Contrary to the generally accepted volcanic model for the Iberian Pyrite Belt stratiform massive sulphide deposits, Rio Tinto, the largest deposit, formed in a sill-sediment complex. An appreciation of processes unique to such an environment demonstrates that the mineralization event at Rio Tinto was initiated as exhalation of ore fluids into an evolving sedimentary basin. Ore-fluid circulation was, therefore, first driven by extensional tectonics, which in turn triggered decompressional melting. The resulting magmas are inferred to have driven fluid circulation adjacent to the magma plumbing network, contributing significantly to the total fluid discharge. The major stratiform deposits in the Pyrite Belt display a spectrum of characteristics intermediate between volcanic-hosted and sediment-hosted styles; variable combinations of the two fluid convection models implicated at Rio Tinto are largely capable of explaining this spectrum.