Evolución metalogenética de complejos carbonatíticos en contexto hipoabisal y plutónico: Bonga y Monte Verde (Angola)

[spa] En Angola afloran complejos carbonatíticos en todos los niveles de emplazamiento tanto plutónico, hipoabisal como volcánico. Recientemente, se han caracterizado en detalle algunos de estos complejos debido a las mineralizaciones en HFSE y REE que presentan así como su comportamiento en función del nivel de emplazamiento. Sin embargo, aún no se conoce con precisión la distribución de elementos raros en ambientes subvolcánicos explosivos. Los complejos de Monte Verde y Bonga muestran mineralizaciones en las manifestaciones explosivas de modo que su estudio ha permitido describir y comprender la distribución de elementos raros en tales condiciones. De forma paralela, los resultados obtenidos en esta tesis han permitido entender la relación entre carbonatitas y lamprófidos ultramáficos en el complejo de Bonga, asociación que hasta ahora ha sido descrita en escasos complejos. El complejo de Bonga presenta cuerpos intrusivos mixtos de composición carbonatítica – lamprofíricos. Éstos fueron formados en estadios de cristalización tardíos por fenómenos de magma mingling como resultado de una intrusión de magma aillikítico y alnöítico dentro de una cámara magmática con magma carbonatítico residente. Esta mezcla de magmas dio lugar a estadios explosivos que no concentran especialmente elementos raros como HFSE y REE. De forma similar, las brechas subvolcánicas estudiadas en el complejo de Monte Verde contienen cantidades secundarias de estos elementos. En ambos complejos se concentran en cantidades traza de pirocloro empobrecido en F. El estudio de la distribución de elementos raros y la evolución de la carbonatita de Bonga se ha realizado mediante la caracterización de la geoquímica del pirocloro. Se ha determinado pirocloro primario de Na y F en los diques anulares alrededor de la intrusión que indica una composición rica en álcalis en el magma carbonatítico. El contenido en Na y F disminuye debido a la progresión de la fenitización. Los estadios posteriores más tardíos que dan lugar a rocas de magnetita-apatito, al cuerpo calciocarbonatítico central y a la matriz carbonatítica, presentan pirocloro pobre en F con una proporción de Na inferior. Los mayores enriquecimientos de elementos raros HFSE y REE se registran en los pirocloros secundarios formados en procesos hidrotermales muy tardíos y que están asociados a la invasión de la carbonatita por fluidos procedentes de las rocas encajantes. En estos estadios es donde se concentran carbonatos y fosfatos de REE. En consecuencia, el potencial metalogenético de las rocas de Bonga en ambientes explosivos no es destacable. Sin embargo, los elementos raros se suelen concentrar en estadios anteriores, en la formación de los diques anulares y rocas de magnetita – apatito. El potencial en elementos raros en las brechas carbonatíticas subvolcánicas de Monte Verde es muy bajo. Tampoco las rocas plutónicas alcalinas asociadas contienen cantidades de elementos raros importantes, como indica la escasez en carbonatos, fosfatos de REE así como en silicatos y óxidos agpaíticos (loparita, eudialita y sorosilicatos de Na-Ti-Zr). Éstos además presentan intervalos de agpaicidad bajos

Phosphate sequence study of Boqueirâo pegmatite (Rio Grande Do Norte State, Brazil)

