Academia.eduAcademia.edu

Figure 6 – uploaded by Young-Seog Kim

See full PDFdownloadDownload figure
generated by Ivanhoe Mines Ltd.). d, e The WNW-ESE long sections show contoured Au and Ag values, lithological contacts and faults that are interpreted from drill hole intersections. Steeply dipping F, faults offset strata, V; veins and Au—Ag ore zones. mRL refers to distance relative to sea level Fig. 6 Geological sections for Eunsan: a-c The SW-NE cross- sections demonstrate the spatial and timing relationships between Au-Ag-bearing V, veins and subparallel F, faults. The SW-dipping strata and near-vertical V, veins are displaced vertically about 100 m by normal-sinistral F, faults (modified after unpublished sections

Figure 6 generated by Ivanhoe Mines Ltd.). d, e The WNW-ESE long sections show contoured Au and Ag values, lithological contacts and faults that are interpreted from drill hole intersections. Steeply dipping F, faults offset strata, V; veins and Au—Ag ore zones. mRL refers to distance relative to sea level Fig. 6 Geological sections for Eunsan: a-c The SW-NE cross- sections demonstrate the spatial and timing relationships between Au-Ag-bearing V, veins and subparallel F, faults. The SW-dipping strata and near-vertical V, veins are displaced vertically about 100 m by normal-sinistral F, faults (modified after unpublished sections

Related Figures (14)

Fig. 1 The Seongsan district is located in the Haenam—Jindo basin, in the south-western part of the Korean peninsula. Cretaceous rocks are indicated in the figure by different colours while all other rocks are represented by shades of grey
Fig. 1 The Seongsan district is located in the Haenam—Jindo basin, in the south-western part of the Korean peninsula. Cretaceous rocks are indicated in the figure by different colours while all other rocks are represented by shades of grey
The Eunsan and Moisan low-sulphidation epithermal deposits are located in nam—Jindo province ( 1 km apart, are hosted the Seongsan district of the Hae- Fig. 2). These deposits are about by similar rock types, display com- parable hydrothermal alteration patterns and formed at about 77.5 Ma (Bowden 2007). These similarities suggest that the deposits are the produc of the same district-scale mineral- isation event. Accordingly, the aims of this study are to (1) compare the structural development of the deposits, (2) identify similarities and differences in ore fluid pathways and focussing mechanisms at the two deposits, and (3) use this information to devise improved exploration strategies for the district. The present study benefits from severa existing studies in the documents stratigraphic ation relationships in the district and relates the formation o low- and high-sulphidat tion and alteration events in the district. Choi et al. (2005 examine vein paragenesis, hydrothermal alteration minera Seongsan district. Bowden (2007 , Structural and hydrothermal alter- ion epithermal deposits to deforma- assemblages and the physicochemical properties of the ore
The Eunsan and Moisan low-sulphidation epithermal deposits are located in nam—Jindo province ( 1 km apart, are hosted the Seongsan district of the Hae- Fig. 2). These deposits are about by similar rock types, display com- parable hydrothermal alteration patterns and formed at about 77.5 Ma (Bowden 2007). These similarities suggest that the deposits are the produc of the same district-scale mineral- isation event. Accordingly, the aims of this study are to (1) compare the structural development of the deposits, (2) identify similarities and differences in ore fluid pathways and focussing mechanisms at the two deposits, and (3) use this information to devise improved exploration strategies for the district. The present study benefits from severa existing studies in the documents stratigraphic ation relationships in the district and relates the formation o low- and high-sulphidat tion and alteration events in the district. Choi et al. (2005 examine vein paragenesis, hydrothermal alteration minera Seongsan district. Bowden (2007 , Structural and hydrothermal alter- ion epithermal deposits to deforma- assemblages and the physicochemical properties of the ore
occurs as discontinuous lenses throughout the Seongsan district (Fig. 3). “°K/*°Ar whole-rock dating of basalt from the Hwawon Formation indicates a crystallisation age of 103.442.5 Ma (Kim and Nagao 1992; Moon et al. 1990). The Hwawon Formation is unconformably overlain by 250— 450 m of black shale, laminated siltstone and mudstone, tuffaceous sandstone, chert and calcareous siltstone and minor intercalated volcanogenic units of the Uhangri For- mation (Kim 1991; Koh 1996). The formation locally con- tains plant fossils (Chun and Chough 1995) as well as trace fossils of dinosaurs, pterosaurs, birds and arthropods (Hwang et al. 2002). 40K Ar whole-rock dating of andes- ite from the Uhangri Formation indicates a crystallisation age of 95.4+2.9 Ma (Kim and Nagao 1992). The Haenam Formation unconformably overlies the Uhangri Formation and includes four dacitic to rhyolitic tuff members: from oldest to youngest, the Hwangsan, Haenam, Seoho and Yongdang members (Koh 1996). The Hwangsan Tuff of the Haenam Formation is the principal unit exposed in the Seongsan district and is the main host to epithermal depos- its (Park et al. 1997). The Haenam Formation is locally
