GENESIS OF UNIDIRECTIONAL SOLIDIFICATION TEXTURE IN HIGHLY DIFFERENTIATED GRANITIC ROCKS AT KHARAATYAGAAN, CENTRAL MONGOLIA

Kharaatyagaan unidirectional solidification texture (UST)-bearing aplite unit at central Mongolia, 0.4-200 m thick and up to 250 long generally northeast-southeast trending, intruded Neoproterozoic granite. Litho-geochemical prospecting result shows well determined anomalies of gold, tungsten and molybdenum mineralization in the area.

Based on the field relationship between the leucocratic granite and host rock, the UST is considered to have formed in the roof of the small intrusion. UST-bearing aplite is characterized by multiple sequences of four main textural units: (i) common UST, (ii) brain rock, (iii) quartz-rich UST, and (iv) fine-grained aplite units. Outcrop distribution showed that early stage units of the UST sequence were exposed at the bottom and late stage units were crop out at the top of the Kharaatyagaan hill.

The base of each UST quartz layers, which appears sharp in hand specimen, is irregular on a microscopic scale and conforms to grain boundaries in the aplite.

UST quartz grains are oriented with their c-axes perpendicular to the layering. The interstice between UST quartz layers is filled by aplite. The aplite interlayers typically consist of 10-20 µm sized grains of quartz, sodic plagioclase and alkali feldspar. Most phenocrysts within the aplite layers are relatively unaltered. Alkali feldspars are perthitic and weakly sericitised.

Geochemical feature indicates that the Kharaatyagaan UST-bearing aplitic body is alkaline, enriched in LREE continents and formed in volcanic arc setting. Bulk compositions of aplite layers are consistent with the UST-bearing aplite and aplite dyke from Karaatyagaan have normal magma compositions. However, if assuming that the UST-bearing rocks which are very quartz rich were formed from magma melting temperature unreasonably may have been very high, regardless of the amount of H₂O present. The upper part of the felsic intrusion was formed at relatively high T (>600^°C), high P vacuum condition. When the T and P were fluctuating, volatile activity was increased and results the UST. During the post magmatic stage, the closed system was opened and hydrothermal activity took place; the volatile and mineral-rich fluid resulted in ore mineralization. The Kharaatyagaan Au-W mineralization might be the one example of such process.