Genesis of Carbonate Breccia Containing Invisible Gold in Taebaeksan Basin, South Korea

: The Yemi breccia developed and is distributed within the Paleozoic carbonate rock (Maggol Formation) in the central part of the Taebaeksan Basin, South Korea. Explanation for the genesis of the Yemi breccia has been controversial. We investigated the petrological and mineralogical properties of the breccia and the matrix materials at 60 outcrops. The Yemi breccia is divided into crackle, mosaic, and chaotic breccias based on morphology. In addition, these are divided into blackish, reddish, grayish, and white to pinkish matrix breccias according to the materials of the matrix. Quartz, calcite, pyrite, hematite (after pyrite), and minor epidote, chlorite, and opaque materials mainly comprise the matrix materials. The pyrite grains from the Yemi breccia can be divided into two types based on the mineral texture: diagenetic and hydrothermal. We analyzed the chemistry of pyrite and hematite (after pyrite) from the Yemi breccia with an electron probe X-ray microanalyzer (EPMA). Invisible gold was detected within the pyrite grains by EPMA and disseminated micron-sized isolated gold particles were discovered by backscattered electron (BSE) images. The texture of Au-bearing pyrite and gold particles in the Yemi breccia is especially well matched with pyrite and gold from the Shuiyindong Carlin-type hydrothermal gold deposits, China. Therefore, we suggest an important role of hydrothermal fluid in karstification within the Paleozoic carbonate rock.


Introduction
The carbonate Yemi breccia developed in the Taebaeksan Basin in the northeastern part of the southern Korean Peninsula (Figure 1). It is distributed within an area that extends approximately 10 km north-to-south and 3 km east-to-west in the Yemi area ( Figure 2). The genesis of the breccia has been reported variously as intraformational brecciation or intrusion of argillaceous materials in the cracks made by deformation; synsedimentation breccia related fault activities; syngenetic fault breccia during formation of a sediment basin; solution collapse breccia formed by dissolution of pre-existing evaporites; or diagenesis based on the occurrence of clasts, matrix mineralogy, fossils, and sedimentary structures [1][2][3][4][5]. On the other hand, it is reported that some breccias can be formed by the introduction of hydrothermal fluid along bedding or strata boundaries in carbonate rocks [6][7][8][9][10][11][12][13][14]. Breccias may have hydrothermal and hydraulic fracturing origin [15,16], formed by processes similar to typical karstification [6]. Their morphology is very similar to typical karst breccias formed by meteoric water [17,18]. However, previous studies did not consider the theory that brecciation originated by hydrothermal fluid. Previous studies have reported the genesis of this breccia, but it is still controversial. Here we investigated the morphology and mineralogy of the infilled matrix of the breccia to clarify the genesis. In particular, we discovered invisible gold in pyrite and isolated gold particles in the Yemi breccia. As a result, we suggest that hydrothermal fluid acted in the development of the Yemi breccia within carbonate rock.  [19]. (b) Geological map showing stratigraphic units of Taebaeksan Basin (modified from [20]). Bold rectangle (Yemi area) shows the study area.  (1962) [5,21]. Study area shows tectonic imbrication, formations that are repeated by thrust faults, anticlines, and synclines.

Regional Geology
Three major Precambrian massifs comprise the Korean Peninsula: the Nangrim, Gyeonggi, and Yeongnam massifs ( Figure 1). The Gyeonggi and Yeongnam massifs are separated by the Okcheon Belt, which comprises the weakly metamorphosed Paleozoic Taebaeksan Basin and the Okcheon Metamorphic Belt (OMB). The Taebaeksan Basin includes early Paleozoic marine sedimentary rocks (Joseon Supergroup) and late Paleozoic nonmarine sedimentary rocks (Pyeongan Supergroup), which were folded during the Triassic and Jurassic periods [19]. The Joseon Supergroup is mostly composed of carbonate rock formed mainly in shallow marine and tidal environments, reflecting sea-level changes [19]. The Pyeongsan Supergroup formed mainly in shallow marine, deltaic, and fluvial environments [19]. The Late Triassic to Early Jurassic Daedong Group is composed of conglomerate, sandstone, and shale, which unconformably overlie the Pyeongsan and Joseon Supergroups [22]. Most of the sedimentary rocks in the Taebaeksan Basin are folded, including many well-developed high-angle faults. Cluzel et al. [23,24] insisted that the NS-trending folds and thrust faults were formed by the Songrim Orogeny and the EW-trending folds and faults were developed by the Daebo Event [25]. As a result of these two events, the major geologic structures in the Taebaeksan Basin are commonly NNE-to NE-striking thrusts and associated folds (Figure 1b). Westward-dipping thrust faults dominate in the western part of the basin (Yeongwol area), while oblique-slip faults and synclines with minor thrusts dominate in the eastern part (Taebaek and Jeongsun areas).

