Abstract The Boléo copper-cobalt-zinc district occupies over 60 km 2 on the eastern coast of Baja California Sur, Mexico, and was an important producer of copper for more than 100 years, until 1985. Recently, detailed exploration has delineated a new, large, and potentially exploitable resource. The Boléo stratiform deposits occur in the upper Miocene Santa Rosalía basin, which formed in response to early rifting in the Gulf extensional province at some time after 10 Ma. All of the known Boléo ore zones, or mantos, occur within one restricted trough, the Boléo subbasin, which constitutes part of the Santa Rosalía basin. The subbasin was filled by epiclastic sediments from highlands to the west and by dis-tally erupted air-fall ash. Infilling of the basin took place in five coarsening-upward cycles, each representing one transgression-regression episode culminating in deltaic conglomerate. It is thought that the initiation of each cycle was controlled by episodic fault reactivation that resulted in basin subsidence. The fine-grained basal sediments of each cycle constitute a manto, which is particularly rich in distal air-fall ash. Primary sulfide minerals in Boléo ores are complex and imperfectly understood. They include fram-boidal pyrite rimmed by later metal sulfides including low-temperature chalcocite, together with bornite, chalcopyrite, carrollite, sphalerite, and possibly other, rare, species. The ore minerals are finely disseminated over intervals reaching 20 m in thickness which comprise a laminated basal zone and overlying intra-basin slump breccias, all variably altered to montmorillonite. Isotopic studies suggest that the sulfur in the sulfides is biogenic, derived from the bacterial reduction of sulfate. Secondary ore minerals, of which there are many, are products of sulfide degradation above the water table. The principal economic metals exhibit vertical and district-wide lateral variation. In individual mantos, copper is normally dominant near the base and zinc near the top, whereas cobalt is somewhat more evenly distributed. There is also an overall trend toward zinc enrichment from the lowest to the uppermost manto. In terms of lateral zoning, zinc has an inverse relationship with copper. Generally speaking, a central copper-dominant core is flanked by marginal zinc-dominant zones. Cobalt is less predictable, being largely independent in a proportional sense of the distributions of copper and zinc. Boléo deposits are considered to be early diagenetic in origin, similar in many respects to other sedimentary copper deposits. The ore minerals formed in fine sediments laid down cyclically on the floor of the restricted Boléo subbasin, where reducing conditions promoted by sulfate-reducing bacteria gave rise to an ideal environment for metal-sulfide precipitation. Early framboidal pyrite was succeeded by sulfides of copper, cobalt, and probably zinc. The presence of chalcocite and montmorillonite, as well as the likelihood of bacterial activity, suggest low ore-forming temperatures, possibly less than 100°C. A source of metals beneath the basin is implied by the presence of mineralized faults and, tentatively, an association with coeval alkaline magmatism is suggested.