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Article

Neoproterozoic Aksu Diabase Dyke, Chinese South Tianshan: Magma Sources and Implications for Regional Gold Metallogeny

by
Guoxiong Ma
,
Xiaobo Zhao
*,
Chunji Xue
and
Renke Wang
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Xueyuan Road 29, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(3), 326; https://doi.org/10.3390/min13030326
Submission received: 2 February 2023 / Revised: 21 February 2023 / Accepted: 22 February 2023 / Published: 25 February 2023
(This article belongs to the Special Issue Mineralogy and Geochemistry of Mafic-Ultramafic Assemblages and Associated Ore Deposits)
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Abstract

:
Tianshan is one of the world’s largest gold provinces; however, the relationship between gold mineralization and metasomatized subcontinental lithospheric mantle (SCLM) remains poorly understood. To improve our understanding, we present new bulk-rock geochemistry and platinum group element (PGE) concentrations of the SCLM-sourced Aksu Neoproterozoic diabase dykes in Chinese South Tianshan. These data, combined with in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of hydrothermal pyrite grains in the diabase dykes, are used to discuss the SCLM source characteristics in the region and their potential links to formation of gold deposits. The diabase dykes exhibit high Th/Yb (0.47–0.62) and low Nb/U (13.4–16.3) ratios, indicating that magma evolution involves subduction-related fluid metasomatism and limited contamination of the continental crust. This is consistent with little variation in whole-rock Pd/Zr, Cu/Zr, and Ni/MgO ratios, suggesting that no sulfide segregation was caused by crustal contamination and magma mixing. In addition, the diabase dykes show low PGE and Au contents, with high Cu/Pd (>105) and low Cu/Zr (<0.5) ratios, indicating that magmas were derived from low-degree partial melting of the SCLM under S-saturated conditions. Such source characteristics indicate residual sulfides and chalcophile elements (e.g., PGEs, Au, and Cu) were concentrated at the SCLM reservoir in South Tianshan. Hydrothermal pyrite in the studied dykes has similar Au/Ag ratios and trace element distribution patterns to gold-bearing pyrite of lode gold deposits in Chinese South Tianshan, indicating that metasomatized SCLM may have contributed ore metals during the formation of these gold deposits. Adding to the available data, our study highlights that the SCLM may be a potential metal source reservoir, and it may have contributed to formation of the lode gold deposits in Chinese South Tianshan.

1. Introduction

The relationship between subcontinental lithospheric mantle (SCLM) and formation of gold metallogenic provinces has received considerable attention in the last 20 years [1,2,3]. For instance, the SCLM has been suggested to be a prominent source in some major gold provinces, such as Jiaodong [4,5], Deseado Massif [6], and the western margin of Yangtze Craton [7]. However, there are also major gold provinces that are thought to be unrelated to the SCLM, e.g., Yilgarn Block [8], Central Victoria [9], Lena [10,11], Otago Schist [12,13], and the Abitibi greenstone belt [14].
Tianshan is one of the world’s largest gold provinces and hosts an array of world-class lode gold deposits, e.g., Muruntau, Kumtor, and Unkurtash [15,16,17]. However, the relationship between formation of these gold deposits and metasomatized subcontinental lithospheric mantle (SCLM) remains poorly understood [18]. Mantle-derived components have been recognized in ore-forming fluids of the giant Muruntau gold deposit based on low radiogenic initial Os isotope ratios of auriferous arsenopyrite and elevated 3He/4He ratios of fluid inclusion in gold-bearing quartz [19,20]. Moreover, Mao et al. (2004) confirmed comparable ca. 296-298 Ma ages for gold ores and alkalic granitoids at the Kumtor gold deposit, indicating a mantle-related source [21]. Conversely, Wall et al. (2004) argued that mantle-related magmatic rocks have no genetic links with gold deposits, but instead provided magmatic heat to generate gold-bearing metamorphic fluids (TAG model) [22,23]. Uncertainty about a metal source derived from mantle also concerns lode gold deposits in the Chinese part of Tianshan, e.g., the partial mantle-related C–H–O isotopic characteristics of the Sawayaerdun gold deposit [24]. These controversies are largely due to relatively poor exposure of SCLM-derived rocks in the region, and limited attention has been paid to their relationship with the formation of gold deposits.
The Neoproterozoic Aksu diabase dykes (ca. 760–745 Ma) occur sporadically in southwestern Chinese Tianshan and are thought to be derived from a metasomatized SCLM source [25]. Contemporaneous lode gold deposit, i.e., Djamgyr gold deposit (ca. 802 Ma), has also been reported elsewhere in Kyrgyzstan’s Tianshan [26]. In the Chinese part of Tianshan, although the Aksu diabase dykes formed several hundred million years earlier than the regional Au mineralization (ca. 280–290 Ma [16,20]), host rocks of these diabase dykes also host the Awanda lode gold deposit, which represents the second-largest gold deposit in the region [27]. The potential link between these dykes and regional gold metallogeny requires further evaluation.
In this study, we report new whole-rock geochemical and PGE data on the Aksu diabase dykes, as well as in situ LA-ICP-MS trace element analyses of pyrite grains in the Aksu diabase dykes. Based on the findings, we discuss the SCLM source characteristics in the Chinese Tianshan region and their potential links to large-scale gold mineralization. Adding to the available data, we advance the current understanding of the metal sources of the lode gold deposits in the Tianshan gold province.

