Genesis of the Qingchayuan Flake Graphite Deposit in the Huangling Dome of Yangtze Block, South China

Abstract

The Qingchayuan flake graphite deposit is located in the Huangling Dome, which represents a part of the Yangtze Block in South China. This deposit is a major and highly typical flake graphite deposit within this metallogenic region. The graphite ores are found within graphite-bearing mica schist and graphite-bearing biotite–plagioclase gneiss. The fixed carbon content varies from 3.52 to 13.78% with an average of 7.83%. The major element analysis shows that the main chemical components of the Qingchayuan flake graphite ore are SiO₂, Al₂O₃, TFe₂O₃, and K₂O. The carbon isotope study of the graphite ore indicates light carbon values ranging from −22.80 to −26.72‰, suggesting that it has a biogenic origin. In addition, the sulfur isotope values of the graphite samples range from −10.67 to −14.58‰, indicating the formation of the graphite deposit is related to biological processes. The presence of traces of migmatization around the graphite deposit indicates that the graphite has undergone ultra-high temperatures during the formation process. The origin of the Qingchayuan flake graphite deposit is explained by a two-stage genetic model, which involves material deposition and regional metamorphism (including migmatization). Firstly, after the deposition of carbonaceous material and its conversion into graphite by regional metamorphism, the graphite might have undergone recrystallization, resulting in the development of big flakes due to migmatization. This model is supported by previous studies and newly collected information.

1. Introduction

Graphite is one of the most commonly observed forms of elemental carbon. It can be found alongside other forms, such as graphene, lonsdaleite, diamond, fullerenes, and numerous amorphous forms [1]. Even though it is a non-metallic material, graphite exhibits excellent metallic properties, making it one of the most useful non-metallic minerals. Graphite possesses attributes such as coatability, lubricity, plasticity, thermal conductivity, and electrical conductivity. It is also chemically stable and resistant to high temperatures, corrosion, and acids and alkalis [2]. Due to its unique structure, graphite has distinct physicochemical properties, making it widely used in refractory materials, brake linings, casting molds, lubricants, batteries, steelmaking, and more, accounting for approximately 85% of its total usage [3,4]. Naturally, graphite occurs in metamorphic rocks as flake-like, striated, blocky, or earthy aggregates and rarely in veins [5]. It is classified into crystalline (flake) and cryptocrystalline (earthy) graphite, with crystalline graphite holding significant industrial value due to its ease of separation in industrial processes [6]. Crystalline graphite, with its larger flakes and better selectivity, holds significant industrial value and is considered a non-renewable resource [5]. As a result, the European Commission classified graphite as a rare mineral in its report titled “Critical Raw Materials Resilience: Charting a Path towards Greater Security and Sustainability.” Similarly, flake graphite was included among the twenty-four key mineral species in China according to the “National Mineral Resources Planning (2016–2020)” report. Recognized for its importance, flake graphite is considered a critical mineral in China and the rest of the world [7,8,9,10], contributing to increasing global demand, especially in the lithium-ion battery sector, which has surged by 200% since 2019 [4].

China’s flake graphite deposits are primarily distributed around ancient continental blocks, such as the Yangtze Block and the North China Craton, with major formation periods from the Neoarchean to the Early Cambrian. The most extensive and high-quality deposits are associated with the Neoproterozoic, particularly in the Yangtze Block, which hosts significant deposits like the Zhongba and Sanchaya graphite deposits [11,12].

The Huangling Dome, located in the Yangtze Block, is geologically significant for its complex stratigraphy and deformation features. It contains regional metamorphic crystalline graphite deposits, with large flakes up to four to five mm in diameter [13]. Understanding the genesis and characteristics of these deposits is essential for further graphite exploration and development.

This study presents a detailed analysis of the Qingchayuan flake graphite deposit, focusing on its mineralogy, petrography, and carbon and sulfur isotope data. Based on previous research and the thorough analyses completed in our work, a genetic model for the formation of graphite is proposed. Gaining a thorough comprehension of the ore-forming environment and characteristics of the Qingchayuan flake graphite deposit would facilitate the identification and investigation of graphite deposits in the area.