occurs as discontinuous lenses throughout the Seongsan district (Fig. 3). “°K/*°Ar whole-rock dating of basalt from the Hwawon Formation indicates a crystallisation age of 103.442.5 Ma (Kim and Nagao 1992; Moon et al. 1990). The Hwawon Formation is unconformably overlain by 250— 450 m of black shale, laminated siltstone and mudstone, tuffaceous sandstone, chert and calcareous siltstone and minor intercalated volcanogenic units of the Uhangri For- mation (Kim 1991; Koh 1996). The formation locally con- tains plant fossils (Chun and Chough 1995) as well as trace fossils of dinosaurs, pterosaurs, birds and arthropods (Hwang et al. 2002). 40K Ar whole-rock dating of andes- ite from the Uhangri Formation indicates a crystallisation age of 95.4+2.9 Ma (Kim and Nagao 1992). The Haenam Formation unconformably overlies the Uhangri Formation and includes four dacitic to rhyolitic tuff members: from oldest to youngest, the Hwangsan, Haenam, Seoho and Yongdang members (Koh 1996). The Hwangsan Tuff of the Haenam Formation is the principal unit exposed in the Seongsan district and is the main host to epithermal depos- its (Park et al. 1997). The Haenam Formation is locally
during basin development and associated sub-volcanic and volcanic activity during the Cretaceous (Choi et al. 2003). The onset of high-sul pre-dates that of low-su phidation epithermal mineralisation phidation systems (81—70 and 78— 70 Ma, respectively). Both forms of epithermal mineralisa- tion temporally overlap 98-71 Ma) and plutoni suggesting that minera felsic to intermediate volcanism (ca. sm (ca. 86-68 Ma) in the district, isation coincided with prolonged subduction-related plutonic and volcanic activity (Bowden 2007). Oxygen, H, and district suggest that met S isotope data for the Seongsan eoric-derived fluids were important for transporting metals in low-sulphidation epithermal sys- tems, whereas magmatic fluids were involved in high- sulphidation systems (Bowden 2007).
during basin development and associated sub-volcanic and volcanic activity during the Cretaceous (Choi et al. 2003). The onset of high-sul pre-dates that of low-su phidation epithermal mineralisation phidation systems (81—70 and 78— 70 Ma, respectively). Both forms of epithermal mineralisa- tion temporally overlap 98-71 Ma) and plutoni suggesting that minera felsic to intermediate volcanism (ca. sm (ca. 86-68 Ma) in the district, isation coincided with prolonged subduction-related plutonic and volcanic activity (Bowden 2007). Oxygen, H, and district suggest that met S isotope data for the Seongsan eoric-derived fluids were important for transporting metals in low-sulphidation epithermal sys- tems, whereas magmatic fluids were involved in high- sulphidation systems (Bowden 2007).
The Eunsan deposit was mined from 2002 to 2009 as two open pits, followed by underground workings. The inferred All rock types are cut by 0.5—100-cm thick, Au—Ag-bearing quartz veins that strike WNW and dip steeply to the NE (e.g. 300°/75° NE) (Figs. 5 and 8a). The veins mostly consist of quartz, with minor adularia, pyrite, sphalerite, chalcopyrite, galena, electrum, native silver, proustite, argentite, aguilarite,
The Eunsan deposit was mined from 2002 to 2009 as two open pits, followed by underground workings. The inferred All rock types are cut by 0.5—100-cm thick, Au—Ag-bearing quartz veins that strike WNW and dip steeply to the NE (e.g. 300°/75° NE) (Figs. 5 and 8a). The veins mostly consist of quartz, with minor adularia, pyrite, sphalerite, chalcopyrite, galena, electrum, native silver, proustite, argentite, aguilarite,
movement. Vertical displacements of shallowly dipping strata are up to | m, whereas lateral displacements are less than a metre. These structures are interpreted to be the product of the third deformation event at the Eunsan deposit (Dg3). 45° to vein margins. They are interpreted to represent subsid- iary R fractures to the main WNW-striking D fractures in a dextral transtensional wrench system (cf. Riedel 1929). Based on these vein geometries, the interpreted movement sense for veins is dextral-SSW side up, with undetermined displace- ment magnitudes. These structures are interpreted to be the product of the earliest deformation event at the Eunsan deposit (Drz}).
