sumed
in perfecting the method of concentration in use and in exploratory
work in the mine. The production of sapphires suitable for cutting
amounted to over 100,000 carats between April and December of 1907. In
addition, several thousand ounces of culls for watch jewels, bearings,
and instruments of precision, were obtained at the same time.
Variegated Sapphire Company.—The
auriferous placer sapphire deposits on Dry Cottonwood Creek, 16 miles
N. 70° E. of Anaconda, were exploited with a dredge during 1907 by the
Variegated Sapphire Company under the management of Mr. William Dodd.
The deposits are located at an elevation of over 6,000 feet, nearly
1,500 feet above the valley of Deerlodge River, to which Dry Cottonwood
Creek is tributary. The company owns some 2 miles of gulch land with
beds of gravel 40 to 100 feet wide and from 10 to 14 feet thick. The
gravels in some of the gulches to the side of the company's land and in
the flats below are also reported as carrying sapphires.
The
country rock in the region around the mine is a quartz porphyry, in
places nearly a biotite granite. This porphyry is rather fine grained
and composed of quartz and feldspar phenocrysts,with biotite laths and
crystals in a ground mass. The quartz occurs in clear, glassy crystals
and rounded grains, some of them fractured, thickly scattered through
the rock. The feldspar, chiefly a plagio-clase, has largely decomposed
to kaolin in the surface rock examined. This kaolinization has taken
place both within the crystals, on their exterior, and in the ground
mass. The biotite also seems to have been
partially
hydrated, and in thin section under the microscope has a low
birefringence and a strong dark-green to brownish-black pleo-chroism.
The
source of the sapphires is not known. Mr. Dodd reports their occurrence
farther up the gulch than the part examined, in a rock like the
porphyry described above. The gravels in the gulch consist chiefly of
blocks of porphyry, some of them rounded into cobbles, others flat and
slab-like with but partially rounded corners. The overburden or top of
the deposit, consisting chiefly of black muck with but little gravel
through it, is 3 to 4 feet thick.
The
dredge used by the company is of the bucket type, and has a capacity of
750 cubic yards in twenty-four hours. It is operated by a steam engine,
and has a dynamo for its electric-light equipment. The material from
the dredge buckets goes to a revolving screen from which everything
over 1 inch in diameter is separated and turned into the pond under
water at the back of the dredge, while everything under 1 inch in
diameter is run over 56 feet of riffles. The debris from the sluice and
the riffles is piled on the coarse material at the back of the dredge.
In this way a dam is built which retains the water in the pond on which
the dredge floats. By excavating before and constructing a dam behind,
the dredge will be worked up the gulch. The grade of the gulch is not
light, and the flow of the creek during the summer is quite small. The
dredge cuts a square face in the gravels across the gulch. The
overburden is first removed for a width of 6 feet upstream, being run
directly through the dredge without washing. Mercury is placed in the
riffles to catch gold, and clean-ups are made weekly. The sapphire
concentrates are washed from the sluices of the dredge into a bin and
are later sieved and panned down.
