Geochemistry of Cryogenian Datangpo manganese deposits in the southeastern Yangtze Platform of South China: implications for the origin of metallogenesis and depositional environment

ABSTRACT Sedimentary manganese (Mn) deposits in the southeastern Yangtze Platform of China have been newly discovered with considerable potential for resource exploration. Such deposits formed due to dynamic changes in the global environment, including the break-up of the supercontinent Rodinia, Snowball Earth glaciation, and the Neoproterozoic Oxygenation Event. The Mn-bearing sequences are hosted in the basal Datangpo Formation and are interbedded with black carbonaceous shale in a series of graben sub-basins in the Nanhua Rift Basin. We investigated the origin of these Mn deposits by analysing the major and trace elements, total organic carbon, and stable carbon and oxygen isotopes. Positive Eu anomalies were observed, indicating that the Mn was derived from a hydrothermal system. Redox-related elements, including Ce, Mo, U, V, S, and P, indicated that the depositional environment of the Mn layers was oxidative and that Mn2+ was initially oxidized to Mn oxides. However, the host rock (i.e. black carbonaceous shale) was deposited under dysoxic – anoxic and even sulfidic water conditions. The alternating distribution of Mn ore and host rock suggested alternating redox conditions in the sub-basins. A negative correlation between the Mn content and δ13Ccarb values (−10‰–−7‰) and a deficiency of sulphide minerals indicate that the diagenetic degradation of organic matter (δ13Corg from − 34‰ to − 31.9‰) took place through Mn oxide reduction rather than microbial sulphate reduction and can contribute to a considerable source of bicarbonates. The approximately homogeneous δ13Ccarb depletion and their apparent discrepancy with δ13Corg indicate that the post-depositional genesis of the Mn carbonate ore occurred in a relatively open early diagenetic pore water system with a considerable contribution from seawater-derived carbon sources. GRAPHICAL ABSTRACT


Introduction
Throughout their geological history, sedimentary manganese (Mn) deposits experience various accumulation environments (Bühn et al. 1992;Roy 2006).Similar to other transition elements such as iron (Fe), the metallogenesis of sedimentary Mn follows the evolution of Earth's systems, and it is primarily controlled by changes in redox conditions established over various periods (Roy 2006).The redox conditions are strongly linked with tectonic forces (Campbell and Allen 2008) such as supercontinent break-up and rapid seafloor spreading, the impacts of climate change (e.g.glacialinterglacial cycles) (Och and Shields-Zhou 2012), and endogenic (e.g.volcanism and hydrothermal activities) and exogenic (e.g.changes in the composition of the atmosphere and hydrosphere) processes (Roy 2006;Lyons et al. 2015;Myrow et al. 2018).Such processes, which are fundamentally driven by tectonic forces, have been proposed to jointly create geologically and geochemically favourable environments for the supply, migration, and accumulation of sedimentary Mn deposits (Roy 2006).
The Cryogenian Period (~720-635 Ma) in the mid-Neoproterozoic Era was a critical geological interval that witnessed the break-up of Rodinia (Goddéris et al. 2003;Donnadieu et al. 2004), the development of Snowball Earth (Donnadieu et al. 2014), and a significant rise in atmospheric O 2 called the Neoproterozoic Oxygenation Event (Campbell and Allen 2008;Och and Shields-Zhou 2012).Concurrently, metallogenesis produced Mn carbonate deposits interbedded with black shale in the Datangpo Formation in the Nanhua Rift Basin, which is located in the southeastern Yangtze Platform of South China.The proven reserves here contain up to 832 million tons of Mn and account for 60.29% of China's total Mn reserves based on 2013 statistics (Zhou et al. 2019).These Datangpo-type Mn ore deposits, which are named after the place they were initially explored, are mostly distributed in the eastern Guizhou Province and adjacent areas (Zhou et al. 2019).Previous studies related to Datangpo-type Mn deposits have provided valuable information on Cryogenian interglacial ocean chemistry (Feng et al. 2010;Ma et al. 2019) and clarified its role in Mn metallogenesis (Wu et al. 2016;Yu et al. 2016;Xiao et al. 2017).However, researchers have not yet reached a consensus on Mn ore genesis that describes the source and formation of Mn carbonates.Different contrasting genetic theories have been proposed that can be classified as follows: biogenic enrichment of Mn regardless of source (Liu et al. 1983), hydrothermal Mn ore deposition (Chen and Chen 1992;Xie et al. 1999), direct deposition of hydrothermally derived Mn from seawater reduction (Yang 2002), and reduction of Mn oxides coupled to organic matter oxidation in anoxic pore water with high alkalinity generated by microbial sulphate reduction.
In this research, we collected Mn ore and host rock samples (i.e.interbedded manganiferous shale and black carbonaceous shale) from the basal Datangpo Formation and performed a series of whole-rock geochemical and lithological analyses to characterize the major and trace elements, total organic carbon (TOC), stable carbon, and oxygen isotopes.We also collected trace element and carbon isotope data of certain typical Mn carbonate deposits in geological time for comparison with the Datangpo-type Mn deposits.Our objectives were to gain a deeper understanding of Mn sources, palaeo-redox conditions during the deposition of the basal Datangpo Formation, the depositional processes, and the diagenetic origin of Mn carbonate ore.

