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Geosciences 2018, 8(4), 120; https://doi.org/10.3390/geosciences8040120
Geology and Isotope Systematics of the Jianchaling Au Deposit, Shaanxi Province, China: Implications for Mineral Genesis
School of Jewellery, Guangzhou College South China University of Technology, Guangzhou 510800, China
Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China
Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou 510006, China
Centre for Exploration Targeting, The University of Western Australia, Crawley, WA 6009, Australia
MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China
Institute of Deep-Sea scicence and Engineering, CAS Sanya 572000, China
Guangdong Institute of Marine Geological Survey, Guangzhou 510080, China
Authors to whom correspondence should be addressed.
Received: 9 March 2018 / Accepted: 29 March 2018 / Published: 3 April 2018
The giant Jianchaling Au (52 t Au) deposit is located in the Mian-Lue-Yang Terrane in the southern part of the Qinling Orogen of central China and is hosted by metamorphosed carbonate rocks of the Late Neoproterozoic Duantouya Formation. The deposit consists of multiple generations of mineralised quartz(-carbonate) veins in WNW-trending extensional ductile-brittle shear zones. Based on the mineral assemblages and cross-cutting relationships between the quartz(-carbonate) veins, the paragenesis is characterised by an early coarse-grained pyrite-pyrrhotite-pentlandite-dolomite-quartz assemblage (I), followed by pyrite-sphalerite-galena-carbonate-arsenopyrite-fuchsite-carbonate-quartz containing gold (II), and fine-grained pyrite-dolomite-calcite-quartz-realgar (As2S2)-orpiment (As2S3) (III). The H-O-C isotope systematics for the three vein sets indicate that the mineralising fluid is probably sourced from the metamorphic dehydration of carbonate rocks in the Duantouya Formation, and gradually mixed with meteoric water during the emplacement of the third vein set. The δ34S values for sulfides (6.3–16.6‰) from the second auriferous vein set are greater than zero, indicating sulfates reduction from the Neoproterozoic metamorphic rocks (Duantouya Fm). The (206Pb/204Pb)i ratios from pyrite (17.521–18.477) from each of the vein sets overlap those of the ultramafic rocks (18.324–18.717) and the Bikou Group (17.399–18.417), indicating that the units are possible sources for the sulfides in the mineralisation. Both εNd(t) and Isr(t) of sulfide overlap with the meta-ultramafic field and Duantouya formation and dominated with mature Sr-Nd character, which indicated that the Duantouya may play an important role during the ore formation and there may exist a minor ultramafic source that is involved in the ore fluid. The S-Pb-Sr-Nd isotopic ratios are closely related to those of the Bikou Group and Duantouya Formation, which indicates that the mineralised fluid has interacted with both units. Combining the previously published data with data from this study on the mineralised area, we surmise that Jianchaling is characteristic of an orogenic-type gold deposit related to the Triassic Qinling Orogeny associated with continental collision.
Keywords:Jianchaling Au deposit; isotope geochemistry; ore genesis; orogenic-type gold deposit; Qinling Orogen; central China
The different tectonic events are accompanied by different types of mineralisation and associated hydrothermal alteration of different ages [1,2,3], including orogenic-Au deposits [4,5,6]. Many of these deposits record multiple deformation events, making then difficult to study and leading to uncertainty in their genesis [7,8,9]. Nonetheless, some of the deposits have been identified as orogenic-types, such as the Juneau Gold Belt in America , the Sukhoi Log gold deposit in Russia , the Victorian gold province in SE Australia , the Jiaodong gold province in China  and the Shanggong, Wenyu and Huachanggou Au deposits, and Tieluping, Weishancheng, Yindonggou, Wangpingxigou, Dahu and Zhifang Ag-Au-Mo deposits [14,15,16,17,18,19,20,21,22,23,24,25,26] in China of the Qinling Orogen.
Other deposits present in the Qinling Orogen include porphyry-, skarn-, breccia pipe-, hydrothermal-, Carlin-, and epizonal-types, which have been studied in detail [1,27,28,29,30,31,32,33,34,35,36,37,38]. Orogenic gold deposits recognised in the orogen include Jianchaling, Lijiagou and Huachanggou [8,25,39], of which Jianchaling is the largest (Figure 1). However, the genesis of the giant Jianchaling Au deposit is still poorly understood.
The Jianchaling Au deposit is located in the triangular area marked by Mianxian, Lueyang, and Yangpingguan towns (MLY), in the South Qinling Terrane of the Qinling Orogen (Figure 1c). The deposit has a measured resource of 52 t Au with an average grade of 5 g/t hosted by quartz-veins in a ductile-brittle shear zone [21,39,40,41,42,43]. Even though the genesis of the mineralisation at the deposit has been studied, the source of the metals and nature of the mineralising fluids have remained unclear.
This contribution summarises the geology, mineralisation, and H–O–C–S stable and Pb–Sr–Nd isotopic systematics for the giant Jianchaling deposit with the aim to clarify the source of the metals and nature of the mineralising fluid. This, in turn, provides a better understanding of the tectonic and mineralising processes in MLY.
2. Geological Setting
The Qinling Orogen is located south of the North China Block (NCB) and has experienced a complex tectonic history [44,45,46]. It is located in the central portion of the east-trending Central China Orogen, and developed during the Mesozoic collision between the North China and Yangtze blocks (Figure 1a,b) [1,47,48,49]. The Luanchuan Fault is the northern boundary of the orogen, and the southern boundary is the Longmenshan Fault. Deformation associated with the orogen extends into the Huaxiong Domain that is also included in the southern part of the Precambrian North China Block. The subdivisions of the orogen are shown in Figure 1b.
