In the Echo Bay region of Great Bear Lake the Echo Bay Group has been intruded by Precambrian granites, granodiorites, aplites, and diabases. This group, which has undergone zeolite-facies regional metamorphism, has a lower dominantly sedimentary-pyroclastic sequence (about 1320 m thick) and an upper volcanic sequence (> 1500 m thick). The Lower Echo Bay sub-Group consists of tuffs and cherts with a 190 m thick intermediate lava flow and occasional thin limestones. A sequence of andesites and pyroclastics comprises the Upper Echo Bay sub-Group. Contact metamorphism of the hornblende hornfels and albite–epidote hornfels facies has been induced by the granitic intrusions. A Rb–Sr isochron for the Upper Echo Bay sub-group volcanics gives an age of 1770 ± 30 m.y. K–Ar apparent ages for the granite, granodiorite, and volcanics average around 1650 m.y., possibly reflecting local argon loss up to this date. Mineralized veins with a pitchblende-Co,Ni arsenide-silver-Fe, Cu, Ag, Bi, Zn. Pb sulfide para genesis are mainly confined to the sedimentary-pyroclastic horizons of the Lower Echo Bay sub-Group. It is possible that the mineralization was effected by fluids generated during the low-grade regional metamorphism and that some contribution was made to ore genesis by the thermal regime developed during the diabase intrusion.