2. Regional Geological Setting

The Qingchayuan flake graphite deposit is located in the northern part of the Yangtze Block, which is bordered by the Cathaysia Block to the southeast and separated by the Jiangnan Orogen (Figure 1; [14,15]). The Yangtze Block, with its basement composed of Archean to Proterozoic rocks, is overlain by Neoproterozoic strata. The Kongling metamorphic complex in the Northern Yangtze Block represents the oldest exposed unit, consisting of Archean-Paleoproterozoic high-grade metamorphic rocks, including TTG gneisses and amphibolites [16].

The Huangling Dome, about 200 km north of the Yangtze Block, consists of the Archean-Proterozoic Kongling terrane and the Neoproterozoic Huangling igneous complex, which includes various intrusive rock types formed between 860 and 790 Ma [17,18,19,20,21]. This region is significant for its complex tectonic history and stratigraphic deformation, with the Huangling Dome exposing the Yangtze Block’s earliest basement metamorphic rock series dating to 3290 Ma [22,23,24].

The Qingchayuan graphite deposit, hosted within the Paleoproterozoic strata of the Huangling anticline, is composed of large-flake graphite ore with flakes ranging from 0.15 to 5 mm [13]. The graphite ore is associated with intense migmatization and structural features, including isoclinal folding and ductile shear zones, which align with regional northeast-trending and east–northeast-trending faults. Mafic dikes, such as diabase and lamprophyre, intruded the graphite orebodies, further complicating the structural geology of the area.

All the layers of high-quality graphite ore in this region are found in the lower part of the Huanglianghe Formation. The magmatic rocks related to the occurrence of graphite are categorized into two types. One type is demonstrated by Quanyi potassium feldspar granite, which is a large-scale formation with a zircon U-Pb age of 1854 Ma [25]. This finding could have important consequences for the process of migmatization that follows. The other type consists of dikes (diabase dikes and lamprophyre dikes) commonly observed in the mining area, which intruded and disrupted the orebodies after the formation of the graphite ore. Migmatization is widespread and intense in this area, with all mineralized rocks undergoing migmatization, resulting in migmatized graphite schists and migmatized graphite-bearing biotite–plagioclase schists. Migmatitic granites are spatially closely related to the graphite ore, with most graphite deposits containing migmatitic granites (leucogranite, syenogranite, and syenite pegmatite) that are in direct contact with the ore layers, often occurring as interlayers within the ore layers. The graphite ore areas generally exhibit monoclinic structures, with the structural lines trending in the same direction as the regional ductile shear zones, primarily northeast and east–northeast. Later, northwest-trending regional faults (such as the Jiaozhanya Fault and the Wuduhe Fault) dislocated the ore-bearing rock series and the ductile shear zones. Secondary northwest-trending brittle faults within the mining area directly disrupted the graphite orebodies and were often later filled by dikes.

3. Qingchayuan Flake Graphite Deposit

The central part of the Huangling Dome basement is one of the major concentration areas for regional metamorphic crystalline graphite deposits in China and the only large flake graphite deposit area in the Northern Yangtze Block. The Huanglianghe Formation, which serves as the ore-bearing horizon, comprises a Khondalite series of graphite-bearing schists, gneisses, marbles, and calc-silicate rocks, with the ore-bearing rock series widely distributed in the region (Figure 2).

Graphite orebodies typically have stratiform, stratiform-like, and lenticular shapes, generally exhibiting a stable distribution. The orebodies usually extend for hundreds of meters to over 1000 m along the strike, with some of them reaching more than 3000 m (Figure 3). The thickness of the ore strata ranges from a few meters to more than 30 m. The hanging and footwalls primarily comprise biotite–plagioclase gneisses, diopside rocks, diabase, marble, or leucogranite (Figure 3). The orebodies’ occurrence typically aligns with the stratigraphic layers, with the contact between the hanging wall and footwall showing either a gradual or abrupt transition. In certain regions, the boundaries are clearly defined.

