Gubong, Cheongyang Gold District

South Korea

Main commodities: Au Ag
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The Gubong mesothermal quartz vein gold deposit occurs within the Cheongyang Gold District, in the central section of the western coastal zone of the Republic of Korea, approximately 100 km south of Seoul.

The deposit consists of five massive, gold-bearing mesothermal quartz veins that fill NE and NW oriented fractures along shear zones distributed over an area of 14 sq. km in Precambrian meta-sedimentary rocks of the Gyeonggi Massif.

In the Gubong mine area the meta-sediments of the Gyeonggi Massif are mainly banded gneisses or granitic para-gneisses and partly migmatitic gneisses. Granitic para-gneisses and migmatitic gneiss bodies with gradational contacts are found within the banded gneisses, which are the most abundant rock in the area and are commonly intercalated with hornblende schist, serpentinite, and marble. These bodies strike at 20 to 80° and vary in dip from 30 to 70°SE to 30 to 70°NW. The banded gneiss is composed of quartz, biotite, muscovite, plagioclase, orthoclase, microcline, hornblende, sericite and epidote. The granitic para-gneisses within the banded gneiss commonly has poorly defined contacts due to migmatisation and is composed mainly of quartz, plagioclase and biotite. The hornblende schist is made up principally hornblende with minor quartz, orthoclase, plagioclase, biotite, muscovite, and sericite.

The Jurassic Daedong Supergroup, which unconformably overlies the Precambrian meta-sedimentary rocks, consists of sedimentary rocks and is distributed along a north-northeast trend east of the mine. It includes  i). breccia bearing sandstone and conglomerate zones, with intercalated black shale and sandstone, that are in fault contact with the Precambrian banded gneiss; and is composed of sandstone, mudstone and  ii). sandstone, siltstone and shale intercalated with conglomerate and coal. Jurassic granites intrude the Precambrian gneiss and consist of a small NE-SW trending stock of biotite granite composed of quartz, orthoclase, plagioclase, microcline, biotite, sericite and calcite.

The main structural feature of the Cheongyang gold district is a series of major NNE­trending folds and faults which crosscut Precambrian meta-sedimentary rocks, Jurassic granitoids and partly offset the Jurassic Daedong Supergroup. The major faults are thrust structures with 30 to 80 km of strike extent. These sinistral strike-slip faults are believed to have been active from the Late Jurassic to Early Cretaceous.

The gold deposits of the Cheongyang Gold District are largely associated with Jurassic to Cretaceous (195­127 Ma) orogenic and igneous activity. The ore bearing veins are locally both concordant and discordant with the foliation of the Precambrian gneiss and are divided into three groups, based on their orientation and location and have a ribbon texture interpreted to reflect repeated hydraulic fracturing events, namely:
i). The Danbong veins which consist of two parallel gold-bearing quartz veins that strike at 10 to 20° and dip at 40 to 60°NW. These veins are continuous for about 900 m along strike and extend to more than 130 m vertical depth. They vary from 0.3 to 2.0 m in thickness and contain up to 33 g/t gold and 228 g/t Ag.
ii). No. 6 veins - neither of the two parallel, gold-bearing quartz veins that comprise this set is continuous along strike. They strike from east-west to between 20 to 60° and dip at 20 to 50°S to 25 to 35°SE. The individual segments are about 400 to 500 m in strike length and extend to more than 600°m vertical depth. They vary from 0.6 to 1.5 m in thickness and contain up to 80 g/tAu and from 5 to 229 g/t Ag.
ii). Bongam 1 vein which comprises one discordant gold-bearing quartz vein that has a strike length of about 300 m, extends to over100 m-vertical depth and strikes 290 to 310°, and dips at 25° W.

Mineral deposition is interpreted to have been associated with hydrothermal fluid over-pressuring within and below the active shear zones. The vein mineralogy and paragenesis suggest two separate discrete mineralising episodes, separated by a major faulting event. Ore minerals are contained within quartz and calcite associated with fracturing and healing of veins formed during both mineralising episodes.

Wall rock alteration related to the first (or main) ore stage include sericite and chlorite with minor pyrite and carbonates. Sulphide minerals deposited with electrum during this stage included arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena and argentite. Electrum was also deposited during the second stage mineralisation in a single group of veins, accompanied by pyrite, sphalerite, chalcopyrite and galena, while the second stage of deposition in other veins was barren.

The Gubong mine has been the largest gold producer in the Republic of Korea, with over 0.1 tonnes of gold produced per year from 1930 to 1943. The ore had an average grade of about 6.4 g/t Au and an average Au/Ag ratio of about 3:1. Remaining ore reserves are estimated at about 0.15 Mt of ore.

The most recent source geological information used to prepare this summary was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Yoo B C, Lee H K and White N C,  2006 - Gold-Bearing Mesothermal Veins from the Gubong Mine, Cheongyang Gold District, Republic of Korea: Fluid Inclusion and Stable Isotope Studies: in    Econ. Geol.   v101 pp 883-901

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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