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Article

Composition and Geochemical Characteristics of Pyrite and Quartz: Constraints on the Origin of the Xinjiazui Gold Deposit, Northwestern Margin of the Yangtze Block, China

1
Xi’an Center of Mineral Resources Survey, Xi’an 710100, China
2
School of Earth Sciences and Resources, Chang’an University, Xi’an 710054, China
*
Author to whom correspondence should be addressed.
Minerals 2022, 12(6), 688; https://doi.org/10.3390/min12060688
Submission received: 13 April 2022 / Revised: 20 May 2022 / Accepted: 26 May 2022 / Published: 29 May 2022

Abstract

:
The Xinjiazui gold deposit, a newly discovered deposit, is situated in the northwestern margin of the Yangtze Block, China. The source and genesis of gold mineralization are poorly understood. It is urgent to use the H–O isotopic composition of quartz and geochemistry of pyrite to evaluate the origins of the Au and ore-forming fluids of this deposit. Three types of pyrite were identified, including synsedimentary framboidal pyrites (Py0), the directional arrangement of pyrites in pre-mineralization stage (Py1), and euhedral coarse-grain pyrites in the quartz–sulfide veins of the mineralization stage (Py2). The As content in Py2 is relatively higher than Py0 and Py1, indicating that the ore-forming fluids are strongly enriched in As. The δ34S values of Py2 (+5.50–+13.34‰) overlap with the S1–2M phyllite (+7.25‰–+8.70‰). This result is consistent with the Pb isotopic composition of Py2, showing that the source of ore-forming materials was derived from the S1–2M phyllite. Meanwhile, the variations in quartz’s H and O isotopic composition suggest that the ore-forming fluids were derived originally from metamorphic fluid. Additionally, the Au mineralization is strictly controlled by the shear zone. Above all, we would like to classify the Xinjiazui deposit as an orogenic gold deposit.

1. Introduction

Orogenic gold deposits are an essential type of global gold deposit, with wide formation ages, wide occurrence depth, high grade, and large scale, accounting for more than 30% of global gold resources [1]. They have also become the research focus of mineral deposit and geotectonic studies [2,3,4]. In China, most orogenic gold deposits are structurally hosted in metamorphic rocks, with lodes existing as quartz–carbonate veins or veinlets, and gold commonly occurs as disseminations in hydrothermal sulfides in the surrounding area’s rocks [5]. However, some significant issues, including the sources of ore-forming components (e.g., fluids, metals, sulfur) of these gold deposits, have been debated for decades [6,7,8,9,10]. Pyrites (FeS2) are typical major mineral phases and are known for their close association with Au in gold deposits [11,12,13,14,15]. Their texture, chemical, and isotopic variations make them ideal ore-forming condition indicators [11,16,17,18] and the genesis for various metallic deposits [19,20,21,22,23,24,25,26,27,28]. Moreover, the isotopic systems (such as H–O) with specific differences in different geological reservoirs are an important means to define the source of ore-forming materials and ore-forming fluids, and play a vital role in indicating and discriminating the genetic types of ore deposits [6,29,30,31,32,33,34,35,36]. Therefore, the composition of pyrite and the H–O isotopic composition of quartz veins may provide new information for the debate regarding of the origins of Au and ore-forming fluids of the gold deposits.
The Longmenshan orogenic belt, located in the northwestern margin of the Yangtze block, is surrounded by Bikou terrane, South Qinling orogenic belt, Songpan–Garze block, and Hannan–Micangshan tectonic belt [37] (Figure 1b). This orogenic belt consists of several gold deposits, such as the Dingjialin, Taiyangping, Dongjiayuan [38,39,40], and Xinjiazui gold deposits. The ore-hosting strata of the Dingjialin, Taiyangping, and Dongjiayuan gold deposits are Silurian Maoxian group (S1–2M) sericite phyllite [38,39,40] (Figure 2a). However, the ore body of the Xinjiazui gold deposit is mainly distributed in Cambrian Niutitang formation (Є1n) carbon–silicon–slate, and a small amount in S1–2M phyllite [41] (Figure 3a–c). Although the Xinjiazui gold deposit exhibits a close spatial association with the Dingjialin–Taiyangping metallogenic belt, it is unclear whether they were formed in the same metallogenesis. Therefore, it is critical to compare the source of ore-forming metals and fluids between the Xinjiazui gold deposit and Dingjialin–Taiyangping metallogenic belt.
The mineralization of primary ores in the Xinjiazui gold deposit is dominated by quartz-vein-type gold ores, and the gold is mainly hosted by pyrite [41]. Therefore, we chose the quartz-vein-type gold ores as the focus of our study and carried out detailed petrographic observation. Meanwhile, new data from this study include the electron probe component analysis, S and Pb isotopes of the primary gold-bearing pyrites, and the H and O isotopic composition of quartz. The results and new findings are reported in the paper.

