![]() The Freda Sandstone is a red/brown ferruginoussandstone, siltstone, and mudstone. This area has produced about equal amounts of copper sulfide and native copper ores. ![]() These were normally oxidized, but nearthe White Pine Mine south of the district, the shales becomeblack and reducing. The Nonesuch Shale is a mixture of siltstones,shales, carbonates, and minor sandstone. They exhibit more variability in composition and rangefrom Fe-rich olivine thoelites to thoelitic andesites at the top of the section.These sediments were deposited in an arid climate with seasonally high rainfalls. The Lake Shore traps have an ageof about 1087 ± 1.6 Ma. ![]() There are up to 31 lava flows in the Lake Shore Traps, which arenear the top of the Copper Harbor Conglomerate. It overlays and interfingers with the top of the PortageLake Volcanics. The Copper Harbor Conglomerate varies between 100and 1800 meters thick. The Porcupine volcanics are similar to the PortageLake volcanics, but have been erupted in the southwest portion of the map in thePorcupine Mountains. These sedimentaryrocks comprise less than 5 % of the total volume, but contained about 40 % of thecopper production from the district and act as important marker beds in theotherwise similar appearing volcanic rocks. There are minor amounts ofconglomerate in the section ranging in thickness from a few cm to 40 m thick,with increasing amounts as one goes higher in the section. These rocks were derived from magma generated in the upper mantle and becameincreasingly more primitive (chemical composition like the mantle) as the rocks evolved. The basalts vary from a magnesium -aluminum rich olivine thoelite with minor amounts of basaltic andesite,andesite, and dacite. ![]() There are more than 200 individual flows thatcomprise the Portage Lake Volcanics. The period of rift volcanism extended from 1109 to 1089 Ma, with thePortage Lake Volcanics erupting over a two or three million year span around1095 Ma. All rocks are of Keweenawan age (Pre Cambrian)Įrupted rocks, but only about 5 km is exposed on the Keweenawpeninsula. Geologic section in the Keweenaw peninsula. ![]()
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