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Article

The Contribution of Carbonaceous Material to Gold Mineralization in the Huangjindong Deposit, Central Jiangnan Orogen, China

by
Yueqiang Zhou
1,
Zhilin Wen
1,*,
Yongjun Liu
1,
Jun Wu
1,
Baoliang Huang
1,
Hengcheng He
1,
Yuxiang Luo
1,
Peng Fan
1,
Xiang Wang
1,
Xiaojun Liu
1,
Teng Deng
2,3,
Ming Zhong
1,
Shengwei Zhang
1 and
Mei Xiao
1
1
Hunan Institute of Geological Disaster Investigation and Monitoring, Changsha 410014, China
2
School of Earth Sciences, East China University of Technology, Nanchang 330013, China
3
State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(10), 1042; https://doi.org/10.3390/min14101042
Submission received: 3 September 2024 / Revised: 9 October 2024 / Accepted: 9 October 2024 / Published: 17 October 2024
(This article belongs to the Special Issue Microanalysis Applied to Mineral Deposits)
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Versions Notes

Abstract

:
The Huangjindong gold deposit in northeastern Hunan is one of the most representative gold deposits in the Jiangnan Orogenic Belt. The orebodies are mainly hosted in the Neoproterozoic Lengjiaxi Group, which comprises carbonaceous slates. Abundant carbonaceous material (CM) can be found in the host rocks and ore-bearing quartz veins, but its geological characteristics and genesis, as well as its association with gold mineralization, are still unclear. Systematic petrographic observation demonstrated two types of CM in host rocks and ores, i.e., CM1 and CM2. Among them, CM1 is the predominant type and mainly occurs in the layered carbonaceous slates, while CM2 is mostly present in quartz veins and mineralized host rocks. Laser Raman spectroscopic analyses of CM1 were performed at higher temperatures (376–504 °C), and CM2 was generated at similar temperatures (255–435 °C) to gold mineralization. Combined with previous studies, we can conclude that CM1 was produced by Neoproterozoic to early Paleozoic metamorphism before gold mineralization, while CM2 is of hydrothermal origin. Geochemical modeling indicates that CM1 could promote gold precipitation through reduction, as well as facilitate structure deformation and metal absorption as previously proposed. However, hydrothermal CM2 is favorable for gold mineralization because it triggers sulfidation, similar to other Fe-bearing minerals (such as siderite) in the host rocks. Consequently, both types of CM in the Huangjindong deposit are favorable for gold mineralization and carbonaceous slates could be important gold-bearing units for future ore prospecting in the Jiangnan Orogen as well as other places in South China.

1. Introduction

Many orogenic gold deposits [1] exist on the fault zone in the form of auriferous quartz veins or various alterations, which contain a large amount of carbonaceous matter [1,2,3,4] (CM). CM refers to C-H-O compounds with soluble–insoluble macromolecular structures, which are solid black substances with heterogeneous components [5]. It is generally accepted that CM can be made of organic and inorganic sources [6], of which the former is formed by biological sedimentation during diagenesis, tectonics, and metamorphism [7], while the latter can be generated by chemical reactions of CO2 and CH4 in hydrothermal solutions [8,9]. Previous studies have shown that CM is closely related to gold mineralization, mainly as an efficient reductant and adsorbent for mineral precipitation [10]. In addition, the hydrothermal CM can co-precipitate with sulfide, leading to the destruction of Au-HS aqueous complexes and therefore promoting mineralization [6,11].
There are many gold (polymetallic) deposits in the Jiangnan Orogenic Belt with proven and predicted gold resources of over 970 t. Previous studies indicate that there are abundant carbonaceous slates and phyllites in this district, which are important host rocks for gold deposits. The formation of these carbonaceous strata is closely related to ductile shear deformation, and the carbon content is often proportional to the intensity of regional tectonic movement [12]. Simultaneously, auriferous quartz veins also have a high carbon content, which may be the product of hydrothermal processes. All these geological features indicate that CM may play an important role in the mineralization of the Jiangnan Orogenic Belt. However, no systematic studies have been conducted on carbonaceous materials and their relationships with gold mineralization in the Jiangnan Orogenic Belt.
The Huangjindong deposit is one of the largest gold deposits in the Jiangnan Orogenic Belt. Previous studies demonstrate that some ore bodies are hosted in carbonaceous slates, which makes this an ideal example for studying the geological features of CM and its contributions to gold mineralization. Therefore, this paper selects the Huangjindong gold deposit as the area of study to determine the characteristics of different types of CM and the role of gold enrichment. This is not only helpful in correctly understanding the metallogenic mechanism of large-scale gold (polymetallic) deposits in the Jiangnan orogenic belt but also provides a basis for gold exploration.

