Geochemistry of the late Permian No. 30 coal seam, Zhijin Coalfield of Southwest China: influence of a siliceous low-temperature hydrothermal fluid

Abstract

This paper describes the influence of siliceous low-temperature hydrothermal fluid on the elemental concentrations and mineralogical characteristics of the late Permian anthracitic ( R o,max=3.58%) coal seam (No. 30) from the Zhijin Coalfield in western Guizhou Province, SW China. Coal samples were examined using instrumental neutron activation analysis, inductively coupled plasma-mass spectroscopy, inductively coupled plasma-atomic emission spectrometry, X-ray fluorescence, powder X-ray diffraction, scanning electron microscopy equipped with energy-dispersive X-ray, and isotope analysis. The modes of occurrence of elements were determined using a sequential chemical extraction procedure. The studies indicate that the No. 30 coal seam has a very high content of veined quartz (vol. 9.4%), whose isotope values of δ 30Si and δ 18O are 0.6‰ and 15.4‰, respectively, indicating that the quartz originated from siliceous low-temperature hydrothermal fluid (formation temperature 160–220 °C) rather than detrital material of terrigenous origin or magmatic hydrothermal inputs. Results of scanning electron microscopy equipped with energy-dispersive X-ray and sequential chemical extraction procedure show that the veined quartz is the dominant source of Fe, Cu, U, Pd, Pt and Ir, which are as high as 2.31%, 356, 8, 2.1, 2.43, and 0.006 μg/g in this coal seam, respectively. The studies have also found that elements, such as Fe and Cu are mainly in the veined quartz and they do not occur as sulfides in this coal seam, in sharp contrast to many other coal seams in China. The geochemical and mineralogical anomalies of the coal are attributed to the siliceous low-temperature hydrothermal fluid.