IDENTIFICATION AND ALTERATION TRENDS OF GRANITIC-PEGMATITE-HOSTED (Y,REE,U,Th) (Nb,Ta,Ti) OXIDE MINERALS: A STATISTICAL APPROACH

Abstract

The (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals in REE-enriched granitic pegmatite and A- and I-type granites are primarily comprised of the fergusonite, samarskite, euxenite, aeschynite and pyrochlore groups. They are typically metamict and altered, and can be difficult to identify on a structural basis (XRD); consequently, examination of sample composition may be the most reliable approach to their identification. Canonical discriminant analysis can be used to discriminate among the various groups, and to assess the style and degree of alteration affecting samples. Identification of individual species should be made on the basis of previously published guidelines or, in their absence, on the basis of the “50% rule” of the IMA. Vector analysis shows that alteration typically pushes the compositions of samarskite-, euxenite- and aeschynite-group minerals in a direction that causes them to resemble pyrochlore-group minerals. For this reason, it has been historically difficult to determine the exact nature of “viethofingite”, “hjelmite” and “ampangabeite”; all are shown to be altered varieties of samarskite-group minerals. Yttrotantalite-(Y) is shown to be a member of the samarskite group, not the fergusonite group.