Manganotantalite from the Borborema Pegmatite Province, northeastern Brazil: Remarks on conditions of formation

Abstract

In this study carbon dioxide-rich fluid and carbonate-rich melt inclusions in manganotantalite were investigated to constrain the formation of the host crystals. The results of homogenization experiments on fluid and melt inclusions demonstrate that in the Alto do Giz pegmatite, and possibly in other pegmatite systems, water- and alkaline carbonate-rich fluids and melts are responsible for the transport and deposition of tantalites at temperatures around 600°C and about 4 kbar. Moreover, evidence is presented to show that during crystallization of the manganotantalite there coexisted three different components now trapped as separate inclusions: two immiscible silicate melts (type-A and type-B melt inclusions) and a CO2-rich fluid. We hypothesize that the immiscible separation may have been a critical factor in producing the necessary water- and alkaline carbonate-rich fluids and melts necessary for tantalite transport from a normal, more peraluminous pegmatite melt.