Pyrite Concretions in the Lower Cambrian Niutitang Formation, South China: Response to Hydrothermal Activity

0. INTRODUCTION

The Lower Cambrian, recording pivotal changes in ocean chemistry and biological evolution during the Ediacaran-Cambrian transition (Zhao et al., 2023; Zhang Y N et al., 2022; Fan et al., 2016; Santosh et al., 2014; Zhang X L et al., 2014; Tucker, 1992), were widely distributed on the Yangtze Block, extending from the carbonate platform in the northwest to slope-basin in the southeast. The ocean redox condition during the Early Cambrian has been debated for decades (Liu Y R et al., 2022; Wu Y W et al., 2022; Zhang et al., 2022; Liu Y et al., 2020; Wu C J et al., 2020; Huang et al., 2019; Sawaki et al., 2018; Jin et al., 2016; Xiang et al., 2016; Feng et al., 2014; Och et al., 2013). Consensus has been reached that a stratified ocean existed, with oxic surface water, sulfidic mid-depth water, and ferruginous deep water (Wu et al., 2022; Li Z H et al., 2021; Li C et al., 2020). Also, hydrothermal activity could transport abundant trace elements as well as reducing gases. The former is beneficial for biomass prosperity and could facilitate primary productivity, while the latter promoted water anoxia (Zhang Y et al., 2022; Xie et al., 2021; Zhu et al., 2021; Wu Y W et al., 2020; Gao et al., 2016; Lin et al., 2010). The Niutitang Formation black shale is enriched in metal deposits, such as Ni-Mo sulfide ore deposit, barite deposit, and phosphorite deposit (Shi et al., 2021; Gao et al., 2018; Qiao et al., 2016; Xu et al., 2013; Huang et al., 2010; Jiang et al., 2007; Lehmann et al., 2007; Steiner et al., 2001). However, the pyrite concretions within the bottom of black shale lack detailed investigation.

Pyrite can precipitate from the water column or within sediment porewater during early diagenesis, or it can be produced by hydrothermal activity during late diagenesis (Lang et al., 2018). Thus, two types of pyrite, including syngeneic pyrite and diagenetic pyrite, are generated. Syngeneic pyrite mostly presents framboidal or euhedral with smaller diameters, and the size distribution of framboid pyrite is a useful proxy for indicating redox condition (Rickard, 2019; Gallego-Torres et al., 2015; Berner et al., 2013; Wignall et al., 2005). In contrast, diagenetic pyrite appears as framboids with larger diameters, aggregates, or concretions parallel to the bedding (Lang et al., 2018). Generally, pyrite forms through a reaction between H₂S and active Fe (Ⅱ) that presents either as dissolved Fe²⁺ or as Fe (Ⅱ) in sediments, to form iron monosulfide (FeS) and ultimately pyrite (FeS₂), below the oxic-anoxic redox interface (Bottrell and Newton, 2006; Berner, 1985, 1970). About 75% to 90% of H₂S, produced by sulfate-reducing bacteria (SRB) through dissimilatory sulfate reduction (DSR), is re-oxidized by oxygen in overlying pore water or with Fe (Ⅲ) or Mn (Ⅳ) from within the sediment (Bottrell and Newton, 2006). Thus, only a small part of H₂S participates in the pyrite formation. This reaction of H₂S and Fe (Ⅱ), tested by many laboratory experiments (Peiffer et al., 2015; Wilkin and Barnes, 1996), is fueled by organic matter or hydrogen serving as the electron donor (Lang et al., 2018; Fike et al., 2015). The pyrite formation by bacterial sulfate reduction can exist in closed-bottom and open systems (Schwarcz and Burnie, 1973). The former describes the sulfate reduction rate faster or equal to the sulfate supply, where the diffusive sulfate supply is limited, and sulfate reduction is analogous to the Rayleigh distillation process. The latter is the opposite, found in the bioturbation zone in modern normal marine sediments or an anoxic water column of euxinic basins (Kakegawa et al., 1998).

The pyrite sulfur isotopic composition represents the precursor H₂S due to the negligible sulfur isotopic fractionation during pyrite precipitation (Goldberg et al., 2006). Sulfate-reducing bacteria preferentially utilize ³²S in sulfate (Sim et al., 2011; Kemp and Thode, 1968), leading to isotopically lighter pyrite relative to original seawater sulfate. Under the influence of Rayleigh distillation in a sulfate-limited situation (closed-bottom system), progressive ³⁴S enrichment in pyrite is expected (Goldberg et al., 2006). However, Jørgensen et al. (2004) pointed out that an open system was always maintained to a certain extent in sediment pore water, in which case SO₄^2- and H₂S diffusion impacted the sulfur isotopic composition. Previous studies indicate that sulfur isotope composition of sedimentary pyrite is controlled by organic matter content, seawater sulfate concentration, reactive Fe content, and sedimentation rate (Lang et al., 2020; Canfield, 1989).

Numerous studies of pyrite concretions at centimeters-scale of Nantuo Formation (Lang et al., 2018) and Doushantuo Formation (Xiao et al., 2010). Similar to carbonate concretions, pervasive and concentric growth mechanisms raised by (Raiswell and Fisher, 1999) are applicable for pyrite concretions as well (Gregory et al., 2019). The existence of the pyrite concretions may serve as a record for probing the atmospheric, oceanic, and biological features during geological time (Lang et al., 2018). However, the formation mechanisms of pyrite concretions of the Lower Cambrian Niutitang strata remain poorly understood (Goldberg et al., 2006). Here, we have investigate pyrite concretions of the bottom Niutitang Formation in Hunan Province, South China. By combining trace and major elements, scanning electron microscopy, and laser ablation plasma mass spectrometry, the formation process of pyrite concretions and its implication for the Early Cambrian ocean have been illuminated.