2. Regional Geology

The Jiangnan Orogenic Belt is located on the southeast margin of the Yangtze block [13,14,15] (Figure 1a). It was formed by the collision of the Yangtze plate and the Cathaysia plates during the Neoproterozoic era [16,17,18,19]. Afterward, several stages of tectonic movements occurred in South China, generating basin–range structures [4,20] as well as extensive magmatism in the Neoproterozoic, early Paleozoic, early Mesozoic, and late Mesozoic eras [18,21]. The predominant rocks in the Jiangnan Orogenic Belt are the low-grade metasediments of the Lengjiaxi and Banxi groups. The Lengjiaxi Group and its equivalents are composed of low-grade greenschist facies metamorphic rocks, including (carbonaceous) slates, sandstones, siltstones, and tuffaceous slates. The Banxi Group and its equivalents are mainly composed of sandy slates, greywacke, siltstones, and slates [18]. Hundreds of gold (polymetallic) deposits and occurrences have been discovered in the Jiangnan Orogenic Belt [12], with representative ones being Huangjindong, Wangu, Woxi, and Jinshan.
Northeast Hunan, located in the central Jiangnan Orogenic Belt, is mainly composed of the Neoproterozoic Lengjiaxi group, Banxi group, and Cretaceous sedimentary rocks [24]. The Lengjiaxi group, which is composed of low-density erosion sediments in the plain submarine fan of the semi-deep sea basin, is composed of the Leishenmiao, Huanghudong, Xiaomuping, and Pingyuan formations from bottom to top [24]. The lithology is mainly sericite slates, silty slates, metamorphic siltstones, and metamorphic fine sandstones. Due to the collision between the Yangtze plate and the Cathaysia block, an angular unconformity was formed between the Lengjiaxi and Banxi groups [18,25]. Cretaceous strata can be divided into Daijiaping and Dongtang formations, and the lithologies are mainly sandstone, conglomerate, and greywacke.
This area is characterized by an NE-trending basin and range structures. From west to east, there are three NNE- and NE-trending deep and large strike-slip faults, Xinning–Huitang, Changsha–Pingjiang, and Liling–Hengdong, which separate the early extensional basins and granite mountains. The Northeastern Hunan region has Neoproterozoic to late-Mesozoic magmatic granites [26] (Figure 1b). Among them, the late-Mesozoic S–type granite (about 160–130 Ma) is the most widespread. These granitoids are generally distributed along NE- and SW-trending faults, such as the Lianyunshan intrusion along the Changsha–Pingjiang fault [26]. In addition, a large number of late (about 136–83 MA) felsic and bimodal volcanic rocks have developed in this area. These S–type granites and felsic rocks are interpreted to have been formed in an extensional tectonic environment [27,28,29,30].
Many gold deposits have developed in this area, mainly including Huangjindong, Wangu, and Yanshansi. The gold deposits are mostly distributed near late-Mesozoic granite and controlled by the NE-trending regional fault zone. Wall rock alteration includes sericitization, carbonation, silicification, chloritization, and pyritization. The main types of mineralization are quartz vein, altered slate, and mineralized structural breccia. The main ore minerals include scheelite, pyrite, arsenopyrite, native gold, stibnite, galena, sphalerite, and chalcopyrite. Gangue minerals mainly include quartz, siderite, sericite, dolomite, and calcite.

