Formation Conditions of Early Cambrian Witherite (BaCO₃) Deposit in Chongqing: Implications for Differential Oceanic Changes

Abstract

The discovery of extensive layered witherite (BaCO₃) deposits in the Early Cambrian strata in South China offers valuable insights into the unique paleo-marine environment of this region. Based on stratigraphy, petrography, geochemistry, fluid inclusions, and pervious published multi-isotope geochemical analysis, we aim to explore the distinct genesis mechanism of the witherite deposits in the Chengkou area of South China and unveil the specific paleo-marine environment during their formation. This study concludes that the wide ⁸⁷Sr/⁸⁶Sr ratios (0.708266 to 0.708504) of witherite and barytocalcite (BaCa(CO₃)₂) support the seawater-derived barium. Negative δ¹³C values (−15.6 to −12.5‰) indicate the involvement of organic matter during the formation of witherite. The complex vapor-phase (including CH₄, H₂S, etc.) and HS⁻-containing liquid-phase compositions in the primary liquid–vapor inclusions of the witherite and barytocalcite imply that the two minerals are formed in sulfur-rich euxinic seawater. The broad homogenization temperatures are generated by thermal re-equilibration of the inclusions, rather than the actual temperatures of the trapped fluids. The salinity range of fluid inclusions in the Bashan witherite deposit (0.2 to 16.2 wt.%) records mixing between moderate-salinity basinal-derived fluids and low-salinity seawater-sourced fluids. We propose that the formation of Chengkou witherite deposits is linked to a sulfate-limited euxinic seawater environment, highlighting the spatiotemporal heterogeneity in Early Cambrian paleo-oceanic sulfate concentrations.

1. Introduction

The Cambrian period marks a critical moment in Earth’s history, characterized by a surge in biodiversity linked closely to marine environments [1,2,3]. Geological and biological shifts during the Ediacaran–Cambrian transition set the stage for the ‘Cambrian explosion’ [4,5]. The witherite deposit found exclusively in South China provides crucial geological evidence for understanding Cambrian marine environments. These deposits originated from the Early Cambrian and display temporal correlation and spatial specificity with the marine environment during that period [6,7]. Moreover, Barium (Ba), as a vital element of life, can accurately record variations in the chemical environment of the ocean [8,9]. An investigation of the distinctive formation mechanism of its carbonate rock assembly is essential for unraveling the intricate oceanic geochemical field [10,11].

Ba occurs naturally in the forms of barite (BaSO₄) and witherite (BaCO₃) minerals. Barite, as a common sulfate phase, forms in diverse geological environments such as marine basins [12], hydrothermal systems [13], and cold seep settings [14], serving as a key proxy for reconstructing paleo-oceanic conditions [15,16]. In contrast, witherite deposits represent rare carbonate-hosted barium, with economically significant occurrences restricted to discrete tectonic domains, including the Qinling-Dabashan metallogenic belt (China [7]), Pennines mineralization zones (UK [17]), and California’s Franciscan Complex (US [18]). Owing to the scarcity of witherite, present studies have concentrated on evaluating witherite deposits [7], with limited studies on the metallogenic mechanism of witherite. Recent studies on witherite deposit in the Dabashan region have started to address this gap [6,19,20,21,22], but there is still limited understanding of the ore-forming fluids and formation environments of witherite. The formation mechanism of witherite remains debated, with proposed origins including the metasomatic genesis [23], biogenic genesis [24,25], submarine hydrothermal fluids [26], and cold seeps [7]. Furthermore, the marine environment during the formation of witherite deposits and the aggregation and distribution patterns of barium in ancient seawater are still not well understood.

To address the research gaps identified, this study focuses on the globally significant Early Cambrian witherite deposit in Chengkou, Chongqing. This deposit provides valuable insights into marine environmental shifts during the Cambrian period. Our research investigates the characteristics of the ore-forming fluids and the depositional environments of witherite minerals by analyzing fluid inclusions in high-grade ores from the study area, alongside a comprehensive review of relevant isotope studies. Through this approach, we aim to clarify the distinct genesis mechanism of these deposits and offer a case study that explores the variability of marine systems during the Early Cambrian.

