The
first item takes no account of placer gravel but represents output of
mines producing ore only. The greatest output was from . Arizona, Utah,
Montana, Nevada, and New Mexico—all notable for large yield of copper
ores—and from Alaska.
Many
gold and silver mills employ concentrating apparatus, and the
concentrates they obtain are combined in the table with those from
straight concentrating mills under the heading "Concentrates produced.
The gold and silver included in this item is recovered mainly by
amalgamation and cyanidation, as is shown in detail in the last table
of this report.
The
figures for the quantity of ore treated by concentration only include
the large quantities of copper, lead, zinc, and mixed ores whose
concentrates are smelted primarily for these metals, the gold and
silver being recovered in refining the copper and lead bullion and
smelting the zinc residues. Examples are the copper ores of the
disseminated deposits of Bingham, Utah, and of New Mexico and Nevada;
the copper ores of Butte, Mont.; and the lead and lead-zinc ores of
Coeur d'Alene, Idaho, Butte, Mont., and Leadville, Colo. The quantity
of concentrates produced and the recoverable gold and silver content
represent not only the concentrates from these straight concentrating
mills but also those from gold and silver mills, which form, however, a
very small part of the total quantity of concentrates and are derived
mainly from Alaska, California, and Colorado ores. These concentrates
are mainly smelted but are also treated in gold and silver mills by the
processes named.
The
ore treated in straight concentration mills increased notably in 1920,
mainly because of more activity at copper mines in Montana, Arizona,
and other States. Montana mills produced the largest quantity of
concentrates, followed by those of Arizona, Utah, Nevada, Idaho, and
New Mexico. The recoverable gold content of concentrates came mainly
from Colorado, Alaska, Utah, California, Montana, and Nevada. About 67
per cent of the recoverable silver from concentrates produced came
from Montana and Idaho; and large quantities came from Colorado, Utah,
Arizona, Alaska, and Nevada. The increase was due largely to an
increase in the silver in concentrates derived from ores mined in
Idaho, Utah, and Colorado.
The
quantity and the recoverable precious-metal content of crude ores
shipped from the mines direct to the smelters in general include the
richer gold, silver, copper, lead, and mixed ores from which the gold
and silver is eventually recovered by refining the copper or lead
bullion that collects the precious metals in the smelting. Arizona
produced more than 53 per cent of the ore directly smelted and was
followed by Utah, Tennessee, Colorado, Montana, and Nevada. Utah and
Arizona produced nearly 57 per cent of the silver recovered from crude
ore smelted.
The
old materials, principally tailings and slags, re-treated (with which
are included ore and tailings leached) are partly smelted, often for
their fluxing as well as their metal value; but considerable
quantities of old tailings are now concentrated and cyanided, and the
impounding of mill tailings for future treatment as well as because of
legal requirements has become a general practice. The increase in
quantity of old material treated in 1920 was due mainly to the
treatment of old slag and tailings in Arizona and Montana. Most of the
gold and silver derived from old tailings is included under
