Clay mineralogy, crystallinity, and K-Ar ages of illites within the Polish Zechstein basin: Implications for the age of Kupferschiefer mineralization

In a preliminary study, the K-Ar ages of illite indirectly dated the time of base metal mineralization within the Kupferschiefer shale in the Polish Zechstein basin. To test the validity of these preliminary results and conclusions, additional samples of the Kupferschiefer were collected throughout this basin and studied with re spect to clay mineralogy and K-Ar age. Significant differences in the crystallinity, polytypism, and K-Ar ages of re-illite were noted between samples within, or close to, the mining district at the fore-Sudetic monocline (the mineralized zone) and samples from the barren, pyritic Kupferschiefer from the central and northern part of the Polish Zechstein basin (the nonmineralized zone). In all the shales included in this stud); the clay fractions were mixtures of 2M(1) and 1M, and/or 1M(d) illites. Illite within the mineralized zone showed lower crystallinity and a smaller amount of the 2M(1) polytype compared to illites farther from the mineralized area. The K-Ar ages of the <2-mu m fractions of the Kupferschiefer from the mineralized zone fall in a narrow range around 250 Ma. The K-Ar ages of illites from the nonmineralized zone are greater, ranging from 277 to 348 Ma. The ages of diagenetic illite in five samples from the mineralized zone were estimated by linear extrapolation to 0 percent 2M(1) illite of plots of the It-Ar ages of different grain size fractions against their percentages of 2M(1) illite. The ages so estimated for the diagenetic illites are in the range of 190 to 216 Ma. Likewise, the ages of detrital illite from Kupferschiefer shale across the entire basin were estimated by extrapolation to 100 percent 2M(1) illite of plots of K-Ar age against percentage of 2M(1), illite for different size fractions. These estimated ages for detrital illite increase toward the north, reaching 370 Ma, which is consistent with the idea that Caledonian basement rocks in northern Poland supplied clastics to the Kupferschiefer. The differences in crystallinity polytypism, and K-Ar ages of illite cannot be exclusively explained by differences in burial depth of the Kupferschiefer in the Polish Zechstein basin. The results of this study argue for enhanced neoformation and/or recrystallization of illite at the time mineralization took place and suggest that the formation of diagenetic illite was induced by the mineralizing event.