The mode of occurrence and origin of minerals in the Early Permian high-rank coals of the Jimunai depression, Xinjiang Uygur Autonomous Region, NW China

Abstract

This paper investigates the mineralogy of coal and non-coal samples from the Haerjiao exploration area in the Jimunai depression, Xinjiang Uygur Autonomous Region, northwestern China, using X-ray diffraction (XRD), optical microscopy and scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM-EDX). The minerals in the anthracite proximal to the igneous intrusions are represented by the occurrence of chlorite, muscovite, and illite, which are absent in the semianthracite distal to the igneous intrusions. Kaolinite, the dominant mineral in the semianthracite, decreases in abundance or even disappears in the anthracite. Muscovite and illite in the anthracite are interpreted to result from the alteration of the kaolinite precursor and possibly formed by the interaction of kaolinite and elements (e.g., K, Fe, and Mg) from igneous hydrothermal solutions. The formation of chlorite is the result of the transformation of kaolinite due to the interaction of kaolinite with Fe-Mg-rich solutions during the diagenetic stage and the direct epigenetic precipitation from Fe-Mg-rich igneous siliceous hydrothermal fluids. Calcite, ankerite, and sulfide minerals resulted from epigenetic precipitation from hydrothermal fluids. Cleat/fracture mineralization indicates that the Fe-Mg-rich hydrothermal fluid from which chlorite precipitated was introduced earlier than the Ca-rich fluid from which calcite precipitated. The injection of hydrothermal solutions that formed sulfide minerals occurred at the latest stage.