Multiple proxies indicating methane seepage as the origin of Devonian large barite deposit in Zhenning-Ziyun, Guizhou, SW China

Ore Geology Reviews 80 (2017) 18–26

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Multiple proxies indicating methane seepage as the origin of Devonian large barite deposit in Zhenning-Ziyun, Guizhou, SW China Junbo Gao a, Ruidong Yang a,⁎, Jun Chen a, Lulin Zheng a, Wei Cheng b, Huairui Wei a a b

College of Resources and Environmental Engineering, Guizhou University, 550025 Guiyang, Guizhou Province, PR China Mining College of Guizhou University, 550025 Guiyang, Guizhou Province, PR China

a r t i c l e

i n f o

Article history: Received 7 October 2015 Received in revised form 21 June 2016 Accepted 22 June 2016 Available online 23 June 2016 Keywords: Bedded barite deposits Isotopic composition Sedimentary characteristics Methane seepage

a b s t r a c t Bedded barite deposits are found in Cambrian and Devonian strata in China. In this paper, we report the occurrence of Late Paleozoic methane seeps from bedded barite in one of the largest barite deposits in China. The ore bodies are stratiform and lenticular in shape and are hosted within Upper Devonian cherts. The barite deposits usually have laminar, banded, fragmental, massive and rosette textures, and radial and spheroidal structures were observed in the barite ores. Nodular limestones are associated with the barite deposits; they consist of clotted micrites and framboidal pyrites. The internal structure of framboidal pyrite is simple, in which the sun-flower structure has been observed, similar to modern seep carbonates. The δ13CV-PDB values range from − 8.82‰ to − 10.27‰, and δ18OV-PDB ranges from − 8.10‰ to − 8.20‰ in Xiyahe and from + 0.03‰ to −3.31‰ in Luocheng. The 13C depletion suggests that these carbonates formed in a methane seep through the anaerobic oxidation of methane (AOM). This evidence lends strong support to the interpretation that methane hydrate contributed to the barite deposition. In addition, the δ13CV-PDB values decrease gradually from Xiyahe to Luocheng, which indicates the gradually weakening strength of the methane seep activities. The δ30SiNBS-28 values of the cherts range from − 0.3‰ to + 0.5‰, except for two samples (LJ-24: δ30SiNBS-28 values = +0.9‰; ZL18: δ30SiNBS-28 values = +1.1‰). The lowest δ30SiNBS-28 values reflect silicon derived from hydrothermal fluids ascribed to intense tectonic activity, which consequently caused the rapid maturity of hydrocarbons. The 87Sr/86Sr values (87Sr/86Sr = 0.70863–0.70898, avg. 0.70877; n = 11) of barite are higher than the average value of contemporary seawater, showing that hydrothermal processes played a minor role in barite mineralization. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The generation of bedded barite deposits has been the focus of geological studies for many years. There are three main theories explaining the genesis of stratiform barite deposits, namely: formation of sedimentary hydrothermal exhalative deposits (Emsbo and Johnson, 2004; Fang et al., 2002; Luo et al., 2012; Maynard and Okita, 1991; Nuelle and Shelton, 1986; Tu, 1987; Wang and Li, 1991; Xia et al., 2004), combination of hydrothermal exhalative sedimentation and biogenic processes (Clark et al., 2004; Jewell, 1994; Karunakaran, 1976), and cold seepage generation (Canet et al., 2014; Paytan et al., 2002; Torres et al., 2003). Torres et al. (2003) suggested that bedded barite deposits hosted within Paleozoic organic-rich and chert-rich sediments are closely related to submarine methane seeps. The main evidence for this viewpoint is

⁎ Corresponding author at: Graduate school of Guizhou University, Huaxi District, Guiyang 550025, Guizhou Province, PR China. E-mail address: [email protected] (R. Yang).

http://dx.doi.org/10.1016/j.oregeorev.2016.06.020 0169-1368/© 2016 Elsevier B.V. All rights reserved.

