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Minerals 2018, 8(6), 262; https://doi.org/10.3390/min8060262
Trace Element Analysis of Pyrite from the Zhengchong Gold Deposit, Northeast Hunan Province, China: Implications for the Ore-Forming Process
Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, School of Geosciences and Info-Physics, Central South University, Changsha 410083, China
School of Resources and Safety Engineering, Central South University, Changsha 410083, China
Author to whom correspondence should be addressed.
Received: 1 June 2018 / Accepted: 15 June 2018 / Published: 20 June 2018
The Zhengchong gold deposit is located in the central segment of the Jiangnan Orogen in northeastern Hunan Province, South China. The host rocks of this deposit are the Neoproterozoic slates of the Lengjiaxi Group and granodiorite. The structures in the Zhengchong gold deposit are dominated by NE-trending reverse faults, which control the gold-bearing veins. The orebody consists of NE-trending laminated quartz veins and NW-trending quartz veins. The alteration styles include silicification, carbonatization, sulfidation, sericitization and chloritization. The Zhengchong gold mineralization can be divided into four stages: Quartz-pyrite (stage I), quartz-pyrite-arsenopyrite (stage II), quartz-polysulfide (stage III) and quartz-carbonate (stage IV). Three generations of hydrothermal pyrite were identified: Disseminated euhedral to subhedral cubes in altered wall-rock (PyI), euhedral to subhedral cubes inter-grown with arsenopyrite and tetrahedrite in quartz veins and wall-rock (PyII), and euhedral cubes with microinclusions (native gold, galena, sphalerite, chalcopyrite, tetrahedrite, and pyrrhotite) or metasomatic textures in sulfide-rich veins or veinlets (PyIII). PyII and PyIII are arsenian pyrite and represent the main Au-bearing minerals. PyI records the lowest concentrations of Au; PyII and PyIII record similar amounts of Au, Cu, Pb, Zn, and Bi, but PyIII is more enriched in Co, Ni, Te, and Se. The substitution of As, Se and Te for S and that of Co and Ni for Fe occurs by direct-ion exchange. Invisible gold is uniformly distributed within the arsenian pyrite, and visible gold fills microfractures in PyII or occurs as inclusions in PyIII. Co, Ni, Cu exhibit positive correlations with Au and a negative correlation between Au + Cu + Co + Ni and Fe reflect that Fe vacancies may have been a major cause of the precipitation of invisible Au and other metal elements in pyrite structure. There are systematic trace element differences between the three generations of pyrite (PyI, PyII, PyIII). The more Co, Ni and Se, Te substitution that occurred for Fe and S, respectively, the greater the increase in the Co/Ni ratio (<1) and the decrease in the Se/Te ratio (<10) in stage III, indicating that a more reduced, lower-temperature metamorphic hydrothermal fluid was present in stage III.
Keywords:pyrite; trace element; gold deposit; Zhengchong; Northeast Hunan Province
Gold deposits in metamorphic terranes can be classified as orogenic gold deposits or reduced intrusion-related gold deposits [1,2,3]. Orogenic gold deposits dominantly form in greenstone belts, at or above the brittle-ductile transition, with weakly reduced, low-salinity fluids with near-neutral pH values at approximately 300 °C [1,4,5,6,7,8]. In contrast, reduced intrusion-related gold deposits are intrusion-hosted, sheeted arrays of thin, low-sulfide quartz veins with Au–Bi–Te–W signatures [2,6]. Some models have explained the role of fluids in gold mineralization, including a decrease in solubility that occurs during drops in pressure associated with earthquakes in the case of quartz veins [9,10] and the destabilization of gold bisulfide complexes by a redox reaction that occurs between the transporting fluid and iron in the host rocks, which precipitates both pyrite and free gold [7,11].
Current issues in the study of gold deposits include the source of the ore-forming fluids, the precise tectonic setting and age of mineralization, and the specific depositional mechanisms for gold [3,8]. The first two controversies are mainly due to the abundance of isotope and fluid inclusion data [12,13] and the lack of ore minerals that have been subjected to high-precision geochronological analysis . However, information about the transport and deposition of gold and its fluid evolution can be obtained using the detailed petrogenetic analyses and trace element data of pyrite [15,16,17,18]. Pyrite is stable under a very wide range of sulfur activity and thermodynamic conditions compared to other sulfides [15,19,20]. Gold is preferentially concentrated in the crystal structure of arsenian pyrite or as micrometer-size particles [19,21,22]. It is possible to identify the gold mineralization in the Zhengchong deposit based on the presence of arsenian pyrite.
