There are many siderite-bearing strata in western Guizhou, and the occurrence forms are diverse (Figure 2), mainly including nodular (Figure 2A), lenticular (Figure 2B), irregular (Figures 2B,C), and banded (Figure 2C). The banded siderite is usually surrounded by horizontal bedding and does not cut across the bedding (Figure 2C).

According to the microscopic morphology, the siderites can be generally divided into three types and six subtypes (Figure 2): gelatinous siderites (I), microlite siderites (II₁), powder crystal siderites (II₂), petal-like siderites (III₁), radiating fibrous siderites (III₂), and concentric siderites (III₃).

The results of carbon–oxygen isotopes are shown in Table 3. The samples with type I siderite have uniform δ¹³C_PDB and δ¹⁸O_SMOW values of −9.20‰ to −10.06‰ and 17.34‰ to 20.65‰, respectively. The δ¹³C_PDB values of samples with type II siderite range greatly from −2.80‰ to −10.58‰, which are dependent on the crystal types. The δ¹⁸O_SMOW values are similar ranging from 23.23% to 24.64‰. The samples with type III siderite have δ¹³C_PDB and δ¹⁸O_SMOW of −6.33‰ to −11.94‰ and 25.78‰ to 28.38‰, respectively, with large fluctuation.

The detailed results are shown in Supplementary Appendix S1, and the in situ spots are shown in Figures 5, 6. Among them, the type I siderite has FeCO₃ (wt%) ranging from 26.62% to 51.64%, with an average of 36.63%, average CaCO₃ of 3.56%, and average MgCO₃ of 6.76%. For microlite siderite, the content of FeCO₃ (wt%) ranges from 47.34% to 86.8%, with an average of 72.67%. The contents of CaCO₃ and MgCO₃ vary greatly in different samples. The average content of CaCO₃ and MgCO₃ in J-5 is 4.43% and 2.79%, whereas that in J-11 is 7.64% and 7.99%, and that in J-13 is 8.58% and 10.27%, respectively. The environment formed by microlite siderite may be more extensive than that of gelatinous. The content of FeCO₃ (wt%) in powder crystal siderite ranges from 64.11% to 90.68%, with an average of 76.39%; the content of CaCO₃ ranges from 5.62% to 11.22%; and the content of MgCO₃ ranges from 2.76% to 16.76%, which are similar to the results of microcrystalline siderite samples. The content of FeCO₃ in type III siderite ranges from 71.26% to 91.87%, and the contents of CaCO₃ and MgCO₃ are lower than those in the other two types of siderite.

The Sr/Ba value of type I siderite ranges from 0.79 to 3.75, with an average of 2.26, higher than that of the whole rock. δ Ce is between −0.08 and 0.06, with an average of 0.01; V/(V+Ni) is between 0.58 and 0.90, with an average of 0.73; and V/Cr is between 1.34 and 11.12, with an average of 2.36, which are also close to the whole rock. The V/Sc value is higher than the whole rock, with an average of 35.85. Different samples of type II₁ siderite are different in composition, although the in situ results are similar to that of the whole rock. The Sr/Ba value of type II₂ siderite ranges from 0.38 to 1.66, with an average of 0.58, which is quite different from the whole rock. There is little difference in Sr/Ba values of type III siderite, and the total average value is 0.50. The δ Ce value is between −0.19 and 0, and types III₁ and III₂ are different, reflecting the different diagenetic water where type III siderite formed. The V/(V+Ni), V/Cr, and V/Sc of III₁ and III₂ are also different.