2. Geological Setting

The Tianshan orogen in the southwestern part of the Central Asian Orogenic Belt (CAOB; Figure 1a) extends from the Kyzylkum Desert in western Uzbekistan, through Tajikistan and Kyrgyzstan, to Xinjiang in NW China (Figure 1b) [28]. The Chinese part of the Tianshan orogen has been subdivided into four major suture-bounded tectonic domains from north to south, namely Chinese North Tianshan, the Kazakhstan-Yili Block, the Middle Tianshan Block, and South Tianshan (Figure 1b) [29].
South Tianshan, hosting major lode gold deposits, represents a late Paleozoic accretionary complex bounded by the North Tarim Fault to the south and the Atbashi–Inylchek–South Nalati Fault to the north (Figure 1c) [29]. It is composed of a series of dismembered Precambrian terranes that are overlain by Paleozoic meta-sedimentary sequences [28]. The Precambrian terranes are composed mainly of the Paleoproterozoic Xinditagh Group, and the Neoproterozoic Aksu Group, and they are all exposed in the southernmost part of Chinese South Tianshan. The Lower Paleozoic strata are mainly exposed in the middle part of South Tianshan (Figure 1b) and are composed of Lower Cambrian to Ordovician passive margin sediments and Upper Silurian clastic sedimentary sequences [31,32]. Upper Paleozoic strata occur extensively across South Tianshan (Figure 1c) and mainly consist of Devonian to Carboniferous clastic sedimentary rocks, including carbonaceous rocks, calcareous siltstone, grey siltstone, marine carbonate rocks, sandstone, calcareous sandstone, and shale [31]. The Upper Silurian to Carboniferous metasedimentary formations are the most important gold-hosting sequences in Chinese South Tianshan (Figure 1c) [15,33]. Permian fluvial sediments and rift-type volcanic rocks are tightly overlain by the Pre-Carboniferous strata [34].
A few outcrops of Neoproterozoic mafic and voluminous Early Permian A-type granites and basalts are exposed in Chinese South Tianshan (Figure 1c). The Neoproterozoic mafic dykes cross-cut the Neoproterozoic Aksu Group, cropping out mainly in the southwestern part of Aksu city (Figure 2). The Early Permian (ca. 290–280 Ma) A-type granitoids are widespread across South Tianshan and show both A1 and A2 affinities, which have been interpreted to be related to the Tarim mantle plume [35] or formed in a post-collisional extension setting [36]. Basaltic lavas and mafic–ultramafic complexes (295–285 Ma) are exposed in the southernmost part of Chinese South Tianshan, e.g., the Keping and Bachu areas. These rocks show oceanic island-arc basalt (OIB)-like features, and are accepted to be related to the Permian Tarim mantle plume [37,38].
The Aksu district is localized in the southwestern part of Chinese South Tianshan (Figure 1c). The Precambrian basement belts are composed mainly of the Neoproterozoic Aksu Group and Ediacaran Sugetbrak Formation [41]. The Aksu Group is mainly exposed in southwestern Aksu (Figure 2), including greenschist, blueschist, psammitic schist, pelitic schist, and metagreywacke [39,42]. The blueschist protolith has been dated to be of ca. 806–754 Ma based on whole-rock 40Ar/39Ar and detrital zircon U–Pb dating methods [39,40]. The pelitic schist has a 40Ar/39Ar age of 756–741 Ma [43] or detrital zircon U–Pb age of 730 Ma [42]. The psammitic schist has a detrital zircon U–Pb age of ca. 791 Ma [43]. The deformed sequence of the Aksu Group is crosscut by a series of NW-striking diabase dykes (Figure 2 and Figure 3a), which have U–Pb zircon ages of 759 ± 7 Ma (SHRIMP; [27]) and 757 ± 9 Ma (LA–ICP–MS; [44]). In addition, amphibole from the mafic dyke yielded a 40Ar/39Ar plateau age of 744.5 ± 2.8 Ma [41]. These different ages indicate that the crystallization age of the Aksu diabase dyke is most probably Neoproterozoic (~760–745 Ma; [45]). In addition, these diabase dykes show an enrichment of large-ion lithophile element (LILE) and light rare-earth element (LREE), along with depletion of high-field-strength element (HFSE) and heavy rare-earth element (HREE), which are similar to arc-like geochemical signatures [27]. The large range of radiogenic 87Sr/86Sr(i) from 0.7050 to 0.7074, and 143Nd/144Nd with initial εNd(t) values ranging from −6.4 to 1.5, further indicate that they might have been derived from a metasomatized SCLM source [27,45].

3. Field Geology and Sample Description

Field investigation and sample collection focused on the SCLM-derived Neoproterozoic diabase dykes in southwestern Aksu (Figure 1b and Figure 2). The diabase dykes have NW–SE strikes with variable extended widths ranging from 2 to 10 m (Figure 2 and Figure 3a–c), intruding into quartz mica schist and chlorite schist of the Neoproterozoic Aksu Group (Figure 3a–c). Five fresh diabase samples were collected from the Aksu diabase dykes (Figure 2 and Figure 3a,d). The collected diabase samples were greyish-green in color and free of alteration (Figure 3e). They were composed of plagioclase (50–60%), clinopyroxene (30–35%), and hornblende (5–10%), with some magnetite and pyrite (Figure 3f). Sampling coordinates and petrological descriptions of the diabase samples are given in Table 1.
A large amount of subhedral pyrite grain was found in our diabase samples (Figure 4). Pyrite occurs as an interstitial phase in plagioclase and clinopyroxene (Figure 4), and it is irregular in shape and porous (100–300 μm), suggesting a hydrothermal origin.
Pyrite grains are generally intergrown with magnetite and silicate minerals, and a few of them contain magnetite rims (Figure 4c,d), suggesting dissolution of sulfide melt by a volatile phase. The textural relations among pyrrhotite, pyrite, and magnetite suggest a significant associated transfer of sulfur and chalcophile metals such as Cu and Au to hydrothermal fluids (Figure 4e,f) [47].