that bedded barite deposits generally lack sulfide mineralization, and that they have 87Sr/86Sr values higher than coeval seawater. Canet et al. (2014) studied the Mazatán bedded barite deposits in the Upper Carboniferous in Sonora (northwestern Mexico). They found that the carbonate horizons depleted in 13C, with δ13CV-PDB from − 24.3‰ to − 18.7‰, and 87Sr/86Sr in barite higher than that for coeval seawater. These authors concluded that the barite deposits had been formed as a result of submarine methane seepage. The Qinling and Jiangnan regions in the northern and southern parts of the Yangtze Platform, respectively, are characterized by numerous and important barite deposits (Fig. 1A) (Wang and Li, 1991). The barite deposits in Guizhou Province constitute an important part of the Yangtze Platform barite mineralization zone, where barite occurs in the Early Cambrian and Upper Devonian series. The super large barite deposits occurring in the Lower Cambrian black shales of the Dahebian region are characterized by a stratiform structure, and have reserves of more than 100 Mt, the highest in the world. Published studies suggest a sedimentary-exhalative origin for barite mineralization (Fang et al., 2002; Maynard and Okita, 1991; Tu, 1987; Xia et al., 2004; Yang et al., 2007, 2008).

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Fig. 1. Geological setting and geologic map of the large Devonian barite deposits in Zhenning-Ziyun, Guizhou Province, China. (A) Location of sediment-hosted stratiform barite and witherite deposits in South China (modified after Wang and Li (1991)). (B) Paleogeographic reconstruction of the research area during the late Devonian (modified after Chen et al. (2001)). (C) The regional distribution of the barite deposit.

The barite deposits studied here are located in the Zhenning-Ziyun county, southern Guizhou, and are hosted within Upper Devonian, chert-rich sedimentary series. The deposits have a reserve in excess of 33.02 Mt, which makes them the second largest concentration of bedded barite in the Guizhou province. Gao et al. (2012, 2013a, 2013b) studied the sedimentological and geochemical features of these deposits, and suggested that they are formed by submarine sedimentary exhalative processes. But the study and genetic aspects of the nodular limestones recently discovered at Xiyahe and Luocheng have long been ignored. Based on the previous research results regarding the barite deposit combined with the information known so far, it can be inferred that the formation of these nodular limestones may be related to the leakage of methane. A detailed study of the genesis and formation mechanism of these limestones may change our understanding of the Zhenning-Ziyun Devonian barite deposit, and be significant to expanding and improving the research on ancient methane seeps during Late Paleozoic. In this paper, we report a series of analyses on the sedimentary and lithological characteristics, along with isotope systematics, including

the strontium isotopic compositions of the barite, silicon isotopes of the chert, and carbon and oxygen isotope characteristics of the nodular limestone associated with barite deposits in different outcrops of Zhenning-Ziyun county, Guizhou. On this basis, we try to establish an ideal deposit model. This work is important for further understanding of the genesis, metallogenic mechanism and controlling factors for the Chinese Paleozoic stratiform (bedded) barite deposits, and also it could support the methane-seepage mineralization general theory for bedded barite deposits. 2. Geological characteristics of Zhenning-Ziyun barite deposits 2.1. Geology setting The large Devonian barite deposits of Zhenning-Ziyun are located on the southeastern margin of the Yangtze platform, a transitional zone between that platform and the Youjiang Orogenic Belts. The output and distribution of ore bodies are clearly controlled by the Shuicheng-