The Jiangnan Orogenic Belt (JOB), which is located in South China (Figure 1a), hosts more than 250 Au-(polymetallic) deposits and contains more than 960 t of total gold reserves [23,24]. Northeastern Hunan Province (NEHP) contains approximately 125 Au-(polymetallic) deposits, including the Wangu, Huangjindong and Yanlinsi deposits (Figure 1b) . The Zhengchong gold deposit is a new exploration area in the Yanlinsi orefield. However, the ore genesis of the deposits in the Jiangnan Orogen is not well understood . Various genetic models have been proposed for the deposits in the JOB, including orogenic, epithermal, intrusion-related, and even SEDEX-type models [25,26,27,28,29,30,31,32,33].
Here, we provide textural, EPMA and LA-ICP-MS data for Au and the other trace elements in the pyrite from the Zhengchong deposit. In this contribution, we show that systematic differences exist between different generations of pyrite, and we use these data to better understand the evolution stages of ore systems and characterize their ore-forming processes.
2. Geological Setting
2.1. Regional Geology
The JOB is a Neoproterozoic collisional zone located between the Yangtze Block to the northwest and the Cathaysia Block to the southeast (Figure 1a) [34,35,36,37,38].
The NEHP is located in the central part of the JOB. The lithostratigraphic units in this region consist of Neoproterozoic Lengjiaxi Group (LJXG) low-grade metamorphic volcaniclastic and sedimentary rocks and Meso-Cenozoic red-bed sedimentary rocks, with minor Archean to Paleoproterozoic crystalline metamorphic rocks and Paleozoic sedimentary rocks . A Basin-and-Range-style tectonic setting (Figure 1b), comprising extensional basins and granitic domes, as well as marginal strike-slip faults, formed during the late Mesozoic [24,39,40,41]. Most of the granites in the JOB have been interpreted as S-type granites that formed from the Proterozoic, Early Paleozoic (Caledonian), and Early Mesozoic (Indosinian) to the Late Mesozoic (Yanshanian) [24,39,42].
2.2. Deposit Geology
The Zhengchong gold deposit is hosted in the Neoproterozoic lower and upper Huanghudong and lower Xiaomuping formations of the LJXG (Figure 2). These rocks are all NE-striking and dip to the NW at 26°–65°, indicating that they comprise a reverse anticline. The lower Huanghudong Formation is a suite of flysch turbidites that is mainly composed of greywacke, slate and silty slates. The upper Huanghudong Formation consists of grayish metamorphic quartz greywacke, siltstones, slate and silty slates. The lower Xiaomuping Formation is composed of pale yellow slate and silty slates. The altered granodiorite that is exposed in the central part of the Zhengchong deposit has experienced carbonatization, sulfidation, and sericitization (Figure 2 and Figure 3f). The NW-trending QCVs are always associated with strong alteration.
The orebody contains Au-bearing quartz veins that range in thickness from centimeters to meters and consist of quartz, carbonate, pyrite, arsenopyrite, and native gold. There are two sets of quartz veins in the deposit: (1) NE-trending laminated quartz veins (Figure 3b) and (2) NW-trending quartz veins, which are more complicated and include disseminated pyrite and arsenopyrite in and around veins (Figure 3a), massive pyrite veins (Figure 3d,e) and disseminated pyrite veins (Figure 3c–f). These Au-bearing quartz veins are hosted in strata and altered granodiorite. The gold mineralization is closely associated with carbonatization, sulfidation, sericitization and chloritization (Figure 4).
Based on the mineralogical and textural characteristics of the ore minerals and the crosscutting relationships between the mineralized veins (Figure 3), four mineralization stages were recognized (Figure 5). These stages are the quartz-pyrite (stage I), quartz-pyrite-arsenopyrite (stage II), quartz-polysulfide (stage III) and quartz-carbonate (stage IV) stages. Stages II and III are considered to be the main stages of ore formation.
3. Samples and Analytical Methods
3.1. Sample Sites
To obtain a more complete picture of the evolution of the ore-forming hydrothermal fluids and the genetic model of gold deposition for the Zhengchong Au deposit, three samples were collected from tunnels at different levels (Figure 2b) based on their cross-cutting relationships and differences in mineralization between veins. Detailed descriptions of the analyzed samples are given in Table 1.
3.2. Electron-Probe Microanalyses
Ore minerals and ore textures were observed in polished sections using standard reflected-light microscopy techniques. Selected representative thin sections containing more auriferous pyrite and Au-bearing minerals were analyzed by electron probe microanalysis (EPMA) using a Shimadzu EPMA-1720H housed at the School of Geosciences and Info-physics (SGI), Central South University (CSU), Changsha, China. The operating conditions of the electron microprobe maintained an accelerating voltage of 15 kV, a beam current of 10 nA, and an electron beam diameter of 1 μm. The following X-ray lines were used to analyze different elements: AsL, SK, FeK, CoK, NiK, AuM, and PbM. The mineral and metal standards used for the calibration of elemental X-ray intensities included pyrite (S and Fe), gallium arsenide (As), cobalt (Co), nickel (Ni), gold (Au) and galena (Pb). The resulting data were then ZAF-corrected using proprietary Shimadzu software. The minimum detection limit of Co was 0.03 wt %, that of Ni was 0.01 wt %, that of Au was 0.06 wt %, that of Pb was 0.04 wt %.