2. Regional and Deposit Geology

The Dabashan region, located on the northeastern margin of the Sichuan Craton Basin, is divided into the South Dabashan and North Dabashan tectonic units by the Chengkou-Zhongbao fault [25]. The North Dabashan belongs to the Yangtze platform and experienced stages of extension and tectonic inversion, resulting in the formation of plow-type normal faults and thrust fault tectonics. In the Early Paleozoic, sedimentation was governed by thrust faults, leading to substantial variations in stratigraphic thickness. Following tectonic inversion, the North Dabashan area was characterized by ductile slip and brittle thrust faults. Several large-scale barium mineralization belts have formed this area, and multiple deposits are distributed therein (such as the Ziyang-Huangboshuwan deposit, Chengkou-Bashan deposit, Wanyuan-Miaozi deposit, etc.). Liu et al. (2010) further subdivided the North Dabashan large-scale barium mineralization belt into barite and witherite mineralization sub-zones based on systematic variations in mineral assemblages [25]. The witherite mineralization belt is situated north of the Chengba fault, extended from Shaanxi province along the border between Sichuan and Hubei province, covering approximately 320 km (Figure 1b). These deposits occur in the cherts of the Lower Cambrian unit in a layered or apparently layered form [24].

The Chengkou-Bashan witherite deposit in Chengkou County, Chongqing is located in the Huangxi-Chengkou anticlinorium fold belt within the North Dabashan thrust zone. The area is characterized by a series of tight folds, and the fault structure is well-developed [28]. The magmatic rocks in this region, primarily composed of diabase, are predominantly of Caledonian age [7]. There are Cu, Pt, Pd, Zn, Pb, Sb, Mo, and Ni and other metal mineralization abnormalities in the mining area. Exposed strata include the Baziping Formation (Zb), Bashan Formation (Є₁b), Liujiaping Formation (Є₁l), Jianzhuba Formation (Є₁j), Maobaguan Formation (Є₂m), Baguamiao Formation (Є₂b), and other marine platform carbonate formation and continental shelf siliceous rock-clastic formation sequences. The Baziping Formation (Zb), upper unit of the Sinian specific to Chinese stratigraphy, consists of dolomite with minor lenticular siderite at the bottom, limestone at the intermediate part, and black chert at the top [26]. The Chengkou-Bashan deposit is hosted in the black, thin-layered carbonaceous chert of the Bashan Formation. It consists of chert, siliceous shale, and carbonaceous shale from bottom to top, and orebodies are bedded/stratiform or stratiform-like and lenses strictly controlled by the stratum (Figure 1c). The predominant ore mineralogy in the Chengkou-Bashan deposit is witherite, accompanied by barytocalcite and barite. Gangue minerals mainly include pyrite, quartz, calcite, and organic matter. The deposit is characterized by obvious sedimentation or sedimentation-transformation, which is mainly transformed by later tectonic movements, causing the ore body to be cut and shifted or deformed and reversed (Figure 2a,b).

According to the characteristics of rock assemblage, the Bashan Formation is divided into two lithological sections, which are described separately as follows: the first section of the Bashan Formation (Є₁b¹) has a thickness of 84.12 m. The lower part consists of gray-black thin-layered carbonaceous chert interbedded with gray fine-grained limestone lenticular bodies, which are intermittently distributed along the strata with spacing of 20–30 cm. The middle part is characterized by black thin-layered to massive chert, exhibiting cryptocrystalline texture with massive, striped, or banded structures (Figure 2c,d). The upper part contains thin to thick-layered massive chert interbedded with thin-layered slate, with a total thickness of 24.94 m.

The second section (Є₁b²) ranges from 31.85 to 46.46 m in thickness and hosts the witherite (BaCO₃) orebody. The primary lithology includes black thin-layered chert, carbonaceous shale, and carbon-siliceous shale, intercalated with witherite ore. The stratifications are irregular, showing fold structures and ‘S’-shaped twists (Figure 2e,f). The lower part is composed of black thin-layered siliceous shale and carbonaceous siliceous shale, with individual layers ranging from 1 to 8 cm in thickness. The middle and upper parts host the witherite orebody. The witherite orebody varies in thickness from 0.38 to 7.79 m. Towards the eastern side of the exploration area, the witherite mineralization gradually transitions into dolomite.

3. Methods

3.1. Samples

A total of 12 representative samples, including witherite, calcite, and ore-bearing chert, were collected from the underground mine tunnel of the Chengkou witherite deposit in Chongqing, China (See

Table 1. The source and lithology of the samples.

Sample No.	Lithology	Formation
LH-001B1	Laminated chert	Baziping Fm.
LH-002B1	Chert, C-rich	Baziping Fm.
LH-003B1	Barytocalcite	Bashan Fm.
LH-003B2	Barytocalcite	Bashan Fm.
LH-004B1	Barytocalcite	Bashan Fm.
LH-004B3	Shale	Bashan Fm.
LH-004B4	Witherite	Bashan Fm.
LH-004B5	Barytocalcite	Bashan Fm.
LH-006B2	Witherite	Bashan Fm.
LH-007B1	Barytocalcite	Bashan Fm.
LH-007B2	Barytocalcite	Bashan Fm.
LH-008B1	Dolostone	Lujiaping Fm.

