Origin of Black Shale-Hosted Dagangou Vanadium Deposit, East Kunlun Orogenic Belt, NW China: Evidence from Mineralogy and Geochemistry

Abstract

Little is known of a large black shale belt within the Naij Tal Group in the East Kunlun region, which hosts polymetallic deposits, including manganese, vanadium, and cobalt. The recently discovered Dagangou vanadium mineralization is the first black rock series-type vanadium deposit in the East Kunlun region and Qinghai Province and represents a significant find owing to its intermediate scale. This study investigated the mineralogy, major and trace elements, rare earth elements, and platinum group element geochemistry of the Dagangou vanadium deposit. Scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed that the main vanadium-bearing minerals are micas, followed by limonite, clay minerals, feldspar, and jarosite. The SiO₂/Al₂O₃, Co/Zn, Sr/Ba, and Pd/Ir ratios, as well as the Ir content of the ores, indicated strong involvement of hydrothermal activity in the mineralization process. The V/Cr, Ni/Co, and U/Th ratios, as well as the δU values and significant negative δCe anomalies, suggested that the vanadium-bearing black rock series formed in a strongly anoxic reducing environment. The Al₂O₃/(Al₂O₃ + Fe₂O₃) and MnO/TiO₂ ratios, along with weak positive δEu anomalies and strong enrichment of heavy rare earth elements, indicated that mineralization occurred in an extensional tectonic setting. The black shale-hosted vanadium polymetallic deposit formed in a setting that transitioned from an open oceanic deep-sea environment to a progressively shallower continental margin.

1. Introduction

The black shale series includes dark gray to black siliceous (silica-rich), carbonate (carbonate-rich), and argillaceous (clay-rich) rocks, along with their metamorphic equivalents. These rocks contain high levels of organic carbon and sulfides [1]. These black series used to be rich in vanadium (V), nickel (Ni), Mo, uranium (U), platinum group elements (PGEs), Se, Au, Ag, Cd, Ti, yttrium (Y), rare earth elements (REEs), Ba, phosphorus (P), and coal [2,3,4,5,6,7,8]. It often hosts large to super-large ore deposits and is, thus, known as the “polymetallic enrichment layer.”

The sedimentary environment, a source of metallogenic materials, and the genesis of black shale-hosted vanadium deposits have consistently been focal issues in research. Black shale series can form in stagnant settings such as continental shelf margins and ocean basins [9]. It is widely believed that they develop in anoxic reducing environments resulting from the degradation of organic matter [10]. However, the extent of anoxia remains poorly constrained, ranging from mildly anoxic to severely anoxic conditions [11,12,13,14,15,16]. The sources of metallogenic materials for vanadium deposits primarily include terrigenous clast [12,17,18,19], submarine hydrothermal venting [13,20,21,22,23,24], and seawater (involving the enrichment by organic matter) [14,25,26,27]. The genetic models can be broadly categorized into five types: seawater sedimentary [28,29,30], seafloor hydrothermal sedimentary [31,32,33,34,35,36,37], biogenic sedimentary [38], extraterrestrial impact [39,40], and hydrothermal-dominated mixed [41,42].

Black shale series typically appear dark gray to black due to their high organic matter content and commonly contain fine-grained sulfides. Particularly for black mudstones, the presence of abundant clay minerals (such as illite) further complicates the microscopic identification of minerals. This significantly increases the difficulty of studying the occurrence states of vanadium. It is widely recognized that vanadium enrichment in black shale-type vanadium deposits primarily occurs in three forms: within organic matter, hosted in clay minerals, and as independent vanadium-bearing minerals [28,43,44]. In recent years, with advancements in analytical techniques and testing precision, an increasing number of independent vanadium-bearing minerals have been identified, including vanadinite [45], vanadium garnet [46,47], calciovolborthite [47], Berndlehmannite [48], mannardite, and sulvanite [49,50].

The formation and development of black shale-hosted deposits are often closely linked to major turning points in Earth’s history, as well as to the assembly, breakup, and subsequent relatively tectonically quiescent environments of the supercontinent Pangea [8]. The East Kunlun Orogenic Belt (EKOB), located at the western portion of China’s Central Orogenic Belt (Figure 1a), has undergone two crucial tectonic regime transition periods: the early Caledonian period, marking the shift from a passive continental margin to an active continental margin, and the late Indosinian period, characterized by the transition from compressional orogeny to intraplate lithospheric extension and thinning, associated with the evolution of the Cambrian–Devonian Proto-Tethys Ocean and the Carboniferous–Triassic Paleo-Tethys Ocean [51,52,53,54,55,56,57], It also hosts numerous Ni-Cu-Co-Au-Fe-Pb-Zn-Li-Nb-REE deposits [58,59,60,61,62,63,64,65,66,67,68]. However, the EKOB lacks a significant black shale-hosted vanadium deposit [69]. Recently, several V deposits associated with black shale—Dagangou, North Dagangou, Wenquangou, and North Heishishan—were discovered in EKOB. The Dagangou V deposit in Golmud City has reached a medium-scale status, suggesting some potential for the region, and represents the first discovery of V deposits in EKOB.

However, studies on V metallogenesis are limited, with most addressing only regional patterns [70] while neglecting key aspects such as geological characteristics, ore-forming environments, deposit genesis, and metallogenic material sources. This study investigated the Dagangou V deposit, a black shale-hosted deposit in the EKOB. In this study, detailed geological field investigation, major and trace element analysis of whole rocks and vanadium ore, PGE analysis of vanadium ore, mineralogical observation, and SEM-EDS analysis of vanadium ore were conducted on the Dagangou deposit. This study aims to (1) determine the geological background of black shale in EKOB, (2) investigate the genesis of vanadium deposits, and (3) ascertain the occurrence state of vanadium.

2. Geological Setting

2.1. Regional Geology

The Dagangou V deposit lies on the southern margin of the Qaidam Basin in the Burhan Budai Mountains section of the central East Kunlun Mountains. Tectonically, it is part of the South Kunlun Terrane subduction–accretion complex zone within the East Kunlun Orogenic Belt of the Qin–Qi–Kun system. The East Kunlun Belt has undergone several marginal orogenic stages, evolving from the Pre-Proto-Tethyan Ocean to the Tethyan Ocean [19]. Its tectonic framework is marked by four nearly EW- to NW-trending faults: North Kunlun, Central Kunlun, South Kunlun, and A’nyemaqen. These faults divide the three tectonic zones within East Kunlun and separate them from the Bayan Har Belt to the south (Figure 1b). From north to south, the orogenic belt consists of the North Kunlun Caledonian back-arc rift zone, the Central Kunlun basement uplift granite zone, the South Kunlun composite accretion zone, the A’nyemaqen Ophiolitic Belt, and the Bayan Har Orogenic Belt [71,72].

The Ordovician–Silurian Central Kunlun Ocean subducted northward, forming a typical subduction–accretionary complex wedge on the southern slope of East Kunlun. Its northern boundary is defined by the Central Kunlun fault, adjacent to the East Kunlun magmatic arc, while the South Kunlun fault marks its southern boundary, adjacent to the Muztag–Xidatan–Buqingshan ophiolitic mélange zone and Maduo-Maqin accretionary wedge. This complex wedge represents an important giant suture zone in the northern Tibetan Plateau [73] and plate junction between the Qin–Qi–Kun and Tibet–Sanjiang orogenic systems [74]. The subduction–accretionary complex wedge on the southern slope of East Kunlun generally trends NWW. It comprises the Meso-Neoproterozoic Wanbaogou Group, Early Paleozoic Ordovician–Silurian Naij Tal Group, Early Triassic Hongshuichuan Formation, Early–Middle Triassic Naocangjiangou Formation, Late Triassic Babao Shan Formation, and Early Jurassic Yangqu Formation. Volcanic activity in the area is weak. Magmatic intrusions are common, primarily including Neoproterozoic collision-related granodioritic gneiss and monzonitic granite gneiss, Early Silurian subduction-related peraluminous monzonitic granite, and Late Silurian syn-collisional strongly peraluminous high-K calc-alkaline quartz diorite–granodiorite assemblages. Additionally, Late Triassic extensional gabbro is locally developed.

