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. 2021 Dec 23;7(1):1244–1258. doi: 10.1021/acsomega.1c05874

Genesis and Distribution of Pyrite in the Lacustrine Shale: Evidence from the Es3x Shale of the Eocene Shahejie Formation, Zhanhua Sag, East China

Danish Khan †,*, Longwei Qiu †,*, Chao Liang †,*, Kamran Mirza ‡, Saif Ur Rehman ‡, Yu Han †, Abdul Hannan §, Muhammad Kashif §, Kouassi Louis Kra †
PMCID: PMC8757364  PMID: 35036786

Abstract

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Pyrite is a common mineral in sedimentary rocks and is widely distributed in a variety of different morphologies and sizes. Pyrite is also widely distributed in the Es3x shale of the Eocene Shahejie Formation in the Zhanhua Sag, Bohai Bay Basin. A combination of geochemical and petrographic studies has been applied to address the formation and distribution of pyrite in the Es3x shale. The methods include thin section analysis to identify the representative samples of the shale-containing pyrite, total organic carbon (TOC) content analysis, X-ray fluorescence, X-ray diffraction, electron probe micro-analysis, and field emission scanning electron microscopy (FE-SEM) coupled with the energy dispersive spectrometer, to observe the characteristics, morphology, and distribution of pyrite in the lacustrine shale. The content of pyrite in the Es3x shale ranges from 1.4 to 11.2% with an average content of 3.42%. The average contents of TOC and total organic sulfur (TS) are 3.48 and 2.53 wt %, respectively. Various types of pyrites are observed during the detailed FE-SEM investigations including pyrite framboids, euhedral pyrite, welded pyrite, pyrite microcrystals, and framework pyrite. Pyrite framboids are densely packed sphere-shaped masses of submicron-scale pyrite crystals with subordinate large-sized euhedral crystals of pyrite. Welded pyrite forms due to the overgrowth and alteration of pyrite crystals within the larger pyrite framboids. Pyrite microcrystals are the euhedral-shaped microcrystals of pyrite. The framework pyrite is also observed and is formed due to the pyritization of plant/algal tissues. Based on the growth mechanism, the pyrites can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. The presence of pyrite, especially the abundance of pyrite framboids, suggests that the environment during the Es3x shale deposition in the lacustrine basin was anoxic. Their dominant smaller size suggests the presence of an euxinic water column, which is consistent with the lack of in-place biota and high TOC contents. This research work not only helps to understand the pyrite mineralization, role of organic matter, and reactive iron in pyrite formation in the shale but also helps to interpret the paleoredox conditions during the deposition of shale. This research work can also be helpful to other researchers who can apply these methods and conclusions to studying shale in other similar basins worldwide.

1. Introduction

Pyrite is abundantly present in both recent and ancient sedimentary rocks and is the most stable mineral of iron sulfide in lower temperature anoxic environments.1,2 The distribution of pyrite is pervasive in modern anaerobic sediments and is preserved in various ancient sedimentary rocks.3 The characteristics of pyrite, for example, sulfur isotope, size, and trace elements distribution, are key indicators of the ancient depositional settings in shale. According to the previous literature, pyrite can be divided into two different types including pyrite framboid and euhedral pyrite.4−8 Framboidal pyrites are composed of clusters of pyrite microcrystals in subspheroidal to spheroidal form with a raspberry-like resemblance.3 Pyrite framboids have a clear framework with unvarying pyrite microcrystals, and their framework is not even disturbed by the overburden pressure.9 On the other hand, euhedral pyrite is comprised of a single crystal or a cluster of pyrite crystals of different dimensions.10,11 Framboids pyrites deposit in the early phases of diagenesis during sedimentation,3 whereas euhedral pyrites form in the early to late phases of diagenesis during sedimentary deposition.9,12−15 Both types of pyrites have variable grain sizes, the grains of pyrite framboids are submicron in size, while euhedral pyrite grains are submicron-to-centimeter-sized;6,12,16 thus, it is very stimulating and crucial to discriminate between the two pyrite types along with various other pyrite forms at a submicron level throughout different experimentations. The smaller pyrite size, especially the framboidal pyrites, can tell us about the euxinicity of the depositional environments.17 Welded pyrite forms due to the overgrowth and alteration of pyrite crystals within the larger pyrite framboids. Pyrite microcrystals are the euhedral-shaped microcrystals of pyrite. The framework pyrite is also observed and is formed due to the pyritization of plant/algal tissues.3,5,6 Based on the growth mechanism, the pyrites can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. Syngenetic, early diagenetic, and late diagenetic pyrites can be differentiated on the basis of their size. Therefore, the size not only gives information about the paleoredox conditions but also differentiates the syngenetic, early diagenetic, and late diagenetic pyrites.3,9,12−17

Pyrite with its various textures is distributed abundantly throughout the Es3x shale in Zhanhua Sag, Bohai Bay Basin (BBB). Previous research work focuses on the sedimentology, stratigraphy, structure geology, and reservoir properties of the Eocene shale in the study area.18−22 However, the research regarding pyrite genesis, types, and their distribution sparsely exists to date. We have conducted a comprehensive investigation of total organic carbon (TOC) contents, whole-rock mineralogy, elemental geochemistry (major, minor, and trace elements), field emission scanning electron microscopy (FE-SEM), and electron probe micro-analysis (EPMA) of the Es3x shale in the Zhanhua Sag in order to understand the relationship between TOC and total organic sulfur (TS) with pyrite, and the use of pyrite as a proxy for paleoredox conditions in detail. Our study highlights the formation mechanism and distribution of various kinds of pyrite first time in this area. The prime focus of this study is to understand the following questions, for example, (i) what are the main characteristics of pyrite mineralization? (ii) What is the mechanism of pyrite growth? (iii) From where does the influx of reactive iron come from? (iv) What is the role of organic matter (OM) in pyrite generation? (v) How pyrite can be used to interpret the paleoredox conditions? The results derived from this study would help to understand not only the formation and distribution mechanism of pyrite but also can be used to interpret the paleoredox conditions of the lacustrine shale in the Zhanhua Sag and other similar basins containing pyrite-rich fine-grained sedimentary rocks worldwide.

