Abstract
Geochemical characteristics of rocks in fault zones have been extensively studied, while there are limited studies on coal occurring in fault zones of underground coal mine. In this study, five coal samples were carefully collected from a reverse fault zone in Qi’nan colliery. Systematical detection methods were employed to analyze the different chemical and physical characteristics of fault-related coal samples. Through comparative analysis, the following insights are obtained. Three subdivided fault zones were classified according to the deformation characteristics of coal samples. Frictional heat and strong ductile deformation generated by fault motion led to the dissociation of phenol and carboxyl groups in coal molecules, which sharply decreased the concentrations of elements Co and Mo bound to these functional groups in zone I. The modified pore-cleat system in zone I with higher pore volume and lower permeability allowed solutions containing enriched trace elements to migrate through zone I locally. Concentrations of HREE, MREE and related elements associated with the invasive solutions showed significant positive anomalies in zone I. Precipitation and smearing of clay minerals in zone I led to poorer connectivity. Disruption and delamination of laminar clay minerals by strong compression-shear stress significantly increased the adsorption sites for related elements, especially the HREE and MREE. Nano-scale clay minerals resulting from stress-induced scaly exfoliation also enhanced the retention capability of REE in zone I.
Highlights
Geochemical and geophysical characteristics of fault rocks are primarily controlled by fault zones or fault systems [1,2,3,4,5]
Clay minerals appearing in zone I were broken into porphyritic shapes with smaller particle sizes compared with those in zone II (Figure 7)
Comparison of diverse tectonically deformed coals (TDCs) showed that HREE and MREE were highly concentrated in sample 3
Summary
Geochemical and geophysical characteristics of fault rocks are primarily controlled by fault zones or fault systems [1,2,3,4,5]. The permeability controls the migration of fluid in different subdivided zones, which is important for the chemical alteration of fault rocks [15,16,17]. Fractures and cataclasis within the fault core significantly increase the permeability of rocks, which is beneficial to the fluid migration [18,19]. Permeability evolution, fluid migration, and chemical alteration in fault zones are auxiliary to each other in a faulting deformation process. Faults in coal seams are related to tectonic stress concentration, which could strongly alter intact coals into deformed ones [26]. Based on the knowledge of geochemical characteristics of fault-related coal samples, differentiation mechanisms of trace elements in the fault zone are preliminarily clarified
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