Geochemical evolution of a lateritic Sn–Zr–Th–Nb–Y–REE-bearing ore body derived from apogranite: the case of Pitinga, Amazonas — Brazil

Abstract

The Pitinga region, located 250 km from Manaus in the Amazonas State, northern Brazil, contains primary and supergene mineralization of Sn, Zr, Th, Nb, Y and REE. The principal orebody of supergene mineralization (Serra do Madeira) is represented by a lateritic profile with seven horizons (from bottom to top): parent rock, saprolite, clayey mottled, bauxitic, concretionary, colluvial and latosol. The parent rock is a porphyritic alkaline-granite containing albite, quartz, K-feldspar, lepidolite, riebeckite, fluorite, cryolite, zircon, hematite, cassiterite, thorite, xenotime, pyrochlore, columbite, bastnaesite and fluorcerite. The main minerals in the weathering profile are kaolinite, quartz, gibbsite, hematite + goethite. The bulk chemical composition of the lateritic profile when compared to parent rocks shows higher contents in Al 2O 3, Fe 2O 3, Sn, Zr, Th and Nb with lower SiO 2, FeO, CaO, Na 2O, K 2O, F, Pb, Rb and U. The Y, P and REE have lowest values in the saprolitic and clayey mottled horizon and are highest in the latosol. REE minerals are represented by zircon, xenotime, thorite and bastnaesite (in both the parent rock and in the whole profile) and fluorcerite, probably churchite and ytriofluorite only in the parent rock. The large variety of REE minerals is responsible for the irregular pattern in the distribution of the REE in the profile. Iso-zirconium mass balance techniques showed that the profile developed iso-volumetrically in the saprolitic and clayey mottled horizons while the colluvial and latosol horizons were reduced in volume. Alkalis, F and Rb were strongly leached; Si, Fe 2+, Mn, P, Y, LREE and Pb were moderately leached; Sn, Zr, Nb, Th, Hf, Ga, HREE and Ta were relatively enriched, and Al, Fe 3+ and Ti were strongly enriched.