2. Regional Geology

The BBB is a rift-related basin of the Cenozoic era with a total area of 200,000 square kilometers approximately, positioned on the east coastline of China. From northeast to southwest, BBB is divided into seven depressions (Linqing, Jizhong, Jiyang, Changwei, Huanghua, Bozhong, and Liaohe) and is comprised of four uplifts (Cangxian, Xingheng, Chengning, and Neihuang Uplifts) (Figure 1).18−22 Among these abovementioned seven sub-basins, the Jiyang Sub-basin is a typical rift basin of the Mesozoic to Cenozoic age in the southeast of BBB. The Jiyang Depression is composed of four secondary subdepressions (sags), that is, Dongying, Huimen, Zhanhua, and Chezhen. These sags are separated by the Chengdong Uplift, Chenjiazhuang Uplift, and Yihezhuang Uplift.23,24 The Jiyang Depression has undergone three phases of tectonic evolution, and it was in the depression stage throughout the earlier Eocene time period.25,26 A moderately to strongly reducing, warm-humid climate and semi-deep to deep lacustrine sedimentary environment prevailed at that time.27 Zhanhua Sag (Zhanhua Subdepression) is our study area and it is located on the northeast side of the Jiyang Depression. It is composed of an area of approximately 2800 km2 and is bounded by the Chenjiazhuang Uplifts on the southern side, and the Yihezhuang Uplift and Chengdong Uplift on the northern side, and on the eastward side by the Kendong Uplift. The Zhanhua Sag is constrained by the Yidong and Chengdong Faults on the western and northern sides, respectively. It comprises several subsags including the Gubei, Bonan, Gunan, Fulin, and Sikou28,29 (Figure 1).

Figure 1.

Figure 1

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Location map of the study area in the Zhanhua Sag, BBB, East China.

The Zhanhua Sag consists of a thick sedimentary sequence from the Paleozoic to Cenozoic era, and the strata of the Cenozoic era are composed of the Paleogene to the quaternary system (Figure 2). The strata of the Paleogene system comprise the Kongdian Formation, Shahejie Formation, and Dongying Formation. The Neogene system contains the Guantao Formation and Minghuazhen Formation. The Paleogene shale strata are primarily located in the Eocene–Oligocene Shahejie Formation.30

Figure 2.

Figure 2

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Stratigraphic chart of the study area in the Zhanhua Sag, BBB, East China. Note: the red box in the stratigraphic chart shows the study interval.

The Shahejie Formation is composed of alternating layers of sandstone, siltstone, oil shales, mudstone, and evaporites deposited in fluvial–lacustrine–deltaic depositional settings (Figure 2). The Shahejie Formation is divided into four members including Es1 (Es1s and Es1x), Es2 (Es2s and Es2x), Es3 (Es3s, Es3z, and Es3x), and Es4 (Es4s and Es4x)30 (Figure 2). The Es3x shale (lower third part of the Es3 member) of the Shahejie Formation in the Zhanhua Sag represents our study horizon. A thick sequence of the Es3x organic-rich shale was developed with a higher content of carbonate minerals, stable thickness, and moderate depth in our study area.24 The Zhanhua Sag is considered for the study of pyrite because the shale is composed of an abundant amount of pyrite in this area, yet no studies have been conducted for pyrite genesis in this area to date.

3. Results

3.1. Mineralogical Characterization

The Es3x shale is composed of various minerals including quartz, calcite, dolomite, clay, and pyrite (Table 1). The content of pyrite in the study interval varies from 1.3 to 11.2% with an average of 3.3%. The concentration of quartz ranges from 5.3 to 14.3% with an average concentration of 9.6%. The percentage of calcite minerals in the Es3x shale ranges from 31 to 85.1% with an average of 58.8%. The abundance of dolomite in the Es3x shale varies from 2.7 to 10.6% with an average of 7%. The clay mineral in the Es3x shale varies from 0 to 43% with an average quantity of 20% (Table 1).

Table 1. Whole-Rock Mineralogical Composition (wt %), TOC, and TS Contents of the Es3x Shale in the Zhanhua Sag, BBB.