1. GEOLOGICAL SETTING

Yangtze and Cathaysia blocks were sutured together during the mid-Neoproterozoic, and went through failed rifting during the late Neoproterozoic, thus producing the Nanhua Basin between them (Wang and Li, 2003). Upper Ediacaran to lower Cambrian (~551–529 Ma) strata of the Yangtze Block exhibit three main sedimentary facies from northwest to southeast along the paleo-water depth gradient, including shallow shelf facies, transitional facies, and slope to deep basin facies, respectively (Guo et al., 2007). The Ediacaran-Cambrian (E-C) successions were widely distributed on the Yangtze Platform. Dengying Formation in late Ediacaran and Yanjiahe/Zhujiaqing/Kuanchuanpu Formation in the earliest Cambrian are typical of shallow-water carbonate deposition. Southeastward, Liuchapo/Laobao/Piyuancun formations are characterized by chert and shale (Zhao et al., 2018). Liuchapo Formation is regarded as a diachronous unit straddling E-C transition by geochronology studies (Chen et al., 2015; Wang X Q et al., 2012). Later the worldwide transgression in South China (Goldberg et al., 2007) led to the extensive organic-rich shale deposition, namely, Guojiaba/Shuijingtuo/Niutitang/Xiaoyanxi/Hetang/Shiyantou formations (Yang et al., 2023; Liu Z X et al., 2022). Within an arc-parallel linear belt, the submarine-hydrothermal Ni-Mo sulfide ore layer extends more than 1 600 km (Steiner et al., 2001). It serves as a lithological marker as the boundary between the Cambrian stage 2 and stage 3 (Zhu et al., 2003) or the Late Tommotian biostratigraphically (Steiner et al., 2001). The Re-Os isochronal age of sulfide ore layer of 521 ± 5 Ma is reported in Guizhou and Hunan Province (Xu et al., 2011). The black shales of the Lower Cambrian Niutitang Formation in Guizhou Province yield a Re-Os isochronal age of 535 ± 11 Ma (Jiang et al., 2007). Besides, the U-Pb age of 542.6 ± 3.7 and 522.3 ± 3.7 Ma in the Middle–Upper Liuchapo Formation and base of overlying Niutitang Formation are investigated for basin settings (Chen et al., 2015). Thus, Niutitang Formation deposited about 20 Ma after the E-C boundary and corresponded to stage 2–stage 3, also evidenced by abundant sponge spicules found in the Guchengcun Section (Niu et al., 2018).

The Daobeixi (DBX) Section (N28°23′41.61″, E110°27′50.33″) is located in Yuanling County, Changsha City, Hunan Province, China. Palaeographically, this section was in the deep-water basin (Fig. 1a). The Lower Cambrian Niutitang Formation, overlying the Liuchapo Formation cherts, is dominated by black siliceous mudstone within pyrite concretions at the bottom (Fig. 1b).

Figure  1.  (a) Simplified geographic map of Yangtze Block during Ediacaran-Cambrian transition, modified after (Gao et al., 2016); (b) lithology column of studied section. ED. Ediacaran; LCP Fm. Liuchapo Formation.

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2. SAMPLES AND METHODS

Sixteen unweathered siliceous organic-rich mudstones and three pyrite concretions were collected from the Niutitang Formation. About 20 g of dried powder samples (200 mesh size) of 16 mudstones and one pyrite concretion (C-1) samples were prepared to perform total organic carbon (TOC) contents, trace, and major element concentrations experiments. In the meantime, three pyrite concretions (C-1, C-2, C-3) within the black mudstone in the bottom Niutitang Formation were chosen to perform in-situ sulfur isotope compositions analysis and scanning electron microscopy (SEM) observation.

2.1 Total Organic Carbon Analysis

TOC contents were tested by CHNOS Elemental Analyzer (Vario EL Ⅲ) carbon and sulfur analyzer in the Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences (Wuhan). First, powder samples (200 mesh size) were treated with a 5% hydrochloric acid (HCI) solution to remove carbonate. Then they were washed with distilled water and centrifuged until all the HCI was removed. Last, the samples were dried and analyzed. The analytical precision was better than 10%, based on replicate analyses of the Chinese national standard GB/T 19145-2003.

2.2 Major and Trace Elements Analysis

Major element concentrations were measured in Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). About 5 g powder were analyzed by X-ray fluorescence (XRF, AXIOS max) for major oxide compositions. Pretreatment details are described in (Wang et al., 2019). Trace element analysis of whole-rock was conducted on Agilent 7700e ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. First, 50 mg sample powder was accurately weighed and placed in a Teflon bomb and mixed with 1 mL HNO₃ and 1 mL HF slowly. Next, the Teflon bomb was put in a stainless-steel pressure jacket and heated to 190 ℃ in an oven for more than 24 h. After cooling, the Teflon bomb was opened and placed on a hotplate at 140 ℃ and evaporated to incipient dryness, and then 1 mL HNO₃ was added and evaporated to dryness again. After that, 1 mL of HNO₃, 1 mL of MQ water, and 1 mL internal standard solution were added. Then the Teflon bomb was resealed and placed in the oven at 190 ℃ for more than 12 h. Lastly, the final solution was transferred to a polyethylene bottle and diluted to 100 g by adding 2% HNO₃.

2.3 Scanning Electron Microscopy Observation

Microscopic observation of pyrite morphology and mineralogy compositions were carried out on polished and carbon-coated thin sections by high-resolution field-emission SEM (HITACHI SU-8010 and Quanta 450 FEG) equipped with energy dispersive spectroscopy (EDS) in State Key Laboratory of Biogeology and Environmental Geology and State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). The resolution is 2.5 nm at 30 kV under backscattered electron images in high and low vacuum environments.