2. Regional Geology

The NE-trending Longmenshan thrust–nappe belt formed during the Indosinian collision of the Songpan–Garze block and Yangtze block. Before the Late Triassic, the western margin of the Yangtze block was generally a stable continental marginal development stage, while the Songpan–Garze area was a residual ocean [43]. In the early Indosinian period, the Yangtze Block, Longmen Mountain, and the Songpan–Garze area experienced a tectonic reversal, which caused the tectonic environment to change from early extension to compression. At this time, the Qingchuan–Yangpingguan fault on the western margin of the Yangtze Block also reversed from an early tensile normal fault to a ductile left-lateral strike-slip. These structural inversions resulted in the westward intracontinental subduction of the Yangtze Block along the Beichuan–Yingxiu fault. Meanwhile, the Qingchuan–Yangpingguan fault, Beichuan–Yingxiu fault, and Anxian–Dujiangyan fault divide the northern segment of Longmenshan orogen and its immediate area into four parts, namely the back Longmenshan orogenic belt (I), front Longmenshan fold and thrust belt (II), Anxian–Dujiangyan fault zone (III), and foreland fold belt (IV) [37] (Figure 1b).
The Qingchuan–Yangpingguan fault is a boundary fault that separates the Back–Longmenshan Orogenic belt and Bikou terrane (Figure 1b), which contains multiple secondary faults, such as the Dingjialin–Taiyangping brittle-ductile shear zone (Figure 2). The Dingjialin to Taiyangping brittle-ductile shear fault, trending 40°–50° and dipping 50°–60°, controls the distribution of several gold deposits, such as the Dingjialin, Taiyangping, and Dongjiayuan deposits (Figure 2).

3. Deposit Geology and Mineralization

3.1. Geology of the Xinjiazui Gold Deposit

The Xinjiazui gold deposit is located 8 km northeast of the Dingjialin–Taiyangping gold belt (Figure 2b). The strata in the area mainly consist of Upper Sinian Dengying formation (Z2dn), Lower Cambrian Niutitang formation (Є1n), and Silurian Maoxian group (S1–2M). The ore body of the Xinjiazui gold deposit is vein-like, occurs in Yanzibian–Huashigou brittle-ductile shear fault (FD4), and was distributed in Silurian Maoxian group (S1–2M) and Camcambrian Niutetang formation (Є1n) strata (Figure 3a–c), with a trend of 315°–320° and dip angle of 35°–65°. The ore body has a clear boundary with the surrounding rocks, with an average thickness of 3.53 m and an average grade of 2.90 × 10–6. Notably, the Yanzibian–Huashigou shear fault strictly controls the ore mineralization, and high-grade gold ores are developed only in the position with ductile solid deformation and brittle fracture (Figure 3d).
The mineralization of primary ores in the Xinjiazui gold deposit is dominated by quartz-vein-type gold ores. Gold is mainly hosted by pyrite, quartz, and a small amount of polymetallic sulfides, and it occurs as fracture gold or gold inclusions. The fracture gold (2–10 μm) is mainly distributed in the fractures of pyrite in the form of branching, wheat grain, and veined gold. The gold inclusions (2–8 μm) are embedded in pyrite or hematite in irregular granular and rounded form. Combining the crosscutting relationship of veins, the characteristics of mineral paragenetic association, and typical ore fabric, the mineralization of the Xinjiazui gold deposit can be divided into hydrothermal mineralization and supergene oxidation epoch. The hydrothermal metallogenic epoch is the primary ore event. It can be subdivided into three stages: (I) quartz–pyrite stage, (II) quartz–calcite–natural gold polymetallic sulfide stage, and (III) quartz–carbonate stage [41].

3.2. Texture of Pyrite and Mineral Paragenetic Sequence

Based on petrographic observations and backscattered electron (BSE) analysis, pyrites in the Xinjiazui deposit are classified into three generations (Figure 4), including the framboidal pyrites in the synsedimentary epoch (Py0), the directional arrangement of hydrothermal pyrite in pre–mineralization stage (Py1), and coarse grains disseminated in the quartz–sulfide veins of mineralization stage (Py2).
The framboidal pyrite (Py0), mainly distributed in a relatively fresh Cambrian carbonaceous slate (Figure 4a–c), is comprised of framboidal grained (about 5 μm) sub-euhedral to anhedral pyrite grains. Locally, the size of framboidal aggregate can be 50 μm. The Py0 corresponds to the pre-enrichment stage of syngenetic hydrothermal sedimentation.
The Py1 is mainly produced as quartz–pyrite veins along the phyllite or plate and is formed in the early stage of mineralization, with a particle size of 10–30 μm. The quartz–pyrite veins underwent ductile shear deformation along with the surrounding rock, resulting in the pyrite being elongated and drawn wire-oriented (Figure 4d–f). This stage suffered from regional brittle-ductile shear deformation, conducive to the migration and accumulation of ore-bearing fluids.
The Py2 is developed in the relatively wide quartz vein or the contact area between the vein and surrounding rocks. These euhedral pyrite grains (Py2) have a size of 50 μm–2 mm without zoned texture and contain abundant fracture gold or gold inclusions (Figure 4g,h). Notably, some pyrites have a fragmented structure (Figure 4h,i), indicating that this stage has suffered from brittle fracture. Near-surface pyrites were oxidized to hematites, which retained the pentagonal dodecahedron of pyrite, forming a skeleton texture (Figure 4i).