The Jianchaling deposit is a fault-controlled lode-gold deposit hosted by the Neoproterozoic Duantouya Fm in the eastern part of the MLY (Figure 1). The group unconformably underlies Late Paleozoic turbiditic successions in the Songpan-Ganzi Basin situated between the Mian-lue Suture to the north and Han Jiang Suture to the south (Figure 1c). The area is an important part of the Qinling Orogen and hosts many mineral deposits (e.g., Au, Cu, Ni, Zn, Pb, Mn) in various lithological and structural settings (Figure 1c), and is commonly known as the Golden Triangle .
The rock-types in the MLY area include metamorphosed Neoarchean granite-greenstones in the Yudongzi Complex, Neoproterozoic volcanic sequences in the Bikou Group, Late Neoproterozoic calcareous units in the Duantouya and Jiudaoguai formations, and Paleozoic carbonate in the Lueyang Formation (Figure 2). The Bikou Group is widespread in the area and consists of greenschist to amphibolite facies felsic and mafic volcanic protoliths. Various generations of metamorphism and ductile to brittle NE- and NW-trending faults (including thrusts and basin-forming extensional faults) have developed in the area, which control the location of felsic, mafic and ultramafic intrusive rocks and mineralisation (Figure 2).
3. Geology and Mineralization of the Jianchaling Area
Rocks outcropping in the Jianchaling area Neoarchean Yudongzi Complex, Neoproterozoic Bikou Group, Late Neoproterozoic Duantouya and Jiudaoguai formations, and Paleozoic Lueyang Group (Figure 2). The deposit can be subdivided into northern and southern zones containing five lenticular mineralisation in the WNW-trending Hejiayan and NW-trending Xiqugou faults and their splays. The richest mineralisation is the “No.1 Au orebody” that is 1950 m long, up to 23 m wide, and dips >43° N.
The Hejiayan and Xiqugou faults are brittle-ductile shear zones , and host mineralised lenticular quartz veins at the contact between ultramafic rocks and calcareous beds in the Late Neoproterozoic Duantouya and Jiudaoguai formations (Figure 2 and Figure 3). The quartz veins shallowly dip between 20° and 40° WSW is places in steeply dipping sections of the fault.
The mineralisation includes native gold and lesser amounts of electrum and native silver associated with pyrite and minor amounts of hematite, magnetite, limonite, chromite, marcasite, realgar, and orpiment. The gangue minerals are dolomite, calcite and quartz, with minor amounts of serpentine, fuchsite, and albite (Figure 4a–c) .
Yue [21,39] recognized early deformed and recrystallised quartz—pyrite—carbonate vein crosscut by the quartz—gold—fuchsite—arsenopyrite—pyrite—magnetite second generation vein set, and the third generation of dolomite—calcite—quartz—fine-grained pyrite—realgar—orpiment veins (Figure 4c). The pyrite associated with each of the vein sets are also distinct from each other. The first vein set contains pyrite that is >2 mm across, variably fractured, anhedral to subhedral in shape, and intergrown with euhedral fuchsite and serpentine (Figure 4d,e). The second pyrite generation is not deformed, subhedral, 1–2 mm across, and commonly has cores or inclusions of the first generation pyrite, chalcopyrite, and sphalerite (Figure 4f,g). This pyrite is also commonly altered to hematite with martite pseudomorphing magnetite (Figure 4h). The third generation of pyrite is <10 μm across, subhedral to euhedral, and present in alteration zones containing the third vein (Figure 4i). The paragenesis is illustrated in Figure 5.
4. Sampling and Analytical Methods
Fresh samples were collected from each of the vein sets at depth in the Jianchaling mine. The samples were examined in thin section to confirm their paragenetic relationship. The sample characteristics are listed in Table 1.
Six samples of pyrite from the second (mineralised) vein set and five samples of the Duantouya Formation and meta-ultramafic from the wall-rock were collected for lead, strontium, and neodymium isotopic studies. The samples were crushed into power.
The samples were then crushed to a minus 10 mesh size (420 microns, and sulfide and carbonate fragments were handpicked under a binocular microscope. Between 10 and 50 mg of the powder was leached in acetone and washed in distilled and deionised water to remove contamination, then dried in an oven at 60 °C. The samples were then dissolved in a solution of HF + HNO3 + HClO4, dried, and redissolved in 6 N HCl, redried, and redissolved in 0.5 N HCl (for Sr and Nd separation) or 0.5 N HBr (for Pb separation). The Sr and Nd fractions were separated following standard chromatographic techniques using AG50 × 8 and PTFE–HDEHP resins with HCl as eluent. The Pb fraction was separated using a strong alkali anion exchange resin with HBr and HCl as eluents.
The lead isotopes were analysed using a MAT-261 thermal ionization mass spectrometer with the standard NBS 981 in the Analytical Laboratory at the Beijing Research Institute of Uranium Geology, China. Measurements of the common-Pb standard NBS 981 gave average 208Pb/206Pb values of 2.1681 ± 0.0008, 207Pb/206Pb of 0.91464 ± 0.00033, and 204Pb/206Pb values of 0.059042 ± 0.000037; the uncertainty is of <0.1% at the 95% confidence level. Some of the U, Th and Pb values were used to estimate the Pb isotope ratios assuming an age of 198 Ma, which is the 40Ar/39Ar date determined on fuchsite from the Jianchaling deposit . These Pb isotopic ratios are presented as (208Pb/204Pb)i, (207Pb/204Pb)i and (206Pb/204Pb)i.
