Geochemical characteristics and indication of graphite deposits in Xinrong Region, Shanxi, China

The NE–SW-trending graphite belt found in the Xinrong Region, Datong City is one the richest graphite reserves in China, extending for more than 22 km. The ore-bearing layer of the Huangtuyao Formation is a graphite-bearing gneiss. Based on major and trace element analysis data, we determined from three diagrams that the graphite-bearing gneiss is a parametamorphic rock, and this gneiss was formed by the regional metamorphism of carbonaceous argillite. The detritus of this ore deposit originated from an arc region as seen from the K2O/Na2O–SiO2 diagram. According to the Ta/Yb, Sr/Yb and K2O/Na2O ratios as well as the composition of Gehuyao gneiss, we concluded that this gneiss has typical low-potassium adakitic rock characteristics. Combined with the MgO/SiO2 diagram and the characteristics of low K2O and high Al2O3, the genetic model of Gehuyao gneiss is melted subducted oceanic crust. Based on regional geological data and comparison of samples, we found an inherent relationship in the main composition content between the graphite-bearing gneiss and the Gehuyao gneiss. This indicates that the ore deposit accumulated the weathering products of Gehuyao gneiss during the sedimentary period. This research provides evidence for ore body evolution and makes it possible to establish the sedimentary–metamorphic model of the graphite ore in this region. In light of the range of the ore body, we predict that the potential area for graphite mineralization in the North China Craton was 2 ± 1.5 km away from the boundary. Thematic collection: This article is part of the Applications of Innovations in Geochemical Data Analysis collection available at: https://www.lyellcollection.org/cc/applications-of-innovations-in-geochemical-data-analysis

Graphite has been identified as a strategic mineral resource in China and the USA owing to its excellent physical and chemical characteristics and wide applications Wang et al. 2019). China has the world's largest flake graphite reserves, of which the North China Craton (NCC) accounts for 74% (Xiao et al. 2016). The Xinrong graphite deposit of Shanxi province is located in the SW of the graphite mineralization belt of the NCC. At the end of 2020, 52 million tons of flake graphite mineral reserves were detected in this region, with estimated reserves of up to 100 million tons. Currently, the study of graphite ore mainly focuses on the source of carbon, the characteristics of the ore body and the genesis of the ore deposit. However, the role of the surrounding rocks in the evolution of ore formation is unknown (Snachev et al. 2015;Khanchuk et al. 2018;Yan et al. 2018), and little research has been conducted in this area (Zhao et al. 2019). Therefore, the study of the genetic characteristics of rocks has great significance in this region, especially systematic research on the characteristics of graphite (Liang et al. 2020). This work describes the characteristics of the ore body and surrounding rocks, and indicates the relationship between them, which provides evidence for the evolution of the graphite belt in the NCC.

Geological setting
The Datong Xinrong graphite ore belt is located in the eastern region of the intraplate active belt (II) belonging to the margin of the NCC, where the geological conditions are very favourable for metallogenic rocks. The graphite ore belt is 22 km in length and 40-750 m in width in the NE -SW direction, and its morphology is mainly layered with branching and merging. The inclination of the ore is 272-340°and the obliquity is 60-88° (Liang et al. 2020).
The study area includes the Huangtuyao Formations in the Jining Group of the Middle Archean, Zumapo Formation of the Cretaceous, Yungang Formation of the Middle Jurassic, Hanuoba Formation of the Neogene, and Quaternary epochs (Fig. 1). The graphite-bearing gneiss (GBG) of the Huangtuyao Formation, which is generally a monoclinal structure, comprises an ore-bearing horizon. The Gehuyao gneiss (GHG) is in conformity with the GBG, and contains many lens-like rocks, such as metamorphic mafic dykes, xenoliths and residues (Fig. 2c). The fold structures in this area mainly include the Qihuang-Songjiazhuang anticline, Duijiugou syncline, Lihuazhuang syncline and Beiyujian fold group. The region also developed a large number of small-scale and shape-changing folds. Magmatic rocks are well developed in Xinrong District, primarily consisting of mixed granite of the Wutai stage, diabase of the Luliang stage, and granitic pegmatite. Among these, the NW-trending diabase is larger and divides the ore body, while the pegmatite is very small at 0.1-2 m and contains heavy mineralization (Fig. 2f ).

Major and trace element analysis
The samples for geochemical analysis were crushed and powered to 200 mesh in an agate mill. Major and trace element analyses were carried out at the laboratory of Shanxi Geological Exploration Bureau and China University of Geosciences. Major elements were analysed by wet chemistry and X-ray fluorescence spectrometry with analytical uncertainties ranging from 0.5 to 1.5%, and the total amount between 99.30% and 100.70% (Chen and Zhao 2021;Cho et al. 2021).
The trace element analysis was conducted using inductively coupled plasma mass spectrometry, for which measurements were better, the margin of error was 3-5% Yi et al. 2021).

