Evolution of Pyrite Compositions at the Sizhuang Gold Deposit, Jiaodong Peninsula, Eastern China: Implications for the Genesis of Jiaodong-Type Orogenic Gold Mineralization

Abstract

Gold deposits in the Jiaodong Peninsula represent a primary gold resource in China and mostly exhibit similar ore-forming features related to sericite-quartz-pyrite alteration and other controls from (micro-)structural deformation. This study investigates the pyrite textures and trace elements in the Sizhuang gold deposit (>100 t Au) to document the key factors impacting on the genesis of the Jiaodong-type orogenic deposits. Three main types of pyrite are identified: (1) the first generation of pyrite (Py1) occurs as disseminated euhedral to subhedral grains in K-feldspar-albite-rutile-hematite and sericite alteration (stage 1), (2) Py2 as aggregates in quartz-sericite-pyrite altered rocks or quartz-pyrite veins (stage 2) can be subdivided into Py2a as irregular cores, Py2b as a zoned overgrowth on Py2a, and Py2c as overgrowth on early pyrite, and (3) Py3 as fine-grained crystals in siderite-polymetallic veins (stage 3). Primary gold at the Sizhuang deposit is coevally or slightly later deposited with Py2b, Py2c, and Py3. Laser ablation–inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses show that the highest Co and Ni contents in Py1 and high but variable Co in Py2b favors the involvement of deep high-temperature magmatic waters at stage 1 and middle stage 2. The elevated As contents from Py2a to Py2c and depletion of trace elements (e.g., Co, Ni, As and Te) and high Au/Co, Cu/Ni, and As/Ni values in Py2a and Py3, combined with published H-O isotope data, imply a meteoric water ingress during stage 2–3. Thus, the fluid evolution at Sizhuang is a consequence of pulsed deep magmatic fluid release plus progressive meteoric fluid ingress. The rhythmic Co–As–Ni–Au bands of Py2b additionally suggest episodic changes in the composition of ore-forming fluids. Moreover, the sharp textural features (e.g., pyrite overgrowth on previously cataclastic crystals) of Py2 and As-Cu-rich and Co-poor bands in zoned Py2b probably also reflect rapid metal deposition and self-organization and subsequent mineral crystal growth due to the pressure release during phase separation in the Sizhuang deposit. Considering the significantly concentrated gold (>1300 t) in the regional Jiaojia fault zone and Au-bearing mineral formation related to phase separation (boiling) in the Sizhuang deposit, gold mineralization in the Sizhuang deposit was interpreted to be controlled by the pressure-driver owing to the seismic activities in the Jiaojia fault system.

1. Introduction

The Jiaodong Peninsula located in the eastern portion of the North China Craton (Figure 1) represents the largest gold province in China with a measured resource of >5000 t Au [1,2,3,4,5]. Gold deposits in the Jiaodong Peninsula (Jiaodong-type deposits) are fundamentally similar to typical orogenic gold deposits in the greenstone belt, such as low-salinity, CO₂-rich, and mid- to high-temperature fluid conditions [6,7,8,9]. However, the gold mineralization, dated at about 120 ± 5 Ma, significantly post-dates the age of the host rocks, which include the Precambrian metamorphic rocks, 160–155 Ma Linglong or Kunyushan granite, and 130–125 Ma Guojialing granite [10,11,12]. This is in contrast to the syn-metamorphic origin for orogenic gold deposits in Archean greenstone belts [3,13,14,15]. For this reason, the term “Jiaodong-type orogenic deposit” has been proposed to distinguish these unique Cretaceous gold deposits in the Jiaodong Peninsula [1,7,16,17].

Gold mineralization of many Jiaodong-type orogenic deposits has a number of features in common, including (1) they typically have an early K-feldspar stage, intermediate quartz-sericite-pyrite and quartz-(siderite)-polymetallic stages as the primary ore-forming periods, and a post-ore quartz-calcite stage [7,10,17,19,20,21], and (2) gold emplacement with preferential localization in the hanging or footwall of detachment faults [3,5,22,23]. These common characteristics imply that coupling of fluid evolution and structural deformation likely played a critical role in generating the Jiaodong-type gold deposits.

Pyrite is an ubiquitous mineral associated with gold in several types of hydrothermal gold deposits [24,25,26,27,28,29,30,31]. Numerous studies have demonstrated that pyrite can effectively incorporate a wide range of metal and metalloids (e.g., Au, Ag, As, Cu, Co, Ni, Te, Pb, and Bi), depending on the fluid compositions, geochemical properties of dissolved elements, and the precipitation processes [29,31,32,33,34,35,36]. Furthermore, the diversity of textural features in pyrite (e.g., cataclastic, zonation, replacement, and recrystallization) in hydrothermal gold systems is interpreted as a result of complex growth processes during ore-forming evolution [24,31,36,37,38,39,40,41]. Hence, synthetic studies on pyrite geochemistry and texture at the micron- and/or atomic-scale by advanced analytical techniques, such as scanning electron microscopy, electron probe micro-analyzer (EPMA), laser ablation–inductively coupled plasma mass spectrometry (LA–ICP–MS), secondary ion mass spectrometry, and atom probe tomography, have been popularly applied to probe (1) metals and metalloids incorporation forms [33,34,42,43,44,45,46], (2) pyrite crystallization history and metals and metalloids precipitation, partition, remobilization, and redistribution [36,38,40,41,47], (3) fluid sources, conditions, and evolution [27,28,29,30,33,48], and (4) metallogenesis of mineral systems [28,29,32,35,39,43,47,48,49].

