Review of platinum-group element distribution and mineralogy in chromitite ores from southern Iran

Abstract

The platinum-group element (PGE) potential of four chromite mining districts in southern and south-eastern Iran was investigated using geochemical and mineralogical methods. A total of 20 mines and prospects were sampled in the Neyriz ophiolite, the Abdasht and Sikhuran complexes in the Esfandagheh district, and the Faryab district. Chromitite and dunite were analysed for major, trace element and PGE geochemistry, and the platinum-group element mineralogy (PGM). In all mining districts, chromites analysed from mantle tectonite, probable transition zone and cumulate zone chromitites are refractory, low in TiO2 (<0.3wt.%) as well as Cr- and Mg-rich, with Cr# [100∗Cr/(Cr+Al)] ranging from 70 to 81 and Mg# [100∗Mg/(Mg+Fe2+)] ranging from 40 to 83. Associated olivine is highly magnesian (Mg# 89–98) and Ni-rich (0.1–1.0wt.% NiO).PGE concentrations range from <100 to >5000ppb, with median values around 200ppb in all complexes investigated. The 187Os/188Os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial Os. Mantle-normalised Pt/Ir ratios are generally low (<0.2), but are up to 0.5 in small chromite pods in the harzburgitic mantle section of the Neyriz ophiolite. PGM, mainly laurite–erlichmanite, mostly occur as small (<1 to 30μm) inclusions in chromite. They are associated with hydrous silicates (calcic amphibole), base metal sulphides (Ni- and Cu-sulphides) and other PGM, such as cuproiridsite, Ni–Ir sulphides, and Os–Ir alloys. PGMs interstitial to chromite are represented by irarsite, hollingworthite, Ru–Os–Ir alloys, tolovkite, RhNiAs, Pt–Fe alloy, probable Ru-rich oxides, and others.In the Sikhuran complex, pods of sulphide-bearing chromitite (0.2–1.2% sulphides) occur in a dunite zone interpreted as an ultramafic cumulate zone. Chromites in chromitite are characterised by high Cr# (75–81) and intermediate Mg# numbers (45–65); in associated dunite, chromites (Cr#, 74–80) are more Fe-rich (Mg#, 29–50). Chromitite and dunite carry up to 5.2ppm total PGE and are characterised by high normalised Pt/Ir (up to 5), Pt/Pd<1, and radiogenic Os (moderately suprachondritic initial Os). The interstitial sulphides are dominated by pentlandite that carries on average 0.45% Co, 25ppm Os, 11ppm Ir and 8ppm Rh. Palladium and Pt concentrations in pentlandite are below the detection limit of the Laser Ablation–ICP-MS technique. Pentlandite is associated with rare grains of Pd- and Pt-minerals such as stibiopalladinite and Pt–oxide, and with Cu-rich phases.Large degrees of partial melting in a supra-subduction zone geotectonic setting, probably in a two-stage melting process, produced boninitic melts that crystallised refractory, Cr–Mg rich and Ti-poor chromite. In some cases, sulphide saturation was achieved during and after chromite formation. Saturation during chromite crystallisation is evidenced by Ni–Cu-rich sulphide droplets preserved in some chromitites. Sulphur saturation after chromite crystallisation governed the formation of Ni-rich sulphide liquids that exsolved monosulphide solid solution, which later crystallised to form PGE-bearing pentlandite. Platinum and Pd did not partition into pentlandite but crystallised as Pt- and Pd-rich PGM associated with the pentlandite.