To understand the distribution of the main elements in auriferous pyrite, EPMA X-ray elemental maps were obtained from representative grains.
3.3. LA-ICP-MS Analysis
The analytical instrumentation employed in this study consists of a New Wave UP-213 nm Laser Ablation System coupled with an Agilent 7700s Quadrupole ICP-MS housed in the CODES LA-ICP-MS facility at the University of Tasmania, Hobart, Australia. Depending on the size of the pyrite grain, analyses were performed by laser ablation using spot diameters of 10–35 μm and a repetition rate of 2–5 Hz. The laser beam energy was maintained between 1.6 and 2.5 J/cm2. The analysis time for each sample was 90 s, including 30 s of background measurement with the laser off and 60 s of analysis with the laser on. The acquisition time for all masses was set to 0.02 s, with a total sweep time of ~0.6 s. Data reduction was performed using standard methods [43,44], with Fe as the internal standard. Calibration was performed using the in-house standard (STDGL2b-2), which comprises powdered sulfides doped with certified element solutions and fused to a lithium borate glass disk . The standard was analyzed twice every 1.5 h with a 100-μm beam size at 10 Hz to correct for instrument drift. The accuracy is expected to be better than 20% for most elements . A series of 28 elements was chosen for spot analysis (i.e., Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Zr, Mo, Ag, Sn, Sb, Te, Ba, La, W, Pt, Au, Tl, Pb, Bi, Th, and U) in this study. The minimum detection limit of S was 50.8176 ppm, that of Fe was 2.9364 ppm, that of As was 0.8589 ppm, that of Au was 0.0009 ppm, that of Co was 0.0035 ppm, that of Ni was 0.003 ppm, that of Cu was 0.0738 ppm, that of Zn was 0.1571 ppm, that of Pb was 0.0059 ppm, that of Ag was 0.0099 ppm, that of Se was 2.4309 ppm, that of Te was 0.039 ppm, that of Sb was 0.0223 ppm, that of Bi was 0.0005 ppm.
4.1. Petrography and Mineralogy
The ore minerals are mainly composed of pyrite and arsenopyrite; lesser amounts of galena, sphalerite, chalcopyrite, tetrahedrite, pyrrhotite, rutile and native gold formed during stage II and stage III (Figure 6). The gangue minerals include quartz, carbonate (i.e., dolomite, siderite, calcite), sericite, and chlorite (Figure 4).
Pyrite is a very common mineral in most metamorphosed ore deposits and many types of metamorphic rocks . Once pyrite has formed, it becomes much more refractory and retains many of its characteristics, even in deposits that have undergone penetrative deformation . The pyrite in the Zhengchong gold deposit, which is the most abundant sulfide in these orebodies, is entirely hydrothermal pyrite. Three major crystal growth stages were defined based on their textures and paragenetic sequences observed using optical microscopy and back-scattered electron (BSE) images obtained by EMPA (Figure 6). PyI is characterized by isolated crystals in quartz veins (Figure 6a) or disseminated euhedral to subhedral cubes in country rock (~1 mm across). PyII comprises euhedral to subhedral cubic crystals (~500 μm across); these grains exhibit oscillatory zonation with core-rim structures and are commonly overprinted by other minerals (Figure 6b,d). Three subgenerations (PyII-1, -2, -3) were defined based on BSE images (Figure 6b), and the pores of PyII-2 commonly contain arsenopyrite and tetrahedrite. PyIII, which comprises euhedral and subhedral cubes (100 μm~1 mm across), is characterized by inclusions (e.g., gold, chalcopyrite, galena, sphalerite and pyrrhotite) (Figure 6f,h,i).
4.2. EPMA Data
The EMPA data obtained from different generations of pyrite are listed in Table 2. Point analyses were obtained from the core to the rim on relatively large pyrite grains. The average Fe, S and As contents are 47.18 wt %, 53.74 wt % and 0.37 wt % for PyI; 46.96 wt %, 52.73 wt % and 1.49 wt % for PyII-1; 46.40 wt %, 51.58 wt % and 3.01 wt % for PyII-2; 46.90 wt %, 52.48 wt % and 1.63 wt % for PyII-3; and 46.72 wt %, and 52.34 wt % and 1.72 wt % for PyIII, respectively. The concentrations of Au and other trace elements (Co, Ni, Pb) in pyrite were close to or below the limits of detection.
