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.

Yellow Dog peridotite (plagioclase-bearing lherzolite)

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

The Yellow Dog peridotite, thought to be approximately 1.1 Ga in age, is composed  of partly serpentinized peridotite, containing 40 to 50 percent olivine (1/2 to 2/3 serpentinitzed) and 30 percent pyroxenes consisting of subequal amounts of orthopyroxene and clinopyroxene. (Klasner, et al., 1979)*. In addition, it contains 5 to10 percent plagioclase, which would classify it as a "plagioclase-bearing lherzolite" according to the IUGS criteria.

The peridotite body was first described by William Morris in his 1977 M.S. thesis**. He named it the "Yellow Dog Plains peridotite". The surface outcrop is roughly oval-shaped in horizontal cross-section. Doug Hull's comment below is valid.  Appendix C (Geology) to the mining development permit request is now on the Web. You can get it in PDF format by Googling "Eagle Deposit Geology". This report includes cross-sections, at irregular horizontal intervals and at 50-meter vertical intervals, of what is referred to as the "Eagle Deposit". These diagrams justifiy calling it a dike.The report states that the "intrusion at the surface"  extends 480 meters in length and is 100 meters wide at its thickest point. The peridotite intrudes metasediments that are at least 1.9 Ga in age.  

The image above shows two heavily fractured olivine macrocrysts, which were most likely carried upward by magma originating in the upper mantle. A serpentine meshwork fills most of the fractures, and stringers of magnetite occupy the larger fractures in the crystal on the right. Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 mm.

Below is an optical scan of a fractured surface of a hand sample of Yellow Dog peridotite. 2400 dpi optical scan, Imaged area 17 mm x 17 mm. 

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

*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.

**Morris, William J., (1977) Geochemistry and Origin of the Yellow Dog Plains Peridotite, Marquette County, Northern Michigan, unpublished master's thesis, Michigan State University, 82p. 

Mantle lherzolite xenolith in basalt - full thin section.

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

Spinel lherzolite xenolith, San Carlos Indian Reservation, Arizona. Lherzolite is at left; brightly colored grains are olivine, gray and brown grains are pyroxene.  Felty-textured basalt is at right. XPL macrophotograph. Imaged area 21 mm x 34 mm. Click on the image to enlarge. It's a lot more interesting close up!

"Holly-leaf" spinel pattern in mantle lherzolite xenolith

Click on image to enlarge          Photo © Daniel R. Snyder

Light green olivine , olive-green enstatite (top; bottom center), and a spinel grain with "holly-leaf" stringers extending from the main mass. Cr-diopside lherzolite xenolith. San Carlos Indian Reservation, Arizona. Reflected-light photomicrograph of polished section. Imaged area 2.7 mm x 4 mm.

Mantle xenolith - reflected-light photomicrograph

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

Light green olivine , olive-green enstatite, and emerald-green chromian diopside in a Cr-diopside lherzolite xenolith. I'm leaning toward calling the curved, dark grain in the center spinel. This is, after all, a spinel lherzolite, and in such rocks spinel is usually near pyroxenes. San Carlos Indian Reservation, Arizona. Reflected-light photomicrograph of polished section. Imaged area 2.7 mm x 4 mm.