depth (m) mineral composition (wt %) of the Es3x shale
 TOC (wt %) TS (wt %)
pyrite quartz calcite dolomite clay
2937.8 11.2 12.2 55.8 3.8 17 6.5 1.8
2960 2.4 12.8 52.6 9.3 22.9 3.4 1.4
2990.05 3.4 9.6 53.7 6.9 26.4 2.3 2.1
2994.3 5.8 13 31.3 6.9 43 3 3.9
3012.2 4.9 11.9 53.6 5.4 24.2 4.4 2.3
3013.2 3.5 10.4 58.1 6.5 21.5 3.2 3.2
3016.3 5.4 14.3 37.3 6.9 36.1 3.7 3.8
3018.1 2.1 12.1 50.5 8.3 27 4.6 3.3
3023.8 5 14.2 38.4 10.1 32.3 5.1 3.6
3024.1 2.6 9.7 63.5 8.7 15.5 4.9 1.9
3033.5 3.1 11.9 42 10.6 32.4 5.8 3.7
3036.2 3.7 9.1 54.8 9.7 22.7 4.9 2.8
3038 3.1 11 52.6 8.5 23.7 3.9 2.2
3045.3 3.5 9.9 57.6 10.2 18.8 3.7 2.5
3052.5 1.4 8.2 64.7 7.8 17.9 6.4 2
3054.3 2.2 5.3 67.8 8.9 15.8 5.6 1.6
3056.43 2.6 8.9 67.6 6.1 14.8 5.7 2
3056.95 1.6 7.8 70.4 5.9 14.3 5.4 1.9
3063 1.8 8.4 71.2 4.2 14.4 1.8 1.7
3064.3 1.5 7.7 85.1 5.7 0 1.2 0.9
3067.8 2 5.6 75.6 8.2 8.6 2 1.5
3070.9 1.5 11.2 60.3 9.1 17.9 1.7 1.9
3090.7 1.9 6.7 65.8 7.3 18.3 1.6 1.9
3098.96 2.1 6.3 63.7 8.3 19.6 1.1 2.1
3100.68 2.2 9.5 62.6 7.6 18.1 1.6 2.6
3105.2 3.6 11.5 62.4 4.3 18 1.7 2.5
3110.2 1.4 7.5 73.5 3.1 14.5 1.2 1.9
3119 3.5 9 64.2 4.2 19.1 1.3 2.5
3140.25 4.5 11.9 50.1 2.7 29.4 1.5 5.4
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3.2. TOC and TS Content Analysis

The Es3x shale is manifested by a higher TOC content ranging from 1.12 to 6.5 wt % with an average concentration of 3.4 wt %. The TOC content in the Es3x shale decreases with increasing burial depth. The TOC content is directly proportional to quartz, clay, and dolomite but inversely related to calcite with increasing burial depth. The reason behind the positive relationship of TOC with quartz and clay is that these minerals are attributed to terrigenous input, which contains a high amount of OM. Whereas, the positive relationship with dolomite shows that authigenic dolomite in the Es3x shale is microbially mediated dolomite, and the OM associated with the mediated microbial organisms encourages the formation of dolomite.31 The positive association of authigenic dolomite with OM shows that the OM played a vital role in the precipitation of dolomite. The negative relationship of OM with calcite is due to the dilution effect of carbonate influx. The TS content is ranging from 0.99 to 5.4 wt % with 2.5 wt % of an average value in the Es3x shale (Table 1).

3.3. Characteristics of Pyrite under the Polarizing Microscope

The distribution of pyrite can also be observed under a polarizing microscope. Pyrite is scattered in almost all the samples of the Es3x shale. Under the cross-polarized light of the polarizing microscope, it shows the black color in a spherical shape (Figure 3A), while under reflected light, pyrite shows the dull yellowish or pale brass-yellowish color (Figure 3B–F). Pyrite framboids, pyrite microcrystals, euhedral pyrite, and framework pyrite can also be observed using the reflected light of the polarizing microscope (Figure 3B–F).

Figure 3.

Figure 3

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Photomicrographs of the polarizing microscope presenting the dispersal of pyrite in the Es3x shale in the Luo-69 well, Zhanhua Sag. (A) Denoting pyrite distribution under the cross-polarized light, 2994.3 m, (B–F) Displaying distribution of different types of pyrite under reflected light, e.g., (B) 2994.3 m (pyrite framboids), (C) 3033.5 m (pyrite microcrystals), (D) 3036.2 m (euhedral pyrite), (E) 3067.8 m (pyrite microcrystals), and (F) 3140.25 m (framework pyrite), respectively.

3.4. Different Forms of Pyrite Based on FE-SEM Analysis

The Es3x shale samples comprise five different types of pyrite that are being encountered during FE-SEM analysis (Figure 4), including (1) pyrite framboid, these are densely packed sphere-shaped masses of submicron crystals of pyrite (Figure 4A), (2) euhedral pyrite, this type of pyrite crystals occur as a cube and they are randomly dispersed in the Es3x shale (Figure 4B,E,F), (3) welded pyrite, represents pyrite framboids that show the evidence of the secondary growth of diagenetic pyrite that can fill or weld the interstitial pores among microcrystallites and/or partially to completely framboids overgrowth3 (Figure 4C,D), (4) pyrite microcrystals, which are small-sized spherical euhedral crystals of pyrite scattered randomly in the Es3x shale (Figure 4G), and (5) framework pyrite, formed due to the pyritization of plant tissues or algae (Figure 4H,I). The elemental composition of pyrite crystals and framework pyrite is a little bit different from each other as shown by EDS analysis (Figure 4E,F,H,I). The concentration of carbon is very low in pyrite crystals as compared to framework pyrite. Apart from this, the framework pyrite contains oxygen and zirconium, while these elements are absent in pyrite crystals (Figure 4F,I, respectively).

Figure 4.

Figure 4

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FE-SEM photomicrographs representing the mineralogical characteristics of the pyrites in the Es3x shale using EDS showing the elemental composition of various forms of pyrites. (A, B) 3012.2 m, well = Luo-69, (C) 3023.8 m, well = Luo-69, (D–F) 3033.5 m, well = Luo-69, (G) 3070.9 m, well = Luo-69, and (H, I) 3140.25 m, well = Luo-69.

3.5. Inorganic Geochemical Study

The concentration of Ca (calcium) ranges from 40.2 to 79.8 wt % (average of 60.3 wt %) and is the maximum of all the major elements, followed by Si, Al, Fe, S, Mg, Ru, and Sc (Figure 5A), which confirms the higher concentration of calcite mineral contents in the Es3x shale. The average contents of Fe and S in the studied shale are 2.78 and 2 wt %, respectively. Pd (palladium) is the most abundant minor element in the Es3x shale, varying from 0.9 to 2 wt % with an average of approximately 1 wt %, followed by Na, Ti, Sr, Mn, Th, Zr, P, and Cr (Figure 5B). Zinc (Zn) is the most abundant trace element in this shale having a concentration range from 0.007 (99.13 ppm) to 0.013 wt % (130 ppm) with an average of 0.0099 wt % (99 ppm), followed by Pb, Ga, Rb, Ni, Cu, and U (Figure 5C).