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Ziyun-Nandan fault, which was formed during the Late Palaeozoic; it is a NW-SE deep fault that cuts into the basement and even into the upper mantle (Mao et al., 1997). The fault zone is representative of faults formed in the Late Caledonian to the Hercynian, and it controls the sedimentary facies and thickness of the Silurian, Devonian and Carboniferous series. It is also a tectonic boundary along the northern margin of the Youjiang Orogenic Belt (Fig. 1B). During the Early to Middle Devonian, the Shuicheng–Ziyun–Nandan rift basin, including the entire Youjiang Orogenic Belts, is characterized by an extension and a faulted depression stage. This resulted in the gradual opening of the Paleo-Tethys ocean (Hara et al. 2010). The Late Devonian strata in the Southern Guizhou and Northern Guangxi area are mainly associated with the basin, isolated carbonate platform, slope and platform margin reef and bank facies (Guizhou Bureau of Geology and Mineral Resources, 1987). Due to the continuing transgression during the early Late Devonian, the spatial distribution of the basin facies was gradually expanded, replacing part of the original carbonate platform. The results show that the basin facies was more concentrated along the Shuicheng-Ziyun-Nandan fault which is distributed in the NW-SE direction. The Zhenning-Ziyun barite deposits hosted in the cherts were mainly formed in the Late Devonian Frasnian. In this period, the cherts and limestones formed at depth in the marine basin were widely deposited in the Ziyun area. These bedded barite deposits formed under deep-water platform trenches, controlled by the Late Palaeozoic tectonic depression of Shuicheng–Ziyun–Nandan. The barite ore bodies are stratiform to lensoid in shape, and they extend for 40 km in the NW-SE direction from Leji in Zhenning to Luocheng in Ziyun (Fig. 1C). The host rocks are mainly black or gray-to-black cherts. The barite ores have a relatively simple mineral assemblage, consisting of barite, some micro-fine pyrites and clay minerals. Lead and zinc are not present

in the barite ores. These features are consistent with the “continental margin” type of barite deposits of Maynard and Okita (1991). 2.2. Host rocks and barite ore bodies The Zhenning-Ziyun area is typical for one of the regions where the Devonian strata are relatively well developed in Guizhou. The barite deposits are hosted in the Upper Devonian Xiangshuidong Formation. The combination of rock types of this formation in Ziyun mainly consists of deposits of intercalated layers of dark gray micritic limestone and thin-medium layers alternated with cherts, with clear rhythmicity in the middle to upper part, as well as and thin-medium gray black cherts with limestones and clay rocks in the lower part, with a total thickness of 319 m. The barite ore bodies are stratiform to lensoid in shape, and they are distributed throughout the NW-SE direction from Leji to Luocheng (Fig. 2). The host rocks are mainly composed of black or gray-black thin-medium layers of chert (Fig. 3A) (layers ranging from 5 to 15 cm), which is mainly composed of microcrystalline quartz. Siliceous organisms (e.g., radiolarians) from some chert samples have been found. In the Leji section, the thickness of the barite ore bodies ranges between 9 and 10 m, including laminar (Fig. 3B), fragmental (Fig. 3C and D), banded (Fig. 3E) and massive structures (Fig. 3F). Moreover, this paper reports the discovery of rosette-shaped barites (approximately 1–3 cm in size) for the first time in the Leji section (Fig. 3G and H). These barite rosettes have features that are similar to barites occurring in modern cold seepages (Aloisi et al., 2004; Torres et al., 2003) and in Neoproterozoic cap carbonates (Wang et al., 2008; Zhou et al., 2010). In contrast, the barite in Mohao shows banded, lamellar and massive structures, whereas that in Luocheng exhibits only massive and laminar structures. The thickness of the barite beds reaches 4 m, and the beds

Fig. 2. Lithostratigraphic columns of large Devonian barite deposits in Zhenning-Ziyun County, Guizhou Province, China.

