Abstract Alteration halos in wall rocks are potential indicators of rare element pegmatites; however, halos associated with niobium-yttrium-fluorine (NYF) pegmatites are poorly documented. It is also unclear whether such halos can survive subsequent orogenic overprinting. In this contribution, we examine the wall rocks of Paleoproterozoic rare element pegmatites that underwent amphibolite facies metamorphism during the Scandinavian Caledonian orogeny. These pegmatites represent the largest known intraplutonic NYF-type bodies globally. Radiometric and geochemical profiles of the surrounding granitic gneiss wall rocks reveal ~2- to 15-m-wide dispersion halos marked by radiometric anomalies and enrichment in rare and radioactive elements, including Cs, Li, F, Sn, Ta, U, Th, Nb, Rb, and Tl. These elements are carried by Caledonian metamorphic minerals but originated from the interaction between Paleoproterozoic pegmatite-derived fluids and the granitic protolith. Remarkably, the Caledonian profiles preserve the geochemical signatures of the original Paleoproterozoic pegmatite halos. Our findings demonstrate that the geochemistry of alteration halos of rare element pegmatites can persist through high-grade metamorphic overprinting. These halos can be traced using radiometry or bulk and mineral geochemistry, offering a promising vectoring tool for exploring ancient NYF-type pegmatites in younger orogenic belts.
Zhou et al. (Fri,) studied this question.