Pyrite is commonly precipitated during several ore-stages of the Jiaodong-type gold deposits and gold occurs as free milling inclusions within pyrite or fillings into pyrite cracks, implying a simultaneous or slightly lag precipitation of gold and pyrite [18,25,50,51]. Moreover, pyrite often exhibits the cataclastic, zoned, and/or overgrowth textures associated with multiple micro-structural deformation events that, at least partly, control gold deposition from fluids in the Jiaodong gold deposits [18,51,52], providing an excellent record of the fluid kinetic environment during mineralization. The Sizhuang gold deposit with a proven reserve of more than 100 t gold is located along the Jiaojia fault, northwest Jiaodong Peninsula [53]. Here we present a detailed investigation of pyrite textures and trace element chemistry of the Sizhuang pyrite to understand gold precipitation and fluid evolution of the Sizhuang deposit as well as the ore genesis of the Jiaodong-type orogenic deposits.

2. Geological Background

The Jiaodong Peninsula consists of two major blocks: the Jiaobei terrane and Sulu orogenic belt. They are separated by the Wulian-Yantai fault (Figure 1). Most of the orogenic gold deposits in the Jiaodong Peninsula are hosted in NNE- or NE-striking detachment faults, such as the Sanshandao, Jiaojia, and Zhaoping faults, in the west Jiaobei terrane (Figure 1). These detachment faults developed as a result of three-stages of deformation: (1) compressional to transpressional deformation under NW–SE principal stress related to paleo-Pacific subduction in the pre-ore stage (>130 Ma), (2) transtensional to extensional deformation under NW–SE minimum principal stress during slab rollback in the syn-ore stage (120 ± 5 Ma), and (3) compressional to transpressional deformation under WNW minimum principal stress in the post-ore stage [22,54]. The NNE- to NE-trending Jiaojia fault extends over more than 60 km and is associated with more than 20 gold deposits (e.g., Sizhuang, Matang, Jiaojia, and Xincheng) with a total gold resource of 1300 t [18,53].

The Sizhuang gold deposit is located in the southern part of the Jiaojia ore-field (Figure 1). Major lithological units exposed around Sizhuang are the Malianzhuang amphibolite (meta-gabbro) of the Archean Jiaodong Group and the Late Jurassic Linglong biotite monzonitic granite (Figure 2a and Figure 3a). The Linglong granite hosts the most ore at Sizhuang and consists of plagioclase (~40 vol%), K-feldspar (~23 vol%), quartz (~28 vol%), and biotite (~5 vol%) with minor pyroxene, zircon, titanite, and apatite [55].

Gold mineralization is structurally controlled by the NNE-striking and NW-dipping Jiaojia fault and secondary faults or joints in the Linglong granitoid (Figure 2). The Jiaojia fault displays hybrid ductile-brittle features with dip angles ranging from 30–45° [55]. The main orebodies can be divided into three groups: (1) the No. 1 orebodies comprise disseminated ore hosted in quartz-sericite-pyrite alteration zone near to the Jiaojia fault, (2) No. 2 orebodies are found in quartz/siderite-sulfide veins and veinlets in sericite-quartz altered rocks, and (3) No. 3 orebodies form distal mineralization consisting of quartz-sulfide veins in K-feldspar-albite-rutile alteration [53,56]. The No. 1 orebodies account for more than 40% of total gold resource of the Sizhuang deposit and are found at depths of less than 900 m. They vary in thickness from 1 to 24 m, extend 200 to 1200 m down plunge with a strike of 2–30° and a NW dip of 21–41° [56]. The gold grade varies from 1.2 to 5.5 g/t. The No. 2 orebodies account for only 1.4% of the known gold resource [56], and mainly consists of 20 small-scale orebodies at the −159 to −920 m levels. The No. 3 orebodies include more than 130 orebodies and contains about 58% of the gold at Sizhuang. The orebodies generally occur as veins, stratiform-like, or lenses, striking 342–028°, dipping 23–47° NW, extending 905 m along strike, with a thickness of 0.8–16.1 m and gold grade of 1–26 g/t.

