Distributions and associations of selected trace elements in the lower Kittanning seam, western Pennsylvania, U.S.A.

Abstract

Trace elements within the Lower Kittanning seam (western Pennsylvania) show significant basinal variations. Many of these variations can be related to differences in the depositional environments of the peat, as inferred by the nature of the overlying shales, in that the distribution of minerals in this seam, particularly that of clay minerals, is controlled by depositional environment. Chromium, Cu, V and Zr show significant increases towards the northwestern and southeastern margins of the basin. These trends can be explained by an association with clay minerals (Cr, V, and, in part, Cu), or with minor amounts of other detrital minerals (Zr). Clay composition of the Lower Kittanning seam reflects proximity to the sediment source and stability of the various clay minerals in the swamp environment. Kaolinite, which increases in concentration towards the basin margins, is primarily authigenic and shows a distribution that reflects the stability of this mineral in the relatively low pH, freshwater areas of the peat swamp. In this seam, illite-mica is predominantly detrital, as inferred by polytype studies, and increases along isolated sections of the basin margins where a greater detrital influx was experienced. Within the <2 micron fraction, illite-mica increases towards the center of the basin, at the expense of kaolinite, due to increased stability of this clay in more neutral, potassium-saturated waters. The distribution of Rb follows that of K, suggesting an association with illite-mica or mixed-layer clays. Vanadium, which shows an association with the clay fraction may have been incorporated originally into the peat as organic complexes; during coalification, this element may have become liberated and adsorbed onto the clays. Zirconium increases towards the basin margins where detrital zircon would be higher. Strontium, which increases towards the basin margins, does not correlate with aluminosilicate elements and its distribution may be related to geochemical conditions within the swamp conducive to the precipitation of a phosphate mineral phase. Beryllium and Ni also increase towards the basin margins. These elements may have been trapped at the margins of the basin by the organic fraction of the peat or coal. Similarly, Cu may owe its distribution, at least in part, to organic trapping. A decrease in Be content along the southeastern margin of the basin may be due to the increase in coal rank observed in that area. No basin-wide trends were seen for Ba, Mn and Zn and their concentrations are controlled by local mineral occurences. Manganese variations, for example, can be related to local occurrences of siderite.