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Article

Enrichment Mechanism of Polymetallic Elements at the Base of the Niutitang Formation in Southeast Chongqing

by
Guozhi Wang
1,2,3,
Can Zhang
3,4,
Dayong Liu
1,2,3,*,
Linfei Qiu
5,
Ziying Li
5 and
Ping’an Peng
1,2,3
1
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
2
CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
3
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
4
Real Estate Registration Center of Zhaoqing City, Zhaoqing 526060, China
5
Beijing Research Institute of Uranium Geology, China National Nuclear Corporation, Beijing 100029, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(10), 978; https://doi.org/10.3390/min14100978 (registering DOI)
Submission received: 26 July 2024 / Revised: 25 September 2024 / Accepted: 25 September 2024 / Published: 28 September 2024
(This article belongs to the Special Issue Mineralogical and Geochemical Characterization of Geological Materials)
Browse Figures
Versions Notes

Abstract

:
Polymetallic enrichment layers are commonly found at the base of the Lower Cambrian and extensively distributed across the Upper Yangtze Platform, yet their genetic models remain controversial. This study systematically collected samples from a typical section in the southeastern Chongqing region for mineral, organic, and inorganic analyses. It investigates the relationship between the abundance of various trace metal elements and organic matter at the base of the Niutitang Formation, as well as the vertical distribution characteristics of organic carbon isotopes and organic matter features. The results indicate that the Niutitang Formation shale exhibits a distinct three-part structure from bottom to top. Various metal elements are enriched in the lower interval, showing a close correlation between the abundance of polymetallic elements and the carbon isotopes of shale organic matter. The middle interval contains the highest TOC value and the lowest Ti/Al ratio, while the upper interval shows a significant decrease in organic matter abundance, with a clear positive correlation between the excess silicon content and Ti/Al ratio. Additionally, the mixing effect of deep-sea upwelling is the primary control on the formation of polymetallic enrichment layers in the lower interval, followed by the adsorption of organic matter under anoxic conditions. The sedimentary environment of the upper interval of the Niutitang Formation trends toward oxidation, with paleoclimate shifting toward colder and drier conditions, exhibiting aeolian sedimentary features that are unfavorable for the enrichment of trace metal elements. Consequently, upwelling is a key factor in the enrichment and mineralization of trace metal elements at the base of the Lower Cambrian in the Upper Yangtze region.

1. Introduction

The Niutitang Formation, a Lower Cambrian unit in southern China, is a widely distributed, thick, and highly mature marine shale formation [1,2] in the Upper-Middle Yangtze Platform. The organic matter in the shale primarily originates from lower aquatic organisms and algae, classified as Type I organic matter [1,3,4,5]. The Niutitang Formation’s abundant organic content makes it a key target for shale gas exploration and development. In terms of lateral distribution, it can be compared to the Qiongzhusi Formation in other regions and the Hetang Formation in the lower Yangtze region [6].
The Lower Cambrian Niutitang Formation is known for its abundance of polymetallic sulfides and rare metal elements, such as nickel, molybdenum, vanadium, and uranium [7,8]. This Formation has the potential to be a low-grade uranium deposit [9], as well as hosting other valuable metal deposits. Previous research has suggested that the enrichment of rare metal elements can be attributed to hydrothermal fluids brought by upwelling ocean currents [7,10], which also contribute to the accumulation of organic matter in the basin [9,11]. As a result, there is a positive correlation between the abundance of organic matter and rare metal elements in the shale of the Niutitang Formation [9].
One distinctive feature of the Niutitang Formation shale is the presence of “excess silica”. However, the source of this excess silica is still a subject of intense debate. Some researchers argue that it is derived from hydrothermal processes [3,9,12,13], while others propose that it originates from biogenic sources [14,15,16]. The variability in the source of excess silica may also be influenced by co-deposition processes and subsequent tectonic activities. Previous studies have shown that Ti/Al may be closely related to aeolian deposits: Ti is often found in heavy minerals and serves as a parameter for arid climatic conditions. The positive correlation between the two in the Qiongzhusi Formation has been explained as a result of aeolian deposits bringing in more Ti and excess silica under relatively cold and dry climatic conditions [17,18]. Additionally, it can also serve as a parameter for sediment input from offshore distances under relatively humid climatic conditions.
δ13Corg depletion has been associated with the expansion of microbially mediated carbon cycling processes in modern eutrophic lakes, high-productivity marine environments, and ancient sediments with high TOC deposited under euxinic conditions [19]. Similarly, negative δ13Corg excursions have been reported in sediments from the Yangtze Platform and other global locations during the same period. This evidence points to the presence of bottom anoxic conditions on the Yangtze Platform during the late Cambrian Stage 2 to early Cambrian Stage 3 [20,21].
This study aims to analyze the variations and correlations of different parameters with depth along the profile of the Lower Cambrian Niutitang shale samples through relevant tests. By considering the paleogeographic location of the Lower Cambrian within the profile, the characteristics of the shale will be discussed, with a focus on the impact of upwelling events on sedimentary environments, trace metal elements, and organic matter characteristics.

