Seon Gyu Choi

First Finding of Eclogite Facies Metamorphic Event in South Korea and Its Correlation with the Dabie‐Sulu Collision Belt in China

Metabasite lenses occur within granitic gneiss at Bibong in the southwestern part of the Gyeonggi massif, South Korea. Bibong metabasites experienced an initial eclogite facies metamorphism (17.0–20.9 kbar, 835°–860°C), which was succeeded by a granulite facies metamorphism (11.5–14.6 kbar, 830°–850°C) and finally overprinted by an amphibolite facies metamorphism (6.7–11.0 kbar, 570°–740°C). The metabasites have Sm‐Nd internal isochron ages at 225–258 Ma, and the major elements of the metabasites display an island arc affinity. The petrochemical and geochronological data suggest the southwestern part of the Gyeonggi massif as an extension of the Triassic Dabie‐Sulu collision belt in China.

Metamorphic Evolution of the Baekdong Metabasite in the Hongseong Area, South Korea and its Relationship with the Sulu Collision Belt of China

Abstract Ultramafic rocks in the Hongseong area, southwest Gyeonggi massif, South Korea, occur as lenses within Precambrian granitic gneiss. The ultramafic bodies contain metabasite boudins that have undergone at least three stages of metamorphism. The first stage mineral assemblage is garnet + augite (X Jd+Aeg = 0.06–0.11) + hornblende + plagioclase + rutile + titanite, and represents a transitional condition between the eclogite (EG), high-pressure granulite (HG) and amphibolite facies (AM). The second stage is characterized by an assemblage of garnet + augite (X Jd+Aeg = 0.01–0.06) + hornblende + plagioclase + titanite, representing the transitional condition between low-pressure granulite facies (LG) and AM. A third stage is identified by an AM assemblage of hornblende + plagioclase ± garnet in symplectites formed during garnet breakdown. The P-T conditions of the first, second, and third stages are 11.8–16.3 kb 690–780°C, 8.2–8.7 kb 660–820°C, and 4.0–6.3 kb 490–610°C, respectively. Cores and rims of two garnets in the country rock granitic gneiss give P-T estimates of 8.9–13.5 kb 640–815°C and 5.5–6.3 kb 575–680°C, indicating a retrograde P-T path similar to that of the metabasite boudins. Ti-Zr-Y data indicate that the Baekdong metabasites probably had an island arc paleotectonic setting. While it may be concluded that the Hongseong area in the Gyeonggi massif is a possible extension of the Sulu collision belt in China, Sm-Nd whole-rock - garnet isochron ages of the Baekdong metabasites (268–297 Ma) are significantly older than the age of UHP metamorphism (208–245 Ma) in the Dabie-Sulu collision belt. Hence, collision between the North and South China blocks may have occurred earlier in the Korean peninsula than in China.

Ridge Subduction-related Jurassic Plutonism in and around the Okcheon Metamorphic Belt, South Korea, and Implications for Northeast Asian Tectonics

The Okcheon metamorphic belt (OMB) in central South Korea is surrounded by Middle Jurassic granitoid batholiths that intruded South Korea extensively; the granitic bodies form a complex about 200 km long and 150 km wide as part of a Mesozoic granite belt along the East Asian continental margin. Middle Jurassic magmatism was related to ridge subduction that occurred around 200 to 166 Ma, with the main magmatic period between 175 and 166 Ma; main cooling ages range from 168 to 152 Ma. The magmatism was divided into two stages: (1) a deeper, earlier stage, which resulted in emplacement of diorite, granodiorite and granite as shown in the northeast OMB; and (2) a shallower, younger stage, which resulted in emplacement of granite and two-mica granite as shown in the southwest OMB. Most granitoids are peraluminous to metaluminous I-type granitoids that originated in a volcanic arc; an exception is an S-type two-mica granite. Inherited cores of 998 and 262 Ma U-Pb SHRIMP II zircon ages from the two-mica granite indicate that two-mica granite is reworked crustal material formed by earlier magmatism before the Middle Jurassic event. Together with previous studies on the Middle Jurassic granitoids, the present result indicates that subduction of the Farallon-Izanagi ridge beneath Asia caused widespread igneous activity throughout South Korea, especially during Middle Jurassic ridge subduction.

