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764 MINERAL RESOURCES, 1918----PART I.
concentrating,
as in many States; simply concentrating ores, as in parts of Colorado
and Arizona; all-sliming and cyaniding ores; and finally smelting ores.
Tailings both from old dumps and from present millings are largely
reworked by concentration and subsequent cyanidation.
The
loss in tailings from gold mills is being constantly cut down, and the
most serious loss remains in tailings from concentrating plants. The
chlorination process is of decreasing relative importance, having been
used in only a few plants in California in 1917 and 1918. Smelting is
mainly of concentrates and of siliceous and pyritic ores, which are
also valuable as fluxes. Exact figures of relative output by methods
will appear in detail by States in another table.
As
most of the production of gold in the United States is derived from
placer gravels and dry or siliceous ores, including true gold,
gold-silver, and silver ores, the following table for certain States,
prepared by V. C. Heikes, of the United States Geological Survey, is of
special interest. The table shows the quantity of each class of ore
treated and the average content in gold per ton for the period 1913 to
1918, inclusive, for Arizona, Idaho, Montana, Nevada, Utah, and
Washington. None of the States had a large yield of gold from
gold-silver ores, and Nevada, with a yield much less in 1917 and 1918,
than in 1913, was the only State that produced a large quantity of gold
from silver ores. Arizona showed an increase in quantity both of gold
ore treated and of gold recovered in 1917 and 1918, and the average
content per ton in 1918 was considerably more than in 1913. In Idaho
the quantity of gold ore treated in 1918 was only 4,752 tons more than
in 1913, but the average recovery decreased from 0.410 ounce per ton in
1913 to 0.332 ounce per ton in 1918. In Montana about 5,100 tons more
of gold ore was treated in 1918 than in 1913 and the average yield per
ton of ore decreased from 0.369 ounce in 1913 to 0.320 ounce in 1918.
In Nevada the quantity of gold ore treated in 1918 was about 232,400
tons less than in 1913 and the average recovery of gold per ton
decreased from 0.438 ounce to 0.311 ounce. The yield of gold from gold
ores in Utah has been comparatively small in recent years and there was
a decreased quantity of gold recovered from such ores in 1918 compared
with 1917; the yield was less than $7,500, or 3 per cent of that for
the year 1913. The quantity of gold ores treated in Washington in 1918
was about 10,000 tons less than in 1913 and the average recovery per
ton declined from 0.570 ounce in 1913 to 0.309 ounce in 1918. Nevada
has produced since the working of mines in the Tonopah district the
greater part of the gold recovered from silver ore. Since 1913 there
has been no decrease in the quantity of silver ore treated, but the
gold content is so much smaller than it was in 1913 that the quantity
of gold recovered has decreased from 144,962 ounces in 1913 to 86,027
ounces in 1918.
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