Mn-un Son, Zn-jn Dn,b, Jun-jn Zn, Yn-xn Son, Jun-w Bo, Yu-qun Wn,Hon-bo Lu, S-yon L, J L, Ru-xn L, Bn Wn, Xn-on Lu, Ln-ln Zn,L-l Don, Jn L, Cun-yn H

a Shandong Provincial No.6 Exploration Institute of Geology and Mineral Resources, Weihai 264209, China

b Shandong Provincial Bureau of Geology and Mineral Resources, Jinan 250013, China

c Shandong Provincial No.3 Exploration Institute of Geology and Mineral Resources, Yantai 264003, China

d Shandong Institute of Geological Sciences, Jinan 250013, China

e Shandong Provincial Ruihai Mining Limited Company, Laizhou 261400, China

f Shandong Institute of Geophysical & Geochemical Exploration, Jinan 250013, China

g Hebei Key Laboratory of Strategic Critical Mineral Resources, Hebei GEO University, Shijiazhuang 050031, China

h School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China

i School of Resources and Environmental Engineering, Shandong University of Technology, Zibo 255000, China

Keywords:

Gold deposit

Deep prospecting

Thermal uplifting-extension mineralization

Transformation of mantle properties

Stepped metallogenic model

Mineral exploration engineering

Jiaodong-type gold deposits

Sanshandao

Jiaodong Peninsula

China

A B S T R A C T

The Jiaodong Peninsula in Shandong Province, China is the world’s third-largest gold metallogenic area,with cumulative proven gold resources exceeding 5000 t. Over the past few years, breakthroughs have been made in deep prospecting at a depth of 500-2000 m, particularly in the Sanshandao area where a huge deep gold orebody was identified. Based on previous studies and the latest prospecting progress achieved by the project team of this study, the following results are summarized. (1) 3D geological modeling results based on deep drilling core data reveal that the Sanshandao gold orefield, which was previously considered to consist of several independent deposits, is a supergiant deposit with gold resources of more than 1200 t (including 470 t under the sea area). The length of the major orebody is nearly 8 km, with a greatest depth of 2312 m below sea level and a maximum length of more than 3 km along their dip direction. (2) Thick gold orebodies in the Sanshandao gold deposit mainly occur in the specific sections of the ore-controlling fault where the fault plane changes from steeply to gently inclined,forming a stepped metallogenic model from shallow to deep level. The reason for this strong structural control on mineralization forms is that when ore-forming fluids migrated along faults, the pressure of fluids greatly fluctuated in fault sections where the fault dip angle changed. Since the solubility of gold in the ore-forming fluid is sensitive to fluid pressure, these sections along the fault plane serve as the target areas for deep prospecting. (3) Thermal uplifting-extensional structures provide thermodynamic conditions, migration pathways, and deposition spaces for gold mineralization. Meanwhile, the changes in mantle properties induced the transformation of the geochemical properties of the lower crust and magmatic rocks. This further led to the reactivation of ore-forming elements, which provided rich materials for gold mineralization. (4) It can be concluded from previous research results that the gold mineralization in the Jiaodong gold deposits occurred at about 120 Ma, which was superimposed by nonferrous metals mineralization at 118-111 Ma. The fluids were dominated by primary mantle water or magmatic water. Metamorphic water occurred in the early stage of the gold mineralization, while the fluid composition was dominated by meteoric water in the late stage. The S, Pb, and Sr isotopic compositions of the ores are similar to those of ore-hosting rocks, indicating that the ore-forming materials mainly derive from crustal materials, with the minor addition of mantle-derived materials. The gold deposits in the Jiaodong Peninsula were formed in an extensional tectonic environment during the transformation of the physical and chemical properties of the lithospheric mantle, which is different from typical orogenic gold deposits. Thus, it is proposed that they are named “Jiaodong-type” gold deposits.

1. Introduction

The Sanshandao gold deposit is located on the western side of the Jiaodong gold ore concentration area on the southeastern margin of the North China Craton. With gold reserves of more than 5000 t, and the Jiaodong gold ore concentration area is the world ’s third-largest gold concentration area after the Witwatersrand Basin in South Africa and the Muruntau area in Uzbekistan. The Sanshandao gold deposit was discovered in 1965 and is the first identified altered rock type gold deposit in fractured zones in China. The deposit has over 60 t of gold resources in 1969, while the overall gold endowment had grown to 118 t above the elevation of -400 m by the end of the last century. The Sanshandao gold deposit was put into production in 1989, and currently, the maximum mining depth has reached 1050 m underground. Meanwhile, it is the first undersea mine in the world. The deep prospecting programs conducted since the 21st century contribute to the discovery of two gold deposits.The first is the Xiling gold deposit with proven gold resources of 383 t, which lies in the deep part of the main orebody of the Sanshandao gold deposit that extends along the dip direction.The second one was discovered under the sea area as the extension of the main orebody along with the trend, with proven gold reservoirs of 470 t. According to statistics from S&P 500, the sea-area gold deposit ranks second among the world’s top 10 gold deposits discovered during 2010-2019.Meanwhile, the sea-area gold deposit is also the world’s largest gold deposit under the sea area. Deeply drilled cores have revealed that the Xiling and sea area deposits adjacent to the Sanshandao, which were originally considered independent of each other, are interconnected with each other.Therefore, they combine to form a supergiant gold deposit with gold reserves exceeding 1200 t.

Extensive studies have been conducted on the gold deposits in the Sanshandao area and its vicinity (Lü GX and Kong QC, 1993; Lu HZ et al., 1999; Li HM et al., 2003;Wang JC et al., 2003; Li SX et al., 2007; Deng J et al., 2010,2020a; Song MC et al., 2010a, 2010b, 2012; Niu SY et al.,2019; Jiang XH et al., 2011; Yang LQ et al., 2014; Qiu KF et al., 2020; Sai SX et al., 2020). Consensus has been roughly reached in many aspects, such as ore-controlling factors,mineralization timing, characteristics of fluid inclusions,stable isotopes, ore-forming stages, and alteration. However,in-depth studies are yet to be carried out on the spatial relationships of the main orebodies discovered in various exploration stages and regions, especially on the mineralization styles and patterns of deep orebodies.Meanwhile, a consensus has not been reached on the genesis of the deposits. For example, previous researchers proposed that the gold deposits are magmatic-hydrothermal ore deposits related to Linglong, Guojialing, or Weideshan granitoids(Wang LG et al., 1998; Li SX et al., 2007; Luo XD et al.,2014; Song MC et al., 2014b, 2015a). Another school of thought classified them as orogenic gold deposits related to the subduction of the paleo-Pacific Plate (Zhou TH and Lu G,2000; Goldfarb RJ et al., 2001; Deng J et al., 2010), while some argue that the gold deposits are associated with craton destruction (Zhu RX et al., 2015). Based on the summary of the achievements made in gold prospecting in the Sanshandao area, this study investigates the relationships between major gold orebodies in the shallow mining areas previously discovered and those in deep mining areas explored in recent years in detail. Further, this study describes in detail the main features of the gold deposits and their variation in-depth in the Sanshandao area and its vicinity, which constitute a supergiant gold deposit-the supergiant Sanshandao gold deposit (short for Sanshandao gold deposit). Moreover, the authors summarize previous research results and discuss the structural control on the gold mineralization, tectonic background, and genesis model of the deposit. This study is greatly significant for the deep understanding of the metallogenic regularity and formation mechanisms of Jiaodong-type gold deposits and for guiding the deep prospecting of the deposits.

2. Regional geological background

The Sanshandao gold deposit is located in the Jiaodong Peninsula, Shandong Province, where the circum-West Pacific metallogenic domain, the Precambrian metallogenic domain, and the Qinling-Qilian-Kunlun metallogenic domain overlap. Multiple tectonic superimpositions, especially the Yanshanian tectono-magmatic activities, led to the Mesozoic explosion of different kinds of mineralization (Hua RM and Mao JW, 1999; Zhai MG et al., 2004). The Jiaodong Peninsula is composed of the ancient metamorphic basement and Mesozoic-Cenozoic continental sedimentary strata and magmatic rocks (Fig. 1). The Jiaobei Terrane to the west of the Jiaodong Peninsula is located on the southeastern margin of the North China Craton. The Weihai Terrane to the east of the Jiaodong Peninsula is a part of the Dabie-Sulu orogenic belt. The sediment and volcanic rocks in Jiaolai Basin to the south of the Jiaodong Peninsula overlay the Jiaobei and Weihai terranes.

Fig. 1. Regional geological sketch map and gold deposit distribution of the Jiaodong Peninsula. 1-Quaternary; 2-Cretaceous; 3-Paleoproterozoic and Neoproterozoic; 4-Neoproterozoic with eclogite granitic gneiss; 5-Archean granite-greenstone belt; 6-Cretaceous Laoshan-type granites; 7-Cretaceous Weideshan-type granites; 8-Cretaceous Guojialing-type granites; 9-Jurassic granitoids; 10-Triassic granitoids; 11-geological conformity/unconformity; 12-fault; 13-shallow gold deposits (very large and large/medium-sized and small); 14-deep gold deposits (very large and large/medium-sized and small); 15-gold deposit of altered rock type/quartz vein type/altered breccia type; 16-supergiant Sanshandao gold deposit. ME1-Jiaoxibei metallogenetic sub-region; ME2-Qipengfu metallogenetic sub-region; ME3-Mouru metallogenetic sub-region;F1-Sanshandao fault; F2-Jiaojia fault; F3-Zhaoping fault; F4-Xilin-Douya fault; F5-Jinniushan fault.

2.1. Precambrian crystalline basement

The Precambrian crystalline basement in the Jiaodong Peninsula consists of the Jiaobei and Weihai terranes. The Precambrian crystalline basement of the Jiaobei Terrane consists of the Middle Archean-Neoproterozoic metamorphic rock series. The Mesoarchean-Neoarchean metamorphic rock series is dominated by tonalite-trondhjemite-granodiorite(TTG) gneiss suites. It can be divided into two stages according to isotopic ages, namely the early stage of 2738-2707 Ma and the late stage of 2577-2496 Ma. Besides,there is an age record of about 2.9 Ga (Liu JH et al., 2011; Xie HQ et al., 2013). A small number of Jiaodong rock groups occur in the form of xenoliths in the TTG gneiss suite. They mainly consist of biotite leptynites, amphibolites, and hornblende leptynites interbedded with magnetite quartzites,as well as some visible basic-ultrabasic intrusions such as serpentinized peridotites, meta-pyroxene hornblendes, and metagabbros. The Paleoproterozoic Jingshan and Fenzishan groups, mainly composed of marbles, (graphite-bearing)leptynites, gneiss, and high-alumina schists, possess characteristics similar to those of khondalite series rocks.They are amphibolite-granulite facies metamorphic littoralneritic sedimentary rock series. The SHRIMP U-Pb age of the youngest detrital zircons in the Jingshan Group is 2175 ± 16 Ma. The metamorphic ages of the Jingshan and Fenzishan groups are about 1.8-1.7 Ga (Li SZ et al., 2012). The Mesoproterozoic Zhifu Group is mainly composed of quartzites and K-feldspar quartzites interbedded with magnetite layers, and it belongs to the littoral-facies sedimentary rock series of low amphibolite facies. The Neoproterozoic Penglai Group is mainly composed of phyllites, slates, quartzites, crystalline limestones, and marbles, and it is a set of metamorphic littoral-neritic sedimentary rock series of greenschist facies.

The Weihai Terrane is mainly composed of Neoproterozoic granitic gneiss and minor amounts of Paleoproterozoic metamorphic strata, Mesoproterozoic basicultrabasic intrusions, and Triassic eclogites. The U-Pb isotopic ages of zircon cores of the Neoproterozoic granitic gneiss are 723 ± 36 Ma, 738 ± 17 Ma, and 744 ± 63 Ma (Tang J et al., 2004). The zircon SHRIMP U-Pb dating of the granitic gneiss shows that the metamorphic basement of the Weihai Terrane is 2400 Ma and experienced metamorphic events at 1800-1700 Ma and about 200 Ma (Xu ZQ et al.,2006). The age of ultrahigh-pressure metamorphism is 242-220 Ma and the exhumation age of ultrahigh-pressure metamorphic rocks is 219-202 Ma (Xu ZQ et al., 2006; Liu FL et al., 2003).

2.2. Meso-Cenozoic strata and magmatic rocks

Mesozoic strata are mainly distributed in the Jiaolai Basin,which is a Cretaceous continental facies volcanic-sedimentary basin. The Early Cretaceous Laiyang Group in the lower part of the basin is mainly composed of clastic sedimentary rocks such as sandstones, mudstones, and conglomerates of piedmont alluvial facies, fluvial facies, and lacustrine facies.The Early Cretaceous Qingshan Group in the middle part of the basin is an intermediate-basic-acidic volcanic rock suite interbedded with small amounts of sedimentary rocks. The Late Cretaceous Wangshi Group in the upper part of the basin is composed of the clastic rock series, such as the red sandstones, mudstones, and conglomerates of fluviallacustrine facies. The isotopic ages of volcanic rocks in the Qingshan Group are 123-98 Ma (Song MC et al., 2009). The Quaternary sediments are widely distributed in modern seashores and river valleys, and Paleogene fluvial-lacustrine sedimentary strata are locally distributed.