A TRITON thermal ionization mass spectrometer was used to measure the Sr and Nd isotopes in the Analytical Laboratory at the Tianjin Institute of Geology and Mineral Resources, China. The 87Sr/86Sr isotope ratios were normalized against the 86Sr/88Sr = 0.1194 and 143Nd/144Nd isotope ratios to 146Nd/144Nd = 0.7219. The JNdi Nd-Standard yielded 143Nd/144Nd ratios of 0.512118 ± 0.3 against a reference value of 0.512115 ± 0.7 , and the NBS 987 Sr standard with a reference level of 0.710248 was used yielding 87Sr/86Sr ratios of 0.710250 ± 0.7. The Sr and Nd isotopic compositions were measured with a thermal ionization ISOPROBE-T mass spectrometer.
5. Isotope Results
The following are isotopic values determined from samples from the Jianchaling deposit.
5.1. Carbon, Oxygen and Hydrogen Isotopes
The δ13C, δ18O and δD values from quartz and carbonate from each of the three vein sets and the surroundings rocks are listed in Table 2. These results are published on . The δ13C values for first vein set ranges from −4.4 to 0.6‰, the δ13C values for the second (mineralised) vein set ranges from −2.9 to −0.4‰, and the δ13C value for the third vein set is between −4.4 to 2.2‰ (Table 2).
The δ18O values from carbonate and quartz for the first vein set forms two groups of 18.9–23.4‰ and 14.0–17.3‰. The δ18O values from the second vein set are between 13.8 and 19.0‰ (averaging 16.4‰), and for the third vein set are between 11.9 and 16.3‰ (Table 2).
5.2. Sulfur Isotopes
The δ34S isotopic values for the sulfides samples are listed in Table 3 and cited from . The Jianchaling deposit is characterized by highly δ34S positive values with a narrow range. The first vein set has a δ34S value of 14.3‰ (Table 3). The second vein set has δ34S values between 8.2 and 14.3‰. One shale rock sample of the Duantouya Formation have δ34S values of 16.6‰. Two samples of the metamorphosed ultramafic rocks (listwanite and serpentinite) have δ34S values of 6.1 to 8.6‰.
5.3. Lead Isotopes of Sulfides and Wall Rocks
The Pb isotopic analyses completed for this study and previous data are listed in Table 4. In order to compare the contribution of the wall rocks, we also put the lead isotopes of the wall rocks including Yudongzi Fm, Porphyritic granite, Duantouya Fm and Bikou Group in Table 3. Sulfides from gold deposits have 206Pb/204Pb values between 17.257 and 18.477, 207Pb/204Pb values between 15.530 and 15.704, and 208Pb/204Pb values of 36.927 to 38.757 (Table 4). Their calculated (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i values are the same as the measured values due to their very low U and Th contents. Therefore, the analytical data is interpreted as being reliable and representative of the source for metals in the mineralising fluid. The ultramafic rocks have 206Pb/204Pb values of 17.952–19.193, 207Pb/204Pb values of 15.520–15.785, and 208Pb/204Pb ratios of 36.029–38.920 (Table 4).
5.4. Strontium and Neodymium Isotopes of Sulfides and Wall Rocks
Four pyrite samples from the gold-bearing second generation vein set have ISr values in the range 0.706709–0.715929 (average of 0.711973, Table 5), which is similar to that of the meta-ultramafic rocks. The (143Nd/144Nd)i values of the samples are between 0.511376 and 0.512453 (average of 0.511823, Table 6), and the εNd values are between −19.6 and 1.4.
6.1. Sources of Metals and Mineralising Fluids
6.1.1. Oxygen and Hydrogen Stable Isotopes
The oxygen and hydrogen stable isotopes are often used to indicate the source of mineralised hydrothermal fluids . However, the overlap in the metamorphic and magmatic fields in δD versus δ18O diagrams creates uncertainty in deciphering the genesis of mineralisation . This is the case for the mineralising fluid at Jianchaling, which has been interpreted as being magmatic [64,65], meteoric , mixed metamorphic and magmatic [42,43,51], and primarily metamorphic . This places a significant doubt on the usefulness of the δD and δ18O values in directly pinpointing the origin of the mineralising fluid at Jianchaling.
Using fluid-inclusion homogenisation temperatures and detailed paragenetic studies by Yue  (Table 2), the δ18Owater values were calculated using the quartz–water equation by  and carbonate–water equation by . The calculated quartz-carbonate δ18O values for the hydrothermal fluid associated with the first vein set range from 7.5 to 17.7‰, the mineralised second vein set have δ18O values between 5.7 and 11.1‰, and the third vein set range from 1.3 to 4.3‰ (Table 2). Using this data, the δ18O and δD values for first vein set plot in the magmatic field and close to the metamorphic field (Figure 6). However, assuming that magmatic fluids are generated above the lowest eutectic point at temperatures above 573 °C , continuous cooling and water-rock reactions result in reduction of δ18Owater values during the crystallisation of hydrothermal minerals such as quartz and alkali feldspar . Furthermore, magmatic fluids have average δ18Owater values of 18.8‰ at temperatures of 355 °C , which is the average homogenization temperature of fluid inclusions from first vein generation (Table 2). This means that the initial δ18Owater value must have been higher than 18.8‰ at temperatures above 355 °C, which is higher than the 5.7–11.1‰ range for the mineralised second vein set. In addition, the Late Triassic to Early Jurassic plutons in the southern part of the Qinling Orogen commonly have δ18O values of <18.8‰. The high δ18O values for the early hydrothermal fluids are consistent with a major contribution from metamorphic sources (rather than a minor contribution as suggested by Figure 6). This interpretation is supported by the low salinity and high CO2 content of fluid inclusions in the first vein set [7,39]. Furthermore, the δ18O values for quartz in granite porphyry and aplite in the area range between 11.3 and 14.5‰ . This indicates that the δ18Owater value of 10.0‰ calculated at a temperature of ~573 °C for the first vein set cannot exceed the δ18Owater value of 13.2‰ for the second vein set that crystallised at ~300 °C (as determined from the fluid inclusion homogenisation temperatures). In addition, the regional metamorphism in the Jianchaling area is at the greenschist facies , further supporting a ~300 °C temperature for the crystallisation of the mineralised second vein set.