Local Geology
The Joseon and Pyeongsan Supergroups, and the Bansong Formation mainly comprise the study area. The Joseon Supergroup includes the Hwajeol, Donjeom, Dumugol, Maggol, Jikunsan, and Duwibong Formations, and the Pyeongsan Supergroup contains the Hongjeom, Sadong, Gobangsan, and Nokam Formations ( Figure 2). These units have strikes similar to NE-trending thrusts [26]. The Hwajeol Formation is distributed in the southeastern part of the study area and strikes N 30-60 • E. The lower part of the Hwajeol Formation is dominated by greenish-gray sandy shale, dark gray sandy slate, greenish-gray slate, dark gray sandy shale, and dark gray fine-grained sandstone, while the upper part shows alternating beds of pinkish-gray limestone and greenish-gray slate [27]. The Dongjeom Formation conformably overlies the Hwajeol Formation. It consists of dark gray and brown medium-grained quartzite and dark brown laminated silicic limestone in the lower part, and dark quartzite in the upper part. This unit strikes N 70-80 • W and dips 35-45 • NE on average. The Dumugol Formation overlies the Dongjeom quartzite conformably, and consists of greenish-gray slate, greenish-brown slate, brown-green limestone, pinkish-gray limestone, dark gray tabular limestone, and calcareous shale [22]. The Maggol Formation conformably overlies the Dumugol formation and consists of calcic sandstone, dolomite, interbedded calcic conglomerate, and oolitic limestone. It is divided into a lower limestone part and an upper part of alternating dolomite and limestone. The stratigraphic section and the position of the Yemi breccia are reported by Ryu [5].
The carbonate breccia called the Yemi breccia is now known to be a facies of the upper Maggol Formation. It was identified as a separate formation in 1962 [21], but was shown to belong to the upper part of the Maggol Formation by geological, petrological, and paleontological studies [4,5,27]. However, the origin of the Yemi breccia remains controversial. The Jikunsan Formation conformably overlies the Maggol Formation. A shale formation named the Gosung Shale was reported in the study area [21], but Kim (1969) [27] compared it with the Jikunsan Formation based on biostratigraphical data and argued that the two were the same. The formation consists of black shale and bluish-gray limestone, with the lower part dominated by calcareous rocks and the upper part dominated by siliceous rocks [28]. The Dwibong Formation conformably overlies the lower Jikunsan Formation. It is composed of massive pinkish-gray biogenic clastic limestone and calcareous shale [28]. The Hongjeom and Nokam Formations included in the Pyeongsan Supergroup are composed of shale, limestone, and sandstone [29], and are distributed in the southeast of the study area. The Bansong Formation is widely distributed in the study area; its eastern boundary unconformably overlies the Nokam Formation, but the western boundary is truncated by thrust faults. The Bansong Formation is dominated by conglomerate and sandstone with interbedded shale. The linear structures oriented along the NE-SW direction in the study area include bedding, faults, folds, and thrusts, and generally dip to the NW. Several thrust faults developed parallel to the NE-SW direction. These faults are thought to have developed during the Jurassic Daebo Event, after deposition of the Bansong Formation [26].

Sampling and Carbonate Staining
More than 150 samples were collected during field work for this study. Approximately 60 breccia samples were selected for mineralogical and geochemical studies. Most samples were collected from carbonate units. All collected samples were cut to make slab samples and studied under a stereoscopic microscope. Representative samples were used to make thin sections based on these observations. Thin sections were studied under a petrographic microscope and characterized to determine the genesis of the Yemi breccia, using features such as mineral composition, textures, and mineral relationships.
Samples were stained to distinguish between calcite and dolomite. Alizarin red S stains calcite a red color, while dolomite does not stain; the results depend on the proportion of calcite in the sample and other factors. The Alizarin red S solution was prepared as follows: and, 37% hydrochloric acid was diluted with distilled water to make 2% hydrochloric acid: 100 g 2% HCl = 5.4 mg 37% HCl + 94.6 mg distilled water (2) All slab samples were stained with Alizarin red S solution in this study. Rocks with irregular surfaces were polished and then stained.

Electron Probe Microanalysis
Six polished thin sections were selected for electron microprobe analysis to determine their pyrite and hematite geochemistry and gold content. Pyrite and hematite in representative polished thin sections were classified into several types based on morphology, relative minerals, and host rock. The equipment used was a Shimadzu EPMA-1600 electron microprobe analyzer (Shimadzu Corporation, Kyoto, Japan) at Gyeongsang National University to analyze pyrite, hematite, and gold particles. The analysis conditions were acceleration voltage 15 keV, beam current 20 nA, and spot size 1 µm. The elements analyzed were iron, zinc, copper, oxygen, gold, silver, antimony, sulfur, arsenic, cobalt, and lead. Most ore minerals could be identified on backscattered electron (BSE) images, and others required the use of energy-dispersive x-ray spectroscopy (EDS) with the EPMA. In-house standards were used for the analysis the minimum detection limits of the elements are ≤0.01 wt%.