The Naij Tal Group constitutes the main geological body of this subduction–accretionary complex wedge. From bottom to top, the formations are Shuini Chang, Shihui Chang, and Halabayigou. The Halabayigou Formation is further subdivided into calcareous phyllite, carbonaceous phyllite, phyllite, and sandy-slate members. The calcareous phyllite member consists mainly of gray-black schistose calcareous phyllite, sericite phyllite, calcareous silty slate, and argillaceous–calcareous phyllitic slate, with subordinate gray quartz schist and gray mylonitized schistose metasandstone. The carbonaceous phyllite member is composed of gray-green to gray-black spotted carbonaceous phyllite, schistose phyllite, phyllitized slate, argillaceous slate, and silty slate, followed by metasandstone, gray-spotted meta-siltstone, feldspathic quartz siltstone, minor meta-medium- to fine-grained lithic quartz sandstone, and argillaceous limestone. The phyllite member is dominated by gray-green schistose phyllite, with lesser amounts of gray metasandstone, light-gray quartz–albite schist, and siliceous rock. The sandy-slate member primarily comprises gray meta-medium- to fine-grained feldspathic quartz sandstone, sub-litharenite, sub-arkose, dark gray thin-bedded sericite phyllitic slate, medium-bedded meta-siltstone, calcareous siltstone, and argillaceous–calcareous slate, followed by deformed conglomerate and pebble-bearing coarse sandstone. Overall, this suite represents shallow- to semi-deep marine sedimentary deposits. The mineralization is associated with island arcs, continental margin arcs, and back-arc basins within the active continental margin, predominantly forming submarine hydrothermal sedimentary deposits, such as Tuolugou cobalt, Santonggou manganese, and Dagangou V deposits, with a minor proportion related to mid-ocean ridges.

2.2. Ore Deposit Geology

The mining area is primarily underlain by the Halabayigou Formation of the Ordovician Naij Tal Group, with minor exposure of the Meso-Neoproterozoic Wanbaogou Group (Figure 2). The Wanbaogou Group occurs mainly as lenticular bodies in the northern part of the mining area. The outcrops have a surface width of 50–500 m and a length of ~5 km. The overall attitude of the unit dips northward at an angle of 53–77°. Lithologically, it is composed of microcrystalline marble and metasandstone, with contact between them occurring along ductile faults. The Halabayigou Formation is the dominant stratigraphic unit and serves as the ore-bearing sequence. This formation is divided into three lithological members from bottom to top: The Lower Member consists primarily of a carbonate sequence of light-gray limestone with locally well-developed schistosity, with a thickness exceeding 100 m. The Middle Member is the main V-ore-bearing horizon. The lower part comprises a yellowish-brown to gray-black argillaceous–calcareous slate, which has a fault-contact relationship with the underlying Lower Member. The middle part features interbedded thin-layered black carbonaceous and gray-black carbonaceous siliceous slates with occasional thin limestone interlayers. The upper part consists of interbedded dolomitic limestone and argillaceous–calcareous slate. The Upper Member is characterized by an interbedded sequence of argillaceous–calcareous slate and limestone, with thin sandstone layers exposed in the center.

Six faults were identified within the mining area, all trending NWW (northwest), and most are reverse faults. The development of these fracture structures induces ductile–brittle deformation in rocks, forming large-scale ductile shear or tectonic crush zones. Magmatic rocks and dikes are not extensively developed in mining areas. The dikes are predominantly quartz veins that intrude the strata in banded and lenticular forms.

Overall, 37 V ore bodies were identified within the four V mineralization and alteration zones studied. The V ore bodies generally range in length from 98 to 3720 m and in thickness from 0.7 to 15.03 m, with V₂O₅ grades of 0.5%–2.92%. The ore bodies exhibit simple morphology, layered organization, and significant variations in thickness and grade along the strike and dip, characterized by pinching and swelling. The M1 ore bodies in the HSI V ore zone and the M3-2, M3, and M3-5 ore bodies in the HSIII V ore zone are relatively large in both length and scale, representing the main ore bodies. The V ore bodies are hosted in black carbonaceous and carbonaceous siliceous slates, with the wall rock comprising limestone and argillaceous–calcareous slate. The overall estimated V ore resources in the mining area amount to 15,586 thousand tons, with an average deposit grade of 0.80% and V₂O₅ content of 126,625 tons, representing a medium-sized deposit [75].

3. Sampling and Analytical Methods

3.1. Sampling

Samples were primarily collected from the surface and deep drill holes along exploration line 9, specifically targeting the ore and wall rocks of the main M3-2 ore body (Figure 2b). The lithology of the ore is mainly carbonaceous and carbonaceous siliceous slate, whereas the wall rocks consist primarily of limestone, dolomitic limestone, and argillaceous–calcareous slate. Whole-rock major and trace element analyses were conducted for each sample. Representative samples were selected and polished into thin sections and blocks. After detailed microscopic observations, specific areas of the ores were delineated for scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS).

3.2. Major and Trace Element Analyses

Major and trace elements were analyzed at the Key Laboratory of Mineral Resources Evaluation, Northeast Asia, Ministry of Natural Resources, China. Fresh samples were powdered to 200 mesh using an agate ring mill, and a 0.1 g aliquot of the sample was accurately weighed (precision: ±0.1 mg) and transferred into a 50 mL polytetrafluoroethylene (PTFE) beaker. The sample was first moistened with a small volume of deionized water, followed by the sequential addition of 10 mL nitric acid (HNO₃), 10 mL hydrofluoric acid (HF), and 2 mL perchloric acid (HClO₄). The beaker was placed on an electric hotplate preheated to 250 °C and evaporated until dense perchloric acid fumes evolved; this state was were maintained for approximately 3 min. The beaker was then removed from the hotplate and allowed to cool to ambient temperature.

Subsequently, 10 mL HNO₃, 10 mL HF, and 2 mL HClO₄ were re-added to the beaker, which was then heated on the electric hotplate for 10 min. The power was turned off, and the mixture was left to stand overnight for thorough digestion. After standing, the beaker was reheated on the hotplate until the perchloric acid fumes were completely dissipated and then removed from the heat source. While the residue was still warm, 8 mL of aqua regia was added, and the beaker was returned to the electric hotplate to evaporate the solution to a final volume of 2–3 mL. The inner wall of the beaker was rinsed with approximately 10 mL of deionized water, and the mixture was gently heated for 5–10 min until a clear solution was obtained. The beaker was then removed and allowed to cool. Once cooled, the solution was transferred into a 10 mL graduated stoppered polyethylene test tube, diluted to the marked volume with deionized water, thoroughly mixed, and left to stand for clarification. A 1.00 mL aliquot of the clear supernatant was pipetted into a polyethylene test tube and diluted to 10 mL with nitric acid solution (3 + 97, v/v) for instrumental analysis.

Trace rare earth elements (REEs) were determined using an inductively coupled plasma mass spectrometer (ICP-MS, 7700 series, Agilent Technologies, Santa Clara, CA, USA) under the following optimized operating conditions: radio frequency (RF) power of 1300 W, lens voltage of 6 V, pulse stage voltage of 850 V, quadrupole offset set to standard (0), vacuum pressure in the detection region of 1 × 10⁻⁶ Pa, and sample uptake rate of 1.0 mL/min. Major elements were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, 8300V, PerkinElmer Inc., Waltham, MA, USA) with the following parameters: plasma RF power of 1200 W, peristaltic pump speed of 1.5 mL/min, and 3 replicate measurements per sample to ensure data reliability. The analytical precisions for major and trace elements were >1% and 5%, respectively.

3.3. PGE Analyses

PGE analysis and testing were performed at the Qinghai Provincial Geological and Mineral Testing and Application Center (Xining Mineral Resources Supervision and Testing Center of the Ministry of Natural Resources). A 10.0 g aliquot of the sample was accurately weighed and homogenized with assay flux in a fusion crucible. A small quantity of covering agent was added to the mixture, which was then placed in a high-temperature furnace preheated to 1150 °C for a 1.5 h fusion process. The resulting nickel button was crushed and transferred to a beaker, where 60 mL of hydrochloric acid (HCl) was introduced. The mixture was subjected to dissolution until the solution clarified completely with no further bubbling observed. Subsequently, 0.5 mL of sodium tellurate co-precipitant and 1 mL of stannous chloride (SnCl₂) solution were added sequentially, and the mixture was incubated on a hotplate for 30 min under constant temperature conditions. Filtration was conducted using a 0.45 μm filter membrane, and the collected precipitate was rinsed repeatedly with 2 mol/L HCl and deionized water to remove residual impurities.

