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Article

Research on the Metallogenic Enrichment Model of Poly-Metallic Black Shales and Their Geological Significance: A Case Study of the Cambrian Niutitang Formation

by
Kai Shi
1,2,
Zhiyong Ni
1,2,*,
Ganggang Shao
1,2,
Wen Zhang
1,2 and
Nuo Cheng
1,2
1
State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum, Beijing 102249, China
2
College of Geosciences, China University of Petroleum, Beijing 102249, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(11), 3537; https://doi.org/10.3390/pr13113537
Submission received: 2 September 2025 / Revised: 29 October 2025 / Accepted: 29 October 2025 / Published: 4 November 2025
(This article belongs to the Section Energy Systems)
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Abstract

The Lower Cambrian Niutitang Formation was deposited precisely during the Cambrian Explosion period, a short-lived interval marked by drastic shifts in oceanic chemistry and climate. This stratigraphic sequence preserves a comprehensive record of early-ocean evolution and constitutes a world-class reservoir for rare and precious metals, widely termed the “poly-metallic enrichment layer”. Despite its metallogenic prominence, the genetic model for metal enrichment in the Niutitang Formation remains contentious. In this study, we employed inductively coupled plasma mass spectrometry (ICP-MS), carbon and sulfur analyzer, and X-ray fluorescence spectrometry (XRF) to quantify trace-metal abundances, redox-sensitive element distribution patterns, rare-earth element signatures, and total organic carbon contents. Results reveal that metal enrichment in the Niutitang Formation was governed by temporally distinct mechanisms. Member I records extreme enrichment in As, Ag, V, Re, Ba, Cr, Cs, Ga, Ge, Se and In. This anomaly was driven by the Great Oxidation Event and intensified upwelling that oxidized surface waters, elevated primary productivity and delivered abundant organic matter. Subsequent microbial sulfate reduction generated high H2S concentrations, converting the water column to euxinic conditions. Basin restriction imposed persistent anoxia in bottom waters, establishing a pronounced redox stratification. Concurrent vigorous hydrothermal activity injected large metal fluxes, leading to efficient scavenging of the above metals at the sulfidic–anoxic interface. In Members II and III, basin restriction waned, permitting enhanced water-mass exchange and a concomitant shift from euxinic to anoxic–suboxic conditions. Hydrothermal metal fluxes declined, yet elevated organic-matter fluxes continued to sequester Ag, Mo, Ni, Sb, Re, Th, Ga, and Tl via carboxyl- and thiol-complexation. Thus, “organic ligand shuttling” superseded “sulfide precipitation” as the dominant enrichment mechanism. Collectively, the polymetallic enrichment in the Niutitang Formation reflects a hybrid model controlled by seawater redox gradients, episodic hydrothermal metal supply, and organic-complexation processes. Consequently, metal enrichment in Member I was primarily governed by the interplay between vigorous hydrothermal flux and a persistently sulfidic water column, whereas enrichment in Members II and III was dominated by organic-ligand complexation and fluctuations in the marine redox interface. This study clarifies the stage-dependent metal enrichment model of the Niutitang Formation and provides a theoretical basis for accurate prediction and efficient exploration of polymetallic resources in the region.

1. Introduction

Recent investigations have confirmed that black-rock series represent prolific repositories for both fossil energy and critical-metal resources. Black rock series are defined as organic-rich, fine-grained marine sedimentary rocks that accumulate under persistent suboxic to anoxic bottom-water conditions [1,2,3]. Lithologically they encompass a spectrum of dark gray to black barite, phosphorite, carbonate, chert, siliceous shale, silt-bearing shale and mudstone [4,5,6]. These strata were deposited under anoxic to suboxic bottom-water conditions. Among them, black shales not only concentrate significant quantities of hydrocarbon resources and diverse metallic minerals, but also archive contemporaneous geological processes and biological activity. Consequently, these strata serve as key archives for reconstructing contemporaneous palaeoceanography and biogeochemical cycling [7].
The Cambrian Niutitang Formation, a key Early Cambrian stratigraphic unit on the South China Block, is widely recognized for its laterally extensive, organic-rich black shales and polymetallic enrichment. These black shales are widely exposed along the margin of the Yangtze Platform (Guizhou, Hunan, Yunnan, Jiangxi and Zhejiang). Individual beds can be traced for more than 1500 km, with a total thickness of 30~150 m and mineralised horizons 3~20 cm thick [8]. The formation is conspicuously enriched in Ni, Mo, PGEs, Cr, V, U, Au, Zn, Cd, Se, Re and Co, with Ni + Mo locally exceeding 14 wt% and platinum-group (PGE) + Au surpassing 1 g/t [9,10].
The anomalous enrichment of dozens of elements in these black shales within a geologically brief interval has attracted widespread attention and prompted extensive research into the governing mechanisms. The origin of this extreme polymetallic enrichment is debated. Competing models invoke hydrothermal input, secular seawater enrichment, and organic-matter scavenging. Evidence for hydrothermal activity includes anomalous Hg concentrations [11], phosphate nodules [12,13], elevated initial 87Sr/86Sr ratios coupled with positive Eu anomalies [14], the presence of vent-affinity fauna (large worms, high biomass, tube-dwelling organisms; [15]), and enrichments in classic hydrothermal tracers such as Ba, As, Sb and Bi. These signatures have been related to Early Cambrian Gondwanan rifting or associated submarine hydrothermal venting, which remobilised metals normally depleted in upper-crustal reservoirs [16,17]. Conversely, PGE patterns that mimic modern seawater [9], 187Os/188Os ratios close to contemporaneous ocean values [18], and metal concentrations far exceeding those in modern euxinic Black Sea sediments [19] favor direct precipitation from metal-rich seawater under extremely low sedimentation rates. Organic matter further modulates metal fixation. Diagenetic transformation of organic substrates can concentrate, transport and ultimately mineralise elements supplied by seawater or hydrothermal fluids [4]. Controversy persists regarding the origin of poly-metallic enrichment in the Lower Cambrian Niutitang Formation. Some researchers advocate a seawater model to explain the extreme metal enrichment, attributing it to direct precipitation from seawater under highly restricted, low-sedimentation-rate, and strongly reducing conditions governed by contemporaneous seawater chemistry [9,10,20,21]. However, it has been noted that Mo/TOC, Ni/TOC and U/TOC ratios in the Ni–Mo sulfide ores significant exceed those typical of normal marine sediments, suggesting an additional metal source [19]. The presence of Cl-rich, low-pH, high-temperature basinal hydrothermal fluids ascending along rift structures has been corroborated, supporting a hydrothermal model as a key mechanism for extreme metal enrichment [14,22]. In contrast, Emsbo et al. propose that classical hydrothermal deposits exhibit distinctive metal associations and pronounced zoning not observed in the Niutitang Formation [23]. Hofstra et al. therefore introduce a petroleum-exhalative model, in which metals released from surface oil slicks during evaporation, water washing, oxidation and biodegradation account for the lateral continuity and weak geochemical zoning [24]. Recognizing the limitations of a single mechanism, Jiang et al. propose a hybrid hydrothermal–seawater–organic model, whereby hydrothermal pulses precipitate Ni–PGE sulfides and adsorb REE–Sr onto Fe-oxyhydroxides/organic matter, whereas during intermittent periods, reductive dissolution of Fe-oxides releases P–REE–Sr, leading to precipitation of authigenic apatite under pore-water supersaturation [25].
Despite extensive previous investigations, the enrichment model of the Niutitang Formation remains unresolved, hampering accurate reconstruction of its depositional evolution and constraining effective exploration and development of its metal resources. Therefore, this study is framed within the Early Cambrian rift-basin tectono-sedimentary-metallogenic system of the Yangtze Craton. It addresses the fundamental question of how the Niutitang black shales acquired their exceptional poly-metallic enrichment and spatial distribution. Using ICP-MS, elemental analyzers, and X-ray fluorescence spectrometry, we quantify metal distributions and evaluate the roles of terrigenous input, palaeo-depositional conditions, hydrothermal intensity and organic-matter processes in metal concentration. The resulting genetic model is intended to underpin predictive exploration for poly-metallic resources in the region. Moreover, because this succession archives Early Cambrian oceanic evolution and nutrient cycling, clarifying its metallogenic mechanisms also offers insights into the co-evolution of life and its environment.