The Boqueirão pegmatite is located in the Borborema Pegmatite Province in the state of Rio Grande do Norte (NE Brazil) hosted by Neoproterozoic taconglomerates of Equador Formation. The pegma tite occurs as a concentric body displaying characteristic units according their mineralogy. The border and wall zones are composed by quartz, microcline and muscovite often displaying skeletal and graphic textures with accessory tourmaline and garnet. The intermediate zone is divided in two distinct subzones and shows a complex mineralogy exhibiting a varied phosphate association, beryl, columbite -tantalite and zircon. The innermost zone is mainly constituted by a quartz core. In addition, typical replacement bodies of albite are recognized at the eastern part of the body. Lithiophilite triphylite represents the most abundant phosphate phase exhibiting exsolutions of sarcopside and replaced by sicklerite due to Lileaching alteration, best known as Quensel Mason sequence .The scarcity of heterosite -purpurite series is remarkable. Montebrasite and triplite are also primary phosphates observed. Subsequently ,the presence of alluaudite and varulite occurrences suggests that Na-metasomatism affected the pegmatite. The whole ensemble is crosscut by an extensive alteration that resulted in secondary phosphate minerals . Electron microprobe data of triphylite-lithiophilite shows Mn/(Mn+Fe)ratios between 0.21-0.24 but also up to 0.72 which seems to indicates that the earlier pegmatite magma could be slightly fractionated; consequently,ferrisicklerite shows Mn/(Mn+Fe) values between 0.69 -0.77 in agreement with high evolution degree and suggesting that pseudomorphism may be produced by high rock/fluid ratios. Alluaudite and varulite also record Mn and Fe contents up to 0.71 similar to precursor phosphates. Furthermor eight different secondary phosphates have been identified as replacement of primary phases either as pseudomorphs or crosscutting them. They form euhedral crystals of fine grain size, spherulites or extensive masses displaying vivid colours which are ea sily identifiable. Manrich secondary phases are hureaulite, eosphorite, whiteite, jahnsite and serrabrancaite while lipscombite, phosphosiderite and mitridatite correspond to Ferich secondary association. Supergenic late origin and entrance of Ca-K,Mgrich fluids from host rock may be invoked in order to explain their formation. They do not show any relationship with processes of pegmatite crystallization. Hf content of zircon from the intermediate zone has been analysed by electron microprobe as a pow erful indicator of pegmatite evolution degree together with Mn content of phosphates. Hf content obtained is low up to 5% wt. of HfO which which points out to low fractionated pegmatite magma. Besides, the presence of ferromagnesian silicates forming the pegmat ite units such as Ferich tourmaline, garnet and green beryl is in agreement of that hypothesis. Therefore, Boqueirão corresponds to rare element pegmatite of beryl columbite phosphate subtype

Nb and REE Distribution in the Monte Verde Carbonatite-Alkaline-Agpaitic Complex (Angola)

The Angolan alkaline-carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals; whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, wöhlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected

The distribution of rare metals in the LCT pegmatites from the Giraúl field, Angola

The Giraúl granitic pegmatite field, Angola, is composed of five pegmatite types; the most evolved belong to the beryl-columbite, beryl-columbite-phosphate and spodumene types. Pegmatites are concentrically zoned with increased grain size toward a quartz core; the most evolved pegmatites have well-developed replacement units. These pegmatites are rich in Nb-Ta oxide minerals and the field has a moderate interest for critical elements as Ta and Hf. Pegmatites are concentrically zoned with an increased grain size towards a quartz core. Tourmaline, garnet and beryl micas occur as accessory minerals. The abundance of Zr and Nb-Ta minerals increases with the evolution of the pegmatites, as well as the proportions of beryl and Li-rich minerals. The ; the Ta/(Ta+Nb) ratios in Nb-Ta oxide minerals and the Hf/(Hf+Zr) ratios in zircon also increase with the grade of evolution of the pegmatites and inside within each pegmatite body from border to inner zones, and especially in the late veins and subsolidus replacements. Textural patterns and occurrence of late veins with Ta-rich minerals suggest that Nb and especially Ta can be enriched in late hydrothermal exsolved fluids exsolved from the magma, along with Hf and other incompatible elements as Sn, U, Pb, Sb and Bi.Peer ReviewedPostprint (published version

Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites-The Bonga Complex (Angola)

