Genesis of the Songhung gold-polymetallic deposit in the Hoechang district, the Democratic People’s Republic of Korea: Constraints from Geology, Petrochemistry and Pb–S–C–O isotope geochemistry

Ore Geology Reviews 116 (2020) 103250

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Genesis of the Songhung gold-polymetallic deposit in the Hoechang district, the Democratic People’s Republic of Korea: Constraints from Geology, Petrochemistry and Pb–S–C–O isotope geochemistry

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Chung-Nam O, Ung-Ho Pak , Kwang-U Choe, Do-Jun Ryang, Kwang-Min Kim, Hak-Chol Sim, Yong-Il Jo, Un-Jin Ryang Faculty of Geology, Kim Il Sung University, Ryongnam–Dong, Taesong District, Pyongyang, Democratic People’s, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Metamorphic core complex Stable isotope Lead isotope Gold Songhung

The Songhung deposit is located in the southwestern part of the Yangdok Uplift Zone (YUZ) in the northern part of the Pyongnam Basin in the Sino–Korean Craton. Most of large– and small–scale gold deposits and occurrences were discovered and exploited in the western part of the YUZ. Several tectonic events in this area are the key to understanding the gold and polymetallic mineralization in the Sonhung deposit. The YUZ comprises the Neoarchean Rangnim Metamorphic Belt, Proterozoic and Early Paleozoic rocks in the central domain, and granitoid intrusions of crustal and mantle derivation (Yangdok granitoid intrusions). Two extensive NNE–trending fault zones with mylonitized rocks are interpreted to represent detachment fault and two NE–SW–trending regional rift structures represent zones of subsidence (e.g., the Tokchon–Maengsan to the west, the Kowon to the east) and the Junghwa–Samdung subsidence to the south. It is suggested that the YUZ is a metamorphic core complex (MCC), similar to those in the North China Craton (NCC). Polymetallic mineralization in the Songhung deposit is divided into early and late stages. The early mineralization was formed during compressional deformation in the Triassic accompanied by low–grade metamorphism of the sedimentary host rocks. Variations in Pb, S, C and O isotope ratios indicate that the ore–forming fluids and metals were derived from the country rocks. The Au grade is insignificant in this style of mineralization. The main gold–polymetallic mineralization in the Songhung deposit is related to tectonic extension of the region; the Pb, S, C and O isotope data indicate that these elements, as well as ore–forming fluids, could be derived from both the granitic intrusion (Yangdok granitoid) and the Proterozoic and Paleozoic country rocks.

1. Introduction The Songhung Deposit located in the Hoechang district of the Yangdok Uplift Zone (YUZ) and represents one of the most important producers of gold and polymetallic (Cu, Pb, Zn and Fe) minerals in the DPR Korea. Because the Hoechang district is characterized by complex tectonic relationships (e.g., numerous thrusts, faults and folds in the Proterozoic and Paleozoic sedimentary rocks), various magmatic events of different ages (granite, granite porphyry, gabbro, gabbro diabase and dikes), and numerous metallogenic events (Sn, W, Au, Ag, Pb, Zn and Cu), extensive investigations have been completed in the past two decades. The genesis of gold–polymetallic mineralization in this area is traditionally understood based on its spatial association with the folds, faults, granitoids and stratigraphy of the hosted sedimentary rocks, rather than the clarification of tectonic events in the region. There are

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various debates regarding: (1) the thrusts and faults were formed by regional vertical tectonic events during the Proterozoic and Paleozoic period in association with the formation of anticlinal fold and faults parallel to the bedding. The latter appear to represent an important control on ore localization. These structures may be considered as channels for fluid and metal transfer, mineralization and precipitation of Au and base metals. The metals and ore–forming fluids may have originated from the Proterozoic and Paleozoic sedimentary rocks during low–grade metamorphism (Ju et al., 1997; Peak and Pak, 2007; Pak and Jong, 2004; Yang, 1995). (2) NW–trending normal fault system in this area comprise ductile shear zones with cataclastic rock types, and polymetallic mineralization may be related to formation of these faults (Peak, 2007). (3) Ore bodies are also related to magmatism in association with the Yangdok granitoid (e.g., from Sn and W to Au–Cu–Pb–Zn mineralization). Taken together, these data indicate that the

Corresponding author. E-mail address: [email protected] (U.-H. Pak).

https://doi.org/10.1016/j.oregeorev.2019.103250 Received 20 March 2019; Received in revised form 25 October 2019; Accepted 22 November 2019 Available online 27 November 2019 0169-1368/ © 2019 Elsevier B.V. All rights reserved.

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2005; Peak et al., 1993; Pak and Jong, 2004). Seven gold–polymetallic deposits were discovered in the western and southern parts of the YUZ, which are hosted either in the NRMB or in the Proterozoic and Early Paleozoic rocks. Most of these deposits are auriferous quartz–sulfide type and located in the NW–trending faults.

formation of ore–bearing structures and the mineralization in this area may be related with the emplacement of the Yangdok granitoid intrusions. Moreover, the controlling influence of the Hoechang granite porphyry is possibly important because ore bodies are concentrated around the margin (Choe et al., 2011). The genesis of mineralization in the eastern part of the NCC is considered to be related to tectonic model such as the formation of the MCC, a ductile shear zone, which had been affected by compressional and extensional tectonic deformations from the Early to Late Mesozoic period (Tang et al., 1991). The Eastern Asia is characterized by widespread Late Jurassic–Early Cretaceous magmatism and continental extension (Daoudene et al., 2009; Charles et al., 2012) that produced the NE–SW oriented rift basins (Wang et al., 2012), scattered MCCs (Davis et al., 2002; Liu et al., 2005; Wang et al., 2011), and numerous gold occurrences (Deng et al., 2009; Goldfarb et al., 2014; Song et al., 2015). The Pyongnam basin situated in the eastern margin of the Sino–Korean Craton had been also affected by compressional deformation in the Early Mesozoic and extensional deformation in the Middle and Late Mesozoic period, indicating that the mineralization in this area is also related to regional tectonic events. The Pb, S, C and O isotopic compositions of ores and related rocks are useful to understand the source of metal and ore–forming fluids, genesis and timing of mineralization (Barnes, 1979, 1997; Gariépy and Dupré, 1991; Ohmoto, 1972). In this paper, we discuss the tectonic significance of the YUZ, the petrogenesis of granitoid rocks (Yangdok and Hoechang intrusions) and the Pb, S, C, O isotope characteristics of ores and related rocks to better understand the gold–polymetallic mineralization in the Hoechang district. These data are used together with field observations, analytical data of major elements to understand the relationship between the ores and the host rocks.