“Fm.” stands for “Formation”.

). Since the organic matter content significantly influences the observation of fluid inclusions under the microscope, the samples with lower organic matter were selected for polishing thin sections.

3.2. Fluid Inclusions

The microthermometric studies and laser Raman spectra analysis of fluid inclusions were conducted at the Guangxi Key Laboratory of Exploration for Hidden Metal Mineral, Guilin University of Technology, China. Cooling and heating experiments for the low- and medium-temperature inclusions (−196 to +600 °C) were performed using a Linkam THMS 600 freezing–heating unit. The measurement accuracy varies in different temperature ranges: the estimated accuracy of temperatures is ±0.2 °C for temperatures below 31 °C and ±1 °C for temperatures greater than or equal to 31 °C. During microthermometrics, primary inclusions were chosen for testing. The heating/cooling rates are limited to 10 °C/min and were reduced to 1 to 0.1 °C/min near phase transformations.

Qualitative determinations of the compositions of individual fluid inclusions were conducted using a Renishaw RM-2000 laser Raman probe. The experimental conditions were room temperature 23 °C, and the laser light source was an argon ion laser with a wavelength of 514 nm. The counting time of the measured spectra was 30 s, counting every 1 cm⁻¹ (wave number). The spectral range was from 1000 to 4000 cm⁻¹, with the peaks taken at one time in the whole band. The laser beam spot size was about 1 μm, and the spectral resolution was 2 cm⁻¹.

4. Results

4.1. Petrologic and Petrographic Characteristics

Texturally, the witherite structures include massive, laminated, lenticular, and network-like structures (Figure 3a–d), while its textures include cryptocrystalline-fine-grained, medium coarse-grained, and bundle-radial textures (Figure 3e–h).

A microscopic study indicates that the ore textures include (1) cryptocrystalline to fine-grained texture, composed of tightly packed minerals ranging from 0.01 to 0.03 mm in size, commonly observed in dense massive ore samples (Figure 3e); (2) fine- to coarse-grained inequigranular texture, where mineral grain sizes vary between 0.1 and 3 mm, distributed randomly without a clear directional structure (Figure 3h); and (3) bundled to radiating texture, where mineral crystals form clusters or radiating aggregates, with crystal sizes mostly ranging from 0.2 to 0.5 mm (Figure 3f).

The ore structures include (1) dense massive structures, formed by irregularly distributed minerals of varying sizes without a clear directional arrangement (Figure 3c); (2) laminated structures, composed of oriented microcrystalline or fine-grained minerals, displaying primary sedimentary structure, with the hand specimens showing distinct 1–2 mm laminations (Figure 3b); (3) banded to striped structures, characterized by the regular accumulation of minerals with different colors, sizes, and compositions or by the directional arrangement of carbonaceous and muddy components, with bands typically wider than 2 mm, while stripes 2–3 mm wide, and this structure associated with primary sedimentation or metasomatism (Figure 3a); and (4) network-like structures, formed by intersecting veinlets caused by post-depositional tectonic activity or magmatic intrusion, with vein widths ranging from 1 to 5 mm, often filled with calcite or quartz (Figure 3d).

Barytocalcite is gray or dark gray and belongs to the orthorhombic system. It foams vigorously when treated with 5% hydrochloric acid. Under the microscope, grains are mostly granular or tabular-columns, with crystal sizes ranging from 0.02 to 2 mm, mostly 0.5 to 1 mm. It has a pseudo-dolomite cleavage and often coexists with witherite (Figure 3i).

Barite, like the other two minerals, belongs to the orthorhombic system and has a dark gray color. Under the microscope, grains are granular, short columnar, or tabular, with a particle size ranging from 0.01 to 0.1 mm. The crystal interiors often contain carbonaceous substances. They occur in fascicular, radial, and chrysanthemum-like forms, or exhibit microlaminated and banded structures (Figure 3j). Theses minerals are generally found in carbonate-type barite ores, with higher grades in the upper part than in the lower part. They do not form a separate barite ore body.

4.2. Petrography of Fluid Inclusions

Fluid inclusions (FIs) are well preserved in witherite and barytocalcite (Table 1). According to the vapor–liquid ratio (at room temperature) and classification of FIs [29], the FIs in witherite and barytocalcite are divided into primary, pseudo-secondary, and secondary. We further classify the primary inclusions of witherite and barytocalcite into three types: L-type, V-type, and L-V-type primary FIs (Figure 4).

