South
Dakota, Alaska, and Montana. Compared with the preceding year decreases
are common in the States. The only gains are reported from Arizona,
Georgia, Idaho, Nevada, North Carolina, and Washington. 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 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.
Interesting
developments have taken place in the reduction of ores from western
Nevada. A large tonnage which could not be profitably shipped, and much
of that which formerly was shipped, are now treated in large mills at
Goldfield, Tonopah, and Rhyolite, by several slightly different
processes, including the crushing by stamps or rolls, concentration and
cyaniding of tailings. Full description of these various methods of
treatment are contained in the section of western mine reports under
the heading of Nevada.
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, and the Moccasin Mountains of
Fergus County, Mont.
The
dry or siliceous ores further include the quartzose ores of Cripple
Creek, Colorado, in which the prominent characteristic is the
occurrence of large quantities of gold tellurides. These ores are
partly smelted, partly chlorinated, and partly cyanided, all three
processes being applicable.
There
is, finally, a large class of dry ores which contain pyrite and other
sulphides, and which are best treated by the smelting process, with or
without concentration. Colorado contributes by far the largest quantity
of these ores, among which those of Leadville are of particular
importance.
Copper ores.—In
1907, 272,150 ounces of gold were obtained from copper ores, against
271,197 ounces in 1906. The loss is smaller than would have been
expected, and this is probably accounted for by the fact that toward
the end of 1907 operations were suspended principally in those copper
mines which carry only a slight amount of gold. The important States
rank as follows: Utah, Arizona, Mon-