2.2. Deposit geology 2.2.1. Stratigraphy The Songhung deposit is located in the Hoechang district, southwestern part of the YUZ. The strata exposed in the Songhung deposit are the NRMB, the Mesoproterozoic Jikhyon and Sadang–u groups, the Neoproterozoic Yontan Group and the Early Paleozoic Hwangju Group (Fig. 2a). The NRMB crops out in the northeastern and eastern parts of the study area and was strongly migmatized and scattered in the NRC, and consists of gneiss, schist and migmatite in sequence from bottom to top. The gneiss consists of biotite gneiss, marble, flaky graphite gneiss, fine–grained biotite gneiss and banded gneiss. The schist comprises biotite schist and sericite–quartz schist. The migmatite is only present along the contact with the NRC. The Mesoproterozoic rocks outcrop near to the study area except in the northeastern part and they unconformably overlie the NRMB. These rocks host both quartz–sulfide and metasomatic veins and consist of the Jikhyon and Sadang–u groups. The Jikhyon Group crops out around the Songhung deposit, and in ascending order, consists of the Suan Formation (quartzite and sericite–quartz schist) (Fig. 3a), the Sinsong Formation (chlorite sericite schist, crystalline limestone and micaceous schist), the Mulgumsan Formation (quartzite and calcareous quartzite) and the Hoechang Formation (sericite schist and micaceous limestone) (Fig. 3a–e). The Sadang–u Group is exposed in the southwestern part of the study area and includes the Naedong Formation (dark gray bedded limestone, dark gray massive limestone with laminated texture and white massive limestone) (Fig. 3f–g) and the Obongsan Formation (dark gray sheeted dolomite, white massive dolomite and black slate). The Rungri Formation in the Neoproterozoic Yontan Group is only exposed along the NW direction in the western part of the study area and consists of yellow–brown slate, magnetite–bearing slate and gray dense phyllite. The exposure of the Early Paleozoic Hwangju Group unconformably overlying the Yontan Group consists of quartzite and slate and was strongly eroded (Fig. 3h–i).

2. Geological setting 2.1. Regional geology The YUZ is situated in the northern part of the Pyongnam Basin, in the eastern part of the Sino–Korean Craton and extends to northeast for a distance of about 80 km and a width of about 8–45 km (Fig. 1a). It is adjacent to three zones of subsidence (the Tokchon–Maengsan to the west, the Kowon to the east and the Junghwa–Samdung to the south) and the Rangnim Massif to the north (Fig. 1b). The central part of the YUZ consists of the Neoarchean Rangnim Metamorphic Belt (NRMB, ca. 2800–2500 Ma), Proterozoic (ca. 1200–570 Ma) and Early Paleozoic (ca. 510–470 Ma) rocks. Late Paleozoic rocks are not present in this area, suggesting either the interruption by regional rising over seawater level or the intensive erosion after the Mesozoic period. Folded sedimentary rock cover is present at the southwestern part in this area and are divided into NW– and NE–series; the limbs of NW–trending folds dip at 20–50° to NE and SW, whereas limbs of NE–trending folds dip at 10–20° to NW and SE, indicating that these folds may be formed under a NE–SW–trending compressional stress. The folds are symmetric or asymmetric open plunging folds, or overturned. The series of NNE– and NW–trending faults cut cross this area; two large–scale NNE–trending faults are with mylonitized rocks, indicating the probable NW–SE–trending tectonic extension. The western NNE–trending fault is a flat dip normal fault and often curved at surface, while the eastern NNE–trending fault is a steep dip normal fault and represents nearly straight line on the regional geological map (Fig. 1b). The NW–trending faults in the southwestern part of YUZ are dextral strike-slip faults and generally dip 70°–80° to the northeast with a length of about 8–20 km and width of 1–2 m, indicating that these faults may formed during the NW–SE–trending extensional regime. Four intrusive plutons with various ages are exposed in this area: ca. 2500–2200 Ma Neoarchean Ryonhwasan Complex (NRC), ca. 785–760 Ma Neoproterozoic Yonsan Complex (NYC), ca. 287–170 Ma Jurassic Tanchon Complex (JTC) and ca. 166 ± 4 Ma Cretaceous Amnokgang Complex (CAC) (Han and Kim,

2.2.2. Structures The major large–scale folds in the study area are three NW–trending folds: the Sinsong anticline, the Hoechang syncline and the Sungin anticline (Fig. 2a and Fig. 4c–d). These structures were formed in response to the NE–SW–trending compressional stress regime in the early Mesozoic. The limbs of the Sinsong anticline and Hoechang syncline were also folded extensively at the same time, resulting in the formation of small–scale NW–trending M−type folds that have a wavy configuration. This deformation event resulted in extensive interlayer gliding and foliation, which may be favorable for hydrothermal flow and mineralization. The NW–trending dextral strike steep dip normal faults (Jokpidong, Namhang and Sinjagae faults) (Fig. 4e) and NS–trending Rihwadong normal fault cut through the study area. In addition, thrust faults dip at 15°–20° to the southwest along the boundary between the Proterozoic strata. The NW–trending faults continued in parallel each other and developed in central part of the study area, which are characterized by shattered breccia and they represent ore–controlling structures. These faults generally trend northwest, dip 70°–80° to the northeast and were repeatedly reactivated from the Mesozoic to the Cenozoic era. The area between the Namhang and Sinjagae faults is densely dissected by numerous subsidiary faults and ore bodies are also cut by these faults. Moreover, along the contact between the basement (NRMB) and the Suan Formation (quartzite), cataclastic structures are developed and filled by mylonites (Fig. 4f). The NS–trending normal fault dipping at 60°–70° to the west is exposed in the western part of the 2

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Fig. 1. Location of the Yangdok uplift zone in DPR Korea (a) (modified after Pak et al., 2019) and its simplified regional geological map (b). The blue box indicates the area shown in Fig. 2. TO, Tumangang orogen; KM, Kwanmo massif; HRB, Hyesan–Riwon basin; KMCM, Kilju–Myongchon continental margin; RM1, Rangnim massif; PB, Pyongnam basin; RB, Rimjingang Belt; GM, Gyonggi massif; OB, Okchon basin; RM2, Ryongnam massif; RCM, Rakdonggang continental margin.

chlorite are secondary minerals. The third group outcropped in contact between the first group and the JTC at the eastern part of YUZ, consists of fine– to medium–grained alaskite and coarse–grained alaskite. The main minerals are plagioclase (40%), quartz (30%) and K–feldspar (30%), and the secondary minerals are epidote and sericite. The ages of the NRC obtained by zircon U–Pb ICP–MS dating are in the interval 2500–2200 Ma (Han and Kim, 2005). The JTC (Yangdok granitoid) is widely exposed in the eastern part of the study area. It consists of second and third phases according to the formation order and the petrographic and petrochemical signatures. The older second phase is mostly exposed with an area of about 245 km2, whereas the younger third phase is scattered at the southeastern part of the study area as dikes and small intrusions. The second phase comprises coarse–grained granite, porphyritic granite, medium–grained granite and fine–medium–grained granite. These rocks are composed of quartz (35%), plagioclase (30%), microcline (30%), biotite and muscovite (5%) as main minerals with massive structure and minor magnetite (Fig. 3j). The accessory minerals comprise ilmenite, zircon and rutile, and the secondary minerals are sericite and chlorite. The

study area and cuts the NW–trending faults, and is not related to polymetallic mineralization. 2.2.3. Magmatic rocks There are three main episodes of magmatism in the Hoechang district: the Neoarchean Ryonhwasan Complex (NRC), the Jurassic Tanchon Complex (JTC) and the Cretaceous Amnokgang Complex (CAC). The NRC is divided into two groups based on petrographic and petrochemical features: the first and third groups. The first group is exposed extensively in the northeastern part of study area and displays gradual contacts with the NRMB throughout the migmatite zone, and is cut by the JTC with which it has a relatively sharp contact. The first group consists of gneissic granite, gneissic garnet granite, gneissic garnet–cordierite granite, biotite granite, garnet–cordierite granite and garnet granite. Amongst these rocks, gneissic granite is dominant and consists of quartz (30–35%), plagioclase (40–55%), microcline (10–22%) and biotite (3–5%), and accessory minerals including garnet, cordierite, graphite, apatite, zircon, titanite and monazite. Sericite and 3

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Fig. 2. Simplified geological map (modified after Ju et al., 1997) (a) of the study area and cross section (b) showed the half part of dome structure.

