Metallic copper or copper minerals are locally present, in flows, conglomerate, siltstone, and shale of Keweenawan age that underlie about 3,000 square miles of northwestern Wisconsin. Representative samples taken from 24 of the reported occurrences in lava or conglomerate contain 0.0075 to 0.69 percent copper; 10 of them have more than 0.10 percent. No study was made to appraise the geologic Settings of the sampled sites as guides to possibly more significantly mineralized rock. The siltstone and shale sequence is sparingly exposed in Wisconsin but was not sampled; exploratory drilling has shown that it is mineralized locally.
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USGS Numbered Series
Sampling of copper-bearing Keweenawan rocks of northwestern Wisconsin