Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead and zinc sulfide ores hosted within carbonate (limestone, marl, dolomite) formations and which share a common genetic origin. These ore bodies range from 0.5 million tonnes of contained ore, to 20 million tonnes or more, and have a grade of between 4% combined lead and zinc to over 14% combined lead and zinc. These ore bodies tend to be compact, fairly uniform plug-like or pipe-like replacements of their host carbonate sequences and as such can be extremely profitable mines. This classification of ore deposits is also known as Mississippi Valley Type or MVT ore deposits, after a number of such deposits along the Mississippi River in the United States, where such ores were first recognised; these include the famed Southeast Missouri Lead District of southeastern Missouri, and deposits in northeast Iowa, southwest Wisconsin, and northwest Illinois. Similarly Irish-type carbonate lead-zinc ores, exemplified by Lisheen Mine in County Tipperary, are formed in similar ways. Sources The ultimate source of the mineralizing fluid(s) in MVT deposits is unknown. The ore fluids of MVT deposits are typically low temperature (100 °C-150 °C) and have the composition of basinal brines (10-30 wt.% NaCl equivalent) with pH's of 4.5-5 (buffered by host carbonates). This hydrothermal fluid may or may not carry the required sulfur to form sulfide minerals. Mobile hydrocarbons may have played a role in delivering reduced sulfur to certain MVT systems, while methane and other organic matter can potentially reduce sulfate carried by an acidic fluid. The ore fluid is suspected to be derived from clastic red bed sequences (potential metal source) that contain evaporites (potential sulfur source). Transport Two potential transportation mechanisms for the metal-bearing ore fluid have been proposed. The first involves compaction of sediments in basins with rapid sedimentation. Mineralizing fluids within the basin become trapped within discrete, over-pressured aquifers and escape episodically and rapidly. The second fluid transportation mechanism is topographically-driven gravitational fluid flow. This occurs during uplift that is commonly associated with an orogenic event. One edge of a basin is uplifted during the formation of a foreland fold and thrust belt, and basinal fluids migrate laterally away from the deformation front as the basin is uplifted. Migration of the fluids through deep portions of the basin may result in the acquisition of metals and sulfur contained within the basin. Trap The trap for carbonate-hosted lead-zinc