Figure 5.

Figure 5

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Distribution and abundances of geochemical elements in the Es3x shale from the Luo-69 well. (A) Dispersal of major elements, (B) displaying minor elements, and (C) presenting trace elements.

3.6. Concentration of Oxides in Different Types of Pyrite

Different types of pyrite have a variable concentration of oxides, especially SO3, FeO, and CO2, along with other subordinate oxides (Table 2). The concentration of SO3 and FeO is higher in the framework pyrite than in all the other types of pyrite (Table 2). The concentration of CO2 is higher in pyrite microcrystals, pyrite framboids, euhedral pyrite, and welded pyrite than in the framework pyrite (Table 2). The lower concentration of CO2 in the framework pyrite shows that this type of pyrite has very little association with OM and is formed from the pyritization of plant/algal tissues. The higher concentration of CO2 in all the types of pyrite suggests that these types of pyrite have a close relationship with OM and that the OM plays an important role in their formation. The behavior of various elements especially iron, sulfur, oxygen, and carbon in different forms of pyrite has been shown in the backscattered electron (BSE) microscope maps (Figure 6A–E).

Table 2. Concentration (wt %) of Various Oxides in Different Types of Pyrites in the Es3x Shalea.

oxides concentration (wt %) of various oxides in different types of pyrites
PM PF EP WP FP
CO2 17.29 18.35 13.48 12.20 5.88
Na2O 0.25 0 0.02 0.02 0.19
MgO 1.37 0 0.01 0 0.01
Al2O3 0 0 0 0 0.001
SiO2 0.03 0.02 0.06 0.01 0.14
SO3 47.14 49.80 51.82 53.25 56.89
K2O 0.001 0.002 0.006 0.004 0.03
CaO 2.64 0.16 0.36 0.14 0.23
TiO2 0 0 0.01 0 0
V2O5 0.009 0 0.008 0.02 0.01
Cr2O3 0.02 0.01 0 0.004 0.007
MnO 0.137 0.02 0.05 0.10 0.07
FeO 30.81 31.39 33.85 33.82 36.21
CoO 0.03 0.03 0.007 0.06 0
CuO 0 0 0.04 0.04 0.03
ZnO 0 0 0 0.007 0.05
As2O3 0.21 0.19 0.22 0.27 0.19
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a

Note: PM = pyrite microcrystals, PF = pyrite framboids, EP = euhedral pyrite, WP = welded pyrite, AP = amorphous pyrite, and FP = framework pyrite.

Figure 6.

Figure 6

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BSE maps show the behavior of various elements (iron, sulfur, oxygen, and carbon) in different forms of pyrite in the Es3x shale, (A, A1–A4) showing pyrite microcrystals and their major elements BSE maps, (B, B1–B4) showing pyrite framboids and distribution of their major elements BSE maps, (C, C1–C4) showing euhedral pyrite and their major elements BSE maps, (D, D1–D4) showing welded pyrite and their major elements BSE maps, and (E, E1–E4) showing framework pyrite with their major elements distribution BSE maps.

4. Discussion

4.1. Characteristics of Pyrite Mineralization

Pyrite is present throughout the Es3x shale interval, having diverse and variable textures in terms of lithostratigraphic distribution. As these values show a bulk quantity of pyrite in this shale, therefore it is very important to determine the textural diversity by which pyrite is present in the rock to provide the milieu for the amount of pyrite extant in the rock samples as characterized using X-ray diffraction (XRD) analysis. The existence of metabolizable OM (the type of OM for bacteria that change sulfate into sulfide) and reactive detrital Fe-bearing minerals are the principal restrictive factors for the formation of sedimentary pyrite.7,32

The widespread disparities in pyrite morphology and variable distribution of pyrite framboids’ size in the Es3x shale of the Zhanhua Sag propose that cautions are much required in utilizing these features to identify paleoredox conditions of the ancient depositional environment. In the previous literature, it was suggested that primary pyrite morphologies can be utilized as an indicator for paleo-depositional environments.3 The inclusive variations in the structure of pyrite are due to syngenetic pyritization and diagenetic pyritization. Euxinic bottom lake water conditions (syngenetic pyritization) result in a smaller size of pyrite framboids and most of the pyrites are less than 10 μm with the homogenous distribution of size with depth.3 Diagenetic pyritization below oxic or dysoxic water body results in comparatively bigger framboidal pyrites and euhedral pyrite.33 Euhedral pyrite is a common type of diagenetic pyrite. Wilkin et al.3 also proposed that the size of the framboidal pyrites is preserved after their deposition over a geologic period. In other words, the syngenetic pyrite forms in the water column with smaller size indicating an anoxic water condition, while the diagenetic pyrite forms in the sediments with a large size predicting a dysoxic to oxic depositional environment. The pyrite framboids with smaller sizes are abundantly present in the Es3x shale along with the subordinate euhedral crystals of pyrite.

4.2. Growth of Pyrite

The FE-SEM analysis revealed that the Es3x shale contains pyrite framboids, euhedral pyrite, pyrite microcrystals, welded pyrite, and framework pyrite. However, pyrite framboids and euhedral pyrite are the most abundant types that are encountered in the Es3x shale than any other type of pyrite (Figure 7). Anoxic conditions coupled with OM contribute a significant part to the formation of sedimentary pyrite. H2S is formed from sulfate after its reduction by sulfate-reducing bacterial action. This newly formed H2S reacts with Fe to develop Fe monosulfide, and then pyrite crystals are formed and well-preserved in the sediments.7 Different types of pyrites are correlated with each other based on their reaction types and generation pathways.34

Figure 7.