Evolution of rifting basins
Following the assembly and break-up of Rodinia, the Yangtze Craton was formed by the collision of the South China Plate and Cathaysian Block at 850-820 Ma, which began to split again at ~820 Ma (Li 2011).This created three rift systems on the southeastern, western, and northern continental margins of the Yangtze Craton (Wang et al. 2019).The Nanhua Rift Basin, which belongs to the southeastern margin of the Yangtze Craton, expanded intensively at about 820-700 Ma and experienced a gradual transition to a passive continental margin after the Yangtze Craton drifted away from Rodinia (Wang et al. 2016).The distribution of Datangpo-type Mn ore deposits was strictly controlled by the evolution of the Nanhua Rift Basin (Figure 1a).At about 725 Ma, the E -W trending Nanhua Rift Basin (Level I) (Figure 1a) split into three palaeogeographic units along three ductile shear zones across the crust -mantle boundaries: the Wuling Rift Basin (Level II), Xuefeng Rift Basin (Level II), and intervening Tianzhu -Huitong Uplift Belt (Level II) (Figure 1b).The Wuling Rift Basin (Level II) contains the main ore-hosting zones, and it can be further divided into three sub-basins (Level III) and two uplift regions (Level III) (Figure 2a).The Shiqian -Songtao-Guzhang Sub-basin (Level III) was the rifting centre of the Wuling Rift Basin (Level II) (Figure 2a), where the most intensive Mn metallogenesis occurred.Mn carbonate deposits are distributed in subrift basins (Level IV) controlled by northeast trending paleofaults (Figure 2a).Because they are controlled by equidistant faults, the Mn deposits are strung out like pearls on a string (Figure 2a).Supp. Figure S1 shows the sedimentary lithofacies and palaeogeographic and paleobathymetric maps of the Nanhua Rift Basin as a reference for the water depth of the Mn deposition sites.
Chongqing Province), Datangpo, and Nantuo Formations in ascending stratigraphic order (Figure 2b).The Tiesi'ao Formation is dominated by greyish-yellow tillites representing Sturtian glacial deposits (Wu et al. 2016;Yu et al. 2016).The Nantuo Formation comprises grey-green and dark-green tillites representing Marinoan glacial deposits (Wu et al. 2016).The Datangpo Formation corresponds to the post-Sturtian interglacial period, and it is generally subdivided into three members.The first member consists of 0.5-15 m of Mn carbonate, Mn-bearing shale, and black carbonaceous shale (Figure 2b).The second member comprises 1-20 m of pyritic black or grey shale, and the third comprises 1-700 m of grey and yellow sandy or muddy siltstone (Figure 2b).The U -Pb zircon age of the tuffaceous samples collected from the boundary between the Sturtian diamictites and basal Datangpo Formation is 659.3 ± 2.4 Ma (Figure 2b).In contrast, the U -Pb zircon age of the ash bed atop the Datangpo Formation is 654.5 ± 3.8 Ma (Figure 2b).Each ore-hosting subrift basin (Level IV) can be subdivided into graben and horst areas (Yu et al. 2017;Zhou et al. 2019).Each Mn deposit is strictly confined to the centre of each graben sub-basin.The thickness of each Mn deposit decreases from the centre to the margins of the subrift basin (level IV).The graben areas possess typical Cryogenian successions, including the Tiesi'ao Formation, which contains diamictite, and the overlying Datangpo Formation, which contains Mn ore and black carbonaceous shale members (Figure 2b).However, in horst areas, the Tiesi'ao Formation primarily comprises dolomitic diamictites, and the first member of the Datangpo Formation lacks Mn ore sections.Instead, it contains 2-4 m of Sturtian dolomitic cap carbonates (Yu et al. 2016(Yu et al. , 2017)).
According to our field investigation, most Mn deposits exhibited a quasi-lamellar morphology.The studied deposits were not metamorphosed.Figure 3(a-c) show field photographs of a Mn ore body in an underground mine.Such ore bodies were usually interbedded with one to three layers of black carbonaceous shale, and they mainly exhibited massive (Figure 3e-f) and laminated structures (Figure 3g).In some mining areas, such as Datangpo and Daotuo, some massive Mn ores were found that contain round cavity structures (Figure 3f).

Sample collection
Samples collected from the underground mine sections were selected depending on the rock type: Mn ore, manganiferous shale, and black carbonaceous shale (or black shale).Black shale is easily distinguished from Mn ore owing to its sharper fractures, and it generally exhibits a bright lustre (Figure 3b) and contains more massive or scattered pyrite (Figure 3c).Manganiferous shale forms in transition regions between a Mn deposit and the black carbonaceous shale layer (Figure 3b).Specifically, based on major element tests, samples with a high Mn content of >10 wt% were classified as Mn ore, those with a Mn content of 1-10 wt% were classified as manganiferous shale, and those with a Mn content of <1 wt% Mn content were classified as black carbonaceous shale.Hereafter, manganiferous shale and black carbonaceous shale are referred to as 'host rock.'Some of the samples used in this study were recovered from the first member of the Datangpo Formation in five Mn mining areas: Xiushan (XS), Lijiawan (LJW), Zhailanggou (ZLG), Xixibao (XXB), and Datangpo (DTP) (see Figure 2a for their detailed locations).In total, 50 samples were acquired from these areas: 12 samples from XS (numbered XS2-13), 9 from ZLG (ZLG1-9), 5 from XXB (XXB4, XXB6, XXB7, XXB8, and XXB10), 18 from DTP (DTP1-18), and 6 from LJW (1-6).Hereafter, these samples are referred to as 'Datangpo-type Mn deposits.'To investigate the vertical variations of the geochemical indices of the complete Mn-bearing series, samples from XXB were also acquired at intervals of 10 cm from the bottom up of the two underground mine profiles in the first member of the Datangpo Formation.The sample numbers and rock types are illustrated in Figure 4. Hereafter, these samples are referred to as the 'XXB mine profiles.'Finally, two Mn ore samples, DTP m and XXB m (Figure 5), collected from DTP and XXB, respectively, were microsampled by using a 3-mm microdrill to obtain samples from 32 separate points for further carbonate carbon isotope analyses.

Analytical methods
The selected samples were crushed using a Tema Mill machine and passed through a 200-mesh sieve for whole-rock geochemical analysis.All geochemical analyses were conducted at the Third Institute of Oceanography, Ministry of Natural Resources of the People's Republic of China.For the analysis of major elements, powder samples were mixed with Li 2 B 4 O 7 at two different ratios: 0.7 g of the sample (with Mn <5 wt%) and 7 g of Li 2 B 4 O 7 , or 0.4 g of the sample (with Mn ≥5 wt%) and 8 g of Li 2 B 4 O 7 .All samples were thoroughly mixed and melted in an xrFuse 6 electric melting instrument.The major elements were analysed by using X-ray fluorescence spectroscopy (ARL Perform'X 4200), and the details are documented in Wu et al. (2016).The detection limit for all major oxides was 0.01 wt%, and the analytical errors were below 3%.Trace elements, including rare earth elements (REEs), were analysed by using a Thermo Fisher iCAP RQ ICP-MS equipped with a CETAC ASX-560 AutoSampler.We performed inductively coupled plasma mass spectrometry (ICP-MS) following the modified protocol of Eggins et al. (1997) for trace element analysis as described by Kamber et al. (2003) and Li et al. (2005).The analysis errors were less than 10%.
The total sulphur (S) content was determined by using an element analyser (Vario EL/Micro cube, Hanau, Germany).The TOC and organic carbon isotope (δ 13 C TOC ) were measured by using an isotope ratio mass spectrometer coupled with a Flash elemental analyser (Flash EA 1112 HT-DELTA V Advantage, Thermo Fisher).To remove carbonates, the samples were reacted with 4 mol/L HCl in a water bath heated to 80°C-90°C until the reactions subsided.Then, the residue was washed with deionized water until the neutral condition was reached.The residue was dried at 60°C and was crushed to pass through a 60-mesh sieve for TOC and δ 13 C TOC analysis.The analysis uncertainty of δ 13 C TOC was less than ±0.2‰.
The inorganic carbon isotopic composition (δ 13 C carb ) and oxygen isotopic composition (δ 18 O carb ) were measured by using a Gas Bench II-DELTA V Advantage mass spectrometer calibrated against IAEA-CO-8 and IAEA-CO-1 standards.Powdered samples were placed into 12 mL Labco Exetainers, for which the air was expelled by ultrahigh-purity He (>99.999wt%, 100 mL/min).Then, a few drops of anhydrous phosphoric acid were added to the Labco Exetainers, and the samples were reacted at 70°C.The produced gases were passed into a gas chromatographic column to acquire pure CO 2 for analyses of δ 13 C carb and δ 18 O carb .The carbon and oxygen isotope data were respectively expressed based on the international standard of Vienna Pee Dee Belemnite (V-PDB) values as follows.
Scanning electron microscopy (SEM) utilizing backscattered electrons (BS) and secondary electrons (SE) was used to observe Mn ore, manganiferous shale, and black carbonaceous samples on a JEOL JSM-6460lv SEM at the Second Institute of Oceanography, Ministry of Natural Resources of the People's Republic of China.Energy-dispersive spectroscopy (EDS) was performed to analyse the rhodochrosite crystal.
To acquire the mineralogical compositions of Mn ore, manganiferous shale, and black carbonaceous samples, X-ray diffraction (XRD) was conducted by using a PANalytical X'Pert Pro instrument at the Second Institute of Oceanography, Ministry of Natural Resources of the People's Republic of China.Operating conditions included a continuous scanning rate using a Cu -Ni tube at 40 kV and 40 mA with a speed of 8°/min.