2. Geological Settings

The Early Neoproterozoic Jiangnan orogeny resulted from the collision between the Yangtze and Cathaysia blocks, welding them into the Yangtze Craton (Figure 1a) [26]. Subsequent breakup of the Rodinia supercontinent triggered intense global tectonism and overprinted the South China region with a rift regime that replaced the earlier stable cratonic setting [27]. From the Ediacaran to the Early Cambrian, the Yangtze Block progressively evolved into a passive continental margin [28,29] and underwent differential subsidence controlled by regional extensional tectonics [30]. Paleotopography sloped from northwest (high) to southeast (low), with water depth increasing in the same direction [31,32]. Three distinct sedimentary facies belts developed: platform, slope, and deep-water basin (Figure 1b). During the Early Cambrian, a widespread marine transgression flooded the Yangtze region and established persistent reducing conditions, depositing an extensive black-rock succession dominated by carbonaceous shales, cherts, siliceous shales and phosphorite [18,33].
The study area lies in Kaiyang County, Guiyang City, Guizhou Province, situated on the eastern flank of the Central Guizhou Uplift within the southern Yangtze passive-margin fold-and-thrust belt [34]. It belongs to the Yangtze Region in terms of sedimentation. The succession, from oldest to youngest, comprises the Doushantuo, Dengying, Niutitang, and Mingxinsi formations [9,14,35,36]. In the Niutitang Formation, Member I at the base consists of mudstone and is characterized by rhythmic alternations of organic-rich black shales and cherts with local phosphatic nodules. Member II is dominated by black shales and siliceous shales, whereas the uppermost Member III is composed mainly of black carbonaceous shales intercalated with siliceous shales and black shales (Figure 1c).
During the early Ediacaran the region was topographically high [37]. A subsequent transgressive cycle deposited a phosphorite–dolostone–chert succession rich in stromatolites [37]. The middle–late Ediacaran is represented by platform dolostones, whereas a latest-Ediacaran sea-level fall caused subaerial exposure, leading to the parallel unconformity at the base of the Niutitang Formation on the Dengying Formation [37]. In the Early Cambrian, the Yangtze Block lay between 5° and 25° N in the tropics [38]. Global sea-level rise, combined with nutrient-rich upwelling along the block margin and contemporaneous hydrothermal input, resulted in the deposition of the Niutitang Formation, which consists chiefly of cherts, siliceous shales, carbonaceous shales and organic-rich black shales.
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Figure 1. Simplified paleogeography of South China during late Ediacaran—early Cambrian transition and stratigraphic succession in the study area. (a) Map showing the location of the Yangtze Block in China (modified from [39]). (b) Geological map illustrating the location of the Kaiyang area and the Early Cambrian depositional facies and paleoenvironments of the South China Craton (modified from [19]). (c) Stratigraphic column of the Niutitang Formation in Kaiyang.
Figure 1. Simplified paleogeography of South China during late Ediacaran—early Cambrian transition and stratigraphic succession in the study area. (a) Map showing the location of the Yangtze Block in China (modified from [39]). (b) Geological map illustrating the location of the Kaiyang area and the Early Cambrian depositional facies and paleoenvironments of the South China Craton (modified from [19]). (c) Stratigraphic column of the Niutitang Formation in Kaiyang.
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3. Sampling and Experiments

3.1. Samples

Fieldwork was conducted in the phosphate-mining district of Kaiyang County, Guiyang City, Guizhou Province, on the mid-northern eastern limb of the Yangshui anticline (27°9′4″ N, 106°52′15″ E). The section exposes the Lower Cambrian Niutitang Formation (Figure 2a), which rests with a parallel unconformity on the Ediacaran Dengying Formation dolostones. A total of 53 fresh samples were collected at approximately 30 cm intervals along a 15.28 m stratigraphic succession. The basal unit (Member I, ~0.7 m, Figure 2b) consists of one mudstone bed (Figure 2c) succeeded by seven rhythmic alternations of cherts (25~30 cm, Figure 2d) and organic-rich black shales (20~25 cm, Figure 2e), with intercalated phosphorite (Figure 2f). Overlying this, Member II (~5 m) comprises thinly bedded black shales (Figure 2g) and siliceous shales (Figure 2h). Member III (~9.7 m) is dominated by carbonaceous shales (Figure 2i) with variable bed thickness. Among the 53 samples, the lithological distribution is as follows: 1 mudstone, 5 cherts, 5 organic-rich black shales, 1 phosphorite, 8 black shales, 13 siliceous shales, and 20 carbonaceous shales.

3.2. Experiments and Methods

Total organic carbon (TOC) content was determined at the State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum (Beijing), using a carbon–sulfur analyser following the Chinese national standard “Determination of Total Organic Carbon in Rocks” (GB/T 19145-2022, ref. [40]). Approximately 50 mg of powdered sample was weighed into a ceramic crucible, treated with excess HCl, and digested at 75 °C for ≥2 h to remove inorganic carbon. After decarbonation, the residue was rinsed with deionised water to neutrality and oven-dried at 100 °C. An aliquot of each dried sample was mixed with Fe and W accelerators and combusted in a high-temperature oxygen stream, quantitatively converting total organic carbon (TOC) to CO2. The evolved CO2 was measured by non-dispersive infrared (NDIR) spectroscopy to determine TOC content.
Major element compositions of whole-rock powders were acquired by AXIOS-Minerals X-ray fluorescence spectrometry at the Rock-Mineral Preparation and Analysis Laboratory, the Institute of Geology and Geophysics (IGG; Beijing, China), Chinese Academy of Sciences (CAS). Analyses were performed on a PANalytical AXIOS Minerals spectrometer (PANalytical, Almelo, The Netherlands). equipped with a 4 kW Rh-anode X-ray tube, providing full elemental coverage from F to U at detection limits of 1.0 ppm to 100%. For each sample, 0.6000–0.6005 g of powder was weighed into pre-ignited crucibles and ignited at 1000 °C for 1 h to determine loss on ignition (LOI = (W2 − W3)/(W2 − W1)). The calcined residue was then mixed with a 2:1 Li2B4O7-LiBO2 flux, fused into glass disks using a Claisse Fluxy automatic fusion machine, Corporation Scientific Claisse Inc., QC, Canada, and analyzed for major oxides.
Trace element determinations were carried out at the Analytical Instrumentation Centre of Peking University, using a NexION 350X inductively coupled plasma mass spectrometer (ICP-MS), PerkinElmer, Waltham, MA, USA. Approximately 50 mg of sample powder was transferred to PTFE digestion vessels and predigested overnight with 500 µL HCl, 1500 µL HNO3, and 200 µL HF. Complete dissolution was achieved using an UltraWAVE ECR single-reaction-chamber microwave system (Milestone, Milan, Italy) to decompose organic matter and silicate phases thoroughly. To prevent HF-related instrument damage, residual HF was eliminated by hot-plate evaporation at 150 °C, after which the residue was reconstituted to 50 mL with 2% HNO3. Solutions were filtered through 0.45 µm syringe filters, and 10 mL aliquots were introduced into the ICP-MS. A total of 50 elements were quantified.
Enrichment factors (EF) relative to the average upper continental crust (UCC) were calculated to quantify the enrichment or depletion of selected elements, using the formula EFX = (X/Al)sample/(X/Al)UCC, where the UCC data are from [41,42] and X denotes the elements of interest (e.g., Mo and U). Values of EFX > 1.0 indicate enrichment, whereas EFX < 1.0 signifies depletion relative to the UCC reference. The calculation of Ce, Eu, and Y anomalies (Ce/Ce*, Eu/Eu*, Y/Y*) was conducted in accordance with [43,44]. This involved applying the established formulas—δCe = 2CeN/(La + Nd)N, δEu = 2EuN/(Sm + Gd)N, δPr = 2PrN/(Ce + Nd)N, and δY = 2YN/(Dy + Ho)N—to elemental concentrations normalized to the Post-Archean Australian Shale (PAAS) composite [45], denoted by the subscript “N”. The Chemical Index of Alteration (CIA) provides a quantitative measure of chemical weathering intensity [46]. It is calculated as CIA = [Al2O3/(Al2O3 + Na2O + CaO* + K2O)] × 100 (molar proportions), where CaO* represents CaO hosted solely in silicate phases. When the molar abundance of CaO remaining after correction is lower than that of Na2O, the corrected CaO value is adopted as CaO*; otherwise, CaO* is set equal to Na2O. Paleoproductivity is evaluated using Ba-bio and P/Ti ratios [47,48], which quantify the non-detrital fractions of Ba and P. The biogenic barium proxy is calculated following the equation Ba-bio = Basample − [Alsample × (Ba/Al)PAAS], thereby removing the influence of terrigenous detrital input.