L-type FIs are limpid and only-liquid inclusions, which are widespread and most abundant in barytocalcite. These FIs are in the shape of elongated (Figure 4i), oval (Figure 4a–c), and negative crystal shapes (Figure 4d), with 3~8 μm in diameter. The spatial distribution is characterized by clusters, directional (Figure 4h) and sporadic (Figure 4d) in the witherite and barytocalcite.

V-type FIs are limpid and only-vapor phase with a dark border and bright light reflection in the center (Figure 4a,e). These characteristics may indicate that these bubbles might be CH₄ or CO₂. They are of small quantity in the two minerals. These FIs are commonly oval or sub-oval, with 2–5 μm in diameter. They occur as isolated along the direction of witherite and barytocalcite mineral cleavage (Figure 4a,e).

L-V-type FIs are limpid and liquid and vapor two-phase, aqueous inclusions (at room temperature). A vapor bubble with a dark border can be observed in each of these FIs, which homogenizes to liquid phase upon heating. This type of bubbles in these FIs may indicate the presence of CO₂, so there are probably H₂O-CO₂ FIs, while the darker-colored border bubbles might be H₂O-CH₄-N₂ FIs or other gas types of FIs. The proportion of the vapor phase is usually in the range 5 to 20 vol%. These FIs are commonly oval (Figure 4a,c,e), subcircular (Figure 4b), or triangular (Figure 4g), with 1 to 4 μm in diameter. They occur as isolated and scattered along the direction of the two minerals’ cleavage.

4.3. Microthermometric Results

Microthermometric studies were conducted on primary aqueous inclusions (L + V) from all the minerals in the Chengkou witherite deposit. Microthermometric data are summarized in

Table 2. Microthermometric results (the detailed data can be found in Supplementary Table S1).

Sample Number	Mineral	N	Long Axis Size (um)	Tm, ice (°C)	Th (°C)	Salinity (wt.% NaCl eq.)	Pressure (MPa)	Density (g/cm3)
				Range (Mean)	Range (Mean)	Range (Mean)	Range (Mean)	Range (Mean)
004b5	barytocalcite	27	1.4~7.5	−0.1~−5.0 (−1.7)	62.3~167.7 (109.5)	0.2~7.9 (2.67)	3.89~13.24 (7.84)	0.932~1.027 (0.973)
007b1	barytocalcite	6	1.1~3.1	−0.9~−9.9 (−5.5)	89.4~126.3 (111.9)	1.6~13.8 (5.99)	6.73~12.53 (9.61)	0.952~1.049 (0.994)
007b2	barytocalcite	12	1.2~5.3	−0.2~−4.6 (−1.7)	64.1~140.7 (95.6)	0.4~7.3 (2.85)	4.24~9.07 (6.91)	0.954~1.026 (0.984)
003b1	barytocalcite	10	1.0~2.9	−0.2~−5.0 (−2.3)	102.3~185.7 (142.3)	0.4~7.9 (3.78)	7.51~16.89 (11.07)	0.918~0.992 (0.953)
003b2	barytocalcite	27	1.3~8.5	−1.3~−12.3 (−5.7)	66.1~136.7 (85)	3.1~16.2 (8.48)	5.12~14.87 (8.17)	0.957~1.093 (1.027)
004b4	witherite	14	1.0~2.9	−0.1~−21.0 (−8.4)	68.3~138.3 (88.7)	0.2~23.0 (9.51)	5.08~11.78 (8.18)	0.955~1.141 (1.036)
006b2	witherite	15	1.0~5.0	−0.1~−10.6 (−3.2)	54.2~162.7 (100.8)	0.2~14.6 (6)	5.13~17.35 (8.64)	0.967~1.055 (1.001)
Total		111	1.0~7.5	−0.1~−21.0 (−4.1)	54.2~185.7 (101.3)	0.2~23.0 (5.69)	3.89~17.35 (8.63)	0.918~1.141 (0.995)

N = number of inclusions analyzed, T_{m ice} = last melting ice temperature, T_h = homogenization temperature.

.

Primary aqueous inclusions in witherite have ice-melting temperatures from −0.1 to −21.0 °C (n = 29), with salinities ranging from 0.2 to 23.0 wt.% NaCl equivalent, with a concentration concentrated within the range of 1 to 12 wt.% NaCl equivalent (Figure 5). The homogeneous temperatures varied from 54.2 to 162.7 °C, focused on 60 to 120 °C (Figure 5), all homogeneous to the liquid phase.