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Fig. 3. Photographs of the representative sediment structures of the large Devonian barite deposit of Zhenning-Ziyun County, Guizhou Province, China. (A) Banded cherts within the barite deposit. (B) Laminar barite. (C) and (D) Fragmental barite. (E) Banded barite. (F) Massive barite. (G) and (H) Rosette barite. (I) Outcrop characteristic of nodular limestones. (J) The nodular limestone shows as banded nodular limestone. (K) Nodular limestones in various sizes (about 5–20 cm in diameter) from Xiyahe section. (L) The bedding of surrounding rocks (chert and siliceous mudstone) around the nodules limestone.

laterally extend up to 20 m, thinning down sharply and wedging out (Fig. 2). A nodular limestone occurs interbedded within the barite layer and the siliceous rock formation in Xiyahe and Luocheng (Fig. 3I) with a width of ca. 0.5 m (Fig. 3J). These limestones are of nodular types, and are 5 cm to over 70 cm in diameter (Fig. 3K and L). According to the detailed field observations and sedimentary structures, it can be seen that the bedding of surrounding rocks (chert and siliceous mudstone) are around the limestone nodules, indicating syndepositional characteristics similar to Late Cretaceous seep carbonates in Tibet, China (Tong and Chen, 2012), and 13C-depleted carbonates hosted by bedded barite in Sonora, Northwest Mexico (Canet et al., 2014). The mineralogical features of the barite ores have been further studied by using a scanning electron microscope, on barites and minor pyrites. Schistose-like (Fig. 4A), rosette or radial (Fig. 4B) and spheroidal

barite (Fig. 4C) occur in the barite ores at Zhenning-Ziyun, which are similar to the Ordovician-Devonian barite deposits from Nevada, USA (Shawe et al., 1969), the Lower Cambrian witherite deposits from the Qinling-Daba region (Luo et al., 2012; Pi et al., 2014), and the Paleozoic bedded barite deposits from Sonora, Northwest Mexico (Canet et al., 2014). The pyrite replaced by Fe oxides, which preserve the irregular schistose (Fig. 4D) and framboid forms (Fig. 4E) of pyrite, were observed from the barite ores. Based on the component characteristics of the maceral, the limestone is composed of gray fine crystalline or microcrystalline calcite minerals, and it exhibits bright and dark laminae alternating on millimeter scales (Fig. 4F). The nodular limestones all develop clotted micrite (Fig. 4G) and framboidal pyrite (about 10 μm in diameter) (Fig. 4H). The internal structure of the framboidal pyrite is simple, which has the form of sun-flower (Fig. 4I). The clotted micrite and framboidal pyrite are abundant in the limestone; these features are

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Fig. 4. Photomicrographs of barite and nodular limestone samples. (A) Flake textured barite. (B) Radial/rose petal-like textured barite from a fragmental barite. (C) Spheroidal barite. (D) Irregular schistose Fe oxides among banded structure barite and siliceous sediments. (E) Framboids Fe oxides in siliceous sediments. (F) Nodular limestone consisting of bright and dark laminae alternated on millimeter scales. (G) Clotted micrites are observed in nodular limestone, which have diameters about 50 μm to 150 μm (arrows). (H) Framboidal pyrite crystals (arrows) scatter in nodular limestone. (I) The internal structure of framboidal pyrite is simple, which has the form of sun-flower. Brt, barite; Py, pyrite.

often reported in seep carbonates (Canet et al., 2013; Chen et al., 2006; Peckmann et al., 1999; Tong and Chen, 2012). 3. Sampling and analysis A detailed field survey was conducted on Zhenning-Ziyun Devonian barite deposits, and altogether 12 chert samples and 11 barite ore samples were collected from Leji, Mohao, and Luocheng for silicon isotopes analyses, while 6 limestone samples were collected from Xiyahe and Luocheng for carbon and oxygen isotopes analyses. Carbon and oxygen isotope analyses were carried out in the Key National Laboratory of Environmental Geochemistry of the Institute of Geochemistry under the Chinese Academy of Sciences. The instrument used was a gas-stable isotope mass spectrometer under continuous flow mode. The details of the operating procedure are as follows: (1) measure 1 mg ± 50 μg dried carbonate powder (based on CaCO3), put it into a reaction bulb, tighten the shock insulator, and put the bulb in a sample tray at a temperature of 90.0 ± 0.1 °C; (2) use a 30 ml/min helium flow for 5 min to eliminate air from the reaction bulb; (3) inject 300 μl of 100%-pure phosphoric acid into the branch duct of the reactorthe phosphoric acid and carbonate react immediately, releasing CO2 into the reaction bulb; (4) after the reaction is complete, use helium flow to purge the gas from the reaction bulb through a chromatographic separation column; and (5) guide the CO2 into the IRMS and report as δ13C and δ18O values in ‰ notation relative to the VPDB (Vienna Pee Dee Belemnite) (carbon and oxygen) standard. During the analysis, carbonate solid standards are inserted for the purpose of calibrating the measurement results. The precision of the analysis is higher than