The mining region of the Qingchayuan graphite deposit is situated on the southeastern side of the overturned syncline at Bashansi. It is distinguished by a monoclinic structure and has a stratigraphic sequence that is reversed overall. The exposed strata mainly comprise the lower section of the Huanglianghe Formation and the Dongchonghe Complex, dipping southeast at an angle of approximately 50°. The fault structures in the area are highly developed; however, they are rather minor in size and have a minimal impact on the orebodies.

The mining area contains a variety of magmatic rocks, including granite porphyry dikes and diabase dikes. The enclosing rocks of the orebodies primarily consist of biotite–plagioclase gneisses, calcareous–silicate rocks, and marbles (Figure 4). The occurrence and attitude of the orebodies are generally consistent with the enclosing rocks, exhibiting stratiform and lenticular distribution patterns.

4. Analytical Methods

4.1. Petrography Observation and Analysis

Eleven graphite samples were acquired from the Qingchayuan flake graphite deposit for analysis. The materials were analyzed by examining thin sections using an optical microscope, as well as an electron microprobe in backscattered electron (BSE) mode. The BSE images were produced at the School of Earth Sciences, China University of Geosciences (Wuhan), using a JEOL JXA-8230 electron probe microanalyzer (JEOL, Tokyo, Japan). The imaging parameters consisted of an accelerating voltage ranging from 10 kV to 20 kV, a working distance between 10 and 15 mm, and an electron beam current of 10 nm. Before imaging, the thin sections were coated with carbon. Figures were produced using CorelDraw 2020 and data were calculated using WPS Office 2024.

4.2. Fixed Carbon

Fixed carbon was detected using a muffle furnace at ALS Minerals (Guangzhou). The sample was digested with dilute hydrochloric acid to remove inorganic carbon. Then it was filtered, and the filter residue was washed with deionized water and dried. It was then burnt at 425 °C to remove organic carbon, and a carbon–sulfur analyzer (LECO, model CS844, St. Joseph, MI, USA) was used to determine the fixed carbon content of the remaining residue by high-temperature infrared spectroscopy.

4.3. Major Elements

The samples used for analysis are all fresh rock ores. After removing surface impurities, they are crushed, reduced to 300 g, and ground to below 200 mesh (75 microns) in a pollution-free environment. Major element analysis was performed at ALS Minerals (Guangzhou) using an X-ray fluorescence spectrometer (XRF, Dutch PAN aluminum, model PW2424). The sample was placed in a muffle furnace at 1000 degrees Celsius for aerobic burning, and then a flux containing lithium nitrate was added. After thorough mixing, it was melted at a high temperature. After pouring the molten material into a platinum mold to form a flat glass sheet, the elemental compositions of the glass sheets generated from the samples were measured with the X-ray fluorescence spectrometer at a voltage of 50 kV and a current of 50 mA.

4.4. Carbon and Sulfur Isotope Analysis

Four graphite samples were chosen for carbon and sulfur isotope analysis. The analysis was conducted at Beijing Createch Testing Technology Corporation Limited using a Thermo Fisher Scientific MAT 253 plus gas isotope mass spectrometer (ThermoFisher, Waltham, MA, USA), coupled with a GasBench system and a Thermo Fisher Scientific FLASH 2000 HT spectrometer. The comprehensive analytical methodologies can be succinctly summarized as follows:

The sample is subjected to instantaneous high-temperature combustion in an oxygen-rich environment, producing a mixed gas containing carbon, nitrogen, oxygen, and sulfur components. This gas is then carried by high-purity helium and undergoes an oxidation reaction to form CO₂ (SO₂). All the gases produced during combustion are transported by the helium carrier gas through an oxidation–reduction reaction tube, ensuring complete oxidation of the gases (with the small amount of SO₃ generated being reduced to SO₂ as it passes through a copper wire). The gases then pass through a chromatographic column, where CO₂ and SO₂ are separated, before entering the mass spectrometer for analysis.