SHRIMP zircon age of a Proterozoic rapakivi granite batholith in the Gyeonggi massif (South Korea) and its geological implications

A large rapakivi granite batholith in the Neo-Archaean/Palaeoproterozoic Odesan complex, northeastern Gyeonggi massif, South Korea, has been dated at 1839 ± 10 Ma using SHRIMP U–Pb analysis of zircons. The age, petrological and geochemical characteristics of this batholith are similar to those of the rapakivi granite batholiths exposed in the Rangnim massif of North Korea and in the Miyun–Chengde complex of North China. The country rocks of these rapakivi granite batholiths are also comparable; all are composed of granitic gneisses and banded iron formation (BIF)-bearing supracrustal rocks metamorphosed to amphibolite- to granulite-facies. This study provides new evidence for the suggestion that the Gyeonggi and Rangnim massifs may share an affinity with the Precambrian basement of the North China craton. The study provides new insight into the possible eastward extension of the Sulu orogenic belt in the Korean peninsula and further provides evidence to correlate the Korea basement to a possible global 2.1–1.8 Ga supercontinent.

Genetic implications of two different ultramafic rocks from Hongseong area in the southwestern Gyeonggi Massif, South Korea

Abstract Two distinct ultramafic bodies occur in Baekdong and Bibong in the Hongseong area within Gyeonggi massif of South Korea. The Hongseong area is now extensively documented as an extension of the Dabie-Sulu collision belt in China. The Baekdong ultramafic body has a NWW elongation direction. This elongation trend is similar to the general trend of the Dabie-Sulu collision belt. The Bibong ultramafic body is elongated in a NNE direction and runs parallel to the direction of the main fault in the study area. The Baekdong ultramafic bodies show porphyroclastic and mylonitic textures while those at Bibong exhibit a mosaic texture. Both were grouped into peridotite and serpentinite based on their modal abundance of serpentine. In the olivine (Fo) vs. spinel [Cr# = Cr/ (Cr+Al)] diagram, both ultramafic rocks fall with in olivine spinel mantle array. The compositions of olivine, orthopyroxene and spinel indicate that the Baekdong ultramafic rock formed in deeper parts of the upper-mantle under passive margin tectonic setting. The SREE content of Baekdong ultramafic rock vary from 0.19 to 5.7, exhibits a flat REE pattern in the chondrite-normalized diagram, and underwent 5% partial melting. Conversely, large variation in SREE (0.5 21.53) was observed for Bibong ultramafic rocks with an enrichment of LREE with a negative slope and underwent 17 24% partial melting. The Baekdong ultramafic rocks experienced three stages of metamorphism after a high pressure residual mantle stage. The first stage of metamorphism occurred under the eclogite-granulite transitional facies (1123 911°C, >16.3 kb) the second under the granulite facies (825 740°C, 16.3 11.8 kb) and the third is the retrogressive metamorphism under amphibolite facies (782 718°C, 8.2 8.7 kb) metamorphism. The Baekdong ultramafic rocks had undergone high-P/T metamorphism during subduction of the South China Block, and experienced a fast isothermal uplift, and finally cooled down isobarically. Evidences for metamorphism were not identified in Bibong ultramafic rocks. Hence, the Baekdong ultramafic rocks with in the Hongseong area may indicate a link on the Korean counterpart of Dabie-Sulu collision belt between North and South China Blocks.

Fuzzy logic fusion of W-Mo exploration data from Seobyeog-ri, Korea

There are continuing changes in the mineral exploration methodology, specially in the processing, analysis, and interpretation of geological exploration data. Many new data integration techniques based on GISs allow multiple sets of exploration survey data to be processed and fused quickly and precisely, so that the assessment on the final target(s) deposit can be more accurate than conventional approaches.Raw exploration data gathered in field, or old archived exploration data in local government mining offices, are usually in differing data formats, which usually require editing and preprocessing of the data prior to an information representation step utilizing a chosen mathematical tool. In this study, we collected several sets of old geological exploration data, digitized, processed and geocoded each layer of spatial data, and then digitally represented using fuzzy logic for later fusion. In the Seobyeog-ri (Korea) study area, there was also and old tungsten mine, which we used as a ground-truthing control point. The target proposition adopted is that “there is a tungsten (W) and molibdenum (Mo) mineralization” in the study area and the mineral deposit model used was a simple skarn type contact mineralization.Although several authors have recently used fuzzy logie approaches for fusion of mineral exploration data, selecting an optimum fuzzy operator has always been a difficult task. It has become apparent that fuzzy operators depend very much on the deposit model and types of spatial data to be integrated. In this study, three types of operators, fuzzy OR, fuzzy AND, and fuzzy γ-operators are tested and selected ones are used in combination, some in series and others in parallel. The unbiased processing results obtained in this study indicate that the most probable target area is clearly outlined in the vicinity of the old Sangdong tungsten (W) mine, and have confirmed our initial proposition. However, the initial assumption of a skarn type mineralization could not be further verified requiring further investigation.