0.15‰ for δ13C and higher than 0.20‰ for δ18O (based on standard CaCO3, with 10 parallel analysis precisions as the standard). Analyses results are shown in Table 1. Silicon isotope analyses were performed at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology (ALBRIUG). Below is the detailed test procedure: a 12 mg quartz powder sample were taken and put into a drying oven for 12 h. Then, the dried quartz powder was put into a pure nickel reaction vessel at approximately 550 °C, where the quartz reacts with BrF5 for 6 h to form O2 and SiF4. Then, the reaction residue including BrF5 and SiF4 and other products is frozen using liquid nitrogen, releasing O2. The SiF4 was collected in a liquid-nitrogen cold trap, which was put into the Drikold acetone cooling liquid again. Subsequently, the SiF4 gas was further purified by reaction with zinc granules and heated to 70 °C, yielding pure SiF4 gas. The SiF4 was collected and analyzed for silicon isotopes. Measurements of silicon isotope ratios were performed using a Finnigan MAT-253 mass

Table 1 Carbon and oxygen isotopic compositions of the lensoid limestone from the barite deposits in Zhenning-Ziyun, Guizhou Province, China. Locality

Sample No.

δ13C(VPDB, ‰)

δ18O(VPDB, ‰)

Luocheng

ZL6 ZL11 ZL13 ZL17 XYH10 XYH14

−3.31 0.08 −1.92 0.03 −10.27 −8.82

−7.91 −7.43 −7.49 −7.72 −8.10 −8.20

Xiyahe

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spectrometer. The results are expressed in δ values relative to the NBS28 standard. The analytical precisions of δ30Si and δ18O measurements were 0.1‰ (1 SD) and 0.2‰ (1 SD) (Ding et al., 1988), respectively, based on repeated measurements of standard samples. Analyses results are shown in Table 2.The Sr isotope ratios of 11 barite samples from Zhenning-Ziyun barite deposits are from Gao et al. (2013b) and shown in Table 3. 4. Results and discussion 4.1. Carbon and oxygen isotopes The limestone in Xiyahe and Luocheng were relatively depleted in C. Limestone from Xiyahe to Luocheng exhibit δ13CV-PDB values of − 8.82‰ to − 10.27‰ and + 0.08‰ to − 3.31‰, as well as δ18OV-PDB values of − 8.10‰ to −8.20‰ and − 7.43‰ to −7.91‰, respectively. The carbon isotope compositions of the nodular limestone exhibit a regular increase from Xiyahe to Luocheng, and the highest δ13CV-PDB value (up to + 0.08‰) occurs in Luocheng, which is within the range for Devonian seawater (δ 13C V-PDB values range from − 2‰ to + 8‰) (Van Geldern et al., 2006; Veizer et al., 1999), and marine sedimentary carbonates (δ13C = − 1‰ to + 2‰; average, 0‰) (Zheng et al., 1999) and is close to the carbonates of the Upper Yangtze Platform (δ13CV-PDB average 0.38‰) (Huang, 1997). Previous studies have shown that carbonates with low δ13C (− 11.0‰ to − 53.0‰) often occur in modern hydrocarbon seeps (Campbell, 2006; Canet et al., 2006, 2013; Greinert et al., 2002; Han et al., 2004; Sassen et al., 1993). A characteristic of this environment is carbonate precipitation through anaerobic oxidation of methane, which yields extremely low δ13CV-PDB values (as low as −41‰) (Cavagna et al., 1999). In addition, seep sediments of different ages, from the Cenozoic to the Proterozoic, have also been found and studied (Canet et al., 2014; Himmler et al., 2008; Jiang et al., 2003; Peckmann et al., 2007; Peckmann and Thiel, 2004; Smrzka et al., 2015; Wang et al., 2008). However, so far, the genesis of 13C-depleted carbonates within the bedded barite deposits has not received much attention because the barite to carbonate ratio depends not only on the available sulfate and barium but also has a close relationship with environmental conditions (Aloisi et al., 2004). Canet et al. (2014) discussed the genesis of Paleozoic bedded barite deposits from Sonora, in northwestern Mexico, formed in a hydrocarbon seepage environment. They found 13C-depleted carbonates (δ13CV-PDB values range from − 24.3‰ to − 18.7‰) in the barite deposits. Their observations are significant for the understanding of the genesis and metallogenetic mechanism of the formation of Paleozoic stratiform (bedded) barite deposits. In particular, the δ13CV-PDB values of the carbonates in Sonora are higher than that for the methane-derived carbonates found in the modern cold seeps (δ13CV-PDB values range from − 25‰ to − 50‰) (Canet et al., 2006, 2013; Feng and 13

