Using Mineralogy and Geochemistry of Cheshmeh-Bid Chromitites (Neyriz) for Determination of Tectonic Setting

Dunite and serpentinized harzburgite from northwest of the Neyriz ophiolites are host rocks of some podiform chromite deposits in Cheshmeh-Bid area. These chromite deposits occurred as aligned massive lenses in various sizes. The most important ore textures are massive, nodular, layered and disseminated grains. These textures are goodevidences of crystals settling crystals during the process of magma cooling in the chamber. The mineralogical studies revealed that the pyroxene melting tookplace in the peridotite host rock and olivine is recrystallized in dunite envelope. Mineral chemistry of chromite indicates high Cr number(72-77), high Mg number(69-62) and low TiO2 content(0.04-0.14 %) in chromite samples. These amounts are comparable with those chromitites, which areformed from the high partial melting rate of mantle. The medium amount of PGE determined about 212ppb, which is much lessthan the Tang-e-Hana chromite (1556ppb) deposits. The trace element content of chromitite samples and dunite envelope like Ga, V, Zn, Co, Ni and Mn are between 17-24, 466-842, 852-1048, 22-84, 115-171, and 852-1220 ppm respectively. The U shape of REY pattern indicates the interaction of high volatile and REE bearing melt(boninitic source of parent magma) with host rock. Therefore,the Cheshmeh-Bid chromitite resulted likely from the intrusion of boninitic magma ofprimary mantle with high partial melting in the peridotite host rock. The geotectonic setting is above supra-subduction zone of island arc.