Genetic Mechanism and Environment Implications of Siderites in the Lopingian Coal-Bearing Series, Western Guizhou of China: Constrained by Whole-Rock and In Situ Geochemistry

A large number of siderites have been found in the Lopingian (Late Permian) coal-bearing series in western Guizhou, which occurs in various microscopic morphologies and has potential insights into the sedimentary and diagenetic environments. An integrated set of analyses, such as microscopic observation; X-ray diffraction; whole-rock major and trace element, carbon, and oxygen isotope; and in situ major and trace element, has been carried out to unravel the genetic mechanism of the siderites and their environmental implications. According to the microscopic morphology, the siderites can be generally divided into three types and six subtypes, including gelatinous siderites (I), microcrystal-silty siderite [II; microlite siderites (II1), powder crystal siderites (II2)], and spheroidal siderite [III, petal-like siderite (III1), radiating fibrous siderite (III2) and concentric siderite (III3)]. Whole-rock geochemical results show that the iron source for the formation of the siderites was mainly from extensive weathering of the Emeishan high-titanium basalts in hot climate conditions. The carbon and oxygen isotopic results indicate that the origin of CO2 in type I siderites is derived from the dehydroxylation of organic matter. The CO2 in types II1 and II2 siderites is mainly derived from deposited organic matter and marine carbonate rocks, respectively. The CO2 source of type III siderites is sedimentary organic matter and marine carbonate rocks and is affected by different fluids during diagenesis. The whole-rock and in situ geochemical characteristics further point to that type I siderites were formed in the synsedimentary period most strongly affected by seawater. Redox proxies, such as V/Sc, V/(V+Ni), and δ Ce, constrained their formation in a stable and weakly reduced condition. Type II siderites could have been developed in saltwater. Among them, type II1 siderites were formed in the early diagenetic stage, whereas type II2 siderites originated from recrystallization of type II1 siderites and accompanied by metasomatism with calcites under diagenetic fluids of weak reduction to weak oxidation conditions. Type III siderites were formed under the influence of multistage diagenetic fluids. Among them, type III1 siderites formed by the growth of powder crystal siderites (II2) under diagenetic fluids with a weak reducing condition. Type III2 siderites formed by growth around microlite siderites under weak reducing diagenetic fluids. Type III3 siderites formed by concentric growth in diagenetic fluids with weak reduction to weak oxidation conditions and relatively active conditions.

A large number of siderites have been found in the Lopingian (Late Permian) coal-bearing series in western Guizhou, which occurs in various microscopic morphologies and has potential insights into the sedimentary and diagenetic environments. An integrated set of analyses, such as microscopic observation; X-ray diffraction; whole-rock major and trace element, carbon, and oxygen isotope; and in situ major and trace element, has been carried out to unravel the genetic mechanism of the siderites and their environmental implications. According to the microscopic morphology, the siderites can be generally divided into three types and six subtypes, including gelatinous siderites (I), microcrystalsilty siderite [II; microlite siderites (II 1 ), powder crystal siderites (II 2 )], and spheroidal siderite [III, petal-like siderite (III 1 ), radiating fibrous siderite (III 2 ) and concentric siderite (III 3 )]. Wholerock geochemical results show that the iron source for the formation of the siderites was mainly from extensive weathering of the Emeishan high-titanium basalts in hot climate conditions. The carbon and oxygen isotopic results indicate that the origin of CO 2 in type I siderites is derived from the dehydroxylation of organic matter. The CO 2 in types II 1 and II 2 siderites is mainly derived from deposited organic matter and marine carbonate rocks, respectively. The CO 2 source of type III siderites is sedimentary organic matter and marine carbonate rocks and is affected by different fluids during diagenesis. The whole-rock and in situ geochemical characteristics further point to that type I siderites were formed in the synsedimentary period most strongly affected by seawater. Redox proxies, such as V/Sc, V/(V+Ni), and δ Ce, constrained their formation in a stable and weakly reduced condition. Type II siderites could have been developed in saltwater. Among them, type II 1 siderites were formed in the early diagenetic stage, whereas type II 2 siderites originated from recrystallization of type II 1 siderites and accompanied by metasomatism with calcites under diagenetic fluids of weak reduction to weak oxidation conditions. Type III siderites were formed under the influence of multistage diagenetic fluids. Among them, type III 1
The siderite-bearing strata are widely developed in coal measures in western Guizhou, and some studies have been conducted to determine its characteristics, as well as the genesis (Shen et al., 2017;Shen et al., 2019;Zhang et al., 2018;Zhang et al., 2020). For example, various forms of siderite have been recently identified by Zhang et al. (2020), and the suggested morphology of siderites is controlled by sedimentary sequence. However, how the sedimentary and diagenetic environments control the formation of siderites has always been an extensively debated topic. In this article, we identified multiple siderite-bearing strata with variable micromorphological types of siderite at Lopingian coal-bearing series in Panxian, western Guizhou, China, and performed an integrated analysis on the samples and obtained a set of new petrological, mineralogical, and geochemical data. Based on these data, we analyzed the formation model of siderite and their implications for the depositional and diagenetic environment.