Petrographic observations
The black carbonaceous shale predominately comprised terrigenous siliceous components and pyrite (Figure 6a).The manganiferous shale contained many terrigenous components and pyrite with minor amounts of rhodochrosite (Figure 6b).The Mn ore primarily comprised Mnbearing carbonates with only minor amounts of pyrite and clastics (Figure 6(c-e)).The Mn-bearing carbonate minerals were rhodochrosite and Ca-rhodochrosite (Figure 6e; Supp. Figure S2).Individual rhodochrosite roughly exhibited a spherulite structure (Figure 6(c-e)) cemented by Ca-rhodochrosite, which was darker in colour and lower in Mn content (Figure 6e).Each rhodochrosite grain usually displayed an oolitic texture with three obvious concentric layers (Figure 6e).The dark core was likely an algodetrinite around which Mn-bearing minerals grew.EDS analyses indicated that the rhodochrosite spherulites had different layers, where the core and inner layer had a higher Ca content than the outer layer but a lower Ca content than the cementing Ca-rhodochrosite (Supp.Figure S2).

Mineralogical composition
The XRD results (Supp.Table S1) indicated that the primary carbonate minerals were rhodochrosite and calcite.Compared with the Mn ore, the black carbonaceous shale had a much lower carbonate content, with only 1%-3% rhodochrosite and 1%-2% dolomite.Some terrigenous detrital minerals such as quartz, K-feldspar, and clay were observed.Quartz was common in all samples, with a range of 1%-39%.The Mn ore had a lower quartz content than the black carbonaceous shale.Meanwhile, the black carbonaceous shale generally had higher contents of pyrite, K-feldspar, mica, and phillipsite.

Datangpo-type manganese deposits
The major elemental compositions of the samples collected from the five Datangpo-type Mn deposits are listed in Supp.Table S2.The MnO contents of the Mn ore, manganiferous shale, and black carbonaceous shale were 14.85-38.80wt% (average: 31.4 wt%), 2.01-11.9wt % (average: 6.05 wt%), and 0.03-0.76wt% (average: 0.28 wt%), respectively.The Mn ore was rich in Mn, Ca, and Mg but was deficient in Al, Si, Ti, K, and Na compared to the manganiferous shale and black carbonaceous shale (Supp.Table S2; Figure 7).The average Fe contents of the manganiferous shale and black carbonaceous shale were 4.35 wt% and 4.31 wt%, respectively, which were higher than that of Mn ore at 2.58 wt%.The Fe/Mn ratio of the Mn ore was low, with a range of 0.04-0.22 and an average of 0.11, which indicated superior fractionation between Fe and Mn.The samples showed a significant correlation for Fe 2 O 3 -S, which indicated that Fe may be predominantly associated with pyrite (FeS 2 ) (Figure 7f).The total S content of the Mn ore had a range of 0.02-3.64wt% with an average of 0.89 wt%, which was significantly lower than the average contents of the manganiferous shale at 2.46 wt% and black carbonaceous shale at 2.70 wt%.The average P 2 O 5 contents of the manganiferous shale and black carbonaceous shale were 0.175 wt% and 0.121 wt%, respectively, which were lower than the average P 2 O 5 content of the Mn ore at 0.428 wt% (Supp.Table S2).The P/Mn ratio of the Mn ore had a range of 0.0004-0.0367with an average of 0.0081.

Xixibao mine profiles
The major elemental compositions of the Mn ore, manganiferous shale, and black carbonaceous shale samples collected from the two XXB mine profiles are listed in Supp.Table S3.The Mn contents of the Mn ore, manganiferous shale, and black carbonaceous shale were 12.9-31.1 wt% with an average of 23.10 wt%, 2.42-10.79wt% with an average of 5.51 wt%, and 0.15-0.51wt% with an average of 0.31 wt%, respectively.The Fe contents of Mn ores were 1.69-4.42wt% with an average of 2.25 wt%, the manganiferous shale at 1.3-6.39wt%with an average of 3.01 wt%, and the black shales at 0.67-3.18wt% with an average of 1.78 wt%, respectively.The Fe/Mn ratios had a range of 0.06-0.34with an average of 0.11, which indicated significant fractionation.The P content of the Mn ore was 0.14-0.51wt% with an average of 0.20 wt%, which was slightly higher than that of the host rock.The S content of the Mn ore was 0.29-3.07wt% with an average of 0.97 wt%, which was lower than the average S contents of the manganiferous shale at 2.30 wt% and black carbonaceous shale at 1.54 wt%.Thus, the samples from the two XXB mine profiles had very similar characteristics to the samples from the other Datangpo-type Mn deposits.Compared with the Mn ore, the host rock had higher Al, Ti, Si, K, and Na contents but lower Ca and Mg contents.

Datangpo-type manganese deposits
The trace element contents of the samples collected from the five Datangpo-type Mn deposits are listed in Supp.Tables 4-5.The trace element contents of the samples were normalized against that of Post-Archaean Australian average Shale (PAAS) to facilitate their comparison.The Mn ore was rich in Co, Sr, Mo, and Pb and deficient in V, Cr, Ni, Zn, Rb, Zr, Ba, Th, and U (Figure 8a).The manganiferous shale and black carbonaceous shale showed similar patterns to that of the Mn ore Figure 8(c,  e) but with a lower Sr content, high Mo content, and slight enrichment of V, Ni, Zn, Zr, and Pb (Figure 8(c, e)).
The REE and yttrium (REY) contents of the samples, as well as some additional parameters, are presented in Supp.Tables 4-5.The total REY content (∑REY) of the Mn ore had a relatively wide range of 92.16-372.37 ppm with an average of 213.43 ppm.The host rock exhibited higher ∑REY with manganiferous shale at 157.80-433.73ppm with an average of 250.35 ppm and black carbonaceous shale at 165.39-400.72 ppm with an average of 255.10 ppm.The PAAS-normalized REY patterns of Mn ore exhibited a pronounced enrichment of middle REEs (MREEs) compared to both low REEs (LREEs) and high REEs (HREEs), producing a hat-shaped REY plot (Figure 8b).The PAAS-normalized REY distribution patterns of the host rock were characterized by significantly fewer MREEs and slightly more HREEs, which resulted in a flat REY plot Figure 8(d, f).
The Mn ore had Ce SN /Ce SN * ratios (i.e.PAAS-normalized) of 1.04-1.43with an average of 1.25 (Supp.Table S4), which indicated prominent Ce anomalies.The host rock showed no apparent positive Ce anomalies with Ce SN /Ce SN * ratios of 0.98-1.20 with an average of 1.04 for manganiferous shale (Supp.Table S4) and 0.97-1.10 with an average of 1.01 for black carbonaceous shale (Supp.Table S5).
The Mn ore had Eu SN /Eu SN * ratios (i.e.PAAS-normalized) of 1.02-2.35with an average of 1.46, which indicated weak to strong positive Eu anomalies (Supp.Table S4).However, the host rock showed weak negative Eu anomalies, with manganiferous shale having Eu SN /Eu SN * ratios of 0.82-1.34for an average of 1.07 (Supp.Table S4) and black carbonaceous shale having Eu SN /Eu SN ratios of 0.80-1.06for an average of 0.96 (Supp.Table S5).The Mn ore had Y SN /Y SN * ratios (i.e.PAAS-normalized) of 0.914-1.063with an average of 0.98.The measured (Y/ Ho) SN ratios of the Mn ore were 0.911-1.068with an average of 0.976 (Supp.Table S4).The Y SN /Y SN * ratios were 0.799-1.03with an average of 0.965 for the manganiferous shale (Supp.Table S4) and 0.946-1.016with an average of 0.981 for the black carbonaceous shale (Supp.Table S5).The (Y/Ho) SN ratios were 0.815-1.009with an average of 0.943 for the manganiferous shale and 0.924-0.977with an average of 0.942 for the black carbonaceous shale.