third phase consists of fine–grained granite and fine–grained muscovite granite with massive structure. Main minerals are quartz (22–35%), K–feldspar (30–50%), plagioclase (20–30%), biotite (3%) and muscovite (2%) with minor magnetite. The accessory minerals are ilmenite, zircon and monazite. Based on the K–Ar method, the age of the JTC is ca. 287–170 Ma (Peak et al., 1993). The CAC (Hoechang granitoid) exposed in the western part of the study area has a surface area of about 28 km2, and consists of granite porphyry and quartz porphyry (Fig. 3k and l, Fig. 4b). The granite porphyry intruded the Mesoproterozoic Group during an early stage of the Cretaceous magmatism. It is cut by NW–trending faults which were repeatedly reactivated during the late Cretaceous period. The wall rocks are host to skarns and exhibit serpentinization, silicification, chloritization and sericitization. The age of the CAC determined by the K–Ar method, is ca. 166 ± 4 Ma (Peak et al., 1993).

mineralization, Fig. 5b). These ore bodies have very low Au content and disseminated pyrite. This mineralization is discussed in Section 5 in more detail. The economically important gold–polymetallic mineralization in the Songhung deposit is related to the late mineralization event and occurred during the tectonic extension deformation from the Jurassic to Cretaceous periods. There are 9 main ore bodies that can be divided into two types in terms of mineralization style; the quartz–sulfide vein and the metasomatic vein (Table 1). The tabular or lenticular quartz–sulfide veins (No.2, No.3, No.5, No.7, No.8 and No.9 ore bodies) are strictly controlled by the fault between the basement and sedimentary rocks and the NW–trending fault systems, consisting mainly of quartz and sulfide minerals. They are mainly hosted by the Mesoproterozoic Jikhyon and Sadang–u groups, except for the No.8 and No.9 bodies which are hosted in the NRC. The contacts between ore bodies and wall rocks underwent various alterations. The No.1 and No.4 ore bodies of second type are ferric sulfide metasomatic veins, resulted from interaction between hydrothermal fluid and limestone along the boundary of lower quartzite and lower limestone layers in the Mesoproterozoic Jikhyon Group. In contrast, the No.6 ore body has a second type of siderite–sulfide (lead–zinc) metasomatism between the lower and upper limestone of the Sinsong Formation. The Au grade is generally higher in the first type ore bodies. The main characteristics of the ore bodies listed in Table 1.

2.2.4. Ore bodies, mineralogy and alteration of wall rocks The polymetallic mineralization in the Songhung deposit can be ascribed to two early and late mineralization events. The early mineralization took place with the continental compressional deformation during the early Mesozoic period, in which ore bodies are hosted exclusively in bedded limestone or dolomite of Sinsong Formation and cross–cut by the barren quartz veins formed in the first stage of following magma–derived hydrothermal mineralization (e.g., late 4

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Fig. 3. Typical sedimentary and intrusion rocks in the Songhung deposit: (a) quartzite of the Suan Formation, (b) limestone and (c) micaceous schist of the Sinsong Formation, (d) quartzite of the Mulgumsan Formation, (e) micaceous limestone of the Hoechang Formation; (f) limestone and (g) dolomite of the Naedong Formation; (h) limestone and (i) conglomerate of the Junghwa Formation, (j) granite of the Mesozoic Yangdok intrusion, (k) granite porphyry and (l) quartz porphyry of the Mesozoic Hoechang intrusion. Scale bars are 2 cm.

chalcopyrite, sphalerite, galena and native gold. Pyrrhotite is demonstrated by highly heterogeneous distribution in the ore bodies and has no significant gold content, and is associated with pyrite and chalcopyrite. Pyrrhotite crosscuts the pyrite and arsenopyrite, and was metasomatized by latter–formed sulfides such as chalcopyrite, sphalerite and galena. Chalcopyrite is evenly distributed in all ore bodies and is main auriferous mineral. It is associated with pyrite and pyrrhotite, and in many cases, occurs along the cleavage planes of siderite. The Au grade increases in chalcopyrite with a veinlet and a spotted texture in cataclastic quartz, but decreases in proportion in the massive chalcopyrite. Sphalerite occurs in the metasomatic vein together with galena and their grains contain exsolution dots of chalcopyrite, indicating that these minerals were formed at the same time. Sphalerite in quartz–sulfide vein ore bodies has higher Au content but has lower gold

The main minerals include pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, bismuthinite, tetrahedrite and native gold. Pyrite is one of main auriferous minerals and may account for almost 80–90% of total sulfides. Pyrite occurs as dispersed coarse idiomorphic and hypidiomorphic grains in quartz veins with massive, veinlets and vuggy structures, or as disseminated hypidiomorphic grains in metasomatic vein (Fig. 6a, c and g), and sometimes aggregates intergrown with pyrrhotite and ankerite. Most of the coarse–grained pyrites have a cracking and cataclastic texture, and are cross–cut by chalcopyrite and galena which formed later in the paragenetic sequence. Although the amount of arsenopyrite in ores is lower than pyrite, it is ubiquitous in quartz vein and it represents the most important auriferous mineral after pyrite in terms of Au content. Arsenopyrite occurs as a columnar crystal in quartz vein but their crystals are often cracked and contain 5

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Fig. 4. Geological features of the Songhung deposit. (a) quartzite of the Suan Formation; (b) quartz porphyry of Hoechang intrusion; (c) folded and cataclastic rocks of the Suan Formation; (d) overturned fold in the Sinsong Formation (chlorite sericite schist); (e) Sinjagae normal steep fault; (f) cataclastic structure between the basement and the Sinsong Formation.

Fig. 5. Metasomatic vein from No.4 ore body (from a to c) and No.6 ore body (d); quartz–sulfide vein from No. 7 (e) and No. 9 ore body (f).

quartz stage is characterized by massive white quartz and minor disseminated pyrite and siderite, and silicification. The auriferous sulfide–quartz stage contains grey quartz and sulfides, and is subdivided into ferric sulfide and polymetallic sulfide substages according to the mineral paragenesis. The ferric sulfide substage is characterized by abundant pyrite, arsenopyrite and pyrrhotite as well as minor ankerite, siderite and native gold. The ankerite occurs as a veinlet in the sulfide–quartz vein or independent vein with native gold. Here, native gold did not crystallize with ankerite, but the favorable regime for precipitation of native gold may be created by interaction between ore–forming fluid and the ankerite when auriferous fluid is transferred

content in the metasomatic ore bodies. Galena crosscuts all sulfide minerals, and contains major silver and minor gold. Galena mainly occurs as idiomorphic crystal or interstitial phase among the other sulfides. The sequence of gold crystallization can be considered into two stages: the early stage (ferric sulfide) and the late stage (quartz–sulfide). The gold from the early stage is mainly native gold with angular shape in quartz and pyrrhotite (Fig. 6b), whereas the gold from the late stage is dispersed in sulfides such as pyrite, galena and sphalerite. Three mineralization stages in the Songhung deposit are evident on the basis of mineral assemblages, ore texture and structure: barren quartz, auriferous sulfide–quartz and calcite stage (Fig. 7). The barren

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0.5

0.3 0.9

SW 5–20, 1500

SW 5–20, 50 NE 5–15, 300

3 10

0.9 SW 5–20, 3000

4.5–5

1 SW 5–20,200–2000

10

0.5 SW 5, 80–100

10–15

0.6 SW 5–20, 200–500

1

0.4 SW 5–20, 100–400

15–17

0.3 SW 5–20, 200–500

10–15

Mean thickness /m

2–3

along the contact of ankerite. The ore bodies are dominated by the ankerite and siderite instead of pyrite when hosted rocks are represented by limestone and dolomite. When the ore bodies are hosted in siliceous rocks such as quartzite and siliceous slate, the main mineral is represented by pyrrhotite with low Au content. The more abundant pyrrhotite and siderite in ores, the lower Au grade they are. The wall rocks underwent silicification and chloritization. The polymetallic sulfide substage is the most important for gold–polymetallic mineralization and is dominated by pyrite, chalcopyrite, sphalerite, galena and native gold. The alteration of wall rock is related to this substage, and consists of rocks influenced by chloritization, silicification and sericitization. The calcite stage is characterized by barren calcite vein with minor quartz and pyrite. 3. Sampling and analytical methods

Vein Vein Silici., Serici., Carbon. Silici., Serici.

