Abstract The Admiral Bay Zn-Pb deposit is a large (≥140 Mt), epigenetic deposit hosted in Lower Ordovician carbonate rocks of the Willara Sub-basin, Canning Basin, Western Australia. The deposit is located in the Great Sandy Accommodation Zone (GSAZ) of the Admiral Bay Fault Zone (ABFZ), a structurally complex area that was initiated as an extensional system in the Tremodocian and was compressionally reactivated in the Late Silurian-earliest Devonian. The Admiral Bay deposit consists of four main mineralized zones; (1) an extensively altered and fractured algal bioherm association, (2) a lower, lead-rich zone in the upper Goldwyer Formation and lower Leo Member of the Nita Formation, (3) a hydrothermal dolomite zone that extensively replaces the Leo Member, and (4) an upper, zinc-rich zone in the Cudalgarra Member of the Nita Formation and lower Bongabinni Formation. The complex mineralization of the Admiral Bay deposit is interpreted to be the result of two mineralization events. Syndepositional hydrothermal activity, related to extensional faulting in the Early Ordovician, resulted in extensive alteration and fracturing of the Goldwyer Formation bioherm association. No base-metal mineralization was associated with this hydrothermal event. In the Late Silurian, metal-bearing fluids expelled from the Willara Sub-basin were focused into the extensional faults of the GSAZ. Interaction with evaporitic sulfates and hydrocarbons lead to the extensive base-metal mineralization of the Lower Ordovician Goldwyer, Nita, and Bongabinni formations. The basinal and structural setting, timing of epigenetic mineralization, sulfide and gangue mineral assemblage, and geochemical characteristics of the Admiral Bay deposit, are consistent with the features of Mississippi Valley-type mineralization. However, the spatial and temporal association of a lead-rich ore zone (Zn/(Zn+Pb) = 0.02) and a zinc-rich ore zone (Zn/(Zn+Pb) = 0.74) is highly unusual.