The 3rd vein δ18Owater values are between 1.3 and 4.3‰, with a δD value of −81‰, they are plot in the meteoric field in Figure 6 and Figure 7. The δ18Owater values of carbonate-quartz from the mineralised vein set vary between 5.7 and 11.1‰, which are values between those for the first and third vein sets. Exactly, on the base of the δ18Owater values (5.7–11.1‰), we think the 2nd vein set is similar to lode gold deposits and add the possible range of the 2nd vein set in Figure 6 that based on the feature of many lode gold deposits worldwide. This is indicative of the mixing of metamorphic and meteoric fluids during mineralisation (Figure 6).
The δ 18O values for quartz from lode-gold deposits around the world are higher than 10‰ with the values for the mineralising fluids ranging between 5 and 25‰ . The δ18Owater values of the gold deposits on the Jiaodong Peninsula of eastern China range from 4.9 to 10.9‰, with corresponding δ D values of −78 to −101‰ (Figure 6) [69,70]. The δ 18O and δD values for mineralising fluids at Jianchaling plot in the lode-gold deposit field (Figure 6; Table 2; ). Figure 6 also shows that the first vein set containing higher δ18O and δD values are sourced from metamorphic fluids. In contrast, the mineralised second vein set are similar lode gold deposits, and the fluids related to the third vein set have (again) a composition close to that of meteoric fluid.
6.1.2. Carbon and Oxygen Isotopes
The δ13CCO2 values determined for quartz-carbonate from each of the vein sets are essentially the same, hovering around approximated zero (Figure 7). The corresponding δ18OSMOW values show a significant trend with the first vein set being approximately 15‰, the mineralised second vein measuring around 9‰, and the third being around 3‰ (Figure 7). The implication for this trend is that all of the vein sets have similar sources for carbon with a gradual decrease in available oxygen in the hydrothermal fluid with time. Another implication is that there was only one hydrothermal fluid that has changed in composition with time.
The δ13CCO2 values for the hydrothermal fluids related to the three vein sets are significantly higher than the values for organic matter (averaging −27‰) , the content of CO2 in the atmosphere (−11 to −7‰ ), freshwater carbonate (−20 to −9‰ ), magmatic rocks (−30 to −3‰ ), the continental crustal (−7‰ ), and the mantle (−7 to −5‰ ). However, marine carbonate (i.e., −3 to 2‰ δ13CCO2 ) and carbonate in the wall rock (i.e., −0.4‰ to 2.3‰ δ13CCO2; Table 2), are similar in δ13CCO2 composition. This indicates a local source for the hydrothermal fluids related to each of the vein sets. In addition, the close relationship between the ultramafic rocks and the vein sets shown in Figure 7 indicates that the ultramafic rocks are also local sources for the carbon and oxygen in the hydrothermal fluids.
On the other hand, the δ13CCO2 and δ18OSMOW of the mineralised fluids overlap with those of the forming fluid of Archean Au deposits (Figure 7). δ18OSMOW value of the third vein sets are similar with the meteoric water, indicating meteoric water mixed into the fluid system in late stage.
6.1.3. Sulfur Isotopes
The δ34S signatures for various in the MLY and others from throughout the world are presented in Figure 8. The figure shows that most of the sulfides in the MLY have δ34S signatures greater than zero, are predominantly higher than those for orogenic Au deposits throughout the world, and higher than those for basaltic magmatic rocks, overlap the sulfide values in ultramafic rocks, and are coincide with the high end of sulfides in metamorphic rocks. In fact, the δ34S signatures for the metamorphic rocks are the best fit, which includes the wallrocks at Jianchaling (Table 3). Such heavy δ34S values for the sulfides at Jianchaling is consistent with derivation from sulfates that are characterised by heavy δ34S values [77,78]. It is envisages that organic material derived by the metasedimentary rocks in the study area would have reacted with sulfate resulting in the crystallisation of sulfides [78,79]. Examples of such reducing agents are CH4, C2H6, H2S and graphite, which have been detected in fluid inclusions using Laser Raman . The conversion of sulfate to sulfide will proceed in the following chemical reaction:
CH4 + SO42− ←→ H2S + H2O + CO32− and H2S + Fe2+ ←→ FeS + 2H+
Therefore, the Neoproterozoic metamorphic rocks in the MLY are likely sources for sulfur in the Jianchaling area.
6.1.4. Lead Isotopes
The country rocks in the Jianchaling area have a wider range in Pb isotopic (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) ratios than the sulfides in the Jianchaling mineralisation (Table 4). The range of the Pb-isotopes is shown in Figure 9 that includes trends defining tectonic settings . The range of Pb-isotopes shown in Figure 9 correlates with defined tectonic setting of orogenic and upper crustal fields, even though there is a limited number of 10 data points. This is in agreement with the observations made in the discussion section above.