2.4 In-situ Sulfur Isotope Analysis

In-situ sulfur isotopic composition analysis of three pyrite concretions was performed with Resolution S-155, Nu Plasma Ⅱ laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) in State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). During the experiment, a laser spot size of 33 μm with a laser repetition rate of 40 Hz was adopted, and the ablation process was set to last for 40 s. An in-house pyrite standard named WS-1 calibrated the mass bias for sulfur isotopes (Yan et al., 2020). The δ³⁴S (expressed in standard delta notation in units of ‰ is calculated real time as follows.

V-CDT refers to Vienna-defined Canyon Diablo Troilite (V-CDT) standard.

2.5 Calculation Methods

The Eu anomalies are calculated as

where the calculated ratios are weight ratios (Lawrence et al., 2006), and the PAAS refers to post-Archean Australian shale (Taylor and McLennan, 1985).

The enrichment factor (EF) of trace elements is calculated as

where the X refers to the trace element of interest and Al content is used for normalization (Calvert and Pedersen, 1993).

The non-detrital Ba content is calculated as (Schoepfer et al., 2015; Schroeder et al., 1997)

The chemical index of alteration (CIA) is calculated by molecular proportions as

where CaO* refers to the amount of CaO incorporated in the silicate fraction of the rock (Nesbitt and Young, 1982), which can be corrected by: CaO* = CaO - P₂O₅ × 10/3 (in molecular proportions). If the CaO* is higher than Na₂O, then the CaO* is assumed as equivalent to Na₂O (McLennan, 1993).

3. RESULTS

3.1 Major and Trace Elements of the Niutitang Formation Mudstones

Major and trace element compositions of the Niutitang Formation siliceous mudstone are presented in Table 1 and Table 2. The pyrite concretion is dominated by Fe₂O₃ (56.14 wt.%), followed by SiO₂ (8.43 wt.%) and Al₂O₃ (1.81 wt.%). It displays a relatively high TOC content of 4.50 wt.% as well as enrichment of trace elements (Mo, Cu, Ni, Ba, Sb, Pb, Tl). Besides, a positive Eu anomaly (Eu/Eu* = 2.50) is obvious. For mudstones, major elements are mainly composed of SiO₂ with an average of 73.53 wt.%, then followed by Al₂O₃ (average of 6 wt.%) and K₂O (average of 1.46 wt.%), with minor amounts less than 1 wt.% of Fe₂O₃, MgO, CaO, Na₂O, MnO, TiO_2, and P₂O₅. The TOC contents range from 1.68 wt.% to 15.08 wt.%, with an average of 8.09 wt.%. Trace metal elements such as Mo and U are substantially enriched, and Ba is detectably enriched (Algeo and Tribovillard, 2009) (Table 2). The Eu anomalies fluctuate between 0.88 and 1.59, with an average of 1.19.

Table  1.  Major elements concentrations and associated parameters of Niutitang Formation

Samples	Depth (m)	SiO2 (wt.%)	Al2O3 (wt.%)	Fe2O3 (wt.%)	MgO (wt.%)	CaO (wt.%)	Na2O (wt.%)	K2O (wt.%)	MnO (wt.%)	TiO2 (wt.%)	P2O5 (wt.%)	LOI (wt.%)	Ti/Al	CIA
Concretion 1	0	8.43	1.81	56.14	0.37	0.08	0.06	0.38	0.00	0.06	0.04	32.29	0.04	77
D-1	1	72.10	7.58	1.68	0.94	0.07	0.06	2.00	0.00	0.35	0.07	14.40	0.05	77
D-2	2	78.29	5.10	0.54	0.52	0.06	0.08	1.41	0.01	0.27	0.05	13.50	0.06	76
D-3	2.5	78.58	4.36	0.23	0.37	0.07	0.07	1.11	0.01	0.26	0.02	14.77	0.07	76
D-4	3.4	67.88	10.61	0.46	1.25	0.05	0.08	2.65	0.00	0.51	0.03	16.32	0.05	78
D-5	5	77.04	4.88	1.44	0.51	0.05	0.06	1.13	0.01	0.23	0.08	14.50	0.05	80
D-6	6.5	73.69	3.94	1.06	0.38	0.05	0.07	1.10	0.01	0.30	0.02	19.29	0.09	74
D-7	8.2	78.67	3.32	0.72	0.35	0.06	0.08	0.77	0.01	0.17	0.03	15.74	0.06	77
D-8	9.2	79.53	3.21	0.63	0.27	0.05	0.07	0.82	0.01	0.18	0.06	15.04	0.06	77
D-9	10.7	74.93	4.06	0.17	0.37	0.05	0.08	0.99	0.01	0.26	0.05	18.92	0.07	78
D-10	11.3	78.43	10.41	0.44	1.22	0.07	0.07	2.45	0.01	0.77	0.03	5.69	0.08	79
D-11	18.9	82.87	4.93	0.25	0.54	0.07	0.09	1.15	0.00	0.31	0.13	9.49	0.07	80
D-12	25	84.93	4.51	0.30	0.56	0.06	0.09	1.09	0.00	0.23	0.02	8.10	0.06	76
D-13	38.2	83.77	6.85	0.30	0.82	0.06	0.08	1.59	0.00	0.36	0.09	5.89	0.06	80
D-14	48.5	74.29	7.59	0.35	0.85	0.09	0.07	1.81	0.00	0.43	0.05	14.33	0.06	78
D-15	65	80.73	7.02	0.40	0.72	0.08	0.11	1.57	0.01	0.30	0.05	8.92	0.05	79
D-16	81.1	75.92	11.80	0.73	1.11	0.09	0.18	2.73	0.01	0.55	0.06	6.39	0.05	78