Four mineral sequences have been identified in the Sizhuang deposit (Figure 4). Stage 1 is marked by pervasive K-feldspar-albite-rutile alteration and quartz-sericite alteration in the Linglong biotite granite (Figure 3a,b). The K-feldspar-albite-rutile alteration consists of albite, K-feldspar, sericite, hematite, rutile, and pyrite associated with the dissolution of plagioclase, biotite, titanite, and magnetite [55]. Quartz-sericite alteration with the mineral assemblage of sericite, quartz, and minor pyrite formed slightly later than K-feldspar-albite-rutile alteration. Stage 2 (quartz-sericite-pyrite) is the chief gold mineralization period in Sizhuang. Early stage 2 is characterized by white quartz-pyrite veins that are commonly fractured or brecciated by structural deformation and subsequently cemented or crosscut (Figure 2 and Figure 3c,d) by late alteration and veins. Middle-late stage 2 is marked by intense quartz-sericite-pyrite alteration that almost totally replaced the rock-forming minerals in the host rocks (Figure 3e) or early alteration minerals (Figure 3f). Hydrothermal minerals in this stage include grey quartz, sericite, pyrite, as well as minor siderite, chalcopyrite, and electrum (Figure 4). Stage 3 comprises quartz-siderite-polymetallic sulfide veins, stockworks, or veinlets that overprint on quartz-sericite-pyrite alteration (Figure 3g–i). Sulfide aggregates (pyrite, pyrrhotite, marcasite, chalcopyrite, and galena) of stage 3 occur in the middle of the veins (Figure 3g), suggesting they formed late in the paragenesis. Pyrrhotite as the early phase of stage 3 usually replaces previously formed pyrite (Figure 3i). Barite, magnetite, and marcasite were mainly precipitated in late stage 3 (Figure 4; [57]). Minor Te-Bi minerals are also formed in polymetallic sulfide veins [58]. Stage 4 is dominated by quartz and calcite, representing the post-ore stage. The Jiaodong Group amphibolite in the hanging wall displays K-feldspar, sericite, chlorite, and pyrite alteration, but no clear cross-cutting relationship was observed with the other mineralization phases.

3. Samples and Analytical Methods

A total of 22 polished sections were prepared from 19 samples collected from the Sizhuang deposit. Samples were collected at the −128 m level on the 288 exploration line (Figure 2a,b). These were used for optical microscopy and in situ analysis by electron probe micro-analyzer (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS). The detailed sample locations are shown in Figure 2c.

Pyrite grains were etched for about 20–60 s using bleach (sodium hypochlorite solution, 8–10% NaClO) to reveal internal textures. After etching, the samples were rinsed immediately with water and oven-dried.

Backscattered electron (BSE) imaging was carried out with a 1720 EPMA of Shimadzu Corporation at the Central South University, China. The accelerating voltage was set as 15 kV and the current was adjusted in the range from 5 nA to 150 nA based on the composition difference in objective minerals. The brightness and contrast were manually adjusted to present the clearest images.

The LA–ICP–MS analysis of sulfide minerals was conducted utilizing an NWR ablation system (λ = 193 nm) coupled to an iCAP RQ ICP–MS instrument to acquire ion-signal intensities at the Guangzhou Tuoyan Analytical Technology Co., Ltd., Guangzhou, China. Helium was used as a carrier gas. Line scans for sulfide LA–ICP–MS mapping were carried out using a 10 µm spot at 20 Hz and a scan speed of 30 μm/s. The spot size and analysis frequency for spot analysis were set to 40 µm and 8 Hz, respectively. Each spot analysis incorporated a 15 s background acquisition, 35 s sample ablation, and 30 s washout time. Analyses were calibrated against three reference materials (NIST 610, GSE-1G, and GE7). The trace element mapping was calibrated against the reference materials GSE-1G and NIST 610. Iron content determined by 1720 EPMA of Shimadzu Corporation (acc. Voltage 15 kV, probe current 20 nA, and spot size 5 μm) was chosen as the internal standard for pyrite. All LA–ICP–MS data were processed using the software Iolite (v3, School of Earth Sciences, The University of Melbourne, Melbourne, Australia) (http://iolite-software.com). The data for the standard MASS-1, which was used as an unknown sample for quality control, were mostly within 5% of in-house monitored concentrations. The Sizhuang sulfide had many monitor elements (e.g., As, Co, Ag, Pb, Bi, etc.) with orders of magnitude variations (RMS (root mean squared) up to 3698; Table S1) resulting in large variations of uncertainties (RMS 0.013–343) and thus the analytical uncertainty was insignificant relative to sample heterogeneity. The detection of limits for monitored elements is listed in Table S1.

4. Results

4.1. Pyrite Textures

Based on host rocks, mineral assemblages, textures, and relative relationships (e.g., overgrowth, containing, and filling), three types of pyrite in the Sizhuang deposit have been recognized in this study. The earliest generation of pyrite (Py1; Figure 4) is distributed as disseminated grains in K-feldspar-albite-rutile and quartz-sericite altered rocks of stage 1. The Py1 is spatially associated with altered Fe-bearing minerals (e.g., biotite). The Py1 crystals associated with different alteration share similar textural features such as fine- to middle-sized grains (mostly <300 μm in diameter) euhedral to subhedral (Figure 5a–d) and homogeneous BSE images. Minor amounts of Py1 grains occurred as coarse-grained cubic crystals [55]. Py1 locally displays porous textures due to the trapping of silicate mineral inclusions during growth (Figure 5d). Minor fine-grained free electrum coexists with Py1 grains and sericite.

