Trace Element Geochemistry of Brackish-Water Coals in the Central Alberta Plains, Canada

Abstract

Concentrations of the elements As, B, Br, Cl, Cr, Mn, Na, Sb, Th, U, V, and Zn were determined in ply-by-ply samples from two sections of subbituminous coal from seam 3, Vesta Mine, Alberta Plains. The elemental mean concentrations in section 1 were As l.0 ppm, B 215ppm, Br 2.3ppm, Cl 29ppm, Cr 7.4ppm, Co 1.1 ppm, Mn 24.5 ppm, Mo 2.5 ppm, Na 4900ppm, Sb 0.4 ppm, Se 0.8 ppm, Ti 270 ppm, U 1.4 ppm, Th 2.8 ppm, V 2.9 ppm, and Zn 9.5 ppm. In section 2, the mean concentrations were As 0.7 ppm, B 201 ppm, Br 1.6 ppm, CI 24 ppm, Cr20ppm, Co 1.7 ppm, Mn 30.0ppm, Mo 2.8ppm, Na 4200ppm, Sb 0.3 ppm, Se 0.7ppm, Ti 340ppm, U 0.8 ppm, Th 1.8 ppm, V 4.4 ppm, and Zn 68 ppm. Total sulfur content averaged 0.44% in section 1 and 0.48% in section 2. Mean concentrations of these elements are low when compared to western Canadian coals of similar rank, or to world coals (Swaine, 1990). Antimony, barium, and boron are enriched, with the latter being a reflection of the coal-forming environment. Despite the short lateral distance between the sections (∼100 m), seam 3 in section 1 has slightly higher mean concentrations of As, B, Br, Cl, Na, Sb, Se, U, and Th than in section 2. Seam 3 in section 2 has slightly higher mean concentrations of Cr, Co, Mn, Mo, Ti, V, Zn, and S than in section 1. Elements were differentiated on the basis of their inorganic association, organic association, or intermediate (mixed) association. Thus, As, Ba, Cr, Mo, K, Na, and U in section 1 have a positive correlation with ash and aluminum contents of the coal, while Ca and Co show a weak to moderate relationship with ash content. Iron and Mn do not exhibit a clear relationship to ash in coal from section 1. In section 2, the elements As, Ba, Fe, Mg, Mo, K, Na, Th, U, V, Cr, and Co show a weak to strong relationship with ash content, while Ca and Mn show a moderate relationship. The halogens show a negative relationship with ash content in both sections. An inertinite-rich layer in section 1 has higher concentrations of Ca, Mn, and Fe, when compared to huminite-rich layers above and beneath. It also has the lowest boron concentration (25 ppm), lowest sulfur content (0.19%), and highest fixed carbon value in the section. The high boron concentrations in the Vesta Mine coals (mean is 208 ppm) indicate interaction of the peat with brackish waters during deposition, except for the inertinite-rich layer. The study also shows that sulfur content in coal does not always show a correlation to marine influence during coal formation. Perhaps, differences in the paleoenuiron-mental setting may be responsible for the boron/sulfur relationship in the Vesta Mine coals.