Primary aqueous inclusions in barytocalcite were homogenized to the liquid phase at temperatures of 62.3 to 185.7 °C (n = 81), focused on 60 to 140 °C, with salinity ranging from 0.2 to 16.2 wt.% NaCl equivalent (Figure 5).

4.4. Laser Raman Spectroscopy

The results of laser Raman analysis of the vapor- and liquid-phase components of representative FIs in witherite and barytocalcite show that only the peak of the mineral and broad H₂O peak appears in the vapor-phase laser Raman spectrum of most inclusions (Figure 6). This indicates that the NaCl+H₂O FIs are dominant in the witherite and barytocalcite. In the vapor-phase laser Raman spectra of a few inclusions with large vapor-liquid ratios, in addition to the peak of H₂O and the main mineral, there are also CH₄ (2914 cm⁻¹), H₂S (2592 cm⁻¹), HS⁻ (2572 cm⁻¹), SO₂ (1151 cm⁻¹), CO₂ (1283 cm⁻¹), and N₂ (2328 cm⁻¹). The vapor component in the pure vapor-phase inclusions of the two minerals is CH₄ based on the analysis of laser Raman. The above phenomenon may be due to the relatively tiny vapor–liquid ratios of most inclusions and violent pulsation, which makes it difficult for the laser to focus on the vapor-phase components.

5. Discussion

5.1. Nature of Ore-Forming Fluids

Aqueous inclusions present in the minerals serve as valuable archives of geological history, providing insights into the composition of the ore-forming fluids [30]. The wide range of homogenization temperatures (54.2 to 185.7 °C) measured from FIs in witherite and barytocalcite in the research area (Figure 5; Table 2) may primarily reflect temperature variations of the entrapped fluids. However, several other factors could contribute to this variability, including (1) irregular operations during sampling, sample preparation, and experimentation; (2) re-equilibrium due to stretching of the fluid inclusions with low-salinity or only-liquid during freezing; (3) thermal re-equilibration caused by stretching and necking following inclusions formation; and (4) thermal equilibrium re-established, resulting from external influences (e.g., stratigraphic uplift or burial) after inclusions formation [31,32]. Therefore, we have made every effort to conduct meticulous sampling and precise temperature measurement and have tried our best to effectively avoid wide variations in homogenization temperatures caused by irregular operations. Moreover, the majority of inclusions hosted in minerals are predominantly brine inclusions (Figure 5), which preclude stretching during freezing. The majority of inclusions selected for microthermometric analysis exhibit negative crystal-shaped or spherical morphology, with small diameters, and no necking was observed, ruling out stretching or necking as factors causing homogenization temperature variations. Following entrapment, inclusions experience new temperature and pressure conditions that may stretch, leak, or refill, resulting in significant temperature distribution and convergence trends on the temperature–salinity diagram (Figure 7) [33]. The histogram of inclusions’ homogenization temperatures and the temperature–salinity plot exhibit a notable convergence trend in the temperature–salinity relationship (Figure 7). Therefore, the homogenization temperatures may not represent the actual trapping temperatures of fluids but are more likely to reflect the effects of subsequent thermal re-equilibration. The actual trapping temperature of fluids may have been close to or even significantly lower than the lowest measured homogenization temperature [31]. Furthermore, the absence of hydrothermal alteration in the host rocks also indicates a relatively low temperature for the fluids from which the witherite was precipitated.

The widespread occurrence of aqueous inclusions in both types of minerals are similar to those found in low-temperature sedimentary deposits [34,35]. The salinity of inclusions ranges (0.2 to 23.0 wt.% NaCl equivalent) from less than to greater than seawater values, which typically results from phase separation (boiling), but may also be potentially induced by the fluids mixing [36,37]. During mineralization, fluid mixing and boiling are key ore-forming mechanisms [30]. Mixing occurs when fluids of different compositions or properties combine, disrupting the chemical equilibrium and causing mineral precipitation. In contrast, boiling results from rapid pressure drop, which drastically reduces the solubility of ore-forming components via volatile exsolution, concomitantly triggering mineral deposition and generating vapor-dominated two-phase fluid inclusions [31]. As previously discussed, the actual trapping temperature of the fluids is likely close to or significantly lower than the minimum homogenization temperature (54.2 °C), while boiling is commonly associated with hydrothermal deposits [30]. Thus, boiling is unlikely to be the primary ore-forming mechanism for this deposit. In contrast, fluid mixing is more likely responsible for the broad salinity range measured. Given the low-temperature nature of the deposit and the similarity of FI types to those found in low-temperature sedimentary deposits, it is inferred that the mineralizing fluids underwent mixing of different fluid sources (such as a high-temperature brine with ambient seawater). This process resulted in a wide salinity range (0.2 to 16.2 wt.%), disrupted the chemical equilibrium, and facilitated the precipitation of witherite.