Xixibao mine profiles
The trace element contents of the samples collected from the two XXB mine profiles are listed in Supp.Tables 6 and 7.The Mn ore generally had lower contents of trace elements than the host rock excluding Sr, Co, and Mo.The host rock contained higher contents of V, Cr, Ni, Cu, and Zn than the Mn ore.The PAAS-normalized trace element patterns indicate that the Mn ore was richer in Co and Mo (Supp.Figure S3a) while the host rock was richer in Ni, Cu, Zn, Ba, Pb, and U (Supp.Figures S3c and S3e).The Mn ore exhibited an obvious enrichment of MREEs, whereas the host rock exhibited flat REY patterns (Supp.Figure S3b, d, f).Thus, the samples from the two XXB mine profiles had almost identical PAAS-normalized trace element patterns to those of other Mn deposits collected in the Nanhua Rift Basin.
The Mn ore had ∑REY of 155.42-505.54ppm with an average of 253.02 ppm.The manganiferous shale had ∑REY of 148.47-731.45ppm with an average of 326.30 ppm.The black carbonaceous shale had ∑REY of 196.24-289.55 ppm with an average of 227.95 ppm (Supp.Tables 6-7).The Mn ore had Ce SN / Ce SN * ratios of 1.04-1.43with an average of 1.29, which indicated obvious positive Ce anomalies.The black carbonaceous shale showed no obvious Ce anomalies with Ce SN /Ce SN * ratios of 0.94-1.04 with an average of 1.01 (Supp.Figures S3b, d, f; Supp.Tables S6-7).The Mn ore exhibited obvious positive Eu anomalies with Eu SN /Eu SN * ratios of 0.89-1.37 with an average of 1.13.Meanwhile, the host rock exhibited negative Eu anomalies as manganiferous shale had Eu SN /Eu SN * ratios of 0.78-0.96with an average of 0.83, and black carbonaceous shale had Eu SN /Eu SN * ratios of 0.86-0.98 with an average of 0.93 (Supp.Tables S6-7).The Y/Ho ratios of the Mn ore were 25.52-30.12with an average of 27.64 (Supp.Tables S6-7), which is lower than that of seawater (>36) and close to that of chondrite (~28).

Datangpo-type manganese deposits
Supp.Table S8 presents the TOC contents, δ 13 C carb , and δ 18 O carb of the samples.The Mn ore had a TOC content of 0.93-4.83wt% with an average of 2.85 wt %, which is much higher than the average TOC contents of manganiferous shale at 1.35 wt% and black carbonaceous shale at 1.38 wt%.The organic δ 13 C org was rather uniform.The Mn ore was at − 34.04‰ to − 31.86‰ with an average of − 33.15‰, the manganiferous shale was at − 34.6‰ to − 24.3‰ with an average of − 32.3‰, and the black carbonaceous shale was at − 34.68‰ to − 31.01‰ with an average of − 32.94‰.The carbonate δ 13 C carb of the Mn ore was − 10.35‰ to − 5.63‰ with an average of − 7.81‰, which is slightly more negative than the average compositions of the manganiferous shale at − 7.29‰ and black carbonaceous shale at − 7.21‰.The carbonate δ 18 O carb was − 5.38‰ to − 14.15‰ with an average of 8.49‰ for the Mn ore.It was − 6.26‰ to − 15.97‰ with an average of − 11.75‰ for the manganiferous shale and − 7.83 to − 15.30‰ with an average of − 11.39‰ for the black carbonaceous shale.

Xixibao mine profiles
The Mn ore collected from the two XXB mine profiles had a carbonate δ 13 C carb of − 7.24‰ to − 5.18‰ with an average of − 6.27‰.The δ 18 O carb was − 8.93‰ to − 5.03‰ (Supp.Table S9).Because the TOC (32.49‰ −33.73‰) was analysed by using Mn ore and host rocks collected from the XXB mine area (Supp.Table S8), TOC analysis was not conducted on samples from the two XXB mine profiles.

Microsamples
Microsamples were obtained from 32 separate points of two Mn ore samples (DTP m and XXB m ) (Figure 5) for carbon isotope analyses (Supp.Table S10).The δ 13 C values were highly homogeneous and exhibited very small standard deviations of 0.23‰ for XXB m (N = 16) and 0.18‰ for DTPm (N = 16).

Source of manganese
The compiled geochemical data (Supp.Table S2 and Figure 7) of the samples recovered from the basal Datangpo Formation showed that the Mn content had an inverse correlation with the Al and Ti contents.The Mn ore had relatively low Al and Ti contents (Supp.Table S2).Because Al is mainly introduced to marine sediments by fluvial and aeolian sources, its presence is a reliable indicator of terrigenous clastic input, particularly when Ti exhibits the same enrichment (Ma et al. 2019).Thus, these results indicate that Mn is not associated with continental detrital components.The horsts would have preserved a thicker and higher-grade Mn-bearing sequence than the grabens if Mn originated from terrigenous sources, but this was not the case.
A hydrothermal deposition is evidenced by a (Fe + Mn)/Ti ratio above 20 ± 5 and an Al/(Al + Fe + Mn) ratio below 0.35 (Boström 1983).In this study, the Mn ores exhibited (Fe + Mn)/Ti ratios of 93.14-525.65 and Al/(Al + Fe + Mn) ratios of 0.02-0.16(Supp.Table S2), which suggest that Mn in the basal Datangpo Formation most likely originated from a hydrothermal source.However, the black carbonaceous shale showed (Fe + Mn)/Ti ratios of 2.48-18.37 and Al/(Al + Fe + Mn) ratios of 0.61-0.89(>0.35) (Supp.Table S2), which suggest a terrigenous contribution.Figure 9a assesses the relative contributions of hydrothermal and continental inputs to ocean sediments, and all of the samples exhibited significant distribution patterns.The Mn ore corresponded to the hydrothermal endmember, the black carbonaceous shale was more closely associated with the hydrogenous endmember, and the manganiferous shale was intermediate (Figure 9a).In the SiO 2 -Al 2 O 3 diagram, almost all samples corresponded to the normal deposition field rather than the hydrothermal field (Figure 9b).This implies that the Mn came from a hydrothermal input and that Datangpo-type Mn deposits are not typical hydrothermal products.
The Mn ore had weakly to strongly positive Eu anomalies (average: 1.46) (Supp.Table S4), and a positive correlation existed between the Eu SN /Eu SN * ratio and Mn content (Figure 9(c-d)).The positive Eu anomalies of Mn ore may be closely related to the ratio of hydrothermal to hydrogenetic Mn exhibiting strong local control of the basin configuration (Maynard 2003).In other words, Mn deposits in geological records without positive Eu anomalies may reflect a decline in contributing vent fluids compared to seawater because Mn is dispersed far from the vents.In this study, the ice sheets formed during the Sturtian glaciation resulted in low-sea-level stands and the isolation of small subbasins, which induced the anoxic condition in the Nanhua Rift Basin.The presence of positive Eu anomalies of Mn ore in this study may indicate the presence of an anoxic ocean that allowed for the long-distance transport of Eu 2+ and, therefore, Mn 2+ from a marine hydrothermal vent system likely to be linked to faults that developed beneath the ore-hosting sub-basins (Figure 2a).Interestingly, the Mn carbonate deposit in the Datangpo Formation in the Xiangtan region of Hunan Province (see Supp. Figure S1 for its location), which is controlled by faults in the Wuling Rift Basin that developed at the shallow end, exhibited negative Eu anomalies with Eu SN /Eu SN * ratios of 0.71-0.95(Liu et al. 2022).This may imply that Mn 2+ -rich anoxic water with a hydrothermal signature was preferentially diluted by seawater from the open sea in the aftermath of the Sturtian glaciation.
The above evidence indicates that Mn in the Datangpo Formation likely came from hydrothermal vents at some distance from the main hydrothermal activity.We presume that Rodinia's break-up and the resultant development of rifted basins produced favourable sites and ore-forming fluids for Mn metallogenesis.Frequent rift-related volcanism and/or hydrothermal activities during the Sturtian glaciation or even earlier (Gernon et al. 2016) introduced substantial amounts of dissolved Mn 2+ along the faults (Figure 2a) into the anoxic waters of the Nanhua Rift Basin but prevented its precipitation before the Datangpo-type Mn metallogenic era.Then, Mn transport in seawater was enhanced by the Sturtian ice cover.

