REE mineralization (primary, supergene and sedimentary) associated to the Morro dos Seis Lagos (Amazonas, Brazil)

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Marco E. Ciriotti
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REE mineralization (primary, supergene and sedimentary) associated to the Morro dos Seis Lagos (Amazonas, Brazil)

Messaggio da Marco E. Ciriotti » gio 16 set, 2021 14:28

Referenza:
▪ Lemos Giovannini, A., Bastos Neto, A.C., Porto, C.G., Takehara, L., Pereira, V.P., Bidone, M.H. (2021): REE mineralization (primary, supergene and sedimentary) associated to the Morro dos Seis Lagos Nb (REE, Ti) deposit (Amazonas, Brazil). Ore Geology Reviews, 137, 104308.

Abstract:
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% REE2O3 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% REE2O3 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% REE2O3, the manganese laterite has 1.54 wt% REE2O3, of which 1.42 wt% is Ce2O3. 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% REE2O3; 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% REE2O3.
Marco E. Ciriotti

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