The origin of Madagascar chromitites

Precambrian rocks of Madagascar host numerous chromitite occurrences, ranging from centimeter-thick lenses and seams to orebodies containing millions of tons. Production of chromite concentrates and lumpy, coming from Bemanevika mine that was estimated to have a remaining life of 15 years (Rahaga, 2009), establishes Madagascar as the world 15th chromite producer. The five most important chromitite localities, investigated for this work, are all characterized by outcropping chromitite bodies hosted within mafic/ultramafic intrusions of poorly understood age. They may range from Archean to Cambrian in age although they probably date back to Neoproterozoic to Cambrian. Metamorphism and alteration have variously affected all of the chromitites, but never completely obliterated their primary characteristics. Chromitite host rocks are peridotite, orthopyroxenite or orthoamphibolite, and primary gangue phases are orthopyroxene, olivine, rare plagioclase, ilmenite, rutile, pyrrhotite and pentlandite. Secondary assemblages comprise serpentine, talc, Cr-chlorite, tremolitic to actinolitic amphibole and magnetite. Geologic, textural, mineralogical and mineral chemistry data best fit a layered intrusion origin for North Toamasina, North Belobaka, Antanimbary and Andriamena chromitites, while Befandriana chromitites, even in a general layered intrusion scenario, show some contrasting features more similar to ophiolite chromitites. Differences between the studied chromitites can be ascribed to the position of the chromitite bodies within the stratigraphic sequence of a layered intrusion. The most striking chromitites are those from Antanimbary that show features assimilating them to the Cr-bearing Ti-magnetite layers of the Upper Zone of Bushveld complex. Chromitite alteration mostly affected gangue silicates whose primary assemblage was partially to almost totally obliterated, while chromites underwent at North Belobaka and North Toamasina partial and at Antanimbary complete ferritchromitization.