The precipitate, together with the filter membrane, was transferred into a polytetrafluoroethylene (PTFE) sealed digestion vessel, followed by the addition of 1–2 mL of aqua regia. Digestion was carried out in a temperature-controlled oven at 100 °C for 2 h. After cooling to ambient temperature, the digested solution was transferred to a 25 mL volumetric flask, diluted to the marked volume with deionized water, and thoroughly mixed. PGE concentrations were determined using an ELAN DRC-e inductively coupled plasma mass spectrometer (ICP-MS, PerkinElmer Inc., Massachusetts, USA) operated at a power of 1175 W.

In conventional geological sample analysis, indium (In), cadmium (Cd), thallium (Tl), and rhenium (Re) are widely employed as internal standards. However, carbonaceous slate samples are characterized by elevated contents of metallic elements—particularly selenium (Se), tellurium (Te), Cd, In, lead (Pb), antimony (Sb), zinc (Zn), and Re—which exhibit strong affinities with sulfide minerals. This renders the conventional internal standard method entirely incompatible with black shale series samples. Among these elements, Re and Tl are present in the highest abundances in carbonaceous slates, while lutetium (Lu) occurs at relatively low concentrations and does not undergo enrichment during the nickel sulfide fire assay procedure. Consequently, Lu was selected as the internal standard for this study. The analytical precision for all target PGEs was consistently below 2%, confirming the reliability of the optimized method [76,77,78].

3.4. Mineralogical Observation

Mineralogical observations were conducted at the Comprehensive Rock and Mineral Identification Laboratory, College of Earth Sciences, Jilin University, China, using a Nikon LHS-H100C-1 microscope (Tokyo, Japan) capable of transmitted, reflected, and fluorescent light microscopy, equipped with a 100 W Hg light source. A Nikon DXM 1200 digital camera (Tokyo, Japan) was used for microphotography.

3.5. SEM-EDS

The SEM-EDS measurements were conducted at the SEM Laboratory of Xi’an Zhaonian Mineral Testing Technology Co., Ltd. (Xi’an, China) A high-resolution JSM-6460 scanning electron microscope (SEM, Oxford Instruments, Oxford, UK), coupled with an Oxford Link ISIS energy-dispersive X-ray spectrometer (EDS, Oxford Instruments, Oxford, UK), was employed for the analysis, under high-vacuum mode with an accelerating voltage of 25–30 kV and a beam current of 70 μA. Separately, energy spectrum analysis was carried out using an Oxford X-Max 50 energy-dispersive spectrometer (Oxford, UK), operating at an accelerating voltage of 15 kV, a beam current of 0.3 nA, and a working distance of 8 mm.

4. Mineralogical Characteristics

The primary ore types in the Dagangou V deposit are carbonaceous (Figure 3a–c and Figure 4b) and carbonaceous siliceous slates (Figure 3d and Figure 4a). The carbonaceous slate is typically deep black and exhibits a cryptocrystalline texture and slaty structure. On the surface, it appears as a loose black powder that easily soils hands. The rock is primarily composed of abundant carbonaceous powder (30%–60%) and minor sericite (2%–5%), with subordinate amounts of fine quartz veins (25%–55%) and metallic minerals (5%–10%). The carbonaceous siliceous slate is gray-black with a cryptocrystalline texture and slaty structure. The rock mainly comprises a mixture of carbonaceous powder and clay minerals (57%), chalcedony (30%), and clay and metallic minerals (8%), as well as quartz veins (5%). In the ores, carbonaceous materials and clay minerals often occur as aggregates that are closely intergrown (Figure 4c,d). Under a microscope, they typically display a banded distribution, with bands generally 0.01–0.1 mm wide. Quartz in the ore (Figure 4e) occurs mostly as anhedral granular crystals distributed in fine-grained bands or veins, typically measuring only 0.01–0.1 mm. Pyrite (Figure 4f) is one of the main metallic minerals observed under the microscope. It is fine-grained and mostly disseminated, and its crystal forms are diverse and predominantly anhedral, with some crystals exhibiting cubic or framboidal habits.

5. Results

5.1. Major Elements

The primary ore-bearing lithologies—carbonaceous and carbonaceous siliceous slates—exhibited SiO₂ contents of 27.03%–68.03% (average: 48.67%); Al₂O₃, 1.49%–12.67% (7.15%); Fe₂O₃, 2.91%–10.32% (5.35%); P₂O₅, 0.11%–3.63% (1.23%); and TOC, 1.07%–3.07% (1.75%,

Table 1. Major element composition (wt.%) of Dagangou vanadium deposit samples.

Sample No.	Y1	Y2	Y3	Y4	Y5	Y6	Y7	Y8	Y9	Y10	Y11	Y12	Y13	Y14	Y15
Lithology	Dolomite			Argillaceous Slate			Crystalline Limestone		Carbonaceous Slate					Carbonaceous Siliceous Slate
SiO2	35.55	30.87	28.95	73.94	69.72	61.18	0.85	9.99	37.43	56.44	27.03	32.94	43.07	62.17	68.03
TiO2	0.22	0.19	0.11	0.62	0.58	0.68	0.03	0.03	0.23	0.53	0.16	0.17	0.78	0.68	0.23
Al2O3	5.38	2.73	1.93	13.24	11.55	13.98	0.20	0.30	2.11	6.76	4.27	1.49	9.46	13.16	7.27
TFe2O3	2.43	4.45	1.08	2.57	4.24	8.12	0.18	0.62	10.32	6.70	6.57	5.95	2.91	5.31	3.53
MnO	0.45	1.49	0.42	0.03	0.09	0.18	0.02	0.03	0.09	0.04	0.05	0.04	0.01	0.09	0.11
MgO	10.20	10.48	13.72	0.88	1.68	2.63	21.65	18.76	0.28	0.90	0.61	0.28	1.64	1.95	0.88
CaO	16.80	25.33	22.77	0.46	2.80	0.83	30.15	28.56	8.77	9.31	4.89	1.32	13.39	3.49	4.11
Na2O	0.15	0.13	0.63	0.72	1.07	2.73	0.05	0.02	0.49	0.52	0.10	0.08	0.57	0.08	1.06
K2O	2.51	0.39	0.45	3.01	2.87	3.12	0.01	0.08	1.75	2.23	0.97	0.41	2.96	3.25	1.83
P2O5	0.16	0.17	0.04	0.08	0.09	0.13	0.02	0.02	0.95	0.76	3.63	0.93	0.69	0.11	2.64
LOI	23.32	28.87	31.59	3.73	5.05	4.22	45.95	41.64	22.26	13.43	45.67	49.97	17.73	7.76	4.94
n(SiO2)/n(Al2O3)	6.61	11.32	15.04	5.59	6.04	4.38	4.33	33.51	17.73	8.36	6.32	22.17	4.55	4.72	9.36
n(Al2O3)/n(Al2O3 + TFe2O3)	0.69	0.38	0.64	0.84	0.73	0.63	0.53	0.33	0.17	0.65	0.39	0.20	0.76	0.71	0.67
n(TFe2O3)/n(TiO2)	11.10	23.79	9.60	4.16	7.26	11.93	5.59	23.86	45.42	6.94	117.09	82.51	3.73	7.78	15.29
n(Al)/n(Al + Fe + Mn)	0.58	0.25	0.49	0.79	0.67	0.56	0.43	0.26	0.13	0.43	0.33	0.16	0.71	0.65	0.60
n(MnO)/n(TiO2)	2.06	7.96	3.68	0.05	0.16	0.27	0.53	1.01	0.41	0.08	0.29	0.24	0.01	0.13	0.48

). In contrast, the wall-rock argillaceous slate had a relatively higher SiO₂ content of 61.18%–73.94% (average: 68.23%). Carbonate rocks generally exhibited lower SiO₂ contents (0.85%–35.55%, average: 13.34%). Al₂O₃ content was higher in the argillaceous–calcareous slate (average: 12.92%) and lower in the carbonate rocks (average: 2.10%). The average Fe₂O₃ content was 4.98% in the argillaceous slate and 1.58% in the carbonate rock. Both the argillaceous slate and carbonate rocks had low P₂O₅ contents (average: 0.09%).

5.2. Trace Elements

The trace element normalization diagram (Figure 5) indicates that, relative to the upper crustal abundance [79], the ore-bearing black rock series was significantly enriched in B, V, Ga, Ge, As, Se, Mo, Ag, Cd, Sb, Te, Ba, W, Au, Tl, Bi, U, and Y, whereas it was markedly depleted in Sc, Co, Nb, Rb, Zr, Sn, Hf, Ta, and Th. Specifically, V ranged from 246.5 mg/kg to 6934 mg/kg, with enrichment factors of 1.7–49.5 and an average of 26.8 (

Table 2. Trace element composition (in mg/kg) of Dagangou vanadium deposit samples.