4. Results

4.1. Total Carton and Major Element

As the studied samples were collected from an outcrop section, they may have experienced weathering. The Chemical Index of Alteration (CIA) was therefore employed to evaluate weathering intensity and assess data reliability. CIA values range from 75.31 to 86.05 (mean ≈ 80.10; Table 1). The basal cherts of Member I exhibit the highest CIA (mean ≈ 82.95), indicating moderate to intense weathering, whereas organic-rich black shales and phosphorite show lower values (mean ≈ 76.26). Siliceous and carbonaceous shales of Members II and III yield CIA ≈ 80.25 (Table 1), reflecting moderate alteration. Overall, apart from the basal cherts, the section experienced moderate weathering (Figure 3).
Major element compositions are listed in Table 1. SiO2 is the dominant oxide, ranging from 39.82% to 88.77%. Interbedded cherts in Member I reach 86.53% SiO2, whereas adjacent organic-rich black shales contain 68.21%. In Member II and III, SiO2 decreases to 57.01% in siliceous shales and 51.84% in carbonaceous shales. Al2O3 is the second most abundant component, peaking at 25.51% in mudstones, falling to 5.00% in the Member I chert–mudstone alternations, and then increasing to 12.17% in Member II and III. A similar trend is observed for K2O (0.21~4.97%), TiO2 (0.06~0.96%), MgO (0.11~6.11%), MnO (~0.12%) and Fe2O3 (0.78~11.64%). Al2O3, K2O and TiO2 stabilize in Member II and III, whereas MgO, MnO and Fe2O3 fluctuate. CaO rises from 2.06% in Member I to 3.54% in Member II and reaches 4.54% in Member III. Na2O remains low and erratic (~0.03%), whereas P2O5 is consistently 0.32% except for a sharp spike to 11.14% in the Member I phosphorites.
The Niutitang Formation exhibits high TOC contents, ranging from 1.79 to 10.32% with a mean of 5.56% (Table 1). The TOC content peaks at approximately 9.85% in the organic-rich black shales of Member I, while the intercalated cherts average 4.74%. Notably, TOC decreases to 4.72% in Member II before gradually rising to 5.47% in the upper part of Member III (Figure 3). Overall, the formation is characterized by high organic richness.

4.2. Trace Element

Table S1 and S2 present the trace-element abundances measured for 53 representative samples, covering 35 different elements. Concentrations span several orders of magnitude. V (722.08 ppm), Mo (90.84 ppm), Ni (94.51 ppm), Cr (276.07 ppm) and Ba (3306.59 ppm) are notably enriched, whereas Be (1.00 ppm), In (0.09 ppm), Bi (0.16 ppm) and Ta (0.79 ppm) occur at much lower levels. Enrichment factors (EF) relative to upper continental crust (UCC; [42]) were calculated and classified as strongly depleted (EF < 0.5), depleted (0.5 < EF < 1), weakly enriched (1 < EF < 2), enriched (2 < EF < 5), strongly enriched (5 < EF < 10) or extremely enriched (EF > 10) [39]. The results reveal that each element is enriched to varying degrees relative to the upper continental crust (Figure 4). Be, Zr, Sr, Sc, Mn, Ti, Sn, Pb, W, Se and Co display EF < 1 across the succession, indicating depletion. Sn, Pb, W and Se are weakly enriched in Member I, and Co is weakly enriched in Member III. Ta, Hf, Li, Th, Nb, Ge, Cu and Bi are weakly enriched, whereas B, Cs, Ni, Tl and Zn are enriched. Among these, Ta, Hf, B and Cs are uniformly distributed, whereas the others exhibit stratigraphic variability. Nb and Ge are strongly to moderately enriched in Member I but decline to weak enrichment or depletion in Members II–III. Li and Th are depleted in Members I-II yet weakly enriched in Member III. Cu, Zn and Ni exhibit a “high–low–high” pattern, being enriched or weakly enriched in Members I and III but depleted or unenriched in Member II. Re, As, U, Cd, Ag, Mo and Sb are extremely enriched throughout the succession. Cr, V, In, Ga and Ba are extremely enriched only in Member I, decreasing to strongly or weakly enriched levels in Members II and III. Gold is depleted in Members I and II yet extremely enriched in Member III. Additionally, the average Ga concentration (24.83 ppm) exceeds the cut-off grade for gallium ores (0.002–0.01%) [49].

4.3. Rare Earth Element

Rare-earth element (REE + Y) concentrations and PAAS-normalized patterns are reported in Table S3 and illustrated in Figure 5. Total REE + Y contents vary markedly, ranging from 9.97 ppm to 524.40 ppm (mean = 109.32 ppm). The highest value (524.40 ppm) occurs in basal phosphorite, whereas the lowest (mean = 42.23 ppm) is recorded in chert. Siliceous shale, carbonaceous shale and organic-rich black shale yield 88.94 ppm, 140.09 ppm and 108.35 ppm, respectively.
The PAAS-normalized REE patterns differ among the three Members. In Member I, (La/Yb)N ≈ 0.42 indicates heavy-REE enrichment relative to light-REE, producing a left-inclined profile (Figure 5a). Most samples display a pronounced negative Ce anomaly (δCe ≈ 0.64) whereas the basal claystone shows none (δCe = 1.00). Positive Eu anomalies (δEu ≈ 2.66) typify the cherts yet are absent in phosphorite (δEu = 0.89) and organic-rich black shale (δEu = 1.03). Minor negative Eu anomalies (δEu ≈ 0.59) appear in claystone and sporadically in chert and black shale. The Y anomaly evolves from negative (δY = 0.73) through neutral (δY = 1.04) to positive (δY = 1.34). Member II displays a comparatively flat REE pattern, whereas Member III shows a subtle “hat-shaped” profile (Figure 5b,c). Both members exhibit weak negative Ce anomalies (δCe ≈ 0.94) and slight positive Eu anomalies (δEu ≈ 1.17). The Y anomaly shifts from absent (δY ≈ 0.97) to mildly negative (δY ≈ 0.91) in Member III.

