Newly published fluid-inclusion data from quartz in stockwork veins beneath seven massive sulfide lenses in the Iberian pyrite belt suggest that the lenses were formed from fluids that on reaching the sea reversed buoyancy and ponded in basins. Sulfides quenched in the resulting brine pool would have settled to form a sulfide mud. This process provides a relatively efficient trapping mechanism for metal in the fluids and effectively excludes ambient seawater, accounting for the deposits tending to have the characteristics of large size, sheet-like form, absence of relict chimney structures, and a mineral content characterized by pyrite-arsenopyrite, and absence or scarcity of barite, marcasite, and Fe oxides. If total S was less than total metals in the stockwork fluids, some or all of the more soluble Zn and Pb could have been swept from the basin at the overflow, accounting for the variable but generally low Zn and Pb contents of the ores. The lack of sedimentary source for the high salinities implicates magmatic intrusions, possibly similar to those related to Sn-W mineralization.