2. Sample and Experiments

2.1. Geological Setting and Sample Details at the Sampling Site

The Danquan Profile is located in the Youxiu Fold Belt on the southeastern margin of the Sichuan Basin. It is characterized by open anticlines and tight synclines with compartmentalized folds. The profile is situated between two large-scale faults in the basin, creating a stable tectonic environment with rock layers that have gentle dip angles (Figure 1B).
During the Early Cambrian period in the Yangtze region, there was a notable gradation of sedimentary environments extending from northwest to southeast. This gradation began with shallow-water platform facies, transitioned through intermediary facies, and culminated in deep-water continental shelf and basin facies [22,23,24]. Notably, the shallow water platform facies emerged as the dominant environment. The chosen sampling site is strategically positioned in proximity to the transitional zone of the continental shelf slope (as depicted in Figure 1), a location that is particularly sensitive to sea-level fluctuations.
Minerals 14 00978 g001
Figure 1. Simplified paleogeographic map [25] (A) and simplified geological map [26] (B) of the Yangtze Block around the Ediacaran-Cambrian boundary interval. The three black triangles in (A) represent typical profile locations in this area.
Figure 1. Simplified paleogeographic map [25] (A) and simplified geological map [26] (B) of the Yangtze Block around the Ediacaran-Cambrian boundary interval. The three black triangles in (A) represent typical profile locations in this area.
Minerals 14 00978 g001
The underlying formation of the Niutitang Formation is the Maidiping Formation, which also belongs to the Lower Cambrian. This Formation consists of limestone with phosphatic nodules and is associated with a significant phosphogenesis event known as the Meishucun Period.
Mining activities in the area have led to the excavation of phosphate conglomerates from the Maidiping Formation, which are integrated with the base of the Niutitang Formation. This indicates that the lower part of the Niutitang Formation is fully exposed, and the collected samples are relatively fresh. A total of 31 samples were collected from the base of the Niutitang Formation, with a sampled thickness of 44 m. However, the uppermost part of the Niutitang Formation was not observed to be exposed in the area.

2.2. Organic Geochemistry and Mineralogy Experiments

TOC
The particle size of the samples is crushed to smaller than 100 mesh and then dried for 24 h at 60 °C. The accurately weighed samples are placed into a permeable crucible, and the total weight is recorded. Slowly add approximately 5% dilute hydrochloric acid, allowing the reaction to proceed for over 12 h. Subsequently, place the crucible with the samples onto a tray and immerse it in an 80 °C water bath for 1 h to ensure the complete removal of inorganic carbon from the shale samples. After heating, thoroughly rinse the samples with deionized water until the pH paper indicates neutrality. Additionally, rinse the tray, sample holder, and crucible to neutralize them. Once the samples are dried, add a tungsten-tin alloy combustion aid and iron powder to the surface of the treated samples in the crucible. The total organic carbon (TOC) content of the samples will be measured using the ELTRA CS 800 carbon-sulfur analyzer, ensuring a relative error of less than 2%.
Total sulfur (TS)
Before testing, the shale samples are cleaned, crushed, and ground to a particle size of 200 mesh (less than 74 μm). After drying, the powdered samples are wrapped in tin boats and placed into a VARIO EL cube elemental analyzer to obtain data with a precision of ±0.5%.
Organic carbon isotope (δ13Corg)
The sample amount is calculated to ensure a net carbon content of approximately 40 μg according to the TOC value. Then, the samples are wrapped in tin boats and measured using a Thermo Quest Finnigan DELTA plus XL isotope ratio mass spectrometer (IRMS) from Thermo Scientific, Waltham, MA, USA. The results are referenced to the VPDB standard, with an analytical error of ±0.5‰.
X-ray diffraction (XRD)
X-ray diffraction (XRD) analysis of shale can provide semi-quantitative information about the mineral content of the sample. The specific analysis method involves weighing a sample with a particle size of 200 mesh and vacuum drying it at 70 °C for 24 h. The dried sample is then analyzed for mineral content using a Bruker D8 Advance X-ray diffractometer (Bruker Corporation, Billerica, MA, USA).
The analysis is performed under the following conditions: a working voltage of 40 kV and a current of 30 mA. The scanning range is set from 2θ = 3° to 85° with a slit width of 1 mm, and the scanning rate is 4°/min. The mineral content is calculated using the Lorenz polarization method, which corrects for peak areas of different minerals. This calculation provides semi-quantitative information about the minerals present in the sample.

