Review of the formation of the thick chromitite of the Jacurici Layered Complex (Brazil): new evidences from inclusions in chromite

The Jacurici Mafic-ultramafic Complex, NE of the Sao Francisco Craton, hosts a thick massive chromitite and is considered as a single Paleoproterozoic N-S elongated layered body disrupted into many segments by later deformation. Here we discuss the processes that could explain the formation of the thick chromitite. The parental magma is very primitive based on olivine (up to Fo93) and orthopyroxene (up to En94) compositions, and has been considered as originated from a high degree of mantle extraction with subsequent crustal contamination, based on Nd-Os isotopes. Chromites host abundant inclusions, including hydrous silicates, negative-crystal shaped carbonates, base metals sulfides and laurite, indicating an H2O-rich and 5 saturated resident magma with immiscible droplets of carbonate melt during chromite crystallization. The trigger of chromite crystallization remains debated. The petrologic evolution suggests that a high volume of very hot magma flowed through a conduit-like chamber causing erosion and assimilation of metasedimentary country wall-rocks. The crustal contamination has introduced CO2 and H2O, increasing fO2 and favoring chromite crystallization. We proposed a model where chromite started crystallizing in situ and were then overlaid by semi-consolidated chromite slurry that slumped from the margins of the conduit forming a thick chromitite.