한국지구시스템공학회지 Vol. 43, No. 2 (2006) pp. 106-117 연구논문 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea Chul-Ho Heo1), Jae-Ho Lee2)*, Seong-Taek Yun3) and Chil-Sup So3) 보국 마그마성 열수 코발트 광화작용의 유체포유물연구 허철호1)․이재호2)*․윤성택3)․소칠섭3) 요 약 : 보국 코발트 광산의 열수성 석영 ± 탄산염 ± 녹섬석 맥들은 백악기 후기의 화강암내 열극을 충진했다. 맥의 광물조성은 코발트, 몰리브덴, 구리, 납, 아연, 비스무스 및 금의 광석광물을 함유하고 있는 다금속 성향을 보이며, 광화작용은 5개의 광화시기로 구분된다. 맥상광물은 광화시기에 따라 체계적으로 변화하며 다음과 같은 광물공생군을 보인다: 녹섬석과 석영을 수반한 함코발트 비화물, 유비화물 및 휘수연석 → 천금속 황화물, 금, 철산화물 → 탄산염. 광석광물공생에 대해 평형 열역학을 적용하면 다음과 같다: 광화 1, 2기 코발트 광화작용은 T = 560-360℃, log fs2 = -6.2~-12.0 atm의 광화유체에서 일어났으며, 광화 3기의 천금속 황화물 및 금은 T = 380-275℃, log fs2 = -7.5~-10.6 atm 유체에서 침전되었다. 코발트 광화작용에서 천금속 황화물 침전으로 광화작용 이 진행되면서 온도감소와 산소분압의 증가가 수반되었을 것으로 사료된다. 코발트의 침전은 마그마성 염수의 냉각 및 환원에 의해 야기되었을 것으로 사료된다. 이 냉각 및 희석은 초기 마그마계가 쇠퇴하면서 다량의 천수성 지하수의 혼입에 의해서 발생했으며, 계속해서 천금속 황화물, 금, 비스무스가 침전되었다. 광물학 및 유체포유물 연구에 의하면, 코발트, 비소, 몰리브덴은 마그마 정출작용중 직접 용리된 고온(<~585℃), 고염농도(<67 wt. NaCl) 의 마그마 염수로부터 용리되어 분별된 것으로 사료된다. 마그마 염수가 냉각되면서, 이 금속들은 석영 ± 녹섬석 맥내 비화물과 유비화물로 침전되었다. 약 350℃의 온도에서 마그마성 열수계가 쇠퇴하면서, 천수성 지하수의 거대순환이 마그마성 열수계를 붕괴시키고 점진적으로 열수유체의 냉각, 희석, 산화가 촉진된다. 첨금속, 금, 칼슘 은 천수순환중 주변의 퇴적암에서 용탈되며 광화 3기에서 5기의 광화작용과 관련된 유체를 형성하게 된다. 주요어 : 물질흐름분석, 자원관리, 지속가능한, 자원이용지수 Abstract : Hydrothermal quartz carbonates actinolite veins of the Boguk cobalt mine filled the fractures in a granite stock of Late Cretaceous age. They show the polymetallic nature consisting of Co-, Mo-, Cu-, Pb-, Zn-, Bi-, and Au-bearing ore minerals, and is divided into five stages. The vein mineralogy changes systematically with time: cobalt-bearing, arsenides and sulfarsenides and molybdenite with actinolite and quartz → base-metal sulfides, gold,and Fe oxides → barren carbonates. Equilibrium thermodynamic considerations of ore mineral assemblages are as follows: cobalt mineralization in stages I and II, T = 560-360℃, log fs2 = -6.2 to -12.0 atm deposition of base-metal sulfides and gold in stage III, T = 380-275℃, log fs2 = -7.5 to -10.6 atm. With the transition from cobalt mineralization toward base-metal sulfide deposition occurred the temperature decrease and concomitant increase in fo2. The deposition of cobalt probably occurred as a result of cooling and reduction of the magmatic brines. This cooling and dilution occurred by mixing with progressively larger volumes of meteoric groundwater as an early magmatic system waned, and resulted in successive deposition of base-metal sulfides, gold and bismuth, Fe oxides, and carbonates. By combining the mineralogic, fluid inclusion and petrochemical data, the following model is proposed for ore genesis at Boguk: during the Late Cretaceous, a micrographic granite stock intruded 2005년 5월 27일 접수, 2006년 3월 20일 채택 1) 국립공원관리공단 국립공원연구원 2) 한국지질자원연구원 지질기반정보연구부 3) 고려대학교 지구환경과학과 *Corresponding Author(이재호) E-mail; [email protected] Address; Geology & Geoinformation Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea 106 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea 107 volcanosedimentary rocks at near surface. Cobalt, arsenic, and molybdenum were partitioned into high-temperature (up to ~585℃), high-salinity (up to 67 wt. % NaCl) magmatic brines exsolved directly from the crystallizing magma. As the magmatic brine cooled, these metals precipitated as arsenides and sulfarsenides in quartz actinolite veins. Following the waning of the magmatic hydrothermal system at temperatures around 350℃, a huge circulation of meteoric groundwater formed to collapse the system, resulting in progressively larger degrees of cooling, dilution and oxidation of hydrothermal fluids. Base metals, gold, and possibly calcium were leached from surrounding sedimentary rocks during the meteoric water circulation, and formed the fluids related tostage III to V mineralization. Key words : Cobalt deposit, Mineralogy, Fluid inclusion Introduction Konchonri Formation which consists mainly of shale with minor intercalations of sandstone and limestone. There are few cobalt-bearing deposits in Korea, and The bedding strikes 275° to 320° and dips 5° to can be grouped into two genetic types(Nakamura, 15°SW. The Yucheon group rocks extrude or intrude 1942): (1) deposits associated with hydrothermal Cu, the Konchonri Formation and consist mainly of Zn, Au and Ag mineralization in a genetic tie with andesite and andesite porphyry(Yun and Youm, 1997). felsic igneous rocks; (2) deposits associated with A granite stock with an outcrop size of about 2×4 nickel in basic igneous rocks, where cobalt-bearing km intrudes the Hayang and Yucheon group rocks, and minerals occur only as a by-product. The Boguk cobalt hosts the hydrothermal veins of the Boguk cobalt mine deposits in this study share many features with the (Fig. 2). The granite is composed mineralogically of granite- related hydrothermal deposits. Until the quartz, plagioclase, orthoclase and biotite with minor mining activity was stopped at 1970, the Boguk cobalt amounts of hornblende, apatite, zircon, chlorite