Stage 2 pyrite (Py2) occurs as veins or massive accumulations in the quartz-sericite-pyrite alteration (Figure 3d–f and Figure 5e). It consists of coarse- to medium-grained, subhedral to anhedral grains, with cataclastic textures (Figure 5e–k). They display complex textures since they are typically contained, filled, or sealed by later addition of quartz, siderite, and sulfide. Early-phase Py2 (Py2a) occurs as irregular cores up to 150 μm across that are enclosed by zoned Py2 (Py2b; Figure 5f,g). Py2b is the most common Py2 and is subhedral to anhedral and from tens to hundreds of μm across (Figure 5f–k). The early Py2b has narrow compositionally inhomogeneous growth bands revealed by etching or BSE imaging, whereas late Py2b generally develops wide or no zonation (Figure 5f–h). Py2b grains usually form cataclastic textures due to frequent microstructural deformation and are overgrown by late-phase pyrite (Figure 5i,j and Figure 6a). Py2c occurs as euhedral to anhedral fine to medium grains (mostly several to hundreds of μm in size). Py2c grains exhibit brighter colors in reflected light (Figure 5i) and BSE images (partly) than Py2b (Figure 5k). Metallic minerals coexisting with Py2 are mainly chalcopyrite and galena and occur as inclusions in crystals or veinlets filling fractures (Figure 5e,g,i,k).

The third generation of pyrite (Py3) is mainly distributed in stage 3 breccia and silicified rocks as aggregates or veinlets in carbonate-sulfide veins (Figure 6). Py3 is distinguished from early pyrite by its generally small size (<100 μm in size), anhedral to subhedral crystals, and siderite-marcasite-pyrrhotite assemblage (Figure 6a–d). The syn-mineral pyrrhotite is often replaced by Py3 (Figure 6d), indicating it formed before Py3, which in turn is replaced or overgrown by syn-stage marcasite (Figure 6e) or magnetite. Electrum is recognized as the inclusions within Py3 (Figure 6e) or free forms in siderite veins (Figure 6f).

4.2. Pyrite Geochemistry

Trace element chemistry of pyrite (Py1 n = 31, Py2a n = 16, Py2b n = 39, Py2c n = 13, Py3 n = 19) determined by LA–ICP–MS is listed in Table S1.

Disseminated pyrite (Py1) in stage 1 has higher concentrations of Co (36.7–237 ppm; data between the 25th and 75th percentiles in box-plot), Ni (28.8–172 ppm), and Te (from B.D.L (below detection limit) to 2.62 ppm) but lower Cu, Zn, Ag, Sb, and Tl (Figure 7 and Figure 8a). Gold in Py1 ranges from B.D.L to 0.35 ppm, with a mean value of 0.04 ppm (s.d. (standard deviation) = 0.06, n = 31) and correlates with Co (r = 0.717; Figure 8b) and Ni (r = 0.514; Table S2). Silver concentrations of Py1 are markedly related to the concentrations of Mn, Pb (r = 0.976) and Bi (r = 0.787). Cobalt and Ni are homogeneously distributed within Py1 grains (Figure 9a,b). Tungsten is mostly below the detection limit in Py1 grains and its contents in several spots are associated with Ti (Figure 8c and Figure 9a).

The Py2a is depleted in Co, Ni, As, and Te and enriched in Cu, Mo, Ag, Cd, Sb, and Pb relative to Py1. The concentrations of gold in Py2a are similar to Py1 but do not show any correlation with other elements (Table S2). The Py2a is characterized by low Co and As contents compared to Py2b–c (Figure 7 and Figure 10). Zinc, Cd, and In are consistently variable during laser ablation of Py2a (Figure 9c), which is concordant with the high correlation coefficients (Table S2). Overall, the contents of Ni, As, Ag, Te, Au, Tl, Pb, and Bi in the three types of Py2 gradually increase from Py2a to Py2c (Figure 7). Most trace elements (e.g., Co, Ni, Cu, As, Pb, and Bi) in Py2b have large variations (Figure 7 and Figure 8). The growth zoning of Py2b revealed by NaClO-etching is associated with Co and As content variations determined by LA–ICP–MS mapping (Figure 10). The concentrations of Au and Te are generally consistent with As in the oscillatory zonation (Figure 10), but are not shown in the binary plot of Au-Te (Figure 8f). The high-As bands of Py2b have highly variable Cu, Pb, Bi, and/or Ag and Sb (Figure 9d,e). The Py2c is characterized by relatively high Ni (7.35–60.3 ppm), As (449–1982 ppm), and Au (0.05–0.29 ppm) concentrations. Gold is positively correlated to As (r = 0.751) and Te (r = 0.426) in Py2b and Ag (r = 0.758) and Bi (r = 0.784) in Py2c, respectively (Table S2). The Py2c spot (PyL08) with the highest Au (118 ppm) and Ag (140 ppm) concentrations exhibits parallel time-resolved profiles of Au and Ag (Figure 9f).