Figure 7

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FE-SEM photomicrographs showing the distribution of different types of pyrites in the Es3x shale. (A) 3012.2 m, well = Luo-69, (B, C) 3016.3 m, well = Luo-69, (D) 3033.5 m, well = Luo-69, (E) 3045.3 m, well = Luo-69, (F) 3064.3 m, well = Luo-69, (G) 3070.9, well = Luo-69, and (H, I) 3140.25 m, well = Luo-69.

Pyrite microcrystals are randomly distributed in almost all the samples, but their quantity is lower than pyrite framboids and euhedral pyrite (Figure 7A,C). They are in an euhedral shape and poorly aligned. The size of an individual microcrystal ranges from 0.13 to 0.88 μm. Pyrite framboids are usually formed from pyrite microcrystals after their alignment in a framboidal shape. These microcrystals are typically present close to the large-size framboidal pyrite and euhedral pyrite in the studied shale (Figure 7A,C). Their close association with larger pyrite framboids and euhedral crystals of pyrite demonstrates that these microcrystals formed during the earlier stages of pyrite diagenesis at the oxic–anoxic interface (Figure 8A,B). The formation of microcrystals at the oxic–anoxic interface shows that this interface is the active site that catalyzed the nucleation of pyrite in this shale.

Figure 8.

Figure 8

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Formation of the mechanism of different types of pyrites under variable bottom water conditions in the Es3x shale, (A) genesis of diagenetic pyrite under the oxic–dysoxic lacustrine bottom water conditions, (B) formation mechanism of syngenetic pyrites under the euxinic bottom water conditions, and (C) formation of diagenetic pyrites in the deeper burial stages. The size of pyrite formed here is similar to syngenetic pyrite because the pore volume is significantly reduced at deeper burial conditions.

Pyrite framboids are the early diagenetic sulfide phase that is comprised of aggregates of microcrystalline cubes of pyrite, clustered in a spherical framboidal shape. Pyrite framboids have a more restricted size distribution and are smaller in size in the euxinic sediments than in the overlying sulfide free water columns.3 According to Wilkin et al.,3 pyrite framboids are more readily developed in the water body than a layer of sediments and they were arranged and sorted to the water-sediment borderline during their downward progression. Therefore, the size of the pyrite framboids can be used as a proxy for ancient euxinic conditions. Framboidal pyrite develops as an aggregate of microcrystals of Fe monosulfide in the thin Fe-reducing zone formed at the reducing boundary. Framboidal pyrites stop their growth during intense anoxic depositional conditions of the underlying sulfate-reducing region where amorphous and crystalline grains of pyrite develop.3,33 During euxinic bottom water conditions, framboids with smaller diameters of 5–6 μm develop before their descendance underneath the Fe-reducing region where they cease to grow3 (Figure 8B). Thus, during the euxinic settings, small-sized pyrite framboids form and are well-preserved in sedimentary layers where paleo-ecological catalogs can confirm these conditions (Table S1).35 Whereas, during dysoxic bottom water conditions, framboidal pyrites develop on the surface of the layers of sediments where these framboids are much larger with variable sizes, which is constrained by the availability of local reactants.3 The diameter of the framboidal pyrites is variable and mostly found less than 10 μm in the Es3x shale (Figure 7B,D,G). These densely packed sphere-shaped masses of submicron-scale pyrite crystals are bright and show brass-yellow color under the reflected light of the polarizing microscope. This type of pyrite is abundantly distributed in the Es3x shale in the Zhanhua Sag. Their smaller size suggests that the Es3x shale is deposited in dysoxic to more euxinic bottom water conditions.

As pyrite framboids (Figure 7A–C,E,G) with subordinate euhedral crystals of pyrite are abundantly distributed in our study area, therefore we mainly discussed the framboidal pyrite and euhedral pyrite generation in detail here. There are two schools of thought on the formation of pyrite framboids, that is, (1) organic origin and (2) inorganic origin. According to the organic origin, pyrite framboids are formed as a result of sulfate-reducing bacterial action,36 while according to the inorganic source, framboidal pyrite is developed by sluggish variations of greigite.34,37 Hence, pyrite framboids can form from both processes (organic and inorganic). Because of the variations in the water column during pyrite deposition, framboidal pyrite can also be classified into diagenetic pyrite and syngenetic pyrite. During the oxic–dysoxic environment (Figure 8A), the location of redox border is present below the sediment layer; the diagenetic pyrite is formed during the anoxic conditions in the pore water with size ranging 4–50 μm. The lake’s water column at the bottom attains an anoxic to euxinic settings when the oxygen level in the water decreases (Figure 8B). The redox interface is shifted above the sediment layer and in such conditions, syngenetic pyrite framboids are formed (the modern Black Sea is a good example for syngenetic framboids’ growth). During the euxinic bottom water conditions, the particles of pyrite framboids fall from the water column due to gravity settling to the surficial layers of the sediments and are then concealed and buried there for a short time (Figure 8B). Therefore, syngenetic framboids have a size < 6 μm with a narrow diameter than diagenetic framboidal pyrites.38 Hence, the physiognomies of the size distribution of pyrite framboids can reflect the redox settings of the bottom water column. The diagenetic pyrite is also formed at deeper burial conditions. However, the size of pyrite formed here is very small and is similar to syngenetic pyrite because the pore volume is significantly reduced at deeper burial conditions (Figure 8C).