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Table  2.  TOC contents, trace elements concentrations and associated parameters of the pyrite concretion and Niutitang Formation mudstones

Samples	Depth (m)	TOC (wt.%)	U (ppm)	Mo (ppm)	Cu (ppm)	Ni (ppm)	Baexcess (ppm)	Sb (ppm)	Pb (ppm)	Tl (ppm)	EF (U)	EF (Mo)	EF (Ba)	Eu/Eu*
C-1	0	4.50	14.1	55.5	325.9	226.0	1 992.2	208.7	200.6	4.7	47.4	578.2	32.1	2.50
D-1	1	7.78	31.5	35.4	18.6	47.7	1 291.5	13.7	12.7	2.7	25.3	88.1	5.1	1.41
D-2	2	9.48	18.2	32.0	17.5	40.5	1 536.9	/	28.2	1.6	21.7	118.5	8.9	1.17
D-3	2.5	11.52	16.2	29.3	13.9	29.0	1 338.8	/	28.1	0.8	22.7	127.0	9.1	1.59
D-4	3.4	10.77	37.9	31.0	18.4	35.0	1 151.9	/	24.6	2.4	21.7	55.2	3.3	1.20
D-5	5	10.82	17.6	45.6	54.8	34.6	502.5	/	8.0	0.9	22.0	176.4	3.1	0.97
D-6	6.5	13.13	18.4	25.5	10.6	55.8	679.6	/	23.4	0.7	28.5	122.3	5.2	1.03
D-7	8.2	10.97	13.7	31.0	29.7	59.7	770.7	/	8.5	1.5	25.2	176.2	6.9	1.23
D-8	9.2	10.69	22.1	51.4	16.5	80.7	1 086.1	/	18.8	0.8	42.0	302.5	10.0	1.06
D-9	10.7	15.08	28.6	34.2	15.2	38.5	985.5	/	28.0	0.6	43.0	159.2	7.2	0.98
D-10	11.3	1.68	6.4	5.2	91.8	8.8	3 819.0	/	19.0	1.2	3.7	9.4	10.8	1.46
D-11	18.9	5.53	20.0	20.3	40.5	29.6	1 583.6	/	22.4	0.9	24.7	77.8	9.5	1.17
D-12	25	4.44	12.7	30.5	13.3	27.5	756.4	/	4.7	1.2	17.2	127.9	5.0	1.12
D-13	38.2	2.37	16.1	3.8	167.2	21.0	1 334.2	/	28.4	1.5	14.3	10.5	5.8	1.43
D-14	48.5	8.75	18.9	27.7	17.5	41.1	1 019.5	/	19.2	1.5	15.2	68.9	4.1	1.03
D-15	65	4.27	11.2	20.3	47.7	22.3	713.4	/	27.3	1.1	9.7	54.6	3.1	0.88
D-16	81.1	2.16	11.7	7.9	32.0	8.1	3 644.6	/	47.2	1.3	6.0	12.7	9.1	1.25

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3.2 Petrological Characteristics of the Niutitang Formation Mudstones

The Niutitang Formation is well exposed (Fig. 2a) with mainly thin-bedded black siliceous mudstone (Fig. 2b). Pyrite concretions distributed in the organic-rich siliceous mudstone are well preserved, which show either as a whole or surround the mudstone (Fig. 2c) or strip-like parallel to the bedding with both ends of host mudstone bent (Fig. 2d). Also, the radial pyritized sponge spicules emerged on the bedding were observed (Fig. 2e). Under SEM observations, the Niutitang Formation mudstone is mainly composed of quartz, which is associated with organic matter (OM), either as siliceous OM (Fig. 3a) or dispersive OM (Fig. 3b). Sheet-like illite is the most common clay mineral, bending around the rutile and barite (Fig. 3c) or showing alone (Fig. 3d). Besides, pyrites are common either as small size euhedral or framboidal (Figs. 3e, 3f).

Figure  2.  Outcrop photos of the Niutitang Formation in the Daobeixi Section, Hunan Province, yellow arrows refer to pyrite concretions. (a) The overall picture of the lower part of the Niutitang Formation; (b) black thin-bedded siliceous mudstone; (c) pyrite concretions associated with black shale; (d) strip-like pyrite concretion with clear boundaries; (e) pyritized sponge spicules in a pyrite concretion; (f) pyrite concretion shows little variations in δ³⁴S_py.

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Figure  3.  SEM pictures of the Niutitang Formation mudstone. (a) Siliceous OM with a honeycomb structure between quartz (Q) particles; (b) quartz particles are the main matrix and barite (Brt) is grown within the OM; (c) barite intergrown with rutile (Rt), with clay minerals bent nearby; (d) sheet-like illite (Ill) merged in the quartz matrix; (e) dispersive euhedral pyrite crystals in the background of OM and surrounding quartz; (f) framboid pyrites (f-py) distributed in the quartz matrix. (c) is the backscattered electron (BSE) photo, while others are secondary electron (SE) photos.

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3.3 Mineral Compositions of the Pyrite Concretions

Under the reflected light, the boundary of host mudstone and pyrite concretion is distinct (Fig. 4a). From the pyrite concretion boundary to the center, the pyrite content decreases (Fig. 4b). But the interior of concretion is characterized by compactly arranged pyrites with voids filled with silica cement (Fig. 4c). The detailed mineral compositions of the pyrite concretions are clarified by SEM observation. The pyrite morphology can be divided into pyrite framboids and larger euhedral pyrites or pyrite aggregates. Some framboidal pyrites develop in the center of euhedral pyrite (Fig. 4d) or are associated with organic matter and quartz (Fig. 4e). It is often the case that euhedral pyrites accumulate as aggregates with many voids (Fig. 4f). Besides, euhedral pyrites are often in associations with hyalophane, barite, and tetrahedrite. Hyalophane tends to develop between pyrites (Fig. 4g), in the void of pyrite aggregate (Fig. 4h), or along the pyrite edge (Fig. 4i). Barite develops around the euhedral pyrite (Fig. 4g) or occurs independently (Fig. 4h).