The Jiaodong Peninsula experienced intense Mesozoic magmatic activities. Granitoid intrusions in the peninsula mainly include Triassic quartz syenites-syenogranites,Jurassic monzogranites (Linglong-type granites), Early Cretaceous granodiorites (Guojialing-type granites), Early Cretaceous diorites, Early Cretaceous quartz monzonitesmonzogranites (Weideshan-type granites), and Early Cretaceous syenogranites (Laoshan-type granites).Intermediate-basic to acidic vein rocks mainly include lamprophyre dykes, diorite-porphyrite dykes, monzonite dykes, and granite porphyry dykes. Volcanic rocks are mainly an integral part of the Qingshan Group, and small amounts of Cenozoic basalts occur in the western part of the Jiaobei Terrane.

2.3. Structures

The Mesoarchean-Paleoproterozoic rocks in the Jiaobei Terrane underwent three stages of folding and two stages of ductile shearing at 1956-1875 Ma (Li SZ et al., 2012). The Weihai Terrane is characterized by the development of multistage and multi-facies ductile shear zones. Faults have widely developed in the Jiaodong Peninsula since the Mesozoic,including the NE-NNE-trending faults, followed by nearly EW-NEE-trending faults. The NE-NNE-trending faults are distributed widely and densely and serve as the major orecontrolling structures in Jiaodong gold deposits. Additionally,EW-trending faults are sporadically exposed, with poor continuity.

2.4. Overview of gold resources

There are more than 200 gold deposits (districts) with proven resources in the Jiaodong Peninsula (Fig. 1), which jointly form the Jiaodong gold province or the Jiaodong gold ore concentration area. According to the spatial distribution of the deposits, the Jiaodong gold province is further divided into three metallogenic sub-regions, namely Jiaoxibei(Laizhou-Zhaoyuan), Qipengfu (Qixia-Penglai-Fushan), and Muru (Muping-Rushan). These three regions consist of six metallogenic belts, namely Sanshandao, Jiaojia, Zhaoping,Qixia-Daliuhang, Taocun, and Muru; and 13 gold orefields,namely Sanshandao, Jiaojia, Lingbei, Anshi, Dazhuangzi,Linglong, Dayingezhuang, Jiudian, Qixia, Daliuhang,Laishan, Pengjiakuang, and Denggezhuang (Song MC et al.,2015b). The styles of gold mineralization in the Jiaodong gold ore concentration area primarily include altered rock type in fractured zones (Jiaojia type), quartz vein type (Linglong type), followed by the sulfide-quartz vein type(Denggezhuang type), and small amounts of other mineralization types.

3. Geological background of the northwest Jiaodong Peninsula and Sanshandao ore district

3.1. Granitoids related to gold mineralization

The northwest Jiaodong Peninsula is located on the southeastern side of the Bohai Bay and is adjacent to the famous Tancheng-Lujiang fault zone (which extends into the sea at the Bohai Bay) in the west. It is the most important gold district in the Jiaodong Peninsula, with cumulative proven gold reservoirs of more than 4000 t. The northwest Jiaodong Peninsula is mainly composed of Early Precambrian metamorphic rock series and Jurassic Linglong-type granites,followed by other geological bodies including Jurassic Luanjiahe granites, Cretaceous Guojialing-type granites,Weideshan-type granites, Laoshan-type granites, and various vein dykes. In addition, there are Cretaceous and Quaternary strata in the area (Fig. 2). There is a close spatial-temporal relationship between gold deposits in the northwest Jiaodong Peninsula and Jurassic-Cretaceous magmatic rocks. The orehosting magmatic rocks include Linglong-type granites and Guojialing-type granites, which are referred to as ore-hosting geological bodies (Song YX et al., 2017). Meanwhile, the intrusions coeval with the gold mineralization include Weideshan-type granites, Laoshan-type granites, and intermediate-basic vein dykes.

Fig. 2. Geological maps of northwest Jiaodong Peninsula (a) and Sanshandao gold deposit (b). 1-Quaternary; 2-Cretaceous; 3-Early Precambrian metamorphic rock series; 4-Cretaceous Laoshan-type granites; 5-Cretaceous Weideshan-type granites; 6-Cretaceous Guojialing-type granites; 7-Jurassic Linglong-type granites; 8-geological conformity/unconformity; 9-fault/main ore-controlling fault; 10-shallow large/medium-small gold deposit; 11-deep large/medium-small gold deposit projected to the surface; 12-alteration zone of Sanshandao fault; 13-ore block boundary. F1-Sanshandao fault; F2-Jiaojia fault; F3-Zhaoping fault.

Linglong-type granites are dominated by monzogranites with different textures, structures, or mineral assemblages in terms of lithology (Table 1). The early intrusive bodies mainly include gneissic garnet-bearing monzogranites, while the late intrusive bodies mainly consist of massive lightcolored monzogranites. In terms of chemical composition,Linglong-type granites fall in the granite zone in the SiO2vs.K2O+Na2O diagram (Fig. 3). With high contents of Na2O+K2O, low MgO content, and peraluminous characteristics, they are potassic granites and belong to the high-K calc-alkaline rock series (Lin BL et al., 2013).Moreover, they are rich in LREEs and LILEs (Rb, Ba, U, and Sr) and are depleted in HFSEs (Nb, Ta, P, and Ti) (Yang KF et al., 2012). Their87Sr/86Sr ratio,εNd(t),εHf(t) and Sr/Y ratio are 0.711281-0.712418, 21.6-19.4, 28.7-6.2, and 55.07-214.44, respectively, and theirεHf(t) values fall between the crust evaluation curves of 1.9 Ga and 2.5 Ga(Huang T et al., 2014; Yang KF et al., 2012). They show lowεHf(t) values, high Sr/Y ratio, and no notable negative europium anomalies, which are similar to those of both adakites (Lin BL et al., 2013; Yang KF et al., 2012) and Neoarchean TTG in the Jiaodong Peninsula (Wu M et al.,2014). Magmas in Linglong-type granites were formed in a relatively high-pressure environment and originated from the partial melting of the thickened lower crust (＞ 40 km) in North China (Zhang HF et al., 2004; Yang KF et al., 2012).Therefore, Linglong-type granites are S-type granites. The zircon U-Pb isotopic ages of Linglong-type granites are 163-159 Ma (Table 1). There are inherit zircons with ages of 800-700 Ma, 230-200 Ma, 19-18 Ga, and 28-18 Ga (Song MC et al., 2018; Wang B et al., 2021). This indicates that the melting source areas of Linglong-type granites include Neoproterozoic and Triassic materials from the Sulu orogenic belt and Neoarchean-Paleoproterozoic materials from the northern Jiaodong Peninsula.

Fig. 3. Petrochemical classification diagram of intrusions related to gold mineralization and gold orebodies in the Jiaodong Peninsula(after Song MC et al., 2020a for the ranges of Linglong, Guojialing,Weideshan, and Laoshan-type granites).

Table 1. Characteristics of intrusions related to gold mineralization in the northwest Jiaodong Peninsula.

Table 1. (Continued)

Guojialing-type granites are mainly composed of monzonitic diorites, quartz monzonites, granodiorites, and monzogranites (Table 1), with porphyroid textures. They are monzonite-granite rock series (Fig.3) and show metaluminous characteristics. They are sodic granites and are dominated by the high-K calc-alkaline rock series, with some samples falling in the zone of shoshonite series. They have high contents of CaO, TFe2O3, MgO, K2O, total alkali, Sr, Ba,LREEs, and LILEs and are depleted in Cr, Ni, and HFSEs.Their87Sr/86Sr ratio,εNd(t),εHf(t), and Sr/Y ratio are 0.710175-0.71172, from -21.30 to -11.17, from -25.2 to-13.9, and 114-378, respectively (Yang KF et al., 2012;Wang ZL et al., 2014a, 2014b; Liu Y et al., 2014; Wang LG et al., 2018; Song YX et al., 2020). TheirεHf(t) values fall between the crustal evolution curves of 2.5 Ga and 3.0 Ga(Yang KF et al., 2012). According to a previous study (Wang ZL et al., 2014b), the high Ba-Sr contents mainly originated from the partial melting of basement rocks in the northern Jiaodong Peninsula, with the addition of intermediate magmas derived from the partial melting of juvenile basic lower crust formed by the underplating of mantle-derived magmas, which is related to the subduction of the paleo-Pacific plate toward the North China Craton and upwelling of the asthenosphere.The initial206Pb/204Pb and208Pb/204Pb ratios of Guojialingtype granites are 17.047-17.945 and 37.744-38.389,respectively, indicating that large quantities of lower crust materials were involved in the diagenesis (Wang ZL et al.,2014a; Song YX et al., 2020). TheεNd(t) values of Guojialing-type granites are similar to those of the basic vein rocks in the northwest Jiaodong Peninsula, indicating that the magmas in Guojialing-type granites were derived from the mantle (Yang KF et al., 2012; Liu Y et al., 2014; Wang ZL et al., 2014a; Wang LG et al., 2018; Luo XD et al., 2014).Guojialing-type granites contain rich Neoarchean and Paleoproterozoic inherited zircons and a small amount of Jurassic inherited zircons but bear no Neoproterozoic and Early Paleozoic inherited zircons (Yang KF et al., 2012). This indicates that the crust-derived materials originated from the basement of the North China Craton (Early Precambrian metamorphic rock series in the Jiaodong Peninsula) and lack the crust materials from the Sulu ultrahigh-pressure metamorphic belt and the Yangtze Craton. Overall,Guojialing-type granites are considered the result of mixing of the lower crust acidic magmas formed from the partial melting of the basement metamorphic rock series in the Jiaodong Peninsula with mantle-derived basic magmas.Meanwhile, it is believed that the formation of magmas is related to the subduction of the paleo-Pacific plate toward the North China Craton and upwelling asthenosphere. The zircon U-Pb isotopic ages vary in the range of 133-127 Ma (Table 1).

Weideshan-type granites are widely distributed in the Jiaodong Peninsula, without noticeable gold mineralization inside the pluton and contact zones. However, they are closely related to copper, lead, zinc, and molybdenum deposits (Ding ZJ et al., 2013; Song MC et al., 2017). They have a complex composition and mainly consist of diorites, quartz monzonites, granodiorites, and monzogranites, with porphyric textures (Table 1). They are chemically classified as monzonitic diorite-granite, with a high total alkali content, a low Na2O/K2O ratio (0.3-1.1), low contents of Fe2O3, MnO,MgO, TiO2, and P2O5, and metaluminous characteristics(Goss CS et al., 2010). In the petrochemical classification diagram, they mainly fall into the zones between sodic granites and potassic granites (tending to approach the sodic granite zone), thus belonging to the high-K calc-alkaline rock series and shoshonite series. They are rich in LREEs and LILEs and deficient in HFSEs and show weak-moderate negative europium anomalies. They haveεHf(t) values of-25.52- -11.55 and strong positive Pb anomalies. Their TDM2model ages are 3096-3434 Ma, indicating that their magma sources are related to the Archean basement and ancient recycled continental crustal materials (Goss CS et al., 2010)and that the magmas originated from the partial melting of recycled continental crustal materials and enriched lithospheric mantle. The zircon U-Pb isotopic age of Weideshan-type granites is 125-110 Ma (Table 1).

Laoshan-type granites are mainly distributed in the ultrahigh-pressure metamorphic belts in the eastern and southern parts of the Jiaodong gold ore concentration area. In terms of spatial distribution, they do not show direct relationships with the mineralization of gold and nonferrous metals. They are monzogranite-syenogranite-alkali-feldspar granite series, while only monzogranites are reported in the northwest Jiaodong Peninsula (Table 1). In the petrochemical classification diagram, they mainly fall into the granite zone(Fig. 3). They are distinctly rich in silica and alkali and deficient in calcium, belonging to potassic granites, the high-K calc-alkaline rock series, and the shoshonite series.Therefore, they have typical characteristics of A-type granites.They show noticeable REE fractionation and significant negative europium anomalies and are rich in LREEs.According to the trace element analysis, Laoshan-type granites show the characteristics of low-Ba-Sr granites with a low Ba content and a high Rb content (Goss CS et al., 2010),and thus they are different from Linglong and Guojialing-type granites with high Ba and Sr contents. Similar to the abovementioned Linglong, Guojialing, and Weideshan-type granites, Laoshan-type granites are rich in LILEs and depleted in HFSEs, showing negative Ba anomalies, and positive Sr anomalies. They haveεHf(t) values of -20.32- -16.53 and TDM2model ages of 3096-3434 Ma. Overall, Laoshan-type granites are A-type granites formed in a regional extensional tectonic setting, and they are the result of Mesozoic lithospheric thinning and craton destruction (Yan QS et al.,2019). The magmas of Laoshan-type granites originated from the partial melting of deep crust, which may be related to the subduction of the paleo-Pacific plate and the recycling of the oceanic crust in subduction zones. The zircon U-Pb isotopic age of Laoshan-type granites is 125-119 Ma (Table 1).