5. Discussion

5.1. Detrital-Terrigenous Input

Rare-earth element (REE) signatures differ systematically among sediment types. Terrigenous deposits inherit a felicity-upper-crust composition, yielding high La/Y ratios, whereas marine precipitates track seawater and display correspondingly low La/Y ratios [50]. The La/Y ratio can therefore be used to gauge the relative contribution of continental detritus [51]. In the Niutitang Formation, most samples plot between the seawater and upper-crust end-members without clustering near the crustal field (Figure 6a). Seawater has an Er/Nd value of ~0.27. Detrital input and diagenetic alteration preferentially enrich Nd over Er, driving Er/Nd below 0.1 as terrigenous supply increases. Niutitang shales yield Er/Nd = 0.08~0.26 (mean = 0.14), indicating only modest continental influence (Figure 6b). Stratigraphically, Member I averages ~0.20, whereas Members II and III decrease to ~0.12 and ~0.13, respectively, implying a progressive increase in detrital supply up-section. Overall, terrigenous input exerted limited control on shale deposition in the Niutitang Formation.

5.2. Hydrothermal Activity

Early Cambrian rifting within the South China Block generated a series of intracratonic rift basins that were repeatedly invaded by hydrothermal fluids [28,53]. These fluids delivered abundant nutrients (N, P, Si, Zn, Fe) that fueled biotic blooms and elevated primary productivity [54], while simultaneously introducing large inventories of Ba, Si, Pb, Zn, Mn, Ni, Mo, Se, and PGEs [14,15,55,56,57]. The resulting hydrothermal–sedimentary system produced stratiform Ni-Mo-PGEs deposits and supergiant barite ores along the passive margin [58,59]. Upon discharge onto the seafloor, the accompanying H2S established euxinic bottom waters that enhanced organic-matter preservation and focused metal precipitation [57,60,61,62]. Consequently, black shales exposed in Guizhou, western Hunan, and Jiangxi are silicified and strongly enriched in multiple metals [19,63,64,65].
Comparative studies on East Pacific Rise hydrothermal Mn crusts (high Ni, Zn) vs. deep-sea hydrogenous nodules (high Co) demonstrate that Ni and Zn are dominantly of hydrothermal origin, whereas Co is mainly hydrogenous [66]. Thus, samples plotting near the Ni-Zn join on a Ni-Co-Zn ternary diagram are interpreted as hydrothermal precipitates, whereas those adjacent to the Co apex signify hydrogenous contributions [67]. Our data cluster within the hydrothermal field (Figure 7a), corroborating a hydrothermal imprint on the Niutitang Formation. Because Fe and Mn are preferentially mobilized by high-temperature fluids, whereas Al and Ti remain immobile and reflect detrital input [68,69], the ratios Fe/Ti and Al/(Al + Fe + Mn) serve as proxies for hydrothermal intensity. Hydrothermal sediments typically yield Al/(Al + Fe + Mn) ≤ 0.01 and Fe/Ti > 20 [70,71,72,73]. In the Fe/Ti-Al/(Al + Fe + Mn) diagram (Figure 7b), Member I records variable but overall moderate hydrothermal influence, whereas Members II and III show distinctly weaker signals.
Under highly reducing hydrothermal conditions, Eu3+ is reduced to Eu2+, generating positive Eu anomalies in marine precipitates [74,75]. Accordingly, positive Eu anomalies in REE patterns are widely used to fingerprint hydrothermal input [25,43,63,76,77]. In the Niutitang Formation, pronounced positive Eu anomalies are restricted to cherts of Member I (Figure 5a), whereas phosphorites, mudstones, and organic-rich shales display subdued or even negative anomalies. The rhythmic alternation of chert and black shale (Figure 3) implies episodic hydrothermal pulses. Members II and III exhibit only minor positive Eu anomalies, indicating diminished hydrothermal activity (Figure 5b,c). Because Ba interference can artificially enhance Eu signals [9,78], we evaluated Ba/Eu relationships and found no correlation (Figure 8b), confirming that the Eu anomalies are hydrothermal in origin.
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Figure 7. Hydrothermal activity identification chart. (a) Ternary plot Ni-Co-Zn (modified from [67]). (b) w(Fe)/w(Ti) vs. w(Al)/w(Al + Fe + Mn) diagram (modified from [79]).
Figure 7. Hydrothermal activity identification chart. (a) Ternary plot Ni-Co-Zn (modified from [67]). (b) w(Fe)/w(Ti) vs. w(Al)/w(Al + Fe + Mn) diagram (modified from [79]).
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Figure 8. Plots of (a) Y anomalies to (La/Nd)N ratios, (b) Eu anomalies to Ba/Eu ratios, (c) Ce anomalies to Pr anomalies, (d) Ce anomalies to (Dy/Sm)N ratios calculated from the PAAS-normalized REE abundances of organic matter in sedimentary rocks in Kaiyang, South China. Field I, neither Ce anomalies or La anomalies; Field IIa, positive La anomaly causes apparent negative Ce anomaly; Field IIb, negative La anomaly causes apparent positive Ce anomaly; Field IIIa, real positive Ce anomaly; Field IIIb, real negative Ce anomaly.
Figure 8. Plots of (a) Y anomalies to (La/Nd)N ratios, (b) Eu anomalies to Ba/Eu ratios, (c) Ce anomalies to Pr anomalies, (d) Ce anomalies to (Dy/Sm)N ratios calculated from the PAAS-normalized REE abundances of organic matter in sedimentary rocks in Kaiyang, South China. Field I, neither Ce anomalies or La anomalies; Field IIa, positive La anomaly causes apparent negative Ce anomaly; Field IIb, negative La anomaly causes apparent positive Ce anomaly; Field IIIa, real positive Ce anomaly; Field IIIb, real negative Ce anomaly.
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Collectively, our data indicate that intense hydrothermal activity during deposition of the Member I supplied abundant metals and H2S, creating euxinic conditions that facilitated extreme enrichment of As, Ba, Ag, Cr, V, Re, Cs, Ga, Ge, Se, and In (Figure 4). Hydrothermal fluxes waned up-section (Figure 3), leading to markedly lower metal concentrations in Members II and III (Figure 4). This covariation demonstrates a direct causal link between hydrothermal intensity and metal enrichment, underscoring the pivotal role of hydrothermal processes in generating the polymetallic anomalies of the Niutitang Formation.

