The abundance of iron in Mississippi Valley–type lead-zinc deposits and districts varies greatly; some deposits contain large amounts and others are almost free of iron. Iron in Mississippi Valley–type deposits is largely paragenetically early pyrite or marcasite that was replaced by sphalerite and galena, often in the central part of the deposit or district. Sedimentary exhalative and Irish-type base metal deposits, which also form from basinal brines, have similar variations in iron content. Calculated metal contents of brines in equilibrium with galena, sphalerite, and pyrite show that iron is significantly more abundant than lead and zinc in high-temperature (>200 °C), relatively acid brines with low sulfur contents, whereas zinc dominates under most other conditions, including brines with high temperatures and high sulfur contents. These results suggest that iron-rich Mississippi Valley–type deposits form from brines expelled from the deepest, hottest parts of sedimentary basins.