Economic and Environmental Geology (자원환경지질)

The Korean Society of Economic and Environmental Geology (대한자원환경지질학회)
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Occurrences of Hot Spring and Potential for Epithermal Type Mineralization in Main Ethiopian Rift Valley

주 에티오피아 열곡대 내 온천수의 산출특성 및 천열수형 광상의 부존 잠재성

  • Moon, Dong-Hyeok (Korea Institute of Geoscience and Mineral Resources) ;
  • Kim, Eui-Jun (Korea Institute of Geoscience and Mineral Resources) ;
  • Koh, Sang-Mo (Korea Institute of Geoscience and Mineral Resources)
  • 문동혁 (한국지질자원연구원 광물자원연구본부) ;
  • 김의준 (한국지질자원연구원 광물자원연구본부) ;
  • 고상모 (한국지질자원연구원 광물자원연구본부)
  • Received : 2013.02.08
  • Accepted : 2013.03.16
  • Published : 2013.06.28
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Abstract

The East African Rift System(EARS) is known to be hosted epithermal Au-Ag deposits, and the best-known example is Main Ethiopian Rift Valley(MER) related to Quaternary bimodal volcanism. Large horst-graben system during rifting provides open space for emplacement of bimodal magmas and flow channel of geothermal fluids. In recent, large hydrothermally altered zones(Shala, Langano, and Allalobeda) and hot spring related to deeply circulating geothermal water have been increasing their importance due to new discoveries in MER and Danakil depression. The hot springs in Shala and Allalobeda occur as boiling pool and geyser on the surface, whereas some areas didn't observe them due to decreasing ground water table. The host rocks are altered to quartz, kaolinite, illite, smectite, and chlorite due to interaction with rising geothermal water. The hot springs in MER are neutral to slightly alkaline pH(7.88~8.83) and mostly classified into $HCO_3{^-}$ type geothermal water. They are strongly depleted in Au, and Ag, but show a higher Se concentration of up to 26.7 ppm. In contrast, siliceous altered rocks around hot springs are strongly enriched in Pb(up to 33 ppm, Shala), Zn(up to 313 ppm, Shala), Cu(up to 53.1 ppm, Demaegona), and Mn(up to 0.18 wt%t, Shala). In conclusion, anomalous Se in hot spring water, Pb, Zn, Cu, and Mn in siliceous altered rocks, and new discoveries in MER have been increasing potential for epithermal gold mineralization.

동아프리카 열곡계는 천열수 금-은 광상을 배태하고 있는 것으로 알려져 있으며, 그 대표적인 예로서 에티오피아에 발달된 열곡대는 제 4기 복마그마 화산활동과 관련하여 광화작용이 수반된다. 주 에티오피아 열곡대의 형성과정 동안 발달된 대규모의 지루-지구 구조는 마그마의 관입과 열수유체의 순환을 위한 열린 공간을 제공하였다. 최근 들어, 에티오피아 열곡대 및 다나낄 함몰지 내 천열수형 광화작용이 보고되고 있어, 주 에티오피아 열곡대 내 대규모 열수변질대의 발달(샬라, 랑가노, 알라로베다)과 온천수에 대한 관심이 높아지고 있다. 온천수의 지표산출은 주로 비등천(샬라) 또는 간헐천(알라로베다)의 형태로 이루어지고 있으나, 일부 지역은 지하수면의 하강으로 폭발적인 비등과 분출이 관찰되지 않는다. 온천수의 지표 분출과 관련하여 주변 암석들은 주로 석영, 카올리나이트, 일라이트, 스멕타이트, 녹니석의 광물조합을 갖는 암석으로 변질되어 있다. 온천수는 중성 내지 약한 알카리(pH 7.88~8.83)의 특성을 보이고, 대체적으로 중탄산염 유체로 분류된다. 온천수 내 금과 은 및 기타 수반원소들의 함량은 상당히 낮으나, Se의 함량이 최대 26.7 ppm으로 나타났다. 반면에 온천수 주변 규질 퇴적물들은 Pb(최대 33 ppm, 샬라), Zn(최대 313 ppm, 샬라), Cu(53.1 ppm, 데마에고나), Mn(최대 0.18 wt%, 샬라) 등으로 다소 부화되어 있다. 결론적으로 온천수내 높은 Se 함량과 규질 암석내 Pb, Zn, Cu, Mn 이상 및 최근 천열수형 광화작용의 발견은 주 에티오피아 열곡대내 천열수형 광상의 부존 가능성에 관한 기대를 높인다.

