Located in the Eastern Cordillera of northwestern Argentina, the Aguilar mine and several smaller prospects are aligned north-south in a 30- by 5-km district. The Zn-Pb-Ag sulfide ores are hosted in intensely folded and faulted Lower Ordovician quartzites and hornfelses, near their contact with the Cretaceous Aguilar granite. The orebodies are stratigraphically conformable with the siliciclastic rocks and parallel to the contact metamorphic halo of the granite, in the pyroxene-hornblende and albite-epidote hornfels facies.The ores are associated with sediments formed in local depressions of a tectonically active, shallow-marine environment and consist of strata-bound and stratiform sulfide lenses and layers several hundreds of meters long and wide, and tens of meters thick. A distinctive ore stratigraphy is recognizable: disseminated and stockwork sulfides overlain by breccia-hosted sulfides and banded to massive sulfides. Lead-rich fissure veins and quartz veins cut these types of strata-bound sulfides; barite is abundant in one of the prospects. The ores are made up of fine- to medium-grained intergrowths of sulfides and sulfosalts in quartzite and calc-silicate gangue. The sulfides are intensely recrystallized and annealed in the Aguilar deposit, which is located in the pyroxene-hornblende metamorphic halo. In the Esperanza prospect, located in the lower grade albite-epidote halo, the sulfides are finer grained, less recrystallized, not annealed, and contain abundant framboidal pyrite and textures of soft-sediment deformation.The calc-silicate assemblage in the Aguilar deposit is characterized by subcalcic garnets with significant proportions of spessartine (Sp (sub 26-78) mole %), almandine (Al (sub 5-23) ), grossularite (Gr (sub 8-52) ), and andradite (Ad (sub 0-14) ). Pyroxenes are manganese rich and iron poor, with an endmember range of hedenbergite (Hd (sub 10-55) mole %), johannsenite (Jo (sub 15-35) ), and diopside (Di (sub 20-80) ). Late-stage skarn minerals include calcium-rich bustamite, subcalcic actinolite, chlorite, and vesuvianite with up to 20 wt percent rare earth elements.Sulfur isotope values range from 10.8 to 26.5 per mil for sulfides, and from 32.4 to 34.0 per mil for barite. The strongly positive delta 34 S values for sulfides and barite are consistent with fractionation of sulfide and sulfate from Lower Ordovician seawater. Lead isotope ratios in galenas from all Aguilar ores have means of 206 Pb/ 204 Pb = 18.04, 207 Pb/ 204 Pb = 15.64, and 208 Pb/ 204 Pb = 38.03. Potassium feldspar from the Cretaceous Aguilar granite is far more radiogenic, having ratios of 206 Pb/ 204 Pb = 19.28, 207 Pb/ 204 Pb = 15.67, and 208 Pb/ 204 Pb = 39.00. These isotope ratios suggest an early Paleozoic, crustal source for the lead in the ores, unrelated to the lead in the granite.The overall geologic setting and geometry of the Aguilar ores, the distinctive ore stratigraphy, the mineral composition and textures, the calc-silicate assemblage, and the stable and radiogenic isotope evidence suggest that the sulfides in the Aguilar district formed as exhalative accumulations in a Lower Ordovician, shallow-marine sedimentary basin. The present distribution of the ores and their metamorphic textures indicate overprinting by contact metamorphism during the emplacement of the Cretaceous Aguilar granite.