THE MAGMATIC TO HYDROTHERMAL TRANSITION AND ITS RELATION OF UNIDIRECTIONAL SOLIDIFICATION TEXTURE IN HIGHLY DIFFERENTIATED GRANITIC ROCK AT THE KHARAATYAGAAN, CENTRAL MONGOLIA

Late Permian unidirectional solidification texture (abbreviated as UST hereafter) in highly differentiated granitic rock is identified in the Neoproterozoic lecogranite at the Kharaatyagaan, central Mongolia. Bulk geochemistry indicates that the Kharaatyagaan UST-bearing aplite and aplite dykes are alkaline, and are enriched by light rare-earth elements. UST layered body is composed of the following parts from part-A bottom to part-C top. The part-A is UST-bearing aplite layers about 22 m wide, which consists of mostly thick UST layers. The aplite units are red pinkish colored, and contain higher amount of biotite, apatite and albite than the part B. The part-B is laminated fine-grained layers, about 18 m thick, with all type of UST units. The part-B is characterized by thin aplite units and more similar to part A. The aplite units of the part-C are light pinkish colored and K-feldspar rich. In this unit biotite and apatite is absent, and muscovite occurs.

SEM-CL-imaging of UST quartz from Kharaatyagaan show four types of quartz: euhedral quartz phenocrysts with well-developed concentric growth zoning (Qa1) in the aplite; euhedral quartz with weak growth zoning in the aplite (Qa2); UST quartz exhibiting distinct growth zones (Qu1); and UST quartz showing mosaic texture (Qu2). Contrasting bimodal light/dark-CL zonation is apparent in Qa1 type showing dark cores (low Ti) and with light rim (high Ti). The darker cores crystallized at a lower temperature range (540° to 730°C) whereas rims were formed at higher crystallization temperature (730° to 780°C).

Fluid inclusions in UST quartz from Kharaatyagaan mainly consist of two phase and polyphase fluid inclusions. The homogenization temperature and NaCl equivalent salinity of CO₂-bearing fluid inclusions of the part A and part B UST quartz range from 340°C to >550°C with 4 to 10wt. % NaCl equivalent. The UST quartz from part C homogenization temperature of two phase fluid inclusions range 240°C to 400°C with 5 to12wt. % NaCl equivalent.

Differences in morphology of phases among the various layers suggest changing condition of growth from undercooled melt. When the T and P were fluctuating, volatile activity increased and resulted in the UST. During the magmatic-hydrothermal transition, the closed system was opened and hydrothermal activity took place; the volatile and metal-rich fluid resulted in ore mineralization. A primary goal of this study is to better understand the role of magmatic volatiles and the importance of the magmatic-hydrothermal transition on UST related ore mineralization.