The major element analysis was performed on the Yangdok (12) and Heochang (9) granitoid intrusions. The number in brackets is the number of samples. Samples were crushed into small grains (0.3–0.5 mm), handpicked under a binocular microscope to remove visible impurities, ultrasonically cleaned in distilled water and dried. The samples were ground in agate mortar and the powder was used for analysis. For major element analysis, the samples were mixed well with Li2B4O7 and melted by electric melting instrument. The analysis was carried out using X–ray fluorescence (XRF) spectrometer with 0.1 wt% detection limits at the Analytical Institute of our university. Loss on ignition (LOI) is the weight difference between burning and very high temperature heating.

NW, 50 NW, 500 Quartz–sulfide Quartz–sulfide Gneiss (NRMB) Gneiss and schist (NRMB) No.8 No.9

NW, 2000 Vein Quartz–sulfide Quartzite (Suan) No.7

NW, 4500 Tabular Limestone(Suan) No.6

NW, 1500–3000 Quartz–sulfide Quartzite (Sinsong) No.5

Vein

NW, 100–120 Lenticular Metasomatic Limestone (Sinsong) No.4

Silici., Serici., Carbon., Chlori. Silici., Serici., Carbon., Chlori. Silici., Serici., Carbon.

NW, 400–700 Tabular, Vein

Silici., Serici., Carbon., Chlori. Silici., Serici., Carbon. Quartz–sulfide Quartzite (Mulgumsan) No.3

3.2. Stable isotope 3.2.1. Oxygen and Carbon isotope The oxygen and carbon isotopic analysis was carried out for siderite (10), ankerite (3) and limestone (3). The samples were crushed and powdered, and 20 mg of several samples was decomposed in 100% phosphoric acid at 30 °C in order to obtain carbon dioxide gas. The oxygen and carbon isotopes were analyzed on a MAT–251EM mass spectrometer at the Sukchon Analytical Institute and the isotope data were reported in per mil relative to the Standard Mean Ocean Water (SMOW) standard for oxygen and to the Pee Dee Belemnite (PDB) standard for carbon. The analytical errors of the δ13C and δ18O were less than 0.2‰.

Pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, pyrrhotite, native gold Pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, pyrrhotite Pyrite, pyrrhotite, chalcopyrite with dark grey Pyrite, pyrrhotite, minor galena Metasomatic

Pyrite, arsenopyrite, chalcopyrite, galena, ankerite, native gold Pyrite, pyrrhotite, siderite, minor chalco– pyrite

NW, 300–700 Tabular, Vein Silici., Carbon.

Pyrite, arsenopyrite, magnetite, chalco– pyrite, galena, ankerite Pyrite, arsenopyrite, chalcopyrite, galena Quartz–sulfide No.2

Tabular Silici., Carbon. Siderite, native gold, minor sulfide Metasomatic

Limestone and dolomite (Naedong) Sericite schist (Hoechang) No.1

Ore body morphology Wall rock alteration Ore type Host rocks

Main mineral

NW, 400–700

3.1. Major elements of granitoids

Ore body

Table 1 Characters of the ore bodies in the Songhung deposit.

Strike (length/m)

Dip (°) and length (m)

Au grade (g/ t)

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3.2.2. Sulfur isotope Sulfur isotopic analysis were performed on pyrite (9), galena (2), chalcopyrite (2) and arsenopyrite (2) from No.1, No.4 and No.5 ore bodies (−70, −120 and −210 levels). Samples were crushed to 0.1–0.3 mm and ultrasonically cleaned in distilled water and dried. The pure sulfide minerals were handpicked under a binocular microscope and were converted to SO2 by heating to 1200 °C in a stream of oxygen, and then the SO2 samples were analyzed directly using a MAT–251EM mass spectrometer at the Sukchon Analytical Institute. Isotope data are reported in the conventional δ notation relative to the Vienna Canon Diablo Troilite (V–CDT) sulfide. The analytical error is about ± 0.3‰. 3.3. Lead isotope Ten samples of pyrite (4), galena (2), arsenopyrite (2) and chalcopyrite (2) from the quartz–sulfide vein (of No.5 body) and four samples of pyrite from metasomatic vein (−70, −120 and −210 levels of No.1 and No.4 bodies) are collected for lead isotopic composition analysis. The characteristic of samples shows in Table 3. The samples were crushed to 0.1–0.3 mm and the pure sulfide minerals were handpicked under a binocular microscope to avoid impurity from surrounding 7

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Fig. 6. Main minerals in ores of the Songhung deposit.

respectively) and Na2O/K2O (0.78–0.96, 0.71–0.85 and 0.63–0.66, respectively) ratios. The values of A/NK and A/CNK ratios are 1.3–1.59 and 1.09–1.11 for coarse–grained granite, 1.18–1.4 and 1–1.14 for medium–grained granite and 1.09–1.55 and 1–1.38 for fine–grained granite (Table 2), respectively. The Heochang intrusion (granite, granite porphyry, felsite, fine–grained granite and quartz porphyry) is characterized by high contents of SiO2, Al2O3 and K2O. The contents of Na2O, MgO, MnO, CaO, Fe2O3, FeO and P2O5 are relatively low in comparison with the Yangdok intrusion. The values of CaO/Na2O (0.34–0.89 for granite, 0.28–0.55 for granite porphyry, 0.26–0.30 for felsite, 0.14–0.68 for fine–grained granite and 2.31 for quartz porphyry) and Na2O/K2O (0.48–0.76 for granite, 0.63–0.67 for granite porphyry, 0.53–0.56 for felsite, 0.62–1.02 for fine–grained granite and 0.31 for quartz porphyry) are similar to the Yangdok intrusion. The values of A/NK and A/CNK ratios are 1.01–1.61 and 0.93–1.86 respectively but these values in quartz porphyry are very higher (3.34, 1.86, respectively), which may be caused by high content of Al2O3 and low content of Na2O or by analytical error (Table 3).

minerals and powdered in an agate mortar. The powdered pyrite (50 mg), arsenopyrite (50 mg), chalcopyrite (50 mg) and galena (5 mg) samples were dissolved in Teflon vials with a HF + HNO3 solution on heating plate with 140 °C for 5 days and added in these vials the dilute HBr as eluant with anion–exchange resin to purify Pb. The lead isotopic compositions were measured using MAT–261 mass spectrometer at the Sukchon Analytical Institute with an analytical precision better than ± 0.2‰. The values of standard material NBS 981 measured in this study are 206Pb/204Pb = 16.945 ± 0.003 (2σ), 207 Pb/204Pb = 15.489 ± 0.003 (2σ), 208Pb/204Pb = 36.701 ± 0.005 (2σ). 4. Results 4.1. Major elements of the Mesozoic Yangdok granitoid and Hoechang granite porphyry Analytical data for the major element compositions of the Yangdok and Heochang intrusions are listed in Tables 2 and 3, respectively. The low LOI values (from 0.47 to 1.0%) and the presence of unaltered hornblende, biotite and plagioclase indicate that all samples are relatively fresh. The Yangdok intrusion (coarse–, medium– and fine–grained granite) has high contents of SiO2, Al2O3, K2O and Na2O, while the contents of MgO, MnO and CaO are relatively low. Furthermore, these rocks are characterized by moderate contents of Fe2O3, FeO and P2O5, and low values of CaO/Na2O (0.33–0.69, 0.18–0.44 and 0.16–0.44,

