Authigenic grey monazite from ordovician metasediments of the Iberian massif: the Matamulas placer

Authigenic nodular monazite occurs in dark, Middle Ordovician, low-grade metapelites of the Central Iberian and West Asturian-Leonese Zones of the Iberian Massif. Erosion of monazite-bearing slates and phyllites, and short-range transport resulted in localised recent alluvial deposits rich in millimetre-size monazite, of which the Matamulas site, within the Central Iberian Zone, represents one of the most extensive and potentially exploitable placer-type accumulations of Th-poor grey monazite in western Europe. Nodular monazite from the Matamulas area depicts spheroidal or triaxial ellipsoid shapes and is mostly greyish, although it may display different surface colours from yellow to black. The nodules contain abundant silicate (quartz, feldspar, white mica, chlorite, zircon) and Fe oxides-hydroxides, organic matter, rutile and Mn oxide inclusions, generally oriented at random or showing orientations related to original bedding anisotropy. Nodular monazite displays concentric optical zoning, continuous or oscillatory, associated with the abundance and nature of inclusions. Chemical zoning is typically progressive, with cores enriched in medium and heavy rare earth elements (REE), and rims rich in light REE (La-Ce). Substitution of Pr by La-Ce is decoupled from that of elements with smaller ionic radii. Thorium contents are low (estimated mean 0.16 wt % ThO2) and irregularly distributed across the monazite nodules. The increased concentration of phyllosilicate and other inclusions toward nodule rims, the sparse occurrence of twinning and the observed chemical zoning suggest progressive recrystallisation and refining from precursor gels or cryptocrystalline aggregates, facilitated by localized fluid-assisted element mobility during nodule growth. U-Pb dating of monazite yields an Early Devonian age of ca. 400 Ma, significantly older than the Upper Carboniferous age of regional metamorphism and tectonic foliation in the host metasediments. The age obtained, together with their non-oriented growth within specific levels of the sedimentary pile, indicates that the process of formation of nodular monazite took place through diagenesis or burial metamorphism. Although nodular monazite is absent from equivalent stratigraphic units in more internal zones of the Iberian Massif, its occurrence over a broad area of western Europe warrants further exploration for placer-type monazite deposits analogous to those at Matamulas in comparable geomorphological settings.