• Journal of the Korean earth science society
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• v.43 no.5
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• pp.639-646
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• 2022

Occurrence of black opaque hydrocarbon (pyrobitumen) in some Cretaceous Jinju sandstones of the Sindong Group, Gyeongsang Basin in Korea is first reported in this study. The pyrobitumen is developed on chlorite pore-lining cement, or impregnated into the outer zone of chlorite cement. Therefore, it seems to have been formed after the precipitation of chlorite cement, indicating the former presence of crude oil. The liquid hydrocarbons migrated into sandstones during moderate burial and these sandstones seem to have acted as a liquid hydrocarbon reservoir. The presence of pyrobitumen in the Jinju Formation indicates that this formation underwent deep burial after liquid hydrocarbon migration. As reservoir temperatures increased further, hydrocarbons were cracked and a solid pyrobitumen residue remained in the reservoir.

• The Journal of the Petrological Society of Korea
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• v.25 no.1
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• pp.51-67
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• 2016

Geochemical characteristics of the Early Cretaceous igneous rocks from eastern China and the Gyeongsang Basin, Korean Peninsula has been summarized. They have wide range of lithological variation with extrusive picrite-basalt-andesite-trachyte-rhyolite and lamprophyre, and intrusive gabbro-diorite-monzonite-syenite-granite and diabase in eastern China, mostly belonging to the high-K calc-alkaline or shoshonitic series. The volcanic rocks intercalated with the Hayang Group sedimentary assemblages in the Gyeongsang basin are high-K to shoshonitic basaltic trachyandesites. The Early Cretaceous basaltic rocks studied mostly fall within the field of within-plate basalts on the Zr/Y-Zr and Nb-Zr-Y tectonic discrimination diagrams. On a Sr-Nd isotope correlation diagram, basaltic rocks from the North China block (NCB) and the continent-continent collision zone (CZ) between the North and South China blocks plot into the enriched lower right quadrant along the extension of the mantle array. The initial $^{87}Sr/^{86}Sr$ ratios of basaltic rocks from the South China block (SCB) are indistinguishable from those of the NCB and CZ basaltic rocks, but their ${\varepsilon}_{Nd}$ (t) values are relatively more elevated, plotting in right side of the mantle array. Basaltic rocks from the NCB and CZ are characterized by low $^{206}Pb/^{204}Pb(t)$ ratios, lying to the left of the Geochron on the $^{207}Pb/^{204}Pb(t)$ vs. $^{206}Pb/^{204}Pb(t)$ correlation. Meanwhile, the SCB basaltic rocks have relatively radiogenic Pb isotopic compositions compared with those of the NCB and CZ basaltic rocks. Basaltic rocks from the Hayang Group plot within the field of the NCB basaltic rocks in Sr-Nd and Pb-Pb isotope spaces. Metasomatically enriched subcontinental lithospheric mantle (SCLM) is likely to have been the dominant source for the early Cretaceous magmatism. Asthenospheric upwelling under an early Cretaceous extensional tectonic setting in eastern China and the Korean Peninsula might be a heat source for melting of the enriched SCLM. Metasomatic agents proposed include partial melts of lower continental crust delaminated and foundered into the mantle or subducted Yangtze continental crust, or fluid/melt derived from the subducted paleo-Pacific plate.

• Economic and Environmental Geology
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• v.19 no.spc
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• pp.175-191
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• 1986

The Geodo granodiorite intruded into the Joseon Supergroup is fine-grained at the marginal part, and medium-grained and more leucocratic at the central part. The Quartz monzonite porphyry intruded inte Precambrian granite and Geodo granodiorite has abundant plagioclase phenocryst. The Imog granite intruded into the Yulri Group and the Joseon Supergroup is mediumgrained biotite granite with partly pinkish feldspar phenocryst. The K/Ar ages obtained from the biotite of the Geodo granodiorite and Imog granite are Early ($111{\pm}1{\sim}107{\pm}1$ Ma) and Late ($93{\pm}1{\sim}92{\pm}1$ Ma) Cretaceous, respectively. The K/Ar sericite age of the quartz-sericite zone of the lower Jangsan quartzite occuring in the western area gave much younger age (about 170 Ma) than that of the Jangsan quartzite, that might be reset due to the regional metamorphism of the Daebo orogeny. The granitic rocks of the area are felsic to mafic, metaluminous to peraluminous, calc-alkalic (alkali-lime index${\fallingdotseq}$ 57) and I-type (magnetite-series) based on the chemical data_ And they appear to have been fractionated at the order of Geodo granodiorite, Quartz monzonite porphyry and Imog granite. In terms of mineralogy, geochemistry and K/Ar biotite age, a rock suite of monzodiorite, quartz monzodiorite and quartz monzonite-granodiorite in the Geodo stock was fractionally differentiated from a magmatic body from its margin to inward.