Redox environment for manganese accumulation in the Datangpo Formation
We use a series of redox-related proxies, including Ce anomalies, S and P contents, and the enrichment or depletion of redox-sensitive elements such as Mo, U, V, Cr, Ni, and Cu to show that the Mn accumulation in the deepest subbasins may indicate a transition from the anoxic to oxic condition.Under the oxic condition, the previously stored dissolved Mn 2+ was oxidized to Mn oxides, whereas the interbedded carbonaceous shale layers may have been deposited from the dysoxic -anoxic (even sulfidic) seawater column.The alternating distributions of Mn-bearing and carbonaceous shale layers in the Datangpo Formation suggest alternating redox conditions.

Ce anomalies
We can use the method proposed by Bau and Dulski (1996) to show that the Ce anomalies in the Mn ore are real and not complicated by the anomalous abundance of La (Supp.Figure S4a).The Ce SN /Ce SN * ratio of the Mn ore displayed a moderate correlation (r = +0.56)with the Mn content (Supp.Figure S4b), which indicates that the Mn content has some degree of control over Ce enrichment.Therefore, we propose that the occurrence of Ce anomalies is closely related to the oxygenation of dissolved Mn 2+ to insoluble oxides under the oxic condition.However, the lack of Ce anomalies for the black carbonaceous shale (average: 1.01) (Supp.Table S5) implies that they were deposited in seawater that was in a redox state that inhibited the oxidation of Ce 3+ to Ce 4+ .
The redox evolution of the basal Datangpo Formation can be inferred from the Ce anomalies of the two XXB mine profiles.Figure 10 exhibits the variations in the Ce SN /Ce SN * ratio and Mn content with depth of the two XXB mine profiles.First, the Ce SN /Ce SN * ratio and Mn content showed similar tendencies in their variations.Second, all Mn ore showed obvious positive Ce anomalies (Ce SN /Ce SN * >1), which indicates an oxic depositional environment.Conversely, the black carbonaceous shale showed no Ce anomalies, which suggests an anoxic condition.Therefore, the Ce anomalies provide evidence for the alternating redox conditions of the basal Datangpo Formation.

Sulfur and total organic carbon
Marine oxic sediments usually have a low total S content because the oxidative biodegradation of organic matter is likely to occur (Xu et al. 2019).In this study, the black carbonaceous shale had a much higher total S content than the Mn ore (Supp.Table S2), which indicates that the former was deposited under the anoxic condition.In addition, the consistently low TOC/S ratio provides independent evidence that the black carbonaceous shale of the basal Datangpo Formation was deposited under the anoxic condition (Figure 11a) because such an environment facilitates sulphate reduction to produce abundant sulphides (Xu et al. 2019).However, the relatively high TOC/S ratios of the Mn ore may highlight an increase in the oxygen content of the water column during Mn deposition (Figure 11a).

Phosphorus content
The enrichment of phosphorus (P) is usually linked to redox control.Figure 11b shows that Mn ore recovered from the five Datangpo-type Mn deposits had a high P content compared to the host rock.This suggests that Mn layers may accumulate in an oxic environment, whereas the black carbonaceous shale layers were likely deposited under the anoxic condition.This is because P tends to diffuse upward from the sediment and return to the photic zone under the anoxic condition, so only 1% escapes cycling and is trapped in sedimentary rocks (Span et al. 1992;Tribovillard et al. 2006).However, in the intermittently oxic bottom water, Fe-and Mn-oxyhydroxides that scavenge PO 4 3− from sediment pore waters precipitate above the oxic -anoxic boundary, which gives P sufficient residence time to form authigenic phosphate minerals (Tribovillard et al. 2006).Therefore, the oxic depositional environment for Mn ore may have been more favourable for P enrichment than the depositional environment of the host rock.

