In the Lake Michigamme area, iron ores have been mined from the Negaunee Iron-Formation and from iron-formation members in the overlying Michigamme Formation. One now-inactive iron mine, the Michigamme Mine (aka Mt. Shasta Mine), produced almost one million tons of high-grade iron ore. This ore was in the form of hematite- and magnetite-rich rocks, the result of secondary iron oxide enrichment within the Negaunee Iron-Formation (Paleoproterozoic, 2.11 b.y. or 1.874 b.y.).
Chlorite schist was a common waste rock in the Michigamme Mine. Its mine dumps are full of this material.
Chlorite schist (7.7 cm across at its widest) from the Michigamme Mine, UP of Michigan, USA.
During iron mining days, the most frustrating waste products to the miners were “black diamonds”. They were so abundant that they frequently got caught up in & damaged the mine's rock-crushing equipment. Michigamme Mine's black diamonds are some of the most fascinating minerals in Michigan. The masses have the shape of large, 12-sided garnet crystals, but they're not composed of garnet. The greenish-black material making up these masses is chamosite chlorite ((Fe+2,Mg,Fe+3)5Al(Si3,Al)O10(OH,O)8 - iron magnesium hydroxy-oxy-aluminosilicate). This is a fantastic example of a pseudomorph (“false-form”). Pseudomorphs form if any mineral replaces another mineral, but takes on the crystal form of the original. Chamosite-after-garnet pseudomorphs from Michigamme Mine have been collected & sold for decades.
“Black Diamond” - chamosite chlorite pseudomorph after almandine garnet (2.8 cm across), derived from chlorite schist waste rock at the Michigamme Mine, Michigamme, UP of Michigan, USA.
I'm not exactly sure why chlorite schists & chlorite-after-garnet pseudomorphs occur at the Michigamme Mine. I suspect these rocks are metadiabases (?). The Negaunee Iron-Formation does have many interbeds of metamorphosed diabase (basaltic) sills in the Lake Michigamme area. The diabase sills apparently formed at about the same time as the rocks of the Clarksburg Volcanics Member (lower Michigamme Formation) were erupted. If this correlation is accurate, the diabase sills were intruded at about 1.85-1.86 billion years ago.
The diabase has been subjected to low-grade metamorphism in some areas and higher-grade metamorphism in other places. Lithologically, the metadiabase ranges from coarsely-crystalline, non-foliated metamorphic rock to fine-grained or coarse-grained foliated metamorphic rock (see Cannon & Klasner, 1977 and Klasner & Cannon, 1978). The sample shown at top is a fine-grained chlorite schist inferred to be from a metadiabase “interbed” in the Negaunee Iron-Formation.
Most info. from:
Cannon & Klasner (1977) - Bedrock geologic map of the Diorite and Champion 7˝-minute quadrangles, Marquette County, Michigan. United States Geological Survey Miscellaneous Investigations Series Map I-1058.
Klasner & Cannon (1978) - Bedrock geologic map of the southern part of the Michigamme and Three Lakes quadrangles, Marquette and Baraga Counties, Michigan. United States Geological Survey Miscellaneous Investigations Series Map I-1078.
Heinrich & Robinson (2004) - Mineralogy of Michigan. Houghton, Michigan. Seaman Mineral Museum. 252 pp.
Dana Slaughter (pers. comm.)