The abundance of trace elements and the ratio of some immobile elements such as Al₂O₃/TiO₂, ΣREE, and La/Yb in mudstone can help to perform provenance analysis (Spears and Rice, 1973; McLennan et al., 1993; He et al., 2007; Dai et al., 2017; Xie et al., 2018; Liu et al., 2020). Rare earth elements are not easy to migrate during sedimentation and diagenesis, and the distribution pattern of REE can be used for provenance analysis (Boynton, 1984; Dai et al., 2017; Liu et al., 2019). Figure 3C shows that the REE distribution curves of all of siderite-bearing strata samples are relatively similar, which are characterized by enrichment in LREE and depletion in HREE. Eu has no obvious anomaly similar to Emeishan high-titanium basalt (Xiao et al., 2004). To study the provenance of the siderite-bearing strata, the discrimination diagram of ΣREE versus La/Yb was applied (Allègre and Michard, 1974; Xie et al., 2018) (Figure 7A). Apart from a small number of samples with gelatinous siderite that fall within the region of sedimentary rock, the others fall in the alkaline basalt (Figure 7A). Positive Eu anomalies indicate the existence of a large amount of mafic rock detritus, whereas strong negative Eu anomalies indicate the incorporation of felsic magmatic material (Dai et al., 2017; Liu et al., 2020). Al₂O₃/TiO₂ is also a reliable index used to discriminate the provenance of sedimentary rocks (Spears and Rice, 1973; He et al., 2003; He et al., 2007; Dai et al., 2017; Liu et al., 2020), and previous studies have suggested that Al₂O₃/TiO₂ <7 is characteristic of the Emeishan high-titanium basalt (He et al., 2007). Figure 7B shows that the δ Eu value of samples ranges from 0.74 to 1.21, and Al₂O₃/TiO₂ values are less than 7, falling in or similar to the field of Emeishan high-titanium basalt (Xiao et al., 2004; Dai et al., 2016; Liu et al., 2020). The high-titanium basalt in the east of Emeishan Large Igneous Province located in the Kangdian Upland is the most likely provenance area of the Late Permian coal-bearing strata in western Guizhou (Chung and Jahn, 1995; Xiao et al., 2004). During Late Permian, the South China was hot and humid (Bercovici et al., 2015; He et al., 2020; Zhang et al., 2020). In this case, the source rock suffered strong chemical weathering, and the iron was continuously leached under strong weathering and brought into the sedimentary basin after frequent regression and transgression, providing a sufficient iron source for the formation of siderite.

There are two major possible sources of carbon in sedimentary rocks: sedimentary organic matter and marine carbonate rocks (Ohmoto, 1972; Veizer et al., 1980; Liu et al., 2021). The CO₂ source was determined based on the discrimination diagram of δ¹³C_PDB versus δ¹⁸O_SMOW (Figure 8). The data of gelatinous siderite indicate that the source of CO₂ was derived from the dehydroxylation of organic matter, and its formation was affected by seawater and organic matter. The data distribution of microcrystal-silty siderite (II) is scattered, and its CO₂ source may be sedimentary organic matter and marine carbonate. Microlite siderite is evenly distributed in the strata (Figure 2), and its source is more likely sedimentary organic matter, whereas powder crystal siderite is formed by metasomatic calcite or paleontological shell. The CO₂ sources of spheroidal siderite are sedimentary organic matter and marine carbonate rocks, which are affected by different water during diagenesis. Moreover, the analysis results of spheroidal siderite may indicate the genesis of various spheroidal siderite.