4. Analytical Methods

4.1. Whole-Rock Major and Trace Elements

Five fresh Neoproterozoic diabase samples lacking signs of alteration or wall-rock assimilation were crushed in an agate mill to a powder of 200 mesh size, and they were selected for whole-rock major and trace element analyses at the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS). Before the analyses, all samples were inspected using a general magnifier to select only fresh surface devoid of any weathering (alteration). Major elements were determined by X-ray fluorescence spectrometry (ARL Perform’ X4200) methods. Analytical uncertainties were <5% for major elements. Trace elements were analyzed using inductively coupled plasma mass spectrometry (ICP-MS) (Plasmaquant MS elite), with analytical uncertainties below 10%. The analytical protocol was similar to that described in [48].

4.2. Whole-Rock PGE Elements

Four samples of Neoproterozoic diabase were analyzed for their platinum group element (PGE; Pt, Pd, Ru, Rb, Rh, and Ir) geochemistry at the IGCAS. The PGE elements were determined by the isotopic dilution method coupled with ICP-MS (Plasma Quant ® MS) analysis. Pt, Pd, Ru, and Ir were determined by isotopic dilution, and 194Pt was used as an internal standard for calculating the abundance of the single isotope element Rh. Detailed analytical procedures are described in [49]. The total procedural blanks were lower than 0.0101 ng/g for Pd, 0.0132 ng/g for Ru, 0.0012 ng/g for Os, 0.0053 ng/g for Ir, 0.0019 ng/g for Pt, and 0.0021 ng/g for Rh. Reference standards of TDB-1 (diabase) were used to monitor the accuracy, which was above 90%.

4.3. In Situ LA-ICP-MS of Elements

In situ trace element concentrations of pyrite from the diabase samples were determined using LA-ICP-MS (Coherent, Germany) at the IGCAS. The analytical instrumentation was a Coherent Compex-Pro 193 nm ArF (Coherent, Germany) excimer laser ablation system attached to an Agilent 7700x ICP-MS (Agilent, USA). The operating condition and analytical procedure applied to sulfide were described in [50]. Spot ablation was carried out using a size of 26 μm at 3 J/cm2 and a 5 Hz repetition rate. Each analysis comprised a background acquisition of 30 s (gas blank) and a 60 s analysis of the sample. The integrated count data to concentrations for lithophile elements were calibrated and converted by GSD-1G. Sulfide reference material MASS-1 was analyzed as an unknown sample to check the analytical accuracy [51]. The background and analysis signals comprised off-line selection and integration by Excel-based software IC-PMS DataCal (version 6.37), along with time-drift correction and quantitative calibration for trace element analysis [52]. Concentration data and detection limit calculations were performed following the protocol in [53].

5. Results

5.1. Whole-Rock Major and Trace Elements

Whole-rock major and trace element compositions of the diabase samples are listed in Table 1. The Aksu diabase samples possess low loss on ignition (LOI) values ranging from 1.53 to 1.87 wt%. The samples have low SiO2 (47.26–48.1 wt%) and K2O + Na2O (5.02–5.23 wt%) values, mostly plot within the alkaline series field of a TAS diagram (Figure 5a), and exhibit sub-alkaline characteristics on a Nb/Y vs. Zr/TiO2 plot (Figure 5b). The Aksu Neoproterozoic diabase samples exhibit high (La/Yb)N (2.01–6.03) ratios, and they display enrichment of LREE with flat HREE (Figure 6a). In primitive mantle-normalized diagrams, the diabase samples are enriched in large-ion lithophile elements (LILE) such as Rb, Ba, and Th, and depleted in HFSEs, with remarkable negative Nb and Ta anomalies (Figure 6b).

5.2. Whole-Rock PGE Contents

Whole-rock platinum group element (PGEs) results are listed in Table 2. The Aksu Neoproterozoic diabase exhibits low PGE contents (ΣPGEs= 0.36–0.65 ppb), with Pt at 0.02–0.23 ppb, Pd at 0.04–0.44 ppb, Ir at 0.02–0.12 ppb, Ru at 0.21–1.41 ppb, and Rh at 0.24–1.45 ppb. The primitive mantle-normalized PGE patterns are roughly horizontal for the Aksu diabase, with an anomalous trough at Pt and peaks at Rh and Pd (Figure 7a). They are markedly contrasted with PGE patterns of regional mafic dykes (Zhengyuan and Yilgarn Craton lamprophyres), Victoria gabbro (Figure 7b [58,59,60]), and felsic intrusions (Tuwu porphyry Cu deposits, Figure 7b, [61]). In addition, the sloped patterns of diabase are different from those of orogenic Au deposit (Figure 7a) [5] and porphyry Cu–Au (PGE) deposit (Figure 7b) [62].