2.3. Analysis of Major and Trace Elements

Analysis of Major Elements using Fusion Method: Approximately one gram of sample (with a particle size above 200 mesh) is weighed and incinerated at 1000 °C to eliminate organic matter. The remaining residue is then mixed with a flux containing lithium nitrate and other components, thoroughly blended, and subjected to high-temperature fusion. The molten material is poured into a platinum mold to form a flat glass slide, which is subsequently analyzed using a PANalytical PW2424X X-ray fluorescence spectrometer (Malvern Panalytical, Worcestershire, UK). The precision relative deviation (RD) is <5% and the accuracy relative error (RE) is <2%.
Trace element testing is conducted using two different methods: the dissolution method with HNO3-HClO4-HF-HCl and the fusion method with LiBO2/Li2B4O7. Two dried samples of shale, each weighing 0.5 g and with a particle size above 200 mesh, are prepared.
In the dissolution method, one sample is digested using concentrated nitric acid, hydrochloric acid, and hydrofluoric acid. The mixture is then evaporated to near dryness and dissolved in diluted hydrochloric acid to a specific volume. The analysis is performed using an Agilent VISTA inductively coupled plasma emission spectrometer (Agilent Technologies, Santa Clara, CA, USA) and a Perkin Elmer Elan 9000 inductively coupled plasma mass spectrometer (PerkinElmer Corporation, Waltham, MA, USA).
In the fusion method, the other sample is mixed with a flux consisting of lithium metaborate and lithium tetraborate. The mixture is melted in a furnace at a temperature above 1025 °C, cooled, and then dissolved in nitric acid, hydrochloric acid, and hydrofluoric acid to a specific volume. The analysis is carried out using an inductively coupled plasma mass spectrometer.
Both instruments used in the analysis have a precision relative deviation (RD) of less than 10% and an accuracy relative error (RE) of less than 10%. This ensures reliable and accurate results in the trace element testing of the shale samples.

