Geology and Sulfide Mineralization of the Duluth Complex - Virginia Formation Contact Minnamax Deposit, St. Louis County, Minnesota

Abstract

The Minnamax deposit, near Babbitt, Minnesota, is a large, low-grade magmatic iron-copper-nickel sulfide deposit at the contact of the Duluth Complex and the Virginia Formation. The Virginia Formation consists of pelitic hornfels with locally abundant calc-silicate pods and contains diabase intrusions - all have been deformed and metamorphosed to the pyroxene hornfels facies by the Duluth Complex. The Duluth Complex, emplaced as a crystal mush, consists of sulfide-bearing troctolitic rocks, commonly noritic near the contact, which contain barren gabbro to peridotite xenoliths. The Duluth Complex - Virginia Formation contact is highly irregular, characterized by numerous Duluth Complex apophyses and Virginia Formation xenoliths. Granite to diorite dikes cut all lithologies. Sulfide mineralization, consisting of pyrrhotite, pentlandite, chalcopyrite, and cubanite, is primarily disseminated in the troctolitic rocks within 1000 feet of the contact. Sulfide veins fill fractures and breccia zones in both the Duluth Complex and the Virginia Formation. Pelitic hornfels adjacent to the veins and the sulfide-bearing troctolitic rocks commonly is replaced by sulfides. In one area the veins are sufficiently concentrated to form a high-grade sulfide body, the Local Boy deposit. The veins probably formed by filter-pressing of sulfide liquid from the troctolitic rocks into fractures. Some distinctly chalcopyrite-cubanite rich veins appear to have formed by separation of a copper-rich liquid from a pyrrhotite solid solution at magmatic .temperatures. Late stage hydrothermal solutions deposited sulfides and gangue along fractures and in calc-silicate pods.