An oxidised intrusion-related origin in the controversial Jiaodong gold province (China) for the Shicheng Au-Cu deposit

ABSTRACT

The Shicheng Au-Cu deposit is hosted in the late Early Cretaceous Sanfoshan pluton within the Muping-Rushan (Mu-Ru) metallogenic belt, southeast Jiaodong. The deposit is characterised by Au-Cu bearing quartz-carbonate-sulphide veins that illustrate overprinting sheeted-like and breccia styles. Paragenetically-later veins contain Pb-Zn-(Cu) metal association, suggesting metal zonation at the deposit scale. Quartz from Shicheng shows the cathodoluminescence (CL) textures characteristic of hydrothermal systems with a magmatic influence and exhibits a similar quartz generations sequence. Moreover, this study presents a later overprinting mineralising event, characterised by quartz veins that contain As-rich pyrite, at the Jinqingding deposit in the middle part of the Mu-Ru belt. Jinqingding has a young mineralisation age that overlaps the emplacement age of the Sanfoshan pluton. Quartz from these veins at Jinqingding shows CL textures like those of the Shicheng deposit. The Sanfoshan pluton evolved from highly oxidised, high-K calc-alkaline, I-type magmatism. The early phases of the Sanfoshan pluton have K/Rb, Rb/Sr, and Fe₂O₃/FeO ratios that indicate moderately evolved, less fractionated, and highly oxidised magma, respectively, favouring Au-chalcophile metal association. Therefore, the Shicheng deposit and the overprinting mineralising event at Jinqingding may reasonably represent the products of an oxidised magmatic-hydrothermal system associated with the Sanfoshan-forming magmatism.

KEYWORDS:

• Sanfoshan
• Rushan
• Jinqingding
• CL textures in quartz
• oxidised magmatic-hydrothermal system

1. Introduction

There is a spatial relationship between gold deposits and granitoid rocks throughout the globe. The gold-rich vein-types which do not have, in general, close spatial and temporal relationships with granitoid rocks are categorised as orogenic gold deposits. These deposits tend to be syn-tectonic and form during regional deformation from, substantially, mid- to lower-crustal metamorphic fluids (e.g. Groves et al., 2003, and references therein). However, a significant number of gold deposits show close spatial, temporal, and genetic relationships with the associated intrusions and contain fluids that have a direct connection to them. These are termed as the intrusion-related gold deposits. Most of the intrusion-related gold deposits, whether or not of porphyry type, are characterised by Au-chalcophile metal association (e.g. Sillitoe, 1991; Thompson et al., 1999). Thus, the intrusion-related gold systems can be further classified according to their metal associations. In other words, within the intrusion-related clan, there are two gold mineralising systems the reduced and oxidised ones (Hart, 2007). Oxidised intrusion-related gold deposits are mostly Au-rich variants of the porphyry Cu deposit model associated with oxidised plutons (Hart, 2007). Porphyry Cu and oxidised intrusion-related hydrothermal systems are zoned at the deposit to district scale. Cu-Au core metal association grades into Pb-Zn-rich zone, and the more distal zone is characterised by Au-As association (e.g. Blevin, 2004; Sillitoe, 1991). Moreover, Plumlee et al. (1995) stated that polymetallic vein and replacement deposits are intimately associated with igneous intrusions, and most of these deposits grade laterally (and sometimes vertically) from Cu-Au-rich within and near intrusions to Pb-Zn-rich away from intrusions. It is worth mentioning that the ‘Cu-richest’ (Au-poor) porphyry deposits tend to be related to calc-alkaline magmatism in subduction-related settings, whereas the ‘Au-richest’ porphyry deposits are associated with high-K calc-alkaline to alkaline magmatism in late to post-subduction or post-collision and extensional settings (e.g. Chiaradia, 2020). This study reveals an oxidised intrusion-related origin for the Shicheng Au-Cu deposit, a poorly-investigated mineral deposit within the Muping-Rushan (Mu-Ru) metallogenic belt (southeastern Jiaodong), and presents a detailed description of the Shicheng mineralisation, vein styles, and paragenetic stages. The study highlights a genetic connection between the Shicheng Au-Cu deposit and a later overprinting mineralising event (containing As-rich pyrite) at the structurally controlled Jinqingding deposit in the middle part of the Mu-Ru belt. This study is the first in Jiaodong to test the relationship between a host (or near-synchronous) granitoid composition, its oxidation state, and the corresponding ore element associations. The host to the Shicheng veins is the Sanfoshan pluton, which has zircon U–Pb ages of 114 ± 3 Ma to 116 ± 3 Ma (J. Zhang et al., 2010) younger than other nearby gold deposits hosted by the Kunyushan composite plutons in the same metallogenic belt (Mu-Ru). Thus, it has been essential to investigate the Shicheng deposit, as the younger deposit, i.e. Shicheng, may be the least subjected to possible overprinting events and hence a better opportunity for deciphering the mineralising process.

The old-dated judgement by Sillitoe and Thompson (1998) supported the classification of the Jiaodong gold deposits as intrusion-related vein-type gold deposits. Song et al. (2019) indicated a temporal relationship between the Weideshan granitoids (such as the Aishan and Sanfoshan plutons) that were emplaced at ca. 118.0 to 110.5 Ma and the large-scale gold mineralisation in Jiaodong. It is worth mentioning that Mills et al. (2015a) stated that the JJ3 orebody of the Jiaojia gold deposit (northwestern Jiaodong) is distinct from the main orebody and characterised by steeply dipping dilational veins, and they suggested that the JJ3 orebody represents a later event involving a late-stage oxidised magmatic-hydrothermal fluid. In contrast, a bunch of recent studies supported the classification of the Jiaodong gold veins as orogenic gold deposits or excluded the intrusion-related origin. L. Zhang et al. (2020) stated that the Jiaodong ‘gold-only’ deposits, which are structurally controlled by regional NNE–NE-trending faults, are orogenic gold deposits. They suggested a time lag (~8 m.y.) between the proposed peak of magmatism (ca. 128 Ma, i.e. related to the Early Cretaceous Guojialing granodiorite) and a focused gold mineralisation event (ca. 120 ± 2 Ma). They excluded plutons emplaced after the proposed peak of intrusive activity, i.e. the late Early Cretaceous granites such as the Sanfoshan pluton. However, the Sanfoshan pluton hosts the Shicheng Au-Cu deposit, which appears to be younger than the activation of the regional faults in the Mu-Ru belt (synkinematic biotite ⁴⁰Ar/³⁹Ar age of 122.8 ± 0.3 Ma after H. Zhang et al., 2007).

Furthermore, in northwestern Jiaodong, the Dayingezhuang gold deposit has sericite ⁴⁰Ar/³⁹Ar age of 130 ± 4 Ma (Yang et al., 2014). The Jinqingding gold deposit (also referred to as ‘Rushan’), ~13 km north of Shicheng, represents the largest vein style gold deposit in China and has about 16 orebodies (e.g. Chen et al., 2019) with ‘no. 2ʹ as the main orebody. It was mined originally for Cu and Ag, due to its shallow economic copper mineralisation (e.g. Mills et al., 2015a). Sai et al. (2020) proposed the classic fault-valve model at Jinqingding. They suggested that the different ages of the deposit represent a single, protracted gold event lasting for about 5 m.y. (122 Ma to 117 Ma). Sai et al. (2020) said that the earliest age of the Sanfoshan monzogranite (zircon U–Pb SHRIMP emplacement age of 118 Ma; Goss et al., 2010) is tenuous, and its emplacement age of 116–114 Ma (J. Zhang et al., 2010) is younger than the Jinqingding deposit, suggesting asynchronous magmatic and hydrothermal activities (i.e. ruling out a magmatic-hydrothermal origin). However, Deng et al. (2020) stated 114.2 ± 1.5 Ma gold mineralisation at Jinqingding based on in-situ U–Pb dating of hydrothermal monazite (but no details of the paragenetic context). This mineralisation age clearly overlaps the ages of both the Sanfoshan pluton and, presumably, the hosted Shicheng Au-Cu deposit. Moreover, on the geologic map of the Mu-Ru metallogenic belt in Sai et al. (2020), the Shicheng deposit and other gold deposits in the belt seem to have been controlled by NNE-trending regional faults and hosted mainly by the Kunyushan composite plutons. However, the geologic maps in other published work (e.g. Chen et al., 2019; Hu et al., 2006) show the Sanfoshan monzogranite, which is not cut by these regional faults, as the host for the Shicheng deposit.

