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.

Tremolite in Yellow Dog peridotite

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

Tremolite (center) in Yellow Dog peridotite. Tremolite and actinolite, both calcic clinoamphiboles, are very similiar in appearance, having bladed or fibrous crystals or, as in this image, thin columnar crystals in parallel aggregates. Tremolite is the magnesian end-member of the tremolite-ferro-actinolite solid solution, while iron substitutes for some of the magnesium in actinolite.

Two diagnostic differences are that actinolite is slightly pleochroic and ranges from pale green to deep green in thin section (the greater the iron content, the deeper the green). Conversely, tremolite is colorless in thin section and non-pleochroic. In PPL, this specimen proved to be colorless and non-pleochroic - thus it is tremolite.

Tremolite alters to talc and to chlorite. In the above image, talc borders tremolite on the top and right edges of the bladed portion (granular mineral with very high birefringence), and also forms small clots below it and to the left.  Rounded mineral grains around periphery of image are olivine. Pyroxene (gray) is at right, and mica (red) is at left. XPL. Imaged area 2.7 mm x 4 mm.

Higher magnification image (below) of bladed portion of tremolite bundle and talc border. Gray grain at lower right is serpentine. XPL. Imaged area 1.3 mm x 2 mm.

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

Clinopyroxene altering to tremolite:

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

A large anhedral grain of clinopyroxene (magenta and violet) is undergoing alteration to tremolite (light-colored mineral at left, bottom center, and right). Unlike the tremolite in the example above, the habit in this image is fibrous (asbestiform). The fibers are clearly visible against the background of the dark band of chlorite at the upper right. XPL. Imaged area 1.3 mm x 2 mm.

Marquette County, northern Michigan.

For more information on the Yellow Dog peridotite, see posts from January 28, 2011, and May 27, 2011.  For still more information, scroll to the top of this page and enter "Yellow Dog peridotite" in the search box at the upper right. You can also click on "Yellow Dog peridotite" in the cloud at the bottom of the page. Be sure to look at the last post on a page, and click on "newer posts" or "older posts", since Blogspot doesn't always display everything at once.

Yellow Dog peridotite - chloritization

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

Chlorites are a group of hydrous minerals that occur in igneous rocks, where they are products of the hydrothermal alteration of mafic minerals. Mg-rich clinochlore, Mg5Al2Si3O10(OH)8, is a common alteration product of olivines and pyroxenes. Klasner et al.* reported that chlorite makes up from 5 to 15 percent of the Yellow Dog peridotite. Since olivine and pyroxenes are lacking in aluminum, the aluminum in the chlorite was probably either derived from the small amount of plagioclase in the peridotite, or transported by percolating fluids from the surrounding Michigamme slate, into which the peridotite is intruded.

In this image, a contiguous belt of chlorite (dark blue interference color with light gray patches) runs from upper left to right center. It is irregular in width, with two large areas connected by narrow veins, and extends past the borders of the image. The belt visible in the image is clearly a cross-section of a lumpy layer of chlorite extending beyond the picture plane.  This layer probably follows a former fracture, along which metasomatizing fluids reached the mafic minerals and gradually altered them to chlorite, widening and filling the fracture.

Here, the chlorite is bordered mainly by olivine in the process of alteration. The cores of the olivine crystals are still intact, but the edges adjacent to the chlorite have altered to a white, fibrous mineral, possibly a different form of chlorite, or possibly serpentine, which readily alters to chlorite.

Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 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 - clinopyroxene interstitial to olivine

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

The dark areas are olivine (partially serpentinized with stringers of magnetite), the magenta and violet shapes are clinopyroxene, and a few gray plagioclase laths show at the upper left. There appear to be several varieties of pyroxene in the Yellow Dog peridotite. One of the most noticeable is anhedral clinopyroxene occupying the interstices between fractured olivine crystals, as in this image. Apparently the pyroxene was the last major mineral to solidify from the melt (In his M.S. thesis, Morris (1977)* refers to these occurrences in the Yellow Dog peridotite as poikilitic texture). Also notice the dark rims around the olivine crystals, and the smooth interface between olivine and pyroxene, suggesting a reaction between the early-crystallizing olivine and the melt.  XPL. Imaged area 2.7 mm x 4 mm.

In the PPL enlargement (below) of the right center of the XPL image, the dark rims around the olivine appear to contain an iron-rich alteration product. The olivine fragments remaining in the core of the crystal are transparent in PPL. Imaged area of enlargement 1.3 mm x 2 mm.

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

Marquette County, northern Michigan.

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

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.