Origin of composite fibrous carbonate veins in Middle Jurassic ultra-low-ash coal, Ordos Basin, China: Insights on the interaction between organic carbon and inorganic carbonates after peat burial

Abstract

Valuable geological information on coal is contained within both the organic and inorganic constituents. Buried organic matter and precipitated carbonate minerals, as key elements of the lithospheric carbon reservoir, raise questions about whether their co-occurrence in coal is coincidental or reflects an underlying connection. Authigenic carbonate minerals in coal exhibit diverse species (e.g., siderite, calcite, and the ankerite-dolomite series) and morphologies (e.g., coal balls; nodules; and cell-, cleat- and fracture-filling forms), reflecting various formation processes during coalification. Fibrous carbonate veins in black shales consist of elongated calcite crystals with large aspect ratios (a rare calcite habit), and are associated with hydrocarbon fluid activity. However, the identification, formation mechanism, and material sources of fibrous carbonate veins within coal remains unclear. To resolve this problem, detailed petrographic, mineralogical, and geochemical analyses were conducted on the middle Jurassic No. 2(2) coal of the Ordos Basin, China, including stable isotopic analysis of carbon and oxygen in composite carbonates excluding siderite. The No. 2(2) coal seam is classified as an ultra-low-ash, super-low-sulfur, and high volatile B bituminous coal, according to the Chinese national standards GB/T 15224.1-2018, GB/T 15224.2-2021, and ASTM Standard D388-23, respectively. It contains abundant fusinite and semifusinite fragments dispersed in collodetrinite. Compared with average world coals, only Sr in the coal is slightly enriched. The carbonates in the coal (33.5 %; low temperature ash basis) consist of calcite, ankerite, and siderite. The values of δ¹³C_VPDB and δ¹⁸O_VPDB associated with composite carbonate veins (calcite and ankerite) range from −34.87 ‰ to −11.43 ‰ and from −14.66 ‰ to −10.84 ‰, respectively. Two types of high-ash yield intervals (Si-rich and Ca-rich) were identified within the low-ash coal seam. The Si-rich benches are attributed to the influx of Si–Al-rich fluids during peat accumulation. Composite carbonate veins, which formed during coalification, represent another source for inorganic matter in high-ash intervals (Ca-rich). Optical and electron microscope observations, together with carbon-oxygen isotopic analyses, indicate that overpressurised fluids from coal maturation and methane oxidation led to in situ coal fracturing, followed by the formation of composite fibrous carbonate veins and crack-seal structures in low-ash coal. Based upon these results, a model for the formation of fibrous carbonate veins within low-ash coal from the Cuijiagou mine is proposed to facilitate an understanding of the carbon transformation from organic matter to minerals after peat burial.