3. Results

After comparing various parameters, the Niutitang shale samples were divided into three intervals based on the Ti/Al and U/Th ratios. The lower and middle intervals exhibit relatively high U/Th values, with the lower interval displaying high Ti/Al values, while the middle interval shows noticeably lower Ti/Al values. In contrast, the upper interval has a significantly low U/Th value (Figure 2). The three intervals are roughly distributed from bottom to top at 0–8 m, 8–26 m, and 26–44 m.
Minerals: The mineral composition of the Lower Cambrian Niutitang Formation shale in the Danquan section is mainly composed of quartz, feldspar, and clay minerals. In the lower and middle intervals of the Niutitang Formation, which spans from 0 to 26 m, there are small amounts of pyrite and dolomite present. However, above 27 m from the bottom, pyrite and carbonate minerals are essentially absent (Figure 3A,B).
δ13Corg, TOC, and TS values: The lower interval of the Niutitang Formation is distinguished by negative δ13Corg values, which show a gradual increase from −32.6‰ at the base to −31.0‰ towards the top. In the middle interval of the formation, δ13Corg values demonstrate relative consistency, fluctuating within a narrow span from −31.4‰ to −31.1‰. Progressing to the upper interval, the δ13Corg values vary from −31.7‰ to −31.0‰, overall exhibiting a positive trend with increasing depth (Figure 3C).
In the Danquan section of the Niutitang Formation, the distribution of organic matter within the black shale displays marked stratigraphic variations. The middle to lower interval is characterized by a high concentration of organic matter, in contrast to the upper interval, which presents a comparatively lower abundance, as depicted in Figure 3D. These observations are consistent with the findings of Pi [7] and corroborate the stratigraphic division into the organically rich Niu2 interval and the organically lean Niu3 interval, as defined by Tan [3]. Additionally, the basal Niu1 interval, which is notable for its low total organic carbon (TOC) content, correlates with the underlying phosphatic dolomite-rich Maidiping Formation, providing a foundational context for the Niutitang Formation in this stratigraphic profile.
The variation trends of total sulfur content are similar to that of the TOC and pyrite content (Figure 3E), which indicates that sulfur is tightly associated with organic matter. The differences in TOC and total sulfur abundance are mainly attributed to the anoxic environment rich in sulfur in the lower-middle interval of the Niutitang Formation and the oxygen-deficient environment with low sulfur in the upper interval [7].
Aluminum element: The elevated aluminum concentrations, varying from 6.57% to 7.28% in the lower interval and from 4.83% to 7.88% in the upper interval, indicate a substantial terrestrial clastic contribution (Figure 3F). Conversely, the aluminum levels are primarily confined to a narrower range of 3.8% to 5.91% throughout the middle interval, with a modest peak of 7.08% at its uppermost portion, pointing to a reduced impact of terrestrial clastic influx in this section.
Excess silicon: The origins of silicon in shale can be categorized into three types: terrigenous silicon, hydrothermal silicon, and biogenic silicon. To calculate the excess silicon content (Si_excess), one must subtract the content of terrigenous detrital silicon using the formula:
Si_excess = Sis − [(Si/Al)bg × Als]
Here, Sis represents the silicon element content in the sample, Als represents the aluminum element content in the sample, and (Si/Al)bg is 3.11 (the average content in shale) [27].
The silicon dioxide (SiO2) content in the Niutitang Formation shale in the lower Cambrian of the DanQuan section in Youyang ranges from 51.4% to 71.1%. The Si_excess varies from 5% to 22%. The lowest Si_excess content is observed in the lower section of the Niutitang Formation, gradually increasing from 5.54% to 9.56% over a 5-m interval. The middle and upper sections show relatively higher Si_excess content, ranging from 8.63% to 20.32% and from 10.69% to 21.99%, respectively (Figure 3G).
Trace elements: In the lower section, the concentrations of most trace metal elements, including arsenic (As), molybdenum (Mo), nickel (Ni), rhenium (Re), antimony (Sb), selenium (Se), and uranium (U), progressively diminish toward the upper part (Figure 4A–G). This trend is succeeded by a pronounced enrichment of metallic elements in the base of the middle interval. Conversely, the upper section shows a marked depletion of these trace elements.

4. Discussion

4.1. Characteristics of the Sedimentary Environment of the Niutitang Formation Shale

4.1.1. Redox Conditions of Water Bodies

The U/Th ratio is a parameter used to assess the reducing conditions of water bodies, with higher U/Th values indicating stronger reducing capabilities of the water. This relationship has been established in previous studies [28,29,30].
In the lower interval of the Niutitang Formation, the samples generally exhibit anaerobic conditions, and most of the U/Th values are higher than 4 (except for the sample with a U/Th value of 2.4 at 3.6 m). Additionally, as the reducing conditions of the water increase, the abundance of U also increases. There is a positive correlation between total organic carbon (TOC) and U/Th (Figure 5A), indicating that stronger anaerobic conditions favor the enrichment of organic matter at middle intervals. However, in the lower interval, there is no correlation, which may be attributed to the influence of U from upwelling mixing sources on the U/Th ratio.
Previous studies have suggested that U/Th ratios greater than 1 indicate the influence of hydrothermal deposition during shale sedimentation [31,32]. This suggests that the lower and middle intervals of the profile may commonly be affected by hydrothermal fluids through the upwelling event, while the upper section of the Niutitang Formation shows a significant decrease in hydrothermal fluid influence.
In the lower interval of the Niutitang Formation shale, there is a weak positive correlation between TOC and total sulfur (R2 = 0.32) and iron (R2 = 0.28) (Figure 5B,C), indicating a rough genetic relationship between organic matter enrichment and pyrite. This relationship is attributed to the processes of anoxic sulfidation and deep-sea ironization resulting from hydrothermal mixing. Additionally, the negative correlation observed between δ13Corg values and TS (Figure 6A), Fe (Figure 6B) in the lower section suggests that a decrease in hydrothermal δ13Corg mixing leads to a gradual reduction in the abundance of S and Fe, supporting the theory that pyrite in this section is primarily of hydrothermal origin. On the other hand, the correlation between TOC and sulfur and iron in the middle section of organic-rich shales and the upper section of organic-poor shales is weak, suggesting that sources of S and Fe in these sections are not genetically linked to organic matter enrichment.