The δ¹³C‰ value, which indicates the isotopic ratio relative to the Vienna Peedee Belemnite (V-PDB) standard, was determined using the formula δ¹³C‰ = [(R_sample/R_standard) − 1] × 10³, where R = ¹³C/¹²C. The error margin for δ¹³C is ±0.05‰, and all data are provided using the conventional delta notation relative to V-PDB.

The δ³⁴S‰ value, representing the isotopic ratio relative to the Vienna Canyon Diablo Troilite (V-CDT) standard, was calculated using the formula: δ³⁴S‰ = [(R_sample/R_standard) − 1] × 10³, where R = ³⁴S/³²S. The margin of error for δ³⁴S is ±0.02‰, and all values are reported in the conventional delta notation relative to the V-CDT standard.

5. Results

5.1. Ore Mineralogy

The predominant graphite ore types in the mining area are largely classified into two categories: schist-type rich ore (graphite-bearing mica schist) and gneiss-type poor ore (graphite-bearing biotite–plagioclase gneiss). Their mineral assemblages are as follows:

Gneiss-type (Figure 5a,b): The main ore mineral is graphite, accompanied by gangue minerals, such as quartz, feldspar, biotite, and sericite (Figure 6). Additionally, there are small amounts of titanite, zircon, and garnet. Graphite minerals occur as flakes with diameters ranging from 0.02 to 1.5 mm, associated with biotite and sericite. The ore exhibits a granoblastic flake structure, with foliated and gneissic textures.

Schist-type (Figure 5c): The main mineral found in the ore is graphite, accompanied by gangue minerals, such as quartz, biotite, sericite, and feldspar (Figure 6). There are also smaller quantities of zircon, tourmaline, pyrite, and secondary limonite (Figure 7). Graphite minerals occur as flakes with diameters ranging from 0.01 to 2 mm, which are arranged directionally and are associated with biotite and sericite. The ore exhibits a granoblastic flake structure with a foliated texture, and locally, gneissic and augen textures are observed.

Under a (thin-section) transmission light microscope, graphite is completely opaque, and only its outline can be seen (Figure 6). In reflected light, the morphology of graphite varies with different flake orientations (Figure 6). In sections parallel to the foliation, the basal surfaces of graphite are mostly irregular, occasionally showing hexagonal basal surfaces. In sections perpendicular to the foliation, graphite appears to be flaky, with fine and clear cleavage planes. Graphite has a low reflectivity but exhibits high levels of bireflectance, making it quite prominent under the microscope.

Graphite can be broadly classified into two types based on its occurrence: (1) Graphite is arranged parallel to foliation and gneissosity, densely distributed, closely intergrown with mica, embedded in mica cleavage, or co-crystallized with mica in parallel (Figure 6). (2) Graphite is irregularly distributed among feldspar and quartz or cuts through feldspar and quartz (Figure 6). Due to stress, graphite flakes often bend into arc shapes or even form S-shapes.

5.2. Fixed Carbon

The results for the fixed carbon content of seven graphite-bearing samples from the Qingchayuan flake graphite deposit are shown in

Table 1. Fixed carbon of graphite-bearing samples in the Qingchayuan flake graphite deposit.

Sample No.	Fixed Carbon Content (%)
20QCY01	5.64
20QCY03	13.78
20QCY05	13.54
20QCY06	5.47
20QCY07	7.02
20QCY08	3.52
20QCY11	5.84

. The fixed carbon content varies from 3.52 to 13.78% with an average of 7.83%.

5.3. Major Elements

Typical graphite-bearing mica schist samples were analyzed to investigate the geochemical characteristics of the deposit. The results of the major element analysis are listed in

Table 2. Major element analysis of graphite-bearing mica schist the Qingchayuan flake graphite deposit (wt. %).