Elemental concentration ratios such as Sr/Ba, Sr/Cu, V/(V+Ni) in clastic rocks can indicate the depositional conditions (Hatch and Leventhal, 1992; Mongenot et al., 1996; Rimmer et al., 2004; Akinlua et al., 2010; Zhao et al., 2016; Xu et al., 2017). Generally, Sr is enriched in the marine, and Ba is concentrated in the continental deposition. The Sr/Ba ratio reflects the influence degree by seawater and freshwater during the deposition period. The higher the value, the stronger the influence degree by seawater (Johnsson, 1993; Armstrong-Altrin et al., 2015). The whole-rock Sr/Ba value of samples with gelatinous siderite ranged from 1.14 to 2.18, with an average of 1.45 (Figure 9A). In addition, the gelatinous siderite-bearing strata contain glauconite (Figure 2D), indicating that the gelatinous siderite is strongly affected by seawater (Odin and Matter, 1981; Johnsson, 1993; Armstrong-Altrin et al., 2015; Banerjee et al., 2016). The whole-rock Sr/Ba value of samples with microlite-powder crystal siderite ranges from 0.5 to 0.9, with an average of 0.74, indicating that this kind of siderite was deposited under the joint action of seawater and freshwater (Johnsson, 1993; Armstrong-Altrin et al., 2015). The whole-rock Sr/Ba value of samples with sphaerosiderites ranged from 0.39 to 1.08, indicating that the paleosalinity of the water column during the sedimentary period of this type was relatively low, and the morphology was different under different paleosalinity.

The Sr/Cu ratio can indicate the paleoclimate (Roy and Roser, 2013; Sarki Yandoka et al., 2015). Under warm and humid climatic conditions, sediments usually show a low Sr/Cu ratio (Roy and Roser, 2013; Sarki Yandoka et al., 2015). Figure 9B shows that the whole-rock Sr/Cu of samples with gelatinous siderite ranges from 1.38 to 6.30 (mostly from 1 to 5), with an average of 3.21; that of the microlite-powder crystal siderite ranges from 1.33 to 3.97, with an average of 2.42. The spheroidal siderite has an Sr/Cu ranging from 1.69 to 3.29, and there is no significant difference in the value. The chemical index of alteration (CIA) indicates that the paleoclimate where types II and III siderite formed is warm and humid. The CIA of gelatinous siderite is greater than 80, reflecting the hot and humid climate (Nesbitt and Young, 1982; McLennan, 1993).

The concentrations of V, Ni, and Ce in fine-grained sediments are sensitive to redox conditions and can help to characterize the depositional conditions (Wright et al., 1987; Jones and Manning, 1994; Holser, 1997; Chen et al., 2015). In general, the value of δ Ce > −0.1 indicates the anoxic condition, and less than −0.1 indicates the oxic condition (Wright et al., 1987; Holser, 1997; Chen et al., 2015). V/(V+Ni) ratios >0.84, 0.84–0.6, and <0.6 represent euxinic, anoxic, and dysoxic conditions, respectively (Hatch and Leventhal, 1992; Zhao et al., 2016; Xu et al., 2017). Moreover, V/Sc and V/Cr ratios can also provide information of depositional conditions (Jones and Manning, 1994; Rimmer et al., 2004). Figure 9C shows that the whole-rock δ Ce values of all samples are greater than −0.1, indicating that the water column in the sedimentary period was anoxic. The whole-rock V/(V+Ni) ratio of most samples is between 0.6 and 0.84, showing that they were formed under the weak reducing condition with weak stratification of the water column (Hatch and Leventhal, 1992; Zhao et al., 2016; Xu et al., 2017). The whole-rock V/Sc value of the samples varies little (Figure 9D). Among them, the type I siderite was 9.19 to 16.66, with a mean value of 11.45; the types II and III siderite ranges from 6.53 to 14.65 and 8.29 to 11.07, with an average value of 10.59 and 10.31, respectively. It shows that most siderite-bearing strata are formed in a weak reduction environment (Jones and Manning, 1994). The whole-rock V/Cr values of gelatinous siderite, microcrystal-silty siderite, and spheroidal siderites are 1.36 to 6.12, 1.11 to 4.12, 1.57 to 5.44, with an average value of 2.76, 2.69, and 3.25, respectively. Although the V/Cr values of some samples are low, the existing geochemical indicators show that most siderite-bearing strata samples are formed in a weak reduction to reduction environment (Jones and Manning, 1994; Rimmer et al., 2004).

Siderite is an authigenic mineral, and thus, different micro forms of siderite retain the information of diagenetic water during its formation.

