Gold
dredging has developed into a most important industry in California.
The yield from this branch of mining in that State in recent years has
been as follows: 1904, $2,187,038; 1905, $3,276,143; 1906, $5,098,359;
1907, $5,065,437; 1908, $6,536,189. Sixty-nine dredge boats were
operating in California in 1908. The total yield from all dredging
operations outside of California was only $815,584 in gold, divided
between Alaska, Colorado, Georgia, Idaho, Montana, and Oregon. The
corresponding output for 1907 was $571,659. Details can not be given
without disclosing the production of single operators. Nineteen dredge
boats were in operation as follows: Alaska, 4; Colorado, 4; Georgia, 1;
Idaho, 5; Montana, 4; and Oregon, 1. No gold was reported from dredging
along Snake River.
Drift
mining in frozen ground has yielded most of the gold from the Lower
Tanana, in Alaska, and a large part of the gold from the Seward
Peninsula is also obtained by this method of mining. In California
drift mining still remains an important branch, but the yield is
declining, and was only about $390,545 in 1908.
The
remaining placer gold is won by the hydraulic method or by-surface
sluicing work, which is practiced in all of the placer States. A very
small amount of placer gold is derived from dry washing in New Mexico
and Arizona.
Dry and siliceous ores.—The
production of gold from dry and siliceous ores in 1908 amounted to
$59,578,704, against $54,274,876 in 1907. The States which yielded over
$2,000,000 from this source rank as follows: Colorado, Nevada,
California, South Dakota, and Alaska. Compared with the preceding year,
California, Alaska, Idaho, South Dakota, and Utah showed considerable
gains, while less amount of gold was recovered from this source in
Nevada, Montana, New Mexico, and Oregon.
The
division necessarily includes many varieties of ore, and several
different methods of reduction are applied to them. The gold ores of
California, Oregon, and Alaska are as a rule free milling, though
concentration and cyaniding of tailings are very often combined with
the simple amalgamation process. The ores of the Homestake mine in
South Dakota fall into the same general class as do the Telluride and
Ouray siliceous ores and ores from many scattering occurrences in
Idaho, Colorado, and Arizona.
In
many cases in Colorado, in Yavapai County, Arizona, and in other
places, the siliceous ore contains but little free gold and is
concentrated without amalgamation.
The
pan-amalgamation process for siliceous gold-silver ores has become
almost obsolete, and is used at only a few places in Montana, Nevada,
Texas, and Arizona. The siliceous ores of western Nevada at Goldfield,
Tonopah, Rhyolite, and Silver Peak are now largely reduced in mills
near the mines, and in consequence lower-grade ores can be treated.
Full descriptions of these mills were contained in the report for 1907.
The process often used combines plate amalgamation, concentration, and
cyaniding.
The
quartzose gold ores formed by replacement of limestone are generally
cyanided and yield a total of several hundred thousand ounces. The
three most prominent localities are the Camp Floyd (Mercur) district in
Utah, the Black Hills of South Dakota, the Moccasin Mountains, and the
Little Rocky Mountains of Montana.
