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.