Insights into rare earth elements and other critical raw materials from Castelo Branco massif alluvial deposits (Portugal)

Rute Salgueiro^1*Teresa Silva¹Iván Martín-Méndez²Daniel de Oliveira¹Maria João Batista¹Carlos Inverno¹

• ¹Unidade de Recursos Minerais e Geofísica, Laboratório Nacional de Energia e Geologia, Amadora, Portugal
• ²Departamento de Recursos Geológicos para la Transición Energética, CN Instituto Geológico y Minero de España, Madrid, Spain

The actual and future demands for critical and strategic raw materials (CRM/SRM) justify the global search of potential sources of these elements, and the exploration approaches to reach them. This study discloses the characterization of rare earth elements (REE) and other CRM bearing alluvial heavy minerals samples sourced from the late- to post-tectonic Variscan Castelo Branco Massif (Portugal). Moreover, it contributes to the understanding of REE, Th and U anomalies from local and correspondent stream sediments identified in past geochemical surveys. The alluvial samples were subjected to semiquantitative analysis of the minerals under binocular microscope and chemical analysis through portable X-ray fluorescence. A varied mineralogical association and a geochemical fingerprint compatible with their main granitic source was identified, revealing several minerals carrying CRM, in which REE and titanium minerals have stood out. The titanium minerals have the higher average abundance (54 %), being ilmenite dominant (50 %), as supported by the relatively higher Ti content of these samples (up to 28.88 %). The REE minerals occur as accessories and summing average of 12 %, in which monazite is the most abundant (4.97 %). Geochemical data support that alluvial monazite is the mineral that has the highest REE and Th concentrations, as it is described in published literature on its granitic source. In addition, the magnetic fraction of samples (~85 %) that includes monazite and xenotime presents LREE anomalous contents (up to 4,557 mg/kg) in relation to their granitic source, NASC and even to ΣREE anomalous values revealed by local stream sediments. The Th (up to 1,969 mg/kg) and U (up to 244 mg/kg) contents follow the same trend. Nevertheless, the nonmagnetic mineral fraction that includes apatite and zircon reaches higher U contents (up to 448 mg/kg). This study indicates that the SRM Ti (ilmenite) and LREE (monazite) have potential to form placer deposits sourced from Castelo Branco granitic rocks in a wider area, which is supported by the regional Ti placers exploited in the past. The occurrence of other accessory elements/minerals with industrial applicability associated with those placers can also possibly represent an added value in the future.