5.3. In Situ LA-ICP-MS of Elements

In situ LA-ICP-MS element compositions are listed in Supplementary Table S3. Only 12 of the 28 spot analyses revealed detectable gold. The gold content in pyrite varied from below the detection limit to 0.34 ppm with a mean of 0.04 ppm, while arsenic varied from 12.54 to 603 ppm (mean=244 ppm). The Au/Ag ratios of pyrite from the Aksu Neoproterozoic diabase were between 0.01 and 0.1 (Figure 8) Moreover, these pyrite grains were rich in a host of elements such as As, Bi, Co, Mn, Ni, Sb, Pb, and Ti (Figure 9).

6. Discussion

6.1. Metasomatized SCLM Source for the Aksu Diabase

The Aksu diabase dykes show low LOI values (1.53–1.87 wt%), suggesting that these dykes were largely unaffected by post-magmatic alteration. The Aksu diabase dykes have limited variation in Th contents (1.54–2.35 ppm) and show no correlation between Nb/La and Nb/Th ratios (Figure 10a), suggesting that crustal contamination had limited effect on their composition during magma ascension. In addition, these diabase samples have higher Ba (1708–1864 ppm) and Sr (355–548 ppm) contents than the average continental crust (Ba = 390 ppm, Sr = 325 ppm; [72]), further suggesting that the potential influence of crustal contamination can be excluded. This is also supported by the low radiogenic Pb isotopic compositions of these diabase dykes (206Pb/204Pb: 16.54 ~ 16.92, 207Pb/204Pb: 15.32 ~ 15.40, 208Pb/204Pb: 37.18 ~ 37.37) [73], which are lower than the mantle mean values of 17.51, 15.43, and 37.63, respectively [74]. The diabase samples show relatively low Mg# (39–40), Ni (20.9–22.6 ppm), and Cr (20–23 ppm) contents, lower than primary mantle-derived magmas (Figure 7a). Such compositional variations indicate that their parental magmas might have been influenced by fractionation crystallization. The Aksu Neoproterozoic diabase dykes exhibit positive Eu anomalies (Eu/Eu* = 1.28–1.31), indicating plagioclase fractionation (Figure 6). This conclusion is consistent with the petrological observation that the main components of diabase samples are clinopyroxene and plagioclase (Figure 3f). Moreover, these diabase dykes show high concentrations of fluid-mobile trace elements such as LILE and LREE, and distinctly negative Nb, Ta, Zr, Hf, and Ti anomalies, consistent with derivation from metasomatic SCLM sources. In addition, the similar elemental patterns of the Early Permian (287 Ma) diabase in Keping are interpreted to be derived from a metasomatic SCLM (Figure 6) [38]. This interpretation agrees with the radiogenic 87Sr/86Sr(i) from 0.7050 to 0.7074, 143Nd/144Nd ratios of 0.511933–0.512346, and initial εNd(t) values ranging from −6.4 to 1.5 of these diabase dykes [27,45], indicating that their parental magma may have been derived from subduction-related metasomatized SCLM [75,76]. Moreover, the diabase samples have relatively high Th/Yb ratios and low Nb/U ratios (Figure 10c,e,f), indicating that the SCLM was metasomatized by slab-derived fluids [76,77]. The high Th/Yb ratios and low Nb/U ratios are similar to mafic dyke samples from the SCLM of Jiaodong District, which experienced fluid-related metasomatism. Collectively, post-magmatic alteration, crustal contamination, and partial crystallization processes can be surmised to have had little effect on the composition of the studied diabase sample, the magma evolution of which involved subduction-related fluid metasomatism and limited contamination of the continental crust.