Sample No.	SiO2	Al2O3	TFe2O3	K2O	Na₂O	CaO	MgO	V2O3	P2O5	S	TiO2
20QCY01	61.77	13.63	6.85	2.82	1.28	1.62	2.70	0.02	0.10	1.30	0.63
20QCY05	60.95	15.10	5.22	2.79	0.48	0.70	2.03	0.02	0.04	0.04	0.60
20QCY06	57.39	14.72	7.76	4.16	2.19	2.33	3.89	0.02	0.10	1.08	0.82
20QCY08	59.73	13.73	7.06	3.58	1.50	0.90	3.70	0.02	0.06	0.03	0.67

. The major element analysis shows that the main chemical components of the Qingchayuan flake graphite ore are SiO₂, Al₂O₃, TFe₂O₃, and K₂O with contents in the ranges of 57.39–61.77, 13.63–15.10, 5.22–7.76, and 2.82–4.16 wt. %, respectively. The MgO content varies from 2.03 to 3.89 wt. %, and CaO is relatively low, varying from 0.70 to 2.33 wt. %. Na₂O (0.48 to 2.19 wt. %) and TiO₂ (0.60 to 0.82 wt. %) are less abundant. The ratio between K₂O and Na₂O is in the range of 1.90 to 5.81 with an average of 3.08, which indicates that the ore is K-rich and Na-poor.

5.4. Carbon Isotopes

Table 3. Carbon and sulfur isotope data of graphite samples from the Qingchayuan flake graphite deposit in the Huangling Dome, Yangtze Block. Q-1 data are sourced from [27].

Sample No.	δ13CV-PDB (‰)	δ34SV-CDT (‰)
20QCY01	−22.80	−14.58
20QCY02	−24.98	−12.38
20QCY03	−24.87	−11.77
20QCY04	−26.72	−10.67
Q-1	−22.13	Not provided

presents the carbon isotopic composition of graphite from samples within the Qingchayuan flake graphite deposit. The data reveal a single group enriched in light carbon, with δ¹³C values ranging from −22.80 to −26.72‰ and an average of −24.3‰. The consistent isotopic results indicate that the carbonaceous material has a uniform origin, which will be further elaborated in this paper’s discussion section.

5.5. Sulfur Isotopes

Table 3 details the sulfur isotopic composition of graphite samples from the Qingchayuan flake graphite deposit. The δ³⁴S values range from −10.67 to −14.58‰, with an average of −12.35‰. This consistent isotopic pattern suggests a uniform origin for the carbonaceous material, a topic that will be explored further in the discussion section.

6. Discussion

6.1. Origin of Flake Graphite Deposit Indicated by Carbon Isotopes

The carbon isotopic content of graphite can vary significantly between biogenic and abiotic sources. Biogenic graphite, typically found in metasediments, forms through the metamorphism of organic matter in a process known as graphitization [2]. These graphites often exhibit lighter carbon isotope values, with δ¹³C values for organic matter averaging around or below −25‰ [28]. On the other hand, abiotic graphite, which may be found in meteorites and ultramafic igneous rocks, has higher isotope compositions. The magmatic carbon content in carbonatite and kimberlite is normally around −5 ± 2‰ [29]. Crystallization from magmatic melts [30] or deposition from carbon-bearing fluids [5,11,31,32] are two possible processes that might result in the formation of heavier isotopic graphite.

After graphite crystallizes, its physicochemical stability significantly restricts isotope exchange [33,34]. After complete crystallization, graphite shows very little change in isotopic composition, even when subjected to intense pressure and temperature during metamorphism and deformation [35,36]. Carbon isotopes serve as a dependable geochemical diagnostic for determining the origin of carbon in graphite.

The carbon isotope data obtained from the Qingchayuan flake graphite deposit, as shown in Figure 8, align with values often associated with organic carbon and are similar to the isotope data seen in crude oil from the Bohai Gulf and coal [36]. The Qingchayuan graphite has carbon isotope values that are lighter and more consistent compared to other graphite deposits, indicating it originated from a single organic precursor under specific metamorphic conditions [27]. Consequently, the carbon isotope values range from −22.80 to −26.72‰ (Table 3), and their distribution (Figure 8) suggests that the graphite in the Qingchayuan flake graphite deposit is of biogenic origin. The carbon isotope data of the Qingchayuan flake graphite deposit are not comparable to the isotopic levels of marine sediments (0 ± 2‰) or magmatic carbon (−5 ± 2‰) [37].