• Economic and Environmental Geology
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• v.3 no.3
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• pp.177-186
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• 1970

Bupyong lead-silver mine is located at outskirt of Inchon, a harbor city on the Yellow Sea about 40 km due west of Seoul. The geology of the area is composed of gneisses of pre-Cambrian age, rhyolite of Jurassic to Cretaceous age which extruded over the gneisses and late Cretaceous granite. Small diabasic dike is observed only in the underground. The contact plane between overlying rhyolite and underlain gneiss is sinuous and generally pitches about $30^{\circ}{\sim}40^{\circ}$ toward east. Conjugate joints and fissures are well developed in the rhyolite striking generally north-southward. Three ore bodies are being exploited and three more are under prospecting. These ore bodies range from few tons of hundred thousand to million tons in reserve. These ore bodies occur exclusively in the rhyolite along joints as network and/or desseminated type. The lower limit of ore bodies is always delineated at about 20~30m above the gneiss which might be indicative of ore genesis that has not been clearly explained so far. Two hypothesis on ore genesis could, however, be considered: firstly lithologic difference in the rhyolite might be a manifestation of different flows along which ore solution ascended and replaced along joints; secondly diabasic dike has acted as ore bringer since the dike contains considerable amount of silver, lead and zine. Ore minerals are galena and native silver accompanied by pyrite, argentite, pyragyrite and magnetite. It is believed that pyritization took place in advance to main mineralization, and ore deposit is classified as meso- to epi-thermal type.

• Economic and Environmental Geology
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• v.14 no.1
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• pp.35-49
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• 1981

The studied area is largely occupied by thick piles of the late Cretaceous volcanic rocks of the Yucheon group, which is northeastern border part of the vast volcanic region in the Yucheon basin. The Yucheon group overlies the Geoncheonri Formation and is intruded by granitic and dioritic stocks and dykes. The group can be devided into two parts; the lower is Jusasan andesitic rocks which was called as Jusasan Porphyrite Formation by Tadeiwa in 1929, and the upper is Unmunsa rhyolitic rocks. The volcanic pile consists mainly of various tuffs such as tuff breccia, lapilli tuff, coarse to fine tuff and tuffaceous sediments, and interlayered flows, which range from basaltic andesite to rhyolite in their lithology. The results of petrochemical and volcanostratigraphic studies on the Jusasan andesitic socks suggest that the volcanic rocks were derived from two cyclic evolutions of magmatic fractionation. Systematic study of 5226 joints from the area reveals two sets of steep joints striking $N20^{\circ}-40^{\circ}E$ and $N40^{\circ}-70^{\circ}W$, are dominant and coincide with the fault pattern developed in the area. Three defferent maximum principal stress axes were recognized from conjugate shear joints, which are trending east-west, north-northwest, and north-northeast.

• Ocean and Polar Research
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• v.24 no.3
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• pp.289-300
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• 2002

The Lago Sofia conglomerates encased in the Cretaceous Cerro Toro Formation, southern Chile, represent a gigantic submarine channel system developed along a foredeep trough. The channel system consists of several tributaries along the trough margin and a trunk channel along the trough axis. Voluminous debris flows were generated ubiquitously along the tract of the submarine channel mainly by the failure of nearby channel banks or slopes. The flows transformed immediately into multiphase flows and resulted in very thick-bedded mass-flow deposits with a peculiar structure sequence. The mass-flow deposits commonly overlie fluted or grooved surfaces and consist of a lower division of clast-supported and imbricated pebble-cobble conglomerate with common basal inverse grading, and an upper division of clast- to matrix-supported and disorganized pebble conglomerate or pebbly mudstone with abundant intraformational clasts. The structure sequence suggests a temporal succession of a turbidity current, a bipartite hyperconcentrapted flow with active clast collisions near the flow base, and a cohesive debris flow probably with a rigid plug. The multiphase flow is interpreted to have resulted from transformation of clast-rich but cohesive debris flows. Cohesive debris flows appear to transform more easily into dilute flow types in subaqueous environments because they are apt to hydroplane. This is in contrast to the flow transitions in subaerial environments where noncohesive debris flows are dominant and difficult to hydroplane.

• Journal of the Korean Geophysical Society
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• v.5 no.4
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• pp.329-336
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• 2002

Electrical resistivity dipole-dipole, seismic refraction, and seismic reflection methods were performed to delineate the boundaries the Yongdong basin(Cretaceous) in terms of physical properties and to ultimately identify the margin architectures of the faults or unconformities. Higer resistivities (approximately >2000 ohm-m) most likely originate from the basement of the basin, contrasting with the lower resistivities from infilled sedimentary rocks. Faults at the eastern margin and unconformities at the western boundary are characterized as high-slope($70^{\circ}$) and gentle-slope($30^{\circ}$) gradients in the resistivity sections, respectively Such features for the boundaries are also suggested by the lower values of seismic velocity and resistivity for the western margin.