According to the in situ test results, the FeCO₃ content of type I siderite is the lowest among the three types. The Al and Si contents are high, and the Al/Si ratio is almost the same, close to 0.9. The results show that gelatinous siderite is symbiotic with fine clay. The Sr/Ba value of each spot in type I siderite is between 0.79 and 3.75, with an average of 2.26 (Figure 10A), higher than that of the whole rock (Figure 9A). These characteristics indicate that the diagenetic fluid is mainly seawater with high paleosalinity, similar to sedimentary water (Johnsson, 1993; Armstrong-Altrin et al., 2015). The average value of δ Ce is −0.03 (Figure 10B), which is similar to the whole rock (Table 2). The mean value of V/(V+Ni) of the in situ mineral is 0.73. Compared with the whole-rock V/(V+Ni) of the corresponding samples, the V/(V+Ni) value of the in situ mineral is basically unchanged. The value of V/Sc of the in situ mineral is 10.25 to 79.02, with an average of 35.85, significantly higher than the whole rock (Figure 10D). These characters indicate that the type I siderite was formed in an early diagenetic environment, and the diagenetic water is mainly reduced and stagnant seawater (Hatch and Leventhal, 1992; Zhao et al., 2016). During the gelatinous siderite formation, the content of the clastic particles is low, and a large number of clay minerals are distributed in the sediments. During the syngenetic stage, the cryptic crystalline siderite precipitates from the seawater and filled in the limited space as cements.

The FeCO₃ content of type II₁ siderite is higher than that of type Ⅰ siderite, whereas the content of Al and Si is lower, with average Al/Si value of 0.41. The type II₁ siderite appears hypidiomorphic granular aggregates, and the crystal morphology is not obvious, only with the boundary between crystal particles vaguely observed (Figure 2F). Moreover, the clay mineral content in the pores is relatively low, and the siderite is relatively pure. The above characteristics indicate that the microlite siderite is likely to be formed in the original intergranular pores. The average in situ Sr/Ba of type II₁ siderite is 0.64, and that of the whole rock is 0.66. There is almost no difference between them, meaning that the paleosalinity of the diagenetic water is similar to the sedimentary water body, that is, saltwater (Johnsson, 1993; Armstrong-Altrin et al., 2015). The average value of δ Ce in in situ mineral is −0.06, V/(V+Ni) is 0.93, and V/Sc is 4.80; δ Ce and V/(V+Ni) are similar to the average values of whole rock, whereas V/Sc is obviously lower than the average value of whole rock (Figure 10D; the average value of whole rock is 11.58). Therefore, the diagenetic water where type II₁ siderite formed is weakly reduced (Hatch and Leventhal, 1992; Jones and Manning, 1994; Rimmer et al., 2004; Zhao et al., 2016). The diagenetic environment of type II₁ siderite is similar to its sedimentary environment. It is a saltwater fluid with weak oxidation–reduction, and the diagenetic water is likely to be primary porewater. During the sedimentation of the strata with type II₁ siderite, the fluid washed the original pores, and the intergranular clay minerals decreased. The primary porewater through layers provides Fe-forming materials for the crystallization of siderite, which promotes the precipitation of siderite in intergranular pores and forms microlite siderite. However, because of the limited crystallization space, the siderite is filled in the pores and developed in the form of microlite siderite aggregate.