Table 2 Silicon isotopic compositions of the cherts from the barite deposits in Zhenning-Ziyun, Guizhou Province, China. Locality

Sample no.

δ30SiNBS-28(‰)

Leji

LJ-0 LJ-14 LJ-24 LJ-27 ZM1 ZM5 ZM7 ZM14 ZL2 ZL5 ZL8 ZL18

0.3 −0.1 0.9 0.5 0.3 0.3 0.3 −0.1 −0.3 −0.1 0.3 1.1

Mohao

Luocheng

23

Table 3 Strontium isotope values of the barite deposits in Zhenning-Ziyun, Guizhou Province, China. Sample no.

Description

87

Sr/86Sr

Error(2σ)

Data source

LJ-26 LJ-23 LJ-19 LJ-13 LJ-12 LJ-11 LJ-10 LJ-8 LJ-5 LJ-3 LJ-1 Devonian seawater Crust

Laminar barite Banded barite Brecciated barite Massive barite Laminar barite Laminar barite Fragmental barite Laminar barite Brecciated barite Banded barite Banded barite –

0.70884 0.70877 0.70898 0.70880 0.70872 0.70869 0.70863 0.70868 0.70875 0.70886 0.70880 0.70829

0.00008 0.00003 0.00008 0.00008 0.00001 0.00001 0.00005 0.00006 0.00005 0.00005 0.00007 –

Gao et al. (2013b)

–

0.71190

–

Mantle

–

0.70350

–

Veizer and Compston (1974) Palmer and Elderfield (1985) Palmer and Edmond (1989)

