In the Telychian (Lower Silurian) marine red beds of the Aizpute core (West Latvia) and several cores of Saarcmaa Island (West Estonia) chalcopyrite crystals up to 0.5 mm in size were found in thin red and grey altered volcanic ash layers and in red and yellow terrigenous claystones and marls. The control of sedimentary facies on the distribution of chalcopyrite suggests its formation in early diagenesis. Chalcopyrite finds are more frequent in metabentonites than in terrigenous rocks, presumably due to better Cu-absorbing properties of the authigenic clay, and amorphous ferric and supposedly once existent but not preserved by the present time manganese oxides. In the West Latvian Aizpute core the findings of chalcopyrite are more numerous and Cu contents are higher than in Saarcmaa sections. Correlation of three metabentonite layers in the Aizpute core and the Viki core of Saaremaa hints at earlier onset of red bed facies in the Aizpute area.