3. Ore Deposit Geology

The Huangjindong gold deposit is located to the southeast of the Changsha–Pingjiang fault. The proven gold reserve is 80 t, with an average grade of 5 g/t [13], including in Yangshanzhuang, Jinmei, Jintang, and other mining areas (Figure 2). The Xiaomuping Formation of the Neoproterozoic Lengjiaxi group is the main host rock, which can be divided into two sections, according to lithology. The first member consists of layered calcareous slates, carbonaceous slates, banded slates, metamorphic fine sandstones, and sericite slates. The second section is composed of layered sandy slates, silty slates, carbonaceous slates, spotted slates, phyllitic slates, and metamorphic fine-grained sandstones.
Two groups of faults have developed in the Huangjindong deposit, i.e., WNW- and NE-trending faults. Among them, the WNW-trending faults have similar occurrences to the metasediments and are the main ore control structures. Such faults may have been formed in the early Paleozoic era and been reactivated under the action of regional tectonic stress in the late Mesozoic [32]. The NE-trending faults cut through WNW-trending faults, such as the Niwan fault (Figure 2). In addition, a series of WNW-to-EW-trending inverted folds and several nearly parallel NW- and EW-trending interlayer compressive faults and sliced faults are also present. No magmatic rocks are exposed in this area. The nearest intrusion is the ca. 142 Ma Lianyunshan granite, which is located about 12 km southwest of the mining area.
Orebodies in the Huangjindong gold deposit can be divided into three types, i.e., quartz veins, altered slates, and structure breccias. The first type of ore has a high grade, while the second type has large reserves and a relatively low grade. Different hydrothermal alterations have been found in the Huangjindong deposit, including silicification, pyritization, carbonation, sericitization, and chloritization. Among them, pre-ore carbonation–sericitization is the predominant type, generating widespread bleaching alterations to the host rocks. This large-scale alteration is also an important ore prospecting indicator for Huangjindong. Siderite, produced by such an alteration, constitutes favorable chemical traps for gold precipitation and could facilitate mineralization by triggering sulfidation [20]. Ore minerals mainly include pyrite, arsenopyrite, native gold and minor sphalerite, chalcopyrite, galena, and stibnite. Gangue minerals are quartz, calcite, siderite, and ankerite, and minor sericite, chlorite, and muscovite [33].
Even though previous studies have documented the bleached slates in detail, multi-layer carbonaceous slates have been found in the Huangjindong deposit (Figure 3a). This type of host rock is well-developed in some mining districts, such as Yangshanzhuang and Jintang. Notably, extensive hydrothermal alteration and mineralization are unable to bleach the host rocks, and the black color is still maintained in the mineralized rocks (Figure 3b). Moreover, the strata extend stably in depth, which is closely related to the presence of gold-bearing orebodies. Abundant CM is not only found in the carbonaceous slates but also in the quartz-carbonate veins (Figure 3c) which are closely related to pyrite and other sulfides (Figure 3d).

4. Samples and Analytical Methods

Fifty-two samples were collected from mineralized carbonaceous slate and quartz (carbonate) veins. The petrology and microstructure of the samples were conducted on a Leica DM2700P microscope. According to the observation of these microstructures, different CMs were selected from the thin sections for Raman analysis.

4.1. Raman Spectroscopy

All types of CMs identified by petrographic analysis were analyzed by quantitative Raman spectroscopy. The Raman spectra were measured by the Thermo Fisher DXR2xi Raman spectrometer of the State Key Laboratory for Nuclear Resources and Environment. A 100 mW laser quantum ring composed of a CW single-frequency semiconductor laser generated 532 nm of incident radiation. At the same time, the generated laser was focused on the sample through a 100× objective lens. Before each test, the spectrometer was calibrated with silicon standards. Since RSCM may be affected by several analysis mismatches, we strictly followed the analysis and fitting procedures described by Beyssac et al. [34,35].
The measurement was carried out on a polished sheet. The carbonaceous material (CM) was systematically analyzed under transparent adjacent minerals (usually quartz) and the power was adjusted to avoid damage caused by polishing. In the extended scan mode (50–3400 cm−1), the acquisition time was 30–60 s. The spectra were then processed using peak fit 4.12 software [34,35].
The primary region of graphite carbon consisted of the following: (1) there was one main high-frequency band (in-plane stretching mode of aromatic carbon) near 1580 cm−1, which is called the G–band; (2) several defect bands at about 1150, about 1350 (D1 band), about 1500, and about 1620 cm−1 (D2 band) corresponded to physical and chemical defects in the graphite structure. Only G–bands were observed in the original graphite, but both G–bands and defect bands were found in polycrystalline graphite and/or disordered graphite carbon. The secondary region consisted of the following: S1 (2700 cm−1) representing the two-dimensional range of the graphite layer and the three-dimensional order of the graphite lattice; the graphite lattice mainly changed from two-dimensional to three-dimensional, and the peaks split into two [36]. The S2 (2900 cm−1) peak is considered to be the result of C-H bonding.