4. Analytical Methods

Electron microprobe (EPMA) analysis: In this paper, EPMA was used to analyze the major elements of 98 spots in pyrite samples from the Xinjiazui gold deposit, including 11 spots on Py0, 35 spots on Py1, and 52 spots on Py2. The pyrite’s major and minor element compositions were determined by JXA–8230 electron probe with a WDS detector at Xi’an Geological Survey Center, China Geological Survey, under 20 kV and 10 nA, with a beam size of 1 μm in diameter. The ZAF correction method was used to correct the atomic number (Z), absorption (A), and fluorescence (F) effects for all analyzed minerals.
S Isotope Analyses of Pyrites in the quartz–sulfide veins of the mineralization stage (Py2) were carried out at Xi’an Ruishi Geological Technology Co., Ltd., Xi’an, China, using the DZ/T0184–1997 method. The instrument includes 253plus, Flash EA elemental analyzer, and Conflo IV multi-purpose interface (American Thermoelectric Company). The Iaea–s3, GBW04414, and GBW04415 were chosen as reference materials, and the analytical accuracy of the standard sample was found to be better than 0.2‰. The δ34S analysis was normalized to the Canyon Diablo troilite VCDT value.
Pb Isotope Analyses of Pyrites in the quartz–sulfide veins of mineralization stage (Py2) were completed in the State Key Laboratory of Continental Dynamics, Northwestern University. Pb isotope composition test, consisting of separation and testing, was carried out on the Neptune Plus MC–ICP–MS (ThermoFisher). Firstly, the sample was added to the digestion tank and dissolved with HF and HNO3. The dissolved sample was separated with Sr-specific resin (produced by Triskem, Bruz, France). After pre-cleaning and leaching, based on the Pb concentration of the solution, a standard solution of Tl was added so that the ratio of Pb and Tl was 1:1 [44]. All tests were carried out in static mode, 202Hg+ was used to monitor the interference of 204Hg+ to 204Pb+, 203Tl/205Tl was used as an external standard [45], and the effect of mass fractionation was corrected by 203Tl/205Tl = 2.3889. The Pb isotope ratio was normalized by 203Tl/205Tl = 0.418922.
Hydrogen–Oxygen Isotopes of Quartz in the quartz–sulfide veins of the mineralization stage were analyzed on a 253plus mass spectrometer in the Xi’an Ruishi Geological Technology Co., LTD, China, following the technique described by Gong et al. (2007) [46] and Ding et al. (1994) [47]. The δ18O values of ore-forming fluids (δ18OW) were calculated from the δ18O values of quartz (δ18OQ). δD values were obtained by measuring fluid inclusions in quartz. Analysis procedures followed those described by Ding et al. (1994) [47]. Isotopic ratios for oxygen and hydrogen are presented in standard δ notation (‰) relative to the Standard Mean Ocean Water (SMOW). Analytical precision was ± 0.2‰ for δ18O and ± 1‰ for δD, respectively.

5. Results

5.1. Pyrite Chemical Composition

The EPMA results (Table S1) show that the Fe and S contents in Py0 are slightly lower than those in Py1 and Py2 (Table 1). Compared with the theoretical chemical composition of pyrite (Fe and S are 46.55% and 53.45%, respectively), the three types of pyrites (Py0, Py1, and Py2) have the characteristics of lower Fe (averaging 45.42%, 45.79%, 45.92%, respectively) and higher S (averaging 53.69%, 54.14%, 54.08%, respectively). This chemistry indicates that pyrites in the Xinjiazui gold deposit are generally enriched in S (Figure 5a).
Py0 (0.013% to 0.055%) and Py1 (0.007% to 0.181%) contain similar As content, and both are lower than that of Py2 (0.003% to 2.228%, Figure 5b). The variation of As contents reflect the evolution of the ore-forming fluid composition, so the subtle differences in As content of Py0, Py1, and Py2 indicate that the ore-forming fluids are more enriched in As. Moreover, there is an obvious negative correlation between the As and S in pyrite (Figure 5b), suggesting that As replaces S in pyrite [48].