5.3. Redox Conditions

Seawater itself is a major reservoir of ore forming metals, and its redox stratified chemistry that ranges from suboxic to euxinic creates favorable conditions for metal fixation. Under strongly reducing or sulfidic conditions, redox-sensitive elements such as Ni, Mo, V, Cr, U, and Co are efficiently transferred from the water column to sediments. In oxic settings these elements occur as soluble high-valence species, whereas they are reduced to low-valence forms and immobilized in reducing environments [47]. Modern analogs include the Black Sea sapropels (Mo, U, V, Ni, Co), the eastern Mediterranean anoxic basins, and organic-rich sections in South China and the Yukon.
Under oxic conditions vanadium is soluble as vanadate, yet in anoxic waters it occurs as VO(OH)3− or VO(OH)2 that readily sorbs to organic matter and in the presence of abundant H2S precipitates as unstable sulfides [80]. Ni and Co are both chalcophile elements that form insoluble NiS and CoS under euxinic conditions, with Ni requiring more reducing conditions than Co [41]. Cr occurs as soluble chromate (VI) in oxic seawater yet is reduced to Cr(OH)2+, Cr(OH)3 or (Cr,Fe)(OH)3 in anoxic settings [81]. V and Cr are sequentially reduced at the upper and lower boundaries of the nitrate–nitrite reduction zone, so the V/Cr ratio effectively tracks redox conditions. Oxic conditions correspond to V/Cr < 2, suboxic to 2 ≤ V/Cr ≤ 4.25, and anoxic to V/Cr > 4.25 [82]. Owing to the geochemical contrast between Ni and Co and their covariation, Ni/Co < 5 indicates oxic settings, 5 ≤ Ni/Co ≤ 7 suboxic water, and Ni/Co > 7 anoxic environments [83]. V/(V + Ni) < 0.46 reflects oxic conditions, 0.46 ≤ V/(V + Ni) ≤ 0.60 suboxic, 0.54 ≤ V/(V + Ni) ≤ 0.82 anoxic, and V/(V + Ni) > 0.82 strongly euxinic [84]. Niutitang shales yield mean values of Ni/Co ≈ 23.33, V/(V + Ni) ≈ 0.81, and V/Cr ≈ 4.44 (Table S2), indicating dominantly anoxic deposition. Member I and Member II show V/(V + Ni) values of 0.97 and 0.94, respectively, exceeding the 0.66 recorded in Member III shales (Table S2), indicating that the lowermost Niutitang succession was deposited under euxinic conditions that grade upward into merely anoxic settings (Figure 3). Ni/Co follows the same trend, corroborating more intense anoxia in the lower part of the formation (Figure 3). In contrast, V/Cr rises from 2.24 in Member I to 5.21 in Members II and III, suggesting a transition from suboxic to more pronounced anoxic conditions (Figure 3). The discrepancy is ascribed to detrital dilution of Co [85,86], which biases these ratios.
Cerium anomalies provide a redox indicator. Unlike other REE3+, Ce is oxidized to Ce4+ in oxic seawater and scavenged as CeO2, producing negative Ce anomalies in seawater-derived precipitates (authigenic silica, phosphorites and organic-rich shales). Conversely, reducing conditions yield negligible Ce fractionation. In the Niutitang Formation, Member I displays a pronounced negative Ce anomaly (δCe ≈ 0.67; Table S3) preserved mainly in chert, phosphorite, and organic-rich shale (Figure 5a), consistent with oxic waters. Members II and III exhibit weak anomalies (δCe ≈ 0.93 and 0.96; Table S3), indicating suboxic conditions. To test whether Ce anomalies are artifacts of weathering, we examined δY vs. (La/Nd)N [43]. The absence of a positive correlation rules out significant weathering overprint (Figure 8a). Genuine negative Ce anomalies are defined by δCe < 0.95 and δPr > 1.05 [43]. Member I samples (δPr = 1.05~1.20; δCe = 0.58~1.00) mostly fall in the authentic anomaly field (Figure 8c), whereas Members II and III lack or show minor La-induced anomalies. Moreover, δCe shows no relation to (Dy/Sm)N (Figure 8d), confirming that HREE enrichment does not distort the Ce signal. Thus, the negative Ce anomaly in Member I is real and records oxic surface-water conditions. In oxic seawater the Y/Ho ratio increases because Y is preferentially scavenged by Fe-Mn oxides relative to the rare earth elements. This ratio parallels the Ce profile and supports an upward transition from oxidizing to reducing conditions (Figure 3).
During the deposition of Member I of the Niutitang Formation, trace elements and rare earth elements (REE) yielded conflicting signals, indicating both euxinic and oxic conditions. This seemingly paradoxical situation can be reconciled by invoking a redox-stratified water column [19]. Surface waters oxygenated by the Ediacaran-Cambrian Great Oxidation Event [87] produced the Ce anomaly in the sediments (Figure 5a). Conversely, deeper sulfidic waters, isolated from the atmosphere, are characterized by enrichments of Mo, V, Cr, U, Ni, and other metals. This stratification promoted massive accumulation of organic matter and metals at the basin floor (Figure 3 and Figure 4). Up-section, the redox stratification weakened in Members II and III, shifting to anoxic–suboxic conditions (Figure 3). The diminished reducing power resulted in lower metal enrichments, as seen for Cr, V, and U (Figure 4). Under these euxinic-reducing conditions, redox-sensitive metals are strongly enriched (Ni EF = 3.07; Mo EF = 87.01; V EF = 11.59; Cr EF = 7.96; U EF = 23.49; Figure 4), demonstrating the decisive control exerted by seawater redox structure on metal accumulation.

5.4. Organic Scavenging of Metals

Biological uptake preferentially concentrates bio-essential elements such as Mo, As, Se, Sb, and Fe during primary production [88]. Once deposited, organic matter (OM) sustains strongly reducing conditions [89] and fuels bacterial sulfate reduction, generating H2S that converts chalcophile metals into sulfide phases. In addition, functional groups within OM can directly complex metals, leading to their sequestration. Hydrous-pyrolysis experiments on immature, metal-rich Permian black shales show that Hg, Se, As, V, Ni, Cr, Au, Pt, and Pd partition preferentially into generated oil rather than into the aqueous phase [90]. Coals commonly host Ge, Be, Pt and Pd bound to humic substances, and humic-acid complexation in peat is considered a key mechanism for PGEs enrichment [91,92]. Thus, OM serves a dual role as both a reductant and a ligand in metal enrichment.
The abundance of marine organic matter (OM) is governed by paleoproductivity and preservation efficiency. Paleoproductivity increases OM flux, while preservation efficiency reduces OM remineralization during burial [53,93]. Preservation is linked to water-column redox state and sedimentation rate, while productivity is controlled by nutrient availability and the structure of the pelagic ecosystem [94]. This study employs TOC, Babio, and Porg indicators to reconstruct paleoproductivity, as these are among the most widely used methods [48,95]. Additionally, because Ni, Cu, Zn, and Mo are involved in biological cycles [15,41], cross-plots of (Ni + Cu + Zn) vs. Ba-bio and Mo-bio vs. Ba-bio are used to corroborate productivity trends.
The Niutitang Formation is TOC-rich (mean ≈ 5.56%). Member I and Member III organic-rich horizons average 6.80% and 5.46% TOC, respectively, with peaks up to 10.32%, whereas Member II averages 4.72% (Figure 3). This pattern records an initial productivity bloom, a subsequent decline, and a late resurgence. Because elevated thermal maturity may bias TOC, complementary proxies were employed. Ba-bio ranges from 523.73 to 32,712.58 ppm (mean 2910.63 ppm; Table S2), Ni + Cu + Zn from 23.00 to 1454.50 ppm (mean 213.93 ppm; Table S2), and Mo from 6.32 to 708.68 ppm (mean 90.84 ppm; Table S1). In (Ni + Cu + Zn) vs. Ba-bio and Mo vs. Ba-bio space, samples scatter widely but cluster at moderate productivity levels (Figure 9).
Co-variation in Ba-bio and P/Ti confirms strong primary productivity in Member I, triggered by Early Cambrian sea-level rise and intensified upwelling that delivered deep-water nutrients [55,96], stimulating algal and bacterial blooms [97,98]. A stratified, sulfidic water column (Figure 3) further suppressed OM degradation, leading to exceptional organic enrichment. In Member II, declining Ba-bio and P/Ti imply lower productivity (Figure 3). This coincides with sea-level fall, weakened stratification and increased bottom-water oxygen, leading to OM remineralization. A minor Ba-bio spike likely reflects elevated terrigenous input. Member III shows a modest productivity rebound, consistent with increasing P/Ti and TOC (Figure 3). Although redox conditions were less reducing, higher productivity compensated for greater OM losses, yielding TOC contents comparable to Member II.
Correlation analyses reveal that strongly enriched metals (Ag, Mo, Ni, Sb, Re, Th, Ga, Tl) are intimately linked to OM abundance. In Member I, Ag, Ga, and Re are extremely enriched (Figure 4) but show no correlation with TOC (Figure 10a,c,g), consistent with metal supply dominated by vigorous hydrothermal activity and a sulfidic bottom water. In Members II and III, however, Ag/Al (R2 = 0.41), Re/Al (R2 = 0.21), and Ga/Al (R2 = 0.14) correlate positively with TOC (Figure 10a,c,g), indicating OM-controlled enrichment. Mo, Ni, Sb, Th, and Tl are enriched in both Members I and III and parallel OM abundance (Figure 3 and Figure 4). Ni/Al (R2 = 0.35), Sb/Al (R2 = 0.24), and Th/Al (R2 = 0.20) correlate with TOC in Member III but not in Member I (Figure 10b,e,f). Tl and Mo exhibit positive TOC correlations in Members II and III (Figure 10d,h). Overall, metal enrichment in Members II and III is dominantly mediated by organic matter, whereas Member I metals were supplied primarily by hydrothermal fluids.

