Geologic map of the Rainy Creek Pluton (as of year 1963). The vermiculite mine was situated on top of the intrusion. By the time the mine was closed in 1990, more of the pluton was exposed and the tailings piles had grown considerably. For decades, winds blew tremolite asbestos dust from the tailings piles into the nearby town of Libby, Montana.

Excerpt from Johns (1970, p. 153): "In the Rainy Creek stock, mass-fiber tremolite of contact-metamorphic origin borders syenite dikes and quartz veins. The tremolite has formed from pyroxenite."

>From Johns (1970, p. 148): ".....the more abundant [of the two pyroxenite types] is a very coarse grained pyroxenite composed of pyroxene (probably diopside), mixed vermiculite and hydrobiotite, and accessory fluorapatite; it contains all of the economic vermiculite in the stock...........The second, finer grained type described by Boettcher forms zones within the the coarse-grained pyroxenite. It consists almost entirely of pyroxenite and contains only a small quantities of mixed vermiculite and hydrobiotite, apatite, garnet, magnetite, ilmenite, and sphene."

The vermiculite ore-to-waste rock ratio was reported to be 1:1.4 (Boettcher, 1963, p. 7).

The host rock for the intrusion is the Precambrian Wallace Formation (Belt Series).

References

Boettcher, A. L. 1963. Geology and Petrology of the Rainy Creek intrusive near Libby, Montana. Unpublished M. S. thesis, Pennsylvania State University. 70 pages.

Boettcher, A. L. 1966. The Rainy Creek igneous complex near Libby, Montana. Unpublished Ph.D. Dissertation, Pennsylvania State University. 155 pages.

Johns, W. M. 1970. Geology and Mineral Deposits of Lincoln and Flathead Counties, Montana. Montana Bureau of Mines and Geology Bulletin 79, 182 pages.

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