence for their existence has been variably interpreted from the presence of dismembered ophiolitic sequences scattered throughout central Iran (e.g., Stoneley, 1981).

Figure 1. Plate-tectonic map of the Tethyan belt from Turkey to Pakistan (modified from Glennie, 2000). Areas shown in dark grey contain ophiolitic fragments and are believed to rep-resent former oceanic areas separating continental blocks (shown in lighter grey). Areas in coarse stipple are cratonic blocks.

The present-day geology of central Iran can be divided into several NW–SE-trending belts that par-allel the paleo-Tethyan margin (Alavi, 1994): to the SW, the Persian Gulf represents a depression in the

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Arabian cratonic margin in advance of the collision zone. This margin is overthrust by Mesozoic–Paleogene supracrustal rocks of the Zagros fold-and-thrust belt, in which ophiolitic fragments are found. To the NE of the Zagros belt lies the Sanandaj-Sirjan Zone, a structurally complex belt of Late Paleozoic–Mesozoic rocks that includes some Hercynian metamorphic (greenschist–amphibolite) assem-blages. The NE margin of the Sanandaj-Sirjan Zone features a series of basins that are interpreted to be forearc depressions, laterally equivalent to the Makran (e.g., Stoneley, 1974, 1981; Farhoudi and Karig, 1977; Farhoudi, 1978; McCall and Kidd, 1982; Alavi, 1994; Glennie, 2000). Ophiolitic frag-ments occur sparsely in this belt, which is thought to represent the locus of Neo-Tethys II suture (Fig. 1).

The main volcano-plutonic arc, known as the Urumieh-Dokhtar Belt, forms a narrow NW–SE-trending range of high ground to the north of the forearc depression that stretches the length of Iran (Fig. 2). Magmatism began in the Eocene, and con-tinued into the Quaternary in a series of pulses. The style of magmatism has varied in both space and time along the belt, and includes typical calc-alkaline arc volcanics, shoshonitic suites, and local alkalic centres. Recent volcanic activity appears to reflect post-collisional tectono-magmatic processes, and includes the most alkalic suites. The main por-phyry copper deposits in the belt, Sar Cheshmeh and Meiduk, are of Middle Miocene age. The Urumieh-Dokhtar Belt is bounded to the NE by a zone of ma-jor faults with Neogene dextral displacements; at the NW end of the belt in northwestern Iran, the belt ap-pears to be dismembered by a series of NE-trending cross-faults, and geological reconstructions become very unclear.

Figure 2. Sketch map or Iran showing the Urumieh-Dokhtar belt and porphyry Cu deposits (from Shahabpour, 1994).

The Lut block of central Iran appears to have been a stable cratonic block throughout this period, although extensive Eocene volcanism occurs throughout the area (Jung et al., 1984). Ophiolitic slivers at the margins of the block suggest the former existence of small ocean basins.

The northern border of Iran is marked by another extensive magmatic belt, the Alborz, which stretches in a sinuous arc from Azerbaijan, along the southern margin of the Caspian Sea, wrapping over the north-ern edge of the Lut Block, and continuing into Af-ghanistan (Axen et al., 2001). Central Iran is thus bordered to the north and south by magmatic belts, of presumed subduction origin. The two belts con-verge in NW Iran, but the geology is complicated by extensive collision-related faulting. The Miocene Sungun porphyry deposit occurs in this region, as well as two large Neogene volcanic centres, Sahand and Sabalan.

3 TECTONIC FRAMEWORK OF CENTRAL IRAN

Interpretations of the tectonic framework of central Iran vary widely, from intra-cratonic models, to sin-gle or double subduction zone models. The Uru-mieh-Dokhtar Belt is the only obvious volcanic arc in this region, although Cenozoic magmatism occurs locally to the south in the Sanandaj-Sirjan Zone, and to the north (Anarak region). Glennie (2000) pro-posed the existence of two Neo-Tethys oceans, I and II, to the south and north of the Sanandaj-Sirjan Zone respectively Fig. 1). The Sanandaj-Sirjan Zone is believed to be a continental fragment rifted from the Arabian margin in the Late Permian or Triassic (Hooper et al., 1994; Glennie, 2000).

Closure of Neo-Tethys I began in the Early Cre-taceous in response to South Atlantic opening, and the suture zone may now be represented by the Main Zagros Thrust. Continued contractional deformation since this time has complicated relationships in the Zagros belt, however, such that the exact location of the suture zone is hard to define. Neo-Tethys II be-gan subduction to the north beneath the Urumieh-Dokhtar arc in the Late Cretaceous or Paleocene, and arc magmatism became extensive in the Eocene, continuing through the Miocene. Porphyry copper intrusive activity in the Urumieh-Dokhtar Belt oc-curred towards the end of this period in the Middle Miocene, and appears to represent the final phase of arc magmatism prior to collision.

Final closure of Neo-Tethys II was probably a diachronous event, with docking perhaps beginning in the Miocene in the NW. To the SE, docking has not yet occurred, and oceanic lithosphere still exists beneath the Gulf of Oman (Jacob & Quittmeyer, 1979).

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4 CENOZOIC METALLOGENIC POTENTIAL OF CENTRAL IRAN

Cenozoic mineralization in central Iran can be viewed within the Neo-Tethyan tectonic framework outlined above, from which it is clear that the main period of porphyry copper formation occurred dur-ing the later stages of subduction-related magmatic activity in the Urumieh-Dokhtar arc. Good geologi-cal exposure in the core of this belt means that most outcropping porphyry systems have already been discovered. However, there is potential for explora-tion beneath Pliocene–Quaternary cover at the mar-gins of the belt.

Perhaps of greater potential, however, is explora-tion for epithermal styles of mineralization, particu-larly of the alkalic-gold type associated with post-subduction or collisional tectonics (Richards, 1995). Mildly to highly alkalic volcano-plutonic systems are scattered widely throughout central Iran, and are not restricted to the narrow Urumieh-Dokhtar arc. Understanding these deposits, or predicting their oc-currence, will require a considerably more detailed knowledge of the tectonomagmatic framework than is currently available for this region.

5 CONCLUSIONS

Central Iran marks the locus of collision between the Arabian and Eurasian plates following subduction of the Neo-Tethys ocean. This collision was multi-stage and diachronous, involving the elimination of at least two small oceanic plates (Neo-Tethys I and II) during the Cenozoic. Porphyry copper deposits are associated with pre-collisional arc magmatism, and there is potential for alkalic-type gold and other styles of epithermal mineralization in association with post-subduction alkalic magmatism. The geol-ogy of this region is not well known, however, and its mineralization potential has not yet been thor-oughly tested using modern exploration techniques.

ACKNOWLEDGEMENTS

I am grateful to Rio Tinto Mining & Exploration for field and logistical support while visiting Iran in the summer of 2002. In particular, I would like to thank Neil McLaurin, John Bartram, Hossein Iranmanesh, Esmaeil Heidari, Hooshang Asadi, Kambiz Moha-jeran, and Faranak Parvinpour. Brent McInnes is thanked for stimulating discussions in the field.

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