• Economic and Environmental Geology
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• v.29 no.5
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• pp.639-660
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• 1996

The lowest formation of the Cretaceous Gyeongsang Supergroup, the Nakdong Formation, unconformably overlies the gneiss complex basement in Hadong, Gyeongsangnam-do and Gwangyang, Chullanam-do. The Nakdong Formation of the study area is 500-600 m thick and occurs as a belt shape. Based upon lithology, sedimentary structure, and bedding geometry the formation consists of three conglomerate facies (Gd, Gn, Gic), five sandstone facies (Sh-n, Sh-i, Sp, Sr, Sm), and four mudstone facies (Mf, Mfn, Mc, Mv). Sandstone facies are the most prominent in the study area. The twelve facies can be grouped into five facies associations. The depositional settings are elucidated from analyses of 12 facies and five facies associations of the formation. The lower part of the Nakdong Formation was deposited in alluvial plain, and the middle and upper parts were in a riverine system. The lithologies of the Nakdong Formation of the Gyeongsang Basin have been considered to consist of generally conglomerates and pebbly sandstones that were accumulated in alluvial fans. But the common lithology of the study area is sandstone which was formed in lower part of alluvial fan or fluvial setting. It is supposed that the coarser sedimentary sequence distributed west to the study area should be eroded out after deposition and early uplift, and the finer sandstone sequence in the east remains behind. The mineral composition of sandstones and the clast composition of conglomerates indicate that the Nakdong Formation was derived mainly from the metamorphic source rocks. Some reworked intraclasts were also supplied from the intrabasinal sedimentary layers. Paleocurrent data collected from cross-beddings, ripple marks, asymmetric sand dune suggest that most sediments were transported from north to south during the Nakdong Formation time.

• Economic and Environmental Geology
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• v.27 no.4
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• pp.325-333
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• 1994

Gold and/or silver mineralization in the southeast province, Korea, occurred in hydrothermal quartz vein that fills fracture zones in Cretaceous volcanic and sedimentary rocks of the Gyeongsang basin or granites and Precambrian gneiss. Most of the gold-silver-bearing veins in the province occur in Hapcheon, Suncheon and Haman-Gunbuk area where they are associated with Cretaceous Bulgugsa granites. On the basis of the Ag/Au ratio on amounts produced and ore grades, mode of occurrence, and associated mineral assemblages, hydrothermal Au-Ag deposits in southeast province, Korea, can be classified as follows: pyrite-type gold deposit (Group IIB, Samjeong and Sangchon deposits), antimony-type gold-silver deposit (Group IV, Gisan and Geochang deposits), and antimony-type silver deposit (Group V, Sanggo, Seweon, Seongju and Gahoe deposits). All of the gold-silver deposits in the province are generally characteristics of the gold-silver or silver-dominant type deposit which contains more silver-bearing minerals than those deposits in central Korea. The gold-silver mineralization in the deposits consist of two generation; the early characterized by gold precipitation and the late represented by silver-rich (as silver-bearing sulfosalts minerals) mineralization. All but one deposit (Samjeong deposit) having relatively lower Au content in electrum values between ${\approx}20$ and ${\approx}50$ atomic %. The mineralogical data on electrum-sphalerite and/or arsenopyrite geothermometry and fluid inclusion data indicate that the gold and silver mineralizations were occurred at temperatures of $190{\sim}280^{\circ}C$ and $150{\sim}180^{\circ}C$, respectively. These suggest that the gold-silver mineralization in the province occurred in the lower temperature and pressure conditions as epithermal-type hydrothermal vein deposit.

• Economic and Environmental Geology
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• v.21 no.3
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• pp.223-234
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• 1988

A large number of gold and/or silver-bearing quartz veins occur in or near Mesozoic granite batholith elongated in a NE-SW direction within the Chungcheong Province. Precambrian schists and gneisses, and Jurassic and Cretaceous granitic rocks serve as hosts for gold and/or silver deposits. On the basis of Ag/Au total production and ore grade ratio, 15 mines may be divided into three major groups: gold-dominant deposits, gold-silver deposits, and silver-dominant deposits. The chemical composition of electrum from skarn deposit (Geodo mine), alaskite-type deposit (Geumjeong mine) and 15 vein deposits was summarized. It was found that the Au content of electrum for vein deposits ranging from 5.2 to 86.5 is lower than that for skarn and alaskite deposits. Among 15 vein deposits, the composition of electrum associated with pyrrhotite is relatively high and has a narrow range of 40.8 to 86.5 atomic % Au, but the Au content of electrum with pyrite is in range of 5.2 to 82.8 atomic %, and is clearly lower than that with pyrrhotite. The grouping of ages for these mines indicates that gold and/or silver mineralizations occurred during two periods in the Mesozoic. Daebo igneous activities are restricted to gold mineralization in the range of 158 to 133 Ma, whereas Bulgugsa igneous activities are related to gold and/or silver mineralization ranging from 108 to 71 Ma. Generally speaking, Jurassic gold-dominant veins have many common characteristics; notably prominent association with pegmatites, simply massive vein morphology, high fineness in the ore concentrates, rarity of silver minerals, and a distinctively simple mineralogy, including sphalerite, galena, chalcopyrite, pyrrhotite and/or pyrite. Although individual deposits exhibit widely differing diversity, Cretaceous gold-silver and silver-dominant veins are characterized by features such as complex vein, low to medium fineness in the ore concentrates and abundance of silver minerals including Ag sulfosalts, Ag sulfides, Ag tellurides and native silver.