Redox-sensitive trace elements
In this study, the black carbonaceous shale (i.e.host rock) was richer in U, V, and Mo than the Mn ore (Supp.Figures S5a and b).Moreover, V and Mo were not correlated with Al 2 O 3 in the black carbonaceous shale (Supp.Figures S5a and b), which suggests their authigenic enrichments took place in a reducing water environment.This is because U, V, and Mo behave conservatively in oxic seawater and are predominantly present in stable and soluble forms.Conversely, under the reducing condition, these elements tend to be removed from the aqueous phase and enrich sediments through reduction reactions and adsorption or precipitation as insoluble species (Algeo and Maynard 2004;Och and Shields-Zhou 2012).Previous studies have suggested that the V/ Cr and V/(V + Ni) ratios can be used to reconstruct the redox state during deposition (Tribovillard et al. 2006).
Our results Figure 12(a, b) indicated that black carbonaceous shale accumulated in a dysoxic -anoxic stratified water column and even under a sulfidic condition.This can also be explained by the high total S content and strong Mo enrichment of the black carbonaceous shale.This is because shallowly buried Mo diffuses upward into seawater, except when it is immobilized as a particlereactive component by H 2 S/HS − under a strongly anoxic condition (Bostick et al. 2003;Elbaz-Poulichet et al. 2005).
However, the Mn ore demonstrated different features from the black carbonaceous shale.The Mn ore was deficient in U and V and had a moderate Mo content compared to PAAS (Figure 8a).Moreover, U-Al 2 O 3 and V-Al 2 O 3 had a positive correlation in the Mn ore, which indicates that the U and V contents of the Mn ore mainly came from terrigenous clastics (Supp.Figures S5a and b).This may point to an increase in oxygen content in water during Mn deposition.The profiles of U, V, and Mo from the two XXB mine profiles were used to improve understanding of the variation in redox conditions of the basal Datangpo Formation.
The enrichment or depletion of U, V, and Mo in the Mn ore and black shale of the two XXB mine profiles (Figure 13) was similar to the above results for the five Datangpo-type Mn deposits.First, the Mn ore had lower U and V contents than the host rock.Second, although Mn oxides can absorb Mo, the Mn ore had a lower Mo content than the host rock, which can be attributed to the increased S content of the host rock.These results also support the precipitation of Mn layers under a more oxic condition compared with the host rock.The V/(V+Ni) and V/Cr ratios also indicate that the interbedded black shale of the XXB mine profiles was deposited in a dysoxic -anoxic environment Figure 12(c, d).Therefore, the profiles of redox-sensitive elements such as U, V, and Mo support the alternating redox conditions.
Overall, the positive Ce anomalies, relatively low S and high P contents, depletion of U and V, and enrichment of Mo indicate that the Mn layers in the basal Datangpo Formation were deposited under the oxic condition.The recovery of microorganisms in the aftermath of the Sturtian glaciation boosted primary productivity and enhanced the O 2 levels of the atmosphere and shallow hydrosphere layer in the deep-anoxic Nanhua Rift Basin.The Mn ore can be attributed to Mn 4  + -dominated oxides deposited in an oxic environment rather than Mn enrichment of carbonates in a reducing environment.The alternation of Mn-rich layers and black carbonaceous shale can be explained by alternating redox conditions within the Nanhua Rift Basin.

Depositional process for manganese ore deposits in basal Datangpo Formation
The formation of sedimentary Mn deposits following the evolution of Earth's systems is closely related to the redox-stratified aquatic system (Roy 2006).The enrichment of Mn indicates a transition from anoxic to oxic water.Conventionally, the behaviour of Mn in restricted marine basins suggests a general model in which Mn undergoes a dynamic vertical geochemical cycle, where the solid Mn 4+ phase precipitates at or above the Mn 2 + /Mn 4+ redoxcline and redissolves in deeper anoxic waters without substrates such as a shelf or seafloor (Frakes and Bolton 1984;Maynard 2003).Thus, Mn enrichment has a critical depth that creates a 'bathtub ring' effect around the margins of a basin (Force and Maynard 1991;Maynard 2003;Roy 2006).Alternatively, Mn accumulates when reactive Mn 2+ migrates from deep water into the midwater column, where the upper boundary of the oceanic oxygen minimum zone intersects with the seafloor (Maynard 2010).However, field studies have revealed that Mn deposits in the basal Datangpo Formation are strictly confined to the centres of sub-basins in the Nanhua Rift Basin rather than on the basin margins (Yu et al. 2016;Zhou et al. 2019).Two modern cases that have exhibited similar scenarios are the Holocene Mn-rich sediments of the Gotland Deep in the central Baltic Sea (Huckriede and Meischner 1996) and the interbedded near-surface Mn-rich layers offshore the Beringian shelf in the Bering Sea (Gardner et al. 1982).The exchange of Mn 2+ -rich anoxic deep water with oxic water is driven by various mechanisms.However, the Mn deposits in the Datangpo Formation are proposed to have originated from the intermittent downwelling of cold, dense, and oxygen-rich surface water exchanged with anoxic bottom water during the tortuous Sturtian glacial -interglacial transition, which precipitated precursor Mn oxides at the centres of the sub-basins (Yu et al. 2016).
The alternating Mn-rich and non-Mn sedimentary layers discussed in Section 5.2 may indicate sporadic water exchanges.A swift shift from anoxic to oxic water would result in the low Y-Ho fractionation and small Ce anomalies observed in the Mn ore (Supp.Table S4).The Mn ore displayed a negative correlation between the (Y/ Ho) SN ratio and Mn content (Supp.Figure S6), which suggests that the precipitation of precursor Mn oxides may have selectively scavenged Ho.However, the residence time of the Mn oxides in the water column was probably too brief to cause significant Y-Ho fractionation.The consistent geochemical behaviour of REY during carbonate diagenesis implies that Y-Ho fractionation is less prone to post-depositional diagenesis (Banner et al. 1988).The deposition sites of the Mn ore were shallow at ~100 m (Supp.Figure S1), which also indicates a decreased residence time for the Mn oxides.The Mn ore was inserted into a ternary to determine whether it originated from hydrothermal Fe -Mn deposition or diagenetic deposition Figure 14(a, b).The precipitation of precursor Mn oxides does not appear to have scavenged considerable amounts of trace elements from the ambient seawater, which may be attributed to the rapid depositional rate and/or the shallow water depth at the deposition sites.Additionally, the trace elements incorporated into Mn oxides by absorption may have diffused into the bottom of the water column, which can be attributed to the diagenetic conversion of the precursor Mn oxides to Mn carbonates, which would also contribute to the decreased occurrence of Cu + Co + Ni contents Figure 14(a, b).Nevertheless, some Mn deposits may still retain the signatures of Mn oxide scavenging behaviour on Co, Cu, and Ni as in the XS and XXB Mn deposits (Figure 14(c-h)).

Seawater bicarbonate
Our results indicate that Mn carbonate ore might not have directly precipitated from the seawater column by the combination of Mn 2+ and HCO 3 − .First, the oxic environment of seawater (see Section 5.2) does not support the reaction of Mn 2+ and HCO 3 − constrained by the reducing condition.Second, the δ 13 C carb values of the Mn ore exhibited an obvious positive correlation with the Mn content (Figure 15(a, c)), which implies that the other carbonate minerals adjacent to Mn carbonates had less significant depletion of heavy carbon.For example, Dekov et al. (2020) explored an Oligocene Mn deposit at Obrochishte, Bulgaria, and found no correlation between the δ 13 C carb and Mn content, which indicated that some of the Mn carbonates precipitated directly from seawater through the reaction of dissolved inorganic species (e.g.Mn 2+ and HCO 3 − ).The negative correlation seems more conspicuous when we exclude the data represented by a red circle in Figure 15a.However, this does not mean that the Mn carbonate phase precipitated directly from seawater, as the five different Mn deposits were sampled randomly.Therefore, the direct precipitation of Mn carbonates from seawater is difficult to explain from the above results.We collected considerable data from several sedimentary Mn carbonate deposits in the geological record (Figure 16) whose metallogenesis followed the mechanism of Mn 2+ oxidizing to Mn oxides and precursor Mn oxides reducing to Mn carbonates.They showed a similar REY PAAS pattern to that of the Mn ore (Figure 16), which implies that the metallogenesis of the Mn deposits in the Datangpo Formation had a mechanism similar to that of the selected Mn carbonate deposits.The characteristics of these Mn carbonate deposits resemble those of hydrogenetic (or hydrogenous) crusts/nodules, which indicate precursor Mn oxides.