Witherite was precipitated from fluids with low to moderate salinity (0.2 to 16.2 wt.%). As mentioned earlier, the salinity range of FIs in the Bashan witherite deposit (0.2 to 16.2 wt.%) suggests the presence of at least two distinct fluid sources. Based on the histogram of inclusion salinity distribution (Figure 5), the salinity range indicates the existence of two end-member fluids during the early mineralization stage: one with moderate salinity (~18 wt.%) and another with low salinity (<5 wt.%). The measured salinities of the moderately saline fluids agree well with that of the basinal brines [38]. The salinity of the low-salinity (<5 wt.%) fluids agrees with that of seawater (3.2 wt.% [39]). Therefore, basinal fluids and seawater could be the components of mineralizing solutions in the studied deposit. Additionally, Hu (2021) investigated the metallogenic geological background of the Chengkou witherite deposit and concluded that the Early Cambrian strata in this region represented a marine sedimentary environment, where the ore-forming fluids were closely related to seawater and basinal fluids [40]. Laser Raman analysis results (Figure 6) indicate that the vapor-phase composition of FIs in the host mineral (witherite and barytocalcite) is predominantly H₂O, with reducing gases such as H₂S and CH₄, while the liquid phase contains HS⁻. The presence of gases such as H₂S and CH₄ may suggest that the aggregation and deposition of witherite minerals occurred in a reducing environment.

5.2. Sources of Ore-Forming Materials

Sr isotopes are often used as a proxy for Ba origin due to their similar chemical properties [41]. In the Chengkou area, the ⁸⁷Sr/⁸⁶Sr ratios of witherite minerals range from 0.708266 to 0.708504 (average 0.708339; Supplementary Table S1), while they fall within the range of contemporaneous Cambrian seawater (0.708 to 0.709) [42], global studies of marine carbonates from various regions have shown that the Sr isotope compositions are mostly lower than those of contemporary Cambrian seawater (0.709 [43]). Hydrothermal fluids typically have low ⁸⁷Sr/⁸⁶Sr ratios [44], whereas continental clastic material has high radiogenic ⁸⁷Sr contents [15]. The ⁸⁷Sr/⁸⁶Sr ratios of witherite deposits in South China are generally lower than those of coeval seawater (Figure 8b), indicating that the barium in the Qinling-Dabashan metallogenic belt shares a similar source, which was likely mixing of hydrothermally derived Sr with seawater. In addition, the ⁸⁷Sr/⁸⁶Sr ratios of some witherite deposits in South China deviate from those of coeval seawater, suggesting that the barium sources in the metallogenic belts may have been locally influenced by detrital inputs from terrestrial sources or overprinted by hydrothermal fluids. It is worth noting that the precipitation of Ba as carbonate would require low seawater sulphate concentrations (otherwise, it will precipitate as barite), high rates of bacterial sulfate reduction, and sulfidic conditions. This is supported by the extremely heavy δ³⁴S values of Early Cambrian seawater [45,46].

Carbon and oxygen isotope research on carbonate minerals provides insights into the carbon source during mineral formation and contributes to understanding the carbon cycle [47]. However, oxygen isotopes in carbonate minerals are highly susceptible to diagenesis [48,49,50]. This could lead to a substantial decrease in δ¹⁸O values, impacting the reliability of isotopic information [51]. As depicted in Figure 9, there is no significant correlation between δ¹³C values and δ¹⁸O values (Figure 10A–H), except for the Miaozi and Chiyan areas (where apparent covariation may result from data scarcity in those areas), suggesting minimal diagenetic influence and the credibility of δ¹⁸O values [52].

The C-O isotopes of witherite in the Dabashan region exhibit a marked negative excursion (δ¹³C_VPDB = −25 to −10‰, δ¹⁸O_VPDB = −20 to −10‰; Figure 9b) compared to Early Cambrian dolomites from other regions of the world. Previous studies have demonstrated that C isotopes are closely linked to sea level changes: as sea levels rise, enhanced solar radiation boosts biological productivity, resulting in the preferential absorption of ¹²C by plants [62]. Ultimately, carbonate rocks precipitated from seawater exhibit relative enrichment of ¹³C. Additionally, δ¹⁸O values influenced by hydrothermal fluids typically fall below −10‰ [63]. The variations in C-O isotopes of witherite from the Dabashan region may indicate regional paleo-oceanic changes during the Cambrian period, such as shifts in photosynthesis rates, productivity, or organic matter oxidation, or they may result from localized influences like hydrothermal fluids.

