Alvarez et al. (1980) explained anomalous Ir concentrations in the Cretaceous-Tertiary (K.-T) boundary informal type sediment, the Fish Clay in Denmark, in terms of a global fall-out of extraterrestrial matter. However, geochemical and mineralogical analyses of the various sublayers in the clay, reveal that not only Ir, but also elements such as Ni, Co, Zn, Mo, As, Se, and Sb are substantially enriched in the clay. The enrichments may be due to precipitation of elements as sulfides at a redoxcline. The facts supporting this view are: 1. 1) the over-all chalcophile-element abundance pattern in the clay, 2. 2) the occurrence of a redoxcline, deduced from the distribution of organic matter and oxidized iron in the clay, 3. 3) the large amounts of small pyrite spheroids present at this redoxcline, 4. 4) the high concentration of trace elements in these spheroids. Also Ni, Co, and Ir, elements that have previously been presumed to have a meteoritic origin, are substantially enriched in the spheroids. A model is suggested in which metal-bearing pore solutions lose their metals by precipitation upon passing the boundary between the oxic Cretaceous chalk and the overlying, anoxic and hydrogen sulnde-rich Fish Clay. The great similarity in geochemistry and lithological appearance between several strongly Ir-enriched clays, suggest that the model may also be applied to other K-T boundary clays.