Browsing by Author "Angerer, Thomas"

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    Microanalytical investigation of K-rich fenites from the Catalao II alkaline-carbonatite complex in Central Brazil: Implications for ore-forming processes within the world's largest niobium province
    (2024) Velasquez-Ruiz, Felipe; Reich, Martin; Cordeiro, Pedro; Lagoeiro, Leonardo; Angerer, Thomas
    Alkaline-carbonatite complexes are the main source of critical raw materials such as niobium (Nb) and rare earth elements (REE), which concentrate through a combination of magmatic and carbohydrothermal processes. These systems typically occur in close spatial connection with altered country rocks resulting from metasomatic alteration by exsolved K-Na-rich fluids, a process known as fenitization. Thus, the association between Nb-REE-rich carbonatites and fenites provides a unique opportunity for the investigation of carbohydrothermal alteration processes leading to critical metal enrichment. In this study, we focused on K-rich fenites associated with a shallow dike swarm system in the Boa Vista niobium deposit, the second-largest global producer of Nb, which is hosted within the Catal & atilde;o II alkaline-carbonatite complex in Central Brazil. We used a combination of micro-analytical techniques including EBSD, EMPA, and mu-EDXRF to unravel the complex micro-textural features of the fenites. Our data suggest that alkaline fluids exsolved vigorously from the carbonatite melts upon dike emplacement and pervasively metasomatized the country rocks, forming a melanocratic proximal fenite (phlogopitite), and distal fine-grained orthoclase-phlogopite-calcite fenites. Fluid alteration mobilized soluble cations (K, Fe, Mg, Ba, and Sr) plus S, CO2 and OH-, whereas Nb and REE were retained in the carbonatite dikes. The consistent dike-orthogonal orientation of metasomatic phlogopite and orthoclase grains in the proximal fenite, determined by EBSD, suggests that newly formed metasomatic minerals precipitated from the fenitizing fluids along the flow path. These orientations differ from the isotropic textures defined by primary (magmatic) minerals like pyrochlore, tetraferriphlogopite, dolomite, and calcite. However, and despite the microstructural differences between magmatic and carbohydrothermal phlogopite, their mineral chemistry is similar, suggesting that fenites are almost synchronous to the emplacement of carbonatites. These data indicate that fenites provide a rich archive of alkali-rich fluid infiltration around Nb-REE-rich carbonatite intrusions.
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    The genetic link between kamafugite magmatism and alkaline-carbonatite complexes in the Late Cretaceous Alto Paranaiba Igneous Province, Central Brazil
    (2023) Velasquez Ruiz, Felipe; Cordeiro, Pedro; Reich, Martin; Motta, Joao Gabriel; Ribeiro, Carlos Cordeiro; Angerer, Thomas; Bernardes, Renato Borges
    The Late Cretaceous Mata da Corda Formation, located in the eastern part of the Alto Paranaiba Igneous Province (APIP), Central Brazil, is one of the few places on Earth where kamafugite melts reached the surface generating large volumes of lava, pyroclastic rocks and shallow intrusions over an area of 4,500 km(2). The western part of the APIP, however, is dominated by hundreds of diatreme-like kamafugites and shallow kimberlite intrusions and by the occurrence of multi-stage alkaline-carbonatite complexes. These complexes feature silica-undersaturated K-rich alkaline rocks, such as aillikite, that closely resemble the mineralogy and geochemistry of kamafugite, albeit lacking feldspathoids. The spatial and temporal distribution of kamafugite and aillikite within the APIP suggests a connection between them. In addition, on a regional scale, airborne magnetic data show three highly magnetic dipole-like structures to the south of the Mata da Corda Formation of an undisclosed nature, which bear geophysical similar responses to the neighbouring alkaline-carbonatite complexes. Links between kamafugite and aillikite are evaluated by the following chemical and isotopic evidence: (1) kamafugite and aillikite compositions plot in the kamafugite field of Foley's ultrapotassic rock classification; (2) similar CI chondrite-normalized REE distribution, with aillikite enriched up to 2 times in REE compared to kamafugite; (3) both lithologies share almost the same rock-forming minerals; and (4) similar Nd-143/Nd-144((i)) and Sr-87/Sr-86((i)) ratios for all the APIP alkaline-carbonatite rocks, indicating a common source from an enriched lithospheric mantle. Therefore, silica-undersaturated rocks from alkaline-carbonatite complexes display an evolved ultrapotassic affinity indicative of a genetic link.