Figure  4.  Ordinary microscope and SEM pictures of the pyrite concretions. (a) Reflected light. The boundary of pyrite concretion (pc) and mudstone; (b) reflected light. The center of the pyrite concretion (pc); (c) reflected light. The interior of the pyrite concretion (pc); (d) SE photo. Euhedral pyrites (Py) developed around framboid pyrites (f-py); (e) SE photo. The center of the pyrite concretion. Framboid pyrites linked with OM are in the voids of quartz (Q) and euhedral pyrites; (pc); (f) SE photo. Large pyrite aggregates; (g) BSE photo. Association of euhedral pyrite, hyalophane, and barite; (h) BSE photo. Association of pyrite aggregates, hyalophane (Hy), barite(Brt), and quartz (Q); (i) BSE photo. Association of pyrite aggregates, hyalophane (Hy), and tetrahedrite (Thr).

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3.4 Sulfur Isotope Compositions (δ³⁴S_py) of the Pyrite Concretions

In-situ δ³⁴S_py data of pyrite on three concretions and spatial distribution of Concretion 1 are referred to Table 3 and Fig. 2f. Overall, δ³⁴S_py values are all positive, which approach the value of modern seawater (δ³⁴S_sulfate ≈ 20.99‰) (Rees et al., 1978) but are lower than that of ancient Cambrian seawater (δ³⁴S_sulfate ≈ 33‰) (Seal, 2006). Second, insignificant differences are found in one concretion or three concretions. For C-1, δ³⁴S_py values range from 15.6‰ to 19.4‰ (avg. 18.2‰), and the spatial distribution shows no zonation. For C-2, δ³⁴S_py values vary from 14.2‰ to 17.8‰ (avg. 16.0‰). For C-3, δ³⁴S_py values fluctuate between 13.2‰ and 19.4‰ (avg. 17.8‰). Third, the fractionation between the δ³⁴S_py and the coeval seawater δ³⁴S_sulfate is small, about 14‰–20‰.

Table  3.  In-situ sulfur isotope compositions (‰-VCDT) of pyrites of three pyrite concretions (C-1, C-2, C-3).

Samples	34S/32S	Error	34S (‰)	32S (V)	Conditions
C-1-1	0.049 662	0.000 007	18.6	9.6	3.5 J/cm2, 10 Hz, 33 μm
C-1-2	0.049 704	0.000 007	19.4	9.9	3.5 J/cm2, 10 Hz, 33 μm
C-1-3	0.049 654	0.000 008	18.4	11.0	3.5 J/cm2, 10 Hz, 33 μm
C-1-4	0.049 679	0.000 064	19.0	11.5	3.5 J/cm2, 10 Hz, 33 μm
C-1-5	0.049 674	0.000 004	18.9	11.0	3.5 J/cm2, 10 Hz, 33 μm
C-1-6	0.049 663	0.000 004	18.7	11.1	3.5 J/cm2, 10 Hz, 33 μm
C-1-7	0.049 657	0.000 006	18.6	11.1	3.5 J/cm2, 10 Hz, 33 μm
C-1-8	0.049 673	0.000 004	18.9	11.5	3.5 J/cm2, 10 Hz, 33 μm
C-1-9	0.049 599	0.000 006	17.4	10.7	3.5 J/cm2, 10 Hz, 33 μm
C-1-10	0.049 640	0.000 004	18.2	11.6	3.5 J/cm2, 10 Hz, 33 μm
C-1-11	0.049 667	0.000 005	18.8	11.1	3.5 J/cm2, 10 Hz, 33 μm
C-1-12	0.049 621	0.000 004	17.8	11.6	3.5 J/cm2, 10 Hz, 33 μm
C-1-13	0.049 546	0.000 003	16.3	11.4	3.5 J/cm2, 10 Hz, 33 μm
C-1-14	0.049 650	0.000 004	17.9	11.4	3.5 J/cm2, 10 Hz, 33 μm
C-1-15	0.049 705	0.000 004	19.0	9.1	3.5 J/cm2, 10 Hz, 33 μm
C-1-16	0.049 677	0.000 034	18.4	8.9	3.5 J/cm2, 10 Hz, 33 μm
C-1-17	0.049 541	0.000 004	15.6	11.6	3.5 J/cm2, 10 Hz, 33 μm
C-1-18	0.049 648	0.000 003	18.1	11.9	3.5 J/cm2, 10 Hz, 33 μm
C-2-1	0.049 942	0.000 003	17.8	12.3	3.5 J/cm2, 10 Hz, 33 μm
C-2-2	0.049 872	0.000 005	16.4	12.7	3.5 J/cm2, 10 Hz, 33 μm
C-2-3	0.049 944	0.000 003	17.8	12.9	3.5 J/cm2, 10 Hz, 33 μm
C-2-4	0.049 845	0.000 005	15.4	11.4	3.5 J/cm2, 10 Hz, 33 μm
C-2-5	0.049 788	0.000 005	14.2	10.6	3.5 J/cm2, 10 Hz, 33 μm
C-2-6	0.049 896	0.000 003	16.4	11.4	3.5 J/cm2, 10 Hz, 33 μm
C-2-7	0.049 872	0.000 003	15.9	11.3	3.5 J/cm2, 10 Hz, 33 μm
C-2-8	0.049 807	0.000 005	14.6	11.4	3.5 J/cm2, 10 Hz, 33 μm
C-2-9	0.049 830	0.000 004	15.1	10.4	3.5 J/cm2, 10 Hz, 33 μm
C-3-1	0.049 996	0.000 003	19.4	10.7	3.5 J/cm2, 10 Hz, 33 μm
C-3-2	0.049 972	0.000 003	18.9	10.5	3.5 J/cm2, 10 Hz, 33 μm
C-3-3	0.049 911	0.000 006	17.6	7.2	3.5 J/cm2, 10 Hz, 33 μm
C-3-4	0.049 943	0.000 004	18.3	11.5	3.5 J/cm2, 10 Hz, 33 μm
C-3-5	0.049 930	0.000 003	18.0	10.6	3.5 J/cm2, 10 Hz, 33 μm
C-3-6	0.049 716	0.000 005	13.2	12.2	3.5 J/cm2, 10 Hz, 33 μm
C-3-7	0.050 011	0.000 003	19.2	10.9	3.5 J/cm2, 10 Hz, 33 μm