2. Geological background

The Dabie-Sulu orogen is believed to be a result of the collision between the North China Craton and the South China Craton (Yangtze Block) in the Triassic (e.g. Hacker et al., 2000). The NNE- to NE-striking Tan-Lu fault sinistrally offsets the Dabie and Sulu orogenic belts, with about 550 km apparent displacement (Zhu et al., 2005). The Tan-Lu fault zone (Figure 1) represents the boundary between Jiaodong and the Eastern Block of North China. Jiaodong is divided by the Wulian-Yantai fault zone into the Jiaobei terrane in the west and the Sulu ultra-high pressure (UHP) orogenic belt in the east. The Precambrian basement of the Jiaobei terrane is composed of the Neoarchean Jiaodong Group, the Paleoproterozoic Jingshan and Fenzishan Groups, and the Neoproterozoic Penglai Group (Tam et al., 2011). And the Sulu UHP orogenic belt is dominated by the Neoproterozoic protolith of eclogite and felsic gneiss (Zheng et al., 2006). The Jiaobei terrane is composed of the Jiaobei uplift and the Jiaolai basin. The Jiaobei uplift comprises the Late Jurassic to Early Cretaceous magmatic activities. These magmatic activities are a Late Jurassic magmatism of the Linglong monzogranite (e.g. Hou et al., 2007), a subsequent Early Cretaceous magmatism of the Guojialing granodiorite and monzogranite (e.g. Hou et al., 2007), and a late Early Cretaceous magmatism of the Aishan granite (Goss et al., 2010). In contrast, in the Sulu UHP orogenic belt, an initial magmatic activity is represented by the small, Late Triassic Shidao syenite complex at the southeastern tip of the belt (Siebel et al., 2009). Subsequently, the belt is intruded by the magmatic activities of the Kunyushan-Sanfoshan plutons (Figure 2). The Kunyushan composite plutons (ca. 161–142 Ma; Guo et al., 2005) are composed of the Duogushan granodiorite and the Washan and Wuzhuashan monzogranites, each with a weak gneissic layering locally (e.g. D.-Q. Zhang et al., 1995). Another magmatism, at ca. 116–114 Ma (J. Zhang et al., 2010), led to the NE-trending Sanfoshan monzogranite. The Xiamashan miarolitic alkali feldspar granite occurs as small bodies emplaced in the Sanfoshan pluton. The Kunyushan composite plutons are dissected by subparallel N- to NNE-striking regional faults, whereas the Sanfoshan pluton appears less deformed. The Kunyushan-Sanfoshan plutons host the gold deposits of the Muping-Rushan (Mu-Ru) metallogenic belt (e.g. Fan et al., 2007). The Kunyushan composite plutons, particularly Washan and Wuzhuashan, host the Jinqingding (the largest), Denggezhuang, Sanjia, and Hubazhuang gold deposits, whereas the Sanfoshan monzogranite hosts the Shicheng deposit (the youngest).

Figure 1. Regional structural setting of Jiaodong. By the Late Jurassic, the NNE- to NE-striking Tan-Lu fault accommodated sinistral slip as part of a transpressional regime during oblique subduction of the Izanagi (paleo-Pacific) Plate (Zhu et al., 2010). A switch to extensional tectonics in the Early Cretaceous (Zhu et al., 2010), presumably due to roll-back of the Izanagi plate, may have aided the emplacement of granites (Goss et al., 2010) and the development of extensional veins

Figure 2. Simplified, revised geologic map of the Mu-Ru metallogenic belt. The background is a shaded relief image. Fault mapping is based on the shaded relief image and the geologic map of the Haiyang-Chaoli area (L.-Z. Wang et al., 1994). Boundaries of rock units are adapted from the aforementioned map (L.-Z. Wang et al., 1994). Colours of rock units in the legend are slightly different from those on the map due to the background colour effect

3. Mapping data, samples, and analytical techniques

3.1. Remotely-sensed data and mapping

Remotely-sensed data and digital elevation models (DEMs) provide reliable bases for depicting and mapping structural lineaments that correlate with well-known, large-scale faults and fractures. For instance, Meixner et al. (2018) stated that lineaments extracted from three different remotely-sensed data sets (including DEMs) correlate with well-known, mappable large-scale structures in the southern Black Forest (Germany). Temporal evolution of the lineament trends may indicate the reactivation or repeated reactivation of inherited structures during various tectonic episodes (e.g. Masoud & Koike, 2011; Radaideh et al., 2016). Moreover, Zoheir et al. (2019) used remotely-sensed (radar) data to map structural lineaments and potentially gold mineralised regional structures in an area in the South Eastern Desert of Egypt. In the Mu-Ru metallogenic belt, the large-scale N- to NNE-striking faults are steeply dipping up to 85° (Cheng et al., 2019), and steeply dipping faults occur as linear features that are relatively unaffected by topography (e.g. Drury, 2001; Meixner et al., 2018). Thus, this study has used, in addition to the geologic map of the Haiyang-Chaoli area (L.-Z. Wang et al., 1994), a DEM-derived relief map as a reliable base for fault mapping in the Mu-Ru metallogenic belt. This study has generated the shaded relief map from the DEM included with an Alaska Satellite Facility’s (ASF) ALOS PALSAR radiometric terrain corrected (RTC) product (ASF DAAC,). It is a resampled DEM from the Shuttle Radar Topography Mission (SRTM) DEM at one arc-second (30 m) resolution to 12.5 m by ASF. Shaded relief images for illumination altitude of 35° and azimuths of 270°, 292.5°, and 315° (i.e. perpendicular to regional fault trends) have been derived from this DEM by this study. These shaded relief images have then been mosaicked (mean value) to the final shaded relief image in Figure 2. This final shaded relief image, in combination with the aforementioned geologic map (L.-Z. Wang et al., 1994), has been used in a visual interpretation for fault mapping in the Mu-Ru metallogenic belt. Faults have then been categorised as ‘Certain’ and ‘Probable’ based on the match between traces extracted from the shaded relief image and faults of the geologic map. An ‘Uncertain’ category means fault location or existence is uncertain. The revised geologic map of the Mu-Ru metallogenic belt (Figure 2) shows that the Kunyushan composite plutons are highly dissected by the large-scale faults, whereas the Sanfoshan pluton appears less deformed. The regional structural setting of Jiaodong has been inferred directly from SRTM DEM-derived shaded relief map for illumination altitude of 45° and azimuth of 315°.

3.2. Ore and host rock samples

In 2008, some ore samples representing the different veins were collected from both the underground exposures and broken ore stockpiles at Shicheng by H.-R. Fan. In 2018, other samples (mostly of a breccia vein style) were collected from the broken ore stockpiles (‘S1ʹ in Figure 3) of the closed Shicheng mine. The Shicheng ore samples have been carefully observed in this study at the macro- and microscopic levels to characterise their ore and gangue minerals, establish fine paragenesis, and deduce some aspects related to ore genesis. Moreover, this study has used some samples collected in 2018 from the Jinqingding underground exposures to investigate any possible overprinting events related to the same system. Samples of the host Sanfoshan monzogranite were collected at the site designated as ‘S2ʹ in Figure 3, together with one sample 08RS021 from the broken stockpiles. Two samples 18XM1 and 18XM2 were collected from the Xiamashan outcrop. These samples have been used in the determination of the whole-rock compositions, and as shown below. This study has also compiled some whole-rock geochemical data from previous work (Hu, 2006). The data compiled from Hu (2006) are for six samples of the Sanfoshan host (03R056, 03R057, 03R031, 03R033, 03R084, and 03R086) and two samples of the Xiamashan alkali feldspar granite (03R080 and 03R082). The Sanfoshan samples 03R056 and 03R057 were collected from the small intrusive stock in the northern part of the Mu-Ru belt, whereas the Sanfoshan samples 03R031 and 03R033 were collected from the southern tip of the Sanfoshan pluton. These four samples are the ‘least fractionated’ Sanfoshan samples (q.v. section 7.), and the northern stock has a zircon U–Pb age of 114 ± 1 Ma (Guo et al., 2005).

Figure 3. Geology of Shicheng after J.-J. Zhang (2006) and Zhao (2007)

3.3. Analytical techniques

Tens of polished thin-sections representing the different veins of the Shicheng deposit were prepared. Observations were made in reflected, plane-polarised light using objectives up to 100X mounted on a polarising microscope. Scanning electron microscope (SEM) imaging was applied on some areas of the polished thin-sections, after carbon-coating, using a Nova NanoSEM 450 field emission SEM at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing. The SEM is equipped with an X-MAXN80 X-ray energy dispersive spectrometer (EDS) and a Gatan MonoCL4 cathodoluminescence (CL) detector. SEM-CL imaging, along with secondary electrons (SE) images of the same areas, was acquired with an acceleration voltage of 15 kV, ≈ 13.9 mm working distance, and a spot size of 6. SEM-backscattered electrons (BSE) imaging was obtained with an acceleration voltage of 15 kV, ≈ 6.5 mm working distance, and a spot size of 5.5. The use of SEM-CL imaging helped to distinguish the different quartz generations and their characteristic textures and sequence. SEM-BSE imaging, together with EDS, was used to characterise the different mineral phases. Moreover, SEM-BSE imaging on a few polished thin-sections prepared from the Jinqingding samples was acquired. Hand-specimens of the Sanfoshan host strongly attract a hand-magnet, and a few thin sections were prepared to check their magnetite content and choose the less altered samples petrographically. Then, five samples (18SF1, 18SF2, 18SF3, 08RS021, and 08RS046) were selected. These samples, together with the two samples from the Xiamashan alkali feldspar granite, were prepared for the determination of whole-rock compositions at IGGCAS, Beijing. Major element oxide contents were determined using X-ray fluorescence spectrometry (XRF) on fused glass discs with a PANalytical AXIOS Minerals instrument, which has analytical uncertainties within 0.1–1.0% (RSD). The trace element and rare earth element (REE) concentrations were measured using inductively coupled plasma mass spectrometry (ICP-MS) with a Finnigan MAT spectrometer. The Chinese standards GSR1 and GSR3 were used. The analytical accuracy and precision were generally < 10% for all trace and REE elements. FeO concentrations were determined using titration with potassium permanganate solution after multi-acid digestion with H₂SO₄, HF, and H₃BO₃. Also, the FeO concentrations of the whole-rock geochemical data compiled from Hu (2006) have been calculated in this study using the oxidation ratio graph in Middlemost (1985).