Roberts, 2011; Peckmann and Thiel, 2004). Peckmann et al. (1999) discussed the carbon isotope composition of the Devonian seep-related carbonates from Morocco and reported δ13CV-PDB values ranging from −20‰ to −11‰. Tong and Chen (2012) studied the δ13CV-PDB values for the first discovery of Cretaceous seep carbonates near Xigaze, Tibet, yielding δ13CV-PDB values ranging −27.7‰ to −4.0‰. These observations suggest that seep carbonate carbon, resulting from anaerobic oxidation of methane, may be mixed with inorganic carbon dissolved in seawater (Formolo et al., 2004). Beyond that, seep methane mainly contains thermogenic (δ13CV-PDB values range from − 50‰ to − 30‰) (Sackett, 1978) and biogenic methane (δ13CV-PDB values range from −110‰ to −50‰) (Whiticar et al., 1986). The δ13CV-PDB values of limestone in Zhenning-Ziyun vary from + 0.08‰ to − 10.27‰, suggesting that the carbon source would not be sourced from biogenic methane. Additionally, oil or bitumen remains were not found in the limestones, which points to thermogenic methane as the dominant carbon source. The δ13CV-PDB values decrease gradually from Xiyahe to Luocheng due to a gradual decrease in the methane seep activity. Authigenic carbonates in the modern ocean, related to methane seeps, usually have positive δ 18 O V-PDB values (Aloisi et al., 2000; Bohrmann et al., 1998; Canet et al., 2006, 2013). However, Naehr et al. (2007) reported lower δ18 OV-PDB values for the carbonates (δ18OV-PDB = − 5.5‰) in Monterey Bay, California. In contrast, diagenetic processes related to burial diagenesis can produce negative δ18OV-PDB overprints in ancient carbonates of the Mesozoic and Paleozoic (Canet et al., 2014; Hammer et al., 2011; Himmler et al., 2008; Peckmann et al., 2001; Peckmann and Thiel, 2004; Veizer, 1983). Peckmann et al. (1999) studied the oxygen isotope composition of Paleozoic seep-related carbonates and reported δ18 OV-PDB values ranging from − 2‰ to − 12‰. Canet et al. (2014) reported the occurrence of methane-derived carbonates in Sonora, northwestern Mexico. Similar instances were found in Neoproterozoic postglacial cap carbonates (Jiang et al., 2003) and in the Late Cretaceous seep carbonates of Xigaze (Tong and Chen, 2012). Previous studies in combination with our data indicate a spatial association between the oxygen isotopic composition characteristics of limestones and late processes related to burial diagenesis. 4.2. Silicon isotopes Generally, the silicon isotopic composition of SiO2 is controlled and influenced by temperature, precipitation rate and proportion in the

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Fig. 5. Sr isotope composition of the barite ores of the large Devonian barite deposits in Zhenning-Ziyun County, Guizhou Province, China. Data source: The 87Sr/86Sr values of the ZhenningZiyun barite deposits are from Gao et al. (2013b); the 87Sr/86Sr values of continental from Palmer and Elderfield (1985); the 87Sr/86Sr values of Devonian seawater from Veizer and Compston (1974); and the 87Sr/86Sr values of mantle from Palmer and Edmond (1989).

solution (Li et al., 2014). As recorded, under low temperature, both the precipitation of SiO2 and the fractionation generated by biological activity have important influence on the silicon isotopic composition (Ding et al., 1994; Douthitt, 1982). Nevertheless, it should be noted that hydrothermal silicon and the rocks formed by its precipitation usually have negative δ30SiNBS-28 values (Chakrabarti et al., 2012; Ding et al., 1994; Fan et al., 2013; Li et al., 2014), which is of significance to identifying the source and origin of fluids. For instance, the δ30SiNBS-28 values of the silica chimneys, sulfide-bearing cherts and pure cherts from the Kuroko-type deposit in Gacun, Sichuan Province, are within the range from − 0.9‰ to 0.0‰ (Hou et al., 1996). The δ30SiNBS-28 values of the Precambrian banded iron formations (BIF) in the North China Craton range from − 2.0‰ to − 0.3‰, with an average of − 0.8‰ (Li et al., 2014), and the δ30SiNBS-28 values of the Archean bedded Si-Fe BIF in the Gongchangling Iron deposits of Liaoning range from − 0.9‰ to −2.2‰ (Jiang et al., 1993). Fan et al. (2013) conducted studies of the silicon isotopic composition of cherts of the Ediacaran-Cambrian transition interval of the Yangtze Platform and divided the 30SiNBS-28 values into three different ranges, with δ30SiNBS-28 values of − 0.3‰ to − 0.5‰, + 0.2‰ to + 0.7‰, and + 1.2‰, respectively. The negative δ30SiNBS-28 values were believed to be related to the large input of hydrothermal silicon due to intense tectonic activity, and the positive δ30SiNBS-28 values may indicate a greater influence of seawater. We selected 12 chert samples from Leji, Mohao and Luocheng and measured the silicon isotopic compositions of the cherts. Except for those of samples LJ-24 and ZL18, the δ30SiNBS-28 values of the cherts associated with the barite deposits range from −0.3‰ to +0.5‰, with an average of + 0.1‰. The lowest δ30SiNBS-28 value is similar to that of the dissolved silica in the Mammoth Thermal Spring from California and that of the Thermal Spring of Yellowstone National Park from Wyoming, USA (Douthitt, 1982), as well as the hydrothermal genesis cherts of the Yangtze Platform during the Ediacaran-Cambrian transition interval (Fan et al., 2013). According to the above analysis, we maintain that the silicon of the cherts was derived from hydrothermal fluids with negative δ30Si. Samples LJ-24 and ZL18 have higher δ30SiNBS-28 values up to +0.9‰ to +1.1‰, with an average of +1.0‰. The higher δ30SiNBS-28 group in our samples resembles the Cambrian Storm chert (30SiNBS-28 = +1.3 ± 0.3‰) in New York State, USA (Robert and Chaussidon, 2006), and those of the cherts (30SiNBS-28 = +1.2‰) at the IVth layer of the Ediacaran-Cambrian transition interval of the Yangtze Platform (Fan et al., 2013), which reflect the mixing of seawater with hydrothermal fluids (van den Boorn et al., 2007). The studies of the silicon isotopic composition suggest that the cherts of the barite deposits are a result of the interaction of hydrothermal fluid with seawater, which consequently caused the rapid maturity of hydrocarbons. This idea can provide isotopic