The δ¹³C values of witherite and barytocalcite in the Chengkou area range from −15.6 to −12.5‰ (average −14.0‰; Figure 8a), indicating that the carbon may be derived from the oxidation or dehydroxylation of organic matter [64]. In the C-O isotope illustration (Figure 9a), the data from the Chengkou area are more concentrated in the vicinity of dehydroxylation, similar to the witherite deposits in the Bashan, Chiyan, and Huangbaishuwan areas, suggesting that dehydroxylation of organic matter contributes significantly to the ${\mathrm{CO}}_3^{2-}$ content. During this process, bacteria or microorganisms produce ¹²C-rich CO₂, CH₄, and other gases through degradation of organic matter. These gases are oxidized and dissolved in seawater to form ${\mathrm{CO}}_3^{2-}$ ions during diffusion [24]. In contrast to the Sichuan Basin (19.68~27.52), the δ¹⁸O values in the Chengkou area (15.7~18) exhibit a significantly negative excursion (Figure 9a). During the period of sea level decline, the δ¹⁸O values increased, tectonic movements caused relative uplift in the region, and the sea level decreased subsequently. Therefore, the relatively positive δ¹⁸O values in the Sichuan area may be attributed to intense evaporation resulting from the relative uplift of the area due to tectonic movements. In contrast, the highly negative δ¹⁸O values in the Chengkou region, different from those in the Sichuan Basin, may be related to meteoric diagenesis or hydrothermal effects. We infer that it is the latter because the oxygen isotope values are significantly lighter (by ~15‰) compared to coeval sedimentary carbonates, which is consistent with the research result of the δ¹⁸O of the precipitated witherite at temperatures significantly higher than those of sedimentary carbonates (~25 °C) [65].

5.3. Implications for the Marine Environment

The marine environment played a crucial role in the large-scale precipitation of witherite [6,7]. This environment includes pH, redox conditions, and elemental cycling processes. The rapid precipitation of witherite deposits in South China during the Early Cambrian reflects the significant control of specific paleo-marine environments on witherite mineralization.

The crystallization and precipitation of witherite are primarily governed by the concentrations of Ba²⁺ and ${\mathrm{CO}}_3^{2-}$ in seawater, which are closely linked to the oceanic redox state [6,11]. Although oxygen levels in the atmosphere and ocean had significantly increased by the Early Cambrian [4], locally restricted basins with oxic surface waters and anoxic, sulfidic deep waters developed along the continental slope of the Yangtze Platform [66]. These restricted basins provided relatively stable, low-energy sedimentary environments conducive to the formation of laminated and banded witherite. Additionally, the anoxic and sulfidic waters dissolved substantial amounts of Ba²⁺, providing the necessary material basis for the large-scale precipitation of witherite [7].

In restricted basins, intense microbial reduction significantly lowered the concentration of ${\mathrm{SO}}_4^{2-}$ in seawater. Under these conditions, with limited ${\mathrm{SO}}_4^{2-}$ concentrations in an anoxic environment, Ba²⁺ remained in a dissolved state in seawater [12], potentially leading to the formation of a reservoir of Ba²⁺ in the Early Cambrian marine [12,67]. During the Early Cambrian, the restricted basins along the Yangtze Platform margin may have been intermittently connected to the open ocean [11]. In this context, the degradation of organic matter produced ${\mathrm{CO}}_3^{2-}$-rich fluids, which were introduced into the restricted basins and mixed with dissolved Ba²⁺, providing the necessary conditions for the formation of witherite.