4. Local geology

The NE-striking elongate geometry of the Sanfoshan pluton (Figure 2) reflects structural control on its emplacement. The Kunyushan composite plutons (Washan and Wuzhuashan) are highly dissected by the large-scale N- to NNE-striking faults of the Mu-Ru belt, indicating a prominent structural control on gold mineralisation hosted in these plutons (such as the Jinqingding deposit). In contrast, the Sanfoshan pluton appears less deformed, and this is in agreement with the published deformation/reactivation dating of the faults in the Mu-Ru belt (ca. 122.8 Ma; H. Zhang et al., 2007) and the Sanfoshan emplacement age (ca. 116–114 Ma; J. Zhang et al., 2010). The Shicheng orebodies (Figure 3) occur as subparallel, N-striking dilational veins in the Sanfoshan pluton (the pluton’s western margin). The dilatant nature is illustrated in the next section, and the veining system also seems to relate to a regional structural pattern. For instance, structural measurements (fault-slip data) within the Jinqingding-controlling fault zone (location 12 in Cheng et al., 2019) indicated a WNW–ESE tension component of stress (σ3 axis). At Shicheng (‘S2ʹ in Figure 3), both the late magmatic and hydrothermal stages are illustrated by some features. North–S-directed pyrite veining was found to crosscut aplite dikes that cut the Sanfoshan monzogranite there. These aplite dikes may be considered as the late magmatic features, whereas Au-Cu bearing quartz-carbonate-sulphide veins and associated alterations are the hydrothermal expressions.

5. Ore textural styles

Ore samples (Figure 4) from both the underground exposures and broken ore stockpiles, as shown above, have been polished for better observation. No previous data have been published describing the Shicheng mineralisation. It is characterised mainly by Au-Cu bearing quartz-carbonate-sulphide veins with styles of overprinting subparallel sheeted-like and breccia. Both vein styles represent parallel reactivation of the same fracture system. These styles have been described in detail below. The macroscopic paragenesis that can be generalised within all of the Shicheng samples includes the exploitation of an early mineralised fracture system by carbonate veins (mainly siderite). Then parallel extensional fracturing, probably associated with a relatively high-pressure fluid introduction (cf. Gudmundsson, 2011; Sibson, 1981), has provided open spaces for the deposition of comb quartz (i.e. a large infill component). The system has subsequently been overprinted by pyrite-chalcopyrite-silica fluids. The sulphides, particularly chalcopyrite, show preferential replacement of siderite and precipitate in interstices between (or vugs in) the comb quartz crystals..

Figure 4. Ore textural styles and macroscopic paragenetic relations at Shicheng. (a) Polished slab of an overprinting sheeted-like style. Parallel fracturing with subsequent comb quartz deposition. Dark colour materials are due to the silica-sericite-sulphide association. Note the coarse-grained size of chalcopyrite adjacent to siderite (preferential replacement), (b) Polished slab of an overprinting sheeted-like style. Siderite is overprinted by quartz and subsequent pyrite-chalcopyrite-silica. (c) Subparallel quartz veins cut pre-existing siderite veins in a broken ore sample. Note the overprint of chalcopyrite, (d) Chalcopyrite-rich zone (vein) in an ore sample, and (e) Coarse chalcopyrite in a quartz druse (infill components)

5.1. Overprinting sheeted-like veins

Many samples from Shicheng (e.g. Figure 4(a, b, c)) show examples of the overprinting subparallel sheeted veining, which reflect the re-opening and accompanying infilling of the same fracture system. The samples share the same situation of parallel extensional fractures occupied by comb quartz veins with subsequent sulphides precipitation. In some specimens, such as the spectacular polished piece in Figure 4(a) that has been included to cover nearly the entire picture at Shicheng, the central region of the mineralised fracture system is traversed by eye-catching comb quartz veining. Quartz crystals are medium to fine, with some crystals exhibit growth zoning. Although the quartz veinlets appear to weld together or crosscut at some points, they are not multidirectional like those of stockworks, and overall parallelism predominates. This comb quartz primarily represents an infilled vein-style open space. Intense silicification in the central zone has resulted in highly silicified pale materials (silica-sericite-sulphide association). Away from the centre and adjacent to the highly altered granitic host, there is parallel carbonate veinlet altered by sulphides (pyrite + chalcopyrite). The carbonate (mainly siderite) component represents the earlier phase that can only be distinguished at the macroscopic level from this sample. At the microscopic level, siderite is micro-veined by silica related to comb quartz and altered by silica-sericite-sulphide association. The sulphide stage represented by pyrite and chalcopyrite post-dates both the comb quartz and siderite phases. Pyrite and chalcopyrite infill between the quartz crystals and preferentially replace the siderite stage, as evidenced by the coarsening of chalcopyrite contiguous with siderite. Thereby, the overall picture contains pre-existing siderite veining that has been parallelly fractured with subsequent comb quartz deposition (high-pressure fluids are suspected). The system has subsequently received sulphides precipitation (pyrite and chalcopyrite). Barite, sometimes, exists in these veins representing a late phase that overprints all the pre-existing phases. In other samples (e.g. Figure 4(b, c)), siderite occurs as centimetre-scale veins that have been fractured, veined, and altered. Dilational fracturing has allowed the introduction of silica-bearing fluids, resulting in coarse, white quartz. The quartz vein width (Figure 4(c)) suggests a reasonably high-pressure fluid. Subsequently, pyrite-chalcopyrite-silica infill components have occupied the pre-existing parallel cracks (Figure 4(b)). The sulphide-bearing fluid (mainly chalcopyrite) has also filled in the interstices between quartz and selectively replaced siderite slivers (Figure 4(b)). Sulphide alteration has strongly affected the siderite veins and overprinted earlier silica-sericite alteration (e.g. Figure 4(c)). In some situations, the original channel (sidewall position) which has allowed the entry of the chalcopyrite-rich fluid becomes indistinguishable due to the selective replacement (e.g. Figure 4(d)). Moreover, a few samples exhibit quartz druses to overprint the siderite stage and receive subsequent chalcopyrite precipitation (e.g. Figure 4(e)).

Figure 4. (Continued). (f) An underground exposure of dilational breccia (photographed in 2009), (g) Broken ore sample of a breccia vein style. Note the sharp siderite fragments which are veined by quartz, (h) Breccia vein from the ore stockpiled at Shicheng. Brecciated siderite fragments are held together by vuggy coarse crystalline quartz, (i) Polished slab of a breccia vein. Comb-like quartz with pyrite-chalcopyrite infill. Pyrite and chalcopyrite preferentially attack the siderite component everywhere, (j) Polished slab from a paragenetically-later vein. Galena and barite have infilled pre-existing open spaces. Greenish wall rock alteration is mainly due to the existence of chlorite. Abbreviations: Sd = siderite; Qtz = quartz; Ccp = chalcopyrite; Py = pyrite; Brt = barite; Gn = galena

5.2. Breccia veins

The other important deposit style at Shicheng is the dilatant breccia veins (Figure 4(f, g, h, i)), which are distinct from the previous one by the existence of large brecciated siderite fragments held together by more coarsely crystalline comb-like quartz. Quartz has hexagonal outlines and clear growth zoning (i.e. pure open-space growth). The second key difference is the large cavities exhibited by this dilational breccia. The original cavities have received mainly silica precipitation and subsequent sulphides but without completely filling the cavities, as evidenced by the clean nature of remaining void spaces (i.e. no iron staining due to the leaching of pre-existing iron-rich mineral). Large quartz crystals are found towards the central zones of precipitation where there is sufficient space to develop large crystals. Some of the siderite fragments exhibit sharp edges (e.g. Figure 4(g)) and are vined by silica related to comb quartz, giving the impression of a high-pressure fluid introduction with subsequent comb quartz deposition. Varying degrees of silica-sericite alteration affect the siderite walls and some fragments, with silica increase adjacent to the infill. This dilational breccia seems related to the brecciation of siderite veins as part of dilatant fractures’ activation by a reasonably high-pressure fluid that has deposited comb quartz. The quartz-carbonate breccia vein has subsequently received pyrite and chalcopyrite precipitation. The sulphide-bearing fluids (pyrite-chalcopyrite) have preferentially attacked the siderite component and infilled between the quartz crystals (e.g. Figure 4(i)).