evidence for understanding thermmogenic methane as the dominant carbon source of nodular limestones. 4.3. Strontium isotopes The strontium isotope composition of barite provides a tracer for the source of Sr and Ba, including the formation process of the barite deposits (Clark et al., 2004; Maynard and Okita, 1991; Paytan et al., 2002; Wang and Chu, 1994; Xia et al., 2004). The 87Sr/86Sr ratio of seep-related barites reflects the interaction of fluids with various types of rock and sediment before seeping at the seafloor (Naehr et al., 2000). Regarding the 11 barite samples, the 87Sr/86Sr ratio ranges from 0.70863 to 0.70898, with an average of 0.70877 (Gao et al., 2013b). These 87Sr/86Sr values are higher than those of Devonian seawater (with an average of 0.70829) (Veizer and Compston, 1974), but significantly lower than those of Earth's crust (0.71190) (Palmer and Elderfield, 1985) (Fig. 5). The strontium isotope characteristics are very similar to the barite deposits formed by cold seepage of methane (Canet et al., 2014; Torres et al., 2003), but they are clearly different from the barite deposits formed by submarine hydrothermal venting (Liu et al., 2014; Paytan et al., 2002; Wang and Chu, 1994; Xia et al., 2004), indicating that hydrothermal activity could not have played an important role in the formation of the Zhenning-Ziyun barite deposits. Additionally, it is worth noting that the positive 87Sr/86Sr values of barite could be interpreted as the mixed origin for the ore-forming fluid, i.e., the continental crust and/or coeval seawater (Feng and Roberts, 2011; Hanor, 2000; Liu et al., 2014; Paytan et al., 2002). According to these results, to our knowledge, the more feasible explanation for the strontium isotope characteristics of the Zhenning-Ziyun barite deposits is that they are related to methane seepage. 5. Conclusions (1) The barite bodies are mainly layered and stratiform, and they are hosted in the Upper Devonian cherts. The barite bodies have laminar, fragmental, banded and massive structures. In addition, barite from radial and rosette texture aggregates occur as clasts in the fragmental zones of the deposit. (2) The nodular limestones have negative δ13CV-PDB values ranging from −10.27‰ to +0.08‰, suggesting thermmogenic methane as the dominant carbon source, due to the anaerobic oxidation of methane (AOM). The δ13CV-PDB values decrease gradually from Xiyahe to Luocheng, due to a gradual decrease in the methane seep activity. (3) The negative δ30SiNBS-28 values of the cherts could reflect the active contribution of hydrothermal silica, and the positive δ30SiNBS-28 values indicate a greater seawater influence.

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