4.2. Geochemical Modeling

Geochemical modeling was conducted using Geochemist’s Workbench® Professional 12 (GWB) software package [37,38]. Thermodynamic data were modified based on the default GWB database (thermo.com. v8. R6+, 10 June 2024), with the augment of Au- and As-bearing species from the SUPCRT92 [39,40]. The module PHASE2 was used to create the phase diagram, including the Au solubility and Fe speciation in the pH-log fO2(g) coordinate. The REACT module was used to simulate the fluid–rock interactions between ore fluids and CM (represented by graphite [6]). During the fluid–rock interaction simulation, minerals were suppressed unless they had been reported to have occurred in the Huangjindong deposits, which included quartz, pyrite, arsenopyrite, gold, pyrrhotite, clinochlore, ankerite, kaolinite, albite, K-feldspar, anorthite, muscovite, annite, phlogopite, graphite, and siderite.

5. Results

5.1. Petrographic Work

The minerals in the unmineralized and unaltered carbonaceous slates are mainly CM, quartz, and sericite (Figure 4a). The CM in these rocks is termed as CM1 and is present as layers. Due to its widespread occurrence in carbonaceous slates, this type of CM is volumetrically predominant in the Huangjindong deposit. Due to its occurrence in original host rocks without hydrothermal activities, CM1 is most likely generated by pre-ore metamorphism. In the extensively mineralized carbonaceous slates, abundant CM1 is closely associated with arsenopyrite, pyrite, sphalerite, siderite, and rutile (Figure 4b,c). Apart from that, some CM1 is present in quartz–carbonate veins (Figure 4d).
The hydrothermal activities related to mineralization in the Huangjindong deposit can be divided into five stages, which can be represented by the mineral assemblages of quartz (Q1)–carbonate, quartz (Q2)–scheelite, arsenopyrite–pyrite–quartz (Q3), poly-sulfide–quartz (Q4), and calcite–quartz (Q5). The first stage represented by the quartz (Q1)–carbonate veins is barren, and the minerals generally have larger sizes (Figure 5a). The carbonates of this stage are mainly siderite in the host rocks and ankerite (Figure 5b) in the veins. The second stage is characterized by the locally occurring scheelite–quartz (Q2) veins (Figure 5c), and they are crosscut by the arsenopyrite–pyrite–quartz (Q3) veins which represent the predominant mineralization stage (Figure 5d). In veins in the third stage, minor CM2 can be recognized, being present as spherical particles, which could aggregate into framboidal shapes, and become closely associated with pyrite and arsenopyrite. The fourth stage is auriferous and marked by the occurrence of polysulphides that cut through pyrite or arsenopyrite grains (Figure 5e). The fifth stage, i.e., quartz–calcite (Figure 5f), is barren.
Based on the petrographic work in this study, the paragenesis can be augmented according to Deng et al. [31]. covering the minerals in the hosting carbonaceous slates and hydrothermal activities (Figure 6). Notably, based on their occurrence and mineralogical characteristics, two different types of CMs, i.e., CM1 and CM2, can be recognized in the Huangjindong deposit.

5.2. Raman Analysis of CM

The Raman spectrum characteristics of different types of CM are shown in Figure 7. In the first-order region, the Raman spectra of CMs have two obvious characteristic peaks (g peak and D1 peak), and there is a D2 peak which is not obvious, but the parameters of each peak are different. In the second-order region, different types of CM have different Raman peaks.
The characteristics of the Raman spectrum CM1 (Figure 7a,b) show a D1 peak with a relatively high and narrow intensity and a G peak with a relatively low and narrow intensity. The positions of D1 and G peaks were 1353 ± 5 cm−1 and 1582 ± 13 cm−1, respectively. There is a D2 band around 1618 ± 5 cm−1. In the second-order region, CM1 has a wide S1 peak. The Raman temperature measurement shows that the temperature of CM1 is 376~504 °C.
The Raman spectral characteristics of CM2 (Figure 7c,d) show a D1 peak with high and narrow relative intensity and a G peak with low and wide relative intensity. The positions of D1 and G peaks were 1341 ± 6 cm−1 and 1589 ± 12 cm−1, respectively. There is a D2 band around 1617 ± 5 cm−1. In the second-order region, there are not only S1 peaks with wide intensity but also S2 peaks with low intensity in CM2. Raman temperature measurement shows that the temperature of CM2 is 255~435 °C.