5.3. Paragenetically-later veins

This study has set Au-Cu association to the Shicheng deposit to reflect the metals of above veins (sheeted and breccia styles). However, the deposit also contains substantial amounts of other base metals. All of the above veins contain no substantial galena (i.e. sparse), and galena has been observed at the microscopic level to overprint the pyrite-chalcopyrite infill (i.e. filling in etch-pits; q.v. subsection 6.1.). Relative timing can be established from this sequence of deposition (i.e. paragenetically-later). But some of the quartz veins from Shicheng are characterised by substantial Pb-Zn-(Cu) metal association. Thus, the substantial galena infill is most-likely proximally-situated to the Au-Cu-rich zones, suggesting metal zonation at the deposit scale.

Parallel fracturing and subsequent open-space infill are exhibited again in these later veins (Figure 4(j)). Galena, sphalerite, and barite in these veins are postulated to mark the end of the ore stage and have occupied the remaining open spaces (vugs and fractures) in the same way as the precursor sheeted pyrite-chalcopyrite infill. Intergrowths of prevailing galena with minor pyrite have been observed using the polarising reflected-light microscope (q.v. subsection 6.1.). Different greenish intense wall rock alteration borders the veins due to the existence of chlorite as a first candidate. Late-stage carbonate (calcite) micro-vein networks crosscut the vein system.

6. Ore petrography and fine paragenesis

Nearly all of the veins from the Shicheng Au-Cu deposit contain pyrite and quartz. They are important gold host minerals at Shicheng because gold occurs primarily as inclusions within pyrite and as free gold in quartz (q.v. subsection 6.3.). Therefore, it has been essential to classify and characterise their different types and generations and to know with which stage gold is actually associated (i.e. the relative timing of gold precipitation). Since most of the vein-style gold deposits in Jiaodong contain pyrite that shows As-rich growth zones, such as those of the Mu-Ru belt—the Denggezhuang gold deposit (Mills et al., 2015b) and the Jinqingding gold deposit (subsection 8.4.), the same texture has been come under suspicion at Shicheng and hence an ‘invisible’ gold-As association (Mills et al., 2015b; Reich et al., 2005). But the Shicheng pyrite from tens of the samples studied, particularly those of the paragenetically-later veins, does not display such texture under SEM-BSE. However, the more distal zone might presumably display it. The Shicheng pyrite can still be differentiated based on morphological, crosscutting, textural position, and mineral association aspects, as below. The quartz generations have been identified under SEM-CL beam, each with a distinct texture that is discernible throughout the entire Shicheng Au-Cu deposit. And chalcopyrite, which is the Cu-bearing mineral and the second abundant metallic mineral in the Shicheng Au-Cu deposit, occurs associated with cogenetic pyrite and ore-stage quartz and predominates over pyrite in some veins.

6.1. Pyrite types

Early pyrite veining in the Sanfoshan monzogranite (site designated as ‘S2ʹ in Figure 3) is associated with narrow K-feldspar alteration halo bordering the veins. This pyrite (Py_E — early pyrite) occurs as coarse crystals compared to other types, with cubic forms (up to mm-sized pyrite). Similar pyrite (Figure 5(a, b)) has been observed in the mine ore samples. Py_E crystals are intensely deformed, with fractures filled by siderite. Thus, Py_E pyrite appears to have formed first, then followed by siderite precipitation. Although there is a little doubt as there is no much information about the deformation event, the close association of this deformed pyrite with an early quartz generation suggests that this pyrite represents an earlier event. The occurrence of this deformed pyrite (Figure 5(a)) together with subsequent pristine (undeformed) pyrite types and open-space growth quartz (Figure 5(c)) in some reactivated veins supports such interpretation. The predominant pyrite types in the Shicheng Au-Cu deposit are those of the overprinting sheeted-like veining (Py_V—vein pyrite) and those filling vugs in and interstices between main-stage comb quartz (Py_I—interstice-filling pyrite). Intergrowths of Py_V with cogenetic chalcopyrite and ore-stage quartz occur (Figure 5(d, e)), and the pits within Py_V are siderite-free and filled by galena (Figure 5(d)). Vein pyrite, Py_V, occurs as fine crystals with mainly cubic to pyritohedral crystal forms compared to coarser pyrite at the vein borders. The latter pyrite has formed by the siderite replacement and sometimes contains remaining siderite inclusions. Kusebauch et al. (2018) (and references therein) stated that the sulfidation or siderite replacement by pyrite is considered as a coupled dissolution-reprecipitation reaction and to be pseudomorphic at pH_25°C > 5. The following two reactions (Kusebauch et al., 2018) illustrate the sulfidation of siderite:

Figure 5. (Continued). (l) Superimposition of Qtz_O is at its maximum with the occurrence of visible gold, (m) CL and SE images of the same area in a reactivated vein, with quartz displays many generations, (n) Native gold grain occurs in the comb quartz of a breccia vein, and (o) BSE imaging with contrast adjusted to show gold, galena, and matildite having sealed open spaces within an early pyrite crystal. Abbreviations: Sd = siderite; Qtz = quartz; Ccp = chalcopyrite; Py = pyrite; Brt = barite; Gn = galena; Sp = sphalerite; Au = native gold; Au,Ag = Au-Ag alloy; Cal = calcite; Mat = matildite; Kfs = K-feldspar; Ser = sericite; Mnz = monazite; Rt = rutile. Quartz stages labelled (Qtz_E, Qtz_M, Qtz_O, and Qtz_L) for (early-, main-, ore-, and late-stages). Pyrite types labelled (Py_E, Py_V, and Py_I) for (early, vein, and interstices-filling pyrites)

Figure 5. Collection of photomicrographs (reflected, plane-polarised light) and SEM imaging (BSE, CL, and SE) that illustrate the characteristics of different stages in the Shicheng deposit. Some reflected light photomicrographs are coupled with BSE, CL, and SE imaging of the approximate areas shown by the red dashed boxes. (a) BSE image shows micro-fractures in early pyrite sealed by siderite, quartz, and chalcopyrite. Note the intense degree of brittle deformation recorded by Py_E in contrast to Py_V in the same reactivated vein of (c), (b) Deformed, early pyrite with larger micro-fractures filled by siderite and quartz. The CL image reveals multiple quartz generations, (c) Open-space growth in the same reactivated vein of (a), and (d) Intergrowth of Py_V with chalcopyrite and quartz in a sheeted style veining. Siderite-free etch-pits within Py_V are filled by galena. Note the predominance of Qtz_O

Figure 5. (Continued). (e) Intergrowth of Py_V with cogenetic chalcopyrite and quartz in a sheeted-like veining, (f) Intergrowth of pyritohedral pyrite with chalcopyrite and subordinate galena fills vug in quartz. Pits within pyrite are filled by galena and chalcopyrite. Barite is a late phase, (g) Py_I in the same vein of (f). Galena and barite fill interstitially around Py_I. Close-up shows the occurrence of gold and galena in etch-pits within Py_I, (h) Comb quartz of a breccia vein with a vug filled by Py_I and chalcopyrite. Etch-pits within Py_I are filled by galena, (i) Pyrite and chalcopyrite occur within galena, filling vug in quartz in a paragenetically-later vein. Note where galena predominates, the cogenetic pyrite becomes much smaller and dissolved, (j) Cogenetic pyrite and galena fill interstices between quartz in the same vein of (i). Qtz_M (oscillatory zoned) is overgrown by Qtz_O (dark), and (k) BSE image shows siderite alteration of the host rock overprinted by silica-sericite alteration adjacent to the comb quartz veining shown in Figure 4(a)

FeCO₃ (s) + 2H⁺ (aq) ↔ Fe²⁺ (aq) + CO₂ (aq) + H₂O (1)

Fe²⁺ (aq) + 2 H₂S (aq) ↔ FeS₂ (s) + 2H⁺ (aq) + H₂ (aq) (2)

The interstice-filling pyrite (Py_I) also forms intergrowths with cogenetic chalcopyrite, ore-stage quartz, and subordinate galena (Figure 5(f, g, h)). Py_I shows, particularly with subordinate galena, coarser crystals compared to those of Py_V crystals, with pyritohedral and some cubic forms. G. Wang et al. (2016) (and references therein) stated that the pyritohedral crystals within the Xishan gold deposit (northwestern Jiaodong) reflect sulphur-rich fluids characterising the main Au-ore stage. Some grains of visible gold and galena (Figure 5(g)), together with sulfosalts (mainly matildite), have sealed all the open spaces within Py_I (such as etch-pits, less-dense fractures, and along boundaries). Where the prevalent galena occurs (Figure 5(i, j)), the cogenetic pyrite infill becomes much smaller and corroded or dissolved. In this case, gold-free pyrite together with galena occurs as infill in the paragenetically-later veins.

