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.
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