Furthermore, these Pb signatures of the sulfides from the mineralisation overlap those of the ultramafic rocks and the Bikou Group, indicating that the units are possible sources for the sulfides in the mineralisation. In contrast, the distinct incongruence of the sulfides from the mineralisation and the granitic rocks indicates that the granitic rocks are not the source for the sulfides in the mineralisation (Figure 9).
6.1.5. Strontium and Neodymium Isotopes
The ISr(t) values of sulfide samples from the Jianchaling deposit range from 0.706709 to 0.715929 (Table 5; Figure 10a). These values are higher than the mean values (0.703 to 0.705) established for mantle-derived oceanic basalt (MORB, OIB, IAB) and continental basalt, and are suggestive of a crustal source [84,85].
The ISr(t) values for the sulfide samples from the deposit are also lower than those for the Duantouya Formation (Table 5), and the (143Nd/144Nd)i sulfide ratios for the mineralised second vein set overlap with those of the Duantouya Formation and the meta-ultramafic rocks (Table 6; Figure 10a,b). The εNd (t) and Isr (t) of sulfide are indicative of a crustal source and dominated with mature Sr-Nd character (Table 5 and Table 6; Figure 10a), and εNd (t) overlaps with meta-ultramafic field, indicated that the Duantouya may play an important role during the ore formation and there may exist minor ultramafic source been involved in the ore fluid.
The Bikou Group has ISr(t) values that overlap with those from the Mian-Lue ophiolite, meta-ultramafic rocks, partly with the second vein set, and are lower than those from the Duantouya Formation (Figure 10a; Table 5 and Table 6) [60,61,86]. Although the pyrite in audiferous rocks has ISr(t) values that overlap with porphyritic granite partly (Figure 10c), it is unlikely to come from porphyritic granite, the evidence is mainly from lead isotopes, as mentioned above.
These relationships can be explained with a mixing model where the mineralised sulfide-bearing fluids interacted with lithological units (Figure 10). This interpretation is supported by the conclusion drawn from the Pb isotope data collected from the Huachanggou gold deposit in the northwestern part of MLY [25,59] (Figure 1c).
6.2. Mineralisation Type
Various mineral-types have been proposed for the Jianchaling Au deposit. These include: (1) a magmatic hydrothermal source associated with Triassic magmatism ; (2) Carlin-like affiliations ; (3) orogenic affiliations ; and (4) sourced from structurally controlled altered rocks [40,53].
Fuchsite separated from the mineralised second vein set yield a well-defined 40Ar/39Ar isotopic age of 198 ± 2 Ma, which suggests that the mineralisation took place during the Triassic Indosinian Orogeny . Implications of this are that the Neoproterozoic (ca. 927 Ma) magma-hydrothermal event associated with the emplacement of ultramafic intrusions. Triassic (ca. 216 Ma) porphyritic granites in the area are ~18 Ma older than the Jianchaling mineralisation, again indicating that the granites are not related to the gold mineralisation.
The C, H and O stable isotopes indicate that the ore-forming fluids are predominantly metamorphosed in origin and contaminated with meteoric fluid during the deposition of the gold at Jianchaling. On the other words, the mineralising fluid is probably sourced from the metamorphic dehydration of carbonate rocks in the Duantouya Formation, and gradually mixed with meteoric water. The 87Sr/86Sr, ISr(t), εNd, Pb and S isotopic data (Figure 10) confirm that the sulfides in the mineralised are sourced from the metamorphosed Duantouya Formation, Bikou Group and minor ultramafic rocks.
Hydrogen, oxygen and carbon isotopic systematics indicate that the ore-forming fluids at the Jianchaling Au deposit progressively evolved from an early stage represented by the first vein set associated with deformation and metamorphism of the country rocks. This was succeeded by a late tectonic stage represented by the mineralised second vein set sourced from a mixed metamorphic and meteoric fluid, and progressed to a late stage represented by the third vein set. The S-Pb-Sr-Nd isotopic data indicate that the mineralising fluids were sourced locally around Jianchaling. From these observations, the Jianchaling Au deposit is interpreted as an orogenic Au deposit formed during the Mesozoic Indosinian Orogeny.
The National Natural Science Foundation (Numbers 41403032, 41502070 and 41202050), and the National Crisis Mine Prospecting Foundation (Number 20089934) financially supports this study. We are grateful to Wenping Zhu for help during isotope analyses and the Exploration Team ‘711’ of Northwest Mining and Geology Group Co., Ltd. for their help during field. Yanjing Chen, Huayong Chen and three anonymous reviewers are thanked for their constructive suggestions, which have greatly improved this manuscript.
Su-Wei Yue and Zhen-Wen Lin participated in field samples collection; Su-Wei Yue conceived and designed the experiments; Su-Wei Yue and Jing Fang performed the experiments; Deng-Feng Li and Leon Bagas analyzed the data; Su-Wei Yue wrote the paper, assisted by all other authors.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 4. Photomicrographs of samples from the Jianchaling Au eposit showing: (a) intergrown pyrite and fuchsite in the mineralised second vein set; (b) a second vein set with quartz containing pyrite; (c) third vein set with comb-like quartz, calcite and dolomite vein orpiment and realgar; (d) pyrite in the first vein set intergrowth; (e) fine-grained cataclastic pyrite following schistosity developed during brittle-ductile deformation before emplacement of the mineralised second vein set; (f) pyrite wrapping sphalerite and chalcopyrite in the mineralised second vein set; (g) pyrite in the first vein set overgrown by pyrite related to the mineralised second vein set; (h) hematite formed from pyrite, and martite; (i) euhedral fine-grained pyrite in the third vein set. Abbreviations: Cal = calcite; Car = carbonates; Cpy = chalcopyrite; Dol = dolomite; Fuc = fuchsite; Hm = hematite; Orp = orpiment; Py = pyrite; Q = quartz; Rea = realgar; Sph = sphalerite.