5.3. Geochemical Modeling

Moderate-salinity (<15 wt.% NaCl equiv.) hydrothermal fluids are commonly trapped as fluid inclusions of the Huangjindong gold deposit. The chemical components of the systems investigated are Au-Fe-As-Na-Si-S-C-Cl-H-O. The ore-forming fluids are considered to be reduced due to the abundance of pyrite and arsenopyrite, and acidic because of the occurrence of illite and sericite. The temperature is set as 250 °C, as indicated by arsenopyrite geothermometry conducted by Deng et al. [31].
The Na, SO42−, Au, and AsH3(aq) contents are specified as 1 mol/kg, 0.005 mol/kg, 100 ppb, and 300 ppm, respectively. Fe’s solubility is low in reduced fluids with high ΣS contents like typical auriferous fluids [41]. Consequently, 1 ppm Fe is specified in the fluids. Due to the low CO2 contents in syn-ore fluids [16], the HCO3− concentration is set at a low value of 10−4 mol/kg. The concentration of SiO2(aq) is buffered by quartz. The oxygen fugacity (log fO2(g)) value is buffered by the assemblage of pyrite and arsenopyrite, i.e., about −35.5 (Figure 8a), and pH is constrained by the Fe solubility and stability of sericite.
Au is mainly present as AuHS(aq) and Au(HS)2− under relatively reduced conditions (Figure 8a). Gold solubility is the highest (>300 ppb) with pH = 5–7.5 and log fO2(g) = −34–−40 (from ~HM + 1.5 to ~HM − 4.5). Under such conditions, both increasing and decreasing log fO2(g) could result in gold precipitation. As shown in Figure 8b, the interaction with graphite leads to the decrease of log fO2(g) to about −38.7 (~HM − 3.2). Such reactions would decrease the Au solubility and therefore cause gold precipitation.

6. Discussion

6.1. Characteristics and Genesis of CM

CM is a carbon-rich material containing CO and/or CH bonds [5,42], which are widely developed in a variety of strata and ore bodies. Previous studies have shown that there is abundant organic sourced CM in the metasediments hosting the gold deposits [43,44]. Temperature increase during structural deformation and metamorphism would change the structure of the original organic materials in the host rocks, resulting in an increase in the crystallinity of CM and promoting the graphitization process [45,46,47].
Inorganic CM is also developed in the gold-bearing quartz-carbonate veins and host rocks and is closely associated with sulfide and gold particles. The CM in the mining area is produced by the chemical reaction of the mixing of hydrocarbon gas in the ore-forming hydrothermal solutions [34,35,44]. During the transfer of hydrocarbon gas-containing fluids from deep formations to shallow formations, regional metamorphism will reduce the content of CO2 and CH4 in fluids and produce carbonaceous substances and water [6].
CO2 + CH4 = 2C + 2H2O
There are two types of carbonaceous materials in the deposit, CM1 and CM2, respectively. Among them, CM1 is the predominant type and mainly occurs in the carbonaceous slates. In addition, CM1 is formed at higher temperatures of 376~504 °C, much higher than the hydrothermal fluids in the Huangjindong deposit [48]. Consequently, it is concluded that CM1 is formed during pre-mineralization regional metamorphism. Previous geochronological studies on the detrital zircons and metamorphic mica demonstrate that the Lengjiaxi Group was initially deposited during the Neoproterozoic and might have gone through metamorphism during the tectonic–magmatic activities during the Neoproterozoic to early Paleozoic [25]. As a result, CM1 is produced by pre-ore metamorphism from the Neoproterozoic to the early Paleozoic, which also likely generated the carbonaceous slates in the Jiangnan Orogen.
Different from CM1, CM2 is mainly present as veinlets in quartz-carbonate veins. In addition, the forming temperatures of CM2 (T = 255~435 °C) are lower than that of CM1 but similar to the ore fluids (223–263 °C). Combined with its close association with hydrothermal sulfides (Figure 3c,d), it is concluded that CM2 is produced by ore fluids in the Huangjindong deposit. Previous fluid inclusion studies indicate that the syn-ore fluids are CO2-barren, so CM2 can only be locally found in the veins.

