Wernecke breccia comprises numerous intrusive hematitic breccia zones exposed in the Wernecke and Ogilvie mountains of central Yukon. The breccias were emplaced in Middle Proterozoic time into Middle Proterozoic strata of Wernecke Supergroup, Fifteenmile group, and possibly Pinguicula group. Significant mineralization of Cu, U, Co, Ag and Au within and near breccia zones occurred during widespread Fe, CO2, and Si metasomatism. Following a period of hydrothermal activity and intense fracturing, breccia zones in the study area were generated in open spaces produced by extensional faulting or rapid expansion of volatile-rich fluids. A strong spatial correlation between breccia and crosscutting mafic to felsic intrusions indicates a magmatic linkage. Metasomatism extended from before brecciation to after cooling of the igneous intrusions. The metasomatising fluids may have been partly derived from residual liquids of possible tholeiitic magma chambers fractionating at depth. Regional deformation and metamorphism incurred during Racklan orogeny in Middle Proterozoic time preceded brecciation; the breccias developed in fully lithified rock. Previous models of breccia genesis invoking evaporite or mud diapirism are considered invalid.