Contact Metamorphism of the Virginia Formation in the Minnamax Deposit St. Louis County, Minnesota

Abstract

The Middle Precambrian Virginia Formation, cut by diabase dikes, was intruded and contact metamorphosed by the Late Precambrian Duluth Complex near Babbitt, Minnesota. Sulfide mineralization of magmatic origin, with minor amounts formed by hydrothermal replacement, is concentrated at the irregular contact zone between the Virginia Formation and the Duluth Complex. Five miles south of Babbitt this mineralization constitutes the Minnamax Copper-Nickel Deposit, which is being investigated and evaluated underground by AMAX Exploration, Inc. The Virginia Formation consists of pelitic hornfels, calc-silicate pods, and "reaction" rims around the pods. The pelitic hornfels is dark gray, fine-grained, massive, and composed of plagioclase, hypersthene, and cordierite with local occurrences of orthoclase, biotite, and graphite. The calc-silicate pods are light gray, fine- to coarse grained, are spherical to ellipsoidal and range from 4 inches to 8 feet across. There are three types of pods; homogenous types with no mineral zones developed, layered types with mineral layers developed, and concentric types with mineral zones developed. The primary minerals are diopside, grossular garnet, plagioclase, sphene, wollastonite, and possibly some calcite, and quartz. From strikes and dips of relict bedding in pelitic horn.fels and from the broken, fra.ctured, and jumclad nature of calc-silicate pods deformation of the Virginia Formation appears intense. The "reaction" rims are dark gray, fine-grained, and up to 3 inches wide. They are composed of plagioclase, hypersthene and poikiloblastic clinopyroxene giving a composition intermediate between the pelitic hornfels and calc-silicate pods. The protolith of the pelitic hornfels appears to be a calcareous argillite and the calc-silicates a siliceous dolomitic limestone. The pods are believed to have originally been calcareous concretions in argillite with some being pieces brought up from the calcareous zone at the top of the Biwabik Iron Formation. The "reaction" rims developed after deformation took place, as they surround broken and fractured pods, and formed from diffusion of calcium from the pods into the pelitic hornfels. The metadiaba.se dikes are dark gray, fine-grained, and massive. They are composed of lathy plagioclase, augite, and hypersthene. A relict ophitic texture is evident and relict plagioclase phenocrysts have been resorbed. Sulfides consist of pyrrhotite, exsolved pentlandite, and chalcopyrite in pelitic hornfels and chalcopyrite with exsolved cubanite in calc-silicate pods. Minor ilmenite and magnetite is present in the sulfides. Alteration consists of uralitization of pyroxenes and sericitic and kaolinitic alteration of plagioclase. Quartz, calcite, apophyllite, anhydrite, fluorite, heulandite, laumontite, and prehnite are gangue minerals. This emplacement occurred after the main metamorphic event and formed by hydrothermal replacement. Based on the primary mineral assemblages present, the rocks fall in the pyroxene hornfels facies. The presence of plagioclase and wollastonite in the calc-silicate pods give a minimum temperature of 600 degrees Celsius at 2 kilobars pressure, and a mole fraction of CO2 in the vapor phase less than 0.25. An increase of albite in plagioclase can lower the temperature of the reaction forming plagioclase and wollastonite, and could cause the plagioclase and wollastonite to disappear with quartz and calcite stableo From the presence of laumontite, an upper limit of 350 degrees Celsius at 2 kilobars pressure can be given for the sulfides e:mplaced hydrothermally. A bottom temperature ranging from 250 to 300 degrees Celsius can be given by the presence of exsolved cubanite in chalcopyrite.