4.1.2. Terrestrial Input

The ratio of Ti/Al elements can be used to indicate the distance of the deposition zone from the coastline, with a higher Ti/Al ratio suggesting closer proximity to the coastline and vice versa [28,33]. Previous studies indicate that the Ti/Al ratio may be closely related to eolian sedimentation, and Ti is often found in heavy minerals and serves as a parameter for arid climate conditions [34]. The highest aluminum (Figure 7A) and titanium (Figure 7B) contents of the lower interval show that the palaeoenvironment of the lower interval is closest to the coastline. The middle interval exhibits a sharp decrease in the Ti/Al ratio at higher levels (Figure 7C), indicating a large-scale transgression event. The Ti/Al ratio is generally higher in the upper interval but still lower compared to the lower interval, with only the highest two samples showing a distance from the coastline and sea level similar to the bottom of the lower section.
The phosphatic dolomite of the Lower Maidiping Formation at the base of the Niutitang Formation was predominantly formed in a tidal flat environment. At the base of the Niutitang Formation, the mixing of polymetallic-rich fluids and ancient seawater, triggered by crustal extension, resulted in the deposition of abundant trace metal elements. The anaerobic conditions arising from this mixing, along with volcanic activity, led to the enrichment of polymetallic elements and the formation of shale with high TOC values. As a large-scale marine transgression followed, creating anaerobic conditions in the bottom water, the enrichment of organic matter played a crucial role in the adsorption of metal elements from seawater, serving as the primary factor in the enrichment of trace metal elements. Subsequently, during a marine regression, the shale in the upper section of the Niutitang Formation formed with relatively lower TOC and trace metal element content.