Figure 5. Paragenetic relationship of the minerals at the Jianchaling Au deposit .
Figure 8. δ34S values for hydrothermal sulfides from the Jianchaling Au deposit. Data sources: Basaltic rocks, Granitic rocks, Metamorphic rocks and Sedimentary rocks after [62,72,80]; Juneau gold belt, Bendigo Au, Kumtor Au and Major orogenic Au deposits in the world after [12,81]; Shanggong Au and Tieluping Ag after ; Yindongpo Au after ; Poshan Ag after ; Major Au deposits in the Qinling Orogen .
Figure 9. The plumbotectonic model for Pb isotopic systematics for the Jianchaling Au deposit. (a) 206Pb/204Pb versus 208Pb/204Pb; (b) 206Pb/204Pb versus 207Pb/204Pb.
Figure 11. Tectonic model for the Jianchaling Au deposit .
Table 1. The geological characteristics of samples from the Jianchaling Au deposit (modified from ).
|Sample NO.||Mineral||Sample Description||Stage||Analyzed Isotope|
|800-53-2||Dolomite||Dolomite veins in the Serpentinite with shear deformation||1||C, O|
|860-52-7||Dolomite||Dolomite veins in the Serpentinite with shear deformation||1||C, O|
|J-6||Dolomite||Pale green dolomite veins in the cataclastic dolomite||1||C, O|
|913-32-7||Calcite||Calcite veins in the cataclastic dolomite with a small number of pyrites||2||C, O|
|913-32-6||Calcite||Calcite veins in the cataclastic dolomite with a small number of pyrites||2||C, O|
|913-1||Dolomite||Milky white dolomite vein||3||C, O|
|913-2||Dolomite||Milky white dolomite vein||3||C, O|
|960-38-1||Dolomite||Milky white dolomite vein||3||C, O|
|860-51-5||Dolomite||Dolomite veins in the listwanite with a small number of pyrites||3||C, O|
|860-51-6||Dolomite||Dolomite veins in the listwanite with a small number of pyrites||3||C, O|
|hjy-8||Pyrite||Grain cluster pyrite with shear deformation||1||S, Pb|
|J3||Pyrite||Massive sulfide ore||2||S, Pb, Sr, Nd|
|J3||Pyrrhotite||Massive sulfide ore||2||S|
|hjy-1||Pyrite||Silicified and fuchsite dolomite ore rocks with disseminated pyrites||2||S, Pb, Sr, Nd|
|hjy-12||Pyrite||Silicified and fuchsite dolomite ore rocks with disseminated pyrites||2||S, Pb, Sr, Nd|
|hjy-10||Pyrite||Intensive silicified and fuchsite dolomite ore rocks with disseminated pyrites||2||S, Pb, Sr, Nd|
|hjy-20||Pyrite||Silicified and fuchsite dolomite ore rocks with disseminated pyrites||2||S, Pb, Sr, Nd|
|hjy-14||Pyrite||Weak silicified and fuchsite dolomite ore rocks with disseminated pyrites||2||S, Pb, Sr, Nd|
|H-4||Whole-rock||Shale (Duantouya Formation)||S, Pb, Sr, Nd|
|960-34-1||Whole-rock||Dolomite||S, Pb, Nd|
|960-28-1||Whole-rock||Dolomite||S, Pb, Sr, Nd|
|780-50-3||Whole-rock||Serpentinite||S, Pb, Sr, Nd|
|800-52-1||Whole-rock||Listwanite||S, Pb, Sr, Nd|
Table 2. C-H-O isotope composition of mineralisation at the Jianchaling Au deposit (modified from ).
|Sample No.||Sample||Vein Set||Mineral||δ18Omineral (‰)||Average δ18OWater (‰)||δDWater (‰)||δ13C V−PDB (‰)||Average δ13CCO2 (‰)||T# (°C)||Reference|
|MS-57||Au mineralisation||1||quartz||17.3||10.8 (8.3 to 12.1)||−72||310 (250 to 355)|||
|MS-79||Au mineralisation||1||quartz||17.0||10.5 (8.0 to 11.8)||−85||310 (250 to 355)|||
|MS-52||Au mineralisation||1||quartz||16.6||10.1 (7.6 to 11.4)||−79||310 (250 to 355)|||
|MS-77||Au mineralisation||1||quartz||14.0||7.5 (5.0 to 8.8)||−70||310 (250 to 355)|||
|800-53-2||Au mineralisation||1||dolomite||23.4||17.7 (15.7 to 18.8)||−0.3||1.7 (0.7 to 2.2)||310 (250 to 355)|||
|J6||Au mineralisation||1||dolomite||18.9||13.2 (11.2 to 14.3)||0.6||2.6 (1.6 to 3.1)||310 (250 to 355)|||
|860-52-7||Au mineralisation||1||dolomite||19.1||13.4 (11.4 to 14.5)||−4.4||−2.4 (−3.4 to −1.9)||310 (250 to 355)|||
|Average||18.0||11.9 (9.6 to 13.1)||−77||−1.4||0.6 (−0.4 to 1.1)|
|913-32-6||Au mineralisation||2||calcite||17.0||9.1 (7.2 to 11.2)||−2.9||−1.9 (−3.0 to −0.9)||240 (200 to 300)|||
|913-32-7||Au mineralisation||2||calcite||16.5||8.6 (6.7 to 10.7)||−1.7||−0.7 (−1.8 to 0.3)||240 (200 to 300)|||
|P633-61-I1||Au mineralisation||2||calcite||19.0||11.1 (9.2 to 13.2)||−0.6||0.4 (−0.7 to 1.4)||240 (200 to 300)|||
