The Carbonatite-Related Morro do Padre Niobium Deposit, Catalão II Complex, Central Brazil

Abstract

Abstract The Morro do Padre deposit contains a valuable niobium resource estimated at 14.5 Mt at 1.52 wt % Nb2O5 (at a cut-off grade of 0.5% Nb2O5) hosted in carbonatite-related rocks and their regolith in the southern part of the Catalão II Complex, in central Brazil. Morro do Padre shares numerous geologic features with some of the biggest niobium producers in the world (the Boa Vista mine, also in Catalão II, the Mina II in Catalão I, and the CBMM mine in the Araxá Complex) and can help advance our understanding of the ore formation processes involved. The Morro do Padre hypogene zone is characterized by E-W–trending dike swarms of tetraferriphlogopite phoscorites (magnetite-apatite-carbonate-tetraferriphlogopite-pyrochlore rocks) and carbonatites intrusive within Precambrian rocks. The magmatic origin of these Nb-rich rocks is supported by country rock xenoliths within dikes and ponding into a stratified sill with repetitive cumulus layers. At least two tetraferriphlogopite phoscorite phases (apatite-rich or pegmatoidal P1 and the magnetite-rich P2) and two carbonatite phases (C1 calcite carbonatite and C2 dolomite carbonatite) are present. The bulk of hypogene mineralization is primarily controlled by the emplacement of P2 dikes and secondarily by C1 and C2 dikes where pyrochlore is accessory. Whole-rock and pyrochlore chemistry and textural and spatial relationships suggest that the genesis of P2 (and that of the niobium deposit) is due to the emplacement of a parental dolomite carbonatite magma that crystallized medium- to coarse-grained magnetite, apatite, tetraferriphlogopite, and pyrochlore on dike walls upon cooling, in an elaborate magmatic type of “cumulate dike build-up.” Weathering generated the regolith zone, where the dissolution of barren phases compounded the Nb concentration even further. Morro do Padre showcases the role of carbonatite-phoscorite magmatism in producing Fe-P-Nb–rich rocks and economic niobium mineralization.