Arsenic, cobalt and nickel contents can be mapped using EMPA to interpret the crystallization processes of pyrite . PyI shows no discernible pattern in terms of the distribution of cobalt and nickel, but arsenic and sulfur show a visible negative correlation (Figure 7). PyII displays well-developed primary crystallographic growth bands (Figure 8 and Figure 9) through the maps of Co, Ni and As, as well as the clearly visible core of a preexisting pyrite (Figure 8). PyIII shows low contents and uniform distributions of Co, Ni, and As, with inclusions of galena, chalcopyrite, sphalerite, pyrrhotite and gold (Figure 9 and Figure 10), only the outer of PyIII is characterized by well-developed arsenic, cobalt and nickel zoning (Figure 10b–d). The correlation of element anomalies in one large fractured crystal reflects the replacement and characteristics of the distribution of elements between PyII and PyIII (Figure 9).
4.3. Trace Element Contents in Pyrites
A total of 36 LA-ICP-MS spot analyses were completed on pyrites selected from two typical samples (290-7, 290-8) in the Zhengchong gold deposit (Table 3); these analyses included 4 analyses of PyI, 20 analyses of PyII and 12 analyses of PyIII.
PyI contains low Au (<1 ppm) and As (183~6870 ppm) contents, as well as low contents of the following trace elements: Co (<1 ppm), Ni (<7 ppm), Cu (0.5–301 ppm), Zn (0.5~1241 ppm), Pb (0.1~553 ppm), Ag (5.4~28 ppm), Se (4~17 ppm), Sb (1~26 ppm), Te (<1 ppm), and Bi (0.03~20 ppm).
PyII records relatively higher abundances of Au (2~57 ppm), As (11,324~35,710 ppm), Cu (2~84 ppm), and Bi (0.1~10 ppm), as well as the following concentrations of other trace elements: Co (0.01~99 ppm), Ni (0.1~411 ppm), Zn (<1 ppm), Pb (3~763 ppm), Ag (<1 ppm), Se (3~37 ppm), Sb (0.5~32 ppm), and Te (<1 ppm).
Compared to PyII (Figure 11), PyIII is enriched in Au (2.5~36 ppm), Co (2~1083 ppm), Ni (7.6~2599 ppm), Te (1.2~46 ppm), and Se (28~97 ppm); the other trace elements measured in PyIII include Cu (2.6~33 ppm), Zn (0.1~4.3 ppm), Pb (0.3~644 ppm), Ag (<1 ppm), As (8862~20429 ppm), Sb (0.05~37 ppm), and Bi (0.06~10 ppm).
These As concentrations are generally consistent with those determined by EPMA (Table 2 and Table 3). Differences in As were observed between different types of pyrite.
Pyrite can be subdivided into three group, which are consistent with the ore forming stage (Figure 11). PyI contains low Au, As Co, Ni contents, but a large ranges in concentrations of Pb, Cu, Zn, Bi, Sb due to the presence of inclusions (Figure 12a). PyII and PyIII record relatively higher abundances of Au, As, but PyIII more enrich Co, Ni, Se, Te, sometimes the content of Pb in PyII and PyIII is really high due to the presence of inclusions (Figure 12b).
5.1. Distribution and Correlations of Trace Elements in Pyrites
The trace element concentrations in pyrite depend on both the nature of the ore fluid and the geochemical properties of the elements. Therefore, trace elements in pyrite have been widely used to constrain the characteristics of ore fluid [18,46,47,48]. The correlations between selected major and trace elements in these pyrites are shown in Figure 13.
The EPMA results reveal that nearly all pyrite samples contain significant amounts of As, as well as a wide range of other trace elements, such as Au, Co, Ni, and Pb. PyI contains lower As contents; in contrast, PyII and PyIII-2 record higher As contents (i.e., up to wt % levels), and these pyrites can thus be classified as arsenian pyrite. However, only PyIII is associated with native gold. The EPMA results show a well-defined negative correlation between As and S (Figure 13a), which is consistent with the substitution of As for S as anionic As− in the Fe(S1−xAsx)2 solid solution in reducing environments [49,50].
There is no clear relationship between Pb and Fe (Figure 13b), and a positive correlation was observed between Bi and Pb in pyrite (Figure 13c). Therefore, we speculate that most of the Pb distributed in pyrite comprises galena inclusions and that Bi occurs as a solid solution in galena (Figure 12) .
A well-defined positive correlation exists between Co and Ni (Figure 13d), and a negative correlation exists between Ni and Fe (Figure 13e), indicating that Co and Ni can replace Fe by direct-ion exchange . PyIII is characterized by higher Co and Ni contents than PyII (Figure 13d). Element mapping by electron microprobe also indicates that Co and Ni are incorporated in the pyrite crystal lattice and exhibit compositional zoning in pyrite grains (Figure 8, Figure 9 and Figure 10).
