Multi-stage hydrothermal activity affecting the early Jurassic K7 coal seam from the Gaosheng coal Mine, Sichuan Basin, southwest China: Evidence from whole-rock geochemistry and C-O-Sr isotopes in authigenic carbonates

Abstract

Hydrothermal fluid is one of the major sources of elevated concentrations of elements in coals from southwestern China. However, the sources, timing, migration pathways of hydrothermal fluids remain unclear. This study investigated the petrology, mineralogy, elemental geochemistry of whole rocks, and isotope geochemistry (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr) of carbonate minerals, to reveal the sources and migration of fluids associated with hydrothermal mineralization within the K7 coal seam from northeastern Sichuan Basin, China. This seam was deposited in an intermontane basin under fresh-water conditions. It contains a complex epigenetic mineral assemblage including carbonates, quartz, pyrophyllite, clay, and minor sulfides and sulfates (gypsum, barite). Syngenetic siderite from the roof and floor mudstones with relatively high ⁸⁷Sr/⁸⁶Sr values (0.710993 and 0.710994) was deposited from fresh water. The δ¹⁸O_VPDB and δ¹³C_VPDB values (from −16.4 ‰ to −13.8 ‰ and from −11.7 ‰ to −8.1 ‰, respectively) of epigenetic calcite and ankerite in the coal indicate their formation from hydrothermal fluids originating from igneous, organic-sedimentary, and, less significantly, marine-carbonate sources. The low ⁸⁷Sr/⁸⁶Sr values (0.708556 to 0.708870) and positive Eu anomaly in coal further indicate the influence of hydrothermal fluids. Elevated Sr/Ba ratios (0.9–1.5) and enriched Ca, Fe, and Mg in the lowermost seam section are attributed to the exfiltration of fluids along coal seam boundaries. The fluids probably penetrated the Mesozoic strata of the northeastern Sichuan Basin through fractures connected with the deep-seated detachment faults. This faulting was associated with Palaeo Pacific-Plate subduction and the Yanshanian post-collisional strike-slip movements in the East China region.