6.2. Origin of Flake Graphite Deposit Indicated by Sulfur Isotopes

The sulfur isotope not only provides crucial insight into early life evolution but also offers key information on the source of ore-forming materials and the processes involved in mineralization [41]. As sulfate concentrations in the oceans increased, sulfate-reducing bacteria began to sustain metabolism through the process of “sulfur respiration” (2CH₂O + SO₄²⁻ = 2CO₂ + H₂S + 2OH⁻), which resulted in significant sulfur isotope fractionation. In particular, the appearance of sulfur isotope fractionation in Precambrian sediments marks the widespread emergence of sulfate-reducing bacteria and the onset of sulfur respiration [42], indicating the presence of biological sulfur and its close relationship to the organic origin of graphite deposits.

Furthermore, variations in sulfur isotopes provide clues to the sources of sulfur in ore-forming fluids. Studies have shown that sulfur in hydrothermal fluids primarily originates from several sources: biogenic sulfur in sedimentary rocks (δ³⁴S Σs = −20 to −10‰), sulfur from the mantle and deep crust (δ³⁴S Σs ≈ 0 ± 3‰), and sulfur from seawater and evaporites (δ³⁴S Σs ≈ +20‰) [43]. Under equilibrium conditions, the δ³⁴S values of sulfides are controlled by the physicochemical conditions of the hydrothermal fluid, and the relative mass differences between different sulfur isotopes govern the fractionation process [44]. If the ore-forming fluid contains significant amounts of marine sulfate and seawater, the sulfur isotopes tend to be enriched in heavier S isotopes. In contrast, if biogenic sulfur dominates, the fluid is characterized by an enrichment of lighter S isotopes [45].

The sulfur isotopic variation in the Qingchayuan graphite deposit is relatively small, indicating a homogeneous sulfur isotopic composition in the ore-forming fluids during mineralization, with biogenic sulfur from sedimentary rocks as the primary source, and potentially a minor contribution from granitic rocks. Analyses of pyrite-bearing graphite ore samples show that the sulfur isotope values are concentrated around slightly negative values (with an average of −12.35‰), further supporting the idea that the sulfur in the graphite deposit originated from organic-rich sedimentary rocks, displaying a composite signature of biogenic organic sulfur and sulfur from granitic rocks (Figure 9). Therefore, sulfur isotopes play a vital role in revealing the origin of graphite deposits and their relationship to biological processes.

6.3. Genesis of the Qingchayuan Flake Graphite Deposit

According to the “Geological and Mineral Exploration Standards: Specification for Graphite, Broken Mica Mineral Exploration (Draft for Approval)” published by the Ministry of Natural Resources of the People’s Republic of China, the minimum industrial grade of flake graphite ore is 2.5%. The fixed carbon content in the Qingchayuan flake graphite deposit varies from 3.52 to 13.78% with an average of 7.83%, which is much larger than the minimum industrial grade of flake graphite ore. The mineral survey results show that the amount of graphite ore in the Qingchayuan flake graphite deposit exceeds 100 million tons, and the amount of graphite mineral exceeds five million tons, indicating a super large graphite deposit.

Based on the different host rocks of graphite ores, the graphite deposit can be divided into three types: regional metamorphic (in which host rocks are gneiss, schist, marble, and orthogneiss), contact metamorphic (in which host rocks are slate and phyllite), and hydrothermal (in which host rocks are granite, diorite, and felsic rocks) [48]. The Qingchayuan flake graphite deposit, primarily comprising schists and gneiss, indicates its formation through regional metamorphism. These deposits are typically located around ancient platforms and blocks. The carbon isotope data suggest that organic carbon is the primary source of ore-forming material. The sulfur isotope data indicates that the formation of the Qingchayuan flake graphite deposit is related to biological processes.

