• Journal of the Korean earth science society
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• v.43 no.5
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• pp.618-638
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• 2022

The Middle Jurassic granite dike swarm intruding into the Paleoproterozoic banded gneiss is pervasively observed in Ueumdo, Hwaseong City, mid-western Gyeonggi Massif. Based on their cross-cutting relationships in a representative outcrop, there are four dikes (UE-A, UE-C, UE-D, UE-E), and depending on the direction, there are three granite dike groups, which are NW- (UE-A dike), NW to WNW- (UE-C dike), and NE-trending (UE-D and UE-E dikes). These granite dikes are massive, medium-to coarse-grained biotite granites, and their relative ages observed in outcrops are in the order of UE-A, UE-D (=UE-E), and UE-C. The geometric analysis of the dikes indicates that the UE-A and UE-C dikes intrude under approximately NE-SW trending horizontal minimum stress fields. The UE-A dike, which showed a relatively low average SiO2 content by major element analysis, is a product of early magma differentiation compared to other dikes; therefore, it is consistent with the relative age of each dike. The ²⁰⁶Pb/²³⁸U weighted mean ages for each dike obtained from SHRIMP zircon U-Pb dating were calculated to be 167 Ma (UE-A), 164 Ma (UE-C), 167 Ma (UE-D), and 167 Ma (UE-E), respectively. The samples of the UE-A, UE-D, and UE-E dikes showed very similar ages. The UE-C dike shows the youngest age, which is consistent with the results of the relative age in the outcrops and major element analysis. Therefore, the granite dikes intruded into the Middle Jurassic (approximately 167 and 164 Ma), coinciding with those of the Gyeonggi Massif, where the Middle Jurassic plutons are geographically widely distributed. This result indicates that the wide occurrence of the Middle Jurassic plutons on the Gyeonggi Massif was formed as a result of igneous activity moving in the northwest direction with the shallower subduction angle of the subducting oceanic plate during the Jurassic.

• Economic and Environmental Geology
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• v.17 no.1
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• pp.1-15
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• 1984

The tin and tungsten deposits are embedded around the age unknown Buncheon granite gneiss which intruded the Precambrian schists, gneiss and amphibolites in Bonghwa-Uljin area. Pegmatite dike swarm developed intermittently about 4km along the southern border of Buncheon granite gneiss at Wangpiri area. Thickness of pegmatite dikes range from 0.5 to 15m. Pegmetite is consisted of quartz, microcline, albite, muscovite and frequently topaz, tourmaline, garnet, fluorite, fluorapatite and lepidolite. Pegmatite dikes are greisenized, albitized and microclinized along dike walls. Cassiterites are irregularly disseminated through the intensely greienized and albitized parts of the pegmatite. Cassiterite crystals are mainly black to dark brown and contain considerable Ta and Nb. Average Ta and Nb contents of the four cassiterite samples are 5300 and 3400 ppm. The Ssangjeon tungsten deposits is embedded within the pegmatite dike developed along the northern contact of Buncheon granite gneiss with amphibolite. This pegmatite developed 2km along the strike and thickness varies from 10 to 40m. Mineral constituents of the pegmatite are quartz, microcline, plagioclase, muscovite, biotite, tourmaline and garnet. Ore minerals are ferberite and scheelite with minor amount of molybdenite, arsenopyrite, pyrrhotite, pyrite, chalcopyrite, sphalerite, galena, pentlandite, bismuthinite, marcasite, and fluorite. Color and occurrence of quartz reveals that quartz formed at three different stages; quartz I, the earliest milky white quartz formed as a rock forming mineral of simple pegmatite; quartz II, gray to dark gray quartz which replace the minerals associated with quartz I; quartz III, the latest white translucent quartz which replace the quartz I and H. All of the ore minerals are precipitated during the quartz II stage. Fluid inclusion in quartz I and II are mainly gaseous inclusions and liquid inclusions are contained in quartz III and fluorite. Salinities of the inclusion in quartz I and II ranges from 4.5 to 9.5 wt. % and 5.1 to 6.0 wi. % equivalent NaCl respectively. Salinities of the inclusion in fluorite range from 3.5 to 8.3 wt. % equivalent NaCl. Homogenization temperatures of the inclusion in quartz I, II and III range from 415 to $465^{\circ}C$, from 397 to $441^{\circ}C$ and 278 to $357^{\circ}C$. Data gathered in this study reveals that tin and tungsten mineralization in this area are one of prolonged event after the pegmatite formation around Buncheon granite gneiss.