| The Altay orogenic belt is the main production area of Li, Be, Nb, Ta, Cs, gem and muscovite in china, and the deposits in this area are mainly related to granite pegmatite. The Askartor Be deposit located in the of the altay orogenic belt not only has vertical mineralization zoning characteristics of granite type deposit but also has characteristics of granite pegmatite type deposit. The vertical mineralization zoning are biotite granite, two-mica granite, Be, Nb, Mo mineralized muscovite fine-grained granite and pegmatoid type aquamarine ore body in ascending order. The aquamarine ore body has characteristics of structure zoning and is thicker than 50 m. By far, the differentiation mechanisms of ore- forming melt/fluid for granite type deposit and granite pegmatite type deposit are is controversial. The Arskartor Be deposit has both granite type and granite pegmatite type ore body, is a typical instance to examine this recognition. Based on the study of geological characteristics and chronology of the deposit, fluid and melt inclusions were used to study the formation mechanism of the ore-forming fluids and summarize the metal ogenic model of the deposit.The metallogenic granites in the deposit are characterized by metaluminous, high-K calc-alkaline, enrichment of large ion lithophile elements and depletion of Ba, K, P and Ti. The geochemical change characteristics of different lithological zoning is directional, indicating magmatic differentiation gradually in ascending order.The weighted mean age of molybdenite in the muscovit albit granite is 214.9± 1.2Ma and the weighted mean age of molybdenite in the pegmatitoides is 218.6± 1.3Ma, not only earlier than the molybdenite Re-Os age but also close to zircon U-Pb age, indicating the ore-forming fluids or melt in the pegmatitoides differentiated from magma earlier. So we believe is liquid immiscibility of granite magma is an important mechanism to form pegmatite type ore body in the deposit.Two types of fluid inclusions were recognized in beryl in pegmatitoides, namely, H2O-CO2-NaCl and crystal-rich inclusions. And, few H2O-CO2-NaCl, aqueous two-phase and melt inclusions were observed in the granite. The homogenization temperatures and salinities of fluid inclusions in beryl vary in the range of 290~337℃ and from 3.2~6.3% NaCleqv, respectively. The homogenization temperature of melt inclusions in granite is 750~850℃, and homogenized to silicate melt, while the homogenization temperature of crystal-rich inclusions in pegmatitoides is 700~850℃, and homogenized to aqueous fluid. The melt inclusions and crystal-rich inclusions have close homogenization with contrast homogenization behaviors and different compositions, implying liquid immiscibility of granitic magma occurred in the late stage of magma evolvements. The volatile-rich fluid formed during liquid immiscibility is the main mechanism, and finally crystallized as pegmatoid type aquamarine ore body. |
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