4.2. The Relationship between Negative Excursion of δ13Corg Values and Enriched Polymetallic Elements in Lower Interval

Similar to previous studies [21], this study indicates no direct correlation between δ13Corg and TOC (Figure 8A). However, strong correlations are observed betweenδ13Corg and Mo/TOC, Ni/TOC, V/TOC, As/TOC (Figure 8B–E) as well as Sb content (Figure 8F), suggesting a close relationship between δ13Corg and syngenetic polymetallic elements. In normal euxinic sediments, the Ni/TOC values are generally below 50 [7,35,36]. The extremely high Ni/TOC ratios in the samples of this study (ranging from 1114 to 8840) indicate the presence of enrichment mechanisms beyond normal biological accumulation. This mechanism could involve the release of nickel from hydrothermal sources into sulfide-rich bottom waters, leading to the direct formation of nickel sulfide precipitation [7]. Nickel is an important trace nutrient in water bodies and is transferred to sediments through microbial processes [7]. The correlation between Ni/TOC and δ13Corg (R2 = 0.88) suggests a direct causal relationship between nickel element enrichment and the δ13Corg.
The previous studies suggest a connection between the enrichment of As and Sb elements and hydrothermal fluids in the lower section of the Niutitang Formation in Guizhou and many other areas [24,37]. Similarly, in the profile of the Niutitang Formation, there is a strong correlation between the increasing δ13Corg values from the bottom and the decreasing ratios of As/TOC (R2 = 0.89) and Sb content (R2 = 0.92). This correlation indicates that the negative δ13Corg values are attributed to the influence of deep-seated hydrothermal fluids.
The δ13Corg value of the lower section of the Niutitang Formation in the study area is −32.6‰, which is significantly lighter than the δ13Corg value (−34.4‰) of the same stratigraphic level in the Songlin area of Guizhou [38]. This difference may be attributed to the proximity to the hydrothermal vent. The Songlin profile, located near the hydrothermal vent, exhibits a strong influence on hydrothermal activity, as evidenced by the presence of hydrothermal vent communities [39]. In contrast, the southeastern Chongqing profile shows a relatively weaker influence of hydrothermal mixing. Furthermore, the impact of hydrothermal activity gradually decreases from the bottom to the top of the southeastern Chongqing profile, ultimately becoming relatively stable in the middle section.
The occurrence of hydrothermal events and the release of methane and CO2 gases have had a significant impact on Earth’s climate and biogeochemical cycles. These events have led to changes in nutrient availability, with elements like phosphorus, nickel, iron, and vanadium being provided [40]. Metallic elements such as molybdenum, zinc, vanadium, and uranium are also released. Additionally, these events create a highly reducing environment in the bottom waters.
These hydrothermal activities have caused a notable negative shift in the δ13CPDB values of carbonate cement, with values as low as −48‰ being recorded. This shift in carbon isotopes has contributed to global warming [41,42]. The biogeochemical processes, particularly bacteria-mediated carbon cycling, play a crucial role in transforming inorganic carbon from hydrothermal sources into biogenic organic carbon. As a result, shale organic matter found in hydrothermally influenced shales exhibits significantly lighter δ13Corg values.
The extensive outflow of deep-reducing hydrothermal fluids during the Ediacaran to Cambrian transition zone, which is rich in silicon and depleted in 13C, can cause a negative anomaly in the carbon isotopes of organic matter. This process leads to the preservation of large quantities of organic matter in anoxic environments, giving rise to the formation of organic-rich shales.
Previous studies have revealed that there is a widespread occurrence of δ13Corg value shifts in the lower part of the Lower Cambrian shale in the Yangtze Platform [21,43,44]. Furthermore, this negative shift in carbon isotopes observed in the Lower Cambrian of the Yangtze Platform can be globally correlated [45], as evidenced by similar observations in the Birmania Basin of northwestern India. In the Birmania Basin, the development of phosphorite is accompanied by a negative shift in organic carbon isotopes, followed by a subsequent positive shift near the Pc/C boundary, resulting in relatively heavier δ13Corg values [46]. The findings of this study align with the observed evolutionary trend of δ13Corg values.
From a tectonic perspective, during the late Cambrian’s second stage, the Yangtze Block and the South China Block underwent rift activity as a result of the Rodinia supercontinent’s breakup. This extensional tectonic activity in the Lower Cambrian allowed deep-seated hydrothermal fluids to enter the ocean basin through extensional fractures and subsequently ascend to the continental shelf [40]. Previous research has also indicated that the late Neoproterozoic to early Cambrian period was characterized by the breakup of Rodinia, leading to significant polar wander events across major continents on Earth [47,48], which led to intense extensional tectonic activity within the continents. As a result, the mixing of hydrothermal fluids exhibits a global consistency.

4.3. Relationship between Polymetallic Elements and Si_excess

The most vigorous polymetallic enrichment occurs at the bottom of the Niutitang shale, coinciding with the lowest levels of Si_excess (Figure 9A), indicating that the polymetallic-enrich fluid originating from the depths is silicon-poor. Approximately 5 m above the base, there is a noticeable increase in Si_excess content as the carbon isotopes in the organic matter become heavier, and the two exhibit a closely positive relationship (R2 = 0.71) (Figure 5A) in the lower intervals. This trend suggests that moving upwards from the base, the mixing with polymetallic-rich fluids progressively weakens while the contribution of biogenic Si_excess gradually increases. Accompanying this process is a reduction in the concentrations of hydrothermal-derived elements such as Ni, P, S, Se, and Re (Figure 9B–F).
Previous studies have suggested that the Si_excess content in Early Cambrian South China ranges from 20% to 30%, reaching up to 50%, with the Si_excess primarily derived from hydrothermal fluids [9]. Contrary to the Lower Cambrian strata in South China as a whole, the Si_excess content in this section is notably lower, and there is no clear correlation between TOC and excess silica. This suggests that the enrichment of organic matter is not directly linked to the Si_excess content. In the western Hubei region, there is a significant positive correlation between Si_excess and TOC in the Lower Cambrian shale, indicating that the Si_excess is predominantly of biogenic origin [14].