3.2. Ore-hosting faults

Jiaodong gold deposits are mainly controlled by NNE- to near SN- trending faults. Large and medium-sized gold deposits in the northwest Jiaodong Peninsula are controlled by the Sanshandao, Jiaojia, and Zhaoping faults and their secondary faults developing on their footwalls. Meanwhile,favorable places for mineralization include the places with sudden changes in fault strike along with the regional trend,the parts with variations in dip angles along the faults, and the places where secondary faults develop.

The Sanshandao fault extends along the Sanshandao-Cangshang-Panjiawuzi area in Laizhou City on the coast of the Bohai Sea, most of which is covered by the Quaternary sediments. The exposed portions of the fault have a length of 12 km, a width of 20-400 m, a general strike of 40°-50°, a dip direction of SE, and a dip angle of 30°-40° (up to 80°locally). It is horizontally S-shaped on the plane and shows irregular morphology (Fig. 4). It controls the Sanshandao(including the Xinli and sea-area ore blocks) and Cangshang large-superlarge altered rock type gold deposits in fractured zones from north to south.

Fig. 4. Geological map of bedrocks in the Sanshandao fault.

The Jiaojia fault extends from Ziluojijia in Laizhou City in the south to Yaojia in Longkou City in the north. This fault is about 60 km long and 50-500 m wide, with a dip direction of NW and a dip angle of 30°-50° (up to 78° locally). It shows an S shape on the plane, irregular morphology, and noticeable swelling and shrinkage, with many branches parallel to the fault strike or intersecting the Jaiojia fault in the shape of character “λ” on the footwall of the fault. More than 20 gold deposits such as Jiaojia, Xincheng, Hedong, and Hexi are controlled by the Jiaojia fault or its adjacent secondary faults.

The Zhaoping fault is the largest ore-controlling fault exposed in the northwest Jiaodong Peninsula. It starts from Songgezhuang in the northern part of Pingdu City in the south and extends northward in the NNE-NE direction to Zhaoyuan City. Afterward, it extends in the NEE direction to the vicinity of Yanjiagou in Longkou City and then pinches out. It is 120 km long in total and 150-200 m wide, with a dip direction of SE-E and a dip angle of 30°-70°. It consists of major faults and a series of secondary faults. Among them, the major fault in the northern of Zhaoyuan City is distributed along the Fushan-Jiuqu Jiangjia area and is called the Jiuqu fault. The northern section of the Zhaoping fault and the secondary faults developing on its footwall control the Linglong gold orefield, the middle section of the Zhaoping fault controls the formation of Dayingezhuang gold orefield, and the southern section of the Zhaoping fault and its secondary faults on the footwall control the Jiudian gold orefield.

3.3. Geological characteristics of the Sanshandao ore district

The Sanshandao ore district is located in a coastal and shallow sea area and consists of Sanshandao, Xinli, Xiling,and sea-area ore blocks (exploration areas; Fig. 2b), which were discovered during different times. This area is mostly covered by Quaternary sediments and seawater, with bedrock outcrops retained only in several areas. The Quaternary sediments have a thickness of several meters to tens of meters,which is up to a maximum of 60 m in the sea area. The bedrock in the gold mine is mainly composed of Jurassic Linglong-type granites and Neoarchean metamorphic rocks(amphibolites and TTG gneisses), and there are numerous vein rocks such as lamprophyres, diabase-porphyries, quartz diorite porphyries, and diorite porphyries. Meanwhile, the Linglong-type granites are intruded by medium-grained porphyritic Guojialing-type granites.

The Sanshandao fault serves as an ore-hosted fault and mainly develops along the contact zones between Linglongtype granites and the Neoarchean metamorphic rock series. It possesses the characteristics of multi-stage activities,including left-lateral transpression before the mineralization,the right-lateral transtension during the mineralization, and the left-lateral transpression after the mineralization (Deng J et al., 2010). In the Xinli ore block, the drilling-controlled fault is 1300 m long and 70-185 m wide, with the fault strike shifting on average from 62° in the south to 38° in the north.The fault has a dip direction of SE and a dip angle of 33°-76°(40°-50° mostly; average: 46°). It extends in a gentle wave pattern in both the strike and the dip direction. The main fracture plane marked by grayish white-grayish black fault gouges continuously develop, and the fault gouges are 0.05-0.50 m thick. In the Sanshandao ore block, the drillingcontrolled fault is more than 1000 m long and 50-200 m wide, with an overall strike of 40°, a dip direction of SE, and a dip angle of 35°-45°. It extends in a gentle wave pattern similar to that in the Xinli ore block. Its main fracture plane lies in the upper part of the fractured zone and is covered by 6-40 cm thick fault gouges (Fig. 5a). In the Xiling ore block,the drilling-controlled fault has a length of 2850 m, a fault height of 3215 m, a strike of 35°, and a dip direction of SE,showing a gentle wave pattern along the fault plane. Its dip angle is about 42° in the section at an elevation of above -600 m. It becomes steeper at an elevation of -600- -1000 m, with a dip angle of 70°-80°. Meanwhile, its dip angle is mainly 30°-50° below -1000 m. The fractured zones are 40-400 m wide. White - grayish-black fault gouges continuously develop along its main fracture plane, with a thickness of 0.02-0.50 m. In the sea-area ore block, the drilling-controlled fault has a length of 4420 m long and a fault height of 2156 m, with a strike of about 35°. It generally extends in a gentle wave pattern, with a dip direction of SE. The dip angles are about 40° in the section at an elevation of above -400 m,75°-85° at an elevation of -400- -1000 m, and 35°-43° at an elevation of below -1000 m. The main fracture plane marked by grayish white-grayish-black fault gouges continuously develop, the fault gouges are 0.05-0.50 m thick (Fig. 5b), and the fractured zones are 40-400 m wide.

Fig. 5. Fault gouges on the main fracture plane in the Sanshandao ore block (a) and sea-area ore block (b) of the Sanshandao gold deposit.

4. Alteration and mineralization

4.1. Alteration zoning and styles

4.1.1. Alteration zones

Alteration zones are controlled by the Sanshandao fault as well, with the scale and zonation consistent with those of fractured zones of the fault. Meanwhile, their alteration intensity and zoning are also consistent with the degree of fragmentation of the wall rocks. The alteration zones are generally 50-200 m wide. Mineralized alteration is visible on both the hanging wall and the footwall of the main fracture plane of the fault. However, the hanging wall shows weak alteration and zoning, with no industrial gold orebodies in general. The mineralized alteration zones along the Sanshandao fault are, from the main fault to the footwall,strongly to weakly sericite-quartz-pyrite altered granites with blurred boundaries. The alteration style is mainly sericitequartz alteration above the main fault zone, which is not as widespread as that in the footwall overall (Fig. 6).

Fig. 6. Comprehensive schematic diagram of lithologic zoning of the alteration zones and fractured zones in the Sanshandao fault.

The alteration zone in the Xinli ore block has a length of 1300 m, a width of 70-185 m, and a controlled height of 1000 m. It has an overall strike of 62°, a dip direction of SE, and a dip angle of 33°-67° (average: 46°). It shows relatively stable morphology overall and extends in a gentle wave pattern, with local abrupt changes in the strike.

There are five alteration zones in the Sanshandao ore block. Among them, the No.1 alteration zone develops along the Sanshandao fault and is the main alteration zone, while other alteration zones are secondary alteration zones underlying the Sanshandao fault. The drilling-controlled section of the No.1 alteration zone is 1700 m long, 50-200 m wide, and 1000 m high. The No.1 alteration zone is wider in the middle part of the ore block and gradually narrows down toward the southern and northern sides, showing arc-shaped morphology on a horizontal plane. The alteration intensity displays symmetrical zoning, with the core dominated by a sericite-quartz-pyrite zone and the two sides (the hanging wall and footwall of the fault) exhibiting sericitolites, sericitolited cataclasites, and sericitized (silicified) cataclastic granites in turn.

The alteration zone in the Xiling ore block has a width of 30-580 m, which varies greatly at different depths. The maximum controlled height of the alteration zone is up to 3190 m. The strike of the alteration zone is 34° in general. It is about 42° in the southern part of the ore block and changes into about 29° in the north. The changes in the dip angle and alteration zoning of the alteration zone are the same as those of the alternation zone in the sea-area ore block.

The alteration zone between No. 20 and No. 42 exploration lines in the sea-area ore block is 1710 m long along its strike. The width varies greatly at different depths.Specifically, it is 150-260 m above -400 m but reduces to 80 m generally between -400 m and -1000 m. It increases to 300-400 m at a depth below -1000 m. The maximum controlled height of the alteration zone is 2156 m. The strike and dip angle of the controlled alteration zone is 35° and SE,respectively. The dip angle of the controlled alteration zone varies greatly with the depth. It is about 40° at the depth above -400 m, 75°-85° between -400 m and -1000 m, and 35°-43° at a depth below -1000 m (Fig. 7a). The controlled alteration zone generally shows stable morphology and extends in a gentle wave pattern along its strike and a regular stepped pattern along its dip direction (Fig. 7b). The controlled alteration zone between No. 50 and No. 76 exploration lines has a length of 2040 m in the strike, a width of 234-448 m, and a maximum controlled height of 970 m. It has a strike of 35° overall, a dip direction of SE, and a dip angle of 30°-45°. It shows the noticeable zoning of altered rocks. In detail, a beresitized cataclasite zone exists 0-320 m below the main fracture plane, where alteration and gold mineralization are the most intensive. Outside the beresitized cataclasite zone lie a beresitized granitic cataclasite zone and a beresitized granite zone. In terms of mineralization characteristics, the beresitized cataclasite zone is dominated by disseminated or veinlet-disseminated mineralization, and the beresitized granitic cataclasite zone and beresitized granite zone are dominated by veinlet and stockwork-type mineralization.

Fig. 7. Combined section of exploration lines (a) and 3D view of a fault plane (b) of the sea-area ore block in Sanshandao gold deposit (after Song MC et al., 2015a).

4.1.2. Alteration types

The alteration types in alteration zones of the Sanshandao gold deposit include K-feldspar alteration, quartz-sericite alteration, silicification, carbonation, and chlorite alteration.Among them, the K-feldspar alteration (Fig. 8a, b) has developed at the earliest stage, with a width over 100-200 m.In the main alteration zones, little K-feldspar alteration is observed due to the later superimposition of sericite alteration and silicification, with only a small amount of porphyritic and cloud-like K- feldspar residuals occasionally visible, although it is hard to tell whether they are hydrothermal or magmatic in origin.

Fig. 8. Photos of altered rocks in Sanshandao gold deposit (a-h explanations see the text).

The quartz-sericite alteration (Figs. 8c, d) is the major alteration type in the mining area and is closely related to mineralization. It occurs in a wide range and generally develops along the fractured zone of the Sanshandao fault. It is frequently associated with disseminated fine-grained pyrites, with coarse-grained pyrites occasionally visible.

The silicification (Fig. 8e) is mainly present as the replacement of granites by grayish-white quartz. It tends to occur in the form of quartz blocks, in which the content of quartz is over 90%. However, silicification is weak in gold-rich beresite orebodies.

Carbonation is an alteration type occurring in the late stage of mineralization, with calcites as the main mineral.Carbonation occurs in two states. One is that carbonation is distributed in quartz-sericite alteration zones in the form of blocks (Fig. 8f) or short veins, with little sulfides such as finegrained pyrite or sphalerites. The other is that carbonation fills in rock fissures, cleavages, or faults in the form of coarse veins (Fig. 8g). The late state frequently occurs in weakly potassic zones, with coarse-grained pyrites occasionally visible.

The chloritization (Fig. 8h) is a common alteration type. It develops in both surrounding rocks and orebodies. However,it is not as strong as other types of alteration. It mainly develops in cataclasites and is strong on both sides of the main fracture plane. It is present as a non-penetrating alteration, with penetrating alteration occurring in several areas.

4.2. Characteristics of orebodies

The Sanshandao and Xinli gold mines located in the northern section of the Sanshandao fault were discovered in the 1960s and the 1990s, respectively. They have always been regarded as two independent deposits since they were discovered. They are 1 km apart on the ground surface. Since the beginning of this century, gold resources have been successively discovered in the deep parts of the Xinli,Sanshandao, Xiling, and sea-area ore blocks. As revealed by exploration and projects (Song MC et al., 2019), the main gold orebodies in the Xinli, Sanshandao, and sea-area ore blocks, which are independently distributed in shallow parts,are interconnected with each other in deep parts (Fig. 9). They constitute a supergiant deposit with a tonnage aggregation index (the ratio of the tonnage of economic metal in a deposit to average metal content in the crust) of greater than 100×109,and the cumulative proven gold reservoirs of this supergiant deposit have exceeded 1200 t.

Fig. 9. Vertical longitudinal projection of main orebodies in the Sanshandao gold deposit.

