The Caribou deposit occurs in a mixed volcanic–sedimentary zone which is transitional between graphitic argillite and silicic tuffaceous rocks. The silicic tuffaceous rocks form the core, and the graphitic argillite the outside, of the steeply plunging Caribou synform.The Ore Zone Unit consists of the volcanic–sedimentary transition zone and the sulfides. The Caribou deposit consists of three or more en echelon lenses of pyrite – sphalerite – galena – chalcopyrite and footwall disseminated sulfides. Sulfide zoning in the deposit indicates that the stratigraphic top is towards the silicic volcanic rocks. The deposit has a galena–sphalerite zone on the silicic volcanic side and the hangingwall contact is sharp. There is a chalcopyrite-rich zone on the sedimentary side and the contact is indistinct. Very fine spheres, cubes, and framboids of pyrite in both the deposit and the sedimentary wallrocks, as well as pyrite colloform structures in the deposit, appear to be relict primary or diagenetic structures.All the rocks, including the sulfides, have been regionally metamorphosed to lower greenschist facies and contain a pervasive slaty cleavage or schistosity, which is the earliest imprint of structural deformation. Pyrite, sphalerite, galena, chalcopyrite, arsenopyrite, quartz, stilpnomelane, and sericite are in a granoblastic intergrowth typical of metamorphosed sulfides. This early structural deformation has fractured and granulated pyrite and, to a limited extent, sphalerite; and chalcopyrite, galena, and sphalerite flowed plastically. The early slaty cleavage or schistosity has been deformed by minor structures and folded into the Caribou synform.Many features of the Caribou deposit and similarities to other volcanic-associated stratiform base metal deposits indicate a syngenetic origin involving submarine deposition of sulfides derived from volcanic, exhalative, hydrothermal solutions and possibly as pyroclastic debris.