Morphological Features of the Yemi Breccia
The Yemi breccia is widely distributed along a north-south axis near the center of the study area ( Figure 2). The Yemi breccia within the Maggol Formation is controlled by bedding planes, and some of it is unrelated to stratigraphy. Most of the breccia observed at the outcrops appears to have poor continuity. Its width, extension, and distribution direction of outcrops vary due to the influence of folds and thrusts. We observed the outcrop of Yemi breccia at 60 points. The Yemi breccia shows three distinct morphologies: crackle, mosaic, and chaotic breccia. This morphological classification, based on textural criteria, generally follows the work of Laznicka (1988) [30]. The Yemi breccia generally occurs as lenticular bodies interbedded with limestone or calcareous silt rocks, showing only one morphological type, although some outcrops show more than one morphology with a gradual transition from one to the other. Figure 3a,b shows a typical crackle breccia, as well as the features that are controlled by the types of host rocks. Field relationships demonstrate that the brecciation developed within the sedimentary layer; calcareous strata are intensively brecciated, while the interbedded silty rocks are not brecciated, and this pattern is common in the study area. Figure 3c shows the features of mosaic and chaotic breccia and a well-preserved sedimentary lamination of the host rock. Some of the breccia has monogenic clasts ( Figure 4). Other parts of the breccia, however, have polygenic clasts, including limestone and shale, and dolomitic limestone and shale (Figure 4a,e). These fragments have various shapes, apparently related to the clast mineralogy or the geological environment. Monogenic clast breccias are common throughout the study area. Polygenic clast breccias, however, are generally observed only near the boundary with the overlying Gosung Shale (Jikunsan Formation) (Figure 2). The Yemi breccia shows various colors because of the matrix materials; it is divided into blackish, reddish, grayish and white to pinkish bodies (Table 1). Blackish matrix breccia (BMB) commonly occurs in bodies that are several meters long (Figure 3e), and their clast size is a few centimeters (1-10 cm; Figure 4a). A commonly observed weathering result is that the matrix protrudes more than the clasts at outcrops (Figure 3d,e). Some parts of blackish matrix breccia change to a yellowish color (Figure 3d). The breccia in Figure 4a has a black matrix and deformed limestone clast, with recrystallized pinkish calcite in the clast. Figure 5a shows the matrix composed of quartz, calcite, pyrite, and an unknown material, presumed to be an insoluble substance or organic material. It has a blackish color because of finely crystalline pyrite and perhaps other unidentified materials. Some parts of the matrix were replaced by quartz (Figure 5e). Quartz, calcite, and pyrite veins are also observed in the BMB (Figure 5f), which developed in Goseong shale. Reddish matrix breccia (RMB) occurs over several meters (3-8 m), and its clast size is a few centimeters (Figure 4d) at the outcrops, similar in occurrence to the BMB. A pattern of differential weathering between the matrix and clasts, comparable to that described above, and is just the result of higher solubility of carbonate minerals in the clasts (Figure 3d,e). RMB generally shows a mosaic texture when observed in the field. Clasts are composed of limestone, dolomite, and shale, and reaction rims are clearly present in limestone fragments (Figure 4b). The RMB matrix mainly consists of quartz, calcite, minor dolomite, sericite, pyrite, hematite, limonite, and dark unidentified materials, presumed to be insoluble residue or organic material (Figure 5b). In some outcrops, BMB and RMB show a vertical relationship (Figure 3d). In the yellowish part of BMB (Figure 3d), pyrite has been oxidized to goethite or iron oxi-hydroxides ( Figure 6), but RMB has hematite (altered pyrite), suggesting that the two breccias formed in different environments. While BMB is dominated by pyrite, RMB is dominated by hematite (Figure 5b). Grayish matrix breccia (GMB) is exposed in outcrops several meters across (Figure 3b), and the outcrops seem to resemble massive or laminated limestone. Boundaries between matrix and clasts are unclear in some samples (Figure 4d). The grayish matrix breccia consists mainly of calcite and dolomite, and minor quartz and pyrite (Figure 5c). GMB also has insoluble residues or organic materials, similar to that observed in BMB and RMB.   White to pinkish matrix breccia (WPMB) is common in the study area, and outcrops generally span several meters (Figure 3f). These breccia bodies are noncontiguous and are scattered in the study area. Yellowish clast boundaries indicate oxidation of the disseminated pyrite formed along the clast boundaries (Figure 4d). WPMB is composed of calcite, dolomite, quartz, and pyrite. Coarse-grained calcite is replaced by fine grained quartz, pyrite, and calcite (Figure 5d).

