In-situ sulfur isotope characteristic and genesis of sedimentary pyrite with varying crystal morphologies in coal-bearing strata in North China

Abstract

Sedimentary pyrite is ubiquitous in coal measures and occurs with different crystal morphologies and geochemical characteristics that are typically employed to infer the bottom seawater microenvironment and diagenetic stages. In this study, representative pyrite with four distinct crystal morphologies were identified in Carboniferous coal gangue in North China. The crystal structure, microscopic morphology, in-situ sulfur isotope content, and distribution of pyrite crystals were comprehensively analyzed using polarizing microscopy, X-ray diffraction, scanning electron microscope, and nanoscale secondary ion mass spectrometry to better understand the diagenetic evolution of these Carboniferous rocks. The varying morphologies and δ³⁴S values of pyrite closely correlate to the crystal growth pattern, sulfur source, and precipitation mechanism. Euhedral granular pyrite and framboidal pyrite have relatively small particle sizes and negative δ³⁴S values (average − 6.59 ‰ and − 36.62 ‰) and are interpreted to have formed through the reduction of sulfate in a brackish lagoon from the syn-depositional stage to the early diagenetic stage. In contrast, massive pyrite and cell-filling pyrite exhibit slightly larger particle size and positive δ³⁴S values (average + 9.39 ‰ and + 10.53 ‰), which suggest formation in the diagenetic stage under the action of microbial sulfate reduction. The geochemical characteristics recorded in the microscale pyrites indicate that the primary sulfur source is seawater sulfate where the wide range of pyrite sulfur isotope values reflects substrate depletion effects in an increasingly closed system. This research highlights the vital relationship between grain shape and the mechanism of sedimentary pyrite formation in the coal-forming environment. Thus, different pyrite mineralization processes in the world can be identified, and the environment of crystal growth understood by in-situ sulfur isotope analysis.