Rare-earth elemental systematics of mixed-layered illite–smectite from sedimentary and hydrothermal environments of the Western Carpathians (Slovakia)

Abstract

Mixed-layered illite–smectite separates of various sizes from bentonites and shales collected in sedimentary and hydrothermal units of the Western Carpathians were analyzed for their major-, trace- and rare-earth elements (REEs). The major elements fairly reflect the changes in the clay mineralogy related to illitization, but a lack of correlation between this chemical composition and the REE contents suggests that their incorporation and fractionation by nucleating and growing illite-type particles is complex during both burial-induced diagenesis and hydrothermal activity. During crystal growth of illite-type fundamental particles, illitization driven by burial-related temperature increase does not fractionate the REEs incorporated in the mineral structures, but it increases their contents, suggesting that the bentonite and shale units of the sedimentary sequence act as closed systems. The amount of smectite layers in the I–S does not relate to the sum of REE or to the La/Lu ratio in the bentonites and shales of the East Slovak Basin sedimentary sequence, outlining that REE incorporation and fractionation concern s the illite layers, not the smectite layers. Alternatively, the hydrothermal system of Dolná Ves acts as an open system with supply of the REEs by external hot fluids, so that the REE contents are not determined by the precipitation of illite. The mixed-layers illite–smectite of the shales yield a narrow span of total REE abundances from 17.3 to 36.5 μg/g, compared to those of the associated bentonites, from 1.20 to 93.7 μg/g, of the same basin. These differences result from varied origins of the original REE-carriers. The Eu anomaly relates to alteration of the authigenic minerals in the sedimentary bentonites acting as a closed system, or to the chemistry of the percolating hot fluids in the Dolná Ves hydrothermal deposit. The data also indicate that no-detectable zircon, apatite, monazite, or any other REE-carrier occurred in the size fractions, at least in the finest, and if any, these minerals had no significant impact on the REE contents and distribution patterns of the varied analyzed size fractions consisting mainly if not exclusively of mixed-layered illite–smectite.