DEER LAKE PERIDOTITE
The Deer Lake Peridotite (aka
Deer Lake Complex) is a metamorphosed peridotite sill complex in the
Precambrian-aged Ishpeming Greenstone Belt of Michigan's Upper Peninsula (UP),
USA. The metamorphism has converted the original peridotite (probably
dunite or close to it) into serpentinite. Good exposures can be
seen next to Co. Rd. 573, along the western side of Deer Lake, northwest of the
town of Ishpeming (see
map). Freshly broken surfaces of serpentinized Deer Lake Peridotite
from here can be wonderfully bluish-green. A slight but noticeable tug
can be felt when a magnet is placed next to these rocks. Many
serpentinites have a minor component of magnetite (a chemical
consequence of peridotite metamorphism). The first two rocks shown below
are slightly magnetic, despite the absence of visible magnetite crystals.
The serpentinites along the
western side of Deer Lake often have thin veins of chrysotile asbestos, one of
the varieties of the mineral serpentine. Older references indicate that
pillow basalts are present at the Deer Lake West locality. More recently
published references interpret the pillow basalt-like structures in the
serpentinite as shear polyhedra or contraction joints in a shallowly-emplaced
sill. Examination of the "pillow" structures in summer 2010
revealed that they are simply the result of sinuosity of asbestos (chrysotile)
veins.
Age: late Neoarchean?, ~2.5-2.7
billion years.
Deer Lake Peridotite (metamorphosed into
bluish-green magnetitic serpentinite) (9.2 cm across) from Deer Lake West
outcrop (see
map), Ishpeming Greenstone Belt, UP of Michigan, USA. The striations
seen in this rock are slickenlines (indicating tectonic movement), which are
common on serpentinite samples.
Deer Lake Peridotite (metamorphosed into
bluish-green magnetitic serpentinite) (6.1 cm across) from Deer Lake West
outcrop.
Economic concentrations of
gold are sometimes found in the metamorphosed Deer Lake Peridotite. The
serpentinite sample shown below is from the Ropes Gold Mine, northwest
of Ishpeming (see
map). The gold occurs in quartz veins that intrude the Deer Lake
Peridotite.
Serpentinite (3.6 cm across), composed
of picrolite antigorite serpentine, from picrolite vein in metamorphosed Deer
Lake Peridotite (late Neoarchean) (Ropes Gold Mine, Ishpeming Greenstone Belt,
Marquette County, western Upper Peninsula of Michigan, USA).
In places, the serpentinite
has been metamorphosed further into talcose rocks.
Talc (4.8 cm across) -
metamorphosed Deer Lake Peridotite (late Neoarchean) from the Ropes Gold Mine
(Ishpeming Greenstone Belt, Marquette County, western Upper Peninsula of
Michigan, USA).
Some info. from:
Bornhorst & Johnson
(1993). Geology of volcanic rocks in the south half of the Ishpeming
Greenstone Belt, Michigan. USGS Bulletin 1904-P. 13 pp.