Abstract

The Buick orebody consists of lead, zinc, and copper sulfides in solution-induced collapse breccias. The breccias occur in dolomitized, shelf-facies calcarenite of the Upper Cambrian Bonneterre Formation. The shelf fades is underlain by digitate algal stromatolites and overlain by shale of the Davis Formation. The breccia bodies, in plan view, are sinuous, subparallel, and continuous north-south with occasional confluences and divergences. The Buick geology staff believes they occupy intratidal drainage channels. Individual breccia bodies are up to 300 feet wide and up to 85 feet thick, with a trough-shaped base, and outward-dipping, bounding fractures on the sides. Well-mineralized, inward-dipping slump breccias occur along the boundary of some breccias, while in others undisturbed, unmineralized beds occur at the fracture boundary.Typically there are three breccia bodies, each with characteristic sulfide mineral assemblages and patterns that are traceable along strike through the two miles of mine opening. The west breccia, the most complex, contains galena-pyrite and galena-sphalerite zones on its west side, a central core of sparse pyrite and galena, and an east-side assemblage of galena, sphalerite, chalcopyrite, pyrite, and several cobalt-nickel sulfides. A pyrite halo caps and flanks the west breccia. The central breccia contains galena and sphalerite in varying proportions with only trace chalcopyrite. The flanks contain the major metal concentrations and higher ratios of zinc to lead. The breccia is capped by a pyrite-galena zone. An unbrecciated, satellite ore shoot of massive replacement sphalerite and galena lies along the west side of the central breccia. The east breccia contains a core of galena-sphalerite-chalcopyrite surrounded by a galena-sphalerite assemblage. The margins of the breccia contain galena and pyrite, and the breccia is capped by a thick pyrite zone.Breccia tops are vuggy and porous and contain mainly open-space filling mineralization, while replacement ore is dominant near the base of the breccias. Paragenesis of the ore has not been completely determined. Numerous overgrowths of different sulfide minerals in vugs, leached and etched crystals, prebreccia ore in a matrix of postbreccia ore, and complex zoning of sulfide minerals suggest several mineralizing pulses.

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