4.2. Carbon and oxygen isotope The carbon and oxygen isotopic compositions of minerals (siderite, ankerite) and rocks (limestone) from various ore bodies and levels are shown in Table 4. The δ13C values range from −8.35 to −12.59‰ for siderite, from −2.51 to −2.88‰ for ankerite and from −0.55 to −2.91‰ for 8

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Fig. 7. Mineralization stage in the Songhung deposit.

limestone. As shown in Table 4, the δ13C values decrease generally from upper to low levels and these in siderite are very low (−12.59‰) in comparison with those (−2.88 and −2.91‰, respectively) of ankerite and limestone. The δ18O values of the minerals and rocks have the following ranges: 12.2–15.8‰ with an average 13.82‰ for siderite; 14.1–15.3‰ with an average 14.63‰ for ankerite and 19.8–20.5‰ with an average 20.02‰ for limestone. In general, the δ18O values of limestone are higher than those of siderite and ankerite.

quartz–sulfide vein are low in comparison with those of pyrite. Furthermore, the δ34SV–CDT values of all sulfides display slight differentiation, from upper to low levels. The Pb isotopic data of sulfides in the Songhung deposit show a uniform trend of Pb isotope compositions and 206Pb/204Pb isotopic ratios vary from 17.521 to 18.366, 207Pb/204Pb from 15.610 to 15.659 and 208Pb/204Pb from 38.129 to 39.218 (Table 5). From quartz–sulfide vein, pyrite separates have 18.312–18.366 for 206Pb/204Pb, 15.631–15.656 for 207Pb/204Pb and 39.203–39.218 for 208Pb/204Pb ratios. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios for galena separates are 18.225–18.259, 15.654–15.659 and 38.815–38.835, respectively. From metasomatic vein, pyrite separates have 206Pb/204Pb ratios ranging from 17.521 to 17.758, 207Pb/204Pb ratios ranging from 15.610 to 15.621 and 208Pb/204Pb ratios ranging from 38.129 to 38.147.

4.3. Sulfur and lead isotope The sulfur and lead isotope compositions of sulfides from various levels and ore types in the deposit are listed in Table 5. The δ34S V–CDT values of pyrite in quartz–sulfide vein range from 6.1 to 8.1‰, with an average of 7.04‰, while these in metasomatic vein range from 7.1 to 8.8‰, with an average 8.05‰. On the other hand, the δ34SV–CDT of galena (6.5‰), arsenopyrite (6.3–8.2‰, with an average 7.25‰) and chalcopyrite (6.1–6.5‰ with an average 6.3‰) in 9

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Table 2 Major element compositions of Yangdok intrusion (wt%). Coarse–grained granite

Medium–grained granite

Fine–grained granite

SiO2

72.12

70.88

73.12

70.76

73.84

71.12

75.24

78.20

72.66

73.36

71.31

70.70

Al2O3 TiO2 Fe2O3 FeO MnO CaO MgO K2O Na2O P2O5 LOI Total Na2O + K2O CaO/Na2O Na2O/K2O A/NK A/CNK

14.35 0.85 1.22 2.75 0.05 2.22 0.98 3.37 3.24 0.22 0.99 102.36 6.61 0.69 0.96 1.58 1.1

14.89 0.38 0.84 1.21 0.01 1.73 0.33 4.43 3.45 1.11 0.67 99.93 7.88 0.50 0.78 1.41 1.09

14.08 0.27 0.95 0.33 0.04 1.16 0.45 4.55 3.55 1.10 0.83 100.43 8.10 0.33 0.78 1.3 1.09

15.20 0.54 1.80 0.44 0.06 2.33 0.85 3.60 3.38 0.19 0.72 99.87 6.98 0.69 0.94 1.59 1.11

14.54 0.24 1.33 0.75 0.02 1.46 0.57 4.73 3.40 0.12 0.81 101.81 8.13 0.43 0.72 1.35 1.08

14.61 0.18 0.22 1.63 0.23 1.71 0.70 4.63 3.92 0.06 0.47 99.48 8.55 0.44 0.85 1.26 1

13.22 0.18 1.10 0.90 0.05 0.63 0.30 4.94 3.50 0.14 1.0 101.20 8.44 0.18 0.71 1.18 1.07

14.56 0.36 0.88 1.39 0.05 1.37 0.58 4.58 3.24 0.10 0.72 106.03 7.82 0.42 0.71 1.4 1.14

14.90 0.34 0.39 1.39 0.05 1.37 0.30 4.94 3.10 0.07 1.33 100.84 8.04 0.44 0.63 1.41 1.15

13.43 0.26 0.38 1.32 0.02 0.99 0.33 4.70 3.10 0.11 0.76 98.76 7.80 0.32 0.66 1.31 1.12

14.01 0.20 0.93 1.70 0.21 0.61 0.39 6.00 3.80 0.10 0.78 100.04 9.80 0.16 0.63 1.09 1

16.94 0.10 0.94 1.05 0.20 0.73 0.42 5.15 3.20 0.08 0.76 100.04 9.80 0.16 0.63 1.55 1.38

5. Discussion

Table 4 C and O isotopic compositions of minerals and rocks from the Songhung deposit.

5.1. Petrochemistry of Mesozoic granitods Granites are commonly divided into I–, S– or A–types according to the nature of their protolith, their petrographic and geochemical features (Chappell and White, 1974; Loiselle and Wones, 1979). The Mesozoic granitoids (Yangdok and Hoechang intrusions) display similar petrochmical features. In detail, the Yangdok and Hoechang intrusions have elevated SiO2 with an average 72.78 wt% and 73.59 wt%, respectively and the Haker diagram (Figs. 8 and 9a) show distinct evolutionary trends with increasing SiO2 content: Al2O3, TiO2, FeO, MnO, CaO, MgO, K2O and Na2O contents decrease, except the Fe2O3 content. Although P2O5 contents are very low and have random values, these of the Yangdok intrusion decrease with increasing of content SiO2, while these of the Heochang intrusion sligthly increase, indicating that the former is formed from more siliceous melts and may be belongs to I–type granite, whereas the latter may be belongs to S–type (Chappell and White, 1974). These rocks are K–enriched (K2O contents with an average 4.64 and 4.28, respectively) and show enrichment of K2O over Na2O (the values of Na2O/K2O are 0.735 and 0.625, respectively), indicating either crust–derived rocks or assimilation and fractional crystallization processes (Esperanca et al., 1992). These rocks are alkali–enriched (K2O + Na2O = 6.61–9.80 wt% for Yangdok intrusion and

No

Ore body and Levels

1 2 3 4 5 6 7 8 9 10 11 12 13 14

No.1, No.1, No.1, No.3, No.3, No.4, No.4, No.6, No.6, No.6, No.1, No.1, No.1, No.1,

15

No.1, −210 m

16

No.1, −210 m

−70 m −120 m −210 m −70 m −210 m −120 m −120 m −70 m −70 m −120 m −70 m −70 m −120 m −70 m

Mineral and rock

δ13CPDB (‰)

δ18OSMOW (‰)

