WATER-ROCK INTERACTION IN AN ACIDIC MINE DISCHARGE AS INDICATED BY RARE EARTH ELEMENT PATTERNS

Abstract

The rare earth element (REE) composition of an acidic spring discharging at pH 2.7 was studied in relation to the REE composition of selected extracts of the coal-bearing strata hydrologically above the discharge. The low pH of the discharge and the access to the strata through which it flows provides an excellent opportunity to study the use of REEs as tracers of water-rock interaction. Extraction techniques were used to mimic the process of acidic groundwater generation in coal-bearing strata. Samples were treated to remove exchangeable ions and then treated sequentially with solutions buffered at pH 5.5, 3.6 and 1.6 in order to mimic the natural buffering systems of groundwaters. Finally, samples were oxidised with hydrogen peroxide in an attempt to remove the pyrite.The results show that the composition of the spring is controlled by pyrite oxidation and buffering by Fe oxy-hydroxides, the latter with compositions inherited from carbonates from within the sequence. The similarity of REE patterns of the carbonate and the silicate extracts tends to obscure information concerning the relative roles of carbonate or silicate phases. The source strata of the spring has evolved over time and shows that different coal seams have a unique contribution. This distinct contribution appears more related to the mixture of sedimentary components (i.e. carbonates, silicates, etc.) in the coal seam and the combination of shale and sandstone layers surrounding coal seams than the composition of the components themselves.The similarity between REE compositions of the sequential extracts suggests that relationships discovered for this spring will be applicable elsewhere in the basin. However, this very property will hamper the use of such lithophilic elements in cases where characterisation of the source area is not possible.