A positive correlation was observed between Se and Te on the Se–Te diagram, which can be divided into two distinct sub-areas (Figure 13g): Se-poor and Te-poor PyI and PyII and Se-rich and Te-rich PyIII. An obvious negative correlation exists between Te and S (Figure 13h), indicating that Te and Se distort the S site in a similar way as the AsS dianion  and that more S was replaced by Se and Te in PyIII. Au and Te show no correlation (Figure 13i).
5.2. Gold in Pyrite
The Au–As diagram (Figure 14a) shows that PyI (lower As) and PyII-2 (higher As) plot below the line of gold saturation, which reflects the incorporation of Au into pyrite in a metastable solid solution [50,54]; PyII-1, PyII-3, and PyIII plot near or above the line of gold saturation, which reflects the formation of Au nanoparticles by the cooling of hydrothermal fluid and/or supersaturation with respect to native Au (Figure 14a) [50,54]. EPMA X-ray elemental maps also show that Au is distributed as invisible gold within the arsenian pyrite; additionally, visible gold fills microfractures in PyII or occurs as inclusions in PyIII (Figure 6, Figure 9 and Figure 10). Previous studies have established that invisible gold is most likely incorporated into metastable solid solution, which is controlled by chemisorption at As-rich, Fe-deficient surface sites, temperature, or the amounts of other trace elements [8,10,49,52,54]. It is interesting that Au does not display the strong sectoral preference noted for As in arsenian pyrite. Furthermore, there is a clear negative correlation between Au and Fe concentrations (Figure 14b); Fe vacancies may be a major cause of the invisible Au precipitation in PyII and PyIII, thus indicating the substitution mechanism of Au = Fe . However, the chemical state of invisible gold in pyrite remains controversial . The fact that Co, Ni, and Cu exhibit positive correlations with Au (Figure 13f,k) and that there is a negative correlation between Au + Cu + Co + Ni and Fe reflect the coupled reaction of Au+ + Cu+ + Co2+ + Ni2+ ↔ 3Fe2+ (Figure 14c) .
There are two alternative mechanisms of gold release: (1) gold is remobilized from the sulfide lattice by a solid-state process or a coupled dissolution–reprecipitation reaction (CDRR) [3,55,56,57,58,59]; or (2) it is introduced during the overprinting caused by hydrothermal fluids [3,57]. EPMA X-ray elemental maps also show that invisible gold is uniformly distributed within arsenian pyrite, whereas visible gold fills microfractures in PyII or occurs as inclusions in PyIII (Figure 6, Figure 7f, Figure 8f, Figure 9e and Figure 10e). Co, Ni, and As preserve initial oscillatory zonation patterns in PyII, but they are uniformly distributed in PyIII (Figure 9). A fluid-assisted method of remobilization would explain why some elements are remobilized but others (Te, Se, Co, and Ni) are clearly not, in addition to supporting the observation of micropores tied to the observed nanoscale and fine particles . The concentrations of Co, Ni, Se and Te are higher in PyIII than in PyII (Figure 11). Therefore, we conclude that the possible upgrading of Au may have occurred if either the fluid became supersaturated and precipitated the majority of gold at end of the mineralization or if it underwent remobilization by CDRR (e.g., Figure 6e,i) .
5.3. Implications for Fluid Evolution
The textures and trace element compositions of pyrite can potentially be used as robust indicators of fluid composition [15,43,53,61,62].
More reducing conditions occurred in stage III, as demonstrated by the minor redissolution of pyrite and arsenopyrite (Figure 6d,g) [63,64] and the change from a pyrite-arsenopyrite assemblage to a pyrite-pyrrhotite assemblage [65,66,67]; this would have promoted the deposition of gold and especially caused the conversion of pyrite (high sulfur content) to pyrrhotite (low sulfur content) in stage III, which is associated with native gold (Figure 6i) [65,66,67,68,69].
The Co/Ni ratio in pyrite has been used by many authors as an empirical indicator of the depositional environment [15,70,71,72]. The presence of chemical zoning in the pyrite of the Zhengchong gold deposit may reflect fluctuations in temperature and pH [15,72]. Co/Ni ratios (<1) with low standard deviations are generally accepted to represent pyrite of sedimentary origin [18,46,47,71]. The Co/Ni ratios of the arsenian pyrite in the Zhengchong gold deposit vary; in general, the average Co/Ni values are 0.15 in PyI, 0.16 in PyII, and 0.38 in PyIII (Figure 15a), thus apparently indicating that the mineralization may have been caused by metamorphic hydrothermal fluids . The higher Co and Ni concentrations (Figure 13d) and higher Co/Ni ratios (Figure 15a) observed from PyII to PyIII may result from a decrease in temperature [15,72].