6.2. Quartz generations

Two clear-cut, widespread quartz generations, namely main- (Qtz_M) and ore-stages (Qtz_O), have been identified in the Shicheng Au-Cu deposit. Qtz_M stage has formed the voluminous comb quartz veins at Shicheng and caused the silicification of both the host rock and the siderite walls adjacent to the veins (Figure 5(k, c), respectively). This silica alteration occurs associated with sericitisation and post-dates both the potassic alteration of the granitic host and the siderite alteration adjacent to the siderite veining. Qtz_M quartz is bright under SEM-CL beam and shows oscillatory growth zoning, which is characteristic of open spaces and vugs (Figure 5(j)). This stage also shows a few examples of sector zoning (Figure 5(c)). The occurrence of the pyrite-chalcopyrite infill marks the end of this main-stage quartz. Ore-stage quartz (Qtz_O) has formed intergrowths with this pyrite-chalcopyrite infill, and Qtz_O deposition has continued up until some galena infill. Qtz_O quartz has modified and overprinted main-stage quartz along boundaries. Overprinting/modification of early phases is at its maximum where gold occurs (Figure 5(l)). This Qtz_O quartz is very weakly luminescent under SEM-CL beam and appears so dark (Figure 5(d)), with a few cases of oscillatory zoning due to partial modification or where open spaces have allowed.

There are, besides Qtz_M and Qtz_O, other quartz generations that are either strongly modified or less voluminous. Early-stage quartz (Qtz_E) appears as CL-bright, unzoned generation. But it is rare to be seen clearly due to the intense modification by the subsequent generations of quartz (Figure 5(m)). It is dissolved and then overgrown along boundaries by main-stage quartz (Qtz_M). Late-stage quartz (Qtz_L) (Figure 5(b, m)) is characterised by both CL-dark micro-veins that crosscut pre-existing quartz and a non-luminescent network of healed micro-fractures in all the previous quartz generations.

6.3. Gold occurrence

Gold in the Shicheng Au-Cu deposit occurs mainly as ‘visible gold’ in pyrite (the chief host mineral) and quartz. Gold occurs primarily as an alloy with Ag (high Au content) and also in the form of native gold, as identified by EDS analysis. Free native gold grains (~50 μm) have been observed in the comb quartz veins of both the breccia (Figure 5(n)) and sheeted-like styles. Au-Ag alloys occur as inclusions within pyrite (Py_I and Py_E). Within Py_I and Py_E, visible gold has precipitated in etch-pits (Figure 5(g)), micro-fractures (Figure 5(o)), and along grain boundaries. Galena and sulfosalts have sealed all of the remaining open spaces, and matildite predominates among sulfosalts. Superimposition of ore-stage quartz (Qtz_O) is at its maximum where gold occurs within Py_E (Figure 5(l)). No visible gold has been observed within the smaller pyrite where the prevalent galena occurs. It appears that gold in the Shicheng Au-Cu deposit belongs to one stage (post-pyrite stage—Py_I), and its introduction has been concomitant with ore-stage quartz and before the prevalence of galena.

6.4. Fine paragenesis

Based on macro- and microscopic relationships drawn previously from both the ore samples (section 5.) and the above petrographic characterisation (different pyrite types and quartz generations), a fine paragenesis has been compiled (Figure 6) that is consistent with all the veins of the Shicheng Au-Cu deposit. The early stage of the mineralised system is represented by deformed pyrite-quartz veins (Py_E and Qtz_E), which are associated with narrow K-feldspar alteration borders. Py_E pyrite (e.g. Figure 5(a, b)) shows intense brittle deformation (shear fracturing). This deformed system has (immediately?) been utilised by a carbonate-bearing fluid that has formed the siderite veining and its associated alteration within the host granite (e.g. Figure 5(k)). Then, parallel extensional fractures (dilational fracturing) have affected the entire system, probably associated with a high-pressure fluid introduction, with subsequent main-stage quartz deposition (Qtz_M). Qtz_M quartz forms the major comb quartz veins (sheeted and breccia styles). Silica alteration occurs adjacent to the infill and is associated with sericitisation, with sericite increase towards the walls (host granite and siderite walls). The veining system has subsequently received pyrite (Py_V and Py_I) and chalcopyrite deposition, and the sulphide-bearing fluids (particularly chalcopyrite) have selectively replaced the pre-existing siderite component. The pyrite (Py_V and Py_I) and chalcopyrite components mark the ore stage’s start and form intergrowths with ore-stage quartz (Qtz_O). Qtz_O quartz is at its maximum where gold—native and Au-Ag alloy—occurs (i.e. the ore stage). Qtz_O quartz deposition has continued up until some galena infill. The prevalence of the galena infill and sphalerite marks the end of the ore stage in the Shicheng veins.

Figure 6. An interpreted paragenetic sequence for the Shicheng deposit—the oldest sequence at the top with successively younger sequences towards the bottom. Bar thickness reflects the significance and voluminosity of the phase. The dashed bar shows presumed continuity. ⁺Could involve more than one stage. *Paragenetic position is not well-constrained. ^?No much information about it

7. Whole-rock geochemistry

Goss et al. (2010) and J. Zhang et al. (2010) presented five and seven lithogeochemical data on the Sanfoshan pluton, respectively. The geochemical diagrams (Figure 7, 8, and 9) plot data of rock samples analysed in this study as well as data from Hu (2006), as indicated earlier. These rocks are the early phases of the Sanfoshan pluton (composed mostly of monzogranite with minor granodiorite) and also contain the Sanfoshan syenogranite and the Xiamashan alkali feldspar granite. The lithological names are based on the R₁R₂ chemical variation diagram (Figure 7(a); De la Roche et al., 1980). The samples have SiO₂ concentrations varying from 69.04–72.44 wt.% in the Sanfoshan monzogranite-granodiorite to 76.19–77.12 wt.% in the Sanfoshan syenogranite and the Xiamashan alkali feldspar granite. These names are consistent with the previous work of Goss et al. (2010) in which they described a pink, coarse-grained monzogranite in a sample that has SiO₂ content of 72.81 wt.% and a pinkish, fine-grained syenogranite in a sample with SiO₂ 76.07 wt.%. J. Zhang et al. (2010) stated that the Sanfoshan pluton is locally porphyritic. This study refers to the rock samples with the high SiO₂ range (76.19 ≤ SiO₂ ≤ 77.12 wt.%) as the late-phase rocks, and those with SiO₂ contents ≤ 72.44 wt.% are referred to as the voluminous, early phases of the Sanfoshan pluton. The typical mineral assemblage of the Sanfoshan monzogranite includes plagioclase, quartz, K-feldspar, and biotite with minor amphibole. The accessory minerals are magnetite, allanite, titanite, apatite, and zircon (Figure 8(d)). The abundant magnetite, in addition to titanite, makes the Sanfoshan monzogranite highly oxidised as deduced earlier by the hand-magnet test. The early phases of the Sanfoshan pluton are metaluminous with A/CNK molar ratios of 0.96–0.99 and A/NK molar ratios above 1.0. The late-phase rocks are weakly peraluminous with A/CNK molar ratios of 1.01–1.06 (Figure 7(b); Shand’s index in Maniar & Piccoli, 1989). The rocks are of the I-type group as reflected by the molar A/CNK < 1.1 (Figure 7(b); I–S boundary in Chappell & White, 2001) and the high sodium, i.e. Na₂O > 3.2 wt.% (e.g. Chappell & White, 2001). The rocks tend to be high-K calc-alkaline (Figure 7(c); Rickwood, 1989).

Figure 7. Geochemical diagrams with the data of rock samples analysed in this study (green circles and squares) as well as data from Hu (2006) (orange and black circles, and orange squares). Orange and green circles represent the early phases of the Sanfoshan pluton. Black circles represent the Sanfoshan syenogranite. Orange and green squares represent the Xiamashan samples. (a) R₁R₂ chemical variation diagram (De la Roche et al., 1980), where R₁ (in millications) = 4Si - 11(Na + K) - 2(Fe + Ti), and R₂ (in millications) = 6Ca + 2Mg + Al, (b) Shand’s index (in Maniar & Piccoli, 1989), where A/CNK (molar) = Al₂O₃/(CaO + Na₂O + K₂O), and A/NK (molar) = Al₂O₃/(Na₂O + K₂O). I–S boundary after Chappell and White (2001), (c) The K₂O vs. SiO₂ diagram (Rickwood, 1989), and (d) Chondrite-normalised rare earth element diagram. Orange and black lines after Hu (2006). Normalising values are from Sun and McDonough (1989)

Figure 8. Geochemical diagrams discriminate between the early phases of the Sanfoshan pluton and the highly fractionated, late-phase rocks. For symbol colours, see Figure 7. (a) K/Rb vs. SiO₂ classification plot of granitoids (Blevin, 2004), (b) Plot of Rb/Sr vs. SiO₂, (c) Plot of Ba vs. SiO₂, and (d) Redox classification scheme of igneous rocks (Blevin, 2004). Representative photomicrograph is included for the samples with FeOT values < 2 wt.%. Abbreviations: VSO = very strongly oxidised; SO = strongly oxidised; MO = moderately oxidised; MR = moderately reduced; Mag = magnetite; Bt = biotite; Ap = apatite; Aln = allanite; Ttn = titanite

Figure 9. Redox-fractionation schematic plot of intrusive igneous suites and the core ore element associations in related mineralisation (Blevin et al., 1996) with superimposing of data of the early phases of the Sanfoshan pluton. Approximate commodity boundaries. Samples of the Sanfoshan pluton shown by green circles are data from this study, whereas orange circles are data from Hu (2006)