The fine-grained pyrite (Py3) in stage 3 sulfide-siderite veins is characterized by extremely low Co, As, Cd, and Te and high Tl and W (Figure 8 and Figure 9h). The concentrations of Ni, Zn, and Au in Py3 are generally lower than in Py2 grains. Lead, Bi, and Ag signals are relatively consistent and locally flat in Py3 spots (Figure 9h,i). Correlations also exist between Au and Ag-Pb-Bi (Table S2).

5. Discussion

5.1. Gold Occurrence and Its Controls

Gold can occur in pyrite as free Au-bearing mineral inclusions (e.g., native gold, electrum, petzite, and calaverite) or as solid solution in the pyrite lattice [26,33,35,46,59,60,61,62,63]. Gold in Py1 is positively correlated with Co (r = 0.717) and Ni (r = 0.514; Table S2). The ionic radii of octahedrally-coordinated Fe²⁺, Co²⁺, and Ni²⁺ are 0.92 Å, 0.885 Å, and 0.83 Å, respectively (Co³⁺ 0.75 Å; Ni³⁺ 0.74 Å), whereas that of Au⁺ is 1.51 Å [64]. Therefore, the equivalent substitution of Fe²⁺ ↔ Co²⁺ or Ni²⁺ is more feasible for pyrite and coupled substitution of the three elements (e.g., 2Fe²⁺ ↔ Co³⁺/Ni³⁺ + Au⁺) is less likely. The much lower Au concentrations (from B.D.L to 0.35 ppm) relative to Co (36.7–237 ppm) and Ni (28.8–172 ppm) also exclude the Au entrance into Py1 by the coupled substitution of octahedrally-coordinated Fe²⁺. Arsenic incorporation into the pyrite structure can distort the crystal structure, resulting in the possibility of Au⁺ entering into pyrite [33,59]. Hence the empirical solubility limits accounting for As contents in pyrite allow the assertion, in most cases, that the Au is present as inclusions (Au⁰) or in solid solution form (Au⁺). All Py1 grains are below the lowest limit of Au-saturation proposed by [59], suggesting gold is dominantly found in the Py1 crystal lattice. The gold occurrence within Py1 grains is, thus, likely associated with As incorporation.

Minor electrum grains were observed in close association with Py1 (Figure 5d). Py1 is disseminated distributed in the K-feldspar-albite-rutile and quartz-sericite altered rocks and associated with the alteration of mafic minerals, especially biotite (Figure 5b). The occurrence is comparable to the early-phase pyrite in the Xincheng gold deposit in the Jiaojia belt [18], implying that Py1 was generated as a result of the preferential sulfidation of iron-rich minerals (e.g., biotite), during which the Fe can consume the reduced sulfur (Fe²⁺ + 2HS⁻ → FeS₂ + H₂) causing the precipitation of pyrite from hydrothermal solutions. Overall, ore-forming fluids of the Sizhuang gold deposit represented a low salinity H₂O-CO₂-NaCl ± CH₄ system [58,65]. Gold is transported as Au-bearing complexes (Au(HS)₂⁻) in such a fluid system [66]. Thus, the reducing effect of iron minerals in the wall rocks on migrating hydrothermal fluids which can potentially lead to the precipitation of sulfides and the destabilization of sulfur-complexes, causing the synchronous gold deposition related to Py1.

The major types of Py2 containing gold are Py2b and Py2c. The analyzed Py2b and Py2c grains (except for L08-02) were plotted below the Au-saturation line (Figure 8e), probably also suggesting the incorporation of refractory Au as Py1. Our trace element dataset shows a generally positive association between Au and As in the Sizhuang pyrite (Figure 8e and Figure 10), especially for the zoned Py2b (r = 0.751; Table S2). This observation strongly suggests that structurally bound Au associated with As incorporation exists in these samples. Only the pyrite analyses of No. L08-02 reflects the existence of Au-bearing mineral inclusions (i.e., electrum). Such an interpretation is supported by the parallel time-resolved depth profiles of Au and Ag in Figure 9f. Notably, the existence of Au-bearing inclusions (e.g., electrum and native gold) in Py2 are detected in the LA–ICP–MS mapping (Figure 10), but these pyrite grains are indicated the form of Au⁺ in Figure 8e. Our data reveal the correlation among Au, Ag, and Bi in Py2c (Table S2) and the Au signal is parallel to that of the other two elements. This association is either attributed to the geochemical affinity among Au, Ag and Bi or metals residual in fluids after Py2b crystallization.