Sample No.	Y1	Y2	Y3	Y4	Y5	Y6	Y7	Y8	Y9	Y10	Y11	Y12	Y13	Y14	Y15	Upper Crustal Abundance
Lithology	Dolomite			Argillaceous Slate			Crystalline Limestone		Carbonaceous Slate					Carbonaceous Siliceous Slate
Li	9.63	7.93	1.85	33.42	32.25	36.53	2.23	2.43	15.26	30.75	41.91	7.56	39.84	32.42	14.55	23
Be	0.98	1.56	0.26	2.37	1.91	1.65	0.04	0.11	2.65	4.60	9.77	5.96	6.76	2.09	1.34	3.2
B	35.58	14.67	5.21	77.55	37.19	102.5	3.34	2.76	3.32	54.04	3.22	3.01	90.27	33.65	53.09	12
Sc	5.96	4.93	3.32	13.38	10.26	18.23	0.68	0.70	7.56	15.92	6.68	2.03	16.68	12.43	6.80	14
V	58.48	23.99	67.60	103.4	69.46	83.10	16.81	12.25	2368	3234	6934	5830	3913	87.14	153.7	140
Cr	53.67	36.28	21.74	97.39	106.3	68.57	30.10	15.63	165.7	215.4	332.7	160.3	215.9	107.3	69.24	69
Co	10.25	34.89	6.48	6.20	4.60	52.79	1.30	1.19	3.98	7.42	20.71	18.56	11.51	15.45	12.50	17
Ni	18.73	41.09	13.49	23.18	26.82	189.2	17.12	9.22	45.02	98.46	479.3	551.7	91.21	52.57	34.51	55
Cu	22.09	4.37	8.82	43.90	15.24	465.9	2.03	2.67	159.7	207.9	329.2	165.8	162.6	34.18	71.24	39
Zn	46.80	25.25	27.86	42.54	38.80	110.3	40.46	15.60	97.16	190.7	1615	1612	170.6	61.83	62.50	67
Ga	918.7	131.0	139.6	353.3	78.53	790.2	1.49	16.95	247.8	959.1	172.6	303.0	1082.0	153.6	890.8	18
Ge	2.32	3.55	1.18	3.62	3.88	8.04	0.22	0.54	6.23	5.39	4.59	4.57	5.28	4.61	3.43	1.5
As	30.91	32.05	27.88	35.80	27.90	47.69	25.96	29.53	139.8	161.70	90.08	166.3	155.0	27.67	29.44	1.6
Se	1.00	0.98	0.87	3.12	1.29	1.73	0.78	0.80	17.29	23.28	34.77	31.62	15.11	2.94	2.80	0.14
Rb	67.55	27.65	12.73	150.7	137.90	93.04	1.02	3.03	59.00	96.74	42.69	22.23	119.8	145.8	64.23	110
Sr	391.3	418.4	303.2	46.01	81.92	58.25	76.66	189.00	746.9	479.3	416.3	997.1	503.4	77.83	367.7	350
Zr	58.45	42.40	25.69	173.8	190.10	248.2	1.13	2.06	95.73	151.70	80.87	57.37	188.70	176.7	75.58	170
Nb	6.48	4.82	2.71	13.87	12.44	17.53	0.14	0.21	6.95	20.65	7.69	6.06	28.88	14.54	6.52	15
Mo	1.36	0.91	0.61	1.31	0.83	0.87	0.26	0.63	112.00	170.70	375.50	395.10	221.60	3.89	4.11	1.6
Ag	0.12	0.06	0.07	0.61	0.21	0.28	0.02	0.07	6.68	1.32	5.39	10.83	1.57	0.29	0.38	0.06
Cd	0.60	0.44	0.81	1.14	1.19	1.70	0.26	0.10	2.72	5.70	23.81	21.77	6.71	1.20	0.98	0.1
Sn	1.68	1.12	0.56	3.78	3.15	3.00	0.10	0.33	1.82	3.45	0.93	0.70	3.78	3.71	2.44	3.3
Sb	6.96	6.94	8.29	4.55	3.53	3.81	9.22	9.76	29.53	60.65	16.74	69.62	69.73	5.04	6.18	0.2
Te	0.06	0.08	0.06	0.08	0.07	0.33	0.02	0.04	0.24	0.41	0.88	0.28	0.58	0.05	0.06	0.003
Cs	2.86	1.73	0.29	9.00	5.10	5.51	0.03	0.06	3.07	4.78	2.72	2.01	7.12	5.29	3.18	3.7
Ba	2185	2976	3166	7465	1425	17,220	41.32	530.20	5423	21,930	3635	6368	25,250	3029	19,860	570
Hf	1.60	0.96	0.67	5.04	5.34	6.83	0.03	0.06	1.96	3.38	1.44	0.66	4.55	5.04	2.19	4
Ta	0.61	0.41	0.30	1.28	1.16	1.41	0.29	0.18	0.59	1.51	0.39	0.16	2.08	1.36	0.67	1.5
W	1.69	1.13	0.91	2.23	1.78	0.77	0.09	0.58	1.91	4.70	3.56	4.14	5.63	2.26	1.76	1.3
Au	0.03	0.03	0.02	0.04	0.04	0.05	0.02	0.02	0.03	0.07	0.04	0.05	0.09	0.05	0.03	0.0023
Tl	0.58	0.16	0.16	0.93	0.75	0.69	0.03	0.05	2.71	2.02	3.75	3.07	2.42	0.86	0.71	0.53
Pb	14.94	7.69	22.07	17.87	7.13	11.77	1.14	2.48	74.78	118.10	50.99	56.58	136.20	11.01	16.19	17
Bi	0.21	0.11	0.17	0.47	0.22	0.45	0.01	0.01	0.43	0.53	0.51	0.35	0.64	0.30	0.33	0.05
Th	6.08	3.34	2.83	16.80	14.21	13.10	0.12	0.45	5.32	10.30	3.17	1.17	15.30	16.35	8.67	11
U	2.17	0.67	0.97	3.04	1.80	1.90	1.75	0.37	46.02	36.53	255.10	282.90	64.68	2.85	6.06	2.8
U/Th	0.36	0.20	0.34	0.18	0.13	0.15	15.14	0.82	8.66	3.55	80.42	241.59	4.23	0.17	0.70
V/V + Ni	0.76	0.37	0.83	0.82	0.72	0.31	0.50	0.57	0.98	0.97	0.94	0.91	0.98	0.62	0.82
V/Sc	9.81	4.87	20.39	7.73	6.77	4.56	24.80	17.59	313.35	203.14	1037.4	2874.7	234.59	7.01	22.59
V/Cr	1.09	0.66	3.11	1.06	0.65	1.21	0.56	0.78	14.29	15.01	20.84	36.37	18.12	0.81	2.22
Ni/Co	1.83	1.18	2.08	3.74	5.83	3.58	13.21	7.73	11.31	13.28	23.14	29.73	7.92	3.40	2.76
Sr/Ba	0.02	0.14	0.10	0.01	0.06	0.00	1.86	0.36	0.14	0.02	0.11	0.16	0.02	0.03	0.02
Co/Zn	0.22	1.38	0.23	0.15	0.12	0.48	0.03	0.08	0.04	0.04	0.01	0.01	0.07	0.25	0.20

). Mo ranged from 3.2 to 395.1 mg/kg, with enrichment factors of 2–247 and an average of 133.1. Se ranged from 6.12 to 34.77 mg/kg, with enrichment factors of 43.7–248.4 and an average of 158.4. Additionally, As, Ga, Ag, Cd, Sb, Te, Ba, Au, Bi, and U exhibited enrichment factors exceeding 10. Overall, the enrichment patterns of V, Se, Mo, Ba, Cd, As, Sb, Ga, and U in this rock suite align with the elemental characteristics of polymetallic black shales [80]. In contrast, the wall rocks were significantly enriched in Ga, As, Se, Cd, Sb, Te, Ba, and Au, whereas they showed marked depletion in Be, Sc, Ni, Cu, Pb, Zn, V, Rb, Sr, Mo, Sn, Ta, U, and other elements.

