Mineralogy and geochemistry of laterites from the Morro dos Seis Lagos Nb (Ti, REE) deposit (Amazonas, Brazil).

The Morro dos Seis Lagos niobium deposit (2897.9 Mt at 2.81 wt% Nb 2 O 5 ) is associated with laterites formed by the weathering of siderite carbonatite. This iron-rich lateritic profile (>100 m in thickness) is divided into six textural and compositional types, which from the top to the base of the sequence is: (1) pisolitic laterite, (2) fragmented laterite, (3) mottled laterite, (4) purple laterite, (5) manganiferous laterite, and (6) brown laterite. All the laterites are composed mainly of goethite (predominant in the lower and upper varieties) and hematite (predominant in the intermediate types, formed from goethite dehydroxylation). The upper laterites were reworked, resulting in goethite formation. In the manganiferous laterite (10 m thick), the manganese oxides (mainly hollandite, with associated cerianite) occur as veins or irregular masses, formed in a late event during the development of the lateritic profile, precipitated from a solution with higher oxidation potential than that for Fe oxides, closer to the water table. Siderite is the source for the Mn. The main Nb ore mineral is Nb-rich rutile (with 11.26–22.23 wt% Nb 2 O 5 ), which occurs in all of the laterites and formed at expense of a former secondary pyrochlore, together with Ce-pyrochlore (last pyrochore before final breakdown), Nb-rich goethite and minor cerianite. The paragenesis results of lateritization have been extremely intense. Minor Nb-rich brookite formed from Nb-rich rutile occurs as broken spherules with an “oolitic” (or Liesegang ring structure). Nb-rich rutile and Nb-rich brookite incorporate Nb following the [Fe 3+ + (Nb, Ta) for 2Ti] substitution and both contain up to 2 wt% WO 3 . The laterites have an average Nb 2 O 5 content of 2.91 wt% and average TiO 2 5.00 wt% in the upper parts of the sequence. Average CeO 2 concentration increases with increasing depth, from 0.12 wt% in the pisolitic type to 3.50 wt% in the brown laterite. HREE concentration is very low. [ABSTRACT FROM AUTHOR]