and mine has produced an average 0.5 to 1.0 wt. % Co per hematite. Along the intrusive contacts with sedime- metric ton of ores, with trace amounts of gold. ntary rocks occur the intrusion-related prophyllitic Only a few studies of cobalt mineralization in South alteration assemblages. The calc-alkaline granite stock Korea were carried out. Park(1990) and Yun and is occasionally uneven in grain size, ranging from fine- Youm(1997) have described a xenothermal feature of to medium-grained. Inward from the margin, the grain the Boguk cobalt deposits, based on ore mineralogy. size tends to be increased. The granite also shows However, the source and physicochemical conditions miarolitic cavities and micrographic texture, suggesting of the cobalt ore mineralization have not been their epicrustal emplacement and the presence of understood. The purposes of this study are to describe abundant volatile components in magma. A Rb-Sr age the complex ore mineralogy, to elucidate the fluid dating of the granite suggested a Late Cretaceous age evolution and to propose genetic model for the (around 86 Ma) of the intrusion and associated ore Co-bearing hydrothermal system. mineralization (Yun and Youm, 1997). Geologic Setting Ore Veins and Mineralogy The Boguk cobalt mine, located at latitude of 35° The hydrothermal mineralization of the Boguk mine 47’N and longitude of 128°45’E, is situated within the consists of narrow(each 0.1-0.5 m thick), fracture- middle western part of the Gyeongsang Basinin which filling quartz, carbonate and actinolite veins. These occur the non-marine, sedimentary and volcanic- veins occur within a calc-alkaline granite stock. The plutonic rocks of Cretaceous age (Fig. 1). The geology ore mineralogy is relatively complex and consists of of the mine area is composed mainly of volcano- cobalt-bearing arsenides or sulfarsenides(in the decre- sedimentary rocks of the Hayang and Yucheon groups asing order of amounts, loellingite, cobaltite and glau- that are intruded by a small granite stock (Fig. 2). The codot), arsenopyrite, molybdenite, base-metal sulfides Hayang group rocks inthe mine area belong to the (chalcopyrite, sphalerite, pyrite, pyrrhotite, etc.), and 제43권 제2호 108 Chul-Ho Heo․Jae-Ho Lee․Seong-Taek Yun․Chil-Sup So Fig. 1. Simplified geologic map of the Republic of Korea, showing the location of the Boguk cobalt mine within the Cretaceous Gyeongsang Sedimentary Basin. Fig. 3. Generalized paragentic sequence of minerals in veins of the Boguk cobalt mine. Temperature scale (*) is based on fluid inclusion temperatures and on thermo- dynamic considerations of ore mineral assemblages. See the “Fluid Inclusions” section for fluid inclusion types. During stages I and II, cobalt was deposited as loellingite, cobaltite and glaucodot. These cobalt-bearing minerals are associated intimately with arsenopyrite, molybdenite and pyrrhotite. Stage I veins are charac- Fig. 2. Geologic map of the Boguk cobalt mine area terized by the occurrence of green-colored amphibole (modified after Yun and Youm, 1997). Hydrothermal veins are developed restrictedly within the granite stock. (actinolite) in association with minor amounts of quartz. Actinolite occurs as massive aggregates which contain cobalt-bearing ore minerals, and is commonly rare amounts of oxides(magnetite and hematite) and replaced by stage IV brown carbonates(siderite and electrum. Gangue minerals are quartz, carbonates and dolomite). Ore minerals consist mainly of Co-rich actinolite. loellingite(2.3-11.5 wt. % Co, average 6.9 %) and Based on investigation of the mineral assemblages arsenopyrite(up to 8.9 wt. % Co) with rare amounts of and textural relationships(e.g., cutting, banding) of cobaltite, glaucodot, molybdenite and pyrrhotite. Stage veins, the vein miner alization at Boguk is divided into II mineralization is characterized by less amounts of five mineralization stages(I to V; Fig. 3). Co-bearing loellingite in clear quartz veins without 한국지구시스템공학회지 Fluid Inclusion Study of a Magmatic Cobalt Mineralization at the Boguk Mine, Korea 109 actinolite. Toward the stage II mineralization, cobalt- alization are carried out to trace the changes in bearing minerals abruptly decrease in amounts, whereas physicochemical conditions of hydrothermal fluids at arsenopyrite and other sulfides increase. Stage II ore Boguk. Representative ore mineral assemblages accor- mineralogy consists dominantly of Co-rich arsenopyrite ding to mineralization stages in veins are summarized (up to 8.8 wt. % Co) and loellingite(4.7-10.2 wt. % Co, in Table 1. average 7.1 %) with rare amounts of molybdenite, Within stage I and II veins, arsenopyrite is coexisting pyrrhotite and pyrite. with loellingite and pyrrhotite. Chemical compositions Stage IIImineralization is represented by deposition and elemental substitutional relationships of arsenopyr- of relatively abundant base-metal sulfides within white ites have been studied by Yun and Youm(1997).