Detailed FE-SEM analysis shows that the euhedral pyrite crystals (Figure 7A–D) occur as a cube in the Es3x shale and are diffusely dispersed. The size of the euhedral pyrite crystals is ranging from 1 to 7 μm. This type of pyrite is mostly developed during diagenetic processes. Euhedral pyrite crystals can be detected in those samples where a less reactive iron develops from direct contact with H2S in pore water under a dysoxic water body.3 The development of euhedral pyrite crystals is slower at saturation levels less than that required for the formation and growth of diagenetic framboids. The euhedral pyrite crystals are generally believed to form through nucleation and evolution process from water solution.34,39 According to Schoonen and Barnes,40 the euhedral pyrite is formed directly from the solution but is formed due to the conversion of unstructured FeS that is produced from primary super-saturated water solution. At this stage, the degree of amorphous pyrite nucleation is higher than pyrite, therefore the faster amorphous FeS nucleation rate controls the euhedral pyrite nucleation. The euhedral pyrite is formed directly from nucleation and growth processes from a solution or due to the transformation of amorphous FeS. Therefore, euhedral pyrite can develop in the pore water of anoxic layers of sediment beneath oxic–dysoxic water column and in the euxinic to the anoxic water column and then undertake subordinate development after burial (Figure 8A–C).

In addition, some other forms of pyrite are also observed in the Es3x shale during FE-SEM analysis, that is, welded and framework pyrites. Welded pyrite (Figure 7A–D,F–I) is a type of diagenetic pyrite that is formed due to the overgrowth and alteration of pyrite crystals within larger framboidal pyrite below the oxic–anoxic interface underneath the sediment layer (Figure 8A). This type of pyrite framboids shows evidence of secondary development of diagenetic pyrite that can fill or weld the interstitial pores among microcrystallites and/or partially to completely framboids overgrowth.3 They are distributed close to the large-sized pyrite framboids (Figure 7A–C,F–I). On the other hand, framework pyrite is also a type of diagenetic pyrite that is formed due to the pyritization of plant tissues or algae under the oxic–anoxic interface (Figures 7H,I and 8A). The presence of different oxides in the framework pyrite shows evidence of the pyritization of plant tissues/algae because the presence of these different oxides shows a close resemblance with those in plant tissues/algae (Table 2). Pyritization is one of the most significant modes of plant preservation. Deterioration of plant tissues and resulting early pyrite distribution have also been known in lab work in the previous literature.41

4.3. Influx of Reactive Iron

In the modern euxinic environment, the proportion of pyrite having syngenetic origin (those developed in the water body) varies greatly than pyrite having diagenetic origin (those developed in the sediments). The reason behind this variation is related to the amount of Fe that is scavenged in the sulfur-dominating water column,38 and is a function of the reactive Fe-bearing detritus (clay and quartz), the transportation pathway of sediments, as well as physiochemical and biogenic conditions in the chemocline. Similar variations in the ratios of syngenetic to diagenetic pyrites are observed in the Es3x shale deposited under different bottom water conditions in the lacustrine basin (Figure 8A,B). The iron (Fe) content of all the representative samples from the Es3x shale displays a positive correlation with quartz and clay contents (Figure 9A), demonstrating that virtually almost all amount of Fe was introduced to the basin with that quartz and clay fractions, most probably as ferruginous grain coatings. The carbonates show a negative relationship with Fe (Figure 9B), indicating that the carbonate fraction contains negligible iron. The lower total Fe/Al content of the Es3x shale (Fe/Al = 0.45) proposes a low delivery of reactive iron to the basin floor during the Es3x shale deposition (Figure 9C). The lower concentration of quartz and clay minerals in the Es3x shale also supports this assumption. Moreover, the average concentration of Al (6.73 wt %) in the Es3x shale is less than the average value of shale (8.8 wt %).42,43 This also shows a substantial reduction of detrital influx along with the required reactive Fe for the formation of diagenetic pyrite. Based upon the shreds of these evidences, it is suggested that the influx of reactive Fe is low in the studied shale and our interpretations are consistent with other researchers.42,43

Figure 9.

Figure 9

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(A) Relationship between quartz and clay with iron, (B) correlation between carbonates and iron, and (C) relationship between Al and Fe.

4.4. Role of OM in Pyrite Generation

The biologic action subsidizes the generation of framboidal pyrite and, therefore, it is proposed that the OM plays a pivotal role in the generation of framboidal pyrite, and this statement is also consistent with other researchers.4,44−46 The theory of the organic origin of pyrite (especially the pyrite framboids) was initially anticipated by various authors after its discovery.4,47,48 Generally, OM is filled within the pores of the framboids in the studied shale (Figure 7E) and this peculiarity is also commonly reported in different shales.6,40,49,50 According to the previous literature,4,36 the structure of the pyrite framboids preserves the OM. The existence of metabolizable OM and reactive detrital Fe-bearing minerals are the main controlling factors for the development of sedimentary pyrite.7,32 Sulfur might be provided by the sulfate-reducing bacteria for pyrite generation (Figure S31).7 In the Es3x shale, many pyrite framboids are associated with OM (Figure 7E). Therefore, it can be assumed that the OM contributes a vital part in the generation of framboidal pyrite in the Es3x shale.

On the other hand, after the successful development of synthetic pyrite from inorganic sources,32,37 it was proposed by Sweeney and Kaplan37 that the pyrite framboids were also developed by cracked greigite grains replacing pyrite. Greigite framboids were formed first by monocrystals aggregates of greigite and then pyrite framboids were developed when pyrite mineral was substituted by greigite framboids.38 However, these ideal lab settings are hard to achieve in natural sedimentary courses, therefore these conditions can not signify the pyrite framboids generation completely in the natural sedimentary environments. In our study area, most of the framboidal pyrite’s association with OM also confirms that the OM is responsible for pyrite generation and growth (especially the pyrite framboids).

