|
130
|
MINERAL RESOURCES.
|
||
|
|
|||
|
companies
were operating in the United States, as follows: California, 24;
Montana, 4; Idaho, 3; Alaska, 2; Colorado, 3; Oregon, 2. In California
and Montana, however, one company in some cases operated several
dredge boats.
Opinions
conflict about the possibility of large dredging operations in Alaska.
An increased yield from this source may be expected from other States,
but it is not likely that any of them will approach California in
yield. The results of dredging along Snake River in Idaho have been
disappointing, and the whole yield from placers along this stream has
diminished to about $23,000 from $27,018 in 1905.
Drift
mining in frozen ground has yielded most of the gold from the lower
Tanana in Alaska. About two-thirds of the gold from the Seward
Peninsula is derived from this method of mining. In California drift
mining at great depths below the surface remains an important branch,
but the yield in 1906 was only about $600,000, a decrease of
approximately $200,000 from 1905.
The
balance of 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
States which yielded over 100,000 ounces from this source rank as
follows: Colorado, California, Nevada, South Dakota, Alaska, and
Montana. 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,
and Arizona. The rich siliceous ores of western Nevada were to a great
extent sold to lead or copper smelters, although the high contents in
alumina are objectionable in some of them. The rich Goldfield ores were
widely distributed in the fall of 1906 among the smelters from
California to Denver. The Tonopah ores were also largely shipped to
smelters. Freight and treatment charges on this class of ores ranged
from $12.50 to $42 per ton, the latter for the richer ores, containing
$100 per ton. Beyond a tenor of $100 per ton the charges increase still
further.
Large
mills are being built at Tonopah for the handling of these gold-silver
ores by stamp or rolls, followed by concentration and cyaniding of
tailings. At Goldfield a part was treated in local mills by
amalgamation, concentration, and cyaniding. The partly free-milling
ores near the surface are now changing to ores requiring concentration
and cyanide treatment.
The quartzose gold ores formed by replacement of limestone are generally cyanided and yield a total of several hundred thousand
|
|||
|
|
|||