Diagenetic degradation of organic matter
Organic carbon is typically light (−20‰ to − 30‰) (Okita et al. 1988;Liu 1990), and it has been considered a potential carbon source for Mn carbonate formation.A review of the geological settings and depositional features of most sedimentary Mn carbonate deposits in geological records (Table 1), including those in the basal Datangpo Formation, demonstrates that they are associated with organic-rich black shale.This is consistent with the recovery and diversification of microbial communities and increased organic carbon burial in the aftermath of the Sturtian glaciation (Piper and Calvert 2009;Och and Shields-Zhou 2012;Yu et al. 2020).Meanwhile, the high flux of organic matter may also profit from the restricted stratified basin.During the early diagenesis of organic matter, the thermodynamic energy yield from the oxidation of sedimentary organic matter coupled with various oxidants (i.e.electron acceptors) decreases in the following order: O 2 > NO 3− > Mn 4+ > Fe 3+ > SO 4 2− > CO 2 .
These oxidants are used in the above sequence (Reeburgh 2007).The reduction of Mn oxides coupled with organic matter oxidation can release CO 2 from the organic matter, which causes negative δ 13 C values and Mn carbonate mineralization.This process can be expressed as follows (Froelich et al. 1979 Bicarbonate generated from the reduction of sulphates combined with the oxidation of organic matter under the anoxic condition can develop carbonate minerals with δ 13 C values of − 14‰ to − 18‰ (Froelich et al. 1979;Okita et al. 1988).However, this was only a minor carbon source for the Mn ore in this study.First, significant sulphate reduction in Mn ore may correspond to a higher abundance of pyrite (Okita et al. 1988).The S content in our Mn ore exhibited a positive correlation with Fe 2 O 3 and no correlation with Al 2 O 3 Figure 17(a, b), which indicates that Fe was present as authigenic pyrite (FeS 2 ).Second, the average S content (0.89 wt.‰) in our Mn ore was much lower than that in the manganiferous shale (2.46 wt‰) and black carbonaceous shale (3.80 wt ‰) (Supp.Table S2) and exhibited no correlation with the Mn content (Figure 17c).Third, pyrite was scattered in the Mn ore and showed an affinity with terrigenous clastics Figure 6(c, e).Therefore, sulphate reduction was likely contraindicated during MnCO 3 formation.However, if the sediment is poised an oxidation state too high for sulphate reduction, no pyrite forms, and there is no diffusive flux of S into the sediment.The rapid deposition of Mn oxide provides such a poised system.Furthermore, if sufficient MnO 2 exists in the sediment, the FeS produced by sulphate reduction oxidizes to Fe oxide and sulphate (Okita and ShanksIii 1992).Thus, sulphate reduction does not produce sulphide minerals until MnO 2 is exhausted.By the time the MnO 2 is exhausted, the burial is too deep for sulphate to diffuse from seawater, and no pyrite can form.Moreover, Neoproterozoic seawater has been proven to have a low sulphate content of <2 mM compared with 28 mM in modern seawater (Sim et al. 2011;Loyd et al. 2012).The extremely low seawater SO 4 2-content has been evidenced by the superheavy δ 34 S in pyrite in Mn ores (Sim et al. 2011).Thus, the minor amounts of pyrite and superheavy δ 34 S suggest that organically derived CO 2 was more likely produced by the reduction of Mn oxides rather than sulphates.Notably, although the δ 13 C carb values in the Mn ore were negative, they are much heavier than the organicderived δ 13 C values of − 34.0‰ to − 31.90‰(Supp.Table S8).This indicates an additional carbon source with heavy δ 13 C.Moreover, the δ 13 C carb values of the   Mamatwan, South Africa Paleoproterozoic (2.64-1.9Ga) Cratonic shelf; Mn oxide and Mn-carbonate ore interbedded with BIF −13.9‰ to − 1.8‰ Kuleshov (2003) Mn ore were much more homogeneous (within −10‰ to − 7‰) than those of other Mn carbonate deposits throughout geological history (Table 1).The two Mn ore samples collected the DTP and XXB Mn deposits (i.e.DTP m and XXB m , respectively) (Figure 5) indicated that the δ 13 C values were highly homogeneous with very small standard deviations (Supp.Table 10).Therefore, seawater may have supplied a considerable amount of bicarbonate for Mn carbonate formation.Mn carbonate minerals may not have precipitated in the enclosed sediment pores because the δ 13 C DIC derived from the decomposing organic matter in the sediment pore water varied from − 25.2 to − 19.4‰ (Fry 1986;Havig et al. 2018;Zhang et al. 2020).Thus, Mn carbonates probably precipitated in an open early diagenetic pore water system (e.g. the uppermost sediments, sediments near the water -sediment interface) where seawater significantly contributed to bicarbonate reservoirs.Some studies (Irwin et al. 1977;Fan et al. 1999) have proposed that fermentation can impose extensive carbon fractionation between the bacterial methane δ 13 C value of − 75‰ PDB and the carbon dioxide δ 13 C value of + 15‰ PDB, which may have been a potential 13 C-rich bicarbonate source for Mn carbonate mineralization.We detected no Mn oxide grains in the Mn ore, which may indicate that the organic matter was superfluous and that the precursor Mn oxides did not survive early diagenesis.However, age-equivalent Mn deposits from the Neoproterozoic in Namibia and Brazil were preserved as Mn oxides rather than Mn carbonates, which is probably because there were no organic-rich sediments in the Mn deposition areas.The genetic interpretation of the spherulite structure of rhodochrosite (i.e.whether or not it replaced the precursor Mn oxides) is beyond the scope of this paper.However, it is an intriguing topic, and it requires special and further focus.

Role of microbial activity in manganese metallogenesis
We observed what we suspect are mineralized filamentous bacterial forms in the Mn ore (Figure 6(g, h)), which implies that microbial activity may have played a somewhat cooperative role in the metallogenesis.First, microbially mediated Mn fixation is considered a crucial mechanism for Mn accumulation in marine sediments.Under the oxidative condition, Mn 2+ oxidation by aerobic microbes in an aquatic environment starts with the activation of the multi-copper oxidase enzymatic process through autotrophic activity (Polgári et al. 2012;Yu et al. 2019).Microbial fossils, biomarker data, and microbially produced microtextures indicative of microbial activity have been reported frequently in some Mn carbonate deposits (e.g.Polgári et al. 2012;Yu et al. 2019).The filamentous mineralized bacteria discovered by Polgári et al. (2012) are similar to those preserved in our Mn ore Figure 6(g, h).Second, the negative δ 13 C carb values in our Mn ore are evidence of microbial activity participating in metallogenesis because microbes preferentially utilize light carbon isotopes.However, determining the nature of the products of the precursory microbial cycle is a challenge because the Mn carbonate deposits would have been severely overprinted by the secondary microbial process.1996).Note that the REY SN pattern of each Mn carbonate deposit was based on the average values calculated from the collected data.

Model of metallogenesis of Mn deposits in the Datangpo Formation
The break-up of Rodinia led to the formation of a series of rifted subbasins, suitable sites for the formation of Mn deposits in the Nanhua Rift Basin.The recovery of microorganisms following the Sturtian glaciation led to an increase in marine primary productivity, leading to elevated levels of atmospheric oxygen.However, the Sturtian deglaciation might not have occurred abruptly.The formation of additional sea ice caused surface water to become denser due to brine rejection, possibly contributing to the sporadic exchange of oxic surface water with anoxic bottom water enriched with dissolved Mn 2+ from hydrothermal sources (Figure 18).Initially, Mn 2+ was precipitated in the form of Mn oxides, which later transformed into Mn carbonates through coupling with early organic matter diagenesis in an open early diagenetic pore water system (Figure 18).The repeated redox exchanges in the bottom basin resulted in the alternating distribution of Mn layers and black shales.