6.2. Mantle Source Evolution

The PGEs and Cu have distinctly different partition coefficients between sulfide and silicate melt, e.g., DPGE (105–106) > DCu (500–1500) [84]. Thus, sulfide saturation will increase Cu/Pd ratios so they are higher than in primitive mantle (~7000) [85]. The Cu/Pd ratios of the studied diabase dykes are significantly higher than those of the primitive mantle (Figure 8a,b), indicating that the diabase dyke rocks may have crystallized from S-saturated melts. In contrast, evolving magma under S-undersaturated melts may lead to elevated Pd/Ir ratios [86]. The Aksu Neoproterozoic diabase dykes have an extremely narrow range of Pd/Ir ratios (Figure 11b), which can be easily distinguished from the S-undersaturated basalt from the Deccan Traps [87]. This evidence further suggests that the parent magma of the diabase dykes is likely to have been S-saturated. Meanwhile, when the magma reaches sulfide saturation, the content of Cu in the magma will decrease, whereas the content of Zr will increase. In the Cu versus Zr diagram, the negative correlation between the Cu and Zr contents of these diabase dykes (Figure 11c) suggests the significant sulfide saturation of diabase dykes during magma evolution. In addition, these diabase dykes share a similar evolution trend to the sulfide-saturated Tarim basalt, Jiaodong lamprophyres, and MORBs (Figure 11c) [5,81,88]. This evolution trend may reflect sulfide saturation of the mafic dykes during magma evolution. S-saturated magmas may lead to low Cu/Zr ratios of below 1 [89]. The Cu/Zr ratios of our diabase dykes are <0.5 (Figure 11d), analogous to those of sulfide-saturated Tarim basalt, Jiaodong lamprophyres, and MORBs [81,82,88]. In comparison, the Deccan Trap basalts show high Cu/Zr ratios and show positive correlation between Cu and Zr contents, meaning they are interpreted to be mainly crystallized from S-unsaturated melts (Figure 11c,d) [89]. These results further indicate that the parental melts of the diabase dykes could have been sulfide saturated.
The S-saturation of magma can be caused by several different processes, such as crustal contamination, magma mixing, and low-degree partial melting. Theoretically, crustal contamination or magma mixing can cause the segregation of an immiscible sulfide melt [81]. The Ni/MgO and Cu/Zr ratios are also a good indicator to determine whether the parental magmas of mafic rocks met segregating magmatic sulfides during their ascent in the crust [87]. This is because Ni and Cu are partitioned into segregating magmatic sulfides, resulting in depletion of Ni and Cu in the parental magma [87]. However, the Ni/MgO ratios of the analyzed diabase dykes do not correlate with Cu/Zr ratios (Figure 8e), indicating that no segregation magmatic sulfides occurred during their melt evolution. This result is consistent with the absence of magmatic sulfides in these diabase dykes (Figure 3f). In addition, the variations in Pd/Zr and Cu/Pd ratios are sensitive to sulfide segregation because the partition coefficients of PGE in magmatic sulfides are two orders of magnitude larger than those of Cu and Ni [90]. The limited variation of Pd/Zr and Cu/Pd ratios in the studied diabase dykes (Figure 11f) further implies that the influence of crustal contamination and magma mixing on S-saturation and sulfide segregation could be excluded. This is consistent with the above results demonstrating that the diabase melt did not experience significant crustal contamination. It also conforms to the fact that the major element data of the studied diabase dykes did not abruptly increase or decrease, indicating that no magma mixing occurred. In addition, the SCLM-derived Jiaodong lamprophyres also show little change in Pd/Zr and Cu/Pd ratios (Figure 11f), which were explained initially as S-saturated, with no sulfide segregation caused by crustal contamination [81]. The variation in Pd/Zr and Th/Nb ratios of the Tarim basalt and East Pacific MORB (Figure 11f) were explained as S-saturated, with sulfide segregation accompanying magma mixing [83,88]. Therefore, the crustal contamination and magma mixing did not trigger sulfur saturation and sulfide segregation for the diabase magma in the crust. Low-degree partial melts are usually S-saturated as they leave the mantle, and they produce chalcophile-depleted melt [91]. The PGE patterns of the Aksu Neoproterozoic diabase dykes are markedly different from the Au–sulfide ores from the porphyry Cu–Au and orogenic Au deposit (Figure 7a) and the high-degree partial-melting felsic intrusions of the Tuwu Cu–Au deposit (Figure 7b). This result implies their derivation from low-degree melting of the SCLM source. Moreover, the Au, Pd, and Pt contents of these diabase dykes are lower than the SCLM-derived low-degree partial melting mafic dykes of Yilgarn Craton and Victoria gold province [58,60], which further indicates that the chalcophile elements are locked in the residue phase by low-degree partial melting of a metasomatized SCLM. Furthermore, the diabase dykes have low PGEs and are strongly depleted in Au contents (Figure 7a,b), indicating that their magmas were produced from S-saturated melts derived from low-degree partial melting of the SCLM. In this scenario, the formation of the Aksu diabase dykes will have left residual sulfide and chalcophile elements (PGEs, Au) in the SCLM source region, leading to elevated sulfide and Au contents in the SCLM, thus increasing the fertility of the SCLM beneath South Tianshan.

6.3. Contribution of Metasomatic SCLM on Regional Gold Metallogeny

Metasomatic processes can result in enrichment of Au of the SCLM, as evidenced by native Au nanoparticles (Patagonia [6]; Beiya [92]) and the high gold contents in mantle xenoliths elsewhere [82]. Neither metasomatized SCLM nor mantle-derived magmas have Au contents remarkably higher than that of global mantle peridotite (1.2 ppb) or normal mafic magmas such as MORBs (1–4 ppb) [82]. Although detailed inspection of the pyrite grains did not reveal native Au nanoparticles (Figure 3), the relatively high Au concentrations (up to 0.34 ppm) obtained by LA-ICP MS analyses indicate a discernible Au addition to their source. It should be noted that these pyrites contain significant amounts of Ag (up to 17.5 ppm, Supplementary Table S3). The Au/Ag ratios of pyrite from the analyzed diabase dykes are between 0.01 and 0.08, while the Au/Ag ratios of the Patagonian mantle sulfides vary from 0.02 to 0.96 (Figure 8) [6]. Thus, the Au/Ag ratios of the pyrite and the mantle sulfides are of a similar magnitude. Such similar metal ratios indicate that the metasomatized SCLM may exert important control on the formation of the pyrite and gold contents in these diabase dykes. Furthermore, the Au/Ag ratios of the pyrite in the analyzed diabase dykes are similar to those of gold-bearing sulfides of the Awanda gold deposit (Figure 8; Au/Ag~0.01–4.59, our unpublished data). Similar Au/Ag ratios of gold-bearing sulfides are also observed in the lode gold deposit in the Jiaodong gold province (Figure 8; Supplementary Table S4). This similarity suggests that metasomatized SCLM may have contributed additional Au to the formation of the lode gold deposits in the region.
Moreover, pyrite of the Aksu diabase dykes is enriched in a host of trace elements such as Ti, Co, Ge, As, Ag, Pb, and low Au contents (0.01–0.34 ppm) while depleted in other metals such as V, Mn, Ga Ti, Mo, and Bi (Figure 9). In addition, their distribution pattern is comparable to those from the gold-bearing sulfides of the Sawayaerdun gold deposit (Figure 9), indicating that the metasomatized SCLM may have contributed ore metals into the ore-forming fluid of the Sawayaerdun gold deposit.