Py3 has extremely low As contents (mostly below detection limit), meaning no significant incorporation of Au⁺ resulting from As substitution in pyrite (inference from As-Au bi-plots). Gold in Py3 shows significant correlation with Ag, Pb, and Bi (Table S2; Figure 9i) that are contained in galena inclusions by substitution mechanisms of Ag⁺ + (Sb, Bi)³⁺ ↔ 2Pb²⁺ or (Ag, Tl, Cu)⁺ + (Sb, Bi)³⁺ ↔ 2Pb²⁺ [67,68]. The positive correlation thus likely reflects the gold storage with galena, which is consistent with the pyrite LA–ICP–MS mapping results that galena contains much fine-grained gold (Figure 10). Previous studies have shown that galena crystal can contain tens to several hundred ppm of gold [61,69,70] and it is also determined as the most favorable sulfide matrix (except for arsenopyrite) for locating gold (Au solubility up to 240 ppm) due to the high metallicity of chemical bond [61]. Besides, galena can capture additional gold by absorption of elements in crystal boundaries during crystallization in terms of heteroepitaxial structures [61,71]. The association of Au with Ag, Pb, and Bi most likely reflects the controls on gold enrichment from galena. The close relationship between gold and galena in the Jiaodong-type gold deposits was documented in [18,72] and our recent works [73,74], implying a common function of galena for gold mineralization. Apparently, galena should be reasonably estimated as a gold concentrator at the Jiaodong-type gold deposits.

5.2. Fluid Evolution Recorded by Pyrite Compositions

The incorporation of trace elements into pyrite is closely associated with complex fluid conditions, including fluid source, composition, and nature (e.g., temperature, pressure, pH), and element partition among co-crystallization minerals or fluids [29,33,40,41,75,76,77]. The earliest generation of pyrite (Py1) has relatively high concentrations of Co and Ni, and low Cu, Zn, Ag, Sb, and Tl (Figure 7), similar to the Xincheng pyrite [18]. Co- and Ni-rich pyrite grains are widely reported in the early stages of hydrothermal systems with relatively high-temperature conditions [26,29,30,31,78,79]. Major factors favoring the enrichment of Co and Ni relative to other metals in pyrite in hydrothermal systems include high fluid temperatures [80,81,82] and/or special host rock composition (e.g., mafic to ultramafic lithology or metasedimentary; [83,84]). Due to the granite as host rocks for the Sizhuang gold deposits, the host rocks maybe less important for the Co and Ni enrichment in Py1. Two-feldspar (K-feldspar and plagioclase) thermometer calculation and fluid inclusion analysis of the K-feldspar alteration in the Sizhuang deposit suggest the early alteration happened under high-temperature (~300 to 450 °C) conditions [58,65,85]. The quartz H-O isotopes from stage 1 are close to the primary magmatic water source region [55]. Thus, high Co concentrations in Py1 suggest early high-temperature fluids that were likely derived from deep magmatic intrusions.

Considering the relative late sequence (decreased temperature) of polymetallic-style mineralization in hydrothermal gold systems, rare Au deposition at high-temperature conditions, and a geochemical affinity of As for low density and low salinity fluids [27,86,87], the diagrams of Cu/Ni vs. Au/Co [78,88] and As/Ni vs. Au/Co potentially reflect the progressive evolution involving the decreasing temperature conditions (likely caused by external cool fluid ingress [89]) in a magmatic hydrothermal system (Figure 11). The diagrams are consistent with the pyrite geochemistry and the changes of fluid conditions in the Sizhuang gold deposit, supporting the dominant controls of temperature on the entry of Co and Ni into early pyrite in the Sizhuang and other Jiaodong-type gold deposits (e.g., Xincheng, China).

The Py2a is depleted in most pyrite-compatible elements (e.g., Co, Ni, and As) and has higher Cu/Ni, As/Ni, and Au/Co ratios relative to Py1 (Figure 11), likely implying a decreasing temperature condition during early stage 2. The Py2a of the early quartz-sericite-pyrite stage (stage 2) is mainly found in white quartz veins or breccias (Figure 3c,d), which typically have high fluid/rock ratio conditions (e.g., [93,94]). The documented H–O isotope shift to meteoric waters at the time of stage 2 [55,58,65] suggests the large meteoric water ingress at Sizhuang during stage 2. Thus, the external fluid influx likely decreased the fluid temperature and concentrations of metals and metalloids, resulting in the element-poor pyrite (Py2a).

The Py2b has high but variable trace elements relative to Py2a (Figure 7). The early phase of Py2b had significantly higher Co concentrations, while the late phase of Py2b was As(-Au) rich (Figure 10), combined with the similar mineral assemblages (Figure 4) and closely spatial relationships (Figure 5f,g), meaning the transition of fluids from Co-rich to As-rich occurred during the middle of stage 2, as stated in [44]. As discussed above, the high Co concentration likely reflects higher temperatures, suggesting the influx of a new pulse of magmatic derived fluids during middle stage 2. In contrast to Co, arsenic preferentially partitions into low density and low salinity aqueous fluids [27,86,87]. Taking account of the decreasing δD_SMOW and δO_H2O over time at Sizhuang [58,65], the As enrichment in late-phase fluids during middle stage 2 was possibly promoted by the decreasing density, salinity, and temperature of ore-forming fluids in response to the input of diluted meteoric water. Similar processes were also documented in [26,27].