Siderite Siderite Siderite Siderite Siderite Siderite Siderite Siderite Siderite Siderite Ankerite Ankerite Ankerite Limestone (wall rock) Limestone (wall rock) Limestone (wall rock)

−10.2 −11.09 −11.78 −9.55 −12.59 −9.31 −10.03 −9.63 −8.35 −9.28 −2.51 −2.88 −2.53 −0.55

13.6 15.8 13.9 12.2 13.8 13.3 14.3 13.5 12.3 15.5 14.5 14.1 15.3 20.5

−2.91

19.8

−2.85

20.3

Table 3 Major element compositions of Heochang intrusion (wt%). Granite SiO2 Al2O3 TiO2 Fe2O3 FeO MnO CaO MgO K2O Na2O P2O5 LOI Total Na2O + K2O CaO/Na2O Na2O/K2O A/NK A/CNK

71.88 13.36 0.01 1.60 0.67 0.06 1.92 0.20 4.45 2.15 0.01 2.39 98.70 6.60 0.89 0.48 1.59 1.13

Granite porphyry 74.33 13.68 0.24 1.65 1.10 0.07 1.1 0.17 4.2 3.20 0.63 0.58 100.95 7.40 0.34 0.76 1.38 1.15

73.78 13.36 0.01 1.16 1.57 0.09 1.37 0.1 3.95 2.5 0.01 0.5 98.40 6.45 0.55 0.63 1.58 1.22

Felsite 74.84 13.43 0.01 0.54 1.17 0.02 0.89 0.29 4.75 3.2 0.05 0.5 99.69 7.95 0.28 0.67 1.28 1.11

74.21 13.24 0.3 2.37 1.43 0.06 0.68 0.05 4.75 2.66 0.23 0.65 100.63 7.41 0.26 0.56 1.38 1.22

10

73.88 13.83 0.01 1.83 0.46 0.07 0.69 0.07 4.36 2.3 0.02 0.63 98.15 6.66 0.30 0.53 1.61 1.41

Fine–grained granite

Quartz porphyry

73.0 15.56 0.01 0.59 0.35 0.01 2.06 0.51 4.87 3.04 0.04 0.78 100.82 7.91 0.68 0.62 1.5 1.11

70.28 17.37 0.01 0.25 0.01 0.01 2.31 1.18 3.25 1.0 0.01 1.7 97.38 4.25 2.31 0.31 3.34 1.86

76.14 11.06 0.07 3.04 0.71 0.03 0.56 0.25 3.93 4.0 0.03 1.5 101.32 7.93 0.14 1.02 1.01 0.93

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Table 5 Sulfur and lead isotopic compositions of sulfide minerals from the Songhung deposit. No

Level

Ore type

Mineral

δ34SV–CDT (‰)

206

207

208

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

−70 −70 −120 −120 −210 −70 −120 −70 −120 −70 −120 −70 −70 −120 −210

Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Quartz–sulfide vein Metasomatic vein Metasomatic vein Metasomatic vein Metasomatic vein

Pyrite Pyrite Pyrite Pyrite Pyrite Galena Galena Arsenopyrite Arsenopyrite Chalcopyrite Chalcopyrite Pyrite Pyrite Pyrite Pyrite

8.1 6.1 6.2 7.1 7.7 6.5 6.5 6.3 8.2 6.5 6.1 8.2 7.1 8.1 8.8

18.315 18.321 18.366 18.312

15.631 15.656 15.632 15.642

39.218 39.215 39.205 39.203

18.225 18.259

15.654 15.659

38.835 38.815

17.758 17.749 17.717 17.521

15.610 15.618 15.621 15.611

38.147 38.138 38.133 38.129

Pb/204Pb

Pb/204Pb

Pb/204Pb

4.25–7.95 wt% for Hoechang intrusion) and classified as high–K calc–alkaline in a K2O vs. SiO2 diagram (Fig. 9b). On a Na2O + K2O vs. SiO2 diagram, these rocks plot in the field of granite (Fig. 9c). The Yangdok intrusion has higher MgO content (0.52 wt% in average) than the Heochang intrusion (0.31 wt% in average). On an A/NK vs. A/CNK diagram (Fig. 9d), most of the values of A/CNK for the Yangdok intrusion fall in metaluminous and weakly peraluminous field (e.g., 8 of 12), while for the Hoechang intrusion, six values of A/CNK fall in peraluminous field and three values belong in metaluminous and weakly peraluminous. These intrusions are K-rich calc-alkaline series, indicating that these intrusions may originate not only by convergence of two continental lithospheres, but also by convergence of oceanic and continental lithospheres or relaxation of a continental lithosphere. Furthermore, the values of Fe2O3/FeO (average 1.16 for Yangdok and 2.09 for Hoechang intrusion, except value of quartz porphyry expected abnormal) indicate that these intrusions may be I-type granite (Abdel, 2016). It suggests that the Yangdok and Hoechang intrusions are a crust–mantle derived or a presence of assimilation and fractional crystallization processes and may be related with the gold–polymetallic mineralization, as well as Sn and W.

(mean = +8.05‰) of pyrite from metasomatic vein, indicating that the oxygen fugacity of the ore–forming fluid for the quartz–sulfide vein was higher than those for metasomatic vein. On the other hand, the positive δ34S values of sulfides may be caused by local reduction of the fluid due to fluid–carbonate wall rock interaction or by reactions of the hydrothermal fluids with 34S–enriched sulfides in wall rocks (McCuaig and Kerrich, 1998). In general, a magmatic hydrothermal deposit is closely associated with igneous intrusions that were emplaced at relatively shallow depths and the felsic intrusive rocks have δ34SV–CDT values of +0.2 to +5.8‰ and the δ34S value of the fluid in equilibrium with granite magma (δ34S = 0.0‰) is about 5.0‰ (Ohmoto and Rye, 1979). It indicates that the source of S in the Songhung deposit may be related with granitoid. From above discussion and Fig. 10 modified after Chai et al. (2016), the characteristics of sulfur isotopic compositions in the Songhung deposit are as follows: (1) sulfur within sulfide minerals was derived not only from a magmatic source, but also the wall rocks with 34S–enriched sulfides and the meteoric origin. (2) the ore–forming fluid had low oxygen fugacity, i.e., reduced regime and the fluid from which is deposited the metasomatic vein was characterized by much lower oxygen fugacity.

5.2. Possible sources of S and Pb

5.2.2. Lead The genesis of lead in gold deposits can be illustarated by the lead isotope ratios in sulfide minerals (Gulson, 1986; Zhang et al., 2014). The concentrations of U and Th in sulfide minerals are usually very low, therefore, the radiogenic Pb isotopes are also insignificant (Zhang, 1992). Furthermore, the initial Pb isotopic composition of the ore–forming fluids in gold deposit can be represented by the least radiogenic composition of sulfide minerals (Ding et al., 2014; Ho et al., 1994). McNaughton and Groves (1996) suggested that galena is potential mineral for determination of initial Pb isotopic composition of lode–gold systems. Pyrite may be also as the second mineral to establish the initial Pb isotopic composition (Ho et al., 1994). Lead isotope ratios of pyrite and galena in quartz–sulfide vein and those of pyrite in the metasomatic vein are listed in Table 5 and illustrated in Fig. 11. The pyrite and galena in quartz–sulfide vein have large variation in Pb isotopic composition, whereas those of the pyrite in metasomatic vein is represented by limited variation and a uniform trend. Pyrite in the quartz–sulfide vein generally more enriched in radiogenic lead than the pyrite disseminated in the metasomatic vein (Table 5), indicating that the Pb is derived from a source enriched in both U and Th and a mixing source of magmatic and assimilated continental upper crust. Meanwhile, the depletions in radiogenic lead of the pyrite in the metasomatic vein indicate another source of the Pb. 235 U and 238U decay to 207Pb and 206Pb daughters with various half–life of 0.7038 Ga for 235U and 4.468 Ga for 238U, respectively, and therefore, 207Pb/206Pb ratios can be used to determine whether the Pb

