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[Display omitted] • There are 4 types of REE mineralization: primary, supergene, clastic and authigenic. • Breakdown of Ce-pyrochlore structure formed a very pure cerianite-(Ce) in laterites. • The richest REE ore occurs in karstic basin (authigenic florencite-(Ce)). • Authigenic florencite-(Ce) is related to the reworking of the upper laterites. • The sedimentary record shows the evolution of lateritization and relief. In the Morro dos Seis Lagos Nb (Ti, REE) deposit (MSLD), Amazonas state, Brazil, there are four types of REE mineralization: primary, associated to siderite carbonatite; supergene, associated to laterite profile; and sedimentary (detrital and authigenic). The mineralogical and geochemical evolutions of the REE in these domains are integrated into a comprehensible metallogenic model. The main primary ore in the core siderite carbonatite is 52 m thick with 1.47 wt% REE 2 O 3 mainly in monazite-(Ce) and bastnäsite. However, considering the entire section intersected in the core siderite carbonatite, the average grade drops to 0.7 wt% REE 2 O 3 mainly contained in thorbastnasite. In the border siderite carbonatite, the REE mineralization is hydrothermal [rhabdophane-(Ce) and REE-rich gorceixite]. The LREE and phosphates are concentrated at the reworked laterites from where the HREE were leached. With the advance of lateritization, pyrochlore was completely decomposed. The final secondary Ce-pyrochlore was progressively enriched in Ce4+ with loss in REE3+, resulting in the breakdown of the structure and release Ce under strongly oxidizing conditions (high Ce4+/Ce3+) thus forming extremely pure cerianite-(Ce). This mineral occurs intercalated with goethite bands in the lower part of the weathering profile, represented by the brown laterite, and forms intergrowth with hollandite in the manganiferous laterite, formed in a more alkaline environment closer to the water table. The brown laterite has 1.30 wt% REE 2 O 3 , the manganese laterite has 1.54 wt% REE 2 O 3 , of which 1.42 wt% is Ce 2 O 3. Tectonic and karstic processes over the carbonatite formed several sedimentary basins. In the Esperança Basin, the sedimentary record (233 m thick) shows the whole evolution of the MSLD. The base of the basin (layer 5) is formed by abundant carbonatite fragments, have florencite-(Ce) mineralization with 1.07 wt% REE 2 O 3; layer 4 is formed by carbonatite fragments interbedded with clayey bed; layer 3 is a rhythmite deposited in a lacustrine environment, with clasts of ferruginous materials related to early stages of carbonatite alteration; layer 2 is made up by clays, is rich in organic matter, has authigenic florencite-(Ce), florencite-(La) and base metals. This layer marks the inversion of the relief and the input into the basin of REE leached from the upper laterites, carried by the groundwater flow; layer 1 was formed by the oxidation of the upper part of layer 2. Layers 1 + 2 have 73 m thick and average of 1.72 wt% REE 2 O 3. [ABSTRACT FROM AUTHOR]

The Morro dos Seis Lagos niobium rare earth element, Ti-bearing lateritic deposit (Amazonas, Brazil) is derived from a primary siderite carbonatite. The complex is the only example of a Nb deposit in which Nb-rich rutile is the main Nb ore mineral. Apart from the laterites, at the current level of exposure the complex consists only of siderite carbonatite; silicate rocks are absent. Three types of siderite carbonatite are recognized: (1) a brecciated and oxidized core siderite carbonatite consisting of up to 95 vol% siderite together with: hematite; pyrochlore; Nb-brookite; Ti-maghemite; and thorobastnäsite; (2) a REE- and P-rich variety of the core siderite carbonatite consisting of siderite (up to 95 vol%), hematite, minor pyrochlore, monazite and bastnäsite; (3) a border hydrothermal siderite carbonatite with ~70 vol% siderite, barite (~15 vol%), gorceixite (~7 vol%) and minor rhabdophane and pyrochlore. The country rock gneiss in which the carbonatite was emplaced was affected by potassic fenitization, with the formation of phlogopite and orthoclase together with monazite, fluorapatite and bastnäsite. The siderite carbonatites exhibit a wide variation of δ13C (−5.39‰ to −1.40‰), accompanied by a significant variation in δ18O (17.13‰ to 31.33‰), especially in the REE-rich core siderite carbonatite, and are explained as due to the presence of both H 2 O and CO 2 in the magma. The core siderite carbonatite is the richest in Fe (48.64–70.85 wt% Fe 2 O 3) and the poorest in Ca (up 0.82 wt% CaO) example of a siderite carbonatite yet recognized The ferrocarbonatite has significant contents of Mn, Ba, Th, Pb and LREE, and a very high Nb (up to 7667 ppm) content due to the presence of Nb-brookite. The substitution 3Ti4+ = Fe2+ + 2Nb5+ recognized in Nb-rich brookite explains enrichment of Nb in the core siderite carbonatite and indicates formation in a reducing environment. The high Nb/Ta ratio (1408–11,459) of the carbonatite is compatible with residual liquids derived by fractional crystallization. The 87Sr/86Sr (0.70411–0.70573) and 144Nd/143Nd (0.512663–0.512715) isotopic data suggest the carbonatite is mantle-derived with essentially no crustal contamination and is younger than the maximum age of 1328 ± 58 Ma (U Pb in zircon). We suggest that the Morro dos Seis Lagos carbonatite complex represents the upper-most parts of a differentiated carbonatite magmatic system, and that the siderite carbonatite is related to late-magmatic-to-carbo-hydrothermal processes. • Portrait evolution of the Morro dos Seis Lagos carbonatite complex. • The siderite carbonatite is the primary rock of the world's largest Nb-deposit. • The complex represents the upper-most parts of a carbonatite magmatic system. • The siderite carbonatite is related to late-magmatic-to-hydrothermal stage. • First isotopic data on the Morro dos Seis Lagos carbonatite. [ABSTRACT FROM AUTHOR]