Notably, the Py2b displays oscillatory zoning of As, Co, Ni, and Au (Figure 10) and cataclastic textures (Figure 5f–h,j). Oscillatory zoning of pyrite can be attributed to multiple hydrothermal fluid inputs or variable growth rates driven by periodic changes of fluid conditions (e.g., pressure, temperature, and pH [25,36,42,43,47]). A previous study proposed the formation of pyrite and other sulfide minerals of the Sizhuang deposit from a single hydrothermal fluid by H₂S continuous separation into vapor phase through fluid immiscibility [57]. But pyrite precipitated from a single fluid rarely exhibits a correlation among major and trace elements. Iron of Py2b is weakly associated with As (r = 0.455) and Au (r = 0.530; Table S2), and there are many mineralized veins and breccia occurrences in stage 2 (Figure 3). This excludes the possibility of a single hydrothermal event and alternatively suggests an open hydrothermal system. The sharp textural features formed during Py2b growth (e.g., irregular bands and overgrowth on cataclastic texture; Figure 5) and corresponding compositional transition (Figure 10) are consistent with an open fluid environment with large fluid fluxes or multiple fluid charging events. Thus, the oscillatory zoning at Sizhuang likely reflects episodic changes in the composition (As, Co, Ni, and Au) of pyrite-precipitating fluid. However, the homogeneous distribution of the S isotope in individual pyrite crystals at Sizhuang [57] requires a similar source for multiple ingresses of fluids. However, the changes or fluctuations in fluid compositions can explain only a subset of the variations observed in pyrite geochemistry. The variation of fluid pressures (about 0.5–3.5 kbar) estimated by fluid inclusions from auriferous quartz veins suggests the pressure fluctuations during mineralization [58,65]. Thus, the complex pyrite textures with respect to micro-structural deformation and subsequent pyrite overgrowth and other mineral filling or healing (Figure 5e–g; [57]) additionally reflect effects on pyrite growth by fluid pressure changes, as suggested in [28,95,96].

The Py2c, as the late phase of Py2, is marked by high As-Au concentrations and other trace elements (e.g., Ni, Ag, Sb, and Te). Pyrite in-situ S isotope data show the pyrite formed in stage 2 and As-rich pyrite have similar isotopic compositions (8.1–9.6‰, mean 8.8‰, and 7.5–9.8‰, mean 9.0‰, respectively; [57]), suggesting a similar source for fluids precipitating Py2b and Py2c. There are two possible sources of As-rich fluids during late stage 2: (1) mobilization of As from early-formed pyrite via dissolution-reprecipitation processes, and (2) progressive inputs of external waters, similar to that proposed for Py2b. The remobilization and redistribution of metals (e.g., Au, As, Pb) has been recognized in many orogenic gold systems as the result of progressive ore-forming evolution or reworking processes [37,38,41,97,98]. However, given the local sharp boundaries between Py2b and Py2c (Figure 5g), we suggest that the Py2c geochemistry was dominantly controlled by hydrothermal fluids rather than metal redistribution. This is consistent with the progressive compositional changes (e.g., progressively elevated Ni, As, Ag, Au, and Te) from Py2a to Py2c during stage 2. Such processes were likely linked to the continuous interaction between fluids and country rocks as the characteristics of the protolith had locally or totally disappeared due to quartz-sericite-pyrite alteration of stage 2 (Figure 3c,e–h).

Py3 exhibits fine-grained anhedral crystals, reflecting rapid precipitation from solutions. The occurrence of abundant syn-stage siderite suggests a carbon- and Fe-rich fluid. The mineral replacement of pyrrhotite by pyrite (Py3) during early stage 3 generated the dissolution and colloform textures (Figure 6d), which could release metals into fluids for late sulfide growth [99]. But the high Cu/Ni, As/Ni, and Au/Co ratios of Py3 and the formation of marcasite required a fluid with low temperature (likely <240 °C; [92]) or low metal budgets. As discussed previously, an input of meteoric water after an intermittent episode provides a reasonable mechanism for temperature decreasing and/or dilution of ore fluids.

