CHROMITE: FROM THE MINERAL TO THE COMMODITY

Abstract

Chromite belongs to the spinel group with the general chemical formula XY2O4, where X and Y represent divalent and trivalent metal ions, respectively. Four types of chromite ore deposits occur as either lode or secondary deposits. Lode chromite ore deposits comprise stratiform and podiform deposits, whereas secondary chromite ore deposits comprise laterite and placer deposits. Chromite is an important industrial mineral used in the refractory industry. Moreover it is the only industrial source of chromium for chemical and metallurgical industries and to be marketed as a valuable commodity it requires very demanding quality parameters that differ according to the kind of application. Chromite ore cannot be usually sold as it is and, therefore, some kind of ore beneficiation is required in order to separate chromite from gangue minerals. The most commonly used beneficiation methods for chromite ores are the gravity methods, such as the shaking table, jig, spiral and Reichert cone methods. The present work deals with the geologic studies that, coming after prospection, lead to the evaluation of the chromite ore quality and to planning and/or improvement of beneficiation plants. The quality of chromite ore deposits, in order to rank them for possible exploitation, was studied in five deposits of Madagascar (Andriamena, Antanimbary, North Befandriana, North Belobaka and North Toamasina), in collaboration with UT Group s.r.l., for the development of chromite mining in the country. The basic geological, mineralogical and geochemical study of the deposits led to the reconstruction of genetic models for each of them. This was the starting point for a more detailed study on the quality of the chromite ore. The five most important chromite ore localities, investigated for this work, are all characterized by outcropping chromitite bodies hosted within mafic/ultramafic intrusions of probable Neoproterozoic to Cambrian age. Metamorphism and alteration affected, at different degrees, all chromitites, but never completely obliterate their primary characters. Chromitite host rocks are peridotite, orthopyroxenite or orthoamphibolite, while primary gangue phases are orthopyroxene, olivine, rare plagioclase, ilmenite, rutile, pyrrhotite and pentlandite. Secondary assemblage comprises serpentine, talc, Cr-chlorite, tremolitic to actinolitic amphibole and magnetite. Geologic, textural, mineralogical and mineral chemistry data are compatible with an ophiolite origin for North Befandriana chromitites and a layered intrusion origin for Andriamena, North Toamasina, North Belobaka and Antanimbary chromitites. These latter show differences that can be related to a different position of the chromitite bodies within the stratigraphic sequence of a layered intrusion. North Befandriana is a high quality deposit that could be exploited without any beneficiation of the ore, Andriamena needs beneficiation to reach market standard, Antanimbary and North Belobaka is low quality for metallurgical or chemical use but could be a good prospect for…