Mineralogical and geochemical investigations of chromite ores from ophiolite complexes of SE Iran in terms of chrome spinel composition

Abstract

Ten chromite ores from ophiolite complexes in SE Iran were analyzed mineralogically by XRD, chemically by XRF, and mineral chemistry by EPMA. The identified paragenesis of silicate minerals of chromite ores with the X-ray diffraction is pronounced differently. It consists of secondary phases formed as serpentine, Cr-containing chlorite (kaemmererite), chromic garnet (uarovite) with preserved partly primary minerals of peridotite parent rocks such as olivine and diopside. From the total chemical analysis by XRF results, a good correlation exists between the Cr₂O₃ and SiO₂ content (as an index of the sum of the silicate minerals). This allows an easy decision for mine ability of chrome ores. In a relatively good correlation are also the Mg and Fe oxide contents. The mineral chemisty (EPMA analysis ) of spimel mineral provides valuable information about the the octahedral layer of the spinel,. The results of the microprobe analysis show a variation in the chemical composition of the spinel phase of a mixed crystal formation consisting of: chromite (Fe²⁺Cr₂O₄), magnesiochromite (MgCr₂O₄), spinel (MgAl₂O₄), and hercynite (Fe²⁺Al₂O₄). This becomes even more complex by the mixed crystal relationship with picotite [(Mg, Fe²⁺)(Cr,Al,Fe³⁺)₂O₄], which contains Fe³⁺ in the tetrahedral position. The chrome spinel vary inCr-numbers [100 ×(Cr / Cr + Al) = 75-92] and Mg-numbers [100 * (Mg / Mg + Fe²⁺) = 38-57]. The partition of iron between Fe³⁺ und Fe²⁺ was made by assuming normal spinel structure, using the formula AB₂O₄. Correlations of microprobe analysis indicate that the mineral chemistry of the studied spinel is characterized mainly by the divalent elements of Mg and Fe²⁺ in the A position and trivalent elements Cr and Al in the B position.