Analysis result
Nineteen samples were obtained for the whole-rock geochemical analysis and 10 were obtained for trace elements from the GBG (Tables 1 and 2). From the GHG, which represents the surrounding rocks, the corresponding numbers of samples were 24 and 10 (Tables 3 and 4).

Results
The geochemical compositions of rocks are crucial for the identification of protoliths , and trace elements are effective indicators of environmental tectonics (Cheng et al. 2018). Although the lithology of the ore body and surrounding rocks is gneiss in this region (Fig. 2), we can use major and trace element data to distinguish the genesis and tectonic setting.

Graphite-bearing gneiss
Trace element ratios are typically used to discern positive and paranormal metamorphic rocks. The elemental ratio combination of Sr/Ba < 1, Fe/V < 250 and Ca/Sr > 200 is considered an inherent feature of parametamorphic rocks (Geology of Nanjing University 1984; Wang et al. 2021). Based on the trace element data of 10 samples from the GBG (Table 1), we determined that the Fe/V values of the samples were <250, which is similar to the characteristics of parametamorphic rocks. The Sr/Ba values were less than 1 and the Ca/Sr values exceeded 200 in nine of the 10 samples. Therefore, it was assumed that the GBG is a type of parametamorphic rock.
Based on this inference and the research of Zhou (1984), the TiO 2 -F diagram (Misra 1971), (Al + Fm) -(C + Alk)/Si diagram (Simone 1953) and (Al-Alk) -C diagram (Winchester 1971), which use whole-rock chemical composition data, have a notable effect on determining whether metamorphic rocks have a sedimentary protolith. In the TiO 2 -F diagram (Fig. 3a), most of the examined samples are distributed in the paranormal metamorphic rocks category, and five samples are distributed near the boundary of positive metamorphic rocks. This indicates that the protolith of the GBG is likely to be of sedimentary origin. The samples have rightdipping rare-earth element patterns with strong negative Eu anomalies (Fig. 3b). This indicates that there are a lot of marine materials in this area, which have the characteristics of littoral and shallow-sea sediments (Liang et al. 2020). Most samples plotted in the (Al + Fm) -(C + Alk)/Si diagram (Fig. 3c) also affirmed the possibility of a mudstone protolith, whereas few were categorized as volcanic rocks, also suggesting a mudstone protolith. The (Al-Alk) -C diagram was used to further determine the type of mudstone as argillite (Fig. 3d). By comparing the data in Figure 3, we determined that the samples obtained from the boundary area ( Fig. 3a) are highly consistent with those of the positive metamorphic samples in others. However, the Al-Alk values of those samples (except one, No. 14) exceeded 20, which indicates that they may be calcareous claystone (Winchester 1971). The protoliths of these samples were likely to be sedimentary rocks modified by pegmatite in the later stages of metamorphism. Overall, these observations explained the increase in graphite scales and grades towards pegmatite veins in this graphite ore (Li 2015). We conclude that the protolith of the GBG is carbonaceous argillite.
The ratios of major elements are important indices for studying the tectonic environment of sediments (Rehfeldt et al. 2006). Using the characteristics and components of sedimentary rocks, we can determine the tectonic setting from ω(Al 2 O 3 )/ω(Al 2 O 3 + Fe 2 O 3 ). When the ratio is between 0.6 and 0.9, it is generally the continental margin environment. If the ratio is between 0.4 and 0.7, it indicates pelagic environments, and between 0.1 and 0.4 it indicates oceanic ridge environments (Jewell and Stallard 1991). Figure 3e indicates that the discriminant ratio of those samples is mainly in the range 0.70-0.87. This suggests that the original rocks of the graphite ore were deposited in the continental margin environment. Because K + is easily absorbed and Na + is readily leached, the K 2 O/Na 2 O-SiO 2 diagram reflects the elemental composition of rocks as well as the denudation source (Roser and Korsch 1986). According to the result in Figure 3f, the detritus of this ore deposit originated from an arc region.

