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816
MINERAL, RESOURCES.
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SAPPHIRE.
MONTANA.
There
was much activity in sapphire mining in Montana during 1907, with a
consequent large production, both of the yogo-blue sapphires and of the
varicolored sapphires found in other parts of the State. Two large
companies operated mines containing blue sapphire in its original
matrix, and two other large companies and smaller or individual
producers worked auriferous placer deposits containing varicolored
sapphires. The blue sapphire in matrix was worked in the Judith River
region, in Fergus County, at points about 11 and 13 miles
west-southwest of Utica, by the New Mine Sapphire Syndicate and the
American Sapphire Company. Placer deposits of varicolored sapphires
were operated on the head of Dry Cottonwood Creek, Deerlodge County, by
the Variegated Sapphire Company, and along the West Fork of Rock Creek,
in Granite County, by the American Gem Mining Syndicate. A little
mining was done and a few finds reported from the auriferous sapphire
deposits along the Missouri River, below Helena, once so extensively
worked.
Yogo blue sapphires.—The
blue sapphires of Fergus County, often called "Yogo sapphires," occur
in a dike of basic igneous rock ° cutting nearly perpendicularly
across the bedded limestone country rock. The dike crosses the canyon
of Yogo Creek (the north fork of Judith River) and the rolling country
sloping eastward from the crest of Yogo Canyon to the bottom lands of
Judith River, a distance of nearly 4 miles. The limestone country rock
belongs to the Madison limestone formation of Carboniferous age, as
mapped by W. H. Weed.6 This formation is over 1,000 feet
tluck, and consists of thinly bedded strata of light-grayish limestone
which dip rather gently to the east. There are a few minor folds in the
limestone, some of which can be seen in the walls of Yogo Canyon near
the mine of the American Sapphire Company. The sapphire-bearing dike is
slightly sinuous and has a strike a little north of east with a nearly
vertical dip. In the canyon, however, it seems to split up into two or
more parts (one of which pinches out in the limestone) or to be
intersected by another dike. The thickness of the main dike throughout
its known length varies from 2 to over 14 feet.
The
rock of the sapphire-bearing dike has been described by Prof. L. V.
Pirsson. When fresh and unaltered it has a dark-gray color with a
greenish or bluish cast. The principal constituents are biotite mica
and pyroxene, of the diopside variety, with minute and large inclusions
of calcite, quartz, pyroxene, and pyrite. Some of the biotite occurs in
phenocrysts of 2 or 3 mm. diameter, though the greater part is in small
shining flakes, thickly scattered through the rock. The glistening
scales of biotite and some of the inclusions are the principal
constituents that can be recognized in hand specimens. The inclusions
of calcite and quartz are surrounded by reaction rims of pale and
sometimes bright emerald green pyroxene. This pyroxene sometimes occurs
scattered through the smaller inclusions, or even constitutes the mass
of them. The dike rock contains numerous seams and veinlets of calcite
and quartz as well as large inclusions
a Somewhat fully described by Weed and Pirsson: Twentieth Ann. Eept. U. S. Geol. Survey, pt. 3 1898-99, pp. 454-459 and 552-557. b Geologic Atlas U. S., lolio 56 [Little Belt Mountains], U. S. Geol. Survey, 189a
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