Transformation For Clastic Rocks By Spontaneous Combustion Of Coal Seams In Terrigenous Basin

The spontaneous combustion of coal seams is a common phenomenon during the geological history.Ordos Basin is one of the major coal-producing areas in northern China.Due to the coal seam spontaneous combustion,a large scale of terrigenous clastic rocks from the Jurassic Yan'an Formation and Zhiluo Formation turned to burnt rocks.The products of the coal seam spontaneous combustion are described and systematically classified based on the study of the Jurassic coal seam in the northeastern Ordos Basin.The products of coal spontaneous combustion are divided into the following three categories:(1)Coal ash and burnt coal(remoulded product of coal seams).(2)Burnt rock(remoulded product of overlying and adjacent terrestrial clastic rocks).(3)Supergene hydrothermal fluid(formed because of the alternation of dry and wet climate).We established a systematic classification of the burnt rocks on the basis of previous studies and the research of the outcrop characteristics of burnt rock from Jurassic Yan'an Formation in Shenshangou,Bailiutugou and Daliuta area in northeastern Ordos Basin and the study of lithology,mineral and the characteristics of drilling core.The burnt rocks can be divided into three different types: low temperature baked rock(layered baked rock),grilled rock(layered grilled rock and slab-like grilled rock)and melt rock.According to the investigation,the characteristics of different burnt rocks were described and classified individually.Also,the red sandstone and white or gray-white sandstone from "oxidation zone" formed by spontaneous combustion coal seam are regarded as a special type of burnt rock.We summarized four kinds ofmodification effects that spontaneous combustion coal seamacted onthe terrigenous clastic rocks by using microscopic analysis,scanning electron microscopy,organic matter maturityanalysis,clay mineral analysis,inclusions test and other means of analysis.The modification effectsinclude: dehydration,oxidation,thermal metamorphism,hydrothermal alteration andelement migration.We also putted forward a mode of the modification effectthat coal seam spontaneous combustion act on terrigenous clastic rocks on the basis of the research in Shenshangou area.It is also helpful to study the evolution of the basin and the change of the paleoclimate by studying the coal seam spontaneous combustion and itsmodification to terrigenous clastic rocks in coal bearing basins.The supergenehydrothermal metallogenic mode of the uranium deposit was proposed in the Zhiluo Formation in Dongsheng area,Ordos Basin.Due to the arid climate,the Jurassic coal seam in the northeastern Dongsheng self-ignited during Late Mesozoic to Cenozoic,and the supergene hydrothermal fluid was formed because of the climate changed from arid to wet.The supergene hydrothermal fluid flows along the sandstone layerswhich have high permeability,and elements,such as Se,U6+ and Mn,in spontaneous combustion coal seam and high temperature surrounding rocks were taken out by supergene hydrothermal fluid.With the gradual reduction of the temperature of supergene hydrothermal fluid or its migration are blocked,these elements will precipitate from the fluid and enrich in somewhere.