Variations in Se and Te have commonly been attributed to changes in fluid temperature and redox conditions [53,73,74,75,76,77]. Slightly neutral, mildly reduced fluids can efficiently transport Au, as Au(HS)2− can also carry ~100 ppb Te at 300 °C [77,78]. Te-rich pyrite precipitates from reduced fluids due to fluid boiling and fluid-rock interactions [53,79]. In contrast, Se-rich pyrite usually forms from low-temperature fluids, regardless of the fO2 and pH values of the fluids [53,79]. The concentrations of Se and Te are higher in PyIII than they are in PyII (Figure 11). The average Se/Te values are 87.19 in PyI, 72.48 in PyII, and 5.36 in PyIII (Table 3). The Se/Te ratios decrease from PyI to PyIII (Figure 15b,c). The clear differences in the Se and Te concentrations and Se/Te ratios between PyII and PyIII represent two different conditions. These data reveal that temperature is the key factor in this change and that the temperature was lower during stage III [5,79].
Field and petrographic observations of most gold deposit in NEHP generally reveal four-stage mineral paragenesis, especially the quartz-pyrite-arsenopyrite and quartz-polysulfide stages, like the Zhengchong gold deposits (e.g., Wangu, Huangjindong) , which also consistent with worldwide orogenic gold deposit belts (e.g., the Yilgarn Craton) [80,81]. Belousov et al. (2016) subdivide orogenic gold deposit in the Yilgarn Craton into Au-As and Au-Te groups based upon pyrite geochemistry, which may correspond to the PyII and PyIII (Figure 11 and Figure 13a) . For example, in the Yilgarn Craton, 65% of orogenic gold deposit pyrites have Co/Ni < 1, Au–As association orogenic gold deposits show Se/Te > 5, in contrast of Au-Te ores . These characteristics fit the Zhengchong gold deposit (Figure 15).
Our textural observations, pyrite generations, EPMA data, LA-ICP-MS data and elemental maps demonstrate the complexities of the pyrites at Zhengchong, which reflect the evolution of hydrothermal events. The results can be used to pinpoint the evolution of fluid during the genesis of the Zhengchong orogenic gold deposit.
- Four stages and three generations of pyrite can be distinguished at Zhengchong. The earliest sulfide mineralization produced unzoned, trace element-poor pyrite (PyI). Significant amounts of oscillatory zoned and As-, Co-, and Ni-rich pyrite (PyII) formed coevally with arsenopyrite and tetrahedrite mineralization. PyII is overprinted by later hydrothermal fluids. Then, inclusion-rich PyIII precipitated in massive sulfide veins or veinlets. PyII and PyIII are arsenian pyrite and represent the main Au-bearing minerals.
- PyI records the lowest concentrations of Au. PyII and PyIII have similar amounts of Au, Cu, Pb, Zn, and Bi, but PyIII is more enriched in Co, Ni, Te, and Se. The substitution of As, Se and Te for S and that of Co and Ni for Fe occurred by direct-ion exchange.
- The EPMA X-ray elemental maps and LA-ICP-MS point analyses show that invisible gold is uniformly distributed within the arsenian pyrite and that visible gold fills microfractures in PyII or occurs as inclusions in PyIII. Co, Ni, Cu exhibit positive correlations with Au and a negative correlation between Au + Cu + Co + Ni and Fe reflect that Fe vacancies may have been a major cause of the precipitation of invisible Au and other metal elements in pyrite structure.
- There are systematic trace element differences between the three generations of pyrite (PyI, PyII, PyIII). The more Co, Ni and Se, Te substitutions that occurred for Fe and S, respectively, the greater the increase in the Co/Ni ratio (<1) and decrease in the Se/Te ratio (<10) in stage III, respectively, indicating that more reduced, lower-temperature metamorphic hydrothermal fluid was present in stage III.
Y.-J.S. and W.-S.W. conceived and designed the experiments; Y.Z. performed the experiments; W.-S.W. and Q.-Q.L. analyzed the data; W.-S.W., Q.-Q.L. and Y.-J.S. wrote the paper.
This research was jointly funded by General Financial Grant from the China Postdoctoral Science Foundation (2017M622596) and Hunan Province Geological Scientific Research project of Land and Resources of Hunan Province (2016-04).
We sincerely appreciate the detailed and constructive reviews and suggestions from two anonymous reviewers, which greatly improved this paper. We especially thank Yu Zhang for helping with the LA-ICP-MS trace element analyses.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1. (a) Simplified map of South China; (b) geological sketch map of northeastern Hunan Province showing the tectonic, magmatic and stratigraphic framework as well as the dominant Au-, Cu- and Co (-polymetallic) ore deposits (modified after ).
Figure 2. (a) Geological map of the Zhengchong gold deposit; (b) cross-section showing geological features of the ore and the host rocks of the deposit, including sampling location.
