MICHIGAMME MINE
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.)