 | Show Table

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4. DISCUSSIONS

4.1 Depositional Condition of Niutitang Formation

Redox condition in the paleo-water column plays a vital role in the timing and depth of the concretion growth, even the formation process (Wei et al., 2020). Previous studies have provided petrographic and geochemical evidence for reconstructing the paleoenvironment conditions of the Niutitang Formation based on TOC and TS contents, iron speciation, sulfur, molybdenum, and carbon isotope analyses, as well as framboid pyrite size distributions (Pašava et al., 2021; Zhang et al., 2018; Zhao et al., 2018; Jin et al., 2016; Feng et al., 2014; Zhou and Jiang, 2009). During the deposition of the Niutitang Formation, the bottom water was anoxic (ferruginous) and even euxinic (containing H₂S) in the lower part and then transitioned into dysoxic and oxic in the upper part (Liu et al., 2020; Wang S F et al., 2015; Wang J G et al., 2012; Goldberg et al., 2007; Lehmann et al., 2007). In this study, two redox-sensitive trace elements, Mo, and U, were adopted to investigate the bottom water redox condition. Their enrichments in sediments can be attributed to authigenic uptake from seawater, largely influenced by oxygen-depleted conditions (Algeo and Tribovillard, 2009). U uptake starts as the Fe(Ⅱ)-Fe(Ⅲ) redox boundary, while Mo enrichment requires the presence of H₂S. Moreover, the particulate Mn-Fe-oxyhydroxide shuttle plays a role in the uptake of Mo to sediments, while U uptake is free of it (Zheng et al., 2000).

Mo concentrations can be taken as a proxy for bottom water conditions (Scott and Lyons, 2012). Mo concentrations exceeding 100 ppm denote a permanently euxinic condition with abundant Mo dissolved in the water column. An intermediate concentration (25 ppm–100 ppm) reflects intermittent euxinic, or dilution effect caused by high sedimentation rate, or depletion of dissolved Mo from the water column, or the effect of pH. A low concentration (< 25 ppm) indicates non-euxinic settings (Scott and Lyons, 2012). The mudstone samples show a decreasing trend of Mo concentrations from lower to the upper part, and C-1 also presents intermediate Mo concentration (55.5 ppm), demonstrating intermittent euxinic bottom water. However, the existence of sponge spicules (Fig. 2f) may demonstrate transient bottom water oxygenation caused by occasional currents, as sponge metabolism requires free oxygen (Zhou and Jiang, 2009), but the early sponge may have a small demand for oxygen (Zhao et al., 2018). Besides, the Mo-U covariation patterns can offer information about variations in benthic redox conditions, the operation of the particulate shuttle, and the evolution of watermass chemistry (Algeo and Tribovillard, 2009). Combined with previous data from slope-basin sections, the Mo-U covariation pattern shows an aqueous Mo/U similar to or above the Mo/U ratio of seawater, indicating an overall euxinic water condition during the Niutitang Formation deposition (Fig. 5). In a restricted basin, the Mo concentration in the water column can be strongly reduced due to rapid removal to the sediment, and lead to a flat Mo-U pattern. However, for studied samples, the Mo-U enrichment pattern doesn't display differential depletion, and the normal-marine-like Mo/U ratios indicate the Early Cambrian Nanhua Basin was connected to the open ocean, but the semi-restricted condition can't be precluded (Cheng et al., 2020). Collectively, these results support an overall euxinic with intermittent anoxic bottom water condition during the mudstone deposition.

Figure  5.  EF (Mo)-EF (U) covariation of Niutitang Formation mudstones. C-1 refers to Concretion-1. Grey dashed lines show Mo/U molar ratios equal to the seawater value (1 × SW) and fractions (3 × SW, 0.3 × SW). ZNC. Zhongnancun Section (Liu et al., 2020); SDP. Siduping Section (Zhao X K et al., 2018); FT. Fengtan Section (Fang et al., 2019); YJ. Yuanjia Section (Cheng et al., 2020).