5.3. REE + Y Composition

The total REE (∑REE) contents in different lithologies of the Dagangou area were generally low (3.44–440.46 mg/kg, average: 171.36 mg/kg). In the V-bearing black rock series (carbonaceous slate), the ∑REE ranged from 133.37 to 440.46 mg/kg, averaging 291.92 mg/kg. Compared with the Post-Archean Australian Shale (PAAS), all the elements except Ce showed varying degrees of enrichment. The europium anomaly (δEu) was calculated using δEu = 2 × w(Eu)_N/[w(Sm)n + w(Gd)_N], where “N” denotes PAAS-normalized values. The Eu values ranged from 0.68 to 2.86 (

Table 3. Rare earth element composition (in mg/kg) of Dagangou vanadium deposit samples.

Sample No.	Y1	Y2	Y3	Y4	Y5	Y6	Y7	Y8	Y9	Y10	Y11	Y12	Y13	Y14	Y15
Lithology	Dolomite			Argillaceous Slate			Crystalline Limestone		Carbonaceous Slate					Carbonaceous Siliceous Slate
La	18.73	13.01	5.33	38.65	38.15	42.89	0.40	1.18	26.68	79.59	71.98	63.12	110.20	41.78	30.75
Ce	37.90	28.37	13.83	72.87	75.46	85.76	0.77	2.23	32.25	91.37	48.57	31.65	127.60	80.18	59.40
Pr	4.27	3.02	1.22	8.41	8.67	9.60	0.11	0.30	7.13	17.96	16.74	9.55	25.29	9.27	7.53
Nd	15.88	11.23	4.35	29.43	30.81	34.39	0.39	1.15	31.84	67.52	72.18	38.40	92.94	32.67	27.67
Sm	3.38	2.28	0.88	5.49	5.94	7.37	0.12	0.26	7.69	12.64	16.28	8.20	15.99	6.32	5.97
Eu	3.35	0.96	0.58	1.75	1.29	3.44	0.05	0.18	2.27	5.23	4.41	3.03	6.13	1.53	3.79
Gd	3.77	2.47	0.86	4.91	5.41	7.52	0.19	0.27	7.43	13.99	22.26	14.38	15.89	5.79	6.62
Tb	0.49	0.37	0.12	0.70	0.80	1.16	0.04	0.04	1.11	2.27	3.49	2.48	2.36	0.84	1.01
Dy	2.72	2.22	0.70	4.03	4.58	6.76	0.32	0.22	6.54	15.01	23.16	18.61	15.08	4.84	6.05
Ho	0.58	0.50	0.15	0.84	0.96	1.45	0.10	0.05	1.40	3.60	5.58	5.03	3.57	0.99	1.26
Er	1.61	1.52	0.44	2.54	2.73	4.31	0.36	0.12	4.13	11.26	17.00	16.62	11.17	2.87	3.65
Tm	0.23	0.22	0.07	0.39	0.41	0.65	0.06	0.02	0.59	1.67	2.40	2.43	1.69	0.42	0.54
Yb	1.39	1.43	0.39	2.55	2.51	4.21	0.47	0.09	3.73	10.58	14.73	14.73	10.83	2.73	3.39
Lu	0.21	0.22	0.06	0.39	0.38	0.65	0.08	0.02	0.58	1.65	2.20	2.27	1.72	0.41	0.53
Y	16.48	15.22	4.54	20.59	24.53	36.17	2.97	1.28	41.11	115.60	207.20	218.80	115.80	25.39	34.62
w(ΣREE)	94.51	67.82	29.00	172.95	178.10	210.16	3.44	6.10	133.37	334.33	320.98	230.49	440.46	190.63	158.15
w(ΣLREE)	83.50	58.87	26.19	156.59	160.32	183.45	1.81	5.28	107.86	274.31	230.16	153.95	378.15	171.75	135.11
w(ΣHREE)	11.00	8.95	2.81	16.36	17.78	26.71	1.63	0.82	25.51	60.02	90.82	76.55	62.30	18.88	23.05
δCe	0.98	1.05	1.26	0.93	0.96	0.98	0.88	0.88	0.55	0.56	0.33	0.28	0.56	0.94	0.91
δEu	2.86	1.23	2.00	1.01	0.68	1.40	0.92	2.01	0.91	1.20	0.71	0.84	1.16	0.76	1.83
Y/Ho	28.50	30.73	29.75	24.38	25.59	24.93	31.25	27.70	29.36	32.14	37.12	43.47	32.43	25.71	27.56
La/Yb	13.47	9.09	13.63	15.13	15.18	10.18	0.84	13.32	7.15	7.52	4.89	4.29	10.18	15.30	9.07
(La:Sm)N	3.49	3.60	3.81	4.43	4.04	3.66	2.16	2.90	2.18	3.96	2.78	4.84	4.34	4.16	3.24
(La:Yb)N	9.08	6.13	9.19	10.20	10.23	6.86	0.56	8.98	4.82	5.07	3.29	2.89	6.86	10.31	6.11
(Sm:Nd)N	0.65	0.62	0.62	0.57	0.59	0.66	0.92	0.68	0.74	0.58	0.69	0.66	0.53	0.59	0.66
(Gd:Yb)N	2.19	1.39	1.78	1.55	1.74	1.44	0.33	2.49	1.60	1.07	1.22	0.79	1.18	1.71	1.58

). The Ce anomaly (δCe) was calculated using δCe = 2 × w(Ce)_N/[w(La)_N + w(Nd)_N], with “N” also representing PAAS-normalized values. The Ce values ranged from 0.28 to 1.26 (Table 3). The normalized REE patterns were nearly flat and slightly left-leaning (Figure 6), indicating that the V-polymetallic mineralization in the black rock series of the Dagangou area is characterized by strong enrichment in heavy REEs (HREEs), weak enrichment in light REEs (LREEs), and a low degree of fractionation between LREEs and HREEs.

5.4. PGEs

The concentrations of PGEs exhibited significant variation between different lithologies (

Table 4. Platinum group element compositions (in ppb) of Dagangou vanadium deposit samples.

Lithology		Pt	Pd	Os	Ir	Ru	Rh	w(Pd)/w(Pt)	w(Pt + Pd)/w(Os + Ru + Ir + Rh)	w(Pt)/w(Ir)
Carbonaceous slate 01		22.32	90.2	0.26	0.08	2.25	4.99	4.04	14.85	293.3
Carbonaceous slate 02		11.7	51.3	1.22	0.29	0.95	2.27	4.38	13.32	39.93
Carbonaceous slate 03		10.6	40	0.57	0.38	0.97	2.01	3.77	12.86	27.6
Carbonaceous slate 04		10.3	37.1	1.19	0.57	0.49	1.65	3.6	12.15	18.01
Carbonaceous slate 05		12.07	36.1	0.37	0.08	1.33	1.65	2.99	14.06	157.3
Average		13.4	50.94	0.72	0.28	1.2	2.51	/	/	/
Carbonaceous siliceous slate 01		6.68	18.9	0.6	0.43	0.75	1.28	2.83	8.36	15.53
Carbonaceous siliceous slate 02		6.42	15	0.17	0.49	0.34	0.73	2.34	12.38	13.10
Average		6.55	16.95	0.39	0.46	0.55	1.01	/	/	/
Argillaceous slate 01		4.09	5.32	0.11	0.3	0.63	0.15	1.30	7.91	13.63
Argillaceous slate 02		1.92	3.47	0.2	0.92	0.71	0.21	1.81	2.64	2.09
Argillaceous slate 03		2.24	3.15	0.16	0.3	0.72	0.2	1.41	3.91	7.47
Average		2.75	3.98	0.16	0.51	0.69	0.19	/	/	/
Dolomite 01		1.35	3.09	0.14	0.11	0.56	0.15	2.29	4.63	12.27
Dolomite 02		1.21	3.09	0.2	0.1	0.2	0.09	2.55	7.29	12.10
Dolomite 03		1.06	2.68	0.12	0.15	1.12	0.19	2.53	2.37	7.07
Average		1.21	2.95	0.15	0.12	0.63	0.14	/	/	/
Concentration coefficient	carbonaceous slate	2.05	4.63	3.61	0.28	11.98	0.5
	Carbonaceous siliceous	1	1.54	1.93	0.46	5.45	0.2
	argillaceous slate	0.42	0.36	0.78	0.51	6.87	0.04
	dolomite	0.18	0.27	0.77	0.12	6.27	0.03
Crustal abundance		6.53	11	0.2	1	0.1	5

). In terms of rock types, carbonaceous slate was the preferred horizon for PGE enrichment, followed by carbonaceous siliceous slate. In contrast, the argillaceous slate and dolomite rocks showed essentially no PGE anomalies. Based on concentration coefficients relative to the crustal abundance in [82], the carbonaceous and carbonaceous siliceous slates showed varying degrees of enrichment for all elements, except Ir and Rh. The concentration coefficients decreased in the order Ru > Pd > Os > Pt > Ir > Rh. In argillaceous slate and dolomite, all elements except Ru showed varying degrees of depletion. Notably, the carbonaceous slate exhibited anomalous enrichment in Pd and Pt, classifying it as palladium–platinum enriched.

