The Calvert Tungsten Mine

Garnets and Epidote

The Calvert Mine near Wise River, Beaverhead County, Montana USA, was an important open-pit tungsten mine that yielded about 113,000 tons of ore with 1.1% tungsten oxide between 1956 and 1962 (Walker, 1963, Tungsten resources of western Montana: US Bureau of Mines Rept. Invest. 6334).

The ore is in a skarn. A skarn is an altered rock in which the chemistry has changed because of introduced material from an external source, such as a granitic body. So it’s not just metamorphosed (“changed form” of the original rock without changing its overall composition), it’s also metasomatized (“changed body,” where new chemicals come in to react with the original rock to make new minerals).

At Calvert, the country rock is sandy carbonates of the Pennsylvanian Amsden Formation (Messenger and others, 2017, Geology, fluid inclusion, and stable isotope study of the Calvert Tungsten Mine, Pioneer Mountains, Montana: Montana Bur. Mines & Geology Open-File Report 685), intruded and probably altered to marble by a tonalite (a rock similar to granite but with more plagioclase feldspar) about 72 million years ago.

Garnets and epidote, both calcium-bearing, are common in skarns especially those in carbonate rocks.  

This garnet, collected in 2016, is probably grossular, calcium-aluminum garnet, but it would take analysis to be certain which of the six common garnet species it is. The word “grossular” is from the scientific name for the gooseberry, Ribes grossularium. Gooseberry-green calcium-aluminum garnets occur in Siberia, although Abraham Gottlob Werner, who named the mineral in 1808, had originally called different specimens of it kanelstein, German for “cinnamon stone.” Although green is common, grossular comes in many colors.

“Garnet” probably comes ultimately from the same root word as pomegranate, for the similarity in color of red varieties and/or the grain-like nature of garnet crystals and pomegranate seeds (Latin granum, “grain”).  

The crystal forms present are the dodecahedron and trapezohedron, with some second order trapezohedrons on some of the trapezohedral faces.

The 5-cm epidote crystal above was the largest one I found when I was there in 2016. It’s a twin, with two crystals sharing a {100} face, the monoclinic front pinacoid. The reentrant on the crystal termination at left is typical of epidote twins.

You can see the planes perpendicular to the elongation direction; those show the perfect cleavage of epidote. Because epidote has this perfect way of breaking and is quite brittle, I was afraid to trim the rock any more than I did, so this crystal sits in a big chunk of rock about 20 cm across.

Although the pit now is a 30-meter-deep lake, it is remarkably unpolluted, with “near-neutral pH, exceptional clarity, and extremely low concentrations of nutrients, sulfate, and most metals” (Gammons and others, 2013, Geochemistry, water balance, and stable isotopes of a “clean” pit lake at an abandoned tungsten mine, Montana, USA: Applied Geochemistry 36, p. 57-69). That seems to be because of a lack of sulfides and a hydrologic system that prevents stagnation.