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

Abstract

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 lateralization have been extremely intense. Minor Nb-rich brookite formed from Nb-rich rutile occurs as broken spherules with an "oolitic" (or Liesegang ring structure). Nbrich 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.

Critical Elements in Supergene Phosphates: The Example of the Weathering Profile at the Gavà Neolithic Mines, Catalonia, Spain

“…In fact, it is well known that the mineral structures of aluminophosphates of the alunite supergroup, such as florencite or goyazite, can concentrate REE at economic grades in lateritised carbonatites and karstic bauxites [74,78,79]. Similar enrichments at subeconomic grades are also described in other carbonatites [80] and kimberlites [81]. Gallium can also enter the structure of various aluminophosphates, typically by Al substitution, being dominant in some minerals such as galloplumbite [82].…”

The Petyayan-Vara Carbonatite-Hosted Rare Earth Deposit (Vuoriyarvi, NW Russia): Mineralogy and Geochemistry

“…High-Ti carbonatites are of particular interest due to the discovery of the unique Morro dos Seis Lagos Ti-Nb-REE deposit (Brazil) [11], wherein Ti-oxides are the main hosts of Nb. High-Ti carbonatites are reported at Magnet Cove (USA) [91], Gross Brukkaros (Namibia) [92], and Salpeterkop (South Africa) [93].…”

“…Therefore, rare earth carbonatites are a major focus of critical metal research. Over the past five years, several dozens of papers have been published on the subject of rare earth carbonatites (RE carbonatites) in Angola [5,6], Australia [7][8][9], Brazil [10,11], Canada [12][13][14][15][16][17], China [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37], Finland [38], India [39][40][41][42], Italy [43,44], Malawi [45][46][47][48][49][50][51], Mongolia [52,53], Namibia…”

Ti-Nb Mineralization of Late Carbonatites and Role of Fluids in Its Formation: Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

“…The appearance of excessive iron is a common feature that can be explained by mechanically included Fe oxides-hydroxides. Examples include brookite and rutile from the Nb (Ti, REE) deposit Morro dos Seis Lagos ( [25]; see their Figure 12). The unusual pattern of the excessive Nb entry that is not compensated by Fe requires further research [39] Later generation brookite, Brk-2 overgrowths both Brk-1, and the rutile developed on it.…”