PRELIMINARY INVESTIGATION OF HYDROTHERMALLY DEPOSITED CARBONATE VEINS WITHIN A GABBROIC ANORTHOSITE OF THE DULUTH COMPLEX, NORTHEASTERN MINNESOTA

An intrusion of gabbroic anorthosite of the Duluth Complex with veins of hydrothermal carbonate, was recently exposed in a road outcrop near the Lincoln Park Middle School in Duluth, Minnesota. The vertical veins are bright pink against the dark green color of the anorthosite, vary in width from 1 mm to 20 cm and occur over a distance of 30 to 40 meters along the roadcut. Centers of the veins contain pink calcite (Mn-rich?) along with white quartz and cavities lined with clear quartz. Outer edges of the veins have rhombic to pseudorhombic crystals of an unknown white mineral. Alteration zones in the host rock vary from 5 mm to 30 cm in thickness, and pieces of brecciated, altered host rock, ranging in size from 2 cm to 15 cm, are incorporated in the veins. The gabbroic anorthosite host rock is dark green with large striated plagioclase crystals. Whole rock X-ray diffraction of vein centers, shows the carbonate consists of calcite, Mg-rich/Mn-rich calcite, ankerite and dolomite. Thin section analysis shows the silica is probably late stage void-filling, and consists of mono-quartz, chert, chalcedony and spherules of radial-fibrous quartz (chalcedony?). Some prismatic ferroan? calcite displays fasicular extinction. The zoned rhombic dolomite? (white in hand sample) is partially to completely replaced by chalcedony, chert and possibly an oxide or kaolinite. Several zoned dolomite? rhombs have completely dissolved interiors, showing subsequent collapse of more stable dark outer zones into the voids, which chert and radial-fibrous quartz later filled. The gabbroic anorthosite is medium to coarse grained and consists of large (up to 5 cm long) euhedral to subhedral crystals of plagioclase feldspar, with most showing Carlsbad and albite twins. Composition of the plagioclase is An_62-65. Plagioclase makes up approximately 90% of the rock. Darker mafic minerals are augite, other clinopyroxenes, Fe-oxides and an unidentified metallic opaque mineral.