Application of arsenopyrite geothermometry and sphalerite geobarometry to the Taebaek Pb-Zn(-Ag) deposit at Yeonhwa I mine, Republic of Korea

The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, Republic of Korea, occurs in a broadly folded and reverse-faulted terrain of Paleozoic sedimentary rocks: the Taebaeksan basin. The orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks. The Pb-Zn(-Ag) mineralization can be divided into four distinct stages based upon the mode of occurrence of ore minerals, ore textural relationships and their composition. Based on temperatures inferred from arsenopyrite compositions by means of electron microprobe and fluid inclusions, the estimated temperatures for the stages I, II, III and IV reach 330 to 350 °C, 270 to 340 °C, 230 to 250 °C, and <220 °C, respectively. The sulphur activity (atm) of ore formation at the Taebaek deposit was estimated for each stage as 10−11 to 10−11.5, 10−9.5 to 10−13, 10−13.5 to 10−15, and <10−15, respectively. Even though application of sphalerite geobarometry is problematic because of the absence of good mineral assemblages, sphalerite coexisting with pyrite but not with pyrrhotite was used to estimate the minimum mineralization pressure (about 1 kbar).

Geochemistry of the Granitoids hosting the Seolhwa Au mine, Asan district, Chungcheongnamdo province, Korea : Genetic implication on the mesothermal gold mineralization

ABSTRACT Gold-silver vein deposits in South Korea are associated intimately with major periods of granitic plutonism. Among these, mesothermal-type gold-rich deposits appear to be associated genetically with Jurassic Daebo Series (210 to 150 Ma) granitoids. The Seolhwa mesothermal gold mineralization were deposited in quartz veins which filled the fault shear zones in the granitoid within the Gyeonggi Massif. These granitoids(161±4 Ma) have the following geochemical characteristics: 1), the relatively high initial 87Sr/86Sr ratio of 0.7224 of S-type nature; 2), the trace-element characteristics of continent- continent collision tectonic environment.; 3), the high A/CNK value of 1.4 to 1.6 of peraluminous nature; 4) the nondectectable gold of granitoid. The mineralogy is simple and consists mainly of rare sulfides and gold. Gold deposition occurred at temperatures and salinities of 250°C to 430°C and 0.4 to 4.9 wt% NaCl. and most fluid inclusions contain XCO2 of 0.16 to 0.62. Recently, it has been conjectured that the Chinese collision belt crosses Korea as either the Imjingang belt or a suture complex, including not only the Imjingang belt but also the Gyeonggi massif and the Ogcheon belt. And Mesozoic (ca. 160–150 Ma) granitoids in East Shandong within this Chinese collision belt contain some 30 Moz gold, making East Shandong the largest granitoid-hosted gold province in the world. Geochemically, the Mesozoic granitoids in East Shandong have the following characteristics: 1), the relatively high initial 87Sr/86Sr ratio of 0.7095 to 7273 of S-type nature; 2), the trace-element characteristics of continent-continent collision tectonic environment.; 3), the relatively high A/CNK value of 0.92 to 1.06 of peraluminous nature. Most of the gold occurs as quartz-veins in the granitoids. The major gold mineralization generally display the characteristics of gold-only provinces, namely CO2-rich, low salinity and reduced fluids. Gold mainly occurs as gold and electrum in the quartz veins. The main minerals closely associated with gold mineralization are pyrite and pyrrhotite, and locally chalcopyrite, galena and sphalerite. Fluid inclusion studies indicate that the fluid temperature range from 170°C to 350°C, salinity varies from 5 to 13 wt% NaCl equivalent (mostly 10%), and that most fluid inclusions contain 10 to 25% CO2. The gold mineralization may be more tectonically and structurally controlled rather than have a direct genetic relationship with the granitoid itself in terms of nondetectable gold. The major gold in East Shandong may have been formed during the subsequent exhumation stage after the collision of the North and South China cratons. Considering to the above-mentioned similarities between the Seolhwa mine district and the East Shandong mine district, it may be conjectured that the major gold in the Seolhwa mine district may have been formed during the subsequent exhumation stage after the collision of the Sino-Korean and Yangtze cratons.