4.4. Sources of Polymetals and the Migration Mechanisms Controlled by Upwelling

During the transition from the Neoproterozoic to the Cambrian, oceanic hypoxia led to the production of large amounts of organic matter and hydrogen sulfide (H2S) in the ocean. When high-salinity brines reached the seafloor, rapid changes in Eh-pH conditions caused the accumulation of metals at the sediment-seawater interface, a process further enhanced during diagenesis [49,50]. Some studies suggest that multiple sources, including seafloor hydrothermal and biological contributions, may have contributed to the formation of polymetallic nickel-molybdenum-precious metal (PGE) mineralization in the black shales of the Lower Cambrian Niutou Formation in southern China [51,52]. Other research indicates that the deposition of vanadium-rich black shales in the Yangtze platform slope basin results from upwelling in deep-water basins, which first passes through mildly reducing, organic-rich North Sea Anoxic Seawater (NSASW), where vanadium (V) is reduced to vanadium (IV) and preferentially removed from seawater through the formation of organic metal complexes [53].
During the early Cambrian, hydrothermal and upwelling processes in the Upper Yangtze region affected the abundance of organic matter and the migration of trace metal elements [33,51,54,55]. In contrast, the weak upwelling in the western Sichuan region is associated with not only low organic matter abundance but also relatively low trace metal element abundance [56]. Upwelling under transgressive conditions is closely related to organic productivity. Furthermore, hydrothermal processes result in silica-rich fluids that concentrate trace metal elements. The negative correlation between the abundance of trace metal elements and excess silica in the lower section of the Niutitang Formation in the study area suggests that trace metal elements do not originate directly from hydrothermal sources but are more likely derived from upwelling migration.
The CoEF*MnEF parameter is considered an effective indicator for assessing upwelling; values greater than 2 suggest a restricted marine sedimentary environment, while values below 0.5 indicate a clear influence of upwelling.
XEF(X) = (X/Al)sample/(X/Al)ucc, where XEF is the enrichment factor of element X, (X/Al)sample is the ratio of element X to Al in the sample, and (X/Al)ucc is the ratio of element X to Al in the average upper continental crust (UCC) [57].
Previous studies on the same profile in the research area also indicate that the lower section of the Niutitang Formation is significantly influenced by upwelling [58]. In this study, the CoEF*MnEF values for the lower section of the Niutitang Formation range from 0.4 to 1.3 (Figure 10), indicating that it was affected by upwelling during sedimentation and was situated in a restricted basin environment. The influence of upwelling is most pronounced in the upper interval of Niutitang shale.

5. Conclusions

1.
Based on the Ti/Al ratio, accompanied by δ13Corg values and TOC characteristics of the shale, the Danquan Niutitang shale section in the Youyang area of Southeast Chongqing in the Upper Yangtze region can be divided into three intervals: a lower interval with highest Ti/Al ratio and lightest δ13Corg values over approximately 8 m from the base; a middle interval rich in organic matter with lowest Ti/Al ratio and relatively stable δ13Corg values from 8 to 27 m; and an upper interval above 27 m with moderate Ti/Al ratio and low TOC and TS values;
2.
The negative offset of the δ13Corg values in the lower interval of the Niutitang shale in the study area, along with the strong negative correlation with Ni/TOC, Mo/TOC, V/TOC, U/TOC, as well as P and S, indicates that the mixing of upwelling fluids with anoxic sedimentary environment is the controlling factor for the negative drift of δ13Corg values and the enrichment of trace metal elements in the lower interval of the Niutitang Formation;
3.
From the bottom to the top of the lower interval of Si_excessNiutitang Formation, as the influence of hydrothermal mixing weakens, the Si_excess content gradually increases, suggesting that Si_excess is primarily biogenic in origin rather than sourced from the hydrothermal fluids which are not silicon-rich;
4.
Previous studies and the present research both indicate that the mixing of upwelling and local marine sediments is the primary factor for the enrichment of polymetals in the lower section of the Niutitang Formation shale in the study area.

Author Contributions

Conceptualization, D.L. and P.P.; methodology, C.Z.; software, G.W.; validation, G.W., D.L. and Z.L.; formal analysis, C.Z.; investigation, C.Z.; resources, P.P.; data curation, D.L.; writing—original draft preparation, D.L.; writing—review and editing, G.W. and D.L.; visualization, G.W.; supervision, L.Q.; project administration, L.Q.; funding acquisition, Z.L. 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-Enterprise Joint Fund (Grant No. U2167210), Special project of CAS (THEMSIE04010104) and Self-Directed Research of the State Key Laboratory of Organic Geochemistry (SKLOG2024-03). This is contribution No. IS 3560 from GIGCAS.

