Loparite-(Ce) in rocks of the Lovozero layered complex at Mt. Karnasurt and Mt. Kedykvyrpakhk

Abstract

Following from a mineralogical and petrographic study of loparite-bearing units I-4 at Mt. Karnasurt and II-4 at Mt. Kedykvyrpakhk, loparite-(Ce) is found to be concentrated in thin (10–40 cm) malignite-ijolite layers at the boundary between the underlying nepheline syenite and the overlying foidolite. Skeletal loparite-(Ce) metacrysts occur as inclusions in nepheline, sodalite, natrolite, aegirine, eudialyte, and lomonosovite or within the intergranular space between them. In turn, the characteristic segregations of skeletal loparite-(Ce) metacrysts contain inclusions of natrolite, lomonosovite, rhabdophane-(Ce), labuntsovite, and other relatively low-temperature minerals typical of pegmatites and hydrothermally altered rocks. The chemical composition of loparite-(Ce) varies within narrow limits (Lop59–70Per11–18Lue5–11Tsn4–7 due to an increase in Ca, Ln, Al, and Ti contents and a decrease in Na, Mn, Th, Sr, Fe, and Ta contents in the transitional zone between the underlying nepheline syenite, ore-bearing foidolite-malignite, and overlying ijoliteurtite. Variations in composition are also caused by the lateral isomorphic replacement of Na and Nb with Ln and Ti. The data obtained show that loparite mineralization is related to a pneumatolytic-hydrothermal alteration of foidolite and nepheline syenite along the contact between them.