Abstract At the Beddiane Mississippi Valley-type (MVT) Pb-Zn deposit, two types of internal sediments were formed: detrital and chemically precipitated. They are irregularly conformable karst cavity fillings consisting mainly of carbonate and sulfide constituents (mostly galena and sphalerite) found in their immediate host rocks. Lead-rich internal sediments occur in the vicinity of primary lead mineralization and zinc-rich internal sediments near zinc-rich primary mineralization. They exhibit distinctively different petrographie textures, lithofacies associations and geochemical characteristics which relate them closely in timing to the emplacement of primary zinc-dominant and lead-dominant mineralization. In effect, the precipitation of primary sulfides, an acid-generating process, resulted in disequilibria in the host dolomitic rocks represented by the solution of enclosing carbonates, and ultimately resulted in the liberation of sulfide fragments, sands and silts and to the solution of primary sulfides, all of which contributed to the subsequent detrital and chemical internal sediments. Detrital sediments represent physically liberated wall rock and its contained sulfides redeposited with typical current transported features (e.g., graded bedding, dropstone effects). The dissolution of enclosing coarse-grained (porous) dolostones and their sulfides ultimately led to supersaturation in Ca, Mg, Pb, Zn and CO 2 and deposition of chemical internal sediments. The fluids responsible for the disintegration of the host dolostones and the emplacement of internal sediments were late, relatively hot, and oxidizing. Under these conditions, organic matter generated organic acids which played an important role in increasing dolomite porosity and liberating sulfides physically and chemically from their carbonate host rocks.