7. Conclusions

The evolution of the SCLM-sourced Aksu Neoproterozoic diabase magma involved subduction-related fluid metasomatism with limited crustal contamination. Diabase magma might have been derived from low-degree partial melting an under S-saturated condition, leaving PGEs and Au in the residual source region. Metasomatized SCLM may have contributed additional ore metals to the lode gold deposits in Chinese South Tianshan.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min13030326/s1, Table S1: Compilation of published whole-rock major (in wt.%) and trace element (in ppm) data for mafic dykes in Chinese South Tianshan; Table S2: Compilation of PGE, Cu, Ni, and S concentrations in ore and ore-related rocks from porphyry Cu–Au deposits and lode gold provinces worldwide; Table S3: In situ trace elements of pyrites from the Aksu Neoproterozoic diabase dykes and lode gold deposits in Chinese South Tianshan (data in ppm); Table S4: Compilation of in situ Au and Ag contents of pyrite from Patagonian mantle xenoliths and Jiaodong gold provinces (data in ppm).

Author Contributions

Data curation, G.M.; funding acquisition, C.X. and X.Z.; investigation, G.M., X.Z., and R.W.; project administration, C.X. and X.Z.; supervision, X.Z. and C.X.; writing—original draft, G.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Open Research projects from the State Key Laboratory of Geological Processes and Mineral Resources (GPMR202117), the MNR Key Laboratory of Metallogeny and Mineral Assessment (ZS2204), and the State Key Laboratory for Mineral Deposits Research (2021-LAMD-K09) and Zijin Mining Group Co., Ltd.

Data Availability Statement

Data are contained within the supplementary materials.