5.4. The Mechanism of Ore-Forming

The marine environment surrounding the Yangtze Block during the Early Cambrian was marked by oxygen deficiency and sulfur enrichment, with ancient seawater showing a redox-stratified structure [68,69]. This environment played a crucial role in sedimentary and geochemical processes and influenced the formation of witherite deposit. Under the influence of tectonic activity and submarine hydrothermal processes, the sulfate concentration in seawater of the Qinba region significantly decreased, while hydrothermal fluids introduced H₂S, leading to anoxic and sulfidic conditions in the seawater (Figure 11A). Additionally, contemporaneous faults controlled the formation of sub-basins within the Yangtze Block [70]. High-grade witherite ore bodies are linearly distributed in the Chengkou-Zhenba area (Figure 1), indicating venting of hydrothermal fluid along fault conduits [71]. This further suggests the Early Cambrian extensional sub-basins in Chengkou, creating a euxinic seafloor environment. In such conditions (seawater rich in H₂S and HS⁻), Ba was partially enriched as ions (Ba²⁺) (Figure 11B). The extreme conditions of oxygen deficiency and sulfur enrichment led to widespread massive biological mortality on the seafloor. Simultaneously, bacteria or microorganisms produced CO₂, CH₄, and other gases through degradation of organic matter. Then, this methane is reduced to ${\mathrm{CO}}_3^{2-}$ in an environment where ${\mathrm{SO}}_4^{2-}$ is present. This process ultimately led to the precipitation of witherite. The carbon isotopic data from the Chengkou witherite deposit provide additional support for this process. According to the δ¹³C and δ¹⁸O relationship diagram (Figure 9a), the carbon in witherite originates from biogenic sources and is converted from reduced organic carbon (CH₄) to oxidized carbon (${\mathrm{CO}}_3^{2-}$). The fractionation process can be described as follows:

Organic matter → CH₄ + CO₂

(1)

$${\mathrm{CH}}_4+{\mathrm{SO}}_4^{2-} \rightleftharpoons {\mathrm{CO}}_3^{2-}+{\mathrm{H}}_2\mathrm{S}+{\mathrm{H}}_2\mathrm{O}$$

(2)

$${\mathrm{Ba}}^{2+}+{\mathrm{CO}}_3^{2-}\to {\mathrm{BaCO}}_3$$

(3)

The continuous precipitation of BaCO₃ (witherite) requires these reactions to proceed unidirectionally, leading to carbon enrichment essential for witherite formation. This process is accompanied by significant carbon isotope fractionation, producing a certain cumulative fractionation effect [72,73]. Given that biogenic methane typically exhibits light δ¹³C values (−110‰ to −50‰) [74], the resultant carbonates (BaCO₃) are expected to have δ¹³C values ranging from −33‰ to −7‰. The measured δ¹³C values of witherite in the Chengkou deposit (−15.6‰ to −12.5‰, average −14.0‰) fall within this predicted range, confirming that the carbon source originated from microbial degradation of organic matter during early diagenesis.

Compared with modern seawater, the higher bacterial or microbial reduction rate and sulfidation conditions during the Cambrian period, as well as the lower concentration of sulfate in the seawater, created a more favorable marine environment for the deposition of barite (as confirmed by the laser Raman analysis of inclusions in Section 4.4, which also suggests the association of witherite with environments containing H₂S and HS⁻) [75]. In summary, the formation of witherite deposits in the Chengkou region reflects key transitional periods in Earth’s history. The heterogeneous distribution of H₂S, HS⁻, or sulfate content in seawater played a crucial role in their formation. The formation and evolution of ancient continents significantly influenced the geological and marine environments in the Sichuan Basin during the Early Cambrian, resulting in a heterogeneous ocean. This oceanic heterogeneity, especially in sulfur-rich and oxygen-deficient marine environments, facilitated the enrichment and deposition of witherite.

6. Conclusions

• Strontium isotope data indicate that the Chengkou witherite deposit shares a common seawater origin with other deposits in the Qinling-Dabashan region. The δ¹³C values of witherite fall between marine carbonate and organic matter, suggesting a specific contribution of organic matter to the formation of witherite. The wide range of homogenization temperatures (54.2 to 162.7 °C) does not reflect the original trapping temperatures of the ore-forming fluids but rather results from post-entrapment thermal re-equilibration of the inclusions. Fluids contain H₂S, CH₄, HS⁻, etc., indicating the formation of witherite in a sulfur-rich and oxygen-deficient stratified water environment, revealing the complexity of the marine environment in the study area during the Early Cambrian.
• The large-scale precipitation of witherite deposits in South China during the Early Cambrian was controlled by unique paleo-marine sedimentary environments. Although atmospheric and oceanic oxygen concentration had risen substantially during this period, the restricted marginal basins of the Yangtze Platform evolved a distinct stratified water column characterized by oxic surface waters overlying euxinic (anoxic and sulfidic) deep waters. This persistent physicochemical stratification created a stable, low-energy sedimentary environment, which is conducive to witherite formation.
• During the Early Cambrian, Chengkou was situated within a restricted marginal basin, where the low sulfate content in the ocean favored the enrichment of Ba+. The extensive proliferation and subsequent death of Cambrian organisms led to the accumulation and degradation of a large amount of organic matter in the Chengkou region, creating a limited marine environment rich in ${\mathrm{CO}}_3^{2-}$. This unique restricted marine environment ultimately became a crucial control factor for forming the largest witherite deposit in the world.