5.2. S and Pb Isotopes

Sulfur isotope compositions of pyrite in Xinjiazui, Dingjialin, and Taiyangping gold deposits are listed in Table 2. As shown, the Py2 exhibits positive δ34S values ranging from 5.50‰ to 13.34‰, with an average of 10.58‰ and a median of 12.89‰. Moreover, the δ34S ratios of Py2 have relatively narrow ranges, indicating that the sulfur isotope ratio of pyrite has reached equilibrium. These δ34S ratios are also similar to the δ34S ratios of pyrite from the Taiyangping (8.50‰–9.90‰; [38,39]) and Dingjialin gold deposits (6.60‰–10.20‰; [38]).
Lead isotope data (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204P ratios) of Py2 in the Xinjiazui gold deposit are mainly concentrated in the ranges 18.177–18.204, 15.688–15.696, and 38.595–38.652, respectively (Table 3). On 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–206Pb/204Pb plots, most data are plotted in a field representing the upper crust (Figure 6).

5.3. Hydrogen and Oxygen Isotopes

Hydrogen and oxygen isotope data are important monitors for the source and evolution of fluids. The H and O isotopic compositions of quartz from the Xinjiazui, Dingjialin, and Taiyangping gold deposits are listed in Table 4. The Xinjiazui gold deposit lacks magmatic activities, which excludes a magmatic source of the ore-forming fluids. The δ18O and δD values of ore-forming fluids vary, respectively, from +19.5‰ to +22.1‰ and from −79‰ to −69‰ for the main stage of mineralization in the Xinjiazui gold deposit. These ranges are consistent with the Dingjialin (δ18O = +15.5‰ to +17.1, δD = −67‰ to −66‰, [38]) and Taiyangping (δ18O = +18.9‰, δD = −67‰, [38]) gold deposits in this region. The reliable δD and δ18O ratios of ore-fluids mostly fall close to the lower side of the metamorphic field (Figure 7), possibly with a small amount of meteoric water. The results of these H–O isotope analyses are consistent with the composition range of orogenic gold deposits (H–O isotopes of water-bearing minerals mainly range from –80 ‰ to –20 ‰, and +6‰ to +13‰ [3,6,50]), reflecting that the ore-forming fluid was derived from metamorphic fluid in this region. As for the temperature and salinity of the ore-forming fluids, measuring the temperature of the fluid inclusions in the future will help solve this problem.

6. Discussion

6.1. Source of Sulfur and Metal

These δ34S ratios of gold-related pyrites in the Xinjiazui gold deposit (+5.50‰–+13.34‰) are similar to the δ34S ratios of pyrite from the Taiyangping (8.50‰–9.90‰ [38,39]) and Dingjialin gold deposits (6.60‰–10.20‰ [38]) (Figure 8), indicating that the same dominant sulfur source for Xinjiazui, Dingjialin, and Taiyangping gold deposits. Positive δ34S values reflect the characteristics of formation sulfur. The ore body of the Xinjiazui gold deposit occurs in S1–2M phyllite and Є1n carbon–silicon–slate, while the ore bodies of Dingjialin and Yangyangping gold deposits only occur in S1–2M phyllite (Figure 3). The protolith of S1–2M phyllite has the character of turbidite. The turbidite is rich in iron, sulfur, and other gold-loving elements, which is conducive to the pre-enrichment of gold. Importantly, the Maoxian group contains nearby 10.0 ppb Au [51], twice the Clark value (4 ppb [52]), indicating that the S1–2M phyllite has the potential to provide gold for the gold mineralization in the Xinjiazui deposit. However, the S1–2M phyllite shows slightly lighter δ34S ratios (+7.25‰–+8.70‰ [38,39]), while the Є1n carbon–silicon–slate exhibit heavier δ34S ratios (+25‰–+30.5‰ [53]) than that of pyrite in the Xinjiazui deposit. This chemistry indicates that the sulfur source of pyrite in the ore-forming stage of Xinjiazui gold deposit is mainly from S1–2M phyllite. The wide δ34S of the Xinjiazui gold deposit is probably related to minor sulfur additions from the Є1n carbon–silicon–slate.
In the main stage of the Xinjazui gold deposit, pyrite is closely related to gold and is the main gold-bearing mineral [41], indicating that pyrite and gold were formed in the same fluid system, and the lead isotope of pyrite (Py2) can reflect the source region of gold [54]. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of Py2 changed little, and were relatively stable in the ore-forming stage, indicating that the source of ore-forming materials in the area was consistent. Pyrite (Py2) lead isotopes are all located in the upper crust (Figure 6a) and concentrated along and above the evolution line of the orogenic belt (Figure 6b), showing the characteristics of orogenic lead isotopes.

