The Khoy Ophiolitic Complex in northwestern Iran represents one of the key remnants of the Neotethys Ocean. The main lithological units exposed in the study area comprise ultramafic rocks, including lherzolite, harzburgite, and dunite, which are strongly serpentinized in some parts. Field and petrographic investigations indicate that these units may represent different stages in the evolution of a primary lherzolitic mantle, with lherzolites representing products of limited melting, whereas harzburgites and dunites reflect moderate to extensive partial melting. Chromite mineralization in Anomaly 4 of the Qeshlaq deposit occurs as lenticular, massive, disseminated, and vein-veinlet forms within serpentinized harzburgites and dunites. The ore mineralogy is dominated by chromite and ferrochromite.
Electron microprobe analyses of chromite from Anomaly 4 of the Qeshlaq deposit indicate that the chromites are Al-rich. The studied chromites contain 33.23-39.53 wt.% Cr2O3, 20.37-30.30 wt.% Al2O3, and less than 0.28 wt.% TiO2. The Cr# values range from 0.37 to 0.46, whereas Mg# values range from 0.66 to 0.70, placing all samples within the field of Al-rich chromites.
In the Cr# versus Mg#, TiO2 versus Cr#, and Al2O3 versus Cr2O3 diagrams, all studied samples plot within the field of podiform ophiolitic chromites. The measured Cr# values, when evaluated using existing petrogenetic models, are consistent with moderate to high degrees of partial melting (approximately 15-20%) of a lherzolitic mantle, leaving a residue that evolved toward harzburgite and, locally, dunite. The very low TiO2 contents coupled with relatively high Al2O3 contents of the studied chromites indicate crystallization from an Al-rich tholeiitic melt. In tectonic discrimination diagrams, the data consistently plot within the fields extending from mid-ocean-ridge basalts to back-arc basin basalts. This characteristic is consistent with a back-arc or transitional MOR- back-arc setting for mantle melting and chromite formation.