The thorite mineralization in the Madeira Sn-Nb-Ta world-class deposit (Pitinga, Brazil)

Abstract

The world-class Sn-Nb-Ta Madeira deposit is located in the Pitinga mine (northern Brazil). The deposit is associated with the albite-enriched granite facies of the A-type Madeira Granite (∼1820 Ma). The mine commercially extracts tin (cassiterite), Nb and Ta (U-Pb-pyrochlore and columbite). Fluorine (cryolite), Y, REE (xenotime), Zr (zircon), U (U-Pb-pyrochlore and zircon) and Th (thorite) are potential byproducts. This work presents a detailed study of thorite mineralization in the albite-enriched granite subfacies: the albite-enriched granite core, the albite-enriched granite border and associated pegmatites. The Madeira deposit is revealed in this work to be among the largest Th deposits in the world, with 164 Mt of homogeneously dispersed ore, average grade of 759 ppm ThO2 in the rock, and higher concentrations (up to 1.8 wt% ThO2) in small pegmatites (average of ∼0.51 wt% ThO2). Thorite compositions are either close to the thorite pole or correspond to relatively limited substitutions in the thorite-zircon-xenotime-coffinite solid solution system. The concentration of Fe in thorite ranges from 0.11 wt% to 29.56 wt% Fe2O3, and in many cases is considered to be of structural nature, as well as part of the content of F (up to 6.02 wt% F). Thorites of all subfacies were strongly affected by hydrothermal alteration related to F-rich, low-temperature aqueous fluids. The hydration of thorite allowed the introduction of M3+ cations (Y, REE, Fe, and Al) and F, and caused losses in Si and Th contents (average of ∼48 wt% ThO2). The alteration also created a Fe-rich halo in thorite, with associated secondary minerals, likely Th-Fe-hydroxyfluorides and Y-Th-Fe-fluorcarbonates. The Th/U average ratios in bulk-rock are 1.85 in the albite-enriched granite border, 3.82 in the albite-enriched granite core, and 19.85 in the associated pegmatites. This variation reflects a pattern of magmatic evolution, with higher availability of U in earlier stages and U depletion in later stages. In Pitinga, the Th and U mineralization are divided into different minerals formed at distinct stages of magma evolution. This feature is related to fluorine enrichment and high alkalinity of the magma that strongly inhibited the early crystallization of zircon, as well as columbite, and favored the early appearance of U-Pb-pyrochlore. When crystallization of hydrous silicates reduced alkalinity, crystallization of zircon (from a magma depleted in U, Nb, Ta and LREE) became intense, accompanied by thorite and xenotime.