6.2. Deposit Type

The Xinjiazui gold deposit is located in the junction area of the Yangtze Block, Bikou terrane, South Qinling orogenic belt, and Songpan–Garze block. The NE-trending Qingchuan–Yangpingguan ductile-brittle shear fault strictly controls Au mineralization distribution, shape, and occurrence. The gold orebodies mainly occur in the Yanzibian–Huashigou fault, a secondary fault of the Qingchuan–Yangpingguan ductile-brittle shear fault (Figure 2, Figure 3). Most of the orebodies are NE-trending “entering”-type auriferous quartz complex vein, and high-grade gold ores are developed only in the position with strong ductile deformation and brittle fracture (Figure 3d). Thus, the Xinjiazui deposit is controlled by ductile-brittle shear fault and brittle fracture [41].
Above all, the Xinjiazui gold deposit is considered to be an orogenic gold deposit, the characteristics of which are similar to those of typical examples [3,5,6,55,56,57]:
(1) The deposit occupies a spatial and temporal position consistent with the Longmenshan Orogen, a collisional orogenic belt;
(2) The wall rocks were deformed and metamorphosed to phyllite or slate;
(3) Mineralization is not stratigraphically selective. Ductile shear zone and brittle fracture structurally controlled the Au mineralization;
(4) Pyrite is the dominant sulfide mineral in the ores, and gold is mainly hosted in pyrite as fracture gold or gold inclusions;
(5) The ores exhibit a simple element assemblage of Au(–Ag), and gold occurs in the form of native gold;
(6) Sulfur and gold were derived from the shallow metamorphic and strongly deformed sedimentary rock series (S1–2M);
(7) The ore-forming fluids had a metamorphic source.

6.3. Mechanism of Mineralization

Based on the analyses presented in this paper, together with previous studies [38], a genetic model has been constructed for the Xinjiazui deposit (Figure 9). Metallogenic materials of the Xinjiazui gold deposit mainly come from the surrounding rock (Figure 6, Figure 8), and gold pre-enrichment occurs during the deposition and diagenesis of the original surrounding rock (Figure 9a). After the Qingchuan–Yangpingguan fault entered the Silurian Maoxian formation, a series of brittle and ductile shear faults were produced, such as the Yanzibian–huashigou ductile-brittle shear fault. Deep metamorphic fluids move upward along the ductile–brittle shear fault and extract metallogenic material from the surrounding rocks (S1–2M phyllite and Є1n carbon–silicon–slate) through water–rock exchange. Furthermore, they then lead to the element Au being remobilized, forming Au-bearing tectonic-metamorphic hydrothermal fluids (Figure 9b). Moreover, the superposition of brittle fractures in the later stage leads to the further enrichment of gold and other metal-forming materials, and high-grade gold ore is finally formed at the superposition of strong ductile deformation and brittle fractures (Figure 9c). It can be seen that natural gold (electrum) occurs in pyrite in the form of fracture gold or gold inclusions (Figure 4h).
Figure 8. Box-plots of δ34S values of ore-forming pyrites from the Xinjiazui, Taiyangping, and Dingjialin gold deposits. Base map modified after Zheng et al. (2000) [58]; data sources of pyrite δ34S values for Carbonaceous siliceous slate are from Zuo (2020) [53]; those for Taiyangping deposit and phyllite are both from Wei (2008) and Zhong (2012) [38,39]; those for Dingjialin deposit are from Wei (2008) [38].
Figure 8. Box-plots of δ34S values of ore-forming pyrites from the Xinjiazui, Taiyangping, and Dingjialin gold deposits. Base map modified after Zheng et al. (2000) [58]; data sources of pyrite δ34S values for Carbonaceous siliceous slate are from Zuo (2020) [53]; those for Taiyangping deposit and phyllite are both from Wei (2008) and Zhong (2012) [38,39]; those for Dingjialin deposit are from Wei (2008) [38].
Minerals 12 00688 g008
Figure 9. Superimposition model for ore mineralization in the brittle-ductile shear zone in the Xinjiazui gold deposit. (a) gold pre-enrichment occurs during the deposition and di-agenesis of the original surrounding rock; (b) deep metamorphic fluids move upward along the ductile–brittle shear fault and lead to the element Au being remobilized, forming Au-bearing tectonic-metamorphic hydrothermal fluids; (c): the superposition of brittle fractures in the later stage leads to the further enrichment of gold and other metal-forming materials, and high-grade gold ore is finally formed at the superposition of strong ductile deformation and brittle fractures.
Figure 9. Superimposition model for ore mineralization in the brittle-ductile shear zone in the Xinjiazui gold deposit. (a) gold pre-enrichment occurs during the deposition and di-agenesis of the original surrounding rock; (b) deep metamorphic fluids move upward along the ductile–brittle shear fault and lead to the element Au being remobilized, forming Au-bearing tectonic-metamorphic hydrothermal fluids; (c): the superposition of brittle fractures in the later stage leads to the further enrichment of gold and other metal-forming materials, and high-grade gold ore is finally formed at the superposition of strong ductile deformation and brittle fractures.
Minerals 12 00688 g009