The type II₁ siderite appears as silty-sized crystal aggregates, and some develop into petal-like siderite (Figures 2L,J). The FeCO₃ content is approximately 80 wt%. The Sr/Ba ranges from 0.38 to 1.66, with an average value of 0.58 (Figure 10A), different from the average Sr/Ba of the whole rock of 0.96. The Sr/Ba value indicates that the type II₂ siderite was transformed by freshwater in the later diagenetic stage (Johnsson, 1993; Armstrong-Altrin et al., 2015). The in situ V/Sc values are 2.9 to 9.0, with an average of 5.24, which are lower than those of the whole rock (Figure 9B) (averagely 11.52). The V/(V+Ni) value is between 0.7 and 0.97 (Figure 10C), with an average of 0.85, which is higher than the whole rock (average 0.75). The δ Ce values range from −0.1 to −0.01 (Figure 10B), which is slightly lower than whole rock (from −0.03 to −0.06, average −0.04). These characteristics reveal the diagenetic water where type II₂ siderite formed is different from its sedimentary water; that is, the diagenetic water is not seawater sealed in sedimentary period or primary porewater, but relatively freshwater with weak oxidation to weak reduction (Hatch and Leventhal, 1992; Jones and Manning, 1994; Rimmer et al., 2004). During the burial period, the siderite had been transformed by the diagenetic fluid, which promoted the recrystallization of siderite crystals with smaller particles formed in the sedimentary period and formed powder crystal siderite with silty size and better crystal morphology.

The type III siderite can be divided into three subtypes based on microscopic morphology. According to the observed type III₁ siderite, the morphologies range from powder crystals through fan-shaped to petal-like morphologies (Figures 2K,L); it is likely to be formed by a single crystal, and the morphology is affected by pores and diagenetic fluid (Passey, 2014). According to the in situ results (Figure 10), the Sr/Ba ranges from 0.38 to 0.63, indicating that the formation environment was affected by freshwater. The in situ V/Sc, V/(V+Ni), and δ Ce values are 5.6 to 9.3, 0.74 to 0.94, and −0.21 to −0.04, respectively. These characteristics indicate that the type III₂ siderite grows radially around the core with weak oxidation to weak reduction. The variation curves of many geochemical indexes from the core to the rim of type III₃ siderite show obvious symmetry (Figure 11), indicating that its growth is slow and continuous and carried out together with the diagenesis. Under the repeated action of diagenetic fluid, the type III₃ siderite grows in a circle around the cryptocrystalline core in a fluid with the weak oxidation–reduction property.

Siderite in the study area shows various forms and has been affected by sedimentary environment and diagenetic environment. Regarding the sedimentary environment, the Upper Permian coal-bearing strata developed under carbonate tidal flat-barrier-lagoon-shallow delta conditions (Wang et al., 2011; Shen et al., 2016; Shen et al., 2019; Qin et al., 2018), and the climate was hot and humid. Such sedimentary condition was beneficial to the formation of siderite (Bercovici et al., 2015). The development and evolution model of siderite is shown in Figure 12. The formation of type I siderite is strongly affected by seawater during transgression, and the sedimentary water is stagnant seawater. During diagenesis, the diagenetic water along the fissures and beddings promoted the recrystallization of the gelatinous siderite to form the powder siderite (Figure 12). The sedimentary water of type II siderite gradually changed from seawater to brackish water in the transitional environment. During diagenesis, the diagenetic fluid of freshwater promoted the recrystallization of siderite along fissures and beddings or metasomatism with calcite to evolve into type II₂ siderite and sometimes into petal-like siderite (Figure 12). The type III₂ siderite grows radially around the core. The type III₃ siderite grows in a circle around the cryptocrystalline core in a relatively active fluid with weak oxidation–reduction under the repeated diagenetic fluid. Under the influence of multistage diagenetic fluid, the type III₃ siderite grows around the cryptocrystalline core.

The formation of siderite is obviously controlled by the different sedimentary and diagenetic environments. Among them, types I and II₁ siderite mainly formed in the syndiagenetic and early diagenetic stages, and their morphology is controlled by the sedimentary environment. Therefore, types I and II₁ siderite can indicate the sedimentary environments. Gelatinous siderite was most affected by seawater, mostly formed in the transgression or near the maximum flooding surface (Shen et al., 2019; Zhang et al., 2020). Microlite siderite was influenced by freshwater during its formation and was most developed in the early stage of transgression or high-stand system tract.