Mineralogical and geochemical characteristics of pyrometamorphic rocks induced by coal fires in Junggar Basin, Xinjiang, China

Abstract

The mineralogical and geochemical characterizations of the pyrometamorphic rocks caused by coal fires are discussed. The minerals in the combustion metamorphic rocks, as analyzed by X-ray diffraction (XRD), are anorthite, hematite, tridymite and cristobalite, quartz in clinkers; and tridymite, sekaniaite, sanidine, mullite, cristobalite, and quartz in paralavas. Tridymite and sekaniaite account for the largest mineral proportion in paralava. The major elements and rare earth elements (REEs) were determined by X-ray Fluorescence (XRF). Combustion metamorphic rocks are characterized by the major elements and REEs. Three methods to evaluate obtained normalized REE distribution patterns were used to evaluate characteristics of combustion metamorphic rocks. Chondrite-normalized distribution characteristics exhibit intense negative anomalies Eu and lack a Ce anomaly. North American Shale Composite (NASC) normalized patterns show a slight negative anomaly in δEu and lack anomalies in δCe for clinkers, but the paralavas show a positive δEu anomaly. Compared with Upper Continental Crust (UCC) normalized patterns, there is a slight difference (LREE-depleted, but HREE-enriched), and it is similar to NASC-normalized patterns. Primitive mantle-normalized trace elements show significant differences in combustion metamorphic rocks, which the clinkers show larger variations in Pb than paralavas, and paralavas have significant negative Dy anomalies. High content of Fe element may result in enrichment in Ni, Co, and Cu.