Fluctuation of Depositional Environment in the Bang Mark Coal Deposit, Krabi Mine, Southern Thailand: Stable Isotope Implication

Abstract

The isotopic composition of organic sulfate-sulfur and pyrite in coal from the Bang Mark coal deposit in the Krabi mine, southern Thailand, was analyzed to determine the depositional environment of the coal. High-sulfur content in coal has δ34S values that range from +1.6 to +9.8 . Organic sulfur in coal has δ34S values that vary narrowly from -1.4 to +4.7 . The δ34S values of pyrite range from -3.0 to +1.4 . The δ34S values indicate that most of the sulfur in pyrite and coal was derived from a magmatic or hydrothermal source. This sulfur was associated with organic matter during or after deposition. This also suggested that the source of sulfur could have come from hydrothermal mineralization surrounding the area. Twenty-four fossil shell specimens of Viviparus sp., Melanoides sp., and Mya arenaria were analyzed for their content of isotopic δ13C and δ 18O. The δ13C in these shells have negative values that range from -5.9 to -0.1 and average - 2.9 ± 2.8 . The δ 18O in these shells has a wider range of negative values, ranging from -11.7 to -2.9 . It averages -8.1 ± 5.2 . The δ13C and δ 18O values indicate freshwater shells in the lower sequence and brackish water shells in the upper sequence. Therefore, the depositional environment of the Bang Mark coal deposit based on stable isotopic study was dominated by freshwater in the beginning and later became brackish water as a result of marine invasion under warmer conditions in a tropical region.