4.2.1. Overall characteristics of orebodies

There are more than 80 orebodies in the Sanshandao gold deposit. Most of them occur in the beresite on the footwall of the main fracture plane of the Sanshandao fault, and very few of them occur on the hanging wall of fracture planes. The main orebodies are mostly concentrated in the I orebody groups, which primarily occur in the alteration zone of beresitized cataclasites on the footwalls of main fracture planes of faults. The main orebodies include the I-1 orebody of the Xinli ore block, the I-1 and I-2 orebodies of the Sanshandao ore block, I-1-I-7 orebodies of the Xiling ore block, and the I-1-I-9 orebodies of the sea-area ore block. The orebodies in the I orebody groups of these ore blocks are connected. As a result, they constitute a giant gold orebody with a total length of 8 km, with a deepest controlled depth of-2312 m, and a maximum controlled height of more than 3 km (Fig. 9). The giant gold orebody is present in the form of a huge vein shape mostly and in a stratoid shape locally. It is distributed in a gentle wave pattern along its strike and dip direction, with branches and combination as well as swelling and shrinkage. Its attitude is essentially consistent with that of the main fracture plane of the Sanshandao fault, with a strike of 30°-80° (average: 35°), a dip direction of SE, a dip angle of 25°-78° (average: 40°), and it generally inclines towards NE (Fig. 9).

The thickness of single-drilling-controlled orebody is 1.00-122.83 m, with an average of 3.04 m and a coefficient of variation of 112.05%, indicating that the orebodies have stable thickness. There are three parts with thick gold orebodies in the Sanshandao gold deposit. Among them, the shallow parts are mainly located at an elevation of 0--600 m in the Xinli and Sanshandao ore blocks, while the deep parts mainly lie at an elevation of -1000- -2000 m (Fig. 10a). The single-drilling-controlled gold grade is 1.00-35.32 g/t, with an average of 3.61 g/t and a coefficient of variation of 71.37%,indicating that useful components are evenly distributed in the orebodies. There are three parts with the enrichment of high gold grade in the orebodies. Among them, the shallow parts are located at an elevation of 0- -500 m in the Xinli and Sanshandao ore blocks, while the deep parts lie at an elevation of -1000- -2000 m in the Xiling ore block and the sea-area ore block in northern Sanshandao (Fig. 10b). Therefore, the orebody-rich parts highly coincide with the parts with thick orebodies. This indicates that there is a positive correlation between the thickness and gold grade of the orebodies. That is, the thicker the orebodies, the higher the gold grade. The weakly mineralized sections between the shallow parts and the deep parts of orebodies are located at an elevation of-600- -800 m (Fig. 10c).

Fig. 10. Vertical thickness and grade contour maps of the Sanshandao gold deposit. a-thickness contour map, b-grade contour map,c-grade×thickness contour map.

4.2.2. Characteristics of main orebodies in each ore block

The I-1 main orebody in the Xinli ore block is controlled by 85 boreholes. It occurs at an elevation of -30- -710 m,with a maximum controlled length along its strike and dips direction of 1145 m and 900 m, respectively. It has an overall strike of 62°, a dip direction of SE, and a dip angle of 33°-67°(average: 46°). It is present as a huge vein mostly and is stratoid and lensoid in shape locally, and it extends in a gentle wave pattern along the strike and dip direction. Its thickness is 0.48-28.96 m, with an average of 7.42 m and a coefficient of variation of 78.27%, indicating a relatively stable thickness.The gold grade of single-drilling-controlled orebody of the orebody is 1.52-12.53 g/t, with an average of 3.26 g/t and a coefficient of variation of 156.09%, indicating that useful components are unevenly distributed in the orebody.

The I-1 orebody in the Sanshandao ore block occurs at an elevation of -10- -1950 m, with a maximum controlled length along its strike and dip direction of 1020 m and 1450 m, respectively. It has an overall strike of 35°, a dip direction of SE, and a dip angle of 37°-44°. It is presented as irregular veins and is discontinuously distributed along the strike and dip direction. It reappears after pinching out in some parts. Its thickness is 0.95 m-12.08 m, with an average of 6.65 m and a coefficient of variation of 68.9%, indicating a stable thickness. The ore grade of the single-drilling-controlled orebody is 1.74-5.65 g/t, with an average of 3.25 g/t and a coefficient of variation of 70.6%, indicating that useful components are unevenly distributed in the orebody.

The I-2 orebody in the Xiling ore block is controlled by 42 boreholes and occurs at an elevation from -920 m to-2250 m. It connects the deep main orebody of the Sanshandao ore block in the south and the deep main orebody of the sea-area ore block in the north (Fig. 9). The drillingcontrolled length along the strike and the maximum depth along the dip direction of the orebody is 1940 m and 1500 m,respectively. The controlled orebody is present in the shape of a huge vein as a whole and is stratoid and large-lensoid in shape locally. It has an overall strike of 25°-36°, a dip direction of SE, and a dip angle of 26°-53°. Its thickness ranges from 1.61 m to 97.55 m, with an average of 10.50 m and a coefficient of variation of 123%, indicating a relatively stable thickness. The gold grade of a single sample of the orebody is 0.05-35.19 g/t, with an average of 4.30 g/t and a coefficient of variation of 210%, indicating that the components unevenly change in the orebody.

The sea-area ore block consists of a shallow and a deep orebody group. There are eight gold orebodies in the shallow orebody group. All of them occur on the footwall of the main fracture plane of the Sanshandao fault except for the II-1 orebody, which occurs on the hanging wall of the main fracture plane of the Sanshandao fault (Fig. 11a). There are five gold orebodies in the deep orebody group, and they are nearly parallelly distributed on the footwall of the main fracture plane of the Sanshandao fault. The shallow and deep orebody groups occur in the parts of the Sanshandao fault with relatively gentle dip angles and constitute two orehosting steps. The ore-free interval between the shallow and deep orebody groups along the No. 30 exploration line lies at an elevation from -700 m to -1000 m (Fig. 11b). The I-4 orebody in the deep orebody group is the main orebody. It is controlled by 29 boreholes and occurs at an elevation from-796 m to -1736 m. The drilling-controlled lengths along the strike and the dip direction are 1446 m and 1072 m,respectively. It is present in a large vein shape generally and is stratoid and large-lensoid in shape locally. Meanwhile, it extends in a gentle wave pattern along the strike and dips direction. It has an overall strike of about 35°, a dip direction of SE, and a dip angle of 21°-52° (average: 39°). Its thickness is 1.07-101.86 m, with an average of 30.91 m and a coefficient of variation of 78%, indicating a stable thickness.The gold grade of a single sample of the orebody is 0.05-213.32 g/t, with an average of 5.23 g/t and a coefficient of variation of 202%, indicating that the components are unevenly distributed in the orebody.

Fig. 11. Map showing the horizontal projection of the middle section at an elevation of -400 m (a) and the section of No. 30 exploration line(b) of the sea-area ore block in the Sanshandao gold deposit.

4.2.3. Three-dimensional spatial distribution of orebodies

According to the three-dimensional geological model of the Sanshandao gold deposit established based on the data of 311 boreholes, the Sanshandao fault is generally in the shape of a dustpan consisting of a steep upper part and a gentle lower section (Fig. 12a), and the main orebodies and thick orebodies are distributed in parts with low dip angles along the fault (Fig. 12b). There are a shallow and a deep orebody group along the fault. Among them, the shallow orebody group is mainly located at an elevation of above -800 m, and it includes 11 gold orebodies, including the II-1, I-1, I-2, I-5,I-6, I-7, I-8, and I-9 orebodies in the northern sea area and the I-1, I-2, and I-4 orebodies in the Sanshandao ore block. The deep orebody group is located at an elevation of below -1000 m,and it consists of 12 main gold orebodies, including the I-3, I-4-1, I-4-2, I-4-3, and I-4-4 main gold orebodies in the northern sea area and the I-1, I-2 (deep), I-3, I-4, I-5, I-6, and I-7 orebodies in the Xiling ore block.

Fig. 12. Three-dimensional perspectives of ore-controlling faults (a) and main orebody distribution (b) of the Sanshandao gold deposit.

Almost 3000 pieces of ore data were evenly extracted from the three-dimensional model after interpolation (2849).According to these data, the orebodies have a large thickness range of 1.00-122.83 m (average: 12.60 m) dominated by 1.00-10.00 m (Fig. 13a). The three-dimensional thickness distribution map (Fig. 14a) shows an area with thick orebodies, indicating that thick orebodies are intensively distributed. Meanwhile, the orebodies show alternating high and low thicknesses along with their strike and dip directions.

Fig. 13. Histograms of the distribution of orebody thickness (a) and gold grade (b) of the Sanshandao gold deposit.

Fig. 14. Three-dimensional distribution maps of the thickness (a) and gold grade (b, grade values magnified by 10 times) of orebodies in the Sanshandao gold deposit.

According to these data, the ore grade ranges from 1.00 g/t to 35.32 g/t, with an average of 2.43 g/t. The grade data are evenly distributed but slightly vary (Fig. 13b). As shown in the three-dimensional gold grade contour map (Fig. 14b), the gold grades of the orebodies are evenly distributed, with small relative differences. Gold grade tends to be high in areas with thick orebodies, and there is a noticeable positive correlation between the thickness and gold grade of the orebodies.

The value of gold grade × thickness can reflect the enrichment of mineralization. In detail, the higher the values,the richer the mineralization. A total of 2849 values of gold grade × thickness were evenly extracted after interpolation.According to these data, the values of gold grade × thickness fall in the range of 1.00-892.58 m g/t (average: 51.83 m g/t)dominated by 1.00-100 m g/t. The orebody model shows an ore-rich section with the values of gold grade × thickness of greater than 240 m g/t, which are much higher than those of its surrounding areas (Fig. 10c).

4.3. Mineralization characteristics

4.3.1. Ore types

The ores in the Sanshandao gold deposit can be divided into the following three natural types according to their mineralization characteristics and their alteration and fragmentation degrees.

(i) Disseminated to veinlet-stockwork beresite type (Fig.15a, b). The ores are grayish-green -dark gray, and its original rock is mostly granite. They were fractured into fine-grained and uniform cataclasites and then experienced alteration and mineralization later. The ore textures mainly include lepidogranoblastic texture, cataclastic texture, and porphyroclastic texture, along with interstitial and poikilitic textures. In terms of structures, the ores are dominated by fine-grained (pyrite) disseminated structure and densely disseminated structure, as well as veinlet-stockwork structure.The gangue minerals mainly include quartz, sericites, with small amounts of calcites. Meanwhile, the metallic minerals mainly include pyrite and a small amount of chalcopyrite,galena, and sphalerite.

Fig. 15. Photos of main gold ore types in the Sanshandao gold deposit. a-disseminated beresite type; b-veinlet-stockwork beresite type;c-veinlet-stockwork beresitized granite cataclasite type; d-veinlet-stockwork potassiumized and beresitized granite type.

(ii) Veinlet-veined beresitized granitic cataclasite type(Fig. 15c). The ores are grayish-white, gray, and light flesh red and show cataclastic textures and a taxitic structure. Pyrite is fine-grained disseminated and forms veinlet and stockwork distribution together with silicified quartz, forming veinlet disseminated and stockwork structures. The gangue minerals mainly include quartz, feldspar, sericite, and a small amount of calcite. Meanwhile, the metallic minerals include mainly pyrite, with minor chalcopyrite, harrisite, and sphalerite.

(iii) Veinlet-stockwork and veined beresitized and potassiumized granite type (Fig. 15d). The ores are light flesh red and grayish-white and show blastogranitic textures. Pyrite and gray silicified quartz are distributed in veins and veinletstockwork, forming a veined and veinlet-stockwork structure.The gangue minerals include mainly feldspar and quartz, with minor sericite. The metallic minerals are dominated by pyrite,with chalcopyrite occasionally visible.

4.3.2. Ore mineralogy

The major metal minerals are pyrite, and the secondary ones are galena, sphalerite, chalcopyrite, arsenopyrite,pyrrhotite, limonite, and magnetite. Primary non-metallic minerals include quartz, sericites, and residual feldspars, and secondary non-metallic minerals are carbonate minerals(calcites, dolomites, and siderites). Gold minerals in the ores mainly include electrum, followed by native gold and kustelite (Table 2). Among them, pyrite is the main goldbearing mineral, followed by galena and quartz.

Table 2. Mineral composition of ores.

4.3.3. Chemical composition of ores

According to the whole-rock analysis results of ore samples (Table 3), the SiO2content is high (59.82%-72.96%),with an average of 67.54%. Most of the ore samples fall into the zones of granodiorite and granite, and their K2O + Na2O content is lower than that of normal granitoid ore-hosting rocks (Fig. 3). The Fe2O3+ FeO content is high, and there is a significant positive correlation between it and S (Fig. 16a),with a correlation coefficient ofR= 0.954 (R2= 0.911). These indicate that the Fe2O3+FeO content is mainly correlated with pyrites. Meanwhile, the K2O content is also high, which is due to the high sericite content.

