Abstract

Polycrase-(Y) and euxenite-(Y) represent the euxenite-group minerals (EGM) that formed as magmatic minerals in the sequence of (Y,REE,U,Th)–(Nb,Ta,Ti) oxides: columbite-(Fe) → Ti-bearing ixiolite ± ferrowodginite → samarskite-group minerals [ishikawaite → samarskite-(Y)] → EGM in the moderately fractionated granitic pegmatites of the hybrid NYF + LCT Julianna pegmatitic system at Piława Górna, Sudetes, SW Poland. The EGM crystallized with increasing Ti activity and decreasing Ta/(Ta + Nb) and Mn/(Mn + Fe) from F-poor and LREE-depleted melt. The variation in their composition results mainly from substitution along the A(U,Th)BTiA(Y,REE)–1B(Nb,Ta)–1 vector.

Metasomatic alteration of the EGM resulted in the formation of a variety of secondary pyrochlore supergroup minerals (PSM). The chemical composition of the PSM depends on the access gained by the metasomatic fluid to the replacement front. Alteration at sites of restricted access of the fluid led first to the formation of atypical Ca-enriched EGM evolving to zero-valence-dominant betafite with Y + REE as the second-rank A-site component, and finally to (Y,REE)-rich zero-valence-dominant pyrochlore. These PSM closely reflect the compositions of the parental EGM as indicated by similar Ti/(Ti + Ta + Nb), Ta/(Ta + Nb), and Mn/(Mn + Fe) values. If the transport of ions to and from the replacement front was relatively easy, the compositions of the alteration products were less limited by the composition of the primary oxides and zero-valence-dominant pyrochlore with Ca or U + Th as the second-rank A-site components formed. This type of alteration took place with decreasing Ti/(Ti + Nb + Ta), Y/(Y + REE), and Mn/(Mn + Fe) but highly variable Ta/(Ta + Nb).

The addition of Ca, Fe, Mg, Sn, and Pb to the pegmatitic system during metasomatic alteration of the EGM could have been achieved through: (1) reaction of the orthomagmatic fluid with the amphibolite wall rock, circulating in a convective cell around the pegmatites; (2) assimilation of the wall rock by the pegmatite-forming melt; or (3) autometasomatic processes. However, given the current state of knowledge about the geochemical evolution of the Julianna pegmatitic system, the first scenario seems most likely.

You do not currently have access to this article.