Mineralogical composition of Late Permian coal seams in the Songzao Coalfield, southwestern China

Coals from three seam sections in the Songzao Coalfield, SW China, are mainly high-ash, high-sulphur semianthracites. Minerals within the Songzao coals are mainly kaolinite, pyrite (or marcasite in some cases), and quartz, with various proportions of non-kaolinite clay minerals, carbonates, feldspars, and anatase. The illite and mixed-layer illite/smectite (I/S) are Na-rich in some of the Datong coal samples. The I/S in the lower coals of the Datong section is most likely an alteration product of dispersed volcanic ash, due to the availability of necessary ions (e.g. K, Na, and Mg) in the marine-influenced coal swamp. Organically-bound Na, which was expelled from the organic matter with coal rank advance, especially with anthracitization, may have supplied additional Na for the formation of Na-rich illite. Authigenic I/S also occurs in a Tonghua coal ply that is overlain by a mafic bentonite and underlain by an alkali tonstein. Potassium, Na, and Mg for the formation of such I/S were probably derived from the leaching of the adjacent alkali tonstein and mafic bentonite. Although the marine water was also a possible supplier of the alkali elements, authigenic I/S is rare in coal plies that occur further away from the altered volcanic layer. Leaching of the volcanic claystones in the Tonghua coal seam probably led to the formation of relatively abundant anatase and rhabdophane in the underlying coal ply. Fracture-filling REE minerals (probably REE-hydroxides or oxyhydroxides) also occurring in that coal ply crystallized from ascending REE-rich hydrothermal fluids, probably associated with contemporaneous volcanic activity.

► The illite and I/S are Na-rich in some Songzao coals.
► I/S in some coals is the alteration product of volcanic ash and sea water.
► Authigenic I/S was derived from the leaching of adjacent volcanic layers.
► Leaching of volcanic ash led to the formation of anatase and rhabdophane.
► REE-hydroxides/oxyhydroxides were derived from hydrothermal fluids.