7. Conclusions

The Xinjiazui gold deposit is located in the northeastern section of back Longmenshan orogenic belt. The Yanzibian–Huashigou shear zone strictly controls the ore mineralization, and high-grade gold ore is developed only in the position with strong ductile deformation and brittle fracture, implying structural control of gold mineralization. Moreover, the host rock of the Xinjiazui gold deposit is composed of S1–2M phyllite and Є1n carbon–silicon–slate, indicating that the gold mineralization is not selective to the stratum. The mineralization of primary ores in the Xinjiazui gold deposit is dominated by quartz-vein-type gold ores, and gold is mainly hosted by pyrite, occurring as fracture gold or gold inclusions. In addition, the H–O isotopes of quartz suggest that the ore-forming fluids were derived originally from metamorphic fluid. The δ34S value and lead isotopes of pyrite show that the ore-forming elements were derived mainly from low-grade metamorphic sedimentary rock. Above all, the Xinjiazui gold deposit is considered an orogenic gold deposit.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min12060688/s1, Table S1: Electron probe analyses of different types of pyrites in the Xinjiazui gold deposit.

Author Contributions

J.L., X.B., Y.G. and K.Y. conceived and designed the ideas; J.L., H.C., S.K. and W.Y. performed the experiments; J.L., X.B., S.K., Z.W., J.H. and Y.Z. analyzed the data; J.L. prepared the original draft; J.L. and X.B. reviewed and edited the draft. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Project of Xi’an Mineral Resources Survey Center, China Geological Survey (ZD20220313), the China Postdoctoral Science Foundation (300204000298), and National Gold Resources Potential Evaluation of China (ZD20220318).

Data Availability Statement

Not applicable.

