CorundumYogo Gulch, Yogo District, Little Belt Mts, Judith Basin Co., Montana, USA
Latitude: 46°52'35"N
Longitude: 110°18'27"W
A gemstone deposit located on the eastern flank of the Little Belt Mountains, about 95 km southeast of Great Falls.
An ultramafic lamprophyre (ouachitite?) dike has been mined for xenolithic sapphires of gem quality ("Yogo sapphires"). The sapphires are usually small, but free of zoning and inclusions.
This lamprophyre is very close to a carbonatite in composition. It shows phlogopite and clinopyroxene megacrysts in a matrix of phlogopite, clinopyroxene, calcite, analcime, magnetite and apatite.
The rock contains ocelli composed of calcite, dolomite, ankerite, baryte, analcime, pyrite and amphibole.
It also contains xenoliths :
- Pyrope-cpx-plagioclase granulites (12-20 kb / 850°C)
- Sapphire-pyroxene-plagioclase assemblage
- Pyroxene-phlogopite-sapphire assemblage
References
- USGS Bull 269
- Canadian Mineralogist (1988): 26: 81-88.
External Links
-
http://www.eos.ubc.ca/personal/groat/sapphires.htm [Link dead. Nov 2011]
Mineral List
22 entries listed. 15 valid minerals.
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