Abstract The Cretaceous Tambogrande volcanogenic massive sulfide (VMS) deposits of northwestern Peru represent some of the largest Cu-Zn-Au-Ag bimodal-mafic VMS deposits in the world. There are currently three known deposits each with approximately 100 million metric tons (Mt) of massive pyrite-rich sulfide. The deposits are intimately associated with dacite lava dome complexes and were deposited within steep-sided basins on the sea floor. Reconstructed sea-floor paleogeomorphic models suggest that sulfide deposition was concentrated in the deepest parts of the basins. Sulfide deposition accompanied synvolcanic faulting and episodic dacitic and basaltic eruptions. A series of time-stratigraphic horizons are defined at the TG1 and TG3 deposits and mark stages in the development of the volcanic complex and massive sulfide bodies. There is only limited evidence for replacement of host rocks during formation of the Tambogrande deposits, in contrast to many other large massive sulfide deposits. The deposits at Tambogrande resulted from focused hydrothermal fluid flow along synvolcanic faults with deposition of sulfide minerals within deep and restricted basins. These depressions, the results of the structural and volcanologic setting, acted as efficient traps for sulfide deposition and were also important for the preservation of the sulfide masses as they acted to shield them from submarine oxidation and weathering. Steep basins and episodic bimodal lava eruptions are key geologic attributes of the depositional setting at Tambogrande and may be necessary for the formation of anomalously large VMS deposits in a volcanic-rock–dominated environment.