|J163||Au mineralisation||2||dolomite||16.4||8.3 (6.4 to 10.4)||−1.6||−0.8 (−1.8 to 0.3)||240 (200 to 300)|||
|J170||Au mineralisation||2||dolomite||13.8||5.7 (3.8 to 7.8)||−0.4||0.4 (−0.6 to 1.5)||240 (200 to 300)|||
|J180||Au mineralisation||2||dolomite||14.6||6.5 (4.6 to 8.6)||−1.2||−0.4 (−1.4 to 0.7)||240 (200 to 300)|||
|J14||Au mineralisation||2||ferrocalcite||17.5||9.6 (7.7 to 11.7)||−1.1||−0.1 (−1.2 to 0.9)||240 (200 to 300)|||
|Average||16.4||8.4 (6.5 to 10.5)||−1.4||−0.4 (−1.5 to 0.6)|
|913-1||Au mineralisation||3||dolomite||13.8||3.2 (0.4 to 4.8)||1.6||1.0 (−0.5 to 1.9)||190 (150 to 220)|||
|913-2||Au mineralisation||3||dolomite||11.9||1.3 (−1.5 to 2.9)||2.2||1.6 (0.1 to 2.5)||190 (150 to 220)|||
|960-38-1||Au mineralisation||3||dolomite||13.1||2.5 (−0.3 to 4.1)||0.6||0 (−1.5 to 0.9)||190 (150 to 220)|||
|860-51-5||Au mineralisation||3||dolomite||14.8||4.2 (1.4 to 5.8)||−4.4||−5.0 (−6.5 to −4.1)||190 (150 to 220)|||
|860-51-6||Au mineralisation||3||dolomite||14.9||4.3 (1.5 to 5.9)||−4.1||−4.7 (−6.2 to −3.8)||190 (150 to 220)|||
|MS-69||Au mineralisation||3||quartz||16.3||3.9 (0.8 to 5.8)||−81||190 (150 to 220)|||
|Average||3||14.1||3.2(0.4 to 4.9)||−0.8||−0.7(−2.9 to −0.5)|
Notation: # the peak of homogenization temperatures (the range of homogenization temperatures) are from . The δ18Owtaer values were calculated using equations for quartz–water and carbonate–water [54,55]. The δ13C in CO2 equilibrated with dolomite and calcite were calculated using the equations of [56,57].
Table 3. Sulfur isotope composition of sulfides and whole rocks in the Jianchaling area (modified from ).
|Sample||Mineral||δ34Sv-CDT (‰)||Sample||Mineral||δ34Sv-CDT (‰)|
|hjy-1 (second vein set)c||Py||11.3||Carbonate (Duantouya Fm) a||Py||16.4|
|hjy-12 (second vein set) c||Py||10.0||Carbonaceous shale (Duantouya Fm) a||Py||5.4|
|hjy-10 (second vein set) c||Py||10.8||Carbonaceous shale (Duantouya Fm) a||Py||10.6|
|J3 (second vein set) c||Po||8.2||Carbonate (Duantouya Fm) a||Py||12.5|
|J3 (second vein set) c||Py||8.7||Hc-Py1 (serpentinite) a||Cpy||12.0|
|hjy-20 (second vein set) c||Py||13.2||L-Py-Mt (listwanite) a||Po||10.5|
|hjy-14 (second vein set) c||Py||14.3||Ni-bearing ultramafics a||Py||9.3|
|hjy-8 (first vein set) c||Py||14.3||Ni-bearing ultramafics a||Py||11.3|
|H-4 Shale (Duantouya Formation) c||Whole rock||16.6||Ni-bearing ultramafics a||Py||13.2|
|800-52-1 (listwanite) c||Whole rock||6.1||Ni-bearing ultramafics a||Py||10.9|
|780-50-3 (serpentinite) c||Whole rock||8.6||Ni-bearing ultramafics a||Py||11.1|
|965-Y-45-46 II||Py||8.5||Ni-bearing ultramafics a||Py||9.5|
|PD503-Y-Py1-2 II||Py||15.4||Ni-bearing ultramafics a||Py||11.7|
|PD404-cm43-Py a||Py||13.9||Ni-bearing ultramafics a||Py||10.5|
|PD406-cm45N-Py a||Py||14.2||Ni-bearing ultramafics a||Py||12.3|
|PD404-cm45S-Py a||Py||18.5||Ni-bearing ultramafics a||Py||14.1|
|PD503-Ym-Py3 a||Py||17.3||Ni-bearing ultramafics a||Py||12.1|
|PD404-cm43B-Py1 a||Py||12.8||Ni-bearing ultramafics a||Py||11.7|
|965-47E-Ym a||Rea, Orp||10.9||Ni-bearing ultramafics a||Py||11.7|
|PD383-cm58-H1 a||Rea, Orp||10.5||Ni-bearing ultramafics a||Py||11.4|
|PD633-cm43B-Py1 a||Py||12.6||Ni-bearing ultramafics a||Py||10.0|
|G-E-1 a||Py||17.0||Ni-bearing ultramafics a||Py||10.9|
|PD383-Y-Py1 a||Py||12.6||Ni-bearing ultramafics a||Py||6.1|
|PD633-Ym-59 a||Py||12.0||Ni-bearing ultramafics a||Py||12.9|
|97-ZHE-H1 a||Py||11.2||Ni-bearing ultramafics a||Po||7.8|
|Au mineralisation||Py||6.3||Ni-bearing ultramafics a||Po||10.6|
|Au mineralisation||Py||15.3||Ni-bearing ultramafics a||Po||9.1|
|Sm-P (albite porphyry) a||Py||11.0||Ni-bearing ultramafics a||Po||10.0|
|N124 (albite porphyry) a||Py||14.6||Ni-bearing ultramafics a||Po||12.5|
|L-Py (albite porphyry) a||Py||12.9||Ni-bearing ultramafics a||Po||10.7|
|Porphyritic granite a||Py||11.8||Ni-bearing ultramafics a||Po||12.3|
|Porphyritic granite a||Py||12.8||Ni-bearing ultramafics a||Po||11.4|