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References

  1. Abbate, E., Passerini, P. and Zan, L. (1995) Strike-slip faults in a rift area: a transect in the Afar Triangle, East Africa. Tectonophysics, v.241, p.67-97. https://doi.org/10.1016/0040-1951(94)00136-W
  2. Aquater (1996) Tendaho geothermal project, final report. EIGS-Government of Italy, Ministry of Foreign Affair, San Lorenzo in Campo.
  3. Cooke, D.R. and McPhail, D.C. (2001) Epithermal Au-Ag-Te mineralization, Acupan, Baguio district, Philippines: Numerical simulations of mineral deposition. Economic Geology, v.98, p.109-131.
  4. Cooke, D.R. and Simmons, S.F. (2000) Characteristics and genesis of epithermal deposits. Economic Geology Review 13, p.221-244.
  5. Corti, G. (2009) Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa. Earth Science Reivews, v.96, p.1-53. https://doi.org/10.1016/j.earscirev.2009.06.005
  6. Drummond, S.E. and Ohmoto, H. (1985) Chemical evolution and mineral deposition in boiling hydrothermal system. Economic Geology, v.80, p.126-147. https://doi.org/10.2113/gsecongeo.80.1.126
  7. Gibson, I.L. (1967) Preliminary account of the volcanic geology of Fantale, Shoa, Haile. Selassie I University.
  8. Gibson, I.L. (1970) A pantelleritic welded ash-flow tuff form the Ethiopian Rift Valley. Contr. Mineral and petrology, v.28, p.89-111. https://doi.org/10.1007/BF00404992
  9. Gichile, S. (1992) Granulites in the Precambrian basement of southern Ethiopia: geochemistry, P-T conditions of metamorphism and tectonic setting. J. African Earth Sci., v.15, p.251-263. https://doi.org/10.1016/0899-5362(92)90072-K
  10. Hofmann, C., Courtillot, V., Feraud, G., Rochette, p., Yirgu, G., Ketefo, E. and Pik, R. (1997) Timing of the Ethiopian flood basalt event and implication for plume birth and global change. Nature, v.389, p.838-841. https://doi.org/10.1038/39853
  11. Kazmin, V. and Seifemichael, B. (1978) Geology and development of the Nazareth area, Northern Ethiopia Rift. Geol. Surv. of Ethiopia.
  12. Kovalenker, V.A. and Plotinskaya, O.Y. (2005) Te and Se mineralogy of Ozernovskoe and Prasolovskoe epithermal gold deposits, Kuril-Kamchatka volcanic belt. Geochemistry, mineralogy and petrology, v.43, p.118-123.
  13. Lindgren, W. (1993) Mineral deposits. 4th ed. McGraw-Hill, New York, 930p.
  14. Mamo, T. and Teclu, A. (2007) Compilation of the geoscientific study of the Dofan geothermal prospect, Ethiopia. Mineistry of Mines and Energy and Geological Survey of Ethiopia, 46p.
  15. Megersa, G. and Getaneh, E. (2006) Geological, surface hydrothermal alteration and geothermal mapping of Dubti-Semera area, Tendaho geothermal field. Geological Survey of Ethiopia, 62p.
  16. Mohr, P. A. (1962) Surface cauldron subsidence with associated faulting and fissure basalt eruptions at Garibaldi pass, shawa, Ethiopia. Bull. Volcan, v.24, p.421-428. https://doi.org/10.1007/BF02599358
  17. Mohr, P.A. (1963) The geology of Ethiopia. Haile Selassie I Univ., Addis Ababa, 268p.
  18. Moussa, N., Fouquet, Y., Gall, B.L., Caminiti, A.M., Rolet, J., Bohn, M., Etoubleau, J., Delacourt, C. and Jalludin, M. (2012) First evidence of epithermal gold occurrences in the SE Afar rift, Republic of Djibouti. Mineralium Deposita, v.47, p.563-576. https://doi.org/10.1007/s00126-011-0397-9
  19. Reed, M.H. and Spycher, N.F. (1985) Boiling, cooling and oxidation in epithermal systems: A numerical approach. Reviews in Economic Geology, v.2, p.249-272.
  20. Seward, T.M. (1973) Thio complexes of gold and the transport of gold in hydrothermal ore solutions. Geochimica et Cosmochimica Acta, v.37, p.379-399. https://doi.org/10.1016/0016-7037(73)90207-X
  21. Seward, T.M. (1989) The hydrothermal chemistry of gold and its implications for ore formation: Boiling and conductive cooling as examples. Economic Geology Monograph 6, p.398-404.
  22. Seward, T.M. and Barnes, H.L. (1997) Metal transport by hydrothermal ore fluids, in Barnes, H.L., et., Geochemistry of hydrothermal ore deposits, 3rd ed., New York, John Wiley and Sons, p.435-486.
  23. Sillitoe, R.H. (2002) Rifting, bimodal volcanism, and bonanza gold veins. SEG letter, n.48, p.24-26.
  24. Simmons, S.F. and Brown, K.L. (2000) Hydrothermal minerals and precious metals in the Broadlands Ohaaki geothermal system: Implications for understanding low-sulfidation epithermal environments. Economic Geology, v.95, p.971-999. https://doi.org/10.2113/gsecongeo.95.5.971
  25. Simmons, S.F. and Brown, K.L. (2006) Gold in magmatic hydrothermal solutions and the rapid formation of a giant ore deposit. Science, v.314, p.288-291. https://doi.org/10.1126/science.1132866
  26. Simmons, S.F. and Brown, K.L. (2008) Precious metals in modern hydrothermal solutions and implications for the formation of epithermal ore deposits. SEG Newsletter, n.72, p.1, 9-12.
  27. Simmons, S.F., White, N.C. and John, D.A. (2005) Geological characteristics of epithermal precious and base metal deposits. Economic Geology 100th Anniversary Volume, p.485-522.
  28. Stratex International (2011) Discovering the future. Annual report, 60p.
  29. Tadesse, S., Milesi, J.P. and Deschamps, Y. (2003) Geology and mineral potential of Ethiopia: a note on geology and mineral map of Ethiopia. Jour. of African Earth Sciences, v.36, p.273-313. https://doi.org/10.1016/S0899-5362(03)00048-4
  30. White, N.C. and Hedenquist, J.W. (1995) Epithermal gold deposits: styles, characteristics and exploration. SEG Newsletter, n.23, p.1, 9-13.
  31. Woldegabriel, G., Aronson, J.L. and Walter, R.C. (1990) Geology, geochemistry, and rift basin development in the central sector of the Main Ethiopian Rift. Geol. Soc. Am. Bull., v.102, p.439-458. https://doi.org/10.1130/0016-7606(1990)102<0439:GGARBD>2.3.CO;2
  32. Zanettin, B. (1993) On the evolution of the Ethiopian volcanic province. In: Geology and mineral resources of Somalia and surrounding regions. Ist. Agron. Oltremare, Firenze, Relaz. e Monogr., v.113, p.279-310.