Depositional setup of the faunal coal balls from Bichom Formation of Lower Gondwana Group of Arunachal Himalaya: insights from EPMA and Raman Spectroscopy

An attempt is made to study the composition of the carbonate minerals and silica in the faunal coal balls of Lower Gondwana Group of rocks of Arunachal Himalaya using Electron Probe Micro Analyser (EPMA) and Raman Spectrometer (RS). It is observed that, the carbonates developed in the coal balls contain Fe, Mg and Mn which corresponds to the concentration of dolomite and indicates a relatively high salinity in the sedimentary environment or of the formation water. Raman spectrum of silica indicates the presence of hydroxylated nanometre sized α quartz- chalcedony in the coal balls. We construe the incorporation of electrolyte rich solution in the coal swamps near sea shore took place due to repeated influx of sea water in lagoonal condition facilitating precipitation of chalcedony in a restricted alkaline environment thus created.