||Mineralogical and Geochemical Study on Unidirectional Solidification Texture in Highly Differentiated Granitic Rocks Associated with Molybdenum Mineralization in Mongolia
ERDENEBAYAR, Jamsranエルデネバヤル, ジャムスレン
In Mongolia, ore deposits spatially related with the Unidirectional Solidification Texture (UST) granite include nine rare metal occurrences, one intrusion related REE occurrence and eleven porphyry copper and porphyry gold-copper deposits. In this study a variety of quartz from different three UST granite areas associated with molybdenum mineralization has been systematically studied.Unidirectional Solidification Texture in an aplite body is recognized in the Neoproterozoic highly differentiated granitic rocks at Kharaatyagaan, central Mongolia. On the basis of crystal morphology, two main types of UST were identified in the aplite body: (i) thin crenulate UST layers, (ii) thick intergrowth UST layers. Bulk geochemistry indicates that Kharaatyagaan UST-bearing aplite and aplite dike are alkaline, and are enriched in light rare-earth elements. Scanning electron microscopy and cathadoluminescence (SEM-CL) imaging of UST quartz from Kharaatyagaan show four types 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). Layered body is composed of the following parts (from bottom to top). Part A: UST-bearing aplite layers about 22 m wide. It consists of mostly thick UST layers. Aplite units are red pinkish colored. Biotite and albite are richer than the part C. Contacts between these layers are generally irregular and folded. All layers are directed to top of the hill. Part B: laminated fine-grained layers, about 18 m thick, with all type UST units. Part B is characterized by thin aplite units. All UST quartz layers are also directed toward the top of the hill. Part C: Part C is characterized mostly by brain texture, crenulate UST layers. Aplite units are light pinkish colored and K-feldspar rich.Also in this part REE minerals were identified. UST quartz crystals are directed toward the top of the hill. Here biotite is absent and muscovite occurs. Part D: fine-grained aplite dike, without any oriented structures, was developed in Neoproterozoic lecogranite with variable thickness from several cm to 50c m and 20 m long. Crystallization temperatures were calculated applying the Ti-in-quartz geothermometer assuming ɑTiO2=1 based on the observation that inclusions of rutile is commonly found in quartz phenocrysts and UST quartz. Contrasting bimodal light/dark-CL zonation is apparent in Qa1 type showing dark (low Ti) cores and with light (high Ti) rim. The darker cores crystallized at a lower temperature range (540-730°C)whereas rims were formed at higher crystallization temperature, 730-780°C. Fluid inclusions in UST quartz from Kharaatyagaan mainly consists of two phase and polyphase fluid inclusions. The homogenization temperature and NaCI equivalent salinity of CO2-bearing fluid inclusions of the part A and part B UST quartz range from 120-550°C< with 1-10wt. % NaCI equivalent. The UST quartz from part C homogenization temperature of two phase fluid inclusions range 140-400°C with 0.1-12wt. %NaCI equivalent.At the Zuun Mod porphyry molybdenum prospect, several small intrusions wherein UST is displayed are exposed as a number of small conical hills within the prospect. They are classified into the following two types. (1) Quartz ring dike consisting of a massive milky quartz layer, surrounded by a mixture of coarse-grained quartz-K-feldspar associated with muscovite. (2) Concentric UST-quartz consisting of thinly laminated crenulate quartz layers and K-feldspar-rich aplitic layers. CL imaging documents three type of quartz (Qz1-Qz3). Qz1: Mosaic of euhedrally zoned quartz. Qz2: spider and cobwed texture. Qz3: euhedral quartz with weak growth zoning in the aplite. Crystallization temperatures determined by Ti-in-quartz geothermometer ofall type quartz range between around 600° and 770°C. Fluid inclusions study in UST-quartz from Zuun Mod mainly consists of polyphase fluid inclusions. The homogenization temperature of CO2-free fluid inclusions in UST-quartz from Zuun Mod from range 324° to 598°C< with NaCI equivalent salinity range from 1to18 wt. %.UST-quartz at Undur Tsakhir consisting of thinly laminated crenulate quartz layers and K-feldspar-rich aplitic layers. Aplitic layers are strongly altered by seritization. Two sample collected and described in this study contain molybdenite within the UST-bearing aplite layers. Based on CL studies three types quartz were distinguished in Undur Tsakhir: (I) phenocryst quartz showing strong growth zoning in aplite . (II)Mosaic of euhedrally quartz. (III) Spider and cobwed texture. Crystallization temperatures determined by Ti-in-quartz geothermometer of all type quartz range between around 493° and 756°C. Fluid inclusions in UST-quartz from Undur Tsakhir mainly consists of three phase fluid inclusions. The homogenization temperature of CO2-bearing fluid inclusions of UST-quartz from Undur Tsakhir range from 198 to 398°C with NaCI equivalent salinity ranging from 1to14 wt. %.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 post magmatic stage, the closed system was opened and hydrothermal activity took place; the volatile and metal-rich fluid resulted in ore mineralization.