5.2.1. Sulfur The Songhung deposit is characterized by a dominance of sulfides (pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena and tetrahedrite) as main minerals and a lack of sulfate minerals, indicating that the total sulfur isotope composition of the hydrothermal fluid, i.e., δ34S∑S≈δ34Ssulfide can be represented by the δ34S value of sulfide mineral (Ohmoto, 1972) and the deposit formed under a reduced condition. The absence of sulfate minerals in the deposit indicates that the fO2 of the ore–forming fluid system was low and the sulfur in hydrothermal fluids existed mainly as HS− and S2− (Hoefs, 2004; Ohmoto and Rye, 1979). As shown Table 5 and Fig. 10, the δ34SV–CDT values of pyrite, galena, arsenopyrite and chalcopyrite in this study have relatively enriched heavy sulfur isotope with δ34S values in a narrow range of +6.1 to +8.8‰ (Fig. 10). In detail, the δ34SV–CDT values of pyrite (+6.1 to +8.1‰, mean = +7.04‰, n = 5), galena (+6.5‰, n = 2), arsenopyrite (+6.3 to +8.2‰, mean = 7.25‰, n = 2) and chalcopyrite (+6.1 to +6.5‰, mean = 6.3‰, n = 2) from the quartz–sulfide vein and pyrite (+7.1 to +8.8‰, mean = 8.05‰, n = 4) from metasomatic vein, indicating that these values are inconsistent with the mantle–derived S (~0‰; Chaussidon and Lorand, 1990; Hoefs, 2015), suggesting that the fluid redox state was below the SO2/H2S boundary and the dominant sulfur species in the fluids was H2S (Ohmoto and Rye, 1979). Furthermore, the δ34SV–CDT values (mean = +7.04‰) of pyrite from quartz–sulfide vein are slightly lower than those 11

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Fig. 8. Chemical variation diagrams for the Mesozoic granitoids in the Hoechang district.

between the upper crust and the orogen Pb evolutionary curves (207Pb/204Pb vs. 206Pb/204Pb) and between the lower crust and the orogen Pb evolutionary curves (208Pb/204Pb vs. 206Pb/204Pb), indicating that the lead was originated from crustal reservoirs but other Pb sources can be existed. These Pb isotope ratios of two different ores have a clear linear relationship, indicating a similar Pb isotope evolution. Although the Pb isotopic composition of the Proterozoic and Paleozoic rocks and the Mesozoic intrusions in the study area is not

within a sample is derived from an old or new source (Faure, 1986; Zhou et al., 2001). The average 207Pb/206Pb ratios of pyrite and galena in the quartz–sulfide vein are 0.85 and 0.86 respectively, whereas those of pyrite in the metasomatic vein show slightly high value with an average 0.88. This suggests that the Pb in the metasomatic vein were extracted from Proterozoic rocks with an old Pb isotope signature. Based on the plumbotectonic model of Zartman and Doe (1981) (Fig. 11), the Pb isotope values in all sulfide minerals in this study plot 12

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Fig. 9. Na2O vs. SiO2 diagram (Rollinson, 1993) (b); K2O vs. SiO2 diagram (c); K2O + Na2O vs. SiO2 diagram (Middlemost, 1994) (d); A/NK vs. A/CNK plot (Maniar and Piccoli, 1989) for the Mesozoic granitoids. A/CNK = Al2O3 / (CaO + Na2O + K2O) molar, A/NK = Al2O3 / (Na2O + K2O) molar.

that some of the Pb could been derived from magma and leached out from sedimentary rocks by circulating fluid, which was incorporated with mineralized fluid. The Pb isotope values of sulfide minerals in the metasomatic vein plot in field of Proterozoic strata, indicating that the source of Pb may be related with the Proterozoic rocks. Generally, this indicates that the source of Pb in the Songhung deposit derived from a mixing two endmembers or a alterative leaching of a single source of Pb (Cunha et al., 2007; Zeng et al., 2014).

considered yet, the source of Pb in sulfide minerals may be expected by the Pb isotope diagrams (Fig. 11) of Chai et al. (2016), on which are exhibited the contours of Pb isotopic compositions in the Proterozoic rocks and Mesozoic intrusions based on the various data set. On these diagrams, the Pb isotope values of sulfide minerals in the quartz–sulfide vein fall not only within the field of Proterozoic strata, but also within and/or near the field of the Mesozoic intrusion, indicating that the source of Pb may be derived from magma and host rocks. This suggests

Fig. 10. δ34S values of sulfides from the Songhung deposit and related lithologies (Chai et al., 2016). Py(q) pyrite from quartz–sulfide vein; Gn galnea; Asp arsenopyrite; Cpy chalcopyrite; Py(m) pyrite from metasomatic vein. 13

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Fig. 11. Lead isotopic compositions of the Songhung deposit (Zartman and Doe, 1981) and related lithologies (Chai et al., 2016).

mantle fields and siderite separates plot into the area between the mantle and sedimantary organic matter fields but it is proximity to mantle field. In detail, the δ13CPDB values of siderite separates (from −8.35 to −12.59‰) are very low in comparison with those of ankerite (from −2.51 to −2.88‰) and limestone (from −0.55 to −2.91‰), indicating that a minor amount of C can be related to the oxidation of oceanic organic matter or seawater bicarbonate (Tang and Liu, 1999). Meanwhile, the δ18OSMOW values of siderite and ankerite are similar (13.82‰ for siderite and 14.63‰ for ankerite with an average), whereas those values of limestone are very high (20.2‰ with an average). This indicates that a certain amount of O in the ore–forming fluid related with siderite mineralization was mainly provided from magma and O isotopic thermal equilibrium with the carbonate host rocks was present during mineralization (Muchez et al., 1995; Zheng, 1990; Zheng and Hoefs, 1993).

5.3. Origin of O and C The carbon isotopic compositions of the ore-forming fluids may vary due to the variation in the redox state, mixing of multiple sources for the carbon, temperature variation, CO2 degassing and calcite precipitation, and a combination of some of the above (Barnes, 1979, 1997). The δ13C values of ore–forming fluid vary according to the organic matter (ca. −27‰), atmospheric CO2 (from −7 to −11‰; Hoefs, 2004), freshwater carbonate (from −9 to −20‰; Hoefs, 2004), igneous rocks (from −3 to −30‰; Hoefs, 2004), continental crust (−7‰; Faure, 1986) and the mantle (from −5 to −7‰; Hoefs, 2004). The δ13CPDB and δ18OSMOW values of mantle, marine carbonates and organic matters are from −4.0‰ to −8.0‰ and from +6.0‰ to +10.0‰ (Taylor et al., 1967), from −4.0‰ to +4.0‰ and from +20.0‰ to +30.0‰ (Veizer and Hoefs, 1976), and from −30.0‰ to −10.0‰ and from +24.0‰ to +30.0‰ (Liu and Liu, 1997; Zeng et al., 2014), respectvely. In the δ13CPDB vs. δ18OSMOW diagram (Fig. 12), these valuses of the limestone (Sinsong Formation, wall rocks) in No.3 ore body of the Songhung deposit fall into the marine carbonate field, whereas ankerite separates plot into the diagram between the marine carbonate rock and