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The primary productivity is a key to controlling the organic matter accumulation in the sediment (Chen et al., 2022; Pan et al., 2020; Pedersen and Calvert, 1990), which participates in the microbial sulfate reduction process and the product H₂S facilitates the formation of pyrites. Primary productivity can be assessed by the TOC contents and nutrient trace elements (Ba, Cu, Ni) concentrations. The TOC contents of the Niutitang Formation mudstone show a declining trend, with a maximum of 15.08 wt.% and an average of 8.09 wt.%, illuminating the abundance of primary organic matter. Concretion-1 also possesses a relatively high TOC content (4.5 wt.%), relatively lower than the mudstones. TOC is commonly affected by diagenesis, redox reactions, biological action, and the dilution of macerals, resulting in either overestimating or underestimating primary productivity (Tan et al., 2021). Alternatively, Ba is used as an indicator of paleoproductivity because of its long residence time in the ocean (Schoepfer et al., 2015). The equatorial Pacific Ocean with high productivity displayed high Ba content (1 000 ppm–5 000 ppm) (Murray and Leinen, 1993). To eliminate the terrestrial influence, Ba_excess is obtained by deducting the detrital part from the total Ba. Ba_excess concentrations of the mudstones are consistently high (528.3 ppm–3 874.1 ppm, averaging 1 455.7 ppm), implying high primary productivity during the mudstone deposition. In addition to Baexcess, Cu and Ni are good proxies for organic matter sinking flux (referred to as productivity) since they are delivered to the sediment mainly in association with organic matter (Tribovillard et al., 2006). In this study, Cu and Ni contents display the highest value in Concretion-1, up to 325.9 ppm and 226.0 ppm, respectively (Table 2). While the Cu and Ni contents of mudstone are 10.6 ppm–167.2 ppm and 8.1 ppm–80.7 ppm. Furthermore, the phosphorite deposits in the adjacent area (Gao et al., 2018; Zhao et al., 2018; Qiao et al., 2016) also prove high primary productivity during the deposition of the Niutitang Formation.

Paleoclimate impacts marine productivity, redox conditions, and terrestrial input (Tan et al., 2021). The chemical index of alteration (CIA) is used to indicate climate change. The CIA in the ranges of 50–60 manifests a cold climate with weak chemical weathering, 60–80 indicates a warm and humid climate with moderate chemical weathering, and 80–100 represents hot and humid climates with strong chemical weathering (Nesbitt and Young, 1982). In this study, the CIA values are between 74 and 80 with an average of 78, higher than those of average shales (70–75) (Nesbitt and Young, 1982), demonstrating a moderate degree of chemical alteration in warm and humid climates during the Early Cambrian, when South China is thought to be located in a subtropical-tropical zone near the equator (McKerrow et al., 1992) The terrigenous input is measured by Ti/Al, with higher values denoting higher terrigenous input. The Ti/Al values of studied samples are stable, with an average of 0.06 (Table 1), suggesting a steady terrigenous input during the mudstone deposition.

4.2 Formation of Pyrite Concretions

The organic matter abundance, dissolved sulfate availability, and reactive iron minerals control the pyrite formation in sediment (Berner, 1984). The first two factors influence the activity of sulfate-reducing bacteria, which converts the sulfate (elector acceptor) into hydrogen sulfide (H₂S) by consuming organic matter (elector donator) (Berner, 1984). With high productivity in the surface water, more reactive organic matter escapes being destroyed by aerobic processes happening at and above the sediment-water interface (SWI). Thus, they can be transferred into the sulfate reduction zone. As a result, more H₂S is generated and available for reaction with iron than it can be dispersed by diffusion, and a greater iron mineral is converted to pyrite (Berner and Raiswell, 1983; Raiswell, 1976). The organic matter was abundant due to the high primary productivity (Table 2). As a result, OM is not the controlling factor of pyrite formation. The reactive Fe mainly comes from dissolved iron in the water column, iron minerals, and possibly external influx. The dissolved iron concentration in a water column is usually the highest in the iron-rich zone below the oxic-anoxic boundary (Brewer and Spencer, 1974). Additionally, the anoxic and ferruginous conditions in Early Cambrian oceans, particularly in the Yangtze basinal area, indicate high Fe²⁺ concentration in porewater. Iron minerals in the sediment participating in the pyrite formation are primary iron hydroxides, and oxides coating on terrigenous grains (Canfield, 1989), which dissolve fast, especially in a reducing environment, and the amount supplied to the ocean may be greater due to the increasing weathering during the Early Cambrian (Li et al., 2019; Zhai et al., 2018). Also, during organic matter oxidation by iron-reducing bacteria, Fe²⁺ could be released (Coleman, 1993). So, the reactive Fe is not the controlling factor of pyrite formation. Pore waters close to the sediment-water interface (SWI) are often iron-dominated. Thus, local supersaturation is created where organic matter decay by sulfate-reducing bacteria generates H₂S (Raiswell et al., 1993). Since the Cambrian ocean was dominated by low sulfate content (probably ~2 mM or lower), this situation persists well into Cambrian Series 3 (Loyd et al., 2012), the sulfate availability becomes the controlling factor of pyrite formation. Nevertheless, the enhanced continental weathering during the Early Cambrian (Li et al., 2019) (Table 1) may deliver abundant sulfate to the ocean, facilitating sulfate reduction and leading to water euxinic.

From the outcrop, these pyrite concretions are usually present either as individuals or surrounding the mudstone (Figs. 2c, 2d) with two ends bent, and no vertical pyrite veins cutting across the mudstone bedding are found. The unevenly distribution of pyrite concretions was attributed to local spatial heterogeneity in extremely small distances in sediment or water (Rickard and Luther, 2007). The pyrite crystals are tightly packed with voids filled with siliciclastic matrix (Fig. 4c) identical to the host rock in composition (Fig. 4e), indicating the concretion formation before the sediment compaction, possibly early diagenetic stage (where porewaters are in connection with overlying seawater). Since taphonomic pyritization involves degradation and the authigenic mineralization of organic matter, preservation of soft tissues requires rapid and efficient pyrite precipitation (Briggs et al., 1996). The pyritized sponge spicules suggest (Fig. 2e) fossil preservation before compaction so that the biological shape was replicated prior to being destroyed. The presence of soft-body fossils offers evidence for concretionary growth close to the sediment-water interface (SWI) in some cases (Raiswell and Fisher, 1999).

