PETROGRAPHIC STUDIES REVEAL A HISTORY OF SUBSOLIDUS FLUID:MINERAL INTERACTION, MINERALOGICAL MODIFICATION AND RARE-METAL ENRICHMENT IN TYPE SAMPLES OF THE CRETACEOUS MONGOLIAN ONGONITES

Ongonites were defined at their type locality at Ongon Kkairkhan, central Mongolia, as pristine topaz-bearing albite-quartz-keratophyres with <4 wt. % F and characterized by phenocrystic albite, K-feldspar, quartz and rare mica in a similar groundmass with topaz microlites. These 120 Ma old dike rocks have narrow chilled margins, were emplaced at a shallow crustal level, and connect along strike to a stockwork zone enriched in W. Detailed petrographic studies integrated with SEM-EDS analysis of samples from the type area indicate the primary mineral phases, both phenocrystic and matrix, record, however, considerable subsolidus exchange with deuteric fluids. The phenocrystic assemblage (5 to 30% modally) is dominated by plagioclase over orthoclase (4:1) with minor quartz and trace muscovite; their textural and chemical features follow: (1) plagioclase is albitic () and often mantled or replaced (30-70%) by orthoclase (Or₁₀₀); (2) K-feldspar is rarely perthitic (film, bleb types) and point (Ab_98-100) and bulk (i.e., raster; Or _96-98 ) analyses are similar; (3) both feldspars are inundated with pitted textures often attributed to dissolution-precipitation reactions; (4) quartz euhedra may contain albite laths oriented in a snowball texture; (5) primary muscovite euhedra (<2 wt. % FeO, 6-8 wt.% F) is replaced along cleavage traces by secondary muscovite (6-8 wt. % FeO, 3-4 wt.% F) along with fluorite; and (6) abundant secondary fluid inclusions (L-V, L-V-solids) occur in both quartz and feldspar. The matrix consists of quartz, pitted albite (Ab₁₀₀) and topaz. The latter has an acicular habit, often in a radial habit, and preferentially occurs in quartz-rich areas. All samples contain a suite of secondary phases, including muscovite, fluorite, monazite, xenotime, zircon (±Hf), U-Th oxides, Nb-Ta oxides with variable Mn, Sn, W, and various Bi phases, including native Bi. The petrographic features and SEM-EDS data indicate that whereas the ongonites have retained primary magmatic textures, extensive subsolidus interaction with deuteric fluids is also recorded and fluid inclusions indicate hese were high T and saline. We suggest the secondary features relate to incursion of fluids derived from a highly differentiated, F-rich granitic magma at depth and that some of the rare-metal enrichment is of secondary origin.