Table 3. Composition of major elements (%) and trace elements (10-6 for Au, Ag, As, Sb, and Bi; 10-9 for Hg, and % for others).

Fig. 16. Curves showing relationships of (Fe2O3+FeO) vs. S (a), Au vs. Ag (b), Au/Ag vs. As/Sb (c), and (Au+Ag) vs. (Cu+Pb) (d) of ores.

The primary useful element in the ores is Au, the associated beneficial elements include Ag, S, Cu, Pb, and Zn,and the hazardous element is As. There is a positive correlation between Au and Ag (Fig. 16b), with a correlation coefficient ofR= 0.7971 (R2= 0.6354). There is also a positive correlation between Au/Ag and As/Sb, with a correlation coefficient ofR= 0.8768 (R2= 0.7688) (Fig. 16c),which reflects that As-bearing minerals such as arsenopyrites are correlated with gold minerals. There is also a certain correlation between Au+Ag and Cu+Pb (Fig. 16d), with a correlation coefficient ofR= 0.9204 (R2= 0.8471), which reflects the relationships of gold and silver minerals with polymetallic sulfides such as chalcopyrites and galenas. The content of hazardous element As is high, with an average of 137.1× 10-6(3.2×10-6-625.0×10-6).

According to the analytical results of combined samples,the average grades of associated components silver, sulfur,and lead are 6.4×10-6(0.5×10-6-130.0×10-6), 2.78%(0.27%-11.59%), and 0.39% (0.001%-4.40%), respectively,which reach the standards for comprehensive utilization.Meanwhile, the contents of copper and zinc are 0.0222%(0.001% -0.280%) and 0.0799% (0.001% -3.020%),respectively, which are below the standards for comprehensive utilization.

4.3.4. Textures and structure of ores

The ore structures mainly include the disseminated structure, followed by stockwork, brecciated, and veinlet structures. The disseminated structure mainly refers to that metal sulfides such as pyrite, chalcopyrite, galena, and sphalerite, which are densely or sparsely distributed in gangue minerals. The stockwork structure refers to that polymetallic sulfides are present in a veinlet form and they fill the fissure network of beresite. The brecciated structure is developed as follows. Some ores are brecciated after fracturing, and they are cemented by quartz or polymetallic sulfides. The veinlet structure (Fig. 18d) refers to that metal sulfides such as pyrite,chalcopyrite, galena, and sphalerite are distributed in altered rocks such as beresite and quartz veins in the form of veinlets with different vein magnitudes.

Fig. 17. Granular lepidoblastic texture (a), cataclastic texture (b), metasomatic texture (c), and veinlet texture (d) of ores.

Fig. 18. Diagrams showing characteristics of gold minerals in the Sanshandao gold deposit. a-line chart of trace element contents in gold minerals; b-histogram of gold mineral contents; c-histogram of grain size of gold minerals; d-histogram of occurrence state of gold minerals.

The ore texture is mainly lepidogranoblastic texture,followed by cataclastic, porphyroclastic, granular, and metasomatic textures. Besides, a small number of ores have emulsion texture. The lepidogranoblastic texture (Fig. 17a) is mainly composed of fine-grained granular minerals (feldspar,quartz, and calcite) and flaky sericite, all of which are closely arranged. The cataclastic texture (Fig. 17b) is developed from the fracturing of minerals in original rocks, which results in irregularly shaped particles. As for the porphyroclastic texture, the porphyroclasts are angular in shape and randomly distributed, and they suffer metasomatic replacement by quartz and sericite, showing unclear boundaries partially. The granulitic texture consists of angular granular minerals, which suffer metasomatic replacement by sericite and quartz into pseudomorph minerals and show unclear or no boundaries mostly. The metasomatic texture is developed by the interpenetration replacement of early metal sulfide minerals by late metal sulfide minerals, such as the metasomatic replacement of pyrites by arsenopyrites and galena (Fig. 17c)and the interpenetration replacement of quartz by chalcopyrite. As for the emulsion texture, chalcopyrite formed from dissolution is distributed in galena, thus forming the emulsion texture.

4.3.5. Mineral characteristics of ores

(i) Metallic minerals

Pyrite. Pyrite is not only the main metal mineral in the ores but also the most important gold-bearing mineral. Its content accounts for more than 90% of the total metallic mineral in the ore. It is xenomorphic-semi-xenomorphic granular and irregularly granular in shape mostly and is distributed in a disseminated or veinlet pattern. Its particle size is mainly 0.01-2.00 mm. Generally, the gold grade is high in parts rich in fine-grained pyrites. It can be classified into early and late generations. The early generation pyrite is irregularly granulitic, with a small amount of cubic euhedral crystals distributed in quartz and sericite. Owing to later hydrothermal transformation and stress, it is often crushed by extrusion, shows crack development, and is replaced by minerals such as late fine-grained pyrite and arsenopyrite or filled by the late generation of pyrite. Coarse-grained pyrite often contains chalcopyrite and pyrrhotite or is interspersed with galena and chalcopyrite veinlets at the stage of goldbearing polymetallic sulfides. The late generation fine-grained pyrite, together with chalcopyrite, galena, and tennantite, is distributed in quartz, and the early generation of pyrite in veinlet and stockwork patterns.

Arsenopyrite. Arsenopyrite is one of the major goldbearing minerals, its content is second only to pyrite, but its content varies greatly in different sections. It generally has automorphic or semi-automorphic granulitic textures, with a grain size of 0.005-1.00 mm. It is distributed in non-metallic minerals or early pyrite and metasomatized early pyrite. It tends to contain minerals such as chalcopyrite, pyrrhotite, and galena. Besides, it bears sphalerite inclusions locally.

Galena. Galena mainly has a particle size of 0.005-0.3 mm. It frequently occurs in pyrite and non-metallic minerals in veinlet and disseminated patterns or produces metasomatized pyrite and tennantite. It commonly coexists with electrums and is distributed in pyrite fissures in a veined pattern. It is also visible that the galena is distributed in a veined pattern alone or together with tennantite and chalcopyrite in the form of xenomorphic crystals.

Sphalerite. Sphalerite is distributed among gangue mineral grains in a xenomorphic granular form or distributed in pyrite fissures in vein aggregates. Besides, a small amount of it,together with chalcopyrite, is distributed among gangue mineral grains in a xenomorphic microparticle shape.

Chalcopyrite. Chalcopyrite is frequently wrapped in pyrite in xenomorphic or irregular forms, fills pyrite fissures or nonmetallic mineral fissures in veined or stockwork forms, or coexists with galena. It is commonly dissolved in sphalerite in the form of emulsion. Sometimes, it is distributed in sericitolite alone in a disseminated form.

(ii) Non-metallic minerals

Quartz. Quartz is the most important non-metallic mineral in the Sanshandao gold deposit and can be divided into primary and secondary types. The primary quartz is automorphic or semi-automorphic in shape, with large grain size. It is mostly distributed in the form of aggregates, lumps,and veins-veinlets. The secondary quartz is xenomorphic crystals with a small grain size or occurs in a sub-rounded shape. It often appears in aggregate with sericite. It is of hydrothermal alteration origin, is closely related to gold mineralization, and contains a small amount of intergranular gold.

Sericite. Sericite is also the major non-metallic mineral in the ore, which is a fine-grained aggregate. It is mainly the product of metasomatic potassium feldspar and plagioclase.Meanwhile, it is a hydrothermal alteration mineral and is frequently associated with fine-grained quartz and disseminated pyrite.

Calcite. Calcite in the ores is scattered in a xenomorphic granular form, with some aggregates distributed in a veined pattern.

(iii) Gold minerals

The gold minerals in the Sanshandao gold deposit include native gold and electrum. Native gold is golden yellow-bright golden yellow, while electrum is light milky yellow with poor polishability. According to the quantitative test results of 139 gold-mineral grains obtained through electron microprobe analysis (EPMA), the chemical components of the gold minerals mainly include Au and Ag, as well as trace elements such as Fe, Cu, Cr, S, Co, Zn, Ni, As, Sb, Bi, Te, and Sb.From shallow to the deep parts of the Sanshandao gold deposit, Au tends to be enriched, Ag tends to be depleted, and all impurity elements tend to be depleted except for Fe, which tends to be enriched (Fig. 18a).

According to the statistics of 39 gold-mineral grains, the gold minerals in the Sanshandao gold deposit mainly include electrum (78.57%), followed by native gold (19.29%), and kustelite (2.14%). Meanwhile, the highest and lowest fineness of the gold minerals is 923 and 372, respectively, with an average of 704. Compared with shallow gold minerals(9.43%), deep gold minerals do not contain kustelite and have a higher content of native gold (29.03%) (Fig. 18b) and a much higher fineness.

According to the statistics of 729 gold-mineral grains, the contents of coarse-grained, medium-grained, fine-grained,micro-fine-sized gold in the deposit are 1.94%, 9.04%,48.43% and 44.59%, respectively, and thus the gold minerals are dominated by fine-grained and micro-fine-sized gold.Compared to shallow gold minerals, deep gold minerals have notably increased grain size. In detail, the content of coarsegrained gold increases from 0.91% to 3.15%, the content of medium-grained gold increases from 6.02% to 11.24%, the content of fine-grained gold increases from 42.15% to 54.38%, and the content of micro-fine-sized gold decreases from 50.91% to 31.24% (Fig. 18c).

According to the statistics of 891 gold-mineral grains,gold minerals in the deposit are mainly in a granular form(Fig. 19a), with granular gold minerals accounting for 73.78%, followed by needle-like, foliaceous, and branch-like gold minerals (Fig. 19b), which account for 26.22%.Compared to shallow gold minerals, deep gold minerals have a lower content of granular gold minerals and higher contents of other forms of gold minerals such as branch-like and linelike gold minerals and tend to have diverse morphologies.

Fig. 19. Gold minerals are distributed in pyrite pores and among pyrite and gangue mineral grains in an angular granular or flaky form (a), and chalcopyrites and gangue minerals in a branch-like form (b).

According to the statistics of 1329 gold-mineral grains,gold minerals in the deposit can be divided into the crack,intergranular, and inclusion types based on their occurrence states. They are dominated by the former two types, which account for 41.56% and 33.59%, respectively. The gold minerals in shallow parts of the deposit are dominated by the crack type (64.91%) and the intergranular type (20.57%),while those in deep parts of the deposit mainly include the intergranular type (60.22%) and the inclusion type (23.82%)(Fig. 18d).

4.4. Ore-forming stages

It can be concluded that mineralized altered minerals were formed in four ore-forming stages from the above-mentioned mineral composition of ores, the mineral paragenesis,crosscutting relationships among minerals, and the textures and structures of ore (Table 4).

Table 4. Ore-forming stages and mineral formation sequence of the Sanshandao gold deposit.

(i) Sericitolite stage: The early stage of mineralization.Sericitolites were formed from hydrothermal metasomatism at this stage, and the mineral assemblage formed is quartz +sericite.

(ii) Gold-quartz-pyrite-arsenopyrite stage: The middle stage of mineralization. At this stage, fine-grained xenomorphic pyrite and arsenopyrite were formed and replaced or filled early pyrite. It is the main ore-forming stage of gold, at which gold minerals mainly included electrum and native gold wrapped in pyrite or distributed in the pores and fissures of pyrite and arsenopyrite. The mineral assemblage formed at this stage is pyrite + arsenopyrite + pyrrhotite +quartz + electrum + native gold.

(iii) Gold-quartz-polymetallic sulfide stage: The middlelate stage of mineralization. At this stage, polymetallic sulfides were fine-grained in shape and were distributed in veinlet and disseminated forms. Owing to the intense filling and metasomatism among minerals, gold (silver) minerals formed at this stage mainly include electrum and kustelite,which are distributed in the fissures of pyrite, galena, and quartz. Therefore, this stage is the late ore-forming stage of gold. The mineral assemblage formed at this stage is pyrite +chalcopyrite + galena + sphalerite + tennantite + freibergite +quartz + electrum + kustelite.

(iv) Quartz-carbonate stage: The end stage of mineralization. At this stage, carbonate minerals such as quartz and calcites were mostly interspersed with ores in the form of veins, with no gold mineralization occurring.

4.5. Resources of the Sanshandao gold deposit

The Sanshandao gold deposit is mainly controlled by intensive drillings. The controlled resources, proven resources, probable mineral reserves, and proven mineral reserves have been explored in different ore blocks individually, with drilling spacing used including (200-240)m× (240-300) m, (100-120) m× (120-160) m, and (20-40)m× (20-60) m (strike × dip direction). The cumulative resources of gold ores and gold of all ore blocks are 333.65×106t and 1240679 kg, respectively, with an average orebody thickness of 7.93 m and an average ore grade of 3.72 g/t. Among them, 81.50×106t of gold ores and 238503 kg of gold are obtained from shallow orebodies, with an average orebody thickness of 8.10 m and an average ore grade of 2.93 g/t. Meanwhile, 252.14×106t of gold ores and 1002176 kg of gold are obtained from deep orebodies, with an average orebody thickness of 7.90 m and an average ore grade of 3.98 g/t. The resources ratio, ore grade ratio, and orebody thickness ratio between deep and shallow orebodies are 4.20, 1.36 and 0.97, respectively. Therefore, the scale and enrichment degree of deep orebodies are both greater than those of shallow orebodies.

