ABSTRACT
Chromitite pods, newly discovered in the Iraqi Zagros Suture Zone, were studied in this research, which can be considered the most comprehensive work conducted in the area so far. The study was carried out in the Kolaka area of the Mergasur district, where the chromitite pods and their host serpentinized peridotite are within the Walash Group. The study covered petrography, mineral chemistry, whole-rock geochemistry (major, trace, REE, and PGEs), and parental melt (magma) compositions.
Cr-spinel (magnesiochromite) is the essential mineral in these chromitites with lesser amounts of serpentine, olivine, pyroxene, and as accessory minerals in serpentinized peridotite. These chromitites display massive, disseminated, fractured, brecciated, and sieve textures. The host rock shows a mesh texture and retains some olivine and pyroxene grains. The magnesiochromites in chromitite consist of 57.80 wt.% Cr2O3, 14.72 wt.% FeOt, 14.11 wt.% MgO, and 12.00 wt.% Al2O3, and minor amounts of MnO 0.21 wt.%, TiO2 0.18 wt.%, NiO 0.17 wt.%; their average Cr#, Mg#, Al#, and Fe3+# values are 0.76, 0.68, 0.23, and 0.04, respectively. The accessory magnesiochromite in the serpentinized peridotite contains 48.39 wt.% Cr2O3, 18.95 wt.% Al2O3, 18.89 wt.% FeOt, and 11.67 wt.% MgO, and minor amounts of MnO 0.29 wt.%, TiO2 0.20 wt.%, ZnO 0.17 wt.%; their average Cr#, Mg#, Al#, and Fe3+# values are 0.63, 0.56, 0.36, and 0.03, respectively. Various tectonic discrimination diagrams, in addition to those based on the parental melt compositions of Al2O3, TiO2, and FeO/MgO, indicate that the Kolaka chromitites are high-Cr, low-Ti, ophiolitic, originated from boninitic magmas within a suprasubduction zone as a part of a Fore-arc tectonic setting during the development of the Taurus (Turkey) – Zagros (Iran/Iraq) Ophiolite Belt. The selective major, minor, and trace elements diagram in the magnesiochromites of the studied chromitite is normalized to the composition of the same elements in chromite from MORB, compared with those from different tectonic settings, such as the high-Cr and high-Al chromitites in ophiolites, komatiites, and layered mafic intrusions, showing that the high-Cr magneisochromites of the studied chromitite are similar to the internationally known high-Cr chromitites.
The whole-rock geochemistry indicates that the studied chromitites are dominantly composed of Cr2O3, MgO, Fe2O3, and Al2O3. The average Cr2O3 contents are 47.09 wt. %, indicating that the high-Cr group has a higher Cr2O3 content (>40%). The ΣREE is very low in the studied chromitites, ranging from 0.02 to 1.56 ppm. The ΣPGE in the chromitites is very low, ranging from 99 to 285 ppb, and the ƩIPGE is enriched relative to ƩPPGE by a factor ranging from 1.86 to 6.31. The chondrite-normalized PGE diagram shows a distinct Ru hump and a Pt trough in high-Cr chromitites, which are similar to those of the corresponding chromitites in NW-Iran, SE-Turkey, and many other regions worldwide, despite the studied chromitites having significantly lower PGE concentrations. The Pd/Ir vs. Pt/Pt* diagram shows that the studied chromitites follow the partial melting trend, they were formed as a result of partial melting of the mantle, rather than by later alteration or other processes. The Pd vs Cu plot indicates that they are S-undersaturated; and the (Pt/Pd)n vs. (Pt/Ir)n classification diagram for PGE shows that the chromite belongs to Group-I.
The Kolaka chromitite of arc origin formed within the suprasubduction zone and was emplaced as a part of the obducted oceanic crust during the collision between the Turkish-Iranian microplate (Eurasian Plate) with the Arabian microplate (Afro-Arabian Plate) during the Cretaceous-Eocene. In general, they share properties with their counterparts in the Taurus-Zagros ophiolite belt, which is a part of the Alpine-Himalayan Orogenic Belt.
