Occurrence of analcime in the middle Jurassic coal from the Dongsheng Coalfield, northeastern Ordos Basin, China

Abstract

This paper describes the occurrence of significant authigenic analcime (NaAlSi2O6·H2O) in the middle Jurassic coal from the northeastern Ordos Basin and discusses its origin. Twelve coal samples from borehole ZK31-19 in the Dongsheng Coalfield were collected and analyzed. The coals are characterized by a low-medium ash yield, high volatile matter and low sulfur contents. The major-element oxides in the studied coals are mostly at similar or much lower concentrations compared to the averages for Chinese coals. Similarly, most trace elements are depleted or in concentrations close to the respective averages for world low-rank coals. Based on Seredin-Dai's classification, the REY (rare earth elements and Y) in the coal mainly display N- and H-type distribution patterns, with no Ce and Eu anomalies. Coal sample ZK31-19-12 from the lower Zhiluo Formation contains analcime up to 23 wt%, with a high Na2O concentration of 1.23% (both on a whole-coal basis). The analcime occurs as lath, as subhedral granular crystals in fissures or pores and as anhedral crystals in the organic matrix. Furthermore, significant analcime has also been observed by SEM in coal from the upper Yan'an Formation (sample ZK31–19-10), which mainly occurrs as anhedral crystals. No evidence of analcime forming from precursors of volcaniclastic material, alkali zeolites or other silicates was detected, therefore, it is deduced that the analcime in the studied coals formed by authigenic precipitation. The Na2O content in the studied coal seams displays a vertical variation different from that of the ash yield (Ad) and those of the detrital-dominated elements, such as SiO2, Al2O3, TiO2, Zr, Nb and Th, indicating a different origin of Na compared with other detrital materials of terrigenous origin. Combined with the evidence of the obvious Na peak in authigenic amorphous material and gypsum mineral by SEM, it can be inferred that the source of Na may derive from the fluids. The paleoclimatic change (from subtropical warm and humid to semi-arid and arid hot, during the deposition period of the Yan'an to the Zhiluo Formation) may provide favorable conditions for analcime authigenesis, leading to its abundant precipitation in the lower Zhiluo Formation and its limited occurrence in the upper Yan'an Formation. Such paleoclimatic change is also significant for the U mineralization in the lower Zhiluo Formation.