The relationship between paleotopography and sulfur distribution in some coals of western Pennsylvania

Abstract

The effects of paleotopography on the distribution of pyritic sulfur were determined by comparing coals deposited in paleotopographic lows with superjacent coals deposited on relatively flat surfaces. The latter coals, used as experimental controls, exhibited little lateral variation and consisted mainly of bright clarain with low pyritic sulfur (< 2%). The coals deposited in topographic lows showed complex lateral and vertical changes in coal petrography which were paralleled by variations in the mineral composition of the coal ash and the percentage of pyritic sulfur. Durains and bone coals located in paleotopographic depressions, and associated with clay partings, contain relatively large amounts of detrital illite and quartz and pyritic sulfur (2-5%). Bright clarains have low sulfur contents (< 2%) and contain relatively little detrital material, the principle clay mineral being authigenic kaolinite. The clay partings are composed mainly of illite and quartz and have sulfur values less than one percent.The abundance of pyritic sulfur in coals with relatively abundant detrital components, is interpreted to mean that the availability of ferrous iron is a critical factor in controlling the distribution of pyritic sulfur. Ferric iron, insoluble in alkaline solutions, is adsorbed on clays and transported into the peat swamp by currents which follow topographic lows. Below the sediment-water interface, ferric iron is reduced to ferrous which then migrates into adjacent, reduced parts of the peat, there combining with sulfide sulfur to form pyrite.