movement. Vertical displacements of shallowly dipping strata are up to | m, whereas lateral displacements are less than a metre. These structures are interpreted to be the product of the third deformation event at the Eunsan deposit (Dg3). 45° to vein margins. They are interpreted to represent subsid- iary R fractures to the main WNW-striking D fractures in a dextral transtensional wrench system (cf. Riedel 1929). Based on these vein geometries, the interpreted movement sense for veins is dextral-SSW side up, with undetermined displace- ment magnitudes. These structures are interpreted to be the product of the earliest deformation event at the Eunsan deposit (Drz}).
Fig. 8 Photographs showing structural relationships at Eunsan: a Heterolithic coarse tuff and overlying volcanogenic sandstone in the western wall of the Eunsan pit are cut by subvertical WSW-striking V, veins with a SW-side-up-dextral displacement. The strata and V, veins are in turn displaced by a subvertical, WSW-striking F2 fault with a SW-side-down (and minor sinistral) displacement of about 100 m. b Plan view of a quartz-carbonate V, vein that cuts volcanogenic sand- stone beds in the north wall of the Eunsan pit. The vein strikes WNW (295°) and dips 80° to the SW. A subordinate horsetail splay strikes NNW (352°) and dips 86° to the E, suggesting a dextral shear compo- nent to the SW-side-up vertical displacement of the V, veins. ¢ Sub- sidiary V, veins in the west wall of the pit demonstrate a SW-side-up movement component to the veins. d A <1 m-wide, subvertical, WNW-striking F, fault cuts shallowly SW-dipping bedding in mud- stone and V vein breccias in the west-facing wall of an underground drive (i.e. in the -80 mRL level). The fault zone is filled with friable, fine-grained, granular fault gouge containing minor fragments of quartz-altered mudstone wallrock. e Steeply SE-dipping, NE-striking, F; faults cut volcanogenic sandstone in the south wall of the pit. Striations on the fault plane pitch 60° to the NE. Fault splays to the main F; fault plane indicate SE-side-up and dextral displacement. f A <5 cm-wide, bedding-parallel normal F, fault offsets a V, vein in volcanogenic sandstone in the west wall of the pit
Fig. 8 Photographs showing structural relationships at Eunsan: a Heterolithic coarse tuff and overlying volcanogenic sandstone in the western wall of the Eunsan pit are cut by subvertical WSW-striking V, veins with a SW-side-up-dextral displacement. The strata and V, veins are in turn displaced by a subvertical, WSW-striking F2 fault with a SW-side-down (and minor sinistral) displacement of about 100 m. b Plan view of a quartz-carbonate V, vein that cuts volcanogenic sand- stone beds in the north wall of the Eunsan pit. The vein strikes WNW (295°) and dips 80° to the SW. A subordinate horsetail splay strikes NNW (352°) and dips 86° to the E, suggesting a dextral shear compo- nent to the SW-side-up vertical displacement of the V, veins. ¢ Sub- sidiary V, veins in the west wall of the pit demonstrate a SW-side-up movement component to the veins. d A <1 m-wide, subvertical, WNW-striking F, fault cuts shallowly SW-dipping bedding in mud- stone and V vein breccias in the west-facing wall of an underground drive (i.e. in the -80 mRL level). The fault zone is filled with friable, fine-grained, granular fault gouge containing minor fragments of quartz-altered mudstone wallrock. e Steeply SE-dipping, NE-striking, F; faults cut volcanogenic sandstone in the south wall of the pit. Striations on the fault plane pitch 60° to the NE. Fault splays to the main F; fault plane indicate SE-side-up and dextral displacement. f A <5 cm-wide, bedding-parallel normal F, fault offsets a V, vein in volcanogenic sandstone in the west wall of the pit
Fig. 10 The simplified map of underground workings situated about 30 m below sea level illustrates the local occurrence of horsetail jog geometries associated with normal-dextral, Au-Ag-bearing V,veins and their offset by normal-sinistral F, faults (modified after an unpub- lished plan generated by Ivanhoe Mines Ltd.) Au—Ag ore zones are widest in volcaniclastic units of the Haenam Formation, which have a higher primary porosity and permeability than the underlying, mudstone and silt- stone beds of the Uhangri Formation. The gentle SW plunge of the Eunsan ore body is interpreted to reflect the intersec- tion of the V; veins and rocks of the Haenam Formation (Fig. 6d and e). The maximum vertical extent of the Eunsan ore body is probably largely controlled by the lower limit of fluid boiling zones, but could also reflect the position of the basal contact of the Haenam Formation with the underlying Uhangri Formation.
