The transgressive basal Zechstein conglomerate penetrated by the Bree borehole (NE Belgium) is 3.15 m thick and has a polymict composition. Originally, clasts consisted predominantly of limestone, dolomite, sandstone and quartzite. Pervasive replacement by ankerite has affected the carbonate clasts, especially in the middle part of the conglomerate layer. Ankerite also forms a pore-filling cement. An origin from Fe-rich waters derived from the underlying Carboniferous clastics is indicated by the chemical and isotopic (depleted deltaO-18 and deltaC-13) characteristics of the ankerite. An evolution in fluid composition due to increasing water/rock interaction is reflected in chemical and isotope variations. The preferential precipitation of both replacive and pore-filling ankerite in the middle part of the conglomerate, reflects channeling of ankerite-bearing waters in this originally highly porous and permeable part.