Figure 3. Photographs of various vein types in the Zhengchong Au deposit. (a) Disseminated pyrite and arsenopyrite in vein and country rock; (b) representative NE-trending laminated quartz veins. (c) Laminated quartz vein (NE) cuts through an earlier quartz-pyrite vein (NW) in granodiorite. (d) Stockwork veins and massive pyrite vein are cut by disseminated pyrite vein, and they are all cut by quartz-carbonate vein. (e) Massive pyrite vein and laminated quartz vein cut by barren quartz-carbonate vein. (f) Disseminated pyrite vein and pyrite pebble.
Figure 4. Photographs of various alteration assemblages in the Zhengchong Au deposit. (a) Chlorite vein near quartz veins near massive pyrite vein and disseminated pyrite vein. (b) Crossed nicols photomicrograph of pyrite with quartz strain fringes exhibiting sericitization. (c) Crossed nicols photomicrograph of carbonatization in country rock. (d) Parallel nicols photomicrograph of chlorite inter-grown with arsenopyrite in quartz vein. (e) Parallel nicols photomicrograph of chlorite and carbonate around breccia pyrite in pyrite pebble. (f) Crossed nicols photomicrograph of altered granodiorite, including carbonatization, sulfidation, and sericitization. Qtz—quartz; Cal—carbonate; Ser—sericite; Ms—muscovite; Chl—chlorite; Ap—apatite; Py—pyrite; Rt—rutile.
Figure 5. Paragenetic sequence of the Zhengchong gold deposit.
Figure 6. Photomicrographs showing textures and features of metal minerals in Zhengchong gold deposit: (a) Reflected light image of isolated gold-barren PyI showing a fractured or corroded pyrite crystal. (b) BSE image of oscillatory zonation of PyII; lighter bands are As-rich. Arsenopyrite and tetrahedrite occur in pores of PyII-2. Chalcopyrite fills the fractures of PyII or occurs as overgrowth on PyIII in a quartz-carbonate vein. (c) BSE image of chalcopyrite and sphalerite cross-cutting arsenopyrite. (d) BSE image of galena filling the fracture of cataclastic pyrite. Sample 290-7 is from a large pyrite crystal in a veinlet. (e) Detail of minerals in fracture in Figure 4d showing the intergrowth of native gold, galena and sphalerite in fracture. (f,g) Earlier-formed chalcopyrite and pyrite, overgrowth of arsenopyrite, tetrahedrite wrapping native gold, and galena finally filling the fracture in a disseminated pyrite vein in stockwork. (h) Galena, pyrrhotite, and chalcopyrite occurring as inclusions in PyIII from a large pyrite pebble in veinlet shown in Figure 3f. (i) Euhedral PyIII with significant amounts of native gold, galena, chalcopyrite and pyrrhotite inclusions in a massive pyrite vein. Py—pyrite; Asp—arsenopyrite; Ccp—chalcopyrite; Gn—galena; Sp—sphalerite; Po—pyrrhotite; Rt—rutile; Gold—visible native gold; Tt—tetrahedrite.
Figure 7. (a) Backscattered electron image of PyI from sample 330-9. (b–f) X-ray element distribution maps of Co, Ni, S, As, and Au in PyI. Cobalt and nickel show uniform distribution, but irregular arsenic enrichment occurs in isolated crystal PyI.
Figure 8. (a) Backscattered electron image of PyII from sample 290-8. (b–f) X-ray element distribution maps of Co, Ni, S, As, and Au in PyII. PyII contains weakly visible growth zoning indicating that cobalt, nickel and arsenic are present in similar concentric zones in these crystals.
Figure 9. (a) Backscattered electron image of PyII and PyIII from sample 290-7. (b–f) X-ray element distribution maps of Co, Ni, As, Au, and Pb in PyII and PyIII. PyIII replace cataclastic PyII, as shown in Figure 6d.
Figure 10. (a) Backscattered electron image of PyIII from sample 330-11. (b–f) X-ray element distribution maps of Co, Ni, As, Au, and Pb in PyIII. The inclusion-rich PyIII contains inclusions of galena, sphalerite, chalcopyrite, tetrahedrite, pyrrhotite and native gold; the distributions of As, Co, and Ni are heterogeneous in inter; oscillatory zonation is present in outer; and the distributions of Au and Pb are present in inclusions.
Figure 11. Comparative box plot of the LA-ICP-MS data of three generations of pyrite, illustrating the concentration ranges of specific trace elements and highlighting significant relative enrichments. The red box indicate PyI, the green box present PyII, and the blue box shows PyIII. The outlier present the anomalous data, error bars represent SD.
Figure 12. Time-resolved record of the signal recorded on the LA-ICP-MS (in counts per second) during a single analysis. (a) PyI-low Au and As, but Pb + Zn + Cu and Bi-bearing inclusions; (b) PyII-high contents of Au, As, Ni, and Pb-bearing inclusion.
