The volcano-sedimentary unit that hosts the stratiform massive sulfide mineralization at Rio Tinto is reinterpreted as a sill-sediment complex that in many respects is analogous to the example that is currently forming in the Guaymas Basin, Gulf of California. However, Rio Tinto is distinctive because (1) it is a product of bimodal magmatism, (2) it has a much higher sill/sediment ratio (4:1), lessening the extent of ore-forming metal depletion during interaction between hydrothermal fluid and wet sediment, and (3) most of the sills were emplaced penecontemporaneously as shown by the ubiquitous development of peperitic sill margins despite the narrowness of the sedimentary screens. These data together with evidence for local extrusive behavior imply rapid formation of the complex at exceptionally high levels of emplacement. It follows that in the complex, variations in thickness of ∼400 m were directly expressed as basin-floor deformation and that troughs in the resultant topography were able to trap exhalative hydrothermal plumes, thus preserving the sulfides from dispersion. Also crucial to the mineralization process was the sheetlike form of the sill complex, which, by acting as a cap, prevented regular convective cell patterns from forming and focused considerable discharge at areas of weakness. The intense magmatic pulse, the creation of pronounced topography, and the capping of the hydrothermal system combined to make a 500 Mt supergiant deposit.