5.4. Tectonic event and metallogenic model The Sino–Korean Craton was collided with the Siberian Craton to the north and the Yangtze Craton to the south during the Middle Triassic, resulting in the intense compressional deformation and in the

Fig. 12. Plot of δ13CPDB vs. δ18OSMOW for the siderite, ankerite and limestone in the Songhung deposit. 14

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from either magma or sedimentary rocks and the ore–forming fluid was a mixing of magmatic with meteoric water. The aqueous nature of the magma–derived hydrothermal fluids was altered depending on the feature of host rocks. The mineralization in this area is considered by three stages with various Au grade: barren quartz, auriferous sulfide–quartz (ferric sulfide, polymetallic sulfide) and calcite. When the host rock is carbonate, the amount of C and H2O increased in the magmaderived ore–forming fluids and caused the mineralization of ankerite, siderite, minor Au and sulfides. In an alternative model, when the ore–forming fluid was transferred along the contact between siliceous rocks such as the quartzite and slate, the Fe and base metals, as well as S, were leached out from the host rocks, and the activity coefficient of S2– was increased. Therefore, major Au and sulfides such as pyrite, pyrrhotite, sphalerite, chalcopyrite and galena could precipitate at this stage. However, a significant role of pH cannot be excluded because the precipitation and concentration of Au is closely related to its variation. It was intensely decreased when the ore fluid interacts with siliceous rocks, resulting in the disequilibrium of aqueous phase and in the precipitation of major Au and sulfide minerals, whereas it was not or weakly changed when the host rock is carbonate, causing the decreasing of the Au grade and the major precipitation of ankerite and siderite with minor sulfides. On the whole, we suggest that the gold–polymetallic mineralization in the Songhung deposit was mainly related with the magmatic hydrothermal event and some of metals may be leached out from the sedimentary rocks by circulating fluid which was incorporated with mineralized magmatic fluid (Fig. 13).

WNW–ESE extensional deformation during the late Jurassic to early Cretaceous. The WNW–ESE extensional deformation in the Early Cretaceous rotated clockwise to NW–SE in the latest Early Cretaceous (Fu et al., 2016 and references therein). In general, typical MCCs from the eastern part of the Sino–Korean Craton are characterized by three essential components, viz: 1) Shallow–moderate dipping detachment fault, 2) A footwall rocks consisting of fault–related mylonitic rocks, and 3) A hanging wall of upper crustal basement rocks (Davis et al., 1996; Fu et al., 2016; Whitney et al., 2013). The MCC consists of crustal thinning, granitic magmatism, formation of the NE–SW striking rift basins (Wang et al., 2012) and many occurrences and deposits of gold mineralization (Deng et al., 2009; Goldfarb et al., 2014; Song et al., 2015; Wen et al., 2015). Most gold deposits from the NCC are associated with brittle to brittle–ductile faults (Goldfarb and Santosh, 2014; Hart et al., 2002; Yang et al., 2016; Yang et al., 2000; Zhang et al., 2011), which are located near–to or within MCCs. The Pyongnam basin situated in the eastern part of the Sino–Korean Craton was also subjected to compressional and extensional deformation during the Triassic to Jurassic and Cretaceous period (Li et al., 2010). The YUZ situated in the northern part of the Pyongnam basin consists of the Neoarchean Rangnim Metamorphic Belt, Proterozoic and Early Paleozoic rocks in a central part, the crust–mantle derived granitic intrusion (Yangdok granitoid), two large–scale NNE–trending faults with mylonitized rocks interpreted as detachment faults and two regional rift–subsidences (Fig. 1b). It indicates that the YUZ can be considered as a Metamorphic core complex. However, the width (ca. 1–3 m) of the ductile shear zone along the detachment fault is very thinner in comparing to other MCCs in the northeastern part of the NCC. We suggest that these relationships may be due to the presence of the quartzite layer of Suan Formation in contact with the footwall and the slow moderate–slip movement of hanging wall during the horizontally extensional stretching deformation. The polymetallic mineralization in the Songhung deposit comprises early and late mineralization. The early mineralization took place with the formation of anticlines, synclines, thrusts and faults in the Proterozoic and Paleozoic thick (about 10–15 km) sedimentary rocks caused by the continental compressional deformation during the early Mesozoic period. This tectonic event could also resulted in low–grade metamorphism of sedimentary rocks, in a extraction and mobilization of metamorphic hydrothermal fluid which includes the high content of C and H2O with metals such as Pb, Zn, Cu, Fe, Au and Ag and in a formation of favorable spaces in core and flank of folds for migration of the mineralized fluid (Li, 2009; Pak et al., 2009). Moreover, this event led to formation of the ore bodies, which are closely controlled by the stratigraphy, mineralogy and fabric of the primary host rocks, and is represented by insignificant Au content and dissemination of pyrite along the bedded limestone and/or dolomite. These ore bodies are cross–cut by the barren quartz veins (Fig. 5b) which formed in the first stage of following magma–derived hydrothermal mineralization. The Pb and S isotopic compositions of pyrite in these ore bodies show that Pb may be derived from the Proterozoic rocks with an old Pb isotope signature and the source of S may be also related with these rocks (Figs. 10 and 11). The late mineralization is associated with the granitic magmatism in this area during the tectonic extension deformation from the Jurassic to Cretaceous periods. The gold and base metal ore bodies are concentrated not only within the detachment faults (No.8 and No.9 ore bodies) where late brittle deformation and alteration overprint the early brittle–ductile transition zone, but also in the NW–trending faults and accompanied subsidiary faults in the hanging wall of upper crustal sedimentary rocks. In general, the Precambrian metamorphic rocks are the most important host rocks for Au and other base metals and have originally contained more leachable gold (Hart et al., 2002; Nie et al., 2003; Zhou et al., 2002). The results of Pb, S, C, O isotope analysis in this study indicate that the Pb, S and other base metals could be derived

6. Conclusions (1) The YUZ situated in the northern part of the Pyongnam basin consists of the Neoarchean Rangnim Metamorphic Belt, Proterozoic and Early Paleozoic rocks in a central part, the crust–mantle derived granitic intrusion (Yangdok granitoid), two large–scale NNE–trending faults with mylonitized rocks interpreted as detachment faults and two regional rift–subsidences (the Tokchon–Maengsan to the west, the Kowon to the east) and Junghwa–Samdung subsidence to the south. It suggests that the YUZ is a metamorphic core complex, similar to those in the NCC. The width of ductile shear zone along the detachment fault is very thin in comparing to other MCCs in NCC because the presence of the quartzite layer of Suan Formation in contact with the footwall or the slow moderate–slip movement of hanging wall during the horizontally extensional stretching deformation. (2) The mineralization in the Songhung deposit is divided into early and late generations. The early mineralization occurred during the tectonic compressional deformation in the Triassic period, and Pb, S, C, O and other base metals may be derived from the host rocks. However, this mineralization is not a significant Au and polymetallic mineralization. The main gold–polymetallic mineralization in the Songhung deposit was related to the initiation and exhumation of Yangdok MCC developed during the tectonic extension deformation of the region, and the ore–forming fluids and Pb, S, C, O could be derived from both the granitic intrusion (Yangdok granitoid) and the Proterozoic and Paleozoic metasedimentary rocks. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was partially supported by the Committee of Education (Grant Number 01–00–10759–2018), DPR Korea. We thank the 15

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Fig. 13. Metallogenic model in the Songhung deposit.

investigators of the Songhung Au–polymetallic mines, researchers of the Analytical institutes for help with fieldwork and analysis. We also thank Prof. Chen Huayong, associate editor Peter C. Lightfoot and two reviewers for their valuable comments and suggestions to improve this paper.

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