MINERALOGY AND GEOCHEMISTRY OF REE AND CR IN PROTEROZOIC CHERTS AT KOKSIN, CZECH-REPUBLIC

Late Proterozoic cherts and associated silicified stromatolites, relat ed to the Koksin andesitic basalt palaeo-volcano near Mitov in Western Bohemia, show the following mineral paragenesis: gorceixite, florenci te, monazite-(Nd), monazite-(La), xenotime, witherite, pyrite, BaCa(CO 3)(2) TiO2, illite, and a mica close to the end-member K2Cr4Al2Si6O20( OH)(4), which is a new mineral of the mica group and the chromium-equi valent of muscovite. These mineral phases occur in microfissures and/o r in voids of the chert. The latter shows an REE distribution characte rized by a negative Ce anomaly, the result of extensive interaction wi th sea-water. The origin of REE- and Cr- bearing phases is attributed to late and shallow hydrothermal fluids, related to subsidence of the Koksin volcano.