5.5. Tectonic Settings and Hydrographic Restrictions

Wen et al. emphasize that black-shale successions typically develop in specific tectonic settings such as passive continental margins and rift basins [8]. These settings have pronounced geochemical barriers that favor the exceptional concentration of strategic mineral resources. Hence, tectonic framework exerts an important control on polymetallic enrichment. Major-element oxides such as SiO2, Al2O3, Fe2O3, and TiO2 are largely insensitive to late diagenetic alteration [99]. Al2O3 and TiO2 track detrital influx, whereas Fe2O3 enrichment is commonly linked to hydrothermal activity at mid-ocean ridges [100]. On the Fe2O3/TiO2 vs. Al2O3/(Al2O3 + Fe2O3) discrimination diagram of [101], almost all samples fall within the continental margin field, with only a few rare analyses from Members II and III extending toward pelagic or ocean-ridge affinities (Figure 11a). Likewise, the K2O/Na2O-SiO2 diagram of [102] confirms a passive-margin provenance for the entire succession (Figure 11b). In summary, the Niutitang Formation was primarily deposited in a passive continental margin setting, which modulated hydrothermal fluxes, redox conditions and paleoproductivity, thereby influencing metal accumulation.
Restricted basins developed along continental margins commonly exhibit limited vertical mixing and pronounced redox stratification, with oxic surface waters underlain by anoxic-euxinic bottom waters [103]. The study area, located adjacent to a palaeo-high (Figure 1b), likely formed such a hydrographic restrictied basin. Mo/TOC is a robust proxy for basin restriction. Restricted settings display low Mo influx and enhanced OM preservation, yielding low Mo/TOC ratios, whereas open-marine deposits exhibit elevated Mo/TOC [104]. This approach is valid only under anoxic conditions [104]. Member I averages Mo/TOC ≈ 4.09 (Table S2), indicative of strong restriction comparable to the modern Black Sea (Figure 12a). Members II and III yield Mo/TOC ≈ 15.75 and 23.80 (Table S2), respectively, reflecting moderate restriction intermediate between Saanich Inlet and Cariaco Basin (Figure 12a).
The UEF/MoEF ratio provides an additional hydrographic constraint [105]. Both U and Mo are enriched under reducing conditions, but U precipitates near the Fe3+/Fe2+ redox boundary, whereas Mo requires free H2S [41,47]. In open marine systems with suboxic bottom waters, authigenic U enrichment exceeds that of Mo, resulting in sediments U/Mo ratios below seawater values. As bottom-water reducing conditions intensify and H2S appears, Mo authigenesis outpaces U, raising U/Mo. All analyzed samples yield UEF/MoEF between 0.1 × SW and 3 × SW (Figure 12b). Only Member I samples fall into the strongly restricted field, whereas the remainder were deposited under moderate to weak restriction (Figure 12b). Collectively, bottom-water restriction was strongest during Member I deposition and progressively waned up-section, within an overarching anoxic-euxinic regime (Figure 12b).
Processes 13 03537 g012
Figure 12. (a) Mo concentration (ppm) vs. TOC content (wt.%) plot for comparison of the degree of hydrographic restriction between the study area and four modern anoxic-euxinic marine systems (modified from [106]. (b) MoEF vs. UEF plot for possible seawater redox conditions constrained by in the Niutitang Formation; dotted lines showing Mo/U molar ratios equal to the seawater values of 3 × SW, 1 × SW and 0.3 × SW, respectively (modified from [105]).
Figure 12. (a) Mo concentration (ppm) vs. TOC content (wt.%) plot for comparison of the degree of hydrographic restriction between the study area and four modern anoxic-euxinic marine systems (modified from [106]. (b) MoEF vs. UEF plot for possible seawater redox conditions constrained by in the Niutitang Formation; dotted lines showing Mo/U molar ratios equal to the seawater values of 3 × SW, 1 × SW and 0.3 × SW, respectively (modified from [105]).
Processes 13 03537 g012
During the early deposition of Member I, pronounced basin restriction and weak water-column circulation intensified stratification, driving bottom-water euxinic conditions. Such conditions promoted metal reduction and precipitation while enhancing OM preservation by shielding organic matter from oxidative degradation. The synergistic interaction between sulfidic redox conditions and abundant OM thus fostered extreme metal enrichment. As sedimentation progressed, hydrological restriction gradually weakened. Members II and III record increasing circulation, higher dissolved oxygen, and nutrient recharge. Although primary productivity rebounded, the concomitant decline in redox intensity compromised OM preservation. Consequently, both organic-carbon and metal abundances decrease relative to Member I, demonstrating that hydrographic restriction indirectly controlled polymetallic enrichment in the Niutitang Formation by modulating redox conditions and organic matter preservation.

5.6. Metal Enrichment Patterns

The origin of metal enrichment in the Early Cambrian Niutitang Formation has long been debated. This study focuses on a new section of the Niutitang Formation in the Kaiyang area. Based on the geochemical characteristics of this section and integrating previous research on the formation, we propose a model for metal enrichment (Figure 13).
During the earliest Cambrian, the Yangtze Block lay between 5° and 25° N in the tropics [13,15,19,53,64]. A rapid global sea-level rise generated vigorous upwelling that transported nutrient-rich deep waters to the photic zone, sustaining exceptionally high primary productivity (Figure 3, Figure 9 and Figure 13a). Consequently, the basal Niutitang Member I accumulated organic-rich black shales with TOC peaks up to 10.32% (Table 1). The basin was strongly restricted (Figure 12a,b), exhibiting low vertical mixing and stable redox stratification, with oxic surface waters overlaying persistently anoxic bottom waters (Figure 3 and Figure 13a). Abundant organic matter reacted with sulfate in the anoxic deep layer, generating abundant H2S that drove the sediment–water interface into euxinia (Figure 13a). This sharp redox gradient efficiently scavenged and focused metals to the seafloor. Geochemical indicators such as Ni-Co-Zn and Al/(Al + Fe + Mn)-Fe/Ti all suggest that the depositional environment was influenced by hydrothermal activity during this period (Figure 7a,b). A particularly pronounced positive Eu anomaly (Eu/Eu* = 2.66; Figure 3 and Figure 5a) further confirms that intermittent hydrothermal pulses injected elements such as Ba, Si, Pb, Zn, Mn, Ni, Mo, Se, and PGEs into the anoxic water column (Figure 13a). Against this backdrop, the coupling of vigorous hydrothermal input, strongly euxinic conditions, and high organic matter flux collectively led to the extreme enrichment of elements such as As, Ba, Ag, Cr, V, Re, Cs, Ga, Ge, Se, and In in Member I (Figure 4).
Subsequently, falling sea level weakened basin restriction (Figure 12a,b) and water-column stratification. Redox conditions shifted from strongly euxinic to anoxic-suboxic (Figure 3), organic matter became more susceptible to degradation, and sulfidic intensity declined sharply (Figure 13b,c). Concurrently, hydrothermal activity waned (Figure 7b), markedly reducing the external metal flux. Nevertheless, geochemical data reveal that Ag, Mo, Ni, Sb, Re, Th, Ga, and Tl in Members II and III still exhibit enrichment factors several times higher than upper-crustal values (Figure 4). These metals display positive correlations with TOC (Figure 10), indicating that metal enrichment was mainly controlled by organic complexation after hydrothermal input waned and redox conditions weakened. Reactive organic functional groups (carboxyl, thiol, pyridyl) formed stable metal–organic complexes that were subsequently delivered to the sediment with OM. Hence, even under progressively less reducing conditions (Figure 3), elevated organic fluxes—driven by high preservation efficiency in Member II or high paleoproductivity in Member III—were sufficient to sustain significant enrichment of critical metals.