There is a pronounced decrease in Ba and Sr contents and increase in Rb content with increasing SiO₂ wt.% of these rocks (q.v. subsection 8.3.), suggesting they relate to one another by fractional crystallisation from a parental magma (e.g. Janoušek et al., 2004, and references therein). Such an explanation of fractional crystallisation can also be inferred from the chondrite-normalised rare earth element (REE) patterns (Figure 7(d); normalising values are from Sun & McDonough, 1989). The rocks in Figure 7(d) show a pronounced increase in a negative Eu anomaly (Eu/Eu* = Eu_N/[(Sm_N)(Gd_N)]^0.5) with the increase in the SiO₂ content, i.e. the degree of fractionation. Europium is referred to as a sensitive monitor of plagioclase fractionation in Gill (2010). The ‘least fractionated’ Sanfoshan samples (orange lines; Hu, 2006) lack a prominent Eu anomaly (0.84–1.17), indicating neither plagioclase accumulation nor fractionation. The Sanfoshan monzogranite (green lines; this study) displays a moderately negative Eu anomaly (0.54–0.73), indicating plagioclase fractionation. The highly fractionated, late-phase rocks have strong and the deepest negative Eu anomalies, ascribing to the significant role of plagioclase fractionation, or plagioclase in the melt residue. There is a decrease in ΣREE with the increase in the degree of fractionation, i.e. SiO₂ content, from 137.82–204.08 ppm in the early phases of the Sanfoshan pluton to 15.69–74.49 ppm in the highly fractionated, late-phase rocks. The rocks are enriched in light REE (LREE) with respect to heavy REE (HREE), but the highly fractionated rocks show the lowest enrichment in LREE with respect to HREE. The REE depletion in the fractionated, oxidised magmas can be attributed to the crystallisation of some specific minerals that have high concentrations of REE, e.g. allanite and titanite, in the least fractionated oxidised granitoids (Watanabe et al., 2017). The Sanfoshan monzogranite contains abundant accessory minerals of allanite and titanite (Figure 8(d)), and this may interpret the REE depletion in the highly fractionated rocks. The chondrite-normalised REE patterns (Figure 7(d)) show nearly subparallel trends, attesting to the role of fractionation from the same parental magma. That is, all rocks have steep negative LREE slopes (La_N/Sm_N) between 6.65 and 15.00, then the HREE segments relatively flatten or upturn slightly (listric-shaped patterns). The HREE slopes (Gd_N/Yb_N) are 1.70–2.34 in the early phases of the Sanfoshan pluton and 0.17–0.73 in the highly fractionated rocks. The listric-shaped patterns likely reflect amphibole fractionation, as the fractional crystallisation of amphibole-dominant systems results in tonalitic to granodioritic rocks that are characterised by listric-shaped REE patterns (e.g. Nandedkar et al., 2016, and references therein).

8. Discussion

8.1. Inference from the ore textural styles

Ore textures bear some information concerning the nature of the hydrothermal mineralising system, structural control, and overprinting and paragenetic sequences (e.g. Taylor, 2009). When combined with ore element associations, ore textural styles could then be utilised to infer the mineral deposit type. Medium- to fine-grained comb quartz of the sheeted-like veining at Shicheng reflects pure open space that has allowed the development of the characteristic comb appearance. This open-space infill texture, the sheeted style, and the Cu-bearing sulphides are characteristics of the porphyry and other intrusion-related mineral deposits (e.g. Dowling & Morrison, 1989). Corbett and Leach (1998) stated that the dilatant sheeted quartz veins occur in the porphyry, porphyry-related and epithermal (e.g. Gülyüz et al., 2018) environments. Thereby, the sheeted-like veining and the overprinting Cu-bearing sulphides at Shicheng are like those of the oxidised intrusion-related gold deposits elsewhere.

To classify the deposit as a ‘typical’ epithermal appears unconvincing because the characteristic epithermal complex banded (colloform) textures (e.g. Dowling & Morrison, 1989; Gülyüz et al., 2020) are absent. Also, the indicative epithermal minerals such as adularia, illite, kaolinite, and alunite (e.g. Simmons et al., 2005) are absent. Furthermore, the proposed orogenic gold deposit model for the Jiaodong gold deposits appears to be not the case for the Shicheng deposit because its mineralisation involves significant copper (chalcopyrite) similar to that of the oxidised intrusion-related gold deposits elsewhere. And it is unusual to find prominent open-space infill textures such as those of Shicheng in orogenic gold deposits. Au-chalcophile metal association tends to be related, in general, to highly oxidised intrusions. Veins in orogenic gold deposits tend to be syn-tectonic and form during deformation, resulting in milky quartz (buck) overprinted by ribbons, stylolites, spider veinlets, and breccias (e.g. Lusty et al., 2011, and references therein). And all of these are undetected in the Shicheng Au-Cu deposit.

8.2. CL-signatures and deciphering the deposit origin

In contrast to volcanic, plutonic, or metamorphic quartz, hydrothermal quartz displays CL textures that include precipitation and dissolution together with microfractures (Frelinger et al., 2015). According to Rusk (2012), CL textures can distinguish among quartz derived from various hydrothermal Au ore deposits. Müller et al. (2010) stated that many porphyry copper ore deposits exhibit a common sequence of quartz generations. Quartz derived from porphyry copper ore deposits displays well-developed oscillatory growth zoning, and involves generations that reflect dissolution, overgrowth, and healed micro-fractures (e.g. Frelinger et al., 2015; Müller et al., 2010; Rusk, 2012). Quartz derived from epithermal deposits exhibits characteristic spherical texture and sector zoning but with very little dissolution textures (Frelinger et al., 2015; Rusk, 2012), whereas quartz derived from orogenic Au deposits appears homogeneous dull grey and shows the least variation in CL textures (Frelinger et al., 2015; Rusk, 2012). Moreover, Wertich et al. (2018) studied CL textures in quartz and stated that the partly preserved growth zones of quartz, together with a late network of micro-fissures, suggest a magmatic influence on the hydrothermal system (i.e. intrusion-related gold such as in Zachariáš et al., 2014). Quartz from Shicheng shows the CL textures characteristic of the porphyry systems (e.g. Müller et al., 2010) and exhibits a similar quartz generations sequence. These textures are the inward euhedral quartz growth projecting into the ore-stage sulphides (e.g. Figure 5(c, j)) and the superimposition of multiple generations. These generations include oscillatory zoned quartz, which is dissolved and overgrown by ore-stage, CL-dark quartz in contact with the ore-stage sulphides (e.g. Figure 5(d, j, m)). Moreover, both stages are overprinted/cut by CL-dark micro-veins and non-luminescent networks of late-stage quartz (e.g. Figure 5(b, m)). In conclusion, these CL textures and the sequence of quartz generations, which are characteristics of the porphyry systems (e.g. Müller et al., 2010), indicate a magmatic influence on the hydrothermal system that formed the Shicheng deposit. Thus, in combination with the ore textural styles and metal associations, the CL textures in quartz can drive to a better understanding of cryptic ore deposits.

8.3. Metallogenic potential of the Sanfoshan pluton

The metallogenic ‘flavour’ or potential of granitic intrusions are, in part, a function of the oxidation state and degrees of compositional evolution and fractionation of these intrusive suites (Blevin, 2004; Blevin et al., 1996). Porphyry Cu and oxidised intrusion-related hydrothermal systems are associated with strongly oxidised and chemically relatively unevolved magmas that have not undergone extensive fractionation (Blevin, 2004; Blevin et al., 1996). Blevin (2004) stated that the K/Rb, Rb/Sr, and Fe₂O₃/FeO ratios serve as metallogenic discriminants and reflect the relative compositional evolution, degree of fractionation, and oxidation state of intrusive igneous suites, respectively. Moreover, the potential for the mineralisation and the ore element association with which a granitoid suite is associated, i.e. granitoid metallogeny, can be determined by comparing the Rb/Sr ratio (degree of fractionation) against the Fe₂O₃/FeO ratio (oxidation state) (Blevin et al., 1996). In other words, low Rb/Sr and high Fe₂O₃/FeO ratios mean less fractionation and strongly oxidised magma, favouring Au-chalcophile metal association. The K/Rb and Rb/Sr ratios have been used to measure the degrees of evolution and fractionation of granitic melts, i.e. an easy detection of residual phases. This is because the concentrations of Rb, Sr, and K, in addition to Ba and Eu, change into the residual liquid in the course of the progressive fractionation of magma, depending on whether the early-formed crystals of specific minerals accept or exclude these elements (compatible or incompatible) (e.g. Faure, 2001). Strontium is compatible in plagioclase and K-feldspar (Faure, 2001). Thus, early-formed crystals of plagioclase will deplete a residual melt in Sr relative to the parent (e.g. Faure, 2001; Gill, 2010; Hanson, 1978). Rubidium is incompatible in all main rock-forming minerals, so it becomes concentrated into the residual melt resulting in late-formed crystals (e.g. feldspar) with higher Rb concentrations (Faure, 2001). Therefore, the Rb/Sr ratio increases rapidly with increasing the degree of plagioclase fractionation beyond silica concentrations of about 65–70 wt.% (Faure, 2001; Hanson, 1978). K-feldspar also increases the Rb/Sr ratio but not as dramatically as plagioclase (Brown, 1991; Hanson, 1978). Potassium and Ba are retained by K-feldspar (Hanson, 1978). Thus, K-feldspar greatly affects the K/Rb ratio and decreases it in the residual melt relative to the parent (Hanson, 1978). And Ba steeply decreases with increasing the degree of fractionation beyond silica concentrations of about 75 wt.% (Blevin, 2004).

