Yellow Dog peridotite - magnetite grains in euhedral olivine crystal

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

Split image of a partially-serpentinized, euhedral olivine crystal (center) enclosing several small grains of magnetite, with other magnetite grains nearby.  Olivine crystals are on the left (XPL) side of the image, orthopyroxene is on the right (PPL) side. The olivine in the Yellow Dog peridotite is relatively iron-rich, with and average composition of Fo81 (Klasner et al., 1979). When olivine alters to serpentine (filling fractures in olivine), the serpentine can't accommodate the the iron, and it is taken up as newly-formed iron-oxide minerals, mainly hematite and magnetite. Some of the other images show stringers of tiny magnetite grains occupying fractures in olivine. In this example, the iron has formed several larger grains of magnetite, many with the diamond shape characteristic of some euhedral magnetite. Yellow Dog peridotite, Marquette County, northern Michigan. XPL and PPL. Imaged area 1.3 mm x 2 mm.

*John S. Klasner, David. W. Snider, W. F. Cannon, and John F. Slack (1979), The Yellow Dog Peridotite and a Possible Buried Igneous Complex of Lower Keweenawan Age in the Northern Peninsula of Michigan, Geological Survey Division, Michigan Department of Natural Resources, 38 p.

Yellow Dog peridotite - full thin section

Click on the image to enlarge. Click twice to enlarge more- it's a lot more 

interesting close up.                                         Photo © Daniel R. Snyder.

The yellow dog peridotite is a pretty scarce rock. It occurs in two small outcrops, about a kilometer from each other, on the Yellow Dog Plains in western Marquette County, in Michigan's Upper Peninsula. But the limited extent of these rock bodies isn't the only reason for the rock's scarcity. The other reason is that the peridotite body hosts a large body of semi-massive and massive sulfides, mainly chalcopyrite and pentlandite. A large mining company is in the process of developing these resources.

This image is a full thin section view of Yellow Dog peridotite (plagioclase lherzolite).  The bright yellow and grayish brown grains are pyroxenes. It's hard to generalize at this scale about which are orthopyroxenes and which are clinopyroxenes. The rest is mostly olivine, much of it serpentinized, in rounded, fractured, remnants of euhedral crystals. If you enlarge the image, you can easily make out the rounded shapes, the fracture networks, and the high birefringence of the individual fragments within the olivine grains. They've had a hard ride up from the mantle. Marquette County, northern Michigan. Macrophotograph,  XPL. Imaged area 24 mm x 39 mm.

Yellow Dog peridotite - digital mosaic.

Click on image to enlarge         Photo © Daniel R. Snyder

Yellow Dog peridotite. Digital mosaic of four 2x-objective images. In this image: rounded and fractured olivine crystals with serpentine meshwork and stringers of magnetite along fractures, pyroxene (yellow), plagioclase (white/gray), and a little mica (red). Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 5.4 mm x 7.8 mm.