Distribution of PGEs and PGMs in chromitites from Abdasht and Soghan mafic-ultramafic complexes (Haji Abad-Esfandagheh district, Southern Iran)

It is well-established that PGEs of ophiolite chromitites mainly form PGMs. Understanding of PGM composition and their microstructural position in chromitites can be used to get information about crystallization processes, physicochemical conditions of chromitites, post-magmatic evolution of chromitites as well as their geodynamic environments. Abdasht and Soghan mafic-ultramafic complexes host major chromite ores that are part of the Haji Abad-Esfandagheh district, the second most important chromite producing area in Iran. They are situated in an E-W trending area 60 km long and 5-10 km wide within a very tectonically active zone, composed of Sanandaj–Sirjan/Bajgan–Durkan highly deformed metamorphic rocks. Abdasht complex is mainly composed of variably serpentinized dunite, harzburgite, websterite and minor wherlite and lherzolite. Chromitite ore bodies are included in mantle harzburgite tectonite and dunites and follow three distinct pseudo-stratigraphic levels for a length of more than 3 km. Soghan complex is located about 3 km East of Abdasht. Minor chromitites are hosted within the Gechin unit, composed of strongly serpentinized dunite and harzburgite. Major chromitite ores are found in the Main Soghan unit, composed of an alternation of dunites and harzburgites. Soghan complex also comprise a Transition Zone and Layered Gabbros. Chromitite samples from 4 different mines (3 in Soghan and 1 in Abdasht) were collected and studied for their metallic mineral assemblages. Of the primary BMS assemblage only few pentlandite relict grains and one bornite were detected in both complexes. The secondary assemblage comprises both low-S sulfides (heazlewoodite) and metallic phases (awaruite, native Ni and Cu) that are indicative of low-S fugacity conditions. PGMs are much more abundant in Abdasht than in Soghan. PGMs are dominated by euhedral laurite inclusions within chromite grains, while PGE alloys show more variable compositions and are usually associated to hydrated silicate phases like amphibole and chlorite. PGE whole rock contents share for all samples a strong depletion in Pt and Pd. PGEtot is higher in Abdasht (173 ppb average and 421 ppb maximum) than in Soghan (97 ppb average and 164 ppb maximum) chromitites. PGE chondritic patterns form two distinct groups of samples. Type I has higher PGEtot values (174 ppb average) and distinct negative Ir anomaly. Type II patterns show much lower PGEtot (20 ppb average) and a flat Os to Rh trend. These two types are equally shared between the two complexes. The whole set of data is consistent with metallic assemblages that can be found in ophiolite mantle sections but in Abdasht and Soghan the secondary transformation of the BMS and PGM assemblages is particularly strong. The main event, probably due to serpentinization, is a strong desulfurization that affected the silicate-hosted BMS more than the mainly chromite-hosted PGMs. This event can be responsible also for a re-distribution of PGEs. Type II PGE patterns are consistent with a stronger desulfurization that in the PGM assemblage mainly affected laurite. Dissolution of laurite flattened PGE patterns and decreased whole rock PGE contents.