6. Conclusions

Geochemical analyses of black shales were employed to constrain the dominant controls and enrichment models of metals in the early Cambrian Niutitang Formation.
  • During deposition of Member I, intense submarine-hydrothermal activity governed metal enrichment. Elements such as As, Ba, Ag, Cr, V, Re, Cs, and Ga are extremely enriched, yet their abundances show no significant correlation with total organic carbon (TOC), indicating that hydrothermal input, rather than organic complexation, was the dominant factor. Highly restricted basin hydrology rapidly established euxinic bottom waters, promoting synchronous precipitation and sequestration of redox-sensitive metals (Mo, U, V) together with organic matter. Thus, the exceptional metal enrichment in Member I reflects a “hydrothermal dominance and sulfide preservation” synergy, with only minor influence from terrigenous fluxes.
  • In Members II and III, waning hydrothermal activity led to a concomitant decline in metal concentrations. Relaxation of basin restriction and enhanced water-column ventilation shifted depositional conditions from euxinic to dysoxic–suboxic, lowering the enrichment of redox-sensitive elements such as Cr and V. In these intervals, TOC is positively correlated with metals (Ag, Re, Ga), implying that organic ligand complexation/adsorption became the principal enrichment mechanism. Although terrigenous input increased (rising Er/Nd ratios), it remained insufficient to dominate metal accumulation. Consequently, enrichment in Members II and III was controlled by the interplay between organic-matter abundance and redox-interface fluctuations.
  • Overall, metal enrichment in the Niutitang Formation follows a stage-dependent, multi-source model: Member I is characterized by hydrothermal–euxinic cooperation, whereas Members II and III are dominated by organic-matter and redox-controlled processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13113537/s1, Table S1. Trace element concentrations (ppm) of the Niutitang Formation shales. Table S2. Trace element concentrations (ppm) of the Niutitang Formation shales. Table S3. Race earth element concentrations (ppm) of the Niutitang Formation shales.