Acknowledgments

We would like to thank Xiaoxing Zhang, Yuan Tian, and Wendi Guo for their help during the field work, as well as Nan Li and Liqiang Yang for their assistance with final editing of the paper. We are grateful for constructive comments by the anonymous reviewers.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Simplified geotectonic map of the Longmenshan orogenic belt [37,42,43]. (a): regional geological map of the Longmenshan tectonic belt and its adjacent areas; (b): the schematic tectonic map of China and the location of the Longmenshan Orogen. Notes: I. Back Longmenshan orogenic belt; II. front Longmenshan fold and thrust belt; III. Anxian–Dujiangyan fault zone; IV. foreland fold belt. Abbreviations: CCO: Center China Orogen; NCB: North China Block; SCB: South China Block; SCS, South China Sea; TLF: Tancheng–Lujiang Fault; ADF: Anxian–Dujiangyan fault; BYF: Beichuan–Yingxiu fault; QYF: Qingchuan–Yangpingguan fault.
Figure 1. Simplified geotectonic map of the Longmenshan orogenic belt [37,42,43]. (a): regional geological map of the Longmenshan tectonic belt and its adjacent areas; (b): the schematic tectonic map of China and the location of the Longmenshan Orogen. Notes: I. Back Longmenshan orogenic belt; II. front Longmenshan fold and thrust belt; III. Anxian–Dujiangyan fault zone; IV. foreland fold belt. Abbreviations: CCO: Center China Orogen; NCB: North China Block; SCB: South China Block; SCS, South China Sea; TLF: Tancheng–Lujiang Fault; ADF: Anxian–Dujiangyan fault; BYF: Beichuan–Yingxiu fault; QYF: Qingchuan–Yangpingguan fault.
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Figure 2. Regional geological map (a) and gold anomalies from stream sediment geochemistry (b) of the Xinjiazui–Dingjialin–Dongjiayuan gold metallogenic belt (modified from 1: 200,000 Bikou geological map; Fu et al., 2014 [40]).
Figure 2. Regional geological map (a) and gold anomalies from stream sediment geochemistry (b) of the Xinjiazui–Dingjialin–Dongjiayuan gold metallogenic belt (modified from 1: 200,000 Bikou geological map; Fu et al., 2014 [40]).
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Figure 3. Deposit geology and soil geochemistry (a), exploration line section (b,c), and cross–section of the exploratory trench TC09 (d) in the Xinjiazui gold deposit (Liu et al., 2022 [41]).
Figure 3. Deposit geology and soil geochemistry (a), exploration line section (b,c), and cross–section of the exploratory trench TC09 (d) in the Xinjiazui gold deposit (Liu et al., 2022 [41]).
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Figure 4. Photographs of three generations of pyrite from the Xinjazui gold deposit. (a) Quartz veins wrapped surrounding rock breccia in the ore-forming stage; (b) framboidal pyrite (Py0) in breccia of surrounding rock (reflected light); (c) framboidal pyrite (reflected light); (d) the pyrite quartz veins developed along the phyllite in the early stage of mineralization; (e) the pyrite (Py1) has elongated and drawn wire deformation and oriented arrangement (back scatter); (f) ductile shear deformation occurs in pyrite (Py1) (reflected light); (g) oxidized gold ores; (h) natural gold is embedded in the pyrite (Py2) fissures (reflected light); (i) pyrite is oxidized to hematite, which retains the pentagonal dodecahedron crystal form and forms skeleton crystal structure (reflected light).
Figure 4. Photographs of three generations of pyrite from the Xinjazui gold deposit. (a) Quartz veins wrapped surrounding rock breccia in the ore-forming stage; (b) framboidal pyrite (Py0) in breccia of surrounding rock (reflected light); (c) framboidal pyrite (reflected light); (d) the pyrite quartz veins developed along the phyllite in the early stage of mineralization; (e) the pyrite (Py1) has elongated and drawn wire deformation and oriented arrangement (back scatter); (f) ductile shear deformation occurs in pyrite (Py1) (reflected light); (g) oxidized gold ores; (h) natural gold is embedded in the pyrite (Py2) fissures (reflected light); (i) pyrite is oxidized to hematite, which retains the pentagonal dodecahedron crystal form and forms skeleton crystal structure (reflected light).
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Figure 5. The plots of main elemental contents of pyrites in the Xinjiazui gold deposit, including (a) w(S) vs. w(Fe) and (b) w(As) vs. w(S) diagrams.
Figure 5. The plots of main elemental contents of pyrites in the Xinjiazui gold deposit, including (a) w(S) vs. w(Fe) and (b) w(As) vs. w(S) diagrams.
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Figure 6. Pb isotopic composition of samples from the Xinjiazui gold deposit plotted with the evolution curve of Zartman and Doe (1981) [49], including (a) 208Pb/204Pb vs. 206Pb/204Pb and (b) 207Pb/204Pb vs. 206Pb/204Pb. LC: lower crust; M: mantle; O: orogen; UC: upper crust.
Figure 6. Pb isotopic composition of samples from the Xinjiazui gold deposit plotted with the evolution curve of Zartman and Doe (1981) [49], including (a) 208Pb/204Pb vs. 206Pb/204Pb and (b) 207Pb/204Pb vs. 206Pb/204Pb. LC: lower crust; M: mantle; O: orogen; UC: upper crust.