5. Discussion

5.1. Metallogenic epoch

There were many disputes about the metallogenic epoch of the Jiaodong gold deposits in the past. Early researchers thought that the Jiaodong gold deposits were formed during the Archean or the Proterozoic. Later, multi-stage mineralization was proposed, that is, Archean, Proterozoic,and the Mesozoic were all the important metallogenic epochs(Li SX et al., 2007). In addition, some studies held that there are three metallogenic events at 150 Ma, 120-110 Ma, and 100-110 Ma (Ding ZJ et al., 2015). At present, most researchers agree that the Jiaodong gold deposits were mainly formed in the Early Cretaceous and concentrated in a short period of about 120 Ma (Yang LQ et al., 2014; Zhu RX et al.,2015; Fan HR et al., 2016).

According to the test results of the Sanshandao gold deposit, the sericite Rb-Sr isochron age and the sericite40Ar-39Ar isochron age are 117.6 ± 3.0 Ma (Hu FF et al., 2013) and 118.56 ± 1.37 Ma (Yang ZY et al., 2020), respectively. As revealed by the test results of the Cangshang gold deposit in the southern part of the Sanshandao fault, the sericite40Ar-39Ar isochron age is 121.1 ± 0.5 Ma (Zhang XO et al., 2003),and the fluid inclusions Rb-Sr isochron age is 113.5 ± 0.6 Ma(Chen YJ et al., 2004).

In recent years, Deng J et al. (2020b) have selected hydrothermal monazites with a symbiotic relationship with natural gold, pyrite, and sericite to carry outin-situU-Pb dating. The SHRIMP U-Pb ages of the Jiaojia and Linglong gold deposits in the western part of the Jiaodong Peninsula are 121.8 ± 3.6 Ma and 120.0 ± 4.6 Ma, respectively, and the LAICP-MS U-Pb ages are 119.8 ± 2.1 Ma and 119.1 ± 1.4 Ma,respectively. The LA-ICP-MS monazite U-Pb ages dated by other researchers are 120.5 ± 1.7 Ma (the Daliuhang gold deposit in Penglai City), 120.0 ± 1.4 Ma (the Xiadian gold deposit in Zhaoyuan City), and 120.0 ± 3.1 Ma (the Hushan gold deposit in Qixia City) (Ma WD et al., 2017; Yang KF et al., 2018; Feng K et al., 2020). All these dated ages fall in the range of 121.8-119.1 Ma.

The40Ar-39Ar ages of the Sanshandao and Cangshang gold deposits are consistent with the monazite U-Pb ages of the gold deposits in the western Jiaodong Peninsula. This paper holds that the40Ar-39Ar age represents the main metallogenic age of the Sanshandao gold deposit, that is, the deposit was formed at about 120 Ma. The Rb-Sr isochron ages are consistent with the early age of Weideshan-type granites and the isotopic ages of silver and nonferrous metal deposits in the Jiaodong Peninsula (118-111 Ma; Song MC et al., 2017), which indicates the time of magmatic-metallogenic events or the age of gold-quartz-polymetallic sulfide stage after the main gold ore-forming stage of the gold deposit.

5.2. Sources of ore-forming fluids and minerals

5.2.1. Nature of ore-forming fluids

Many studies have been conducted on the ore-forming fluids in the Sanshandao gold deposit (Guo CY, 2009; Jiang XH et al., 2011; Deng J et al., 2015a; Wen BJ et al., 2016; Liu YZ et al., 2019), obtaining the following understanding. The ore-forming fluids belong to a medium-low-temperature, lowsalinity, and reducing H2O-CO2-NaCl ± CH4hydrothermal system. The inclusions are generally small, mainly ranging from 3 μm to10 μm. They are mainly of three types, pure CO2, CO2-H2O, and pure water inclusions. Besides,multiphase fluid inclusions containing daughter minerals such as NaCl are also visible. The VCO2+ LCO2inclusions include VCO2+ LH2Otwo-phase and VCO2+ LCO2+ LH2Othree-phase inclusions at room temperature. According to the proportion of vapor phase and liquid phase, the two-phase inclusions can be divided into vapor-rich [V/(V+L)＞ 50%] and liquid-rich[L/(V+L)＞ 50%] subtypes. The pure water inclusions consist of the liquid-phase and vapor-phase types, with the latter accounting for 5%-30% of the total pure water inclusions.The fluid inclusions formed during the early mineralization contain vapor-phase components CO2and H2O, with no CH4and N2being detected. In comparison, rich CH4(Liu YZ et al., 2019) and the presence of N2(Jiang XH et al., 2011) have been detected in the CO2-H2O inclusions formed in the main ore-forming stage. In the liquid phase, the cations mainly include Na+and K+, followed by Mg2+and Ca2+, and the anions mainly include SO42-and Cl-, as well as low content of F-(Guo CY, 2009).

The homogenization temperatures of the fluid inclusions fall in the range of 101°C-400°C, and those at the first,second-third, and fourth ore-forming stages are 239°C-400°C,207°C-336°C, and 101°C-269°C, respectively. The fluid pressure and salinity at the main ore-forming stage were about 90-175 MPa and 2.06%-17.57% NaCleqv, respectively. The fluid density is about 0.61-1.11 g/cm3(Wen BJ et al., 2016;Liu YZ et al., 2019). From the early to the late ore-forming stage, the temperature and pressure significantly dropped,while the salinity was relatively higher at the main oreforming stage (Yang LQ et al., 2014), indicating that the decrease in temperature and pressure is one reason for the gold mineralization.

5.2.2. Sources of ore-forming fluids

The hydrogen-oxygen isotopic components of the gold deposit are widely distributed (Table 5). Some samples fall in the zones of primary magmatic water, metamorphic water,and typical orogenic gold deposits. In comparison, large numbers of the samples fall between primary magma water(or primary mantle water and metamorphic water) and meteoric precipitation and between the mixed trend of chemical precipitation water and that of organic water (Fig.20). The fluid inclusions in different minerals greatly differ in hydrogen-oxygen isotopic characteristics. In detail, the hydrogen-oxygen isotopic compositions in fluid inclusions of quartz mainly fall near zones of primary magmatic water and primary mantle water, those of sericites mainly fall in the zone representing metamorphic water, and those of pyrites mainly fall near the Mesozoic meteoric water and formation water in the Jiaodong Peninsula. Quartz existed throughout the pre-ore and the ore-forming stage, sericites were formed from mineralized alteration at the ore-forming stage, and pyrites were formed at the main ore-forming stage and thereafter. Given these, it is considered that the ore-forming fluids before gold mineralization were dominated by primary mantle water or magmatic water, and metamorphic water appeared in the early stage of gold mineralization. With the progress of mineralization, meteoric water entered the fluids,and the late ore-forming fluids were dominated by meteoric water.

Table 5. Hydrogen and oxygen isotopic compositions of fluid inclusions in the Sanshandao gold deposit.

Fig. 20. δD vs. δ18O diagram of ore-forming fluids in the Sanshandao gold deposit (after Taylor HP, 1974; Sheppard SMF,1986 for the base map).

The3He/4He ratio of fluid inclusions in pyrite is 0.043-2.94 Ra (Han ZY et al., 2019; Wen BJ et al., 2016),which is between the crustal value (0.01-0.05 Ra) and the mantle value (6-9 Ra) (Stuart FM, 1995; Burnard PG, 1999),indicating the characteristics of crust-mantle mixing composition. Mantle-derived He content was calculated to be 0.37%-2.51% (Han ZY et al., 2019) using the formula:

mantle-derived He (%) = [(R-Rc)/(Rm-Rc)]×100,

where Rc end-member value = 2×10-8and Rm endmember value = 1.1×10-5(Han ZY et al., 2019).

This indicates that He in the Sanshandao gold deposit mainly comes from the crust and a small amount of mantle source. Meanwhile, the40Ar/36Ar values range from 488.00 to 5926.44, and the He-Ar isotopic composition of the fluids falls between the crust source and mantle source areas on the relevant diagrams (Han ZY et al., 2019; Wen BJ et al., 2016).As indicated by these He-Ar isotopic characteristics, the oreforming fluids were mainly sourced from the crust and were mixed with varying volumes of deep mantle-derived fluids,and then meteoric water entered the ore-forming fluids during the rise of the fluids.

The carbon isotopic data of calcites, siderites, ankerites,and quartz formed in the late ore-forming stage show that theδ13CPDBvalues fall in the range of-6.6‰ - -1.9‰ (Table 6),which is close to theδ13C values of igneous/magmatic system(-30‰ - -3‰) and theδ13C values of carbon reservoirs of the mantle (-7‰ - -5‰; Hoefs J, 1997). This indicates that the carbon in the ore-forming fluids comes from the magmatic system or the mantle.

Table 6. Carbon isotopic analysis results of fluid inclusions in the Sanshandao gold deposit.

5.2.3. Sources of ore-forming materials

(i) Sulfur isotopes. The Sanshandao gold deposit is enriched inδ34S, with theδ34S values falling in the range from+7.1‰ - +12.8‰ (from +10.1‰ to +12.8‰ mostly; Table 7),which deviates from meteorite sulfur positively, and the sulfur isotopes are highly homogeneous. Theδ34S values are in the order of pyrites ＜ sphalerites ＜ galenas in the deposit,indicating that sulfur isotopic fractionation roughly reached equilibrium during the mineralization. Theδ34S values of the Sanshandao gold deposit overlap with those of relevant geological bodies to a high extent (Fig. 21), indicating that the deposit inherited sulfur isotopes from the relevant geological bodies (Yang LQ et al., 2014). Related geological bodies include Early Precambrian metamorphic rocks, Guojialingtype granites, intermediate-basic vein rocks, Linglong-type granites, and Weideshan-type granites, of which theδ34S values gradually increase. According to statistics, the peakδ34S value of the Sanshandao gold deposit is higher than the averageδ34S value of relevant geological bodies and is closer to the Jurassic crust-derived Luanjiahe granitic pluton(belonging to Linglong-type granite), reflecting that the crustderived materials contribute significantly to sulfur sources.

Fig. 21. Sulfur isotopic compositions of ores and geological bodies related to gold mineralization in the Sanshandao gold deposit (after Li JJ,2006 for geological bodies related to gold mineralization).

Table 7. δ34S value of the Sanshandao gold deposit.

(ii) Lead isotopes. The Pb isotopic compositions of five samples from the Sanshandao gold deposit slightly vary, with the206Pb/204Pb,207Pb/204Pb, and208Pb/204Pb ratios of 17.167-17.464, 15.235-15.766, and 37.269-38.763, respectively(Table 8). The characteristic values of source areas of Pb isotopes in the deposit are as follows. Theμvalue (7.5) is lower than the normalμvalue of the crust (9.58) and close to theμvalue of the mantle (7.3-8.0). Theωvalue (33.32) is lower than that of the crust (36.50), and the Th/U value (4.38)is higher than the normal Th/U value of the crust (3.8),indicating that the ore-forming materials are partially mantlederived. In the tectonic setting discrimination diagram (Fig.22) proposed by Zartman RE and Dou BR (1981), the samples fall into the lower crust and are consistent with the falling scope of Pb from the Jiaodong gold deposits, both indicating the characteristics of Pb isotope in the lower crust.Overall, Pb in the Sanshandao gold deposit was mainly sourced from the lower crust, with a small amount originating from the mantle.

Table 8. Pb isotopic components of ores in the Sanshandao gold deposit (after Wang YW, 1988; Li SX et al., 2007).

Fig. 22. Tectonic setting discrimination diagram of Pb isotopes in ores in the Sanshandao gold deposit (after Zartman RE and Dou BR,1981). Note: the reference ranges of Jiaodong gold deposits are from Yang LQ et al., 2014.

(iii) Strontium isotopes. The initial87Sr/86Sr ratio of calcites in the Sanshandao gold deposit is 0.710657-0.711542(Deng J et al., 2015a), which is higher than 0.710 and overlaps with the87Sr/86Sr ratio ranges of Linglong-type granites, Guojialing-type granites, and mafic dykes formed during the ore-forming stage to a high extent (Yang LQ et al.,2014). This indicates crust-derived characteristics. Therefore,the ore-forming materials may directly come from Linglong and Guojialing-type granites, and some ore-forming materials may be provided by mafic dykes or their magmatic source areas during the metallogenic period.