6. Discussion

6.1. Identification of V-Bearing Minerals

SEM revealed that the ore was primarily composed of mica, quartz, carbonaceous material, limonite, clay minerals, pyrite, and minor amounts of feldspar, jarosite, rutile, and carbonate minerals. Mica occurred mainly at fine scales and was closely intergrown with quartz. Minute flakes were also intergrown with feldspar, kaolinite, and limonite (Figure 7a,b). The grains were generally fine (0.005–0.05 mm), although some aggregates reached ~0.2 mm. Quartz predominantly existed as anhedral granular crystals, including microcrystalline and cryptocrystalline forms. It was closely intergrown with carbonaceous materials and minerals such as mica and feldspar, with grain sizes mainly ranging from 0.01 to 0.50 mm. Carbonaceous material aggregates exhibited irregular granular or patchy forms. The intergranular spaces were primarily filled with quartz and small amounts of mica, calcite, limonite, and halite. Limonite occurred mainly in irregular or granular forms, with some varieties displaying colloform textures or concentric bands. It was distributed within minerals, such as quartz and carbonaceous materials, and closely intergrown with quartz, mica, and feldspar (Figure 7e,f). The grain size typically ranged from 0.01 to 0.30 mm. Clay minerals, primarily kaolinite, were distributed in fine blades or scales with very small particle sizes. They were mainly microcrystalline or cryptocrystalline and closely intergrown with fine-grained quartz, mica, feldspar, and carbonaceous material (Figure 7a). Pyrite, the main sulfide mineral in the ore, occurred as euhedral to anhedral granular crystals distributed within the ore, with grain sizes mainly between 0.01 and 0.20 mm. Feldspar was predominantly anhedral and granular and closely intergrown with fine-grained quartz, mica, and kaolinite (Figure 7c,d). Rutile was primarily distributed in the ores as granular crystals. Carbonate minerals, primarily calcite and dolomite, occurred as granular or vein-like aggregates distributed within quartz and carbonaceous materials.

Figure 7. Backscattered electron images of vanadium-bearing minerals. (a) Vanadium sericite (V-Ser) was closely intergrown with quartz (Qtz) and carbonaceous matter (C). (b) V-Ser (Figure 8a) occurred irregularly within Qtz interstices, with V-Ser closely intergrown with vanadium clay (V-Cly) (Figure 8d) minerals on the left. (c) Vanadium feldspar (V-Fsp, Figure 8c) was closely intergrown with Qtz, with a vanadium rutile (V-Rt) grain on the left. (d) V-Fsp was loosely intergrown with V-Ser and closely intergrown with jarosite (Jrs). (e) Granular vanadium limonite (V-Lim) was closely intergrown with Qtz. (f) Colloform V-Lim (Figure 8b) was closely intergrown with V-Ser and Qtz.

Figure 7. Backscattered electron images of vanadium-bearing minerals. (a) Vanadium sericite (V-Ser) was closely intergrown with quartz (Qtz) and carbonaceous matter (C). (b) V-Ser (Figure 8a) occurred irregularly within Qtz interstices, with V-Ser closely intergrown with vanadium clay (V-Cly) (Figure 8d) minerals on the left. (c) Vanadium feldspar (V-Fsp, Figure 8c) was closely intergrown with Qtz, with a vanadium rutile (V-Rt) grain on the left. (d) V-Fsp was loosely intergrown with V-Ser and closely intergrown with jarosite (Jrs). (e) Granular vanadium limonite (V-Lim) was closely intergrown with Qtz. (f) Colloform V-Lim (Figure 8b) was closely intergrown with V-Ser and Qtz.

Figure 8. X-ray EDS spectra of vanadium-bearing minerals. (a) Sericite. (b) Limonite. (c) Potassium feldspar. (d) Kaolinite.

Figure 8. X-ray EDS spectra of vanadium-bearing minerals. (a) Sericite. (b) Limonite. (c) Potassium feldspar. (d) Kaolinite.

SEM-EDS indicated that the V-bearing minerals in the ore samples were primarily mica, followed by limonite, clay minerals, feldspar, and jarosite (Figure 8a–d). Most mica grains contained measurable amounts of V and were classified as V mica. The V content varied significantly: barium V mica contained over 10% V₂O₅, whereas a small portion contained no V. Some limonite contained V, with V₂O₅ contents generally ranging from 0.55% to 5.05%. Feldspar was primarily classified as potassium feldspar (some containing barium and V) and albite, with minor amounts of barium feldspar. Some potassium feldspar was V-bearing. Some clay minerals contained minor amounts of V.

6.2. Source of Metals

The samples from the study area exhibited strong linear correlations between TiO₂ and Al₂O₃, MnO, SiO₂, and Sc (Figure 9), indicating that these elements are chemically stable, underwent a similar degree of sorting, and can reflect the composition of their source area.

The n(SiO₂)/n(Al₂O₃) ratio is a commonly used indicator for distinguishing rock sources. Taylor proposed using a threshold ratio of 3.6 when studying Si and Al in the continental crust [83]. A ratio > 3.6 suggests the influence of hydrothermal activity or biological processes. A ratio of ~3.6 suggests the primary source of the rock is terrestrial. The n(SiO₂)/n(Al₂O₃) ratios of the ore-bearing and wall rocks in the Dagangou black rock series V-polymetallic deposit (4.33–22.17, Table 1) indicated that both diagenesis and mineralization involved significant contributions from hydrothermal and biological activity. Choi et al. noted that sediments of hydrogenous origin typically have higher Ni and Co contents, whereas hydrothermally formed sediments have higher Ni and Zn contents [84]. In the Zn-Ni-Co diagram (Figure 10), the data points for both the ore-bearing and wall rocks fall within the field representing submarine hydrothermal sedimentation. This suggests that the V-polymetallic mineralization in the black rock series involved submarine hydrothermal sediments.

As terrestrial materials are rich in aluminum and oceanic hydrothermal sediments are rich in iron and manganese, the concentration relationships among Al, Fe, and Mn can be used to trace the provenance of sedimentary rocks. Studying Al-poor Fe–Mn sediments, Jewell and Stallard proposed that Al/(Al + Fe + Mn) > 0.5 represents typical deep-sea sediments with detrital provenance, whereas Al/(Al + Fe + Mn) < 0.35 suggests a predominantly hydrothermal source [85]. The n(Al)/n(Al + Fe + Mn) ratios ranged from 0.13 to 0.71 (average: 0.35, Table 1). Only sample Y13 had a value > 0.5, indicating that the material originated primarily from hydrothermal sources.

The Co/Zn ratio is a sensitive indicator for distinguishing between hydrothermal and authigenic sources. Cronan suggested that while Cu, Ni, and Zn are primarily of hydrothermal origin, Co is mainly hydrogenous, and its enrichment is attributed to the adsorption of trace elements from seawater [86]. Hydrothermally derived materials have relatively low Co/Zn ratios, averaging 0.15, whereas other ferromanganese crusts or nodules typically average ~2.5 [87]. The mineralized area is highly enriched in Ba. In his study of Cenozoic sedimentary strata, Kattyehkob indicated that the Sr/Ba ratio can be used to determine the rock origin: Sr/Ba > 1 indicates a sedimentary origin, whereas Sr/Ba < 1 indicates a hydrothermal origin [88,89]. The ore-bearing rocks had Co/Zn ratios of 0.01–0.07 and Sr/Ba ratios of 0.02–0.16 (Table 2), both indicating a hydrothermal origin. Carbonaceous chondrites can represent extraterrestrial sources, with characteristic ratios of w(Pd)/w(Pt) = 0.57 and w(Pt + Pd)/w(Os + Ru + Ir) = 0.86 [90]. However, the main PGE-enriched horizon (carbonaceous slate, Table 4) showed w(Pd)/w(Pt) ratios of 2.99–4.38 and w(Pt + Pd)/w(Os + Ru + Ir) ratios of 12.15–14.85. These values are not comparable to those in chondrites, eliminating an extraterrestrial origin for PGEs. The Pd/Ir ratio is commonly used to indicate the genesis of deposits. Owing to the different geochemical behaviors of Pd and Ir in magmatic and hydrothermal fluids, where Ir is immobile, hydrothermal sulfides typically exhibit low w(Ir) and high w(Pd)/w(Ir) values, whereas magmatic sulfide ores have relatively low w(Pd)/w(Ir) values [91,92]. Analysis of PGEs in five carbonaceous slate samples showed Ir contents of 0.08–0.57 ng/g and w(Pd)/w(Ir) ratios of 18.01–293.30, indicating a hydrothermal origin.

