Low-temperature hydrothermal alteration of a rare-metal rich quartz-epidote metasomatite from the El'ozero Deposit, Kola Peninsula, Russia

The Keivy peralkaline granite complex, Kola Peninsula, Russia, is the only Archaean A-type granite known in the world. It contains numerous rare-metal occurrences, formed by a complex sequence of magmatic, late magmatic and hydrothermal processes acting on several different host lithologies. During fluid-induced interaction of a peralkaline granite with basic host rocks in the El'ozero occurrence, chevkinite-(Ce), thorite, fergusonite-(Y) and zircon were formed in a rare-metal rich zone within a quartz-epidote metasomatite. These minerals were then altered by lower-temperature fluids to complex assemblages containing ferriallanite-allanite-(Ce), gadolinite-group minerals, titanite, ilmenite, thorite and Nb,Y-oxides. The sequence of alteration events is used to comment on the nature and composition of the fluids involved in the low-temperature alteration of the host metasomatite. It is inferred that the hydrothermal fluids were alkaline, with significant, but varying, amounts of F, CO (sub 2) and S.