Figure 13. Binary plots of selected elements in different generations of pyrite from the Zhengchong gold deposit. (a) S and As data are from Table 2. Correlation between other elements from Table 3: (b) plot of Fe vs. Pb; (c) plot of Pb vs. Bi; (d) plot of Ni vs. Co; (e) plot of Fe vs. Ni; (f) plot of Au vs. Ni; (g) plot of Se vs. Te; (h) plot of S vs. Te; (i) plot of Au vs. Te; (j) plot of Fe vs. Cu; (k) plot of Cu vs. Au.
Figure 15. (a,b) Comparative box plot of the geochemistry of all generations of pyrite (LA-ICP-MS data), illustrating the range of ratios of Co–Ni and Se–Te. (c) Scatter plot of Se/Te and Co/Ni ratios, illustrating the distinct differences between PyII and PyIII (LA-ICP-MS data).
Table 1. Location, host rock, general description and ore minerals of the analyzed samples from the Zhengchong deposit (wt %).
|Sample No.||Location||Host Rock||General Description||Oxide and Sulfide Minerals||Associated Gangue|
|290-7||290 m Level||granodiorite||Coarse-grained pyrite in veinlet (0.5 cm)||Au, Py, Apy, Gn, Sp||Qtz, Cal|
|290-8||290 m Level||granodiorite||Quart-carbonate vein, pyrite alone the boundary||Py, Apy, Tt||Qtz, Cal|
|D036-1||290 m Level||slate||Coarse-grained pyrite in veinlet (2 cm)||Py, Po, Ccp, Rt||Qtz, Dol, Sd, Chl|
|240-3||240 m Level||slate||Quartz veins, including residual slate||Py, Apy||Qtz, Sd, Dol, Chl|
|330-9||330 m Level||slate||Disseminated Sulfides in slate||Py, Apy, Tt||Qtz, Sd, Ser|
|330-10||330 m Level||granodiorite||Disseminated Sulfides in granodiorite||Au, Py, Apy, Sp, Gn, Tt, Rt||Qtz, Ms, Ap, Ser, Cal|
|330-11||330 m Level||slate||Massive pyrite vein||Au, Py, Po, Ccp, Sp, Gn||Qtz, Ser|
|330-12||330 m Level||slate||Quart-pyrite vein from stockwork, Pyrite as a line within vein.||Au, Py, Apy, Ccp, Sp, Gn, Tt||Qtz, Cal|
Abreviations: Au—native gold; Py—pyrite; Asp—arsenopyrite; Ccp—chalcopyrite; Gn—galena; Sp—sphalerite; Po—pyrrhotite; Tt—tetrahedrite; Rt—rutile; Qtz—quartz; Cal—carbonate; Sd—siderite; Dol—dolomite; Ser—sericite; Ms—muscovite; Chl—chlorite; Ap—apatite.
Table 2. Selected EPMA analyses of different generations of pyrite from the Zhengchong deposit (wt %).
|n = 7||Av.||0.37||53.74||47.18||0.05||0.03||0.08||0.14|
|n = 7||Av.||1.49||52.73||46.96||0.07||0.04||0.12||0.09|
|n = 7||Av.||3.01||51.58||46.40||0.06||0.05||0.15||0.10|
|n = 7||Av.||1.63||52.48||46.90||0.06||0.04||0.11|
|n = 15||Av.||1.72||52.34||46.72||0.06||0.06||0.10||0.11|
Table 3. Selected LA-ICP-MS analyses of different generations of pyrite from the Zhengchong deposit.
|Pyrite Type||S, As, Se (wt %)||Fe (wt %)||As (ppm)||Au (ppm)||Co (ppm)||Ni (ppm)||Cu (ppm)||Zn (ppm)||Pb (ppm)||Ag (ppm)||Se (ppm)||Te (ppm)||Sb (ppm)||Bi (ppm)||Co/Ni||Se/Te|
|n = 4||Av.||53.14||46.39||3647.71||0.53||0.40||3.18||76.06||311.00||214.99||0.22||9.63||0.19||7.65||6.08||0.15||72.08|
|n = 6||Av.||52.20||46.08||16,986.38||11.91||7.48||60.62||10.20||0.40||91.81||0.07||18.66||0.27||7.45||2.30||0.12||94.67|
|n = 3||Av.||51.30||45.73||29,191.95||38.01||52.07||225.19||42.98||0.68||161.72||0.12||12.62||0.41||10.51||3.47||0.21||36.86|
|n = 10||Av.||51.92||45.98||20,838.69||9.50||8.92||57.01||18.01||0.44||162.75||0.13||4.97||0.11||10.38||3.10||0.20||51.16|
|n = 12||Av.||52.24||46.03||15,870.44||15.92||164.57||444.62||11.87||0.67||159.95||0.13||66.97||20.80||8.49||3.01||0.37||5.31|
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