First occurrence of gasparite-(Ce), chernovite-(Y) and retzian-(Ce) in Greece: mineral chemistry and implications for REE enrichment in the Varnavas Mn-oxide ore

This work describes the mineral chemistry and mode of occurrence of a complex REE arsenate-phosphate mineral association, namely gasparite-(Ce) Ce(AsO4), chernovite-(Y) Y(AsO4), retzian-(Ce), Mn2REE(OH)4(AsO4), as well as the phosphates, monazite-(Ce), Ce(PO4). The association is reported from the metamorphosed Mn-oxide mineralization within a Triassic volcaniclastic unit from Varnavas, NE Attica (Greece), part of the Attic-Cycladic Blueschist Unit. The REE minerals were investigated using electron probe microanalysis (WDS) and scanning electron microscopy. The Varnavas Mn ores have lenticular or banded textures; the ore bodies, only a few meters wide, are enclosed in piemontite-schists similar to those described from the Alps in Switzerland, Italy, and Austria (Stouraiti et al., 2022; Pagliaro et al., 2022). Gasparite-(Ce) is a common mineral hosting REE in the ore; it occurs as minute isometric grains up to 15 μm in size associated with chernovite-(Y) and retzian-(Ce), enclosed in todorokite-pyrolusite-hollandite ore. The Mn-rich rocks are also profoundly enriched in arsenic reaching concentrations of 2000 ppm. The geochemical data of the previously mentioned phases demonstrate a P-As substitution between monazite-(Ce) and gasparite-(Ce), as well as for the pair xenotime-(Y) and chernovite-(Y) (Pagliaro et al., 2022). The above suggest that both monazite-(Ce) and xenotime-(Y) reacted with As-bearing hydrothermal fluids that converted them to arsenates (Ondrejka et al., 2007). The presence of ardennite-(As) in rocks surrounding the Mn-ore indicates the deposition of As under strongly oxidizing conditions. Notably, all the studied REE-minerals display a pronounced positive Ceanomaly in the normalized REE patterns, which is known to result from oxidation of Ce on Mn-oxides. The inclusions of the REE-arsenates in the Mn-ore and the existence of the monazite and xenotime in the adjacent Mn-rich host rock, may indicate a short fluid transport of leached REE from the REE-phosphate minerals and oxidative deposition in the local Mn-oxide accumulations.