Conclusions
The interplay of the evolution of Rodinia, climatic turbulence during the Sturtian glacial -interglacial transition, and atmospheric oxygen rise contributed to the formation of economically significant Neoproterozoic Mn carbonate deposits in the Cryogenian Datangpo Formation of the Nanhua Rift Basin at the southeastern margin of the Yangtze Craton, South China.Rodinia's assembly and break-up generated a series of Mn-hosting sub-basins in the Nanhua Rift Basin.We concluded that the Mn mainly originated from hydrothermal processes, where the anoxic water column in the stratified Nanhua Rift Basin during Sturtian glaciation may have inherited positive Eu anomalies due to the long-distance transport of Eu 2+ and Mn 2+ from marine hydrothermal vents.The Mnrich layers in the basal Datangpo Formation are interbedded with the host rock.We concluded that the Mnrich layers precipitated under the oxic condition, whereas the host rock layers were deposited in dysoxicanoxic or even sulfidic water.Based on the interbedded  distribution of Mn-rich and non-Mn sedimentary intervals, we concluded that the redox conditions of the depositional environment were alternating.The Mn carbonate deposits showed low degree of positive Ce anomalies and Y -Ho fractionation, which implies a fast deposition rate of precursor Mn oxides that can be attributed to the rapid redox transition and/or shallowness of the deposition site.These conditions may have also reduced the residence time required to significantly scavenge some redox-sensitive elements such as Co, Cu, and Ni.The coupling of Mn oxide reduction and organic matter oxidation provided the primary metallogenic materials for the formation of Mn carbonates.Because of the much lighter and approximately homogeneous δ 13 C carb values of the Mn ore compared to the organic-derived δ 13 C values of − 34.0‰ to − 31.90‰,we concluded that Mn carbonates may have precipitated in a rather open early diagenetic pore water system where seawater may have supplied a considerable amount of bicarbonate.

Figure 2 .
Figure 2. (a) Detailed geological schematic of the Wuling Rift Basins and the distribution of Datangpo-type Mn ore deposits.(b) Stratigraphic section showing the position of the Mn bed (References: (a) was modified from Zhou et al. (2019); (b) was summarized and modified from(Zhang et al. 2008;Yu et al. 2016Yu et al. , 2017;;Wang et al. 2019), andZhou et al. (2019).

Figure 3 .
Figure 3. (a-c) Field photographs from an underground mine: (a) Mn carbonate ore body and (b) boundary (i.e.red line) between the ore body and black carbonaceous shale layer.(c) Profile of black carbonaceous shale in the underground mine.This shale layer contains abundant pyrite.(d) Black carbonaceous shale sample (XXB6).(e) Massive Mn ore sample (XS5).(f) Massive Mn ore sample with round cavity structures (DTP9).(g) Laminated Mn ore sample (ZLG6).

Figure 4 .
Figure 4. Samples collected at 10-cm intervals from the first member of the Datangpo Formation from the two Xixibao mine profiles.

Figure 5 .
Figure 5. Photos of Mn ore collected at sampling points by microdrilling from the (a) Datangpo mine section (DTPm) and (b) Xixibao mine section (XXBm).

Figure 7 .
Figure 7. Correlation diagrams for several major elements in the Datangpo-type Mn ore and host rock (manganiferous shale and black carbonaceous shale).(Please Note: The red circles represent Mn ore, the blue squares represent manganiferous shale, and the black triangles represent black carbonaceous shale.The legends are introduced in Figure 6 7a.These tags are also applicable to the Datangpo-type Mn ore and host rock in subsequent figures).

Figure 9 .
Figure 9. Diagrams of (a) Mn/Ti versus Al/(Al + Fe + Mn) (from Boström 1983) and (b) SiO 2 versus Al 2 O 3 (from Xiao et al. 2017) of the Datangpo-type Mn ore and host rock; (c) Eu SN /Eu SN * versus Mn content of the Datangpo-type Mn ore and host rock and (d) all samples of the XXB mine profiles.

Figure 10 .
Figure 10.(a and b) Variations in the Ce SN /Ce SN * ratio and Mn content over the range of sampling depths for the two XXB mine profiles.

Figure 11 .
Figure 11.Diagrams of the (a) TOC/S ratio and (b) P 2 O 5 content of the Mn ore and host rock collected from the Datangpo-type Mn ore deposits.

Figure 12 .
Figure 12.Diagrams of (a) V/(V + Ni) and (b) V/Cr illustrating the paleo-redox conditions of black carbonaceous shale collected from Datangpo-type Mn ore deposits.Diagrams of (c) V/(V + Ni) and (d) V/Cr illustrating the paleo-redox conditions of the black carbonaceous shale collected from the XXB mine profiles (from Hatch and Leventhal 1992; Jones and Manning 1994).

Figure 13 .
Figure 13.Variations in the U, V, Mo, S, and Mn contents over the range of sampling depths for the XXB mine profiles.

Figure 14 .
Figure 14.Discrimination schemes of (a) Fe-Mn-Co (Boström 1983) and (b) Fe-Mn-(Cu + Co + Ni) (Josso et al. 2017) of the Mn ore collected from the Datangpo-type Mn ore deposits.Diagrams of Mn EF versus (c) Co EF , (d) Ni EF , and (e) Cu EF of Mn ore collected from the Xiushan Mn ore deposit.Diagrams of Mn EF versus (f) Co EF , (g) Ni EF , and (g) Cu EF of the XXB mine profiles.

Figure 15 .
Figure 15.Diagrams of the (a) Mn content versus δ C carb and (b) δ 13 C carb versus δ 18 O carb of the Datangpo-type Mn ore.Diagrams of the (c) Mn content versus δ 13 C carb and (d) δ 13 C carb versus δ 18 O carb of the XXB mine profiles.

Figure 16 .
Figure 16.Examples of typical REY SN patterns of a marine hydrothermal Fe-Mn deposit (data from Bau et al. 2014), hydrogenetic Fe-Mn crust (data from Bau et al. 2014), hydrogenetic-diagenetic nodule (data from Bau et al. 2014), diagenetic Fe-Mn nodule (data from Bau et al. 2014), high-T hydrothermal fluid (data from Bau and Dulski 1999), low-T hydrothermal fluid (data from Bau and Dulski 1999), and seawater (data from Alibo et al. 1999).Examples of REY SN patterns of some selected Mn carbonate deposits from Morro da Mina (data from Cabral et al. 2019), Adilabad (data from Gutzmer and Beukes 1998), Ortokarnash (data from Zhang et al. 2020), and Taojiang (data from Zhu1996).Note that the REY SN pattern of each Mn carbonate deposit was based on the average values calculated from the collected data.

Figure 17 .
Figure 17.Correlation diagrams for the S content with the Fe, Al, and Mn contents of the Datangpo-type Mn ore.

Figure 18 .
Figure 18.Model of metallogenesis of Mn deposits in the Datangpo Formation.

Table 1 .
Age distribution and carbon isotopes of economically important sedimentary Mn carbonate ore deposits around the world.