Platinum group elements geochemistry and composition of podiform chromitites from Khaje Jamali ophiolite complex, Neyriz, southern Iran

Abstract

The subduction of the northern part of the Tethys is documented by arc magmatism from Upper Triassic to Lower Eocene crosscutting the Sanandaj-Sirjan Zone and from Upper Eocene to Plio-Quaternary in the Urumieh-Dokhtar Magmatic Zone, situated north of the SSZ. Neyriz ophiolite is part of Middle Eatern Neo-Tethyan ocean of Mesozoic that geographically is related to ophiolites of NW-SE Iran, along Zagros Suture Zone. The Khaje Jamali ophiolite complex is located in western part of Zagros thrust zone which separates Sanandaj–Sirjan crystalline complex and Zagros thrust belt and is one of the well-exposed mantle peridotites in Neyriz area. A number of rock-types such as isotropic gabbro and sheeted dykes are absent in Khaje Jamali ophiolite complex. Harzburgite is the most abundant ultramafic rock and is associated with the small dunite lenses. Chromite-bearing samples from Cheshmeh Bid and Anjirak areas display various sulfide phases. Such sulfide phases are hosted in serpentinite, chromitite and primary silicates. The most abundant sulfides are pyrrhotite, pentlandite and pyrite. SEM images show sulfide phases such as Os-Cu-Sn alloy, Ni-Fe metallic alloy in serpentinite and Au nuggets. In analyzed samples, a number of elements such as Ti, Al, Ca, Na, P and K are depleted while similar to other peridotites, the ultramafic rocks are enriched in Mg and Co contents and display slight variability (127.8–241.4 ppm). Ni value varies from 1482 to 6101 ppm. Chromite grains show homogeneous composition and are characterized by Cr# , Mg# , Fe# and extremely low content (<0.09 wt.%). High Cr# of spinels in Khaje Jamali ophiolite complex is originally related to boninites or high – Mg arc tholeiites. Total PGE contents in analyzed samples ranges from 80 – 299 ppb. The values change from 0.29 to 4.43. IPGEs contents are systematically reclining in order of layered chromite, nodular chromite, dunite and harzburgite. The changes from 0.11 to 1.96 which can be considered as an indicator feature for podiform chromitites. It is strongly believed that IPGE-enriched chromitites are derived from PGE-enriched magma with high degree of partial melting in supra - subduction environment. A number of samples contain higher values than those of primitive mantle which means PGEs are concentrated in ultramafics from Khaje Jamali ophiolite complex.