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Figure 7. δDH2O and δ18OH2O values of ore-forming fluids measured or calculated from hydrothermal quartz. Fields for magmatic and metamorphic waters are from Taylor (1973) [31].
Figure 7. δDH2O and δ18OH2O values of ore-forming fluids measured or calculated from hydrothermal quartz. Fields for magmatic and metamorphic waters are from Taylor (1973) [31].
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Table 1. Electron probe analyses of different types of pyrites in the Xinjiazui gold deposit.
Table 1. Electron probe analyses of different types of pyrites in the Xinjiazui gold deposit.
Mineral
Generation
Py0 (n = 11) Py1 (n = 35) Py2 (n = 52)
MAXMINAVGSTDMAXMINAVGSTDMAXMINAVGSTDD.L.
As0.055/0.0220.0190.181/0.0400.0492.228/0.2020.417421
Se0.087/0.0200.0260.089/0.0280.0280.168/0.0370.052191
Zn0.081/0.0180.0290.075/0.0160.0200.078/0.0140.021313
Cu0.048/0.0150.0190.055/0.0140.0180.065/0.0120.016299
Ni0.037/0.0070.0121.018/0.1060.1780.370/0.0540.077235
Co0.1120.0240.0690.0290.284/0.0710.0570.6000.0020.0760.093218
Fe46.1844.3345.420.5246.9044.5045.790.6346.6445.0045.920.43218
Cr0.074/0.0220.0270.043/0.0100.0113.860/0.1000.535198
S54.1653.3053.690.3054.8152.5954.140.4754.8552.7554.080.4367
Pb0.115/0.0460.0390.137/0.0340.0460.169/0.0410.048427
Te0.020/0.0040.0070.037/0.0060.0100.053/0.0080.013160
Sb0.037/0.0090.0130.071/0.0110.0160.050/0.0090.014171
Total100.2797.8899.380.69101.6797.90100.290.88101.7799.31100.500.60/
Abbreviations: AVG: average (wt%); MAX: maximum (wt%); MIN: minimum (wt%); STD: standard deviation; D.L.: detection limitation (ppm).
Table 2. Sulfur isotopic composition of pyrite from the Xinjiazui, Dingjialin, and Taiyangping gold deposits.
Table 2. Sulfur isotopic composition of pyrite from the Xinjiazui, Dingjialin, and Taiyangping gold deposits.
SampleLocationRockδ34S/‰Data Source
PD01–11Xinjiazuipyritization quartz vein12.89This study
PD01–12Xinjiazuipyritization quartz vein13.34This study
TC09–19–2Xinjiazuipyritization quartz vein5.50This study
CPD39–26Dingjialin PD39S1–2M phyllite8.70[38]
PD3–3aDingjialin PD3pyritization quartz vein6.60[38]
PD2–5Dingjialin PD2pyritization quartz vein10.20[38]
V4–1Taiyangpingpyritization quartz vein8.50[38]
V9–1Taiyangpingpyritization quartz vein9.90[38]
V9–2Taiyangpingpyritization quartz vein9.30[38]
PD700H4Taiyangpingpyritization quartz vein9.07[39]
PD792H3Taiyangpingpyritization quartz vein9.43[39]
PD878H6Taiyangpingpyritization quartz vein9.84[39]
PD947H2TaiyangpingS1–2M phyllite7.25[39]
PD947H2/2TaiyangpingS1–2M phyllite7.32[39]
Notes: δ34S values were reported relative to the Vienna Cañon Diablo troilite (VCDT). An uncertainty of ± 0.2‰ is recommended.
Table 3. Lead isotopic compositions of stage II pyrite (Py2) from the Xinjiazui gold deposit.
Table 3. Lead isotopic compositions of stage II pyrite (Py2) from the Xinjiazui gold deposit.
SampleLocationRock208Pb/204Pb207Pb/204Pb206Pb/204Pb
PD01–11Xinjiazuipyritization quartz vein38.652 15.696 18.204
PD01–12Xinjiazuipyritization quartz vein38.596 15.689 18.177
TC09–19–2Xinjiazuipyritization quartz vein38.432 15.674 18.148
Notes: Standard errors are within 0.004 for 206Pb/204Pb, within 0.003 for 207Pb/204Pb, and within 0.007 for 208Pb/204Pb.
Table 4. Oxygen and hydrogen isotopic composition of quartz from the Xinjiazui, Dingjialin, and Taiyangping gold deposits.
Table 4. Oxygen and hydrogen isotopic composition of quartz from the Xinjiazui, Dingjialin, and Taiyangping gold deposits.
SampleLaocationRockδ18OV–SMOW(‰)δDV–SMOW(‰)δ18OV–PDB(‰)Data Source
PD01–11XinjiazuiSulfide–quartz vein19.5–79–11.0This study
PD01–12XinjiazuiSulfide–quartz vein19.6–73–10.9This study
TC09–19–2XinjiazuiSulfide–quartz vein22.1–69–8.5This study
CPD39–9DingjialinQuartz-bearing microveined phyllite16.4–67–29.0[38]
CPD39–26DingjialinSulfide–quartz vein15.5–66–20.4[38]
PD3–3aDingjialinSulfide–quartz vein17.1–66–24.5[38]
V4–1TaiyangpingSulfide–quartz vein18.9–67–27.6[38]
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Liu, J.; Bao, X.; Gao, Y.; Yang, K.; Kou, S.; Yang, W.; Wang, Z.; He, J.; Zhang, Y.; Chen, H. Composition and Geochemical Characteristics of Pyrite and Quartz: Constraints on the Origin of the Xinjiazui Gold Deposit, Northwestern Margin of the Yangtze Block, China. Minerals 2022, 12, 688. https://doi.org/10.3390/min12060688

AMA Style

Liu J, Bao X, Gao Y, Yang K, Kou S, Yang W, Wang Z, He J, Zhang Y, Chen H. Composition and Geochemical Characteristics of Pyrite and Quartz: Constraints on the Origin of the Xinjiazui Gold Deposit, Northwestern Margin of the Yangtze Block, China. Minerals. 2022; 12(6):688. https://doi.org/10.3390/min12060688

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Liu, Ji, Xinshang Bao, Yongbao Gao, Ke Yang, Shaolei Kou, Wei Yang, Zhanbin Wang, Jinglong He, Yixiang Zhang, and Hao Chen. 2022. "Composition and Geochemical Characteristics of Pyrite and Quartz: Constraints on the Origin of the Xinjiazui Gold Deposit, Northwestern Margin of the Yangtze Block, China" Minerals 12, no. 6: 688. https://doi.org/10.3390/min12060688

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