Mineral chemistry, isotope geochemistry and petrogenesis of niobium-rich rocks from the Catalão I carbonatite-phoscorite complex, Central Brazil

Abstract

The Catalão I carbonatite complex is part of the Late-Cretaceous Alto Paranaíba Igneous Province (APIP) and consists of a multi-intrusion body zoned from bebedourite–(pyroxenite)-series rocks at the border to carbonatite- and phoscorite-series rocks at the core. The phoscorites consist of apatite, magnetite, and a Mg-silicate (phlogopite or olivine) and are subdivided into early-stage; olivine-bearing (P1); and more evolved, olivine-lacking rocks, dominated either by apatite (P2) or magnetite (P3). P1 rocks are typical phoscorites, whereas the late-stage P2 and P3 rocks are petrographically classified as nelsonites (apatite–magnetite rocks) and host the Nb + P + Fe mineralization of the Catalão I Complex. Dolomite carbonatite (DC) occurs in association with both P2 and P3 late-stage phoscorite, forming paired phoscorite–carbonatite sets. Mica chemistry changes from phlogopite in phoscorite through phlogopite cores with tetra-ferriphlogopite rims in apatite late-stage phoscorite to tetra-ferriphlogopite in magnetite late-stage phoscorite and dolomite carbonatite, which is similar to the Al-depletion seen in micas in phoscorites from the Kovdor and Sokli Complexes. Apatite from phoscorites is enriched in Si, whereas those from late-stage phoscorites and dolomite carbonatites are enriched in strontium. Core chemistry from both apatite and phlogopite crystals shows a trend consistent with evolution from phoscorite to dolomite carbonatite. Ilmenites from phoscorite are Mg-rich while those from late-stage phoscorites and dolomite carbonatites tend to consist of nearly pure FeTiO₃. REE patterns are highly fractionated and indicate that all rock types are cogenetic. The discrepancy in the radiogenic signature between phoscorites and carbonatites from Catalão I suggests that a petrogenetic process, probably related to fluids, can fractionate Nd isotopes, generating different compositions within the province.