5.3. Ore-controlling mechanisms of faults and metallogenic model

5.3.1. The control of faults on gold mineralization

(i) The shielding effect of fault gouges on gold mineralization

The Sanshandao gold deposit has high-number and largescale orebodies, most of which occur on the footwall of the main fracture plane of the Sanshandao fault and are distributed in parallel to the main fracture plane. The main orebodies generally occur in the beresitized cataclasite and beresitized granitic cataclasite near the main fracture plane,while the secondary orebodies occur in the beresitized granitic cataclasites and beresitized granite that are slightly far away from the main fracture plane. In addition, the steep quartzvein-type orebodies that obliquely cross the main fracture plane and the main orebodies occur in the weakly altered granites far away from the main fracture plane. Although sericitized alteration occurs on the hanging wall of the fault,the pyritization on the hanging wall is noticeably weaker than that on the footwall, and only small-scale gold orebodies are locally visible. Fault gouges with a thickness of about 0.05-0.50 m occur continuously and stably along the main fracture plane of the fault. They are mainly composed of clay minerals such as kaolinite and montmorillonite, with no gold mineralization and alteration. The enrichment degree of gold mineralization is positively correlated with the thickness of the fault gouges, and the gold grade tends to be high in parts with thick fault gouges (Guo CY, 2009). The fact that sericitized alteration occurs on both the hanging wall and the footwalls of the fault while gold orebodies mainly occur on the footwall implies that the fault gouges imposed no impact at the sericitolite stage in the early ore-forming stage but blocked and shielded the migration of ore-bearing fluid in the main ore-forming stage. The grayish-black fault gouges in the Jiaojia fault were previously dated to be 131.05-123.53 Ma(Song MC et al., 2010a), which is slightly older than the gold mineralization, also proving that the gold mineralization has not penetrated upward through fault gouges. The reasons for fault gouges controlling the distribution of the Sanshandao gold deposit are as follows. On one hand, the clay minerals dominant in the fault gouges have a dense structure and poor permeability; thus it is difficult for ore-bearing fluid to penetrate the fault gouge zone. On the other hand, as the major component of the hanging wall of the fault, the Early Precambrian metamorphic rock series has experienced longterm deformation-induced reformation, weathering and denudation, and penetration of meteoric water, which combined with a low-temperature form a circulation system of the oxidizing aqueous solution. Meanwhile, the Mesozoic granites that constitute the footwall of the fault have an age close to the gold mineralization and have a high temperature,forming a circulation system of reducing aqueous solution.The junctions of the above two aqueous solution circulation systems serve as parts favorable for the accumulation and precipitation of ore-bearing fluids.

(ii) The effect of the changes in fault occurrence on gold mineralization

The Jiaodong gold deposits are controlled by faults.Massive studies were previously conducted focusing on the relationships between faults and gold mineralization and the ore-controlling regularity of faults, obtaining some understanding such as gold orebodies occurring at the turning parts of faults and fault junctions, pitching regularity of the orebodies along faults, and gold deposits controlling at broom-like structures and the composite parts of NNE-NE-trending faults and EW-trending basement structures, as well as noticing that the Cangshang and Xinli gold deposits in the Sanshandao fault zone both occur in the parts of the NNE-trending fault turning to NE-trending fault and its NE-trending section, respectively (Li HJ, 2002; Li SX et al., 2004;Song MC et al., 2015b; Deng J et al., 2019; Wang SR et al.,2020). Based on deep prospecting results, researchers have proposed some important new understanding of the relationships between the Sanshandao fault and gold deposits,as stated below. It is considered that gold orebodies are mainly enriched in the fault parts with a transition between steep-gentle dip angles and fault sections with gentle dip angles, thus forming a stepped distribution pattern (Song MC et al., 2015b). In the Xinli ore block, the changes in the strike and dip angle of faults control the fluid accumulation and the formation of ore-rich pillars (Yang L et al., 2018). In the Sanshandao ore block, the deposit is distributed in the parts where the Sanshandao fault turns from the NE trending to the NEE-trending, ore zones become wider and gentler and pitch northward, and high-grade ore zones occur in the fault parts with gentle dip angles (Zhang L et al., 2020a).

As stated above, the shallow and deep orebody groups in the sea-area ore block are both distributed in fault sections with a gentle dip angle, thus forming a stepped metallogenic model. The first orebody step in the shallow part is at an elevation of above -800 m, where the dip angle of orebodies is 25°-50°. The second orebody step in the deep part lies at an elevation of below -1000 m, where the dip angle of orebodies is 35°-40°. Weak mineralization or even no ore occurs between the first and second steps, where the dip angle of the Sanshandao fault is 75°-85° and the vertical distance is about 200 m. For the deep orebody-rich parts in the sea-area ore block, fault dip angles and the values of grade × thickness of orebodies were extracted every 20 m along the sections of the dip direction and strike of the ore-controlling fault (Fig. 14)according to the established three-dimensional geological model of deposits. Based on these data, the relationships of the fault dip angle versus the value of grade × thickness of orebodies were plotted (Fig. 23). As shown in the diagram showing the relationship obtained based on the section along the dip direction of the fault (Fig. 23a), the fault dip angle roughly changes in five sections from shallow to deep parts,and the details are as follows. Section I is between the seabed and an elevation of -420 m, where the dip angle sharply changes from low to high and the slope increases from 36.24°to 61.96°, with a slope difference of 25.72°. In this section,the value of grade × thickness of orebodies ranges from 1.00 m g/t to 32.66 m g/t, indicating medium ore-discovery effects.Section II is between -420 m and -800 m in elevation. It is a structurally steep section with a slope of 70.75°-80.78°, and it is an ore-free section. Section III is between -800 m and-1000 m in elevation, where the slope decreases from 77.23°to 42.47°, with a difference of 34.76°. Therefore, it is a section with a steep-to-gentle transition of the fault dip angle.As the slope gradually decreases, the value of grade×thickness of orebodies gradually increases, and it is up to 305.65 m g/t when the slope decreases to the minimum.Section IV is between -1000 m and -1800 m in elevation. In this section, the slope is 39.47°-52.86°, and it is low overall and slightly fluctuates. Therefore, this section is gentle and is also an orebody-rich section, where the value of grade ×thickness of orebodies is 79.72-552.28 m g/t, with an average of 322.48 m g/t. Section V is between -1800 m and -1960 m in elevation, where the slope tends to significantly increase and reaches 51.31° at an elevation of -1960 m. It is an orefree section. On the relationship diagram obtained based on the section along the fault strike (Fig. 23b), the fault dip angle also noticeably changes, and areas with high values of grade ×thickness of orebodies coincide with the areas with a distinct steep-to-gentle transition of the fault dip angle. The above sections along the two directions show that the changes in the fault dip angle have noticeable impacts on mineralization, and the enrichment degree of gold mineralization is high in the sections with a sharp steep-to-gentle transition of the fault dip angle. This relationship between the Sanshandao fault and orebodies strongly supports the stepped distribution model of deep gold orebodies (Song MC et al., 2012).

The controlling effect of the changes in the fault dip angle on the gold mineralization is related to the pressure change in the process of fluid flow. The research shows that water/rock reactions, fluid immiscibility, and fluid mixing are the main mechanisms leading to gold precipitation in the Jiaodong gold deposits (Zhang L et al., 2020b), and pressure fluctuation is the main reason for fluid immiscibility (Yang LQ et al.,2016). When fluids flow through a fault section with a sharp change of fault dip angle, the pressure fluctuation of the fluids increases significantly, resulting in fluid immiscibility, which provides favorable conditions for gold precipitation. When migrating in a fault section with a steep dip angle, oreforming fluids migrate rapidly from the deep area with high stress to the shallow area with low stress. In this case, the fluids are not liable to be deposited and mineralized. When the fluids flow through a fault section with a steep-to-gentle transition of the fault dip angle and then migrate toward a fault section with a gentle fault dip angle, the pressure sharply drops, the flow rate significantly reduces. In this case, fluid immiscibility tends to occur, resulting in gold precipitation and mineralization. Therefore, gold mineralization mainly occurs in fault sections with a changing or relatively gentle fault dip angle.

5.3.2. Tectonic setting of gold mineralization

The massive gold mineralization in the Jiaodong Peninsula occurred under the background of the North China Craton destruction and lithospheric thinning, which is closely related to the spatio-temporal relationship of extensional structures (Qiu YM et al., 2002; Mao JW et al., 2008; Deng J et al., 2015b; Li L et al., 2015; Song MC et al., 2015b; Yang QY and Santosh M, 2015). Since the Late Mesozoic, the eastern part of the North China Craton has experienced largescale lithospheric destruction characterized by lithospheric thinning, forming large-scale extensional structures represented by faulted basins, detachment faults, and metamorphic core complex (Zhang YQ et al., 2004;Ratschbacher et al., 2000; Liu JL et al., 2006; Wang T et al.,2007). Late Mesozoic extensional structures are widely developed in the Jiaodong Peninsula. Among them, the Jiaolai Basin is an extensional superimposed basin featuring northern fault and southern stratigraphic overlap between the North China Craton and the Sulu Orogenic Belt. A very thick volcanic sedimentary rock series were deposited in the basin.Among them, the volcanic interbeds in the Laiyang Group in the lower part of the basin yield isotopic ages of 129.7 ± 1.7 Ma and 129.4 ± 2.3 Ma (Zhang YQ et al., 2008), the volcanic rocks in the Qingshan Group in the central part of the basin yield an isotopic age range from 123.6 ± 3.1 Ma to 98 ± 1 Ma(Qiu JS et al., 2001; Ling WL et al., 2007; Zhang YQ et al.,2008; Liu S et al., 2009; Kuang YS et al., 2012), and the volcanic interbeds in the Wangshi Group in the upper part of the basin yield an isotopic age of 73.2 ± 0.3 Ma (Yan J et al.,2005). In addition to the parallel unconformity between the Qingshan and Laiyang groups, and the angular unconformity between the Wangshi and Qingshan groups, the basin was in a continuous extensional state during 130-73 Ma. Latitic volcanic rocks and bimodal volcanic rocks are developed in the Qingshan Group, indicating a strong crustal extension process. Meanwhile, the geochemical characteristics of these volcanic rocks indicate that the rocks were formed in a backarc extensional active continental margin environment (Liu S et al., 2009; Kuang YS et al., 2012). Metamorphic core complexes in the Jiaodong Peninsula include Linglong,Queshan, and Kunyushan (Yang JZ et al., 2000; Charles et al.,2011), of which the cores consist of Jurassic Linglong-type granites, the inside contains a small amount of Guojialing or Weideshan-type granites, and the edges are surrounded by Early Precambrian metamorphic rocks or Mesozoic sedimentary strata. The detachment faults in the Jiaodong Peninsula include boundary faults of metamorphic core complexes, Mesozoic basin-margin faults, and late bedding slip faults in the Early Precambrian basement. As revealed by the S-wave velocity structure obtained through the ambient noise tomography of short-period dense seismic array profiles(Yu GP et al., 2020), the faults in the northwest Jiaodong Peninsular all show low dip angles and, meanwhile, the basement/shallow seismic discontinuities with high-velocity anomalies on the hanging walls and footwalls of the faults represented by the Sanshandao and Zhaoping faults with a dip direction of SE show remarkable dislocation. All these indicate that the faults in the northwest Jiaodong Peninsular are large-scale extensional detachment faults. Charles et al.(2013) proposed that the extensional structural characteristics in the Jiaodong Peninsular are consistent with the wide rift model.

The mineralization of the Jiaodong gold deposits exactly occurs during the above-mentioned extensional tectonic activity, and the metallogenic epoch is consistent with the isotopic age of the volcanic rocks in the Qingshan Group at the peak of the extensional tectonic activity. All these indicate that the Jiaodong gold deposits were formed in an extensional tectonic setting. The study of extensional structures in East China and its adjacent areas shows that the extensional structures evolved in two stages, namely 140-125 Ma, at which the original extensional detachment structures were initially formed in the middle-lower crust, and 125-110 Ma,at which the detachment structures experienced rapid uplifting and exhumation (Yang Q et al., 2019). The extensional activity mainly resulted from the roll-back of the subducted paleo-Pacific plate, which led to the collapse of the early thickened crust (Davis GA et al., 2001; Yang JH et al., 2007)and also induced back-arc tension or intraplate extension.Therefore, it can be inferred that the Jiaodong gold deposits were formed during the quick uplift and exhumation of detachment structures, which is attributable to the back-arc tension or intraplate extension induced by the roll-back of the paleo-Pacific plate.

The Jiaodong-type gold deposits that were formed in an extensional tectonic setting are different from internationally known gold deposits. Some researchers classified the Jiaodong gold deposits as orogenic gold deposits (Zhou TH and Lu G, 2000; Qiu YM et al., 2002; Goldfarb RJ et al.,2001; Chen YJ et al., 2004), while others pointed out that the Jiaodong gold deposits are distinctly different from typical orogenic gold deposits (Goldfarb RJ et al., 2014; Deng J et al., 2015a; Li L et al., 2015; Zhu RX et al., 2015). For example, Zhu RX et al. (2015) classified the Jiaodong gold deposits as decratonic gold deposits and considered that the essential difference between them and orogenic gold deposits lies in the extensional tectonic setting of the Jiaodong gold deposits. Li L et al. (2015) defined the Jiaodong-type gold deposits as the gold deposits that were formed in an extensional lithospheric tectonic setting and are distributed along the margins, inner ancient suture zones, or microplate junction zones of an activated craton. The comprehensive analysis shows that the Jiaodong-type gold deposits are significantly different from orogenic gold deposits. Typical orogenic gold deposits in foreign countries are primarily related to the setting of continental-margin accretion induced by the subduction of oceanic crust and were formed from the tectonic-magmatic thermal event at 20-200 Ma after regional metamorphism (Goldfarb RJ et al., 2001). They show the zonings of Au-As, Au-As-Te, and Au-Sb from hypogene to hypergene zones. However, the Jiaodong-type gold deposits are quite different. They were formed in the extensional intracontinental lithospheric setting with continental-arc to back-arc extension (Song MC et al., 2015a; Li L et al., 2015),the gold mineralization occurred about 1.9 Ga later than the regional metamorphism, and the associated beneficial components mainly include Au, Ag, Cu, Pb, and Zn.Therefore, this paper believes that it is suitable to refer to all gold deposits formed in the Late Mesozoic extensional tectonic setting in the Jiaodong Peninsular represented by the Sanshandao gold deposit as Jiaodong-type gold deposits.

