STRATIGRAPHIC AND STRUCTURAL CONTROLS ON KUPFERSCHIEFER-TYPE COPPER MINERALIZATION, WHITE PINE, MICHIGAN

In the Upper Peninsula of Michigan, U.S.A., the Mesoproterozoic Nonesuch Formation hosts anomalous Kupferschiefer-type stratabound copper mineralization throughout its exposed strike length with higher grade mineralization concentrated along the flanks of the Porcupine Mountain volcanic edifice. Two deposits in the Porcupine Mountain Copper District (White Pine and Copperwood) are currently being evaluated by Highland Copper Company.

At the White Pine mine 4.5 billion lbs of copper and 50 million oz. of silver were produced between 1953 and 1995. An additional large resource of similar grade remains in a northern extension of the deposit. In the bottom 4 to 6 m of the Nonesuch Formation copper mineralization is concentrated within two distinct dark gray zones of interbedded shale and siltstone separated by lighter colored sandstone and in a 1 to 2 m thick transition zone at the top of the underlying Copper Harbor Formation. Copper occurs as chalcocite (85-90%) replacing very fine grained diagenetic pyrite and organic matter and as native copper (10-15%) primarily introduced along faults during later compressional tectonism. Visible native silver is concentrated in the vicinity of the lower transition zone.

The thickness and extent of copper-bearing beds varies within the approximately 18 x 15 km area of the known deposit due to transgression-regression episodes and synsedimentary faulting. These stratigraphic variations play an important role in determining the minable limits of the deposit. Isopach and grade x thickness diagrams indicate that when all copper-bearing zones are considered mineralizing basin fluids in the vicinity of White Pine had a relatively uniform and finite copper content. As a result, the deposit is economic where copper-bearing beds are concentrated in a potentially minable configuration.

The high-grade zones of structurally controlled copper mineralization attracted early miners to the Porcupine Mountain Copper District and were an important part of the historical White Pine mine. Although there is evidence of significant folding and faulting beyond the known deposits, the potential for blind structurally controlled higher grade copper mineralization remains largely unexplored.