The Bonga complex is composed of a central carbonatite plug (with a ferrocarbonatite core) surrounded by carbonatite cone sheets and igneous breccias of carbonatitic, fenitic, phoscoritic and lamprophyric xenoliths set in a carbonatitic, lamprophyric or mingled mesostase. To reconstruct the dynamics of the complex, the pyrochlore composition and distribution have been used as a proxy of magmatic-hydrothermal evolution of the complex. An early Na-, F-rich pyrochlore is disseminated throughout the carbonatite plug and in some concentric dykes. Crystal accumulation led to enrichment of pyrochlore crystals in the plug margins, phoscoritic units producing high-grade concentric dykes. Degassing of the carbonatite magma and fenitization reduced F and Na activity, leading to the crystallization of magmatic Na-, F-poor pyrochlore but progressively enriched in LILE and HFSE. Mingling of lamprophyric and carbonatite magmas produced explosive processes and the formation of carbonatite breccia. Pyrochlore is the main Nb carrier in mingled carbonatites and phoscorites, whereas Nb is concentrated in perovskite within mingled lamprophyres. During subsolidus processes, hydrothermal fluids produced dolomitization, ankeritization and silicification. At least three pyrochlore generations are associated with late processes, progressively enriched in HFSE, LILE and REE. In the lamprophyric units, perovskite is replaced by secondary Nb-rich perovskite and Nb-rich rutile. REE-bearing carbonates and phosphates formed only in subsolidus stages, along with late quartz; they may have been deposited due to the release of the REE from magmatic carbonates during the hydrothermal processes

Sandstone-Hosted Uranium Deposits as a Possible Source for Critical Elements: The Eureka Mine Case, Castell-Estaó, Catalonia

The Eureka deposit in Castell-estaó in the Catalan Pyrenees is a Cu-U-V deposit, hosted by Triassic red-bed sandstones, and classified here as a low-temperature, sandstone-hosted stratabound metamorphite U deposit. The main mineralisation is stratabound, related to coal-bearing units and produced during the Alpine deformation by migration of hydrothermal fluids. In this stage, the original sedimentary and diagenetic components (quartz and calcite, micas, hematite and locally apatite) were replaced by a complex sequence of roscoelite, fine-grained REE phosphates, sulphides and Ni-Co arsenides and sulpharsenides, Ag-Pb selenides, bismuth phases, sulphosalts and uraninite. The black shales of the Silurian sediments underlying the deposit and the nearby Carboniferous volcanoclastic rocks are interpreted as the source of the redox-sensitive elements concentrated in Eureka. The sulphur source is related to leaching of the evaporitic Keuper facies. The REE transport would be facilitated by SO4-rich solutions. The reduction of these solutions by interaction with organic matter resulted in the widespread precipitation of REE and redox-sensitive elements, including many critical metals (V, Bi, Sb, Co), whereas barite precipitated in the oxidized domains. The occurrence of similar enrichments in critical elements can be expected in other similar large uranium deposits, which could be a source of these elements as by-products. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

The BCN-SGA student chapter: a tool for insertion to research and laboral world in metallogeny

The activity of the BCN-SGA Student Chapter is evaluated. Establishedin 2012 it has grown up to 69 members, organizing 2 international workshops, 8seminars, numerous visits to museums, activities stimulating the use of English in socialevents and microresearch projects. The results of these microprojects are 15 presentations in scientific congresses. These activities enhance teamwork skills anddemonstrate that students can develop high quality research during the whole of theirformation.Peer ReviewedPostprint (published version

The Barcelona SGA-SEG student chapter: New fronts for international cooperation in teaching Geology and student exchanges

The Barcelona SGA-SEG Student Chapter is a student organization created and organized by students from the Faculty of Earth Science (University of Barcelona; UB). It offers to students interested in Mineralogy, Mineral Deposits and Economic Geology the opportunity to participate in research projects, student exchanges, seminars and courses in an international level. The students develop capacity of self-organization, team-work and public social skills. Recent activities are international exchange of student groups and participation in the project to update and replicate the mineral collection of the UB

The pilot project of the mineral collections from the University of Barcelona: An opportunity to create updated teaching material to be shared with other universities

The Mineralogy teaching collection from the University of Barcelona has proved to be a successful tool for students. The urge of its renewal to meet the necessities of modernCindustry brought the idea to replicate this collection in order to offer high quality teaching material to other universities worldwide. This project has led to an international collaboration aiming to enhance international solidarity among universities and make evident the importance of Mineralogy in Geology studies

The BCN-SGA student chapter: a tool for insertion to research and laboral world in metallogeny

The activity of the BCN-SGA Student Chapter is evaluated. Establishedin 2012 it has grown up to 69 members, organizing 2 international workshops, 8seminars, numerous visits to museums, activities stimulating the use of English in socialevents and microresearch projects. The results of these microprojects are 15presentations in scientific congresses. These activities enhance tea