5.3.3. Metallogenic model and process

The genetic model of the Jiaodong gold deposits(including the Sanshandao gold deposit) has always been the focus of geologists, and the understanding of it has been constantly deepened and developed. The Jiaodong gold deposits were considered the gold deposits of the greenstone belt type in the early stage (Yang MZ and Lü GX, 1996; Shen BF et al., 1997). Afterward, they were long regarded as magmatic-hydrothermal gold deposits related to Late Mesozoic granitoid intrusions in the geological exploration of gold deposits in the Jiaodong Peninsular (Li SX et al., 2007).Early in this century, they were classified as orogenic gold deposits by some researchers (Zhou TH and Lu G, 2000;Goldfarb RJ et al., 2001; Qiu YM et al., 2002; Chen YJ et al.,2004; Jiang SY et al., 2009) and the gold mineralization model of the devolatilization of a subducted slab was proposed (Goldfarb RJ and Santosh M, 2014; Groves DI and Santosh M, 2016). In recent years, they were widely considered to be different from typical orogenic gold deposits in terms of the metallogenic tectonic setting, great differences between the metallogenic epoch and the metamorphic period of wall rocks, and mineralized alteration characteristics and were correspondingly divided into Jiaodong-type gold deposits, extensional gold deposits, and decratonic gold deposits (Zhai MG et al., 2004; Li L et al., 2015; Zhu RX et al., 2015).

According to a relevant study, the geochemical properties of rocks in the Jiaodong Peninsular were significantly different before and after the gold mineralization during the Late Mesozoic. Meanwhile, the ancient enriched lithospheric mantle was replaced with the depleted oceanic lithosphere(Wu FY et al., 2000), which was reflected in the significant changes in the geochemical characteristics of magmatic rocks.As revealed by the studies on the intermediate-basic intrusions in the Luxi district and volcanic rocks in the Jiaolai Basin, the mantle peridotite xenolith in Early Cretaceous and Late Cretaceous mantle-derived magmatic rocks have different mineral chemical and geochemical characteristics(Dai FQ et al., 2019). Specifically, for the mantle peridotite xenolith in Early Cretaceous mantle-derived magmatic rocks,their geochemical characteristics are similar to those of island arc basalts and they show the features of the ancient lithospheric mantle (Xu WL et al., 2003). In contrast, for the mantle peridotite xenolith in the Late Cretaceous mantlederived magmatic rocks, their geochemical characteristics are similar to those of oceanic island basalts and they show the features of the new depleted lithospheric mantle (Yan J et al.,2005; Liu JM et al., 2004). Meanwhile, the Cretaceous basic dykes developing in the Jiaodong gold ore concentration area also show different geochemical characteristics. The early basic dykes have typical geochemical characteristics of island arc basalts and were formed from the partial melting of the ancient enriched lithospheric mantle that suffered metasomatic replacement by fluid-rich melts. In contrast, the late basic dykes have the geochemical characteristics of oceanic island basalts and may have been sourced from the partial melting of the lithospheric mantle with mantle convection (Ma L et al., 2014; Deng J et al., 2019). Late Mesozoic granitoids in the Jiaodong Peninsular also show important changes in geochemical characteristics, and their changing trends from the Late Jurassic Linglong-type granites to the Guojialing-type granites of the early stage of the Early Cretaceous and the Weideshan- and Laoshan-type granites of the middle stage of the Early Cretaceous are stated as follows.They are shifted from high-K calc-alkaline series to shoshonite series and from peraluminous to metaluminous rocks in terms of petrochemical composition. Their trace element characteristics are shifted from high Ba and Sr to low Ba and Sr, and from high Sr and low Y to low Sr and high Y.Their REEs characteristics are shifted from no or weakly positive europium anomalies to significantly negative europium anomalies. In terms of zircon types, they are shifted from rocks bearing many ancient residual zircons and ultrahigh-pressure metamorphic zircons to rocks bearing no ultrahigh-pressure metamorphic zircons and to rocks bearing no ancient residual zircons. In terms of granite types, they change from S-type to I-A-type granitoids and from adakitic to arc granites. Furthermore, their diagenetic setting is shifted from a compressional to an extensional setting. Among these granites, Guojialing-type granites have transitional characteristics. Their contents of Ba, Sr and Y are similar to those of Linglong-type granites and have the characteristics of adakites. However, the mantle-derived diorite xenoliths and Sr-Nd isotopic characteristics of the rocks are closer to those of Weideshan- and Laoshan-type granites. The Jiaodong gold deposits were exactly formed after the diagenesis of Guojialing-type granites, which is contemporary with the early magmatic activities of Weideshan-type and Laoshantype granites. This indicates that the transformation of mantle properties and the changes in the geochemical composition of magmas in the Jiaodong Peninsular serve as important factors in the gold mineralization in the area.

Late Mesozoic extensional structures and the transformation of mantle properties jointly constitute the important metallogenic setting of the Jiaodong-type gold deposits. Based on this, the authors put forward the thermal uplifting-extensional metallogenic model of the Jiaodong-type gold deposits (Song MC et al., 2014a, 2015b). During the Late Jurassic, affected by the post-collision compression between the North China Craton and the Yangtze Craton and the subduction of the paleo-Pacific plate or Izanagi plate toward the Asia continent, the Jiaodong Peninsula suffered strong compressional intraplate deformation and crustal thickening (Zhang YQ et al., 2007). This led to the activation of the lower crust and the large-scale remelting of continental crust. As a result, Linglong-type granites were formed. The Cretaceous is an important transformation period for the structural evolution of plates (Wu GY, 2006). During this period, the lithosphere and crust in the North China Craton intensely thinned (Zhai MG et al., 2005) and the Early Cretaceous destruction of the North China Craton reached its peak (Wu FY et al., 2005), which were accompanied by strong tectonism, magmatic activities, metallogenic explosion,and the formation of large basins. Consequently, various extensional structures developed (Wang T et al., 2007; Lin W et al., 2013). The dynamic mechanisms of large-scale regional extensional structures are related to the intraplate or back-arc extension induced by the westward subduction of the paleo-Pacific plate and the collapse of the crust thickened early,which resulted from the roll-back of the subducted plate(Davis GA et al., 2001; Zhu G et al., 2011). In the Jiaodong Peninsula, the subduction and roll-back of the paleo-Pacific plate resulted in lithospheric thinning, asthenospheric upwelling, crust-mantle interactions, and large-scale magmatic activities and extensional tectonism (Fig. 24a).Consequently, a thermal uplifting-extensional structural system was formed (Song MC et al., 2018). In this process,the enriched mantle was transformed into a depleted mantle,and the magmatic rocks were shifted from the adakitic type to the island arc and oceanic island types, resulting in the activation and exchange of crust-mantle elements. Au elements were enriched at the peak of lithospheric thinning and when arc granites completely replaced adakitic granites,which provided abundant ore-forming materials for the gold mineralization. Meanwhile, magmatism provides necessary thermodynamic conditions for gold migration. Owing to the back-arc extension or intraplate extension caused by the rollback of the subducted paleo-Pacific plate, the Jiaodong Peninsula suffered intense extension and quick crustal uplifting, resulting in large-scale interlayer gliding in the crust. The induced detachment faults developing along weak structural surfaces such as the interfaces between Jurassic Linglong granites and Early Precambrian metamorphic rocks provided a favorable space for the migration and accumulation of ore-forming fluids. Due to the shielding of the fault gouge of the detachment fault, the ore-forming materials are mainly enriched in the footwall of the fault (Fig.24b). Owing to inhomogeneous crustal structures, the detachment faults show a stepped pattern of alternate steepgentle dip angles. Deep gold orebodies primarily occur in fault plane parts with a gentle fault dip angle or a steep-gentle transition of the fault dip angle, and thus a stepped metallogenic model was formed (Fig. 25). The tectonic space in the main detachment fault zones was occupied by cataclasite. In this case, ore-forming fluids slowly flowed along loose rock strata, and gradually precipitated and formed altered rock type gold deposits via water-rock metasomatism.Owing to the quick uplifting of the crust and the propping of deep arc granites (Weideshan- and Laoshan-type granites), a large number of tensile fractures obliquely crossing the main detachment faults or featuring a steep dip angle were generated among the adakitic granites (Linglong- and Guojialing-type granites) that were formed earlier on the footwall of the main detachment fault. Then ore-forming fluids filled these faults via the action of pumping, and thus quartz-vein-type gold deposits were formed. The thermal uplifting-extensional and stepped metallogenic models of the Jiaodong-type gold deposits provide direction for the deep prospecting of gold deposits.

Fig. 24. Thermal uplifting-extensional metallogenic model for the Jiaodong-type gold deposits (a) and the Sanshandao gold deposit (b).

Fig. 25. Stepped metallogenic model of Jiaodong-type gold deposits (Song MC et al., 2020b).

6. Conclusions

(i) Multiple adjacent deposits in the Sanshandao area,Jiaodong Peninsula that were previously thought to be independent of each other constitute a supergiant deposit with gold reserves of more than 1200 t (including 470 t Under the Bohai sea area). The main orebodies in the supergiant deposit have a total length of nearly 8 km, a deepest controlled elevation of -2312 m, and a maximum controlled length along their dip direction of greater than 3 km.

(ii) The Jiaodong gold deposits were formed at about 120 Ma and experienced the superposition of polymetallic metallogenic events at 118-111 Ma. The ore-forming fluids are the medium-low-temperature, low-salinity, and reducing H2O-CO2-NaCl±CH4hydrothermal system. Before gold mineralization, the fluids were mainly primary mantle water or magmatic water, metamorphic water appeared in the early stage of mineralization, and meteoric water dominated in the late stage of mineralization. The He-Ar isotopes and C isotopes of the ore-forming fluids reflect the addition of mantle-derived fluids addition. The isotopic characteristics of S, Pb, and Sr of ores are similar to those of ore-hosting rocks,indicating that the ore-forming materials mainly include crustderived materials and minor amounts of mantle-derived materials.

(iii) The orebodies of the supergiant Sanshandao gold deposit mainly occur on the footwall of the Sanshandao fault and the fault sections with a steep-to-gentle transition of the fault dip angle, thus forming a stepped metallogenic model.Fault gouges developing along the Sanshandao fault blocked the gold orebodies onto the footwall of the main fault plane.Owing to the fluctuations in the fluid pressure caused by the changes in the fault dip angle, gold in the fluids was liable to accumulate and be mineralized in the gentle sections with a steep-to-gentle transition of the fault dip angle.

(iv) The Jiaodong gold deposits were formed in the background consisting of extensional structures and the transformation of mantle properties. Therefore, they are different from internationally known gold deposit types such as typical orogenic gold deposits and are referred to as Jiaodong-type gold deposits. The thermal upliftingextensional structures provide thermal dynamic conditions and fluid migration and enrichment space for the gold mineralization. Furthermore, the changes in mantle properties lead to the transformation of the geochemical properties of the lower crust and magmatic rocks. This further led to the activation and exchange of elements, which provided rich materials for gold mineralization. The gold orebodies are distributed in a stepped pattern along ore-controlling faults.

CRediT authorship contribution statement

Ming-chun Song and Zheng-jiang Ding conceived and designed the ideas. Jun-jin Zhang, Ying-xin Song, Jun-wei Bo, Yu-qun Wang, Hong-bo Liu, Shi-yong Li, Jie Li, and Rui-xiang Li participated in field investigation. Bin Wang,Xiang-dong Liu, and Liang-liang Zhang performed the data processing. Lei-lei Dong, Jian Li, and Chun-yan He reviewed and edited the draft.

Declaration of competing interest

The authors declare that they have no competing interests.

Acknowledgment

Professor Jun Deng and Researcher Zhi-cheng Lü reviewed the entire manuscript and proposed valuable comments. Researcher Zi-guo Hao suggested preparing this paper. The authors hereby would like to extend their sincere gratitude to all of them. Meanwhile, this paper was funded by the NSFC-Shandong Joint Fund Program entitled “Control Mechanisms of Faults on Deep Gold Deposits in Jiaodong Peninsula” (U2006201).