As noted earlier, the Sanfoshan pluton is composed mostly of monzogranite with minor granodiorite and evolved from highly oxidised, high-K calc-alkaline, I-type magmatism that underwent fractional crystallisation to produce the highly fractionated, late-phase rocks. Since, in intrusion-related hydrothermal systems, host granitoid composition and the oxidation state of parent magma are the most important in defining the metallogenic potential, the K/Rb, Rb/Sr, and Fe₂O₃/FeO ratios of the Sanfoshan pluton have been investigated, for the first time, herein as follows. The early phases of the Sanfoshan pluton have K/Rb ratios of 267.34 to 377.47, implying that the host pluton only reached a moderate evolution degree (Figure 8(a); Blevin, 2004) similar to that of intrusions associated with Cu-Au systems (K/Rb ratios greater than 200; Blevin, 2004). But the highly fractionated, late-phase rocks have K/Rb ratios of 100.03 to 163.11, i.e. strongly evolved, indicating the prominent role of K-feldspar in the fractionation of granitic melts. The Rb/Sr ratios of the early phases of the Sanfoshan pluton of 0.13 to 0.45 imply a low degree of fractionation (Figure 8(b)), which is roughly similar to that of intrusions associated with Cu-Au systems (low Rb/Sr ratios; Blevin et al., 1996). In contrast, the highly fractionated, late-phase rocks have high Rb/Sr ratios (2.64 to 57.25), indicating prominent plagioclase fractionation. The ‘least fractionated’ Sanfoshan samples that lack a prominent Eu anomaly have the highest Ba contents up to 2604.00 ppm, and samples of the Sanfoshan monzogranite that display a moderately negative Eu anomaly show a slight decrease of Ba contents (up to 1780.90 ppm) (Figure 8(c)). But the highly fractionated, late-phase rocks show Ba removal (13.82–280.90 ppm) consistent with the fractionation of a granitic melt.

Thereby, the whole-rock geochemistry on these intrusive rocks shows two distinct groups. The first is represented by the early phases of the Sanfoshan pluton that have K/Rb, Rb/Sr, and Ba values similar to those of intrusions associated with Cu-Au systems (Blevin, 2004; Blevin et al., 1996). In contrast, the second is the highly fractionated, late-phase rocks that resulted from the residual phases. Finally, the FeO concentrations, as indicated earlier, have been either determined using titration (the samples of this study) or calculated herein using the oxidation ratio graph in Middlemost (1985) (data from Hu, 2006). All data of the early phases of the Sanfoshan pluton have similar, high range of oxidation (Fe₂O₃/FeO) ratios (1.19 to 1.53) that imply an oxidised magma. These rocks fall within the strongly oxidised field of Blevin (2004) (Figure 8(d)). Some samples have FeOT (all Fe in the sample reported as FeO) values < 2 wt.%, and the petrography of these samples shows abundant magnetite content (Figure 8(d)). The strongly oxidised nature of the early phases of the Sanfoshan pluton reflects Cu-Au dominated system (e.g. Baker et al., 2005; Blevin, 2004). In conclusion, the early phases of the Sanfoshan pluton (particularly the ‘least fractionated’ Sanfoshan samples) have the fractionation (Rb/Sr) and oxidation (Fe₂O₃/FeO) ratios that favour Au-chalcophile metal association and overlap the field of intrusions associated with Cu-Au systems of Blevin et al. (1996) (Figure 9).

8.4. Implications for ore genesis in the Mu-Ru metallogenic belt

This study has contributed to the Mu-Ru belt’s overall metallogenic picture by highlighting a genetic connection between the Shicheng deposit and a later overprinting mineralising event in the Jinqingding deposit. At Jinqingding, Sai et al. (2020) stated a single metallogenic event that is closely fitting the classic fault-valve model, with ruling out the intrusion-related origin based on some reasons such as those in the introduction section and, also, the absence of notable, vertical change in alteration, mineralogy, and ore fluid chemistry. They stated that electrum, native gold, and petzite exist but with minor invisible gold. However, according to SEM imaging by this study, the Jinqingding deposit contains significant concentrically zoned As-rich pyrite (Figure 10(a, b, c)), i.e. an ‘invisible’ gold-As association (Mills et al., 2015b; Reich et al., 2005). The As-rich event overprints an early, unzoned, deformed pyrite (Figure 10(d)). The concentrically zoned As-rich pyrite occurs in later overprinting quartz veins, sometimes containing vugs, at Jinqingding. The overprinting quartz veins from Jinqingding display CL textures (Figure 10(e, f)) similar to those recorded from Shicheng. CL textures show oscillatory zoned generation overgrown and cut by CL-dark generation (ore stage). These CL textures, which are characteristics of the porphyry systems (e.g. Müller et al., 2010), indicate a magmatic influence on the hydrothermal system.

Figure 10. Collection of SEM imaging (BSE, CL and SE) shows some specific features displayed by the Jinqingding deposit. (a) BSE image shows concentrically zoned As-rich pyrite with pyritohedral crystal form, (b) SE image shows the same, previous pyrite crystal in vuggy comb-like quartz, (c) BSE image shows monazite occurs in close association with concentrically zoned As-rich pyrite, (d) BSE image shows early unzoned pyrite overprinted by As-rich event (alteration rim) along a fracture, (e) BSE image shows concentrically zoned As-rich pyrite, (f) CL imaging of the approximate area shown in (e) by the red dashed box. Oscillatory zoned quartz is overgrown and cut by CL-dark generation (ore stage), and (g) BSE image shows fractures in early pyrite filled by molybdenite. Abbreviations: Qtz = quartz; Py = pyrite; Mnz = monazite; Mo = molybdenite

Furthermore, at Jinqingding, there are shallow economic copper concentrations reported in the literature (e.g. Mills et al., 2015a). And there is a genetic link between Cu-rich mineralisation and oxidised, I-type intrusions elsewhere (e.g. Sillitoe, 1991). L. Zhang et al. (2020) presented an early age for the Jinqingding deposit (hydrothermal muscovite ⁴⁰Ar/³⁹Ar age of 121.4 ± 0.1 Ma) and also supported the young U–Pb age (114.2 ± 1.5 Ma) on the hydrothermal monazite by Deng et al. (2020). Monazite has been observed closely with the concentrically zoned As-rich pyrites (Figure 10(c)), but not exclusively. This probably interprets this young age (114.2 ± 1.5 Ma). When combined with the overprinting relationship between the different pyrite generations and the shallow economic copper concentrations at Jinqingding, these ages may reasonably reflect overprinting events. The overprinting As-rich event at Jinqingding appears to have occurred at 114.2 ± 1.5 Ma, and this young mineralisation age clearly overlaps the emplacement age of the Sanfoshan pluton (ca. 116–114 Ma; J. Zhang et al., 2010). Thereby, the oxidised intrusion-related hydrothermal origin/model is required at Jinqingding to explain such relationships. It is suggested that Re–Os dating on molybdenite found in close association with the deformed pyrite from Jinqingding (Figure 10(g)) may give additional constraints on the age of an early mineralising event at Jinqingding.

L. Zhang et al. (2020) considered that the absence of metal zonation from proximal Au-Bi-Te to distal Ag-Pb-Zn assemblages is against an intrusion-related origin for the Jiaodong gold deposits. However, such metal zonation is characteristic only to reduced intrusion-related gold systems (e.g. Hart, 2007). In the Mu-Ru belt, the late Early Cretaceous Sanfoshan pluton reflects an oxidised magma. The early phases of the Sanfoshan pluton (particularly the ‘least fractionated’ Sanfoshan samples) show a metallogenic potential for Au-chalcophile metal association. The Shicheng deposit, which appears to grade from Au-Cu into Pb-Zn-(Cu) metal assemblage, emphasises the magmatic association (i.e. an oxidised magmatic-hydrothermal system) that relates back to the Sanfoshan-forming magmatism. According to Corbett and Leach (1998), carbonate-base metal hydrothermal deposits form at crustal levels above the porphyry Cu-Au deposits, and so tend to be associated with higher-level porphyry intrusions. At least, the later overprinting mineralising event, i.e. As-rich pyrite and hence an ‘invisible’ gold-As association (Mills et al., 2015b; Reich et al., 2005), at Jinqingding may reasonably represent a product of the same oxidised magmatic-hydrothermal system within the Mu-Ru metallogenic belt. Thus, the Mu-Ru belt’s overall metallogenic picture indicates that the Shicheng deposit and (at least) the later overprinting mineralising event at Jinqingding are the products of an oxidised magmatic-hydrothermal system associated with the Sanfoshan-forming magmatism in the late Early Cretaceous. It is worth mentioning that the relatively early ⁴⁰Ar/³⁹Ar age (121.4 ± 0.1 Ma; L. Zhang et al., 2020) at Jinqingding appears inconsistent with the magmatic-hydrothermal system associated with the Sanfoshan-forming magmatism. This age (121.4 ± 0.1 Ma) appears synchronous with the published deformation/reactivation dating of the Mu-Ru belt’s regional faults (ca. 122.8 Ma; H. Zhang et al., 2007). In the Mu-Ru metallogenic belt, the youngest mineralisation system (ca. 114.2 ± 1.5 Ma; Deng et al., 2020) relates back to the Sanfoshan-forming magmatism in the late Early Cretaceous.