6.2. Implications for Gold Mineralization

Based on extensive studies of ore structures, fluid inclusion, mineralogy, and isotopes [49], multiple gold precipitation mechanisms have been proposed for gold mineralization in the Jiangnan Orogen, including fluid boiling, cooling, fluid–rock interactions, and fluid mixing. Among them, fluid boiling is mainly inferred from the immiscible fluids’ inclusions and structure deformation [48,50], while cooling is mainly concluded from the measured or calculated temperatures. Due to the different sources of fluids or ore-forming materials based on isotopic studies, fluid mixing has been generally proposed to facilitate gold deposition [23]. Fluid–rock interactions could promote gold mineralization in various ways, including sulfidation, oxidation, and reduction, depending on the lithologies of host rocks [51]. In the Jiangnan Orogen, hematite-bearing host rocks have been proposed to cause fluid oxidation and sulfidation [50]. In addition, sulfidation triggered by the fluid–rock interactions with Fe-bearing carbonates (siderite) is the most important mineralization mechanism for some deposits. Despite the aforementioned studies, CM in host rocks has seldom been studied for the gold deposits in the Jiangnan Orogen, so its role in regional gold mineralization is still unknown.
This study on the Huangjindong deposit conducted the first systematic research on the CM and its association with gold mineralization. As demonstrated by the geochemical modeling, the pre-ore CM1 could significantly decrease fO2(g) in fluids and therefore reduce gold solubility [5,52,53]. Since Au is dissolved as Au+-bearing species in fluids (mainly as Au(HS)2− and AuHS), the reduction is quite efficient for gold precipitation. The deposition process is illustrated in Reactions (1) and (2). Apart from its chemical properties of reduction, CM1 is also a fine adsorbent which could further promote gold precipitation [44]. Moreover, the CM-bearing carbonaceous slates are more susceptible to structure deformation and therefore more likely to focus on ore fluids [10].
4Au(HS)2− + C(s) + 4H+ + 2H2O = 4Au(s) + CO2(aq) + 8H2S(aq)
4AuHS(aq) + C(s) + 2H2O(l) = 4Au(s) + CO2(aq) + 4H2S(aq)
As shown in Reactions (1) and (2), no sulfides are precipitated merely through reduction by CM, which is inconsistent with the abundant sulfides in the mineralized carbonaceous slates (Figure 3 and Figure 4). As shown in Figure 4b and Figure 5b, abundant siderite occurs in the carbonaceous slates. Such Fe-bearing minerals would react with Au-S-bearing ore fluids, causing the co-precipitation of Au and sulfides through Reactions (3) and (4) (only illustrating with AuHS(aq)).
2AuHS(aq) + FeCO3 (siderite) = 2Au(s) + H+ + FeS2 (Pyrite) + HCO3−
AuHS(aq) + FeCO3 (siderite) + As(HS)3(aq) =
Au(s) + O2(aq) + H+ + AsFeS (Arsenopyrite) + H2O + HCO3−
Even though CM2 is volumetrically less important than CM1, it would facilitate gold mineralization [6,53]. The co-deposition of CM2 and sulfides through Reaction (5) would significantly decrease the suffering concentrations in fluids. Therefore, the stability of Au(HS)2− and AuHS(aq) is interrupted, causing a decrease in Au solubility. Such a mechanism can also be termed sulfidation, similar to the role of siderite.
2FeO(s) + 4H2S(aq) + CO2(aq) = 2FeS2 (Pyrite) + C(s) + 4H2O
Based on the discussion above, it is concluded that both types of CMs are favorable for gold precipitation (Figure 9). However, gold mineralization hosted by carbonaceous slates in the Jiangnan Orogen has been previously neglected by both scientific studies and ore prospecting. More importantly, South China is well-known for its widespread carbonaceous rocks, hosting abundant metals such as Au, Ag, Mo, V, Ni, and Cr (also termed black shales) [54,55,56]. As a result, more efforts should be paid towards gold mineralization in the CM-bearing rocks in the Jiangnan Orogen as well as in other places in South China.

