Interaction of Rare-metal Minerals with Hydrothermal Fluids: Evidence from Quartz–epidote Metasomatites of the Haldzan Buragtag Massif, Mongolian Altai

A quartz–epidote metasomatite from the Haldzan Buragtag massif, Mongolian Altai, provides evidence for the low-temperature hydrothermal alteration of rare-metal minerals. In what was essentially a continuous sequence of events during a complex evolutionary history, a peralkaline granitic pegmatite was metasomatized by hydrothermal fluids related to the associated intrusions. Minerals in the pegmatite, including fergusonite-(Y), chevkinite-(Ce), and members of the epidote supergroup, were recrystallized and texturally coarsened at temperatures in the range 318–446 °C. These phases were then altered by lower-temperature fluids (156–160 °C), with replacement of chevkinite-(Ce) by titanite–rutile–cerite-(Ce)–REE-carbonate assemblages and fergusonite-(Y) by complex hingganite–cerite–thorite assemblages. Late-stage veining included cerite-(Ce), fergusonite-(Y), and REE-carbonates. Generally, the light rare-earth elements (LREE) were replaced by Y, heavy rare-earth elements (HREE), and Ca as alteration proceeded. Mineral assemblages are interpreted to show that the hydrous fluids were acidic and oxidizing, contained significant amounts of CO₂, but were poor in F and Cl. The fluids may ultimately have been derived from basaltic magmas emplaced after formation of the pegmatite.