GEOLOGICAL BACKGROUND
The study area is located in Panxian, western Guizhou, at the junction of Yunnan and Guizhou, on the margin of an epicontinental basin covering South China ( Figure 1A). During the late Permian, this area was in the Liupanshui fault depression . The Lopingian is a representative Upper Permian coal-bearing sequence in the region, and there are various sedimentary environments including continental, delta, and marine facies from west to east (Wang et al., 2011;Shen et al., 2016;Shen et al., 2019;Qin et al., 2018). Lopingian can be divided into Wuchiapingian and Changhsingian from bottom to top. The Wuchiapingian is mainly composed of gray, dark gray, and gray-yellow mudstones, silty mudstones, siltstones, fine sandstones, carbonate rocks, and coal seams. Multiple layers of marine key beds and regionally stable coal seams as products of pulsating transgression events can be used as good correlation marker beds (Wang et al., 2011;Shen et al., 2016). Meanwhile, the Changhsingian is mainly composed of gray and dark gray argillaceous siltstone, mudstone, limestone, and fine sandstone, with 6 to 20 coal seams (Shen et al., 2016). According to the sequence stratigraphic framework established by previous studies Shen et al., 2016;Shen et al., 2019), the study strata can be divided into four third-order sequences.
The Y2 and Y3 drillings are located between Tucheng and Panzhou; Y3 drilling is located west of Panzhou ( Figure 1A). The study strata are interpreted as delta, lagoonal tidal flat facies . Note that the siderite-bearing strata are usually grayish black to grayish yellow, which is different from the surrounding rock (Figures 2A-C). The main lithologies are mainly mudstone, silty mudstone, and few siltstones.

SAMPLES AND ANALYTICAL METHODS
A total of 25 siderite samples were taken from three drilling wells in the study area, among which J3 well samples were evenly distributed in the Late Permian coal-bearing series. To investigate the formation model of siderite and their implications for the sedimentary and diagenetic environments the following analytical methods were performed ( Figure 1B).
The petrographical and mineralogical characteristics of the samples were detected by an Olympus BX53M microscope in the School of Resources and Geosciences, China University of Mining and Technology.
The whole-rock X-ray diffraction (XRD) analysis by X′ Pert Pro X-ray powder diffractometer was conducted in Regional Geology and Mineral Resources Institute of Hebei. The test conditions were as follows: (1) working voltage 40 kV; (2) working current 40 mA; (3) Cu-Kα ray, wavelength 0.15416 nm, Ni filter; and (4) continuous step scanning with a step size of 0.017, the scanning speed is of 0.417782/s and the preset time is of 2′ 36″.
Whole-rock major element analyses were conducted on XRF (Primus II, Rigaku, Japan) at the Wuhan Sample solution Analytical Technology Co., Ltd., Wuhan, China. Detailed analysis procedure referred to "Methods for chemical analysis of silicate rocks-Part 28: Determination of 16 major and minor components" in the standard GB/T14506.28-2010.
Whole-rock trace element analyses were conducted on Agilent 7700e ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Detailed analysis procedure referred to "Methods for chemical analysis of silicate rocks-Part 30: Determination of 44 elements" in the standard GB/T14506.30-2010.
The samples for carbon and oxygen isotopic analysis were collected from Y2 and Y3 wells, and the analysis was conducted in the Geological and Environmental Institute of CUG (Wuhan). The siderite samples were first ground to less than 200-mesh and then reacted with phosphoric acid in a constant temperature tank at 72°C for 1 h. The collected CO 2 was transferred to the MAT-253 for the C-O isotopes analysis. The analysis precision better is than ±0.2‰.
In situ major and trace element analysis of siderite by LA-ICP-MS was conducted at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. Beam size of 32 μm and laser frequencies of 5 Hz were used during the analyses. An Agilent 7700e ICP-MS instrument was used to acquire ion-signal intensities. Each analysis incorporated a background acquisition of

Gelatinous Siderite (I)
The gelatinous siderite is yellowish-brown under the polarizing microscope, and it is difficult to observe the grain boundary Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 779991 4 ( Figures 2D,E). Note that they were divided into two forms. One of which is evenly distributed in the pores between clastic particles, and the other is aggregated into blocks. Among them, uniform gelatinous siderite distributed in rocks is often filled in the pores between clastic particles in the form of cement and mixed with clay minerals ( Figure 2D). The block-like gelatinous siderite is often surrounded by clastic particles, clay minerals, and organic matter ( Figure 2E).

