The Mindamar mine is in Richmond County on Cape Breton Island, Nova Scotia. The ore bodies are irregular lenses that lie within a persistent, wide, steeply dipping shear zone cutting mainly sericite schist. The schist was derived principally from siliceous siltstone of probable Middle Cambrian age.The deposits consist of extremely fine-grained pyrite and sphalerite with minor chalcopyrite, galena, and tennantite with a little gold, silver, and realgar in gangue composed mainly of dolomite, magnesite, quartz, sericite, talc, and chlorite. They were formed chiefly in the following overlapping stages:(1) Deposition of most of the fine-grained pyrite and sphalerite by replacement of sericite schist. (2) Deposition of most of the dolomite and magnesite and some of the quartz, partly in fractures and partly by replacement. (3) Deposition of most of the galena, chalcopyrite, and tennantite and some of the pyrite and sphalerite both in the carbonates and in the fine-grained pyrite- and sphalerite-bearing ore.Dolomite and magnesite, in part, appear from their textures to be meta-colloidal. Talc occurs chiefly as randomly oriented flakes which originated by replacement of dolomite and magnesite without recognizable change in volume.Most of the numerous diabase dikes were intruded after deposition of the carbonates and ore minerals. Several, however, were emplaced before completion of carbonate and ore deposition and their margins are intensely carbonatized and locally mineralized.Much of the fine-grained ore shows thin layers differing from one another in the proportions of ore minerals to gangue minerals or of individual ore minerals to each other. This structure may be attributable mainly to selective replacement of relic lamination in sericite schist and partly to replacement controlled by closely spaced parallel fractures.