|Porphyritic granite a||Py||17.5||Ni-bearing ultramafics a||Po||10.9|
|Porphyritic granite a||Po||11.6||Ni-bearing ultramafics a||Po||11.4|
|Porphyritic granite a||Po||13.7||Ni-bearing ultramafics a||Po||11.8|
|Porphyritic granite a||Py||9.0||Ni-bearing ultramafics a||Po||11.1|
|Porphyritic granite a||Py||15.2||Ni-bearing ultramafics a||Po||8.2|
|Porphyritic granite a||Py||15.2||Ni-bearing ultramafics a||Po||10.5|
|Porphyritic granite a||Py||15.2||Ni-bearing ultramafics a||Po||11.6|
|Porphyritic granite a||Py||8.3||Ni-bearing ultramafics a||Po||10.2|
|Porphyritic granite a||Py||10.6||Ni-bearing ultramafics a||Py||10.3|
|Porphyritic granite a||Py||9.5||Ni-bearing ultramafics a||Py||12.1|
|Porphyritic granite a||Py||9.9||Ni-bearing ultramafics a||Py||9.9|
|Carbonate (Duantouya Fm) a||Py||15.0||Ni-bearing ultramafics a||Py||11.4|
|Carbonate (Duantouya Fm) a||Py||18.5||Ni-bearing ultramafics a||Py||11.5|
|Carbonate (Duantouya Fm) a||Po||18.6||Ni-bearing ultramafics a||Py||13.3|
|Carbonate (Duantouya Fm) a||Po||16.5||Ni-bearing ultramafics a||Py||12.3|
|Carbonate (Duantouya Fm) a||Py||10.3||Porphyritic granite b||Py||10.0|
Table 4. Lead isotope composition of ore sulfide and wallrocks at the Jianchaling deposit and in the Bikou Group.
Table 5. The Sr isotope ratios of sulfides and wallrocks at the Jianchaling Au deposit and in the Bikou Group.
|No.||Material||Vein Set||Rb (ppm)||Sr (ppm)||87Rb/86Sr||87Sr/86Sr||2σ||Isr||References|
|Average||N = 4||0.711973|
|Average||N = 2||0.712066|
|Average||N = 8||0.720123|
|Average||N = 15||0.713001|
|H4||Duantouya Fm||105.00||57.20||5.312134||0.734027||0.010856||0.719034||This study|
|960-28-1||Duantouya Fm||0.03||17.90||0.004370||0.720582||0.000493||0.720570||This study|
|Average||N = 4||0.720265|
|Average||N = 9||0.705721|
Table 6. The Nd isotope ratios of sulfides and wallrocks at the Jianchaling deposit.
|Average||N = 5||0.511823||–10.9|
|Average||N = 3||0.511953||–0.28||–8.4|
|Average||N = 2||0.512144||–0.16||–4.7|
Table 7. Geological characteristics comparison between the Jianchaling and Orogenic gold deposit.
|Orogenic Gold Deposit a||Jianchaling Gold Deposit b|
|Tectonic setting||Subduction hyperplasia, continental collision, intracontinental strike-slip and intracontinental compressional orogenic belt||Continental collision|
|Ore-bearing terrane||Metamorphic terrane||Metamorphic and sedimentary terrane|
|Ore-controlling lithology||Ultramafic volcanic rocks, intrusive rocks, miscellaneous sandstones, slate||Ultramafic and dolomite|
|Metamorphism of the host rock||Greenschist facies (low green schist to low granulite)||Greenschist to low amphibolite facies|
|Ore-controlling structures||In the secondary or lower faults of the super-lithospheric fracture zone, the ore-forming structures is mainly the high angle oblique slip belt, the reverse overthrust belt, and also the transverse fracture, the ductile-brittle zone||In the brittle fracture of the ductile shear zone (high angle thrust belt) in the basement fault zone|
|Ore and gangue minerals||Mostly pyrite||Arsenic pyrite, marcasite, arsenopyrite, orpiment, realgar, quartz and calcite|
|Ore type||Quartz vein and altered rock||Altered rock|
|Metallogenic hydrothermal fluids characteristics||Aqueous solution with low salinity and low density, containing CO2 ± CH4 ± N2 ± H2S. The fluids inclusion type has H2O-CO2, rich CO2 (with an unquantifiable CH4 and a small amount of H2O) and gas-liquid two-phase H2O inclusions||Aqueous solution with low salinity and low density, containing CO2 ± CH4 ± H2S. The fluids inclusion type has NaCl-H2O, CO2-H2O-NaCl ± CH4 and pure CO2-CH4|
|Metallogenic fluids salinity||3–12 wt % NaCl equiv.||0.4–15.6 wt % NaCl. equiv.|
|Main metallogenic temperature||350 ± 50 °C||200–320 °C|
|Mineralization pressure||50–400 MPa||117–354 MPa|
|Initial metallogenic fluids feature||Metamorphic fluids||Metamorphic fluids|
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