The tectonic significance of PGM-bearing chromitites at the Ranomena mine, Toamasina chromite district, Madagascar

This paper presents new data on platinum-group-mineral (PGM)-bearing chromitites from the Ranomena mine, the only past-producing chromite mine in the North Toamasina chromite district of Eastern Madagascar. Such data can provide an insight on tectonic setting that led to emplacement of their host ultramafic body and can help to highlight the differences with other chromite deposits of Madagascar. Chromitite host rocks are harzburgite and pyroxenite and gangue minerals are orthopyroxene and minor olivine. Chromite mineral chemistry shows a strong affinity with chromitites from layered intrusions and PGE patterns are similar to those from the stratigraphically lowest chromitites in the Bushveld Complex. PGMs are dominated by laurite and are free of alloys. PGE mineralogy is consistent with early crystallization of laurite–erlichmanite at low fS2. Previous studies have suggested that the North Toamasina ultramafic bodies are fragments of dismembered ophiolites and that their presence within the so-called suture zone supports the suggestion that they mark the closure of an ocean between east and west Gondwana. Even though these ultramafic bodies have been interpreted as ophiolitic, our data better support an origin as small layered intrusions, rather than as fragments of oceanic crust.