Gehuyao gneiss
Based on the trace element data of the GHG (Table 3), we determined that most of the Sr/Ba values were >1, which is a characteristic of positive rocks. The Sr/Ba value of sample 1 was an exception at 0.76. Therefore, it was inferred that the GHG is a positive rock. Furthermore, we found that the GHG is an adak-type of volcanic rock from the Ta/Yb diagram (Fig. 4a) and Sr/Yb diagram (Fig. 4b) (Pearce et al. 1984;Atherton and Petford 1993). Major and trace element analyses of the GHG show that they have high SiO 2 (55.14-69.02 wt%), high Al 2 O 3 (14.22-18.09 wt%), high Sr (≥512 ppm) and low Yb (≤1.92 ppm) (Tables 3 and 4). According to the definition and composition of adakitic rocks (Defant and Drummond 1990), the GHG has typical adakitic characteristics (Zhang et al. 2012). The value of K 2 O/Na 2 O is considered as the scale of the potassium content in the rocks (Whalen et al. 1987). Table 4 shows that the K 2 O/Na 2 O values were <0.65, excluding sample 11. This suggests that the GHG shows the characteristics of low-potassium adakitic rock. Adakitic rock is a type of cryptocrystalline volcanic rock with mafic porphyry, which belongs to the calc-alkaline lithology of arc and is produced by the interaction between the subduction plate and overlying mantle. Two viewpoints for the origin of adakitic rock are widely accepted: (1) melting of subducted oceanic crust (Defant and Drummond 1990); (2) melting of the thickened basaltic lower crust (Kay et al. 1993). Adakitic rock that is formed by melting of the thickening lower crust is generally characterized by high K 2 O (>2.0 wt%), and low Al 2 O 3 (Wang et al. 2004). However, the GHG is characterized by high Al 2 O 3 (14.22-18.09 wt%) and low K 2 O. These values exceeded 2, which is less than 20%, and do not support the theory of melting of the lower continental crust. Most samples plotted in the MgO/SiO 2 diagram (Fig. 4c) also affirmed the possibility of subducted oceanic crust. In summary, we postulated that the GHG was formed by partial melting of subducted oceanic crust.
Owing to the GHG being adakitic rock, we used the total alkali v. silica (TAS) diagram to classify chemical constituents of volcanic rocks (Miyashiro 1974). In the TAS diagram (Fig. 4d), most of the examined samples were distributed in the andesite rock category, and five samples were distributed in the dacite rock category. This indicates that the GHG is mainly composed of andesite rock, with a small amount of dacite rock. Samples 3 and 12 were categorized as basaltic andesite rock (Fig. 4d) and did not fall within the range of the adakitic rock (Fig. 4c), suggesting basaltic rock in the xenoliths may be accumulated in the samples.

Discussion
In the Early Paleoproterozoic era (2.30-2.10 Ga), the NCC was dominated by an extensional tectonic regime with the breakup of the Kenorland continent and evolved into a passive continental margin (Fig. 5). Part of the GHG was formed from arc magmatic rocks (Li et al. 2006). Given the protolith of graphite ore that was deposited and formed in the continental margin during this period (Liang et al. 2021), we compared the compositions of the GBG and GHG (Tables 2 and 4), and found that the main compositional content (e.g. SiO 2 , Al 2 O 3 , and TiO 2 ) of the GBG was slightly lower than that of the GHG, which indicates the phenomenon of inheritance. Combined with the detritus of this ore deposit originating from an arc region (Fig. 3f ), it is assumed that the ore deposit accumulated the weathering products of GHG during the sedimentary period.
Every deposit is unique in some way, which is typically linked to the unique local geological parameters of the rocks that host and enclose a mineral deposit (Yousefi et al. 2021). Based on the above research, it can be considered that the depositional area receiving the detritus of the GHG can be used as an indicator of graphite ore prospecting. From Figure 1, we know that the distance between the GBG and the boundary, which distinguishes the Jining group and the GHG, ranges from 0.5 to 2.7 km. The average distance c. 2 ± 1.5 km can be used for generating an evidence layer for exploration targeting. We predict that the NW area, 2 ± 1.5 km away from the boundary, can be considered as a potential area for graphite mineralization in the NCC (Fig. 5a). Due to scale, the forecast area is not shown on the map.
The Xinrong and Xinghe districts are located at the two ends of the potential area (Fig. 5). In the Xinghe district, many regional metamorphic graphite deposits have been discovered since 1980 (Wang 1989;Yan et al. 2018). The palaeogeographic environment and ore genesis of the graphite deposits in both of the Xinghe and Xinrong districts are consistent, suggesting that the Datong-Xinghe district is an area of great potential for graphite deposits in the NCC.

Conclusions
Major and trace element data are widely used to identify the characteristics of ore deposits, for example the protolith, the genesis and tectonic setting of rock. Using this method, we present the following conclusions for this region's GBG which was formed by the regional metamorphism of carbonaceous argillite. The carbonaceous argillite was deposited in a continental margin environment and received detritus from the GHG. The GHG is a typical lowpotassium adakitic rock, which is mainly comprised of andesite rock and small amounts of dacite rock. The genetic model is melted subducted oceanic crust.
In conclusion, this study provides evidence for ore body evolution and makes it possible to establish a sedimentarymetamorphic model of the graphite ore in this region. Furthermore, we predict the potential area for graphite mineralization in the NCC. This research not only addresses the research gap in this area and provides a foundation for mining exploration, but also proposes an area of great potential for graphite deposits in the NCC. Based on the resources of graphite mineral reserves between the Datong and Xinghe regions, the Datong-Xinghe graphite belt of the NCC has the potential to be the world's largest graphite mining area.
Acknowledgements We greatly appreciate the work of the editor and anonymous reviewers, whose valuable comments improved the manuscript. Competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.