After analyzing new data and reviewing the data of past explorations, we can suggest a two-stage genesis model for the Qingchayuan flake graphite deposit:

Stage I (Sedimentary deposition): The precursors to the present graphite-bearing rocks were likely marine sediments that incorporated organic matter during their deposition. This organic matter eventually underwent metamorphism, leading to the formation of the high-grade flake graphite deposit observed today.

Step 1 of Stage II (Regional metamorphism): In the regional metamorphism phase, carbonaceous material present in sedimentary layers transforms to either earthy graphite or small-sized flake graphite. This transformation occurs due to intense tectonic compression, which aligns minerals according to the applied stress. As regional metamorphism progresses, it often coincides with dynamic metamorphism, elevating the metamorphic grade to that of amphibolite or granulite facies. This advancement in metamorphic grade leads to the formation of graphite-rich layers, especially in carbon-rich sections of the strata. The metamorphic mineralization process modifies the carbonaceous material, promoting its conversion into crystalline graphite with an ordered structure. According to previous XRD measurements, the graphite in this region is of the 2H type, with a space group of P63/mmc, a₀ = b₀ = 0.2462 nm, and c₀ = 0.6711 nm [49]. The calculated graphitization degree in this area is 97.67% [49]. The crystal structure analysis data show that the graphite atom stacking in this area has a high degree of order and was formed in an environment with a high degree of metamorphism.

Step 2 of stage II (migmatization): The graphite deposits in the Huangling area have generally undergone intense migmatization, characterized predominantly by banded injections with a striped structure (Figure 10a). At this stage, all traces of the precursor rock from stage I were unrecognizable. In some local regions, ptygmatic and augen structures (Figure 10b,c) can be observed, indicating significant plastic deformation. The orebodies are well developed with migmatitic granite, exhibiting concordant intrusion features and a multilayered distribution. Migmatization has a pronounced impact on the graphite deposits, leading to a substantial increase in graphite flake size and orebody thickness, albeit with a decrease in ore grade. These characteristics indicate that migmatization has been instrumental in the formation and development of the flake graphite deposits, which is essential for comprehending the geological evolution of this region. The geological setting and formation of the Qingchayuan graphite deposit show a similar formation and tectonic/metamorphic development to what is described in Proterozoic graphite deposits in other parts of the world, e.g., Brazil [50], Canada [39], Finland [51], and Norway [52,53].

7. Conclusions

Recent petrographic, mineralogical, carbon, and sulfur isotopic studies of the Qingchayuan flake graphite deposit in the Huangling Dome (Yangtze Block, South China) indicate a two-stage genesis model for this deposit with our conclusions as follows:

1.

The δ¹³C values of the graphite samples, ranging from −22.80 to −26.72‰, confirm a biogenic origin. This isotopic signature aligns with organic carbon found in sedimentary deposits and provides strong evidence for biological processes involved in the genesis of the Qingchayuan deposit.

2.

The δ^34S values, spanning from −10.67 to −14.58‰, indicate that the sulfur in the deposit originates primarily from organic-rich sedimentary rocks, with potential minor contributions from granitic sources. This signature links the deposit to biological sulfur cycling and further substantiates the biogenic origin of the graphite.

3.

The genesis of the Qingchayuan flake graphite deposit involved an initial phase of the sedimentary deposition of organic materials, subsequently subjected to high-grade regional metamorphism, including migmatization. The intense metamorphism not only transformed carbon-rich sediments into crystalline graphite but also significantly enhanced the size and quality of graphite flakes.

4.

Further explorations in the Huangling Dome and adjacent regions could expand the resource base, as similar tectonic and sedimentary settings may host comparable graphite deposits. Detailed studies of regional migmatization patterns and isotopic compositions will aid in identifying zones with an optimal flake graphite quality, aiding future resource assessments.