Abstract

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.

Highlights

  • Siderite is a common carbonate mineral, and sedimentary siderite is formed by complex reactions of iron and organic matter precipitated during quasi-syngenesis (Sánchez-Román et al, 2014; Wittkop et al, 2014; Weibel et al, 2016)

  • 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)

  • Based on the morphology of siderite in coal-bearing series, it can be divided into three types and six subtypes

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Summary

Introduction

Siderite is a common carbonate mineral, and sedimentary siderite is formed by complex reactions of iron and organic matter precipitated during quasi-syngenesis (Sánchez-Román et al, 2014; Wittkop et al, 2014; Weibel et al, 2016). 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

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