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
1908 gold valued at $4,812,993 was obtained from the treatment of
copper ores, against $5,625,842 in 1907. At first glance it would seem
that with a larger copper production in 1908 than in 1907 a
correspondingly greater quantity of gold should have been extracted.
There was, in fact, practically no change in Arizona and a notable
increase in Montana, the two being the most important copper-producing
States; there were also decided gains in California and Idaho; but all
these could not offset the great loss in Utah, due to a diminished
supply of copper ores rich in gold from the Tintic and Bingham camps.
Utah copper ores yielded only about $2,300,000, or $1,000,000 less than
in 1907. Losses are also noted in the production of Alaska and
Colorado. The relative rank of the principal States in the output of
gold from copper ores was Utah, Arizona, Montana, California, Nevada,
New Mexico, and Alaska.
The
copper ores which contain gold are chiefly of the sulphide class.
Chalcocite is the most abundant mineral, though in some districts
chalcopyrite prevails. Oxidized ores are still extensively mined at
Bisbee, Ariz., and Tintic, Utah; also at the Snowstorm mine in Idaho,
and at some other places. The sulphide ores are ordinarily concentrated
and smelted; the oxidized ores are smelted without concentration. The
gold is recovered by electrolytic refining of the copper.
Lead ores.—From
lead ores and copper-lead ores were recovered in 1908 $1,547,414 in
gold, against $2,669,439 in 1907. In spite of the great reduction of
the lead production in Colorado there was little change in the gold won
from lead ores in that State. At Leadville special effort was made to
mine ores rich in gold. Lessened production is recorded from Montana
and Idaho. The principal cause of the great decrease referred to above
is found in Utah, whence only about $414,000 were obtained from lead
ores, against nearly $1,000,000 in 1907. Lessened activity at Tintic
and Bingham camps was, here too, the responsible factor. The States
producing gold from lead ores ranked as follows: Utah, Colorado, Idaho,
and Montana. The first two States are far ahead of the others. The lead
ores are mainly of the sulphide class; large quantities of oxidized
ores are, however, still furnished by the Tintic district.
Zinc and zinc-lead ores.—Only
small amounts of gold are recovered from zinc ores. Montana, Arizona,
and Utah yielded together $24,674, most of it from Montana. Zinc-lead
ores, chiefly from Colorado, yielded $101,795 in gold, against $42,153
in 1907. Most of the increased Colorado yield for 1908 came from
Creede, not from Leadville.