Data Availability Statement

An Excel document was uploaded in the attachment, containing all the data used in the article.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 2. Correlation between Ti/Al and U/Th of Niutitang shale samples in the study area.
Figure 2. Correlation between Ti/Al and U/Th of Niutitang shale samples in the study area.
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Figure 3. Mineral, δ13Corg,TOC and elemental content variations with depth in the Niutitang Formation profile of Youyang. And the minerals are dolomite (A); pyrite (B); indices related to organic matter are δ13Corg (C) and TOC (D); indices related to elements are total sulfur (E); aluminum (F) and excess silicon (G).
Figure 3. Mineral, δ13Corg,TOC and elemental content variations with depth in the Niutitang Formation profile of Youyang. And the minerals are dolomite (A); pyrite (B); indices related to organic matter are δ13Corg (C) and TOC (D); indices related to elements are total sulfur (E); aluminum (F) and excess silicon (G).
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Figure 4. Trace elemental content variations with depth in the Niutitang Formation profile of Youyang. The trace elements are arsenic (A), molybdenum (B), nickel (C), rhenium (D), antimony (E), selenium (F) and uranium (G).
Figure 4. Trace elemental content variations with depth in the Niutitang Formation profile of Youyang. The trace elements are arsenic (A), molybdenum (B), nickel (C), rhenium (D), antimony (E), selenium (F) and uranium (G).
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Figure 5. Correlation between TOC and U/Th ratio (A), sulfure content (B), and Fe content (C).
Figure 5. Correlation between TOC and U/Th ratio (A), sulfure content (B), and Fe content (C).
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Figure 6. Correlation between δ13Corg and sulfur (A) and Fe (B) concentrations.
Figure 6. Correlation between δ13Corg and sulfur (A) and Fe (B) concentrations.
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Figure 7. Correlation between Si_excess and Ti concentration (A), Al concentration (B), and Ti/Al ratio (C).
Figure 7. Correlation between Si_excess and Ti concentration (A), Al concentration (B), and Ti/Al ratio (C).
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Figure 8. Correlation between δ13Corg value with TOC and metallic parameters. The correlation between the δ13Corg value and TOC is shown in (A), along with the polymetallic/TOC ratios: Mo/TOC (B), Ni/TOC (C), V/TOC (D), and As/TOC (E). (F) The correlation between the δ13Corg value and antimony concentration.
Figure 8. Correlation between δ13Corg value with TOC and metallic parameters. The correlation between the δ13Corg value and TOC is shown in (A), along with the polymetallic/TOC ratios: Mo/TOC (B), Ni/TOC (C), V/TOC (D), and As/TOC (E). (F) The correlation between the δ13Corg value and antimony concentration.
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Figure 9. Correlation between Si_excess with δ13Corg value and concentration of elements. The correlation between the Si_excess and δ13Corg value is shown in (A), along with the polymetallic concentrations: nickel (B), phosphorus (C), sulfur (D), selenium (E) and rhynium (F).
Figure 9. Correlation between Si_excess with δ13Corg value and concentration of elements. The correlation between the Si_excess and δ13Corg value is shown in (A), along with the polymetallic concentrations: nickel (B), phosphorus (C), sulfur (D), selenium (E) and rhynium (F).
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Figure 10. Correlation between aluminum content with CoEF*MnEF of Niutitang shale.
Figure 10. Correlation between aluminum content with CoEF*MnEF of Niutitang shale.
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Wang, G.; Zhang, C.; Liu, D.; Qiu, L.; Li, Z.; Peng, P. Enrichment Mechanism of Polymetallic Elements at the Base of the Niutitang Formation in Southeast Chongqing. Minerals 2024, 14, 978. https://doi.org/10.3390/min14100978

AMA Style

Wang G, Zhang C, Liu D, Qiu L, Li Z, Peng P. Enrichment Mechanism of Polymetallic Elements at the Base of the Niutitang Formation in Southeast Chongqing. Minerals. 2024; 14(10):978. https://doi.org/10.3390/min14100978

Chicago/Turabian Style

Wang, Guozhi, Can Zhang, Dayong Liu, Linfei Qiu, Ziying Li, and Ping’an Peng. 2024. "Enrichment Mechanism of Polymetallic Elements at the Base of the Niutitang Formation in Southeast Chongqing" Minerals 14, no. 10: 978. https://doi.org/10.3390/min14100978

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