6.3. Depositional Environment

V and Ni both belong to the iron family of elements and exhibit multiple ionic valence states that vary with changes in the redox environment. V is readily adsorbed and enriched under oxidizing conditions, whereas Ni is more easily adsorbed under reducing conditions. Both elements are primarily precipitated and enriched by adsorption onto colloids and clay minerals. Therefore, the V/(V + Ni) ratio can be used to infer the redox conditions of a paleodepositional water body [10]. Here, a ratio > 0.46 indicates an anoxic environment, whereas a ratio < 0.46 suggests an oxic environment. Chromium (Cr) is a deep-seated element primarily concentrated in the Earth’s core and mantle. In marine sediments, Cr is primarily derived from detrital material. Both Cr and V are often incorporated into silicate minerals via trivalent isomorphic substitution and transported into the marine environment. However, V is preferentially bound to organic matter, leading to greater enrichment under reducing conditions [93]. Yarin et al. suggested that a V/Cr ratio > 2 indicates deposition in an anoxic environment, whereas a ratio < 2 suggests deposition in an oxic environment [94]. Jones et al. investigated trace elements in British mudstones and proposed that a Ni/Co ratio > 7 indicates a highly anoxic (dysoxic to anoxic) depositional environment, 5–7 suggests a suboxic (oxygen-deficient) environment, and <5 indicates an oxic environment [93]. The ore-bearing rocks exhibited V/(V + Ni) ratios of 0.62–0.98, V/Cr ratios of 14.29–36.37, and Ni/Co ratios of 7.92–29.73 (Table 2). Thus, the water column was highly anoxic during the formation of the ore-bearing black rock series.

The enrichment of some trace elements (U, Mo, Th, etc.) is mainly controlled by redox conditions, with only a small influence from the terrigenous detritus. Therefore, the extent of enrichment of these elements in sedimentary rocks and the ratio between them (Mo_EF, U_EF, Th/U, etc.) can be used as effective indicators of the sedimentary environment [95,96,97]. The elemental association of Mo and U serves as a commonly used indicator system for discriminating redox conditions in sedimentary environments. This is often accomplished by employing a logarithmic scale of Mo and U enrichment factors (Mo_EF, U_EF) for identification [97], which allows for the differentiation among dysaerobic, anoxic, and sulfidic environments. In the ore-bearing rocks of the Dagangou black rock series vanadium polymetallic deposit, the enrichment coefficients of Mo and U are relatively low, plotting within the range of 0.1 to 1 times that of normal seawater. These data are concentrated in the dysaerobic field (Figure 11), representing an oxygen-deficient (anoxic) environment. Wignall proposed a method for determining redox conditions using U and Th, where U + Th/3 represents authigenic uranium content [98]. The sedimentary environment was evaluated using δU = 2U/(U + Th/3), where values > 1 indicate an anoxic environment and values < 1 indicate a normal seawater environment. Jones et al. suggested that the U/Th ratio can also be used to determine redox conditions: U/Th < 0.75 indicates an oxic environment, 0.75–1.25 suggests a suboxic environment, and >1.25 signifies an anoxic environment [44]. Rona et al. concluded that the deposition and enrichment rates of U are accelerated in hydrothermal sedimentation environments [99]; sedimentary rocks formed under hydrothermal conditions exhibit higher U enrichment. Therefore, the U/Th ratio effectively distinguished between normal seawater and hydrothermal sedimentation; hydrothermal sediments typically have U/Th > 1, whereas normal sediments have U/Th < 1. The ore-bearing rocks had δU values of 1.85–2.00 and U/Th ratios of 3.55–241.59, with an average of 67.69—far exceeding 1.25 (Table 2). Therefore, both the δU and U/Th parameters indicate a highly anoxic environment coupled with hydrothermal sedimentation.

Elderfield et al. first used cerium (Ce) to explain the exchange processes of Ce ions during oxidation. In modern oceans, owing to oxidation, cerium exists as Ce⁴⁺, which readily hydrolyzes and is adsorbed and precipitated by iron–manganese hydroxides, clay, organic matter, and colloids [100]. Consequently, seawater exhibits a negative Ce anomaly relative to its neighboring elements La and Pr [101], whereas sediments are enriched, showing either a positive or weak negative anomaly. Under anoxic conditions, Ce is reduced to Ce³⁺ and released into the water column, leading to a positive Ce anomaly in seawater and corresponding depletion in contemporaneous sediments [102,103,104]. As later diagenesis, mineralization, and weathering may alter δCe values, Morad et al. proposed using the (La/Sm)ₙ ratio. The δCe value can reliably indicate paleomarine redox conditions only when the (La/Sm)ₙ ratio is >0.35 and shows no correlation with δCe [105]. Studying the black rock series of California, USA, Murray et al. suggested that δCe values for the mid-ocean ridge, oceanic basin, and continental margin environments were approximately 0.30, 0.55, and 0.79–1.54, respectively [106]. In the Dagangou area, the (La/Sm)ₙ ratios for all lithologies ranged from 2.16 to 4.84, all exceeding 0.35 (Table 3). A poor correlation was observed with δCe (−0.014). This confirms that the δCe values have not been significantly altered by later processes and retain their primary sedimentary signature. The V-bearing black rock series in the Dagangou area exhibited δCe values of 0.28–0.56 (Table 3), indicating a pronounced negative Ce anomaly. This suggests an anoxic depositional environment within the oceanic basin. Therefore, vanadate (V⁵⁺) can be reduced or complexed to form vanadyl ions (V⁴⁺), which can be fixed by forming V porphyrins through association with organic matter. It was then deposited in a reducing environment via biological activity or terrigenous detrital sedimentation. In environments rich in H₂S, V⁴⁺ can be further reduced to V³⁺ and precipitated as V₂O₃ and V(OH)₃ [107]. Therefore, the anoxic reducing conditions of the Ordovician sedimentary water column were likely instrumental in enriching V.

The n(Al₂O₃)/n(Al₂O₃ + Fe₂O₃) ratio is primarily used to determine the tectonic setting in which sedimentary rocks formed. Murray et al. proposed that this ratio falls within the 0.1–0.4 range in mid-ocean ridge settings, 0.4–0.7 in pelagic deep-sea settings, and typically 0.6–0.9 in continental margin settings [108]. The n(Al₂O₃)/n(Al₂O₃ + Fe₂O₃) ratios of the ore-bearing rocks in the Dagangou black rock series V-polymetallic deposit range between 0.39 and 0.76, predominantly indicating a pelagic deep-sea environment. In contrast, the wall rocks (argillaceous slate and dolomitic limestone) have ratios between 0.63 and 0.84 (Table 1), suggesting a continental margin environment. A comprehensive analysis indicated that the tectonic background during the formation of the black rock series V-polymetallic mineralization was a gradually evolving process. It transitioned from a pelagic deep-sea environment to a continental margin, reflecting a tectonic setting with a progressively shallower water depth.

The MnO/TiO₂ ratio is primarily used to determine the sedimentary environment of sedimentary rocks. Sugisaki et al., in their study of manganese content, proposed that a MnO/TiO₂ ratio of 0.5–3.5 suggests sedimentary formation on the ocean floor, whereas <0.5 suggests formation in a marginal sea or continental slope environment close to land [109]. The MnO/TiO₂ ratios of the ore-bearing rocks in the Dagangou black rock series V-polymetallic deposit ranged from 0.01 to 0.48 (Table 1), indicating deposition in a marginal sea or continental slope environment near land.

