Total REEcall: Appraisal of the mineralizaƟon in alkaline-silicate systems from the Ossa-Morena Zone (in Portugal)

Abstract

12ème édiƟon du Colloque InternaƟonal : MagmaƟsme, Métamorphisme et MinéralisaƟons Associées (3MA-12) Errachidia, Maroc – du 23 au 25 avril 2025 Page 73 sur 136 Total REEcall: Appraisal of the mineralizaƟon in alkaline-silicate systems from the Ossa-Morena Zone (in Portugal) José Roseiro 1, Noel Moreira 1, Mercedes Fuertes-Fuente 2, Daniel Oliveira 3, José Mirão 4, Pedro Nogueira 5,6 1 InsƟtuto de Ciências da Terra | InsƟtuto de InvesƟgação e Formação Avançada Universidade de Évora – Portugal 2 Departamento de Geologia | Universidade de Oviedo – Espagne 3 Laboratório Nacional de Energia e Geologia – Portugal 4 Laboratório HERCULES | Universidade de Évora – Portugal 5 Earth Science InsƟtute, Departamento de Geociências da Universidade de Évora, Colégio Luís António Verney, Rua Romão Ramalho, 59, 7000-671 Évora. (ICT, University of Evora) – Portugal 6 InsƟtuto de Cências da Terra - Pólo de Évora (ICT) – Portugal Alkaline silicate melts play a crucial role in the accumulaƟon of rare earth elements (REE), niobium (Nb), and other criƟcal raw-materials deposits. These melts typically originate from mantle- derived magmas enriched in incompaƟble elements that evolve through extensive fracƟonaƟon, leading to them to develop high concentraƟons of REE, Nb, Y, Zr, and other high-fieldstrength elements (HFSE). During late-stage magmaƟc-hydrothermal processes, halogen-bearing fluids further concentrate such elements and migrate to shallower levels. This type of magmaƟsm and associated metallogenic processes are usually associated with riŌing events, in extensional tectonic seƫngs. In the Iberian Massif, a large set of alkaline magmaƟc plutons were emplaced during the riŌrelated magmaƟsm concomitant with the opening of the Rheic Ocean during Cambro-Ordovician Ɵmes, specifically in the Ossa-Morena Zone (OMZ, SW Iberia) and in the Galicia-Trás-os-Montes Zone (NW Iberia). From the alkaline bodies that occur in the OMZ, the Alter Pedroso Massif stands out for its extreme lithological varieƟes that include a range of syenite rocks, including pegmatoids and highly differenƟated facies. The internal structure of this massif is characterized by an apparent horizontal layering, separaƟng a boƩom unit composed of leucocraƟc syenite cumulates, and a top unit characterized by mesocraƟc aegirine- and riebeckite-syenites oŌen intersected by numerous pegmatoid varieƟes. The mesocraƟc syenite unit oŌen show differenƟated rocks composed of disƟnct assemblages of accessory minerals, suscepƟble to be grouped into three types: (i) allanite phenocrysts in aegirine-syenites, (ii) thorite and allanite inclusions in zircons from aegirine-free riebeckite-syenites, and (iii) fluor-apaƟte, REE fluor-carbonates, Nb-Y and Nb-Th oxides in aegirine-riebeckite syenites. It is noteworthy the clear microzoning of the last type, in which the fluor-apaƟte and the REE fluor- carbonates are clearly separated from the HFSE-oxides. Fluorite and zircon are present in all these differenƟated rocks. Whole rock geochemistry studies from all samples show an increase of the criƟcal elements when compared to the "barren" syenite units, especially in REE (sum LREE over 4.000ppm), Nb (up to 1.400ppm) and Y (up to 2400ppm), with Zr over 10.000ppm. Besides the rare metal mineralizaƟon in these unusual syenite facies, the pegmatoid rocks show high accumulaƟon of large euhedral zircon megacrystals. In Iberia, similar REE-Nb-Y mineralizaƟon has only been documented in the Galiñero Complex (NW Spain), while in Alter Pedroso, the accessory mineralogy has been always poorly characterized. New mineralogical and geochemical data of the different facies of the Alter Pedroso Massif thoroughly record and highlight the mechanisms of the consensual metallogenic model for Zr-REE-Nb-Y mineralizaƟon in alkaline-silicate magmaƟc systems, namely the differenƟaƟon enrichments through liquid immiscibility or fracƟonal crystallizaƟon.