Water-rock interaction in an acidic mine discharge as indicated by rare earth element patterns

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 provid es an excellent opportunity to study the use of REEs as tracers of water-ro ck interaction. Extraction techniques were used to min-tic the process of a cidic groundwater generation in coal-bearing strata. Samples were treated t o remove exchangeable ions and then treated sequentially with solutions buf fered at pH 5.5, 3.6 and 1.6 in order to mimic the natural buffering system s 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 inform ation concerning the relative roles of carbonate or silicate phases. The so urce strata of the spring has evolved over time and shows that different co al seams have a unique contribution. This distinct contribution appears mor e related to the mixture of sedimentary components (i.e. carbonates, silica tes, etc.) in the coal seam and the combination of shale and sandstone laye rs 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 lith ophilic elements in cases where characterisation of the source area is not possible. Copyright (C) 2001 Elsevier Science Ltd.