The Cierco Pb-Zn-Ag vein deposits, located in the central Pyrenees of Spain, crosscut Paleozoic metasedimentary rocks and are in close proximity to Hercynian granodiorite dikes and plutons. Galena and sphalerite in the deposits have average δ 34S values of -4.3 and -0.8 per mil (CDT), respectively. Coexisting mineral pairs give an isotopic equilibration temperature range of 89° to 163°C which overlaps with the 112° to 198°C range obtained from primary fluid inclusions. Coexisting quartz has a δ 18O value of 19 ± 1 per mil (VSMOW). The fluid which deposited these minerals is inferred to have had δ 18O H2o and δ 34S H2s values of 5 ± 1 and -1 ± 1 per mil, respectively. Chemical and microthermometric analyses of fluid inclusions in quartz and sphalerite indicate salinities of 3 to 29 wt percent NaCl equiv with Na + and Ca 2+ as the dominant cations in solution. The Br/Cl and I/Cl ratios differ from those characteristic of magmatic waters and pristine seawater, but show some similarity to those observed in deep ground waters in crystalline terranes, basinal brines, and evaporated seawater, Barite, which postdates the sulfides, spans isotopic ranges of 13 to 21 per mil, 10 to 15 per mil, and 0.7109 to 0.7123 for δ 34S, δ 18O, and 87Sr/ 86Sr, respectively. The three parameters are correlated providing strong evidence that the barites are products of fluid mixing. We propose that the Cierco deposits formed along an extensional fault system at the margin of a marine basin during the breakup of Pangea at some time between the Early Triassic and Early Cretaceous. Sulfide deposition corresponded to an upwelling of hydrothermal fluid from the Paleozoic basement and was limited by the amount of metals carried by the fluid. Barite deposition corresponded to the waning of upward flow and the collapse of sulfate-rich surface waters onto the retreating hydrothermal plume. Calcite precipitated late in the paragenesis as meteoric or marine waters descended into the fault system, possibly during a regression in the overlying basin. There are other deposits resembling Cierco elsewhere in the Iberian peninsula. Taken as a group, they are evidence that hydrothermal circulation systems were widespread during Mesozoic extension. Differences among the deposits can be related to the fact that H 2S and other solutes had local and variable sources.