Microcrystal-Silty Siderite (II)
Crystal boundaries can be observed in microlite and powder crystal siderites under the microscope, which appear as hypidiomorphic or automorphic granular single crystal or aggregates with various shapes and sizes (Figures 2F-J). Among them, the microlite siderite often has color of light brown-yellow to light yellow, grain size between 0.004 and 0.03 mm, and mainly hypidiomorphic shapes. This kind of siderite is evenly distributed and mixed with clay minerals and organic matter or appears surrounding clastic particles ( Figure 2F). The powder crystal siderite (II 2 ) is light yellow, and the grain size is between 0.015 and 0.09 mm with a good degree of crystallization. Siderite crystals are mainly automorphic and often develop at the edge of the cavity filled with calcite ( Figures 2G,H) or in the area with high calcite content ( Figures 2I,J).

Spheroidal Siderite (III)
This type of siderite has relatively large grain size, ranging from 0.1 to 1 mm, and appears light yellow to light yellowish brown under plane-polarized light ( Figure 2). Although the petal-like siderite (III 1 ) has similar ring shapes, differences in morphology still exist. The fully developed siderite is spherical, and the incomplete siderite is bow-tie morphology ( Figure 2K). Under cross-polarized light, the interference color is from pink to bright green ( Figure 2L). Radiating fibrous siderite has a radiating texture, internal cleavage, and cross extinction under crosspolarized light ( Figures 2M,N). Concentric siderite (III 3 ) has obvious concentric ring morphology ( Figure 2O), and its distribution in the bedding is highly heterogeneous, usually concentrated in the form of aggregate.
XRD results show that the minerals in the samples include clay, quartz, siderite, plagioclase, calcite, ankerite, anatase, pyrite, hematite, and thenardite ( Table 1). The siderite content ranges from 3.06 to 90.7 wt%. Other main components quartz, clay minerals, and calcite range from 4.4 to 52 wt%, 3 to 77.6 wt%, and 0 to 16.6 wt%, respectively. Other components are low, and some samples contain high ankerite. The content of type I siderite in rocks is relatively low. The XRD results show that the average content is only 26.3 wt%, with an exception of J-3 sample more than 40%. Type II siderite is common in rocks with high relative content ( Table 1). The XRD analysis results showed that the average content in rocks reached up to 48.72% ( Table 1). The distribution of spheroidal siderite in the rock is heterogeneous. The XRD analysis results show that the content varied largely among different samples ( Table 1).

Major Elements
The whole-rock major elements results are listed in Table 2. By normalizing the major elements results based on the upper crust (UC) (Taylor and McLennan, 1985), we find the concentration of main oxides in the studied samples is comparable to that in the UC, except the depletion of K 2 O and the enrichment of TiO 2 , TFeO (total iron), MnO, MgO, and CaO. The type I sideritebearing samples have the highest content of SiO 2 , and the samples containing the other two types of siderite are similar. All of 9.0 39.1 10.9 4.5 11.2 11.5 12.0 Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 779991 5 TABLE 2 | Statistics of major element oxide content (wt%), trace element, and rare earth element concentrations (μg/g) of the studied samples.

Trace Elements
For most sedimentary deposits, some trace elements and aluminum can be well preserved and transported along with clastic materials and are usually immobile during diagenesis. These elements can be used for paleoenvironmental reconstruction only if their contents deviate greatly from average shale and have a good correlation with Al (Tribovillard et al., 2006). The enrichment factors of elements [EF element X X/Al sample /(X/Al average shale )] are obtained by Al normalization, and the results are shown in Figure 3B ( Tribovillard et al., 2006). Among them, the enrichment factors of Cr, Ni, Ga, Sr, Ba, Th, and U are between 0.5 and 2, close to the average shale. Th, Ga, and U have a good positive correlation with Al, whereas Sr, Ba, Ni, Cr, and Rb have a poor correlation (Figure 4).

Rare Earth Elements
The ΣREE values of the siderite-bearing samples are relatively high varying between 121 and 523 ppm. Figure 3C shows the distribution pattern of REE. The REE distribution pattern of all samples shows a significant enrichment of LREE. By comparing them with the values of the high-titanium basalts, trachytes, and rhyolites in the study area (Xiao et al., 2004;Xu et al., 2010) ( Figure 3C), it was found that all samples are similar to the hightitanium basalts. The δ Ce values [δ Ce lg (3 * Ce N )/ (2 * La N + Nd N )] range from −0.09 to 0.02, whereas δ Eu  (Xu et al., 2010), and the blue area is for high-Ti basalt (Xiao et al., 2004

Carbon-Oxygen Isotopic Compositions
The results of carbon-oxygen isotopes are shown in

