Pyroxene grains and pseudomorphs in Josephine serpentinized peridotite.

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

In this specimen,  A few very small remnants of olivine (bright-colored grains) can be seen around the image, but almost all of the olivine has been altered to serpentine. I collected this specimen on the border between the Josephine peridotite and the Onion Camp complex of Yule et al. (2006)*. I decided to call it Josephine peridotite for two reasons: 1. There is little evidence of shearing, and 2. Many large pyroxene grains still persist, and large serpentine pseudomorphs after pyroxene are distributed throughout the specimen. The large gray pyroxene grain at the left side of the image above still appears to be mostly pyroxene, while the broken grain in the center has been altered to bastite-texture serpentine. Hourglass-texture serpentine, probably an alteration product of olivine, occupies the rest of the image. Klamath Mountains, Josephine County, southwest Oregon, XPL. Imaged area 2.7 mm x 4 mm.

In the image below, the white spots on the large hand specimen are reflections from the surfaces of platy pseudomorphs after pyroxene; gray-green grains are pyroxene. Note that there appears to be more remnant pyroxene than pseudomorphosed pyroxene. The brown mesostatis is serpentine. The surface of the small hand specimen has been rough polished, showing dark grains of pyroxene. Lighter gray areas are serpentinized pyroxene grains (pseudomorphs). Scale is in centimeters.

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

*Yule, J. D., Saleeby, J. B., and Barnes, C. G., 2006, A rift-edge facies of the late Jurassic Rogue-Chetco arc and Josephine ophiolite, Klamath Mountains, Oregon, in Snoke, A. W. and Barnes, C.G., eds., Geological studies in the Klamath Mountain province, Californa and Oregon: A volume in honor of William P. Irwin: Geological Society of America Special Paper 410, p. 53-76.

Peridotite in advanced stage of serpentinization - Onion Camp complex, Oregon

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

The serpentinized peridotite of the Onion camp complex, which is in contact with the variably serpentinized Josephine peridotite, is described by Yule et al. (2006)* as "highly serpentinized and sheared dunite, harzburgite, and sparse websterite....".  In the center of the image above, a pyroxene grain has been pseudomorphosed, probably by an amphibole mineral, with the chain structure preserved as parallel strands of the replacement mineral. A large serpentine pseudomorph after olivine occupies the center of the left side of the image, and a less obvious pseudomorph is at the upper right. Elsewhere in the image, serpentine is ubiquitous. Klamath Mountains, Josephine County, southwest Oregon. XPL. Digital mosaic, imaged area 3.6 mm x 6 mm.

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

Hand samples of serpentinized peridotite of the Onion Camp complex. Smaller sample on right has been rough polished on cut surface. Note hematite coating on left sides of both samples. Scale in centimeters.


*Yule, J. D., Saleeby, J. B., and Barnes, C. G., 2006, A rift-edge facies of the late Jurassic Rogue-Chetco arc and Josephine ophiolite, Klamath Mountains, Oregon, in Snoke, A. W. and Barnes, C.G., eds., Geological studies in the Klamath Mountain province, Californa and Oregon: A volume in honor of William P. Irwin: Geological Society of America Special Paper 410, p. 53-76.

Second alteration cycle - serpentine altering to anthophyllite (Presque Isle peridotite)

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

The grain shown in this image, originally olivine, was altered to serpentine, most of which has been altered in turn to anthophyllite. That is, I thought it was anthophyllite, but then I read Michael Lewan's (1972)* M. S. thesis, in which he reported NO anthophyllite in the three samples of Presque Isle peridotite for which he carried out modal analysis. However, all three samples contained small amounts of chlorite (7.3, 7.1, and 5.4 percent volume. I've never seen chlorite that looks like this, but maybe... If you know, please send me a comment.

Tiny red or orange anthophyllite(?) crystals occupy most of the former olivine grain, except for the two remaining areas of serpentine, gray-green in the XPL image above (lower left and upper right). The image below, also XPL, shows most of the anthophyllite(?) crystals (those aligned from upper left to lower right, or vice versa) at extinction - as extinct as they get. The serpentine areas show more clearly in this view, while the anthophyllite(?) crystals that are "at extinction" have dark outlines of varying thickness.

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

In the PPL image below, the serpentine areas are relatively featureless, while elsewhere the elongated crystals of anthophyllite(?) show distinct outlines. Yellow-orange color in PPL image is due to iron-oxide staining from hematite. Black spots are magnetite. Presque Isle peridotite, Marquette County, northern Michigan. Imaged area 0.5 mm x 0.8 mm.

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

*Lewan, Michael D., 1972, Metasomatism and Weathering of the Presque Isle Serpentinized Peridotite, Marquette, Michigan, unpublished M.S. thesis, Michigan Technological University.

Presque Isle peridotite - full thin section

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

This image shows the texture of the peridotite, in which olivine and pyroxene have been almost entirely pseudomorphed by serpentine, much of which has been replaced in turn by amphiboles. Light-colored areas are carbonate veins. Marquette County, northern Michigan. XPL Macrophotograph. Imaged area 25 mm x 41 mm.

Plagioclase in Yellow Dog peridotite (plagioclase-bearing lherzolite)

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 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.

Weathered dunite, Newdale, NC

Click on image to enlarge          Photo © Daniel R. Snyder

Above: Weathered dunite, with small grain size and indented grain boundaries resulting from cataclasis and subsequent weathering. All of the blue-green olivine grains clustered at left center go extinct simultaneously, indicatng that they are fragments of a single crystal. Note the dense network of dissolution channels and voids (black). Newdale, Yancey County, North Carolina. XPL digital mosaic. Imaged area 1.3 mm x 3.2 mm.

Below: Optical scan of a freshly-sawn surface of weathered Newdale dunite, same sample as above (NOT same scale). Yellow-orange color is imparted by hematite, disseminated in serpentine layers between and within olivine grains. In the high-magnification view, many of the more robust olivine grains can be seen to retain a light yellowish-green color. Dark grains at lower left are chromite. Dark green grains in cluster at upper left may be remnants of a fragmented hornblende grain. 2400-dpi optical scan. Imaged area 11.4 mm x 17 mm.

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

Talc in weathered Newdale dunite

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

In the above XPL image, a large talc clot (center) is surrounded by weathered olivine grains. The solid black grains at top center, bottom center, and bottom left are chromite. Linear patterns in the lower part of the talc suggest that this part of the talc is partially-altered tremolite/actinolite. Below is a PPL image of the same area at the same scale, showing clearly the chromite and the serpentine meshwork in the dunite. Newdale, Yancey County, western North Carolina. Imaged area 2.7 mm x 4 mm.

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