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.