In Situ Geochemistry
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 FeCO 3 (wt%) ranging from 26.62% to 51.64%, with an average of 36.63%, average CaCO 3 of 3.56%, and average MgCO 3 of 6.76%. For microlite siderite, the content of FeCO 3 (wt %) ranges from 47.34% to 86.8%, with an average of 72.67%. The contents of CaCO 3 and MgCO 3 vary greatly in different samples. The average content of CaCO 3 and MgCO 3 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 FeCO 3 (wt%) in powder crystal siderite ranges from 64.11% to 90.68%, with an average of 76.39%; the content of CaCO 3 ranges from 5.62% to 11.22%; and the content of MgCO 3 ranges from 2.76% to 16.76%, which are similar to the results of microcrystalline siderite samples. The content of FeCO 3 in type III siderite ranges from 71.26% to 91.87%, and the contents of CaCO 3 and MgCO 3 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 II 1 siderite are different in composition, although the in situ results are similar to that of the whole rock. The Sr/Ba value of type II 2 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 III 1 and III 2 are different, reflecting the different diagenetic water where type III siderite formed. The V/(V+Ni), V/Cr, and V/Sc of III 1 and III 2 are also different.

Provenance Analysis of Siderite-Bearing Strata
The abundance of trace elements and the ratio of some immobile elements such as Al 2 O 3 /TiO 2 , 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 Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 779991 9 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). Al 2 O 3 /TiO 2 is also a reliable index used to discriminate the provenance of sedimentary rocks (Spears and Rice, 1973;He et al., 2007;Dai et al., 2017;Liu et al., 2020), and previous studies have suggested that Al 2 O 3 /TiO 2 <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 Al 2 O 3 /TiO 2 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 hightitanium 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 CO 2 source was determined based on the discrimination diagram of δ 13 C PDB versus δ 18 O SMOW (Figure 8). The data of gelatinous siderite indicate that the source of CO 2 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 CO 2 source Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 779991 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 CO 2 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.

Depositional Conditions of Siderite-Bearing Strata
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 microlitepowder 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 FIGURE 9 | Distribution diagrams of paleosalinity and redox proxies for siderite-bearing strata samples.
Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 779991 samples are formed in a weak reduction to reduction environment (Jones and Manning, 1994;Rimmer et al., 2004).

Diagenetic Environment and Genesis of Siderite
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 FeCO 3 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 wholerock 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 FeCO 3 content of type II 1 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 II 1 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 Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 779991 13 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 II 1 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 II 1 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 II 1 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 II 1 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 II 1 siderite appears as silty-sized crystal aggregates, and some develop into petal-like siderite ( Figures 2L,J). The FeCO 3 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 II 2 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 II 2 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 III 1 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 FIGURE 11 | Variation curves from center to edge of concentric siderite.
Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 779991 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 III 2 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 III 3 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 III 3 siderite grows in a circle around the cryptocrystalline core in a fluid with the weak oxidation-reduction property.

Evolution Model of Siderite and Geological Significance
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 II 2 siderite and sometimes into petal-like siderite ( Figure 12). The type III 2 siderite grows radially around the core. The type III 3 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 III 3 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 II 1 siderite mainly formed in the syndiagenetic and early diagenetic stages, and their morphology is controlled by the sedimentary environment. Therefore, types I and II 1 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.

CONCLUSION
Based on the morphology of siderite in coal-bearing series, it can be divided into three types and six subtypes. There is barely any difference in the sedimentary sources of various siderite-bearing strata, which are Emeishan high-titanium basalt developed in the west of the study area. Type I siderite CO 2 comes from sedimentary organic matter. Types II and III siderite CO 2 is mainly derived from sedimentary organic matter and marine carbonate. The most significant difference in the sedimentary period of various siderite-bearing strata lies in the influence degree of seawater. Among them, type I siderite-bearing strata are strongly affected by seawater, whereas types II and III sideritebearing strata are affected by seawater and freshwater. Different types of siderite bearing strata are formed in depositional conditions from weak oxidation to weak reduction.
The in situ results show that types I and II 1 siderites were formed in the early diagenetic stage. The diagenetic water of type I siderite is sedimentary seawater, and type II 1 siderite is mainly primary porewater. The diagenetic water of type II 2 siderite is freshwater, and type III is multistage fluid.
The siderite morphologies are controlled by the sedimentary environment and diagenetic environment. Among them, gelatinous siderite and microlite siderite are mainly formed in the syndiagenetic stage and early diagenetic stage, thus indicating the sedimentary environment. Gelatinous siderite is strongly influenced by seawater and is mainly formed in the carbonate tidal flat and barrier lagoon. Microlite siderite is formed in the shallow delta with relatively low sea level by the interaction of seawater and freshwater.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.