Archean geology of the northern block of the Ishpeming Greenstone Belt, Marquette County, Michigan

This block, as well as the entire belt, is composed of Archean volcanic and volcaniclastic rocks and is bounded by Archean gneissic granitoid and Early Proterozoic sedimentary rocks. During Archean time, rocks of the belt were metamorphosed to greenschist and amphibolite facies and were subjected to two deformational events. Recumbent folding (D₁) was followed by isoclinal to open folding about east-west axes (D₂). Gold mineralization occurred in sheared, carbonate-altered, and quartz-veined rocks. Anomalous gold commonly is present along the brecciated margins of rhyolite dikes. The gold mineralization is interpreted as being associated with the second (D₂) Archean deformational event. Base metal and silver mineralization occurred in quartz-carbonate veins hosted by brecciated fault zones. This mineralization is paragenetically later than that of the gold: it may not be Archean in age. -from Authors