Geochemistry and metamorphism of stratiform chromitite deposits, Bahia State, Brazil

The Campo Formoso and Jacurici Valley mining districts contain the two largest known reserves of chromite in Brazil. They are usually considered to be of stratiform type, although in the literature each deposit has also been referred to as "podiform" at some stage. Both are hosted by the ultramafic remnants of early Precambrian layered igneous complexes, surrounded by areas of tectonized granulite facies supracrustal gneisses of the Sao Francisco Craton. The form, alteration history and current metamorphic grade of each intrusion varies, although their geochemistry appears to be very similar. Both contain abundant chromite, are orthopyroxene-rich, highly magnesian and were probably derived from rather similar parent magmas of a komatiitic lineage. Chromite mineral chemistry shows high Cr2O3 contents combined with moderate TiO2 levels and gives ambiguous indications of tectonic setting. In part this reflects the primitive parental magma. However, grain core compositions also illustrate the contrasting effects of amphibolite facies versus granulite facies progressive metamorphism.