Acknowledgments

We appreciate Mohamed Zaki Khedr for careful editorial handling, and four anonymous reviewers for their constructive comments and suggestions. We are also indebted to Yun Zhao, Haixia Chu, and Qing Sun for their thoughtful discussion contributions on early versions of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Tectonic location of the Tianshan orogen in the Central Asian Orogenic Belt. (b) Simplified geological map of Tianshan showing the locations of major lode gold deposits (modified from [16]). (c) Geological map of Chinese South Tianshan showing the location of the Aksu terrane (modified from [30]).
Figure 1. (a) Tectonic location of the Tianshan orogen in the Central Asian Orogenic Belt. (b) Simplified geological map of Tianshan showing the locations of major lode gold deposits (modified from [16]). (c) Geological map of Chinese South Tianshan showing the location of the Aksu terrane (modified from [30]).
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Figure 2. Simplified geological maps of the Aksu terrane with locations of samples (modified from [39,40]), as well as published ages for the diabase dykes and Precambrian schist [39,40,41,42,43,44].
Figure 2. Simplified geological maps of the Aksu terrane with locations of samples (modified from [39,40]), as well as published ages for the diabase dykes and Precambrian schist [39,40,41,42,43,44].
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Minerals 13 00326 g003 550
Figure 3. Photographs and photomicrographs showing the occurrence and mineralogy of the diabase dykes in the Aksu area. (a,b) Neoproterozoic diabase dykes intruding the deformed Neoproterozoic Aksu Group; (c) hand specimen of the Aksu Neoproterozoic diabase; (d,e) photomicrograph of diabase showing the mineral assemblage of the diabase under cross-polarized light. Abbreviations: Pl = plagioclase, Cpx = clinopyroxene.
Figure 3. Photographs and photomicrographs showing the occurrence and mineralogy of the diabase dykes in the Aksu area. (a,b) Neoproterozoic diabase dykes intruding the deformed Neoproterozoic Aksu Group; (c) hand specimen of the Aksu Neoproterozoic diabase; (d,e) photomicrograph of diabase showing the mineral assemblage of the diabase under cross-polarized light. Abbreviations: Pl = plagioclase, Cpx = clinopyroxene.
Minerals 13 00326 g003
Minerals 13 00326 g004 550
Figure 4. Photomicrographs showing the sulfide phases in the diabase dykes of the Aksu area. (a,b) Pyrite grains are generally intergrown with magnetite and silicates in plagioclase and clinopyroxene; (c,d) subhedral pyrite grains enveloped by magnetite rims; (e,f) pyrrhotite, pyrite, and magnetite within plagioclase of the diabase dykes. Abbreviations: Pl = plagioclase, Cpx = clinopyroxene, Py= pyrite, Po = pyrrhotite, Mt = magnetite.
Figure 4. Photomicrographs showing the sulfide phases in the diabase dykes of the Aksu area. (a,b) Pyrite grains are generally intergrown with magnetite and silicates in plagioclase and clinopyroxene; (c,d) subhedral pyrite grains enveloped by magnetite rims; (e,f) pyrrhotite, pyrite, and magnetite within plagioclase of the diabase dykes. Abbreviations: Pl = plagioclase, Cpx = clinopyroxene, Py= pyrite, Po = pyrrhotite, Mt = magnetite.
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Minerals 13 00326 g005 550
Figure 5. Rock classification diagrams. (a) Total alkali versus silica (TAS) diagram (from [54]). (b) Nb/Y versus Zr/Ti diagram (modified from [55]). (c) SiO2 versus FeOT/MgO plot [56]. (d) K2O versus silica diagram [57]. Data sources: Neoproterozoic gabbro [27,45]. Data can be found in Supplementary Table S1.
Figure 5. Rock classification diagrams. (a) Total alkali versus silica (TAS) diagram (from [54]). (b) Nb/Y versus Zr/Ti diagram (modified from [55]). (c) SiO2 versus FeOT/MgO plot [56]. (d) K2O versus silica diagram [57]. Data sources: Neoproterozoic gabbro [27,45]. Data can be found in Supplementary Table S1.
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Minerals 13 00326 g006 550
Figure 6. Chondrite-normalized REE patterns (a) and primitive mantle-normalized multiple-trace-element diagrams (b) for Aksu Neoproterozoic diabase dykes. Data sources: N-MORB; E-MORB; OIB values are from [27,45]; and 287 Ma sub-continental lithospheric mantle (SCLM)-like diabase and 290 Ma ocean island-arc basalt (OIB)-like diabase in the Aksu area are from [38]. Literature data are compiled in Supplementary Table S2.
Figure 6. Chondrite-normalized REE patterns (a) and primitive mantle-normalized multiple-trace-element diagrams (b) for Aksu Neoproterozoic diabase dykes. Data sources: N-MORB; E-MORB; OIB values are from [27,45]; and 287 Ma sub-continental lithospheric mantle (SCLM)-like diabase and 290 Ma ocean island-arc basalt (OIB)-like diabase in the Aksu area are from [38]. Literature data are compiled in Supplementary Table S2.
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Minerals 13 00326 g007 550
Figure 7. Primitive mantle-normalized PGE and Au contents for the Aksu Neoproterozoic diabase dykes. Data sources: Danba gold deposit [3], Porphyry Cu–Au deposits [62], Zhengyuan lamprophyres [59], Victoria gabbro [58], Yilgarn Craton lamprophyres [60], and Tuwu felsic intrusions [61]. Literature data are compiled in Supplementary Table S2.
Figure 7. Primitive mantle-normalized PGE and Au contents for the Aksu Neoproterozoic diabase dykes. Data sources: Danba gold deposit [3], Porphyry Cu–Au deposits [62], Zhengyuan lamprophyres [59], Victoria gabbro [58], Yilgarn Craton lamprophyres [60], and Tuwu felsic intrusions [61]. Literature data are compiled in Supplementary Table S2.
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Minerals 13 00326 g008 550
Figure 8. Au versus Ag contents of pyrite from the Aksu Neoproterozoic diabase dykes and lode gold deposits in Chinese South Tianshan. Dotted lines are constant Au/Ag ratios. Data sources: mantle sulfides [6]; Jiaodong gold deposits [63,64,65,66,67,68,69,70]; Awanda gold deposits (our unpublished data). Literature data are compiled in Supplementary Table S4.
Figure 8. Au versus Ag contents of pyrite from the Aksu Neoproterozoic diabase dykes and lode gold deposits in Chinese South Tianshan. Dotted lines are constant Au/Ag ratios. Data sources: mantle sulfides [6]; Jiaodong gold deposits [63,64,65,66,67,68,69,70]; Awanda gold deposits (our unpublished data). Literature data are compiled in Supplementary Table S4.