4.5. Pyrite as an Indicator of Paleo-Redox Conditions

The generation of framboidal pyrite is subtle to the reducing depositional environments, and framboids stop growing once they are concealed and buried.3,38 Therefore, the dispersal of pyrite framboidal size played an important role to predict the paleoredox settings of the bottom water body.3,33,51,52

A whisker plot (Figure 10) is drawn to illustrate a better understanding regarding the distribution of pyrite framboidal size in the Es3x shale. In this plot, the vertical red-colored lines show all the size distributions, and the minimum and maximum sizes are given at the bottom and the top of the lines, respectively. The rectangular boxes show the data from the subordinate quartile (Q = 0.25) to the higher quartile (Q = 0.75), specifically 50% of all the measured framboidal pyrites. The light magenta-colored flat horizontal lines on the inner side of each rectangular box show the median line. The orange-colored small circles show mean values in each box. According to Figure 10, the rectangular area of all the samples except two (2990.05 and 3064.3) are not >10 μm, which clarifies that more than 85% of the framboidal pyrite diameters are <10 μm excluding 2990.05 and 3064.3. Although the tops of the rectangular boxes of 2990.05 and 3064.3 are above 10 μm, their median lines are still below 10 μm, denoting that almost 50% of pyrite framboids’ diameter of 2990.05 and 3064.3 are <10 μm. Based on the values in Table S1,35 we can interpret the bottom water conditions in our study area using pyrite framboids size distribution. We have identified four different zones based on the values in Table 3, for example, euxinic zone (3–5 μm), anoxic zone (4–6 μm), upper dysoxic zone (6–10 μm), and lower dysoxic zone (>10 μm) of the Es3x shale (Figure 10). Hence, it is confirmed from the abovementioned values that the Es3x shale is formed in dysoxic, anoxic to euxinic depositional conditions (Figure 10). The slight variation among the ranges of the representative samples infers that there may have a small fluctuation in the concentration of oxygen in the sedimentary water body. Wilkin et al.3 also proposed that if the framboidal pyrite mean size is 5 ± 1.7 μm then it shows that these framboids were formed in a euxinic water body, and if the mean size is 7.7 ± 4.1 μm then it means these framboids were formed in the non-euxinic water body. In our study area, the mean values of framboidal pyrites are <10 μm and based on this evidence, we inferred that the Es3x shale is formed in a euxinic depositional environment.

Figure 10.

Figure 10

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Whisker plotting displaying the distribution of framboidal pyrite size in the representative Es3x shale samples.

Table 3. Size Distribution of Pyrite Framboids Using FE-SEM Photomicrographs and Image-Pro Plus Measurementsa.

samples MinFD (μm) MaxFD (μm) mean (μm) median (μm)
2937.8 2.39 6.08 3.61 3.01
2990.05 3.1 19.83 9.69 6.59
3012.2 2.11 9.98 6.52 6.03
3016.3 2.36 7.19 4.84 4.87
3018.1 2.57 7.64 4.41 4.07
3023.8 1.73 4.88 3.94 3.4
3033.5 2.27 5 4.35 4.15
3036.2 2.88 4.7 4.95 4.01
3045.3 1.11 11.72 4.95 4.38
3056.95 2.65 5.88 4.87 4.84
3064.3 2.99 16.61 7.28 4.36
3067.8 2.82 6.99 5.46 4.61
3070.9 2.96 11.05 6.56 4.96
3098.96 3.01 11.02 5.38 4.63
3100.68 2.2 5.86 4.25 3.6
3105.2 2.37 11.09 6.17 5.73
3140.25 3.29 7.77 6.94 6.64
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a

Note: MinFD = minimum framboid diameter and MaxFD = maximum framboid diameter.

Apart from the size of framboids, the positive relationship of pyrite with TOC and TS (total sulfur) also shows that pyrite is formed under anaerobic conditions (Figure 11A,B). The paleo-reducing conditions of a water column can also be observed by using a relationship between TOC and TS.53 The TOC/TS ratio < 1.5 shows anoxic conditions, between 1.5 and 5 shows suboxic, and >5 displays oxic water conditions (Figure 11C). Mn/TS and TS/Fe ratios can also be used to interpret the ancient reducing conditions of the water column during deposition.54,55 The lowest values of Mn/TS reflect the anoxic environments, while a higher TS/Fe ratio (also shows the zone of excess sulfur) shows a more reducing environment during deposition of the Es3x shale (Figure 11D). The highest values of TS/Fe from 1 onward show the zone of excess sulfur (euxinic conditions) in the Es3x shale (Figure 11D). Apart from the size of the pyrite framboids, these pieces of evidence also suggest that the Es3x shale is deposited in a reducing and euxinic environment. Hence, it is proved that the pyrite is a reliable source to interpret the paleoredox conditions.

Figure 11.

Figure 11

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(A) Plot shows the positive relationship between TOC and pyrite. (B) Plot shows the positive relationship between TS and pyrite. (C) Plot between TOC and TS for the Es3x shale. (D) Variation between the Mn/TS ratio and TS/Fe ratio, showing the depositional conditions during deposition of the Es3x shale.

5. Conclusions

After a detailed analysis of pyrite in the Es3x shale, the following conclusions are established.

  • (1)

    Five types of pyrite are observed during detailed FE-SEM analysis of the Es3x shale of the Paleogene Shahejie Formation in the Zhanhua Sag, BBB, that is, pyrite microcrystals, pyrite framboids, euhedral pyrite, welded pyrite, and framework pyrite.