7. Conclusions

Abundant CM is found in the Huangjindong deposit that is partly hosted by carbonaceous slates, and two types of CM are recognized, i.e., CM1 and CM2. Among them, CM1 mainly occurs in the host rocks and was formed at higher temperatures, while CM2 is most present in the quartz-carbonate veins and was generated at a similar temperature to the ore fluids. Combined with previous studies in addition to the occurrence and forming temperature, CM1 and CM2 are proposed to be produced by pre-ore metamorphism and syn-ore hydrothermal fluids, respectively.
CM1 could promote gold precipitation through reduction, as well as absorbing and facilitating faulting, while CM2 favors mineralization by triggering sulfidation. Consequently, both types of CM are favorable for gold mineralization, but their roles in gold mineralization in the Jiangnan Orogen have been previously neglected. Considering the widespread carbonaceous rocks in South China, more attention should be paid to gold mineralization in CM-bearing rocks.

Author Contributions

Conceptualization, Y.Z. and Z.W.; methodology, Y.L. (Yongjun Liu); software, J.W.; validation, B.H.; formal analysis, H.H.; investigation, Y.L. (Yuxiang Luo); resources, P.F.; data curation, B.H. and X.W.; writing—original draft preparation, X.L.; writing—review and editing, T.D., Y.Z. and Z.W.; visualization, M.Z.; supervision, S.Z.; project administration, M.X.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Hunan Provincial Natural Science Foundation of China (No. 2024JJ8345) and the Major Scientific Research Program of the Geological Bureau of Hunan Province (HNGSTP202302).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical and legal concerns.

Conflicts of Interest

Yueqiang Zhou and Zhilin Wen are the employees of Hunan Institute of Geological Disaster Investigation and Monitoring. The paper reflects the views of the scientists and not the company.