The pyrite concretion is composed of pyrite, quartz, clay minerals, barite, hyalophane, and tetrahedrite (Fig. 4). The overall process of the pyrite formation includes organic matter consumption, the reaction between H₂S and reactive Fe (Berner, 1984), which is supported by lower TOC content of Concretion 1 compared with that of host mudstone (Table 2). From the SEM observation, two types of pyrite in these concretions were classified. The framboid pyrites exist both in the pyrite concretion and mudstone, with an average diameter of 6.8 μm (n = 35), indicating a euxinic bottom water condition and formation in sulfidic water column. As for euhedral pyrites, they precipitated from the porewater directly (Sawłowicz, 1993). However, euhedral pyrites can be transformed from framboid pyrites (Sawłowicz, 2000, 1993), but such is not the case in this study since there is a distinct boundary between them (Fig. 4d). Euhedral pyrites are prone to form aggregates (Figs. 4d, 4f, 4g, 4h) and associate with hydrothermal minerals, such as hyalophane, barite, and tetrahedrite (Figs. 4g, 4h, 4i), which manifests a close relationship with hydrothermal activity. The trace element mapping of pyrite concretion displays enrichments in Mo, Pb, and Tl with voids filled with Si and O (Fig. 6), which coincides with high concentrations of Mo Pb and Tl (Table 2) and further supports the early diagenetic genesis in porewater within the sediment.

Figure  6.  Elements distribution of pyrite Concretion-1 of the Niutitang Formation.

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The typical growth pattern for concretions includes pervasive growth and concentric growth (Raiswell and Fisher, 1999). The formation style of studied pyrite concretions is discussed here. As the pyrite concretion is mainly composed of densely packed euhedral pyrite without zonation and residual porosity filled with mudstone composition, the pervasive growth pattern is preferred. In addition, the in-situ δ³⁴S_py values show small differences among three pyrite concretions and display no zonation characteristic (Fig. 2f, Table 3), further proving the pervasive growth pattern. The average δ³⁴S_py values for C-1, C-2, and C-3 are 18.2‰, 16.0‰, and 17.8‰, respectively, displaying relatively small fractionation with the coeval δ³⁴S_seawater (33‰) (Seal, 2006). These positive δ³⁴S_py values and small fractionations could be caused by the Rayleigh distillation model in a closed system (Goldberg et al., 2006; Raiswell et al., 2002). Yet, according to the calculation model, the Rayleigh process can only account for < 0.1 vol% of pyrite (Lang et al., 2018), which is inconsistent with our observations of abundant pyrite concretions in the field (Fig. 2). According to Jørgensen et al. (2004), an open system is always maintained to a certain extent in sediment pore water, in which case SO₄^2- and H₂S diffusion impact the sulfur isotopic composition. Thus, the small fractionation was attributed to fast bacteria sulfate reduction limited by sulfate availability in an open system (Xiao et al., 2010; Habicht and Canfield, 1997). As the bottom water became euxinic during the pyrite concretion formation, H₂S diffused from the overlying euxinic water to the sediment and quickly reacted reactive Fe to form pyrites, and pyrite concretions sustained in the SWI.

4.3 Implications for Early Cambrian Hydrothermal Activity

The Ediacaran–Cambrian witnessed the transition from an icehouse to a greenhouse climate and increased plate tectonic activity, which resulted in changes in ocean circulation and nutrient levels (Tucker, 1992). Also, it was characterized by stratified oceans (Li et al., 2020; Babcock et al., 2015). Under such background, a set of organic-rich black shale was deposited widely on the Yangtze Block in the Early Cambrian, induced by the global transgression (Zou et al., 2019; Goldberg et al., 2007). The pyrite concretions in the bottom of the Niutitang Formation of Early Cambrian, similar to the Ni-Mo sulfide layer, mark an interval influenced by special events. Since the pyrite formation requires organic matter, H₂S, and Fe, abundant pyrite concretions imply a large amount of organic matter flux into the sediment, adequate sulfate supply, and high reactive Fe contents. This, in turn, reflects high primary productivity and riverine input of sulfate, which drove bacteria sulfate reduction and led to euxinic bottom water. Also, hydrothermal activities have been reported during the deposition of the Niutitang Formation (Wei et al., 2022; Shi et al., 2021; Han et al., 2020; Wu Y W et al., 2020; Zhang et al., 2020; Steiner et al., 2001). According to geochemical and petrological results, the pyrite concretions formation was associated with hydrothermal activity, as evidenced by positive Eu anomalies (Table 2) and typical hydrothermal minerals of hyalophane, barite and tetrahedrite (Figs. 4g, 4h, 4i). Hydrothermal fluids not only injected reduced gases (CH₄, H₂S), facilitating the water anoxia and even euxinia, but also provided nutrients such as Si, Cu, and Ni to facilitate the primary productivity and heavy metals incorporated in pyrites (Pb, Tl) or forming individual minerals (Ba, Sb).

5. CONCLUSION

To summarize, the Lower Cambrian Niutitang Formation pyrite concretions consist of pyrites, hydrothermal minerals, quartz, and organic matter, which is explained as the formation during the early diagenesis before compaction and influenced of hydrothermal fluid. The δ³⁴S_py values are homogeneous among three concretions, and no zonation is found in one concretion. These positive and consistent values indicate fast bacteria sulfate reduction near the SWI in an open system. Abundant pyrite concretions in the bottom of Niutitang Formation black shale reflect euxinic bottom water, high primary productivity, and high riverine sulfate flux.