Author Contributions

Methodology, K.S. and Z.N.; Validation, W.Z.; Investigation, G.S. and N.C.; Resources, G.S. and N.C.; Data curation, W.Z.; Writing—original draft, K.S.; Writing—review and editing, Z.N.; Project administration, Z.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (Grant No. 42273069).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank the staff of all of the laboratories that cooperated in performing the tests and analyses. We are grateful to the valuable comments of the editor and reviewers that improved the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 2. Field outcrop and hand specimen of the black rock series in the Niutitang Formation in the Kaiyang area. (a) Field outcrop profile of the Niutitang Formation; (b) Field outcrop of Member I of the Niutitang Formation; (c) Hand specimen of mudstone from Member I; (d) Hand specimen of cherts from Member I; (e) Hand specimen of organic-rich black shales from Member I; (f) Hand specimen of phosphorite from Member I; (g) Hand specimen of black shales from Member II; (h) Hand specimen of siliceous shales from Member II; (i) Hand specimen of carbonaceous shales from Member III.
Figure 2. Field outcrop and hand specimen of the black rock series in the Niutitang Formation in the Kaiyang area. (a) Field outcrop profile of the Niutitang Formation; (b) Field outcrop of Member I of the Niutitang Formation; (c) Hand specimen of mudstone from Member I; (d) Hand specimen of cherts from Member I; (e) Hand specimen of organic-rich black shales from Member I; (f) Hand specimen of phosphorite from Member I; (g) Hand specimen of black shales from Member II; (h) Hand specimen of siliceous shales from Member II; (i) Hand specimen of carbonaceous shales from Member III.
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Figure 3. Vertical variation diagram of climatic index of Lower Niutitang Formation in the Kaiyang area.
Figure 3. Vertical variation diagram of climatic index of Lower Niutitang Formation in the Kaiyang area.
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Figure 4. Trace element enrichment factors (EF) relative to upper continental crust [42] of the KY section.
Figure 4. Trace element enrichment factors (EF) relative to upper continental crust [42] of the KY section.
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Figure 5. PAAS [45] normalized REE patterns of shales in Kaiyang, Guizhou province.
Figure 5. PAAS [45] normalized REE patterns of shales in Kaiyang, Guizhou province.
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Figure 6. Discrimination plot for terrigenous input in the Niutitang Formation shales. (a) Bivariate plots of Y-La cross-plot (base map adapted from [51], dashed diagonal lines mark the Y/La ratios of modern seawater (4.7; ref. [50]) and upper continental crust (0.73; ref. [42]). (b) Er/Nd vs. ΣREY diagram (Reference data for seawater, land and detrital come from [52]).
Figure 6. Discrimination plot for terrigenous input in the Niutitang Formation shales. (a) Bivariate plots of Y-La cross-plot (base map adapted from [51], dashed diagonal lines mark the Y/La ratios of modern seawater (4.7; ref. [50]) and upper continental crust (0.73; ref. [42]). (b) Er/Nd vs. ΣREY diagram (Reference data for seawater, land and detrital come from [52]).
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Figure 9. Marine primary productivity discrimination map of Niutuotang Formation shale. (a) Plots of Ba-bio to Ni + Cu + Zn (modified from [15]) (b) Plots of Ba-bio to Mo-bio (modified from [15]).
Figure 9. Marine primary productivity discrimination map of Niutuotang Formation shale. (a) Plots of Ba-bio to Ni + Cu + Zn (modified from [15]) (b) Plots of Ba-bio to Mo-bio (modified from [15]).
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Figure 10. Correlation between TOC with (a) Ag, (b) Th, (c) Ga, (d) Mo, (e) Ni, (f) Sb, (g) Re, (h) Tl for samples in Kaiyang, South China.
Figure 10. Correlation between TOC with (a) Ag, (b) Th, (c) Ga, (d) Mo, (e) Ni, (f) Sb, (g) Re, (h) Tl for samples in Kaiyang, South China.
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Figure 11. Tectonic discrimination plots using major elements. (a) Fe2O3/TiO2 vs. Al2O3/(Al2O3 + Fe2O3) diagram (modified from [101]). (b) K2O/Na2O vs. SiO2 diagram (modified from [102]).
Figure 11. Tectonic discrimination plots using major elements. (a) Fe2O3/TiO2 vs. Al2O3/(Al2O3 + Fe2O3) diagram (modified from [101]). (b) K2O/Na2O vs. SiO2 diagram (modified from [102]).
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Figure 13. Conceptual model showing the metal enrichment of the Niutitang Formation shales (modified from [107,108,109]).
Figure 13. Conceptual model showing the metal enrichment of the Niutitang Formation shales (modified from [107,108,109]).
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Table 1. Total organic carbon (%) and major element concentrations (%) of the Niutitang Formation shales.
Table 1. Total organic carbon (%) and major element concentrations (%) of the Niutitang Formation shales.
LithologySamplesSiO2TiO2Al2O3Fe2O3MnOMgOCaONa2OK2OP2O5LOICIATOC
MudstoneKY-152.67 0.31 25.51 1.41 <0.0012.02 0.06 0.008 4.97 0.37 12.00 82.53 2.28
ChertKY-2.187.95 0.06 1.95 3.35 0.001 0.20 0.02 0.014 0.39 0.10 5.23 80.77 1.79
KY-2.388.77 0.06 1.56 2.26 0.001 0.11 0.07 0.015 0.21 0.14 6.11 84.74 3.95
KY-2.588.43 0.14 1.55 0.89 <0.0010.12 0.06 0.039 0.21 0.10 7.76 81.16 5.36
KY-2.987.95 0.09 1.68 0.78 <0.0010.13 0.06 0.016 0.29 0.09 8.18 82.06 6.87
KY-2.1179.54 0.26 2.44 1.45 <0.0010.14 2.96 0.021 0.30 2.08 10.08 86.05 5.74
Organic-rich blackshalesKY-2.270.44 0.44 8.16 2.15 0.002 0.84 0.04 0.031 2.37 0.24 14.98 75.38 9.84
KY-2.468.30 0.48 9.02 3.11 0.003 0.90 0.05 0.036 2.58 0.31 14.58 75.63 9.51
KY-2.668.70 0.52 10.03 1.49 0.002 1.02 0.03 0.062 2.89 0.11 14.56 75.31 10.32
KY-2.870.74 0.48 8.88 1.88 0.003 0.84 0.09 0.044 2.46 0.21 13.96 76.01 9.95
KY-2.1062.87 0.40 8.01 1.67 0.001 0.66 5.48 0.038 2.04 4.01 14.80 77.47 9.61
PhosphoriteKY-2.757.40 0.12 1.80 1.55 <0.0010.18 15.78 0.045 0.34 11.14 10.90 77.73 6.39
Black shaleKY-366.73 0.77 14.09 2.00 0.005 0.95 0.55 0.042 3.04 0.16 11.90 80.43 5.74
KY-463.40 0.74 13.37 2.92 0.008 0.94 1.71 0.047 2.86 0.18 13.41 80.45 5.80
KY-561.79 0.66 12.23 4.42 0.026 1.40 2.87 0.042 2.65 0.19 13.25 80.28 4.83
KY-657.37 0.70 12.07 5.07 0.004 0.93 2.61 0.055 2.90 0.04 18.35 78.45 5.05
KY-764.88 0.68 12.52 3.41 0.006 0.84 2.05 0.035 2.71 0.20 12.12 80.42 4.88
KY-3467.06 0.84 14.37 1.09 0.003 0.87 0.16 0.042 3.10 0.05 11.76 80.43 5.40
KY-3758.68 0.76 13.29 4.82 0.033 1.09 2.81 0.037 2.96 0.22 15.30 79.99 4.68
KY-4359.53 0.76 13.44 5.10 0.034 2.00 2.64 0.048 3.03 0.23 12.61 79.64 3.91
SiliceousKY-858.66 0.63 11.73 4.25 0.041 2.56 3.73 0.026 2.66 0.18 15.01 79.83 5.08
KY-959.96 0.64 11.59 3.91 0.036 2.15 3.96 0.041 2.62 0.20 14.59 79.60 4.70
KY-1060.07 0.67 11.96 3.90 0.033 1.97 3.94 0.050 2.67 0.20 13.80 79.68 4.36
KY-1154.89 0.63 11.07 4.73 0.057 3.37 5.53 0.022 2.36 0.17 16.43 80.81 3.86
KY-1253.78 0.65 10.99 6.62 0.051 2.87 5.20 0.018 2.36 0.18 16.54 80.79 3.96
KY-1358.01 0.70 12.22 4.12 0.038 2.57 4.14 0.027 2.77 0.20 15.23 79.83 4.18
KY-1457.64 0.69 12.40 4.32 0.032 2.18 3.85 0.026 2.82 0.19 15.18 79.83 4.78
KY-1556.66 0.70 12.13 4.49 0.037 2.61 4.33 0.027 2.65 0.19 16.53 80.39 4.62
KY-1652.36 0.96 12.06 4.13 0.035 2.55 4.41 0.055 2.51 0.20 17.10 80.61 4.36
KY-1754.36 0.79 12.11 4.40 0.034 2.49 4.25 0.030 2.68 0.19 16.54 80.14 4.63
KY-3956.66 0.73 12.71 5.04 0.044 2.41 3.31 0.032 2.79 0.20 15.79 80.25 5.01
KY-4159.94 0.76 13.08 5.48 0.025 1.49 2.19 0.041 2.87 0.23 13.01 80.11 4.54
KY-4258.19 0.76 13.22 5.04 0.041 2.35 3.47 0.039 2.97 0.22 12.84 79.79 3.48
Carbonaceous shaleKY-1856.54 0.69 12.43 4.51 0.034 2.19 4.23 0.034 2.84 0.20 15.73 79.62 4.93
KY-1955.00 0.69 12.38 5.25 0.038 2.75 4.04 0.027 2.81 0.19 16.78 79.80 4.66
KY-2054.52 0.68 12.16 4.47 0.055 2.56 4.76 0.029 2.65 0.19 17.43 80.40 5.18
KY-2149.39 0.62 11.20 11.64 0.038 1.92 3.43 0.011 2.39 0.17 18.78 81.02 4.87
KY-2239.82 0.55 9.79 5.35 0.120 6.11 9.91 0.019 2.20 0.31 24.94 80.04 5.59
KY-2352.21 0.67 11.89 5.45 0.048 2.88 4.71 0.025 2.55 0.23 18.76 80.72 6.80
KY-2444.34 0.58 10.61 4.90 0.086 5.02 8.25 0.032 2.28 0.19 22.96 80.45 6.04
KY-2547.63 0.62 11.11 5.25 0.066 4.15 6.64 0.033 2.40 0.21 21.34 80.42 6.76
KY-2648.12 0.65 11.32 5.34 0.070 3.73 6.39 0.022 2.36 0.27 21.17 81.16 6.74
KY-2747.35 0.62 11.11 5.67 0.066 3.80 6.49 0.030 2.33 0.28 21.62 80.94 6.67
KY-2854.78 0.71 12.29 5.63 0.039 1.89 3.33 0.033 2.49 0.21 18.72 81.42 7.70
KY-2948.99 0.68 11.51 4.52 0.087 4.21 6.98 0.048 2.54 0.43 19.17 79.83 4.69
KY-3053.84 0.72 12.36 4.97 0.044 2.61 3.84 0.027 2.67 0.24 17.93 80.57 6.24
KY-3154.81 0.72 12.25 4.51 0.059 2.89 4.46 0.034 2.58 0.23 17.90 80.84 6.07
KY-3253.83 0.68 11.87 4.68 0.063 2.99 4.89 0.020 2.51 0.24 17.91 81.02 5.86
KY-3352.41 0.68 11.45 5.03 0.066 3.20 4.79 0.025 2.40 0.21 18.99 81.07 7.17
KY-3559.62 0.77 13.13 4.05 0.035 1.62 3.04 0.066 3.04 0.20 13.99 78.94 4.19
KY-3658.54 0.77 13.36 5.40 0.041 1.46 2.54 0.044 3.02 0.21 14.66 79.67 4.79
KY-3858.63 0.76 13.22 5.02 0.033 1.79 2.46 0.036 2.86 0.22 15.83 80.42 5.52
KY-4046.52 0.63 10.80 4.80 0.108 5.07 8.34 0.026 2.39 0.53 20.06 80.16 4.67
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MDPI and ACS Style

Shi, K.; Ni, Z.; Shao, G.; Zhang, W.; Cheng, N. Research on the Metallogenic Enrichment Model of Poly-Metallic Black Shales and Their Geological Significance: A Case Study of the Cambrian Niutitang Formation. Processes 2025, 13, 3537. https://doi.org/10.3390/pr13113537

AMA Style

Shi K, Ni Z, Shao G, Zhang W, Cheng N. Research on the Metallogenic Enrichment Model of Poly-Metallic Black Shales and Their Geological Significance: A Case Study of the Cambrian Niutitang Formation. Processes. 2025; 13(11):3537. https://doi.org/10.3390/pr13113537

Chicago/Turabian Style

Shi, Kai, Zhiyong Ni, Ganggang Shao, Wen Zhang, and Nuo Cheng. 2025. "Research on the Metallogenic Enrichment Model of Poly-Metallic Black Shales and Their Geological Significance: A Case Study of the Cambrian Niutitang Formation" Processes 13, no. 11: 3537. https://doi.org/10.3390/pr13113537

APA Style

Shi, K., Ni, Z., Shao, G., Zhang, W., & Cheng, N. (2025). Research on the Metallogenic Enrichment Model of Poly-Metallic Black Shales and Their Geological Significance: A Case Study of the Cambrian Niutitang Formation. Processes, 13(11), 3537. https://doi.org/10.3390/pr13113537

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