REY+ACT redistribution in polymineral aggregates: a case study on monazite decomposition in pegmatite

Selected pegmatite dyke intruded within the gneiss-marble-amphibolite sequence of the Rhodope Metamorphic Complex was studied for its REY+ACT mineralization in attempt to assess the potential of these pegmatites for critical and strategic raw materials. Mineral composition of K-feldspar and albite is complemented by abundant common accessories of zircon, apatite, monazite, allanite, and titanite. The REY+ACT mineralization is presented as single euhedral crystals (zircon, thorianite, Y-Nb oxide, titanite and fluorapatite) or as complex multimineral nodule-like aggregates. The nodule core typically consists of decomposed Th-bearing monazite-(Ce) altered to different degree by intergrown Th-bearing apatite/thorite assemblage. Fluid-involved reactions led to mineral transformations and redistribution of the elements like LREE, Y, Th, U, and formation of the inhomogeneous allanite-(Ce)/REE-epidote rims enveloping the aggregated earlier phases.