Geochemistry and metamorphic mineral facies of burned rocks in shallow coal beds

The study focuses on the burnt rocks of the 2~2 and 3~1 sections of the Yan'an Formation in Zhangjiamao Coalfield, Northern Shaanxi, and analyzes their geochemical migration patterns and metamorphic mineral facies characteristics. The results indicate that the burnt rocks in this area are low alkaline silicoaluminal metamorphic rocks, with uneven distribution of calcium and alkali horizontally. A small number of burnt rock samples have high loss on ignition and high CaO content, with Fe elements migrate and accumulate. Zr has better compatibility than Hf, and the enrichment of Ba element may be related to later hydrological processes. The trend line of rare earth element distribution is little L-REE enrichment, and the effect of burning on the distribution of rare earth elements is relatively limited. The primary minerals in burnt rocks often exhibit ablative characteristics, with the development of ablative edges and pore structures under the microscope. Newly formed minerals form aggregates and fill mineral spaces or fractures. The mineral facies combination is similar to the diorite facies that undergoes high thermal metamorphism during thermal contact metamorphism, but the metamorphic pressure is lower and the temperature is higher. The burnt rock layers can be divided from bottom to top into a combination of thin plate-like burnt rock, burnt lava, thick plate-like burnt rock, and thick layered burnt rock. tridymite+cristobalite content can better characterize the burning degreecompared to mullite+cordierite+hematite content. The degree of burning in the lower part of the same layer burnt rock is higher than that in the upper part, and the burning degree of plate-like burnt rock in the upper part of the burnt lava is higher than that in the lower part. The burnt rocks in the research area have the "hidden" characteristic and have the same sedimentary period burnt effect. Except for the burnt rock layers exposed in the valley terrace, some of the burnt rock layers are buried under the Pleistocene strata and residual yan'an Formation strata. The early formation of burnt rock layers was later destroyed, and multiple stages of burnt rock layers overlapped to form the current burnt area.