Rare earth elements in supergene zone: a case study of the xenotime-(Y)-chernovite-(Y) solid solution from Rędziny, Sudetes, SW Poland

Rare earth element (REE) mineralization has been documented at Rędziny, located on the eastern margin of the Karkonosze granite (Sudetes, Poland). Minerals of the REE[(As,P)O₄] group with a tetragonal zircontype structure, primarily xenotime-(Y)-chernovite-(Y), Y(PO4)-Y(AsO4) solid solutions, have been identified. These minerals occur as euhedral grains and also as intergrowths with secondary arsenates and silicates, filling cracks, fractures, and voids. Their textural diversity and paragenetic relationships with Ca-, Cu-, Pb-, and Bi-bearing arsenates indicate a crystallization sequence involving successive mineral-forming episodes. Local enrichments in REEs have also been recorded in common supergene arsenates, such as Ca(Pb)-Cu phases including conichalcite (CaCu(AsO4)(OH)) and duftite (PbCu(AsO4)(OH)). Minerals of the xenotime-(Y)-chernovite-(Y) solid solution correspond to intermediate compositions, with the chernovite end-member molar fraction ranging from 0.46 to 0.89. Based on EPMA analyses, yttrium is the dominant cation, accompanied by considerable amounts of middle rare earth elements (MREEs), especially neodymium (up to 12.60 wt.%; 0.21 apfu), samarium (up to 10.39 wt.%; 0.167 apfu), and gadolinium (up to 6.72 wt.%; 0.107 apfu). Some chemical compositions also show a trend towards the gasparite-(LREE) (LREE(AsO4)) compositional field. Xenotime-(Y)-chernovite-(Y) minerals display microporosity, likely resulting from dissolution, metasomatic alteration, and subsequent recrystallization. The REEs at Rędziny likely originated from both evolved, late-stage hydrothermal fluids and rocks of the metamorphic envelope of the Karkonosze granite, where REEs were mobilized by post-magmatic fluids. Subsequent supergene processes may have further enhanced secondary enrichment.