Fig. 10 The simplified map of underground workings situated about 30 m below sea level illustrates the local occurrence of horsetail jog geometries associated with normal-dextral, Au-Ag-bearing V,veins and their offset by normal-sinistral F, faults (modified after an unpub- lished plan generated by Ivanhoe Mines Ltd.) Au—Ag ore zones are widest in volcaniclastic units of the Haenam Formation, which have a higher primary porosity and permeability than the underlying, mudstone and silt- stone beds of the Uhangri Formation. The gentle SW plunge of the Eunsan ore body is interpreted to reflect the intersec- tion of the V; veins and rocks of the Haenam Formation (Fig. 6d and e). The maximum vertical extent of the Eunsan ore body is probably largely controlled by the lower limit of fluid boiling zones, but could also reflect the position of the basal contact of the Haenam Formation with the underlying Uhangri Formation.
WNW-striking F, faults spaced about 50 m apart segment the Eunsan ore body, offsetting Au-Ag zones by up to 100 m vertically (Fig. 9). On some levels in the underground mine, SW-block-down movement along F, faults duplicated V, veins and associated ore zones and significantly increased the total width of the ore body (Fig. 10). By contrast, F; and Fy, faults displace the ore body by <I m and have negligible influence on the geometry of Au—Ag ore shoots.
WNW-striking F, faults spaced about 50 m apart segment the Eunsan ore body, offsetting Au-Ag zones by up to 100 m vertically (Fig. 9). On some levels in the underground mine, SW-block-down movement along F, faults duplicated V, veins and associated ore zones and significantly increased the total width of the ore body (Fig. 10). By contrast, F; and Fy, faults displace the ore body by <I m and have negligible influence on the geometry of Au—Ag ore shoots.
bearing V, veins cut the subhorizontal strata and are surrounded by zoned hydrothermal wall-rock alteration Fig. 11 Geological section for Moisan showing the distribution of rock types, hydrothermal alteration, structures and Au mineralisation (modified after Bowden 2007). Subvertical, WNW-striking, Au-Ag- The Moisan mine stratigraphy is similar to the Eunsan deposit and consists of mudstone and siltstone beds of the Uhangri Formation that are overlain by volcanic and volca- niclastic rocks of the Haenam Formation (Fig. 11). The latter sequence of rocks includes (1) coarse-grained Hwangsan tuff, (2) heterolithic lapilli tuff (Fig. 12a, b) interbedded with
bearing V, veins cut the subhorizontal strata and are surrounded by zoned hydrothermal wall-rock alteration Fig. 11 Geological section for Moisan showing the distribution of rock types, hydrothermal alteration, structures and Au mineralisation (modified after Bowden 2007). Subvertical, WNW-striking, Au-Ag- The Moisan mine stratigraphy is similar to the Eunsan deposit and consists of mudstone and siltstone beds of the Uhangri Formation that are overlain by volcanic and volca- niclastic rocks of the Haenam Formation (Fig. 11). The latter sequence of rocks includes (1) coarse-grained Hwangsan tuff, (2) heterolithic lapilli tuff (Fig. 12a, b) interbedded with
Fig. 12 Photographs of representative rock types in the Moisan de- posit: a coarse-grained, lithic-fragment-rich lapillistone that is matrix- supported and contains <5 vol% plagioclase crystals. Pyrite and chlo- rite replace primary volcanic vesicles, resulting in pyrite-filled amyg- dales with chlorite rims. b A fine tuff comprises mainly ash, with up to 5 vol%, 1 mm-diameter plagioclase crystals, and rare spherical and concentrically layered lapilli. e A green, well-sorted, crystal- and lithic fragment-poor tuff that is moderately hydrothermally chlorite-pyrite- altered. d The coarse tuff is moderately sorted and comprises angular, subhedral plagioclase and amphibole crystals, lithic fragments, and a matrix of plagioclase, quartz and ash. Amphibole is preferentially replaced by pyrite. All photographs are of core from diamond drill hole MS08-03, with the exception of photograph (b), which is from drill hole MS08-02
