Canadian Rockhound: Cordierite

Cordierite
By Richard Gunter

Part II: Alteration Zones

Cordierite is a minor component of several hydrothermal alteration zones associated with copper-zinc deposits in various parts of northern Manitoba.

At the Ruttan Mine near Leaf Rapids, euhedral cordierite crystals up to 6 cm long occur in an alteration zone rich in iron and magnesium. Some euhedral crystals were also be found in the 1980's within the sulphide boulders on the mine dump. They seem to be most plentiful near the sulphide/schist contact.

Other iron-magnesium rich alteration zones occur in the region, notably at the Rat River Zone near Leaf Rapids and the Star Lake Zone east of Sherridon where it occurs with garnet, anthophyllite, spinel group oxides and small orange crystals of the rare mineral wagnerite.

Alteration zones in the Snow Lake area tend to be more aluminous in composition and hence contain sericite, kyanite and gahnite instead of anthophyllite. Very coarse-grained, 5-10 cm long crystals of cordierite were found as a rare constituent of the Snow Lake alteration zones. Crystals and crystal fragments were found in the Anderson Lake Mine and the Rod Mine.

Cordierite crystals from the Ruttan, Anderson Lake and Rod Mines were partially gemmy and have a strong trichroic grey-purple-blue colour change when rotated in a strong light. They also have a strong parting which produces tabular, often gemmy fragments.

Part I

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The preceding article was first published in the May 1997 issue of the Mineral Society of Manitoba Newsletter. Reprinted with permission from the author.