Geology, mineralogy and geochemistry of the Morro Preto Alkaline-carbonatite complex, Goiás, Brazil

Abstract

The Morro Preto Complex, located in the northern portion of the Cretaceous Goiás Alkaline Province, is unique in the region due to the large predominance of carbonatites and phosphate mineralization. It comprises two circular intrusions (Morro Preto North and Morro Preto South) of apatite-rich magnesiocarbonatites gradually transitioning to ferrocarbonatites. The complex also hosts fenites produced by carbonatite metasomatism at the contacts with the Neoproterozoic country rocks. Kamafugite dykes with carbonate globules suggest an immiscibility link between silicate and carbonatite magma.The magnetite-apatite-dolomite carbonatites vary in texture and modal composition from cumulates to magnesiocarbonatites with variable apatite and magnetite, with minor barite and traces of pyrochlore, phlogopite, baddeleyite and zircon. In the ferrocarbonatites, magnetite and apatite are rare or absent, and the dominant carbonate varies from Fe-dolomite to ankerite and siderite, with traces of bastnaesite, monazite, thorium-rich monazite and sulfides. SiO2, Fe2O3, MnO, BaO:SrO and the sum of rare earth elements (REEs) increase from magnesiocarbonatites to ferrocarbonatites, whereas the CaO, MgO and P2O5 contents decrease.Four main apatite types are recognized in the complex: (i) high P-REE primary magmatic sub-euhedral apatites (type 1), (ii) fluorapatite associated with ferrodolomite-rich zones (type 2), (iii) fine-grained, Na-rich metasomatic apatite (type 3) and (iv) secondary fluorapatite filling veinlets and cavities (tuupe 4). In general, the apatite morphological and compositional variations correlate with carbonate evolution and with stable isotope data. Dolomite and type 1 apatite represent the primary group (+9.49 to +11.88‰ δ18O and −5.1 o −4.66‰ δ13C). Ferrodolomite, iron-rich carbonates, along with types 2 and 4 fluorapatite have heavier isotope compositions (up to +21.59‰ δ18O and −3.33‰ δ13C), indicating metasomatic reworking related to late-stage carbonatite-derived fluids.Geological, geochemical and mineral chemistry data are consistent with the evolution of the complex by crystal fractionation, liquid immiscibility, metasomatic overprinting and late hydrothermal events. The K fenitization and late-stage carbohydrothermal events suggest that the Morro Preto Alkaline-Carbonatite Complex represents the uppermost parts of a carbonatite magmatic system.