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Figure 1. (a) Simplified tectonic map of South China showing the location of the Jiangnan Orogen (Modified after Sun et al., 2012 [22]); (b) Geological map of eastern Hunan showing the distribution of structures, lithologies, and major intrusions of different ages, and different types of ore deposits (Modified after Mao et al., 1997 [23]).
Figure 1. (a) Simplified tectonic map of South China showing the location of the Jiangnan Orogen (Modified after Sun et al., 2012 [22]); (b) Geological map of eastern Hunan showing the distribution of structures, lithologies, and major intrusions of different ages, and different types of ore deposits (Modified after Mao et al., 1997 [23]).
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Figure 2. (a) Geological map of the Huangjindong gold deposit and (b) a cross-section showing ore geological features and related host rocks of the deposit (modified from [31]).
Figure 2. (a) Geological map of the Huangjindong gold deposit and (b) a cross-section showing ore geological features and related host rocks of the deposit (modified from [31]).
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Figure 3. Photographs of orebodies and host rocks in the field from the Jinshan gold deposit. (a) carbonaceous slate; (b) bleaching and unaltered carbonaceous slate; (c) CM in the quartz-carbonate veins; (d) CM is associated with sulfides.
Figure 3. Photographs of orebodies and host rocks in the field from the Jinshan gold deposit. (a) carbonaceous slate; (b) bleaching and unaltered carbonaceous slate; (c) CM in the quartz-carbonate veins; (d) CM is associated with sulfides.
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Figure 4. Photomicrographs of ore bodies and host rocks in thin sections from the Huangjindong gold deposit. (a) CM, quartz, sericite, and siderite in the carbonaceous slate; (b,c) CM1 is closely associated with arsenopyrite, pyrite, sphalerite, siderite, and rutile; (d) locally occurring CM in quartz–carbonate veins; (e) the schematic section showing the relationship of the veins and CM. Q, quartz; Ser, sericite; Sd, siderite; Rt, rutile; Py, pyrite; Ccp, chalcopyrite; Sp, sphalerite; Gn, galena.
Figure 4. Photomicrographs of ore bodies and host rocks in thin sections from the Huangjindong gold deposit. (a) CM, quartz, sericite, and siderite in the carbonaceous slate; (b,c) CM1 is closely associated with arsenopyrite, pyrite, sphalerite, siderite, and rutile; (d) locally occurring CM in quartz–carbonate veins; (e) the schematic section showing the relationship of the veins and CM. Q, quartz; Ser, sericite; Sd, siderite; Rt, rutile; Py, pyrite; Ccp, chalcopyrite; Sp, sphalerite; Gn, galena.
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Figure 5. (a) The quartz (Q1)–carbonate veins; (b) ankerite in the quartz (Q1)–carbonate veins; (c) scheelite-quartz (Q2) vein is crosscut by the arsenopyrite–pyrite–quartz (Q3) vein; (d) CM2 is closely associated with pyrite and arsenopyrite; (e) polysulphides cut through pyrite and arsenopyrite; (f) quartz-calcite veins. Apy, arsenopyrite; Ank, ankerite.
Figure 5. (a) The quartz (Q1)–carbonate veins; (b) ankerite in the quartz (Q1)–carbonate veins; (c) scheelite-quartz (Q2) vein is crosscut by the arsenopyrite–pyrite–quartz (Q3) vein; (d) CM2 is closely associated with pyrite and arsenopyrite; (e) polysulphides cut through pyrite and arsenopyrite; (f) quartz-calcite veins. Apy, arsenopyrite; Ank, ankerite.
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Figure 6. Mineralization stages and the paragenesis of the Huangjindong gold deposit.
Figure 6. Mineralization stages and the paragenesis of the Huangjindong gold deposit.
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Figure 7. (a,b) Raman spectra of CM1; (c,d) Raman spectra of CM2.
Figure 7. (a,b) Raman spectra of CM1; (c,d) Raman spectra of CM2.
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Figure 8. (a) Au solubility diagrams in pH-log fO2(g) coordinates at 250 °C with a distribution of Au-bearing aqueous species and Fe-bearing minerals at S = 0.005 mol/kg; (b) mineral precipitation and changes of pH, log fO2(g) and S concentrations in fluids at 250 °C when reacting with graphite.
Figure 8. (a) Au solubility diagrams in pH-log fO2(g) coordinates at 250 °C with a distribution of Au-bearing aqueous species and Fe-bearing minerals at S = 0.005 mol/kg; (b) mineral precipitation and changes of pH, log fO2(g) and S concentrations in fluids at 250 °C when reacting with graphite.
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Figure 9. Mineralization model for the Huangjindong gold deposit. CM1 promotes gold precipitation through reduction, while CM2 favors mineralization by triggering sulfidation. Sd, siderite; CM, Carbonaceous matter; Au, native gold.
Figure 9. Mineralization model for the Huangjindong gold deposit. CM1 promotes gold precipitation through reduction, while CM2 favors mineralization by triggering sulfidation. Sd, siderite; CM, Carbonaceous matter; Au, native gold.
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Zhou, Y.; Wen, Z.; Liu, Y.; Wu, J.; Huang, B.; He, H.; Luo, Y.; Fan, P.; Wang, X.; Liu, X.; et al. The Contribution of Carbonaceous Material to Gold Mineralization in the Huangjindong Deposit, Central Jiangnan Orogen, China. Minerals 2024, 14, 1042. https://doi.org/10.3390/min14101042

AMA Style

Zhou Y, Wen Z, Liu Y, Wu J, Huang B, He H, Luo Y, Fan P, Wang X, Liu X, et al. The Contribution of Carbonaceous Material to Gold Mineralization in the Huangjindong Deposit, Central Jiangnan Orogen, China. Minerals. 2024; 14(10):1042. https://doi.org/10.3390/min14101042

Chicago/Turabian Style

Zhou, Yueqiang, Zhilin Wen, Yongjun Liu, Jun Wu, Baoliang Huang, Hengcheng He, Yuxiang Luo, Peng Fan, Xiang Wang, Xiaojun Liu, and et al. 2024. "The Contribution of Carbonaceous Material to Gold Mineralization in the Huangjindong Deposit, Central Jiangnan Orogen, China" Minerals 14, no. 10: 1042. https://doi.org/10.3390/min14101042

APA Style

Zhou, Y., Wen, Z., Liu, Y., Wu, J., Huang, B., He, H., Luo, Y., Fan, P., Wang, X., Liu, X., Deng, T., Zhong, M., Zhang, S., & Xiao, M. (2024). The Contribution of Carbonaceous Material to Gold Mineralization in the Huangjindong Deposit, Central Jiangnan Orogen, China. Minerals, 14(10), 1042. https://doi.org/10.3390/min14101042

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