Plagioclase in Yellow Dog peridotite (plagioclase-bearing lherzolite)

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

 Orthopyroxene at upper left (gray), enclosing plagioclase laths; clinopyroxene (magenta) at lower right; olivine at lower left, upper right, and top center. Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 mm (2X objective).

This is an unusually rich concentration of plagioclase (center) for the Yellow Dog peridotite. Most of the plagioclase in my samples is scattered as single laths, perhaps two or three small grains in a typical frame this size.  Older geology texts rule out ANY plagioclase if a rock is to be called "peridotite", but the IUGS classification allows up to 10 percent, in which case they are called "plagioclase-bearing ____". However, the consultant's report states that the plagioclase can average 25 to 30 percent "over significant intervals". In order to accommodate this, the consultant uses a maximum of 30% plagioclase for peridotite. The report defines rocks with between 10% and 30% plagioclase as "feldspathic peridotite".  (This report is on the Web. You can get it in PDF format by Googling "Eagle Deposit Geology".) Under the IUGS classification, these rocks would either be olivine gabbro, olivine norite, or olivine gabbronorite.

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.