Economic and Environmental Geology (자원환경지질)

The Korean Society of Economic and Environmental Geology (대한자원환경지질학회)
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Verification of Genetic Process for the High-purity Limestone in Daegi Formation by Oxygen-carbon Stable Isotope Characteristics

산소-탄소 안정동위원소특성을 이용한 대기층 고품위 석회석의 생성기작 해석

  • Kim, Chang Seong (Department of Earth and Environmental Sceinces, Korea University) ;
  • Choi, Seon-Gyu (Department of Earth and Environmental Sceinces, Korea University) ;
  • Kim, Gyu-Bo (Department of Earth and Environmental Sceinces, Korea University) ;
  • Kang, Jeonggeuk (Department of Earth and Environmental Sceinces, Korea University) ;
  • Kim, Sang-Tae (School of Geography and Earth Sciences, McMaster University) ;
  • Lee, Jonghyun (Department of Earth and Environmental Sceinces, Korea University) ;
  • Jang, Jaeho (School of Earth and Environmental Sciences, Seoul National University)
  • 김창성 (고려대학교 지구환경과학과) ;
  • 최선규 (고려대학교 지구환경과학과) ;
  • 김규보 (고려대학교 지구환경과학과) ;
  • 강정극 (고려대학교 지구환경과학과) ;
  • 김상태 (맥매스터대학교 지리지구과학부) ;
  • 이종현 (고려대학교 지구환경과학과) ;
  • 장재호 (서울대학교 지구환경과학부)
  • Received : 2018.10.01
  • Accepted : 2019.01.21
  • Published : 2019.02.28
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Abstract

Two assertions about the process the formation of the high-purity limestone in the Taebaeksan Basin, categorized into syngenetic and epigenetic origin, are verified on the basis of its oxygen-carbon stable isotopic characteristics. The carbonate rocks sampled from the selective six high-purity limestone mines and several outcrops in the Daegi formation are featured by various colors such as the gray, light gray and dark gray. They show a wide range of oxygen stable isotope ratios (4.5 ~ 21.6 ‰), but a narrow range of carbon stable isotope ratios (-1.1 ~ 0.8 ‰, except for vein calcite), which means that they had not experienced strong hydrothermal alteration. In addition, there is no difference in the range of the oxygen stable isotope ratios by mine and color, and it is similar to the range from surrounding outcrop samples. These results indicate that the effect of the hydrothermal alteration were negligible in the generation of high-purity limestone in deposit scale. Whereas, the carbonate rocks can be divided texturally into two groups on the basis of an oxygen isotope ratio; the massive-textured or well-layered samples (>15 ‰), and the layer-disturbed (or layer-destructed) and showing over two colors in one sample (<15 ‰). In the multi-colored samples, the bright parts are characterized by the very low oxygen stable isotope ratios, compared to the dark parts, implying the increase in brightness of the carbonate rocks could be induced by the interaction between hydrothermal fluid and rock. However, these can be applied in a small scale such as one sample and are not suitable for interpretation of the generation of high-purity limestone as a deposit scale. In particular, the high oxygen isotope ratios from the recrystallized white limestone suggest that hydrothermal fluids are also rarely involved during recrystallization process. In addition, the occurrences of the high-purity limestone orebody strongly support the high-purity limestone in the area are syngenetic rather than epigenetic; the high-purity limestone layers in the area show continuous and almost horizontal shapes, and is intercalated between dolomite layers. Consequently, the overall reinterpretation based on the sequential stratigraphy over the Taebaeksan basin would play an important role to find additional reserves of the high-purity limestone.

