Geochemical Characterization of Podiform Chromite Ores from the Ultramafic Massif of Bulqiza (Eastern Ophiolitic Belt, Albania) and Hints for Exploration

Abstract

Four main podiform occurrences of chromite have been distinguished in the ultramafic Bulqiza massif, Albania; they are associated from the bottom to the top with: i) basal harzburgites (lower tectonite sequence), ii) dunitic lens-bearing harzbugites (middle-upper tectonite sequence), iii) layered chromitite-bearing dunites of transitional zone, and, iiii) the stratigraphically lowermost part of the magmatic section. However, only the occurrences hosted by rocks ii) and iii) are economically important. Chromite compositions similar to those of other podiform chromites are bimodal. Thus, chromites from occurrences ii) and iii) are richer in Cr than chromites from the other two occurrences. Accessory chromite disseminated throughout the massif is richer in Fe than orebody chromite, displaying similar range of the Cr/Cr+Al ratios, with accessory residue chromite matching the characteristic (Cr/Cr+Al >0.6) range of type-III Alpine peridotites. Both orebody and accessory chromites show compositional ranges correlating with host-rock compositions, i.e., the Cr-rich chromites are hosted by the most refractory ultramafic rocks. Therefore, the Cr/Cr+Al and Mg/Fe tot (Cr/Fe tot ) ratios of chromite are useful for exploration of high-grade, metallurgical chromite ore at Bulqiza and in other massifs of similar geodynamic setting.