Fig. 12 Photographs of representative rock types in the Moisan de- posit: a coarse-grained, lithic-fragment-rich lapillistone that is matrix- supported and contains <5 vol% plagioclase crystals. Pyrite and chlo- rite replace primary volcanic vesicles, resulting in pyrite-filled amyg- dales with chlorite rims. b A fine tuff comprises mainly ash, with up to 5 vol%, 1 mm-diameter plagioclase crystals, and rare spherical and concentrically layered lapilli. e A green, well-sorted, crystal- and lithic fragment-poor tuff that is moderately hydrothermally chlorite-pyrite- altered. d The coarse tuff is moderately sorted and comprises angular, subhedral plagioclase and amphibole crystals, lithic fragments, and a matrix of plagioclase, quartz and ash. Amphibole is preferentially replaced by pyrite. All photographs are of core from diamond drill hole MS08-03, with the exception of photograph (b), which is from drill hole MS08-02
Fig. 13 Outcrop map and horizontal sections for Moisan. a The simplified outcrop map shows the distribution of major rock types and structures. The lower-hemisphere, equal-area stereographic projec- tions display bedding, vein and fault orientations. b-e The series of horizontal slices through the deposit demonstrate the geometry and position of ore zones that spatially coincide with WNW-trending, sheeted V, veins
Fig. 13 Outcrop map and horizontal sections for Moisan. a The simplified outcrop map shows the distribution of major rock types and structures. The lower-hemisphere, equal-area stereographic projec- tions display bedding, vein and fault orientations. b-e The series of horizontal slices through the deposit demonstrate the geometry and position of ore zones that spatially coincide with WNW-trending, sheeted V, veins
Table 1 A comparison of deformation events for the Eunsan and Moisan deposits adularia from proximal hydrothermal alteration zones at Chunsan has a *°Ar/??Ar crystallisation age of 78.1+0.3 Ma (Bowden 2007), which overlaps the timing of Au-Ag miner- alisation at Eunsan and Moisan. Bowden (2007) interprets the Chunsan low-sulphidation veins and hydrothermal alteration zones to have replaced an earlier, high-sulphidation-related clay alteration in the rocks. Hydrothermal ore fluids associated with low-sulphidation epithermal mineralisation exploited these zones of enhanced secondary permeability in the volca- niclastic rocks (Bowden 2007). of <1 m, but with unconstrained lateral displacements. a disp disp high resu men These faults formed as a result of approximately ENE- SSW-directed shortening (Fig. 14c). Only observed at Eunsan, shallow-dipping, bedding-parallel normal F4 faults ace V, veins with <1 m offsets and indicates a fourth deformation event involving NE-SW-directed extension (Fig. 14d). Although post-Dz event structures generally ay minor displacements, they clearly cut V, veins at angles and have the potential to displace ore shoots, ting in complexity in regard to underground develop- and exploration.
Table 1 A comparison of deformation events for the Eunsan and Moisan deposits adularia from proximal hydrothermal alteration zones at Chunsan has a *°Ar/??Ar crystallisation age of 78.1+0.3 Ma (Bowden 2007), which overlaps the timing of Au-Ag miner- alisation at Eunsan and Moisan. Bowden (2007) interprets the Chunsan low-sulphidation veins and hydrothermal alteration zones to have replaced an earlier, high-sulphidation-related clay alteration in the rocks. Hydrothermal ore fluids associated with low-sulphidation epithermal mineralisation exploited these zones of enhanced secondary permeability in the volca- niclastic rocks (Bowden 2007). of <1 m, but with unconstrained lateral displacements. a disp disp high resu men These faults formed as a result of approximately ENE- SSW-directed shortening (Fig. 14c). Only observed at Eunsan, shallow-dipping, bedding-parallel normal F4 faults ace V, veins with <1 m offsets and indicates a fourth deformation event involving NE-SW-directed extension (Fig. 14d). Although post-Dz event structures generally ay minor displacements, they clearly cut V, veins at angles and have the potential to displace ore shoots, ting in complexity in regard to underground develop- and exploration.
Related topics:
Structural Geology
580 California St., Suite 400
San Francisco, CA, 94104
© 2025 Academia. All rights reserved