태백산분지 북익부 고품위 석회석의 생성은 크게 동생적(syngenetic)과 후생적(epigenetic) 관점으로 나뉘어 있으며, 산소-탄소 안정동위원소 비 특성을 이용하여 이들의 생성환경을 고찰하였다. 북익부의 4개 광산(GMI, 정선, 백운, 백광)과 남익부의 상동(오미아)광산, 그리고, 삼척지역의 쌍용동해광산을 대상으로 하였으며, 이들과의 비교를 위해 정선지역 대기층 노두시료를 포함하였으며, 이들을 색상에 따라 암회색, 회색, 담회색, 백색으로 구분하였다. 산소 안정동위원소 비는 4.5 ~ 21.6 ‰의 넓은 범위의 변화를 보이지만, 탄소 안정동위원소 비는 -1.1 ~ 0.8 ‰ (방해석 맥 제외)로 매우 좁은 범위를 보여, 열수의 작용이 크지 않았음을 지시한다. 또한, 광산별, 색상별 산소-탄소 안정동위원소 비의 분포범위의 차이는 없으며, 주변 노두에서 채취된 탄산염암과 유사한 범위를 보이는 것으로 보인다. 이와 같은 결과는 고품위 석회석의 생성에 있어서 열수의 영향은 거의 없었다는 것을 지시한다. 산소 안정동위원소 비 값은 암석의 색상변화(백색화)보다는 조직의 변화와 관련되는데, 산소 안정동위원소 비 15 ‰ 전후를 기준으로 탄산염암의 산상을 구분할 수 있다. 이보다 높은 경우 괴상 또는 층리가 잘 보존된 특징을 보이는 반면, 낮은 경우 하나의 시료 내에서 두 가지 이상의 색상이 나타나고 있으며, 층리는 교란되거나 거의 소멸하여 흔적만 확인된다. 두 가지 이상의 색상이 나타나는 경우 밝은 부분이 어두운 부분에 비해 현저히 낮은 산소 안정동위원소 비를 보이는 경우가 다수 관찰되며, 이는 열수와의 반응에 의한 현상으로 해석된다. 다만, 이와 같은 현상은 소규모로 나타나고 있으며, 광상 전체적인 고품위 석회석의 생성에 대한 해석으로는 적당하지 않다. 특히, 재결정화한 백색의 탄산염암에서 높은 산소 안정동위원소 비 값을 보이는 것은 이 지역 탄산염암의 재결정화에 열수의 작용이 매우 적었음을 지시하며, 주변 마그마에 의한 열적 영향만을 받은 결과로 해석된다. 또한, 고품위 석회석광상 내 광체의 분포가 수평으로 연속적이며, 고품위 석회석 상하 주변에 백운석이 층상으로 협재되는 등의 산상을 고려할 때, 연구지역 고품위 석회석의 생성은 동생적(syngenetic) 해석이 보다 더 타당할 것으로 판단된다. 따라서, 태백산분지 전반적으로 순차층서적 관점에 의한 퇴적상의 해석이 필요할 것으로 사료되며, 이는 추가적인 고품위 석회석 광체의 확보에 매우 중요한 요소가 될 것이다.

Keywords

JOHGB2_2019_v52n1_107_f0001.png 이미지

Fig. 1. Geologic map of the Taebaeksan Basin with shown representative high-purity limestone and metal deposits (modified from Koh et al., 2001).

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Fig. 2. Slab photographs of the representative carbonate rocks which are indicated the color group and the O-C stable isotopic values. a) dark grey, b) grey, c) light grey, d) white, and e) recrystallized.

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Fig. 3. Slab photographs of the representative carbonate rocks showing the different colors in each sample which are indicated the analysed spots (black or white dots) and O-C stable isotopic values.

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Fig. 4. Distribution of oxygen-carbon stable isotope ratios of the carbonates in the Daegi Formation from some highpurity limestone deposits (a) and by color (b). It is found that the large variation of the oxygen isotopic values but that the small of the carbon except one sample, calcite vein. There is no difference in the oxygen isotpoic ratios by color. Range of sedimentary and magmatic carbonates are from James and Jones (2016) and Tayler et al. (1967). Magmatic water box: δ13CPDB = -8.0 ~ -5.0 ‰; δ18OSMOW = 5.5 ~ 10.0 ‰.

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Fig. 5. XRD results of the white (a) and dark grey (b)carbonate rocks from the Daegi Formation. cal = calcite,dol = dolomite.

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Fig. 6. Distribution of oxygen-carbon stable isotope ratios of the carbonates in the Daegi Formation from some metal deposits.

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Fig. 7. Distribution of oxygen-carbon stable isotope ratios of the carbonates in the Daegi Formation from several outcrops.

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Fig. 8. Microphotographs of the carbonate rocks from the high-purity limestone deposits by color. a) white, b) light grey, c)grey, and d) dark grey. The carbonate rocks of white and light grey color consist only carbonate mineral (a, b) but, the grey and dark grey samples contain some pelitic material (c, d).

Table 2. Ranges of oxygen-carbon stable isotope ratio by color at some high-purity limestone deposits (included the data from the GMI mine)

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Table 3. Values of oxygen-carbon stable isotope ratio of the outcrops in the Daegi Formation

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Table 4. Characteristics of the carbonate rocks grouped by oxygen stable isotopic values

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Table 1. Values of oxygen-carbon stable isotope ratio of the Daegi Formation at some high-purity limestone deposits

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