Eu is particularly important in REE studies because it serves as a sensitive indicator of geochemical conditions and for identifying material sources, tectonic settings, and sedimentary environments. The Eu anomaly is determined based on the equilibrium between its two oxidation states, as it typically exists as a trivalent ion. When the sedimentary environment becomes strongly acidic and reducing, Eu³⁺ is reduced to a divalent cation, leading to the depletion of Eu in seawater and its enrichment in sediments. Conversely, in alkaline and oxidizing environments, Eu²⁺ is oxidized to Eu³⁺, causing the enrichment of Eu in seawater and its depletion in sediments. However, potential interference from barium (Ba) may limit the applicability of Eu in sedimentary environments [110]. In the Dagangou area, Ba contents across different lithologies ranged from 41.32 to 25,250 mg/kg (average: 9344 mg/kg, Table 2), indicating a high overall abundance. The correlation coefficient between δEu values and Ba content in the wall rocks (0.58) and V-bearing black rock series (0.95) indicated that the Eu anomaly during mineralization was influenced by Ba. Additionally, Taylor found that Archean sedimentary rocks exhibit either no Eu anomaly or a positive Eu anomaly, whereas post-Archean sedimentary rocks consistently show a negative Eu anomaly [111]. Zhao proposed that sediments in active continental margins are enriched in HREEs and lack Eu depletion, whereas sediments in passive continental margins are relatively enriched in LREEs and exhibit negative Eu anomalies [112]. In the Dagangou area, δEu values in the wall rocks ranged from 0.92 to 2.86, mostly showing positive δEu anomalies. The δEu values in the ore-bearing black rock series ranged from 0.71 to 1.20 (Table 3), generally indicating weak positive δEu anomalies, coupled with strong HREE enrichment. Thus, ore-bearing black rock series may be formed in a reducing environment associated with an active continental margin.

The Th-Hf/3-Ta diagram was initially proposed to discriminate between different genetic types of basalts [113]. However, non-reactive components of detrital particles ideally are characteristic of the original igneous source and thus are useful in provenance studies with the low-calcic marine black shales. Quinby-Hunt et al. used it to determine the sedimentary tectonic setting and material sources of the marine-formed black rock series [114]. On the Th-Hf/3-Ta diagram, the plotted points fall within the field of divergent plate margins (Figure 12), indicating that the sedimentary tectonic setting during mineralization was an extensional environment. During the Ordovician–Silurian period, the tectonic setting of the East Kunlun region was characterized by bidirectional subduction. With the intensification of rifting and formation of the Proto-Tethyan Ocean, the East Kunlun region evolved from a series of submarine rift valleys into a multi-island ocean. It is widely accepted that the Proto-Tethyan Ocean opened before ~601 Ma [115], underwent subduction by at least ~515 Ma [116], closed around ~440 Ma, and subsequently evolved into a postcollisional extensional setting driven by slab detachment and asthenospheric upwelling between ~430 Ma and 357 Ma [53,117,118,119]. Substantial evidence supports the hypothesis of an extensional setting: (1) the ultramafic–mafic complexes Cu-Ni sulfide deposits (e.g., Xiarihamu) formed under an extensional setting between ~431 Ma and 394 Ma [120,121,122,123,124,125]; (2) eclogite retrograde metamorphism in the CKB, extending ~530 km in length, peaked at ~412 Ma [126]; (3) bimodal volcanic rocks in the CKB formed between 420 Ma and 409 Ma [127]; (4) the Maoniushan Group, composed of continental volcanic rocks, exhibits molassetype characteristics and deposits from the Late Silurian to Early Devonian (423.2–399.6 Ma) [60,117,128]; and (5) several A2-type granitoids formed between 428 Ma and 357 Ma [129,130,131,132,133,134]. The conclusions from this diagram are consistent with the broader tectonic background.

6.4. Comparison of Similar-Type Deposits

China’s sedimentary vanadium deposits are mainly distributed in three metallogenic belts: the northern margin of the Tarim Platform, the northern margin of the Yangtze Platform, and the southeastern margin of the Yangtze Platform. The mineralization age is primarily concentrated in the Cambrian period [135]. Globally, the widespread development of Cambrian black shales is associated with a series of major geological events, including abrupt changes in paleo-oceanic environments, the Cambrian explosion of life, and the formation of large-scale sedimentary mineral resources [15]. This paper selects one typical vanadium deposit from each of the three major metallogenic belts in China and compares them in terms of host lithology, ore mineral characteristics, source of metallogenic materials, ore-forming environment, mode of occurrence of vanadium, and genetic model of the deposits (

Table 5. The similarities and differences between the Dagangou Vanadium deposit and other vanadium deposits.

Deposit	Dagangou	Qianjiaping	Shanglin	Dashuixi
Location	East Kunlun Orogenic Belt	Northern Yangtze Craton	Southern Yangtze Craton	Northern Margin of the Tarim Basin
Formation time	O-S	C	D	C
Metal associated	Mo, Se, Ga, Ag, P, Pt, Pd	Au, Ag, Cu, Pb, Zn, Sb, Ni, Co, PGE	Stone coal, Ag	Ti, Zn, Co, Mo
Host rocks	Carbonaceous slate, carbonaceous siliceous slate	Nodule-bearing carbonaceous siliceous rock, nodule-bearing mudstone, carbonaceous mudstone	Carbonaceous mudstone, carbonaceous siliceous rock	carbonaceous slate, carbonaceous siliceous rock, carbonaceous mudstone
Main ore minerals	Roscoelite, Limonite, Clay, Feldspar, Jarosite	Roscoelite, Illite, Chalcopyrite, Azurite, Vanadium titanomagnetite	Illite, Montmorillonite, Kaolinite, Wavallite, Rutile, Goethite, Monazite	Roscoelite, vanadium-bearing muscovite, barium–vanadium mica, vanadium tourmaline, Limonite
Occurrence of Vanadium	Isomorphic form within mica minerals (65.84%), in adsorbed form within iron oxides and clay minerals (33.90%)	Isomorphic form within mica and illite minerals (70.31%), and adsorbed form within organic matter (20.31%)	Isomorphic form within illite (76.47%), adsorbed form within organic matter (23.35%)	Isomorphic form within mica minerals and kaolinite (96.65%), independent vanadium minerals (3.29%)
Source of metals	Seawater, submarine hydrothermal	Seawater, submarine hydrothermal	Seawater, terrigenous calst	Seawater, submarine hydrothermal
Sedimentary environment	Extremely anoxic sedimentary environment under marginal sea and continental slope settings	Anoxic sedimentary environment in a bathyal-restricted sedimentary basin	Anoxic reducing environment in the neritic trough-basin proximal to the land area	Anoxic reducing environment in a stable marginal shallow marine
Genetic model	Hydrothermal-dominated mixed	Seawater sedimentary	Biogenic sedimentary	Biogenic sedimentary
Reference		[15,23,136]	[137]	[138,139]

). Through comparison, we find that the Dagangou vanadium deposit shares similarities with the other deposits: (1) the ore-forming environments are all anoxic settings ranging from bathyal to neritic; (2) the host rocks are predominantly a suite of organic-carbon-rich slates, with ore minerals mainly consisting of mica-group and clay minerals; (3) the source of metallogenic materials is primarily seawater, with varying degrees of contribution from hydrothermal fluids and terrigenous clastic components; (4) vanadium mainly occurs in isomorphic form within mica-group minerals, followed by adsorption onto clay minerals or organic matter, while independent vanadium minerals are rare; and (5) the genetic models of the deposits include biogenic sedimentation, authigenic marine sedimentation, and mixed origins.

7. Conclusions

(1) The black rock series serves as the primary ore-bearing horizon in the study area, with lithologies dominated by carbonaceous and siliceous slates. The ore is composed mainly of mica, quartz, carbonaceous material, limonite, clay minerals, pyrite, and minor amounts of feldspar, jarosite, rutile, and carbonates. V is primarily hosted in mica, followed by limonite, clay minerals, feldspar, and jarosite.

(2) The ore-forming materials of the Dagangou V deposit have a complex origin resulting from the superposition of biogenic and submarine hydrothermal inputs. The complex source of the metallogenic materials is attributed to the superposition of submarine hydrothermal fluids with biological substances. The genetic model is hydrothermal-dominated mixed.

(3) V mineralization in the black rock series occurred during the northward subduction and consumption of the Central Kunlun Ocean during the Ordovician–Silurian period. The tectonic setting is characterized by an active continental margin that evolved from a pelagic deep-sea environment into a continental margin with progressively shallower water depths. Deposition took place in a proximal marginal sea and continental slope environment characterized by highly anoxic conditions.