Occurrence of Metallic Minerals (Mainly Pyrite and Hematite) in Yemi Breccia
The pyrite and hematite in the Yemi breccia show different morphologies and textures ( Figure 6 and Table 2). Rare euhedral to subhedral pyrite grains (>20 µm) are disseminated in the carbonate clasts of Yemi breccia (Figure 6a) and may have formed during the diagenesis of carbonate rock. In some parts of the Yemi breccia matrix, euhedral to anhedral pyrite of various size are observed (Figure 6b). Some are coarser (50-200 µm) than others. Coarse-grained pyrite show overgrowth zoning and a change to hematite at the margin of the grain (Figure 6b,d,e and Figure 7d). Some pyrite grains are observed with other sulfides, such as pyrrhotite and chalcopyrite (Figure 6c). Pyrite grains are replaced by pyrrhotite, and finally precipitated chalcopyrite. Some pyrite was completely changed to hematite (Figure 6f).

Occurrence of Gold in Yemi Breccia
In the Yemi breccia, we discovered invisible gold, classified into three types. The first type of gold is invisible under optical and electron microscopes, but confirmed by EPMA analysis of pyrite (Figure 7a,b) and hematite (after pyrite; Figure 7c) grains. The gold content of the pyrite (0.01-1.54 wt%) and hematite (0.07 wt%) ranges from 0.01 to 1.54 wt% (Table 3). These properties are similar to the general characteristics of invisible gold from Carlin-type gold deposits ( [31,32] and references therein). The second type of gold is distributed within pyrite and hematite (after pyrite). It is not visible under optical microscopy, but can be confirmed in BSE images from electron microscopy (Figure 7d-f). The particles are irregular, with an average diameter of 0.1 to 5 µm; most of these gold particles are smaller than the beam size (about 1 µm); the gold content is 1.5-12 wt%. Some bright areas in the BSE image are gold particles that occur in zoned pyrite (Figure 7d). The third type of gold is observed at the outer part of pyrite and hematite grains (Figure 7b,e,f). These gold particles range in size from 0.1 to 20 µm. Properties of the second (distributed in pyrite and hematite) and third (isolated grains) types of gold match well with gold from China's Shuiyindong Carlin-type gold deposits [33,34].
The Yemi breccia, observed at 60 points, can be divided into crackle, mosaic, and chaotic breccias according to the morphology. These breccias are common in karst-collapse and hydraulic fracturing areas [15,16]. The term 'karst' includes underground solution cavities, collapsed breccias, and related topographical features consequent to the dissolution of soluble rocks. Although carbonate dissolution is generally caused by groundwater, it is also caused by hydrothermal fluids; the bodies of rock that result are called 'hydrothermal karst' [6,7,11,51] and show the same morphologies as karst produced by shallow meteoric water. The brecciation which developed within the sedimentary layer demonstrate that calcareous strata are intensively brecciated, while the interbedded silty rocks are not brecciated (Figure 3a,b). It means the insoluble silty strata have not been affected by hydrothermal fluid. The matrix of the Yemi breccia is composed of mainly quartz, calcite, pyrite, hematite (after pyrite), and insoluble materials showing irregular shape (Table 1). It is different from traditional carbonate breccia and nontraditional hydraulic fractured breccia. Some clasts in the breccia have an alteration halo (Figure 2b). These features are not observed in sedimentary, nondepositional, tectonic, hydraulic fracturing, or diagenetic breccia, but are similar to hydrothermal karst [6,7,52]. The pyrite from the Yemi breccia shows overgrowth and replacement textures ( Figure 6). The pyrite that occurred by overgrowth was also replaced by hematite. This means the hydrothermal fluid which precipitated pyrite was introduced more than once. In the Yemi breccia, invisible gold was discovered (Figure 7). This phenomenon is reported in hydrothermal Carlin-type gold deposits [31][32][33][34]53,54]. In particular, gold within pyrite and hematite (after pyrite) and isolated gold particles (Figure 7) have been reported in the Shuiyindong deposit [33,34], which is a Carlin-type hydrothermal gold deposit. The evolution model of breccia developed by hydrothermal fluid in limestone and related gold mineralization has been reported [8] in the Carlin-type Conrad zone, Yukon.
In conclusion, the Yemi breccia, developed within the Maggol Formation in the Taebaeksan Basin, has distinct features that can be observed in nontraditional hydrothermal karst. Furthermore, the presence of invisible gold within the breccia is a distinct feature of Carlin-type hydrothermal gold deposits. Thus, we suggest the important role of hydrothermal fluid in karstification within the carbonate Maggol Formation.