5.3. Metallogenic Implications

Gold deposition at Sizhuang occurred in stages 1–3, with stage 2 (quartz-sericite-Py2) being the primary gold-introducing episode. Gold mineralization of stage 2 is marked by the formation of Py2b and Py2c both of which are enriched in As (Figure 7 and Figure 8). In addition, the As-rich pyrite zones have relatively high gold concentrations (Figure 10) and most visible gold grains are hosted in the fractures of Py2 with no evidence of dissolution texture (Figure 5e,i; [58]), suggesting that gold precipitated directly from hydrothermal fluids. This study suggests the ore-forming fluid evolution in the Sizhuang gold deposit began with magmatic water dominant in stage 1, meteoric waters impacting early stage 2, a mixture of magmatic and external waters in middle-late stage 2, and external waters in stage 3. The fluid evolution is in essence the result of a series of pulsed deep fluids release plus progressive ingress of external fluids. We propose that a mix of deep magmatic and shallow external waters (stage 2) generated the orogenic gold mineralization at Sizhuang.

In addition to the composition of fluids (e.g., total metal concentrations) as a prerequisite for mineralization, effective traps and precipitation mechanisms are important factors for controlling economic mineralization [100,101]. We noticed that abundant mineral inclusions, especially of chalcopyrite, are observed in the As-rich and Co-poor bands of Py2 (Figure 10). Chalcopyrite inclusions in pyrite could be attributed to the voids in pyrite crystal structure (e.g., [27,29,34]) or a rapid deposition and crystallization of Cu followed by progressive clusters (e.g., [48]). The first mechanism restricts the size and amounts of inclusions and is thus less possible for the Sizhuang pyrite. Previous studies have demonstrated that there are contemporaneous primary H₂O-CO₂ and aqueous inclusions in auriferous quartz-sulfide veins with variable densities, implying the occurrence of phase separation during mineralization [58,65]. Phase separation with simultaneous decompression and fast volatile exsolution from fluids can lead to the destabilization of complexes and subsequent arsenic and metal (e.g., Au, Cu) separation from fluids (e.g., [25,31,86,102,103]). Due to a lower sensitivity of Co complexes to pressure changes [48], phase separation typically results in the pyrite characteristic of relatively high concentrations of As, Cu, and Au and low Co concentrations [48]. Moreover, the fluid phase separation accompanying vapor escaping would partly decrease the temperature of fluid system, which is also favorable in metal deposition [25,31,48]. Furthermore, the quiet period (aseismic) after fluid pressure releasing during phase separation is conducive to metal self-organization and mineral crystal growth (e.g., [96]). Collectively, it is highly likely that rapid phase separation accompanying pressure releases occurred during Py2b crystallization that formed As-Cu-rich and Co-poor bands. Such a scenario involving rapid pressure drops and subsequent growth is perhaps applied for the free electrum growth in fractures (e.g., [18]).

The alteration-mineralization zone of the Sizhuang deposit as well as most Jiaodong-type orogenic deposits, especially veinlet-disseminated types, was developed as a vein-breccia system in regional detachment fault zones [5,22,23,53,56]. The strong brecciation in the ductile-brittle system of the Jiaojia fault at Sizhuang (Figure 3c,e) would have been a highly efficient mechanism for rapid pressure release (phase separation), resulting in the observed characteristic of pyrite, such as zonation and overgrowth on cataclastic texture and Cu-enrichment by abundant chalcopyrite inclusions in Au- and As-rich and Co-poor bands. Moreover, the dilation portions of fault zones at Sizhuang were also interpreted as preferential locations for fluid flowing and pooling during the dextral and normal movement of the Jiaojia fault [55]. Such a coupled control on fluid pooling and metal deposition from seismic movement along fault zones is consistent with the fault-valve regime of many orogenic gold systems (e.g., [39,41,104,105,106]).

6. Conclusions

(1) Three types of hydrothermal pyrite are recognized at the Sizhuang gold deposit: (I) stage 1 disseminated pyrite (Py1) hosted in K-feldspar-albite-rutile and quartz-sericite altered rocks, (II) stage 2 pyrite in quartz-pyrite veins or quartz-sericite-pyrite altered rocks or breccias (Py2) occurring as irregular cores (Py2a), middle- to coarse-grained and zoned and cataclastic crystals (Py2b) and overgrowth Py2c, and finally, (III) stage 3 anhedral fine-grained pyrite (Py3) in siderite-polymetallic sulfide veins.

(2) Gold occurs as the solid solution (Au⁺) in Py1, Py2b, and Py2c associated with As incorporation, or electrum inclusions in Py2b, Py2c, and Py3 that are formed together with galena.

(3) Py1 with high Co and Ni contents was formed due to the interaction between magmatic waters and wall rocks. Py2a with very low trace element contents (e.g., Co, Ni, As, and Te) was associated with decreased temperature or fluid dilution related to the influx of meteoric waters during early stage 2, whereas Py2b and Py2c were precipitated in association with the mixture of new pulse of magmatic waters and external waters in middle-late stage 2. The composition of pyrite from stage 3 is affected by the input of meteoric waters during the waning stages of the hydrothermal system.

(4) Gold mineralization in the Sizhuang deposit was controlled both by fluid compositions, physico-chemical properties, and shallow-crustal structural deformation. Gold deposition in the Sizhuang and other Jiaodong-type orogenic gold systems was closely associated with the pressure rapid decreases or fluctuations in a fault-valve regime.