Abstract For many years hydrothermal karst phenomena have been proposed as important ore-forming processes in MVT deposits, a concept based upon the assumption that both karst and ores resulted contemporaneously from the same hydrothermal solution. Because the temperature range of sulfide crystallization at 90°-145°C is clearly hydrothermal, only the contemporaneity of karst dissolution and mineralization needs to be demonstrated to validate hydrothermal karst as an ore-forming process. Evidence supporting this relationship is provided by mineralized karst collapse breccias in which successively younger minerals envelop clastic products from earlier mineralization and brecciation episodes. Further evidence of contemporaneity comes from mineralized internal sediments representing three types of deposition: clastic, chemical and residual. Internal sediments were deposited in dissolutional and karst-tectonic openings, and reveal structures which imply phreatic conditions, in solutions capable of precipitating sulfides, barite and carbonates. The chemical and clastic deposition of sphalerite is alternating and both concordant and discordant. The syndepositional deformation of sulfide-bearing internal sediments is gravity-controlled and can be ascribed to the action of dropstones and collapse brecciation, signifying contemporaneity of karstic dissolution and deposition of mineralized internal sediments.