The No. 27 mine lies at the northern end of the Viburnum Trend near a 2.5-mile-long granite high 3 miles west and basinward of the main ore trend.Mineralization, primarily associated with clastic structures, occurs almost exclusively in the digitate stromatolitic reef horizon of the dolomitic Bonneterre Formation. The two most common ore-bearing structures are calcarenite-filled channels between linear algal mounds and calcarenite-filled troughs behind subtidal offshore algal bars. A mineralized local "white rock" facies is present in an embayment on the eastern flank of a granite peak that rose over 500 feet above the Cambrian sea level, and a major non-mineralized breccia within the Bonneterre Formation contains fragments of the Davis Formation as much as 90 feet below their normal position.Two fracture systems are prominent in the mine. Both systems parallel much of the ancient shoreline as do the major region faults.Mineral zonation is well developed with chalcopyrite generally occurring along the basinward flank and at the base of the galena orebodies, while sphalerite is usually found near the top of the orebodies.

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