In
reference to the second item many gold and silver mills employ
concentrating apparatus, and the concentrates obtained are combined in
the table with those from straight concentrating mills under the
heading "concentrates produced." The gold and silver recovery under
this item is mainly by amalgamation and cyanidation, as is shown in
detail in the last table of this report. The total quantity of crude
ore treated in gold and silver mills in 1915 was 11,508,953 short tons,
against 9,761,336 tons in 1914, and was the largest quantity so
treated since these figures were first compiled by the Survey for 1910.
The greatest quantities milled were in Alaska, California, South
Dakota, Colorado, and Nevada. The total recovery of gold from the mills
was 2,416,590 fine ounces in 1915, against 2,224,518 ounces in 1914,
and the production was, as usual, chiefly from Colorado, California,
Nevada, and South Dakota, in the order named. The total silver recover}7
in mills was 16,503,801 fine ounces, against 15,656,831 ounces in 1914,
and came very largely from Nevada (from Tonopah and Goldfield chiefly),
but also in considerable part from New Mexico, Arizona, Texas, and
Colorado.
Under
the third item is given the quantity of ore treated by concentration
only, including 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.; the lead and lead-zinc ores of Cceur
d'Alene, Idaho; Butte, Mont., and Leadville, Colo. The quantity of
concentrates produced and the recoverable gold and silver content,
however, as already stated, represent not only the concentrates from
these straight concentrating mills but also those from gold and silver
mills. 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 from 24,306,504 short tons in
1914 to 29,602,270 tons in 1915, mainly because of the much greater
activity at copper mines. Utah ranked first in quantity of ore
concentrated, followed by Arizona, Montana, Nevada, New Mexico, and
Idaho. These quantities, with the exception of those of Montana and
Idaho, represent largely low-grade disseminated copper ores milled.
The total quantity of gold and silver-bearing concentrates from all
classes of milling ore increased from 3,613,024 short tons in 1914 to
4,127,411 tons in 1915. Montana mills produced the largest quantity of
concentrates, followed by those of Utah, Arizona, Nevada, and Idaho.
The total recoverable gold content of all concentrates was 528,507
ounces in 1915, against 466,620 ounces in 1914, and came mainly from
Colorado, California, Alaska, Utah, and Nevada, in the order named. The
total recoverable silver from concentrates produced was 25,778,129 fine
ounces in 1915, against 24,102,264 ounces in 1914, and came chiefly
from Montana, Idaho, Utah, Colorado, and Arizona.
The
fourth item covers the quantity and the recoverable precious-metal
content of crude ores snipped from the mines direct to the smelters,
and in general includes the richer gold, silver, copper, lead, and
mixed ore from which the gold and silver is eventually recovered by
refining the copper or lead bullion that collects the precious metals
in the smelting. The total quantity shipped of this class of ore in-