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Minerals 13 00326 g009 550
Figure 9. Trace element concentration of pyrite from the Aksu Neoproterozoic diabase dykes and Sawayaerdun gold deposit in Chinese South Tianshan. Data source: Sawayaerdun gold deposit [71].
Figure 9. Trace element concentration of pyrite from the Aksu Neoproterozoic diabase dykes and Sawayaerdun gold deposit in Chinese South Tianshan. Data source: Sawayaerdun gold deposit [71].
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Minerals 13 00326 g010 550
Figure 10. Variation diagrams to discern the petrogenesis for the Aksu Neoproterozoic diabase dykes. (a) Nb/La versus Nb/Th diagram, showing an uncontaminated trend. (b) Elevated Th/Yb at a given Nb/Yb indicates the involvement of subducted components (modified from [78]). (c) Nb/U versus Nb diagram outlines the subduction-related fluid metasomatism from the mantle component [79]. (d) Ba versus Nb/Y plot showing the modification by subduction slab fluids (from [80]). Data sources: Neoproterozoic gabbro [27,45], Early Permian diabase (OIBs and SCLM) [38], Jiaodong mafic dyke [81], and Global MORB [82,83]. Literature data are compiled in Supplementary Table S1.
Figure 10. Variation diagrams to discern the petrogenesis for the Aksu Neoproterozoic diabase dykes. (a) Nb/La versus Nb/Th diagram, showing an uncontaminated trend. (b) Elevated Th/Yb at a given Nb/Yb indicates the involvement of subducted components (modified from [78]). (c) Nb/U versus Nb diagram outlines the subduction-related fluid metasomatism from the mantle component [79]. (d) Ba versus Nb/Y plot showing the modification by subduction slab fluids (from [80]). Data sources: Neoproterozoic gabbro [27,45], Early Permian diabase (OIBs and SCLM) [38], Jiaodong mafic dyke [81], and Global MORB [82,83]. Literature data are compiled in Supplementary Table S1.
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Minerals 13 00326 g011 550
Figure 11. (a) Diagram showing variations in Cu/Pd ratios compared to Pd concentrations [85]. (b) Pd/Ir versus Cu/Pd diagram [88]. (c) Cu versus Zr diagram [89]. (d) Cu/Zr versus MgO diagram [89]. (e) Ni/MgO versus Cu/Zr diagram [87]. (f) Pd/Zr versus (Th/Nb)N diagram [87]. Data sources: Early Permian basalt [89], Jiaodong mafic dyke [5,81], Deccan Trap basalts [87], and Global MORB [82,83]. Literature data are compiled in Supplementary Table S2.
Figure 11. (a) Diagram showing variations in Cu/Pd ratios compared to Pd concentrations [85]. (b) Pd/Ir versus Cu/Pd diagram [88]. (c) Cu versus Zr diagram [89]. (d) Cu/Zr versus MgO diagram [89]. (e) Ni/MgO versus Cu/Zr diagram [87]. (f) Pd/Zr versus (Th/Nb)N diagram [87]. Data sources: Early Permian basalt [89], Jiaodong mafic dyke [5,81], Deccan Trap basalts [87], and Global MORB [82,83]. Literature data are compiled in Supplementary Table S2.
Minerals 13 00326 g011
Table
Table 1. Major oxide (wt.%) and trace element (ppm) compositions of the Aksu Neoproterozoic diabase dykes, Chinese South Tianshan.
Table 1. Major oxide (wt.%) and trace element (ppm) compositions of the Aksu Neoproterozoic diabase dykes, Chinese South Tianshan.
SampleAKSQ-15AKSQ-16AKSQ-18AKSQ-22AKSQ-23
CoordinatesN41°10′05″, E80°03′40″N41°10′07″, E80°03′37″N41°10′08″, E80°03′36″N41°10′11″, E80°03′36″N41°10′10″, E80°03′39″
SiO248.0147.2647.3848.147.81
TiO22.7752.8012.8032.7612.811
Al2O314.0213.8713.9514.0513.93
TFe2O314.314.5314.5414.2914.57
MnO0.2420.2330.2330.2370.234
MgO4.784.884.894.764.85
CaO7.88.238.087.637.83
Na2O3.533.843.453.563.49
K2O1.551.321.661.671.53
P2O50.9430.9460.9020.950.925
LOI1.531.741.871.821.85
Mg#39.8439.8539.9939.7639.74
Total99.7299.92100.01100.03100.03
Sc29.529.632.53232.3
V366366224214231
Cr2222.622.120.823
Co57.3554846.845
Ni21.622.422.620.921.7
Cu41.141.63836.237.6
Zn155151129131128
Ga21.621.320.520.320.7
Rb45.633.228.428.626.9
Sr489548447355377
Y38.437.237.438.338.3
Zr160144146152156
Nb5.95.585.485.75.86
Cs0.4470.3730.3980.4010.422
Ba16911864173617081709
La33.63228.129.228.6
Ce72.668.959.764.154
Pr9.358.958.228.568.6
Nd44.742.9384038.9
Sm109.688.568.988.77
Eu3.963.893.673.843.82
Gd9.329.078.859.188.99
Tb1.371.351.221.281.29
Dy8.478.257.417.647.71
Ho1.681.631.471.541.51
Er4.434.233.783.953.96
Tm0.6170.5920.5270.5480.552
Yb3.793.683.253.353.38
Lu0.5650.5430.4720.4890.489
Hf4.854.53.653.923.95
Ta0.4610.4310.3490.3710.37
Pb6.495.984.344.134.2
Th2.352.151.541.641.68
U0.440.3940.3350.3770.377
Eu/Eu*1.241.251.281.281.31
Notes: Total iron as TFe2O3; TFeO = TFe2O3 × 0.8998; Mg# = MgO/(MgO + TFeO) × 100 in atomic ratio; REE—rare earth elements; LOI = loss on ignition; Eu/Eu* = 2 × EuN/(SmN + GdN), subscript "N" denotes normalization to chondrites (Sun and McDonough,1989 [46]).
Table
Table 2. PGE, Cu, Ni, and S concentrations in the Aksu diabase dykes, Chinese South Tianshan.
Table 2. PGE, Cu, Ni, and S concentrations in the Aksu diabase dykes, Chinese South Tianshan.
Sample No.Pt (ppb)Pd (ppb)Ir (ppb)Ru (ppb)Rh (ppb)ΣPGE (ppb)Au (ppb)Cu (ppm)S (ppm)Ru/IrRu/PtPd/IrPd/Pt
AKSQ-150.0640.1820.0080.0970.0110.362<141.1244612.301.52 22.752.85
AKSQ-160.0710.3730.0080.0910.0140.5573<141.6273510.95 1.28 46.625.27
AKSQ-180.0200.2750.0150.1420.0900.5425<13825609.23 7.11 18.3313.75
AKSQ-22nd0.2430.0080.1320.0110.3942<136.2202215.92 13.21 30.3724.26
AKSQ-230.3130.2190.0070.1040.0100.6529<137.6195515.46 0.33 31.290.70
TDB-1, obtained4.67222.0590.0440.4110.661————————
Primitive mantle *7.13.93.25.00.9————————
* Primitive mantle values are from [46]. “nd” means the content is below the minimum detection limit.
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Ma, G.; Zhao, X.; Xue, C.; Wang, R. Neoproterozoic Aksu Diabase Dyke, Chinese South Tianshan: Magma Sources and Implications for Regional Gold Metallogeny. Minerals 2023, 13, 326. https://doi.org/10.3390/min13030326

AMA Style

Ma G, Zhao X, Xue C, Wang R. Neoproterozoic Aksu Diabase Dyke, Chinese South Tianshan: Magma Sources and Implications for Regional Gold Metallogeny. Minerals. 2023; 13(3):326. https://doi.org/10.3390/min13030326

Chicago/Turabian Style

Ma, Guoxiong, Xiaobo Zhao, Chunji Xue, and Renke Wang. 2023. "Neoproterozoic Aksu Diabase Dyke, Chinese South Tianshan: Magma Sources and Implications for Regional Gold Metallogeny" Minerals 13, no. 3: 326. https://doi.org/10.3390/min13030326

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