9. Conclusions

(1) The Shicheng deposit is characterised by Au-Cu bearing quartz-carbonate-sulphide veins that illustrate overprinting sheeted-like and breccia styles. Paragenetically-later veins contain substantial Pb-Zn-(Cu) metal association, suggesting metal zonation at the deposit scale. There is a genetic connection between the Shicheng deposit and a later overprinting mineralising event at Jinqingding. The overprinting event is characterised by quartz veins that contain As-rich pyrite. This is supported by a clear-cut overlap between a recently reported young mineralisation age for Jinqingding (ca. 114.2 ± 1.5 Ma; Deng et al., 2020) and the Sanfoshan emplacement age (ca. 116–114 Ma; J. Zhang et al., 2010).

(2) Quartz from Shicheng shows the CL textures characteristic of hydrothermal systems with a magmatic influence and exhibits a similar quartz generations sequence. Main-stage quartz shows oscillatory growth zoning under the SEM-CL beam. It is overgrown by ore-stage, CL-dark quartz in contact with the ore-stage sulphides. CL-dark micro-veins and non-luminescent networks represent late-stage quartz. Quartz from the later overprinting veins at Jinqingding shows CL textures like those of the Shicheng deposit.

(3) The Sanfoshan pluton, evolved from highly oxidised, high-K calc-alkaline, I-type magmatism that underwent fractionation to produce other highly fractionated rocks. The early phases of the Sanfoshan pluton are composed mostly of monzogranite with minor granodiorite and have K/Rb, Rb/Sr, and Fe₂O₃/FeO ratios of 267.34 to 377.47, 0.13 to 0.45, 1.19 to 1.53, respectively. This favours Au-chalcophile metal association. In other words, the Shicheng deposit and the later overprinting event at Jinqingding are the products of an oxidised magmatic-hydrothermal system associated with the Sanfoshan-forming magmatism in the late Early Cretaceous.

Acknowledgments

The first author acknowledges the CAS-TWAS President’s Fellowship Programme for financially supporting his PhD. This paper is a part of his PhD study. The first author is grateful to Dr Peter J. Pollard for his advice on the ore textures of some samples and granite metallogeny. Ms Xin Yan is thanked for her tips during using the IGGCAS’s SEM (BSE, CL, and EDS). This paper benefited greatly from the constructive review by Dr Nilay Gülyüz and the valuable role of the Editor-in-Chief Prof Erdin Bozkurt. The work was also financially supported by the National Key Research and Development Program (No. 2016YFC0600105) and National Natural Science Foundation of China (41772080).

Disclosure statement

There are no competing interests.

Additional information

Funding

This work was supported by the National Key Research and Development Program [No. 2016YFC0600105]; the National Natural Science Foundation of China [41772080]; CAS-TWAS President’s Fellowship Programme [2017 CAS-TWAS President’s Fellowship Programme].

Appendix

Table A1. Whole-rock geochemistry—major elements in wt.%, and trace elements and REE in ppm

	Sanfoshan					Xiamashan
	(This study)					(This study)
	18SF1	18SF2	18SF3	08RS021	08RS046	18XM1	18XM2
SiO2	72.15	72.19	72.44	69.95	70.98	77.12	76.77
TiO2	0.23	0.23	0.22	0.30	0.23	0.08	0.07
Al2O3	14.33	13.95	14.02	14.79	14.70	12.61	12.96
TFe2O3	1.82	1.95	1.86	2.42	1.94	0.72	0.79
MnO	0.05	0.04	0.03	0.04	0.04	0.01	0.01
MgO	0.53	0.51	0.47	0.83	0.48	0.08	0.09
CaO	1.40	1.40	1.56	1.89	1.45	0.41	0.45
Na2O	4.05	3.97	4.25	4.25	4.18	4.27	4.23
K2O	4.99	4.77	4.13	4.40	5.08	4.19	4.11
P2O5	0.07	0.08	0.07	0.12	0.08	0.01	0.01
LOI	0.28	0.32	0.27	0.48	0.23	0.22	0.43
TOTAL	99.90	99.42	99.32	99.48	99.40	99.71	99.93
FeO	0.77	0.79	0.80	0.92	0.85	0.27	0.31
FeOT	1.64	1.76	1.68	2.18	1.75	0.65	0.71
Fe2O3/FeO	1.26	1.37	1.22	1.53	1.19	1.54	1.45
Log Fe2O3/FeO	0.10	0.14	0.09	0.18	0.07	0.19	0.16
A/CNK	0.98	0.98	0.98	0.97	0.98	1.02	1.06
A/NK	1.19	1.19	1.22	1.26	1.19	1.09	1.14
R1	2150.22	2227.84	2299.09	2052.82	2001.87	2620.64	2630.11
R2	457.11	448.80	465.25	533.44	467.32	295.10	306.88
Ba	990.56	859.42	946.84	1780.90	909.40	18.28	20.27
Rb	134.15	117.47	122.19	135.88	119.64	327.78	322.65
Sr	298.44	299.01	317.72	467.16	324.67	10.42	14.08
Li	7.65	7.49	6.73	6.43	7.53	0.12	0.82
Be	2.19	2.32	2.45	2.09	2.69	5.19	3.97
V	17.55	19.20	19.14	23.45	19.74	5.23	6.05
Cr	215.98	191.29	160.35	185.83	224.06	201.11	197.72
Co	3.37	3.41	3.13	3.81	3.49	1.70	1.75
Ni	6.19	6.49	5.33	6.69	6.61	4.44	4.16
Cu	8.96	8.01	10.60	5.31	7.98	4.26	6.89
Zn	17.79	16.63	13.65	16.39	17.40	12.14	11.70
Ga	15.39	15.36	15.17	16.38	15.35	16.38	16.20
Zr	135.65	162.43	132.44	126.11	134.32	73.82	88.77
Nb	13.50	12.78	15.08	11.67	14.82	20.16	18.80
Cs	1.67	1.50	2.08	1.64	2.35	6.91	6.50
Hf	4.21	5.06	4.03	3.55	4.22	3.52	3.81
Ta	1.08	1.01	1.18	0.90	1.16	1.29	1.50
Tl	0.68	0.62	0.65	0.72	0.62	1.63	1.54
Pb	20.45	20.22	19.94	17.75	18.77	32.22	31.77
Bi	0.03	0.02	0.04	0.08	0.08	0.18	0.14
Th	22.29	29.36	23.61	22.30	35.80	40.61	36.70
U	2.91	4.41	3.22	4.08	4.66	8.14	6.23
Sc	2.14	2.43	2.55	2.76	2.15	1.73	1.77
Y	10.86	10.89	12.75	8.39	11.19	3.20	3.88
La	42.99	47.17	58.63	40.63	36.08	10.40	12.85
Ce	73.19	77.49	99.41	65.40	62.69	8.43	18.77
Pr	7.55	8.03	10.11	6.61	6.81	1.59	1.96
Nd	24.28	25.57	32.74	21.33	22.64	4.05	4.89
Sm	3.44	3.59	4.62	3.16	3.33	0.48	0.55
Eu	0.59	0.64	0.72	0.65	0.62	0.06	0.07
Gd	2.75	2.91	3.63	2.35	2.75	0.42	0.49
Tb	0.36	0.37	0.46	0.30	0.36	0.06	0.07
Dy	1.90	1.94	2.34	1.54	1.95	0.41	0.45
Ho	0.40	0.41	0.48	0.32	0.41	0.11	0.12
Er	1.14	1.16	1.33	0.88	1.18	0.42	0.48
Tm	0.18	0.18	0.20	0.13	0.19	0.09	0.10
Yb	1.24	1.26	1.34	0.95	1.34	0.86	0.95
Lu	0.22	0.21	0.21	0.16	0.22	0.18	0.21
K/Rb	308.74	337.07	280.33	268.50	352.15	106.04	105.70
Rb/Sr	0.45	0.39	0.38	0.29	0.37	31.47	22.91
Eu/Eu*	0.58	0.61	0.54	0.73	0.63	0.40	0.42
ΣREE	160.20	170.93	216.20	144.40	140.57	27.55	41.95
LaN/SmN	8.07	8.48	8.19	8.31	7.00	13.96	15.00
GdN/YbN	1.84	1.91	2.24	2.05	1.70	0.40	0.43