Deer Lake peridotite - magnetite traces of ancient olivine crystals

Click on image to enlarge.          Photo © Daniel R. Snyder

The Deer Lake peridotite, thought to be Proterozoioc in age, is composed entirely of secondary minerals. Here, the outlines of two adjacent olivine crystals - and their internal fractures - have been preserved by stringers of magnetite that separated out when the olivine was serpentinized. The gray masses appear to be carbonate, possibly magnesite, throughout which magnetite grains have been disseminated, giving the gray color. Deer et al.* state that magnesite may be formed if serpentinites undergo low-grade metamorphism where carbon dioxide is available. A few orange and red wisps of anthophyllite or tremolite signal the beginning of another round of alteration. Marquette County, northern Michigan. XPL. Imaged area 1.3 mm x 2 mm.

Deer, W. A., Howie, R. A., and Zussman, J., (1992), An Introduction to the Rock-forming Minerals,  2nd edition, Pearson-Prentis Hall, 685 p.

Deer Lake peridotite (completely serpentinized)

Cllick on image to enlarge.          Photo © Daniel R. Snyder

The Deer Lake peridotite crops out in a small area north of Ishpeming, in Marquette County, northern Michigan. Although the geologic map of the area labels this exposure as "peridotite" (according to convention), because if its inferred protolith  it is in fact serpentinite. Pictured are two varieties of serpentine, as well as magnetite (black lines and spots) and hematite (reddish-brown areas) from the original olivine and pyroxene, which couldn't be taken up by the serpentine. The magnetite makes this rock weakly magnetic. XPL. Imaged area 1.3 mm x 2 mm. Link to photo of hand samples