Garnet–quartz–muscovite rock stratigraphically underlies a volcanogenic pyritic gold orebody within Bousquet Township, Abitibi greenstone belt, Quebec. Garnet constitutes only 3% of the rock within muscovite-rich layers near its east and west ends whereas the rock contains 12% of garnet near its center. Trails of quartz, carbonate, and sulphide inclusions and parallel bands of an ultrafine black opaque mineral produce a layered appearance in the garnet. Garnet consists of 25% MnO and is zoned with maximum MnO content at its center. Furthermore, MnO content is greatest in garnet near the center and stratigraphic top of the rock and decreases systematically to the east, west, and stratigraphic base. Pb, Zn, and Ba exhibit similar enrichment trends from stratigraphic base to top of the rock; however Ba remains abundant in the overlying orebody.The interpretation is that the garnet rock was a primary Mn-rich sediment that accumulated near a sea-floor hydrothermal vent. By analogy with modern hydrothermal Mn deposits, the garnets are believed to be metamorphosed equivalents of Archean manganese nodules. The spatial relationship of the garnet rock to gold ore, the increase in MnO/FeO in garnet toward ore, and the large areal extent of the rock indicate its usefulness in exploration for gold mineralization.

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