  • (2)

    Framboidal pyrites are composed of densely packed, spherical-shaped masses of submicron crystals of pyrite and are abundantly distributed throughout the Es3x shale, while euhedral pyrite is composed of large-sized euhedral crystals of pyrite and their content is very low.

  • (3)

    The relationship of TOC with pyrite minerals suggests that the contribution of OM in the generation of pyrite is very important and plays a vital part.

  • (4)

    The distribution of pyrite, especially the abundance of pyrite framboids suggests that the environmental settings during the Es3x shale deposition in the lacustrine basin were anoxic, while their dominant smaller size shows their origin from the euxinic (anaerobic and sulfidic conditions) water body, which is reliable with the lack of biogenic contents and the higher contents of TOC.

6. Methodology

6.1. Rock Sample Collection and Thin Section Analysis

The shale samples were collected from the Es3x shale (lower third part of the Es3 member) of the Eocene Shahejie Formation, Zhanhua Sag in the BBB. 43 samples of core were obtained for this research work from the Luo-69 well in the Zhanhua Sag (Figure 1). It is mainly composed of gray to dark gray calcareous shale. Thin sections are prepared from core samples of the study interval. Core samples were repeatedly grounded to achieve a thickness ranging from 0.02 to 0.06 mm, and the area of the core samples is 22 × 22 mm. 43 thin sections were prepared after a detailed systematic analysis. Each sample was analyzed using a “Leica DM4 P” microscope, which had an objective lens with a maximum of 100× magnification, and a standard resolution of 0.26 μm.

6.2. TOC Content Analysis

A total of 29 shale samples were selected and analyzed for TOC contents by using a carbon and sulfur analyzer (model: LECO CS744), which was operated in a closed system, at 1200 °C. At this high temperature, the organic carbon is fully combusted and completely converted into carbon dioxide gas which is detected using the near-infrared detector. The TOC content is calculated based on the CO2 content measured using the infrared detector. The shale samples used for TOC content measurement were crushed and ground to <74 μm in size. 0.13–0.14 g of each powdered sample was added to a crucible and then immersed in a dilute HCl solution (pure HCl was diluted at a volume ratio of 1:7) for 24 h to remove inorganic carbonates. The samples were washed with distilled water to remove acid contaminations and placed in an oven for 2 h at 60 °C. Finally, these samples were placed into the LECO CS744 for TOC analysis.

6.3. XRD and X-ray Fluorescence Analysis

XRD technique is a common technique used in the evaluation of the mineralogical composition of shales. 29 samples were analyzed for clay fraction and whole-rock mineral constituents by using a “Panalytical X’Pert PRO” X-ray diffractometer, equipped with a “Cu X-ray” target (40 kV, 40 mA). Each 5 g sample was dried in an oven at 40 °C for 2 days then crushed and ground to powder form (<44 μm) fraction by using an agate mortar. Identification of various mineral phases and their quantitative relative abundances (weight %) were deduced using computer diffractogram analysis. Major elements (>1% by weight) such as Al, Si, Ca, Fe, Mg, S, Sc, and Ru, minor element (1–0.01% by weight), for example, Cr, Mn, P, Sr, Ti, Zr, Pd, Na, and Th, and trace elements (<100 ppm by weight) such as Ni, Ga, Cu, U, Zn, Rb, and Pb were identified by using an X-ray fluorescence (XRF) spectrometer. A total of 18 Es3x shale samples were selected for XRF analysis and used to identify major, minor, and trace element analysis. The distribution of these elements in the shale samples was estimated by using an “M4 Tornado (Bruker)” micro XRF spectrometer, with a voltage of 50 kV, and a current of 600 μA. A 25 μm beam size, using a time of 5 μs/pixel was used in this analysis.

6.4. FE-SEM and EPMA Analysis

Based on the results of mineralogy from XRD analysis, shale samples having different mineralogical constituents were carefully chosen for FE-SEM observation to study different morphological features of various forms of pyrite and the distribution of OM along with pyrite in the shale samples. For FE-SEM analysis, 17 representative shale samples were selected and polished with argon ions and then coated with gold to increase the smoothness and conductivity, respectively. The FE-SEM analysis was performed using a “Zeiss Crossbeam-550 (Gemini-2)” scanning electron microscope. Additionally, an EDS system (Bruker Nano GmbH, model: Flash Detector 6|100) was also used to specify the elemental constituents in detected minerals. The concentration of various oxides in the different forms of pyrite is calculated using EPMA analysis, and elemental mapping has been done by BSE-elemental mapping. Four representative samples were selected for EPMA for elements identification and mapping. An electron probe micro-analyzer (EPMA-1720 H Series electron probe of the Shimadzu Corporation, Kyoto, Japan) was used to investigate the elemental composition of the different forms of the pyrite in the studied shale. 10 nA electric current with 15 kV of accelerating voltage is used as a standard for EPMA analysis. The diameter of the beam during EPMA analysis was 5 μm.

Acknowledgments

This research work was supported by National Science and Technology Major Project of China (Grant no. 2017ZX05009-002), the National Natural Science Foundation of China (nos. U1762217, 41702139, 42072164, and 41821002), the Taishan Scholars Program (no. TSQN201812030), and the Fundamental Research Funds for the Central Universities (19CX07003A). The authors would also like to acknowledge the School of Geosciences, China University of Petroleum East China for analytical support and financial sustenance.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c05874.

  • Whole-rock mineral composition, TOC, and TS contents in the Es3x shale, showing the XRD raw figures of all 29 representative samples of the Es3x shale, role of OM in the overall process of sedimentary pyrite formation, and summary of characteristics of pyrite framboids used to indicate redox environments during their deposition (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c05874_si_001.pdf (3.2MB, pdf)

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