In
earlier years the Sonora district in Mexico undoubtedly yielded much
gold from these dry placers, and it seems probable that further
production can be expected from these fields and other deposits in the
United States, but much remains to be desired in solving the problem of
profitable extraction. Costly and financially unprofitable experiments
have been reported in attempts at this kind of gold mining, and
intending investors might well consult the small body of literature
already available on this subject. Detailed information relative to the
geology and occurrence of this dry-gravel gold is still largely
lacking. References to brief Survey notes and papers published in the
technical press were given in the general gold and silver report of the
Survey by the writer for 1911. The following notes have been prepared
at the writer's request by his colleagues of the Survey for the present
report:
DRY PLACERS IN ARIZONA. By V. C. Heikes.
In
Arizona extensive dry-placer areas occur in the valleys of both
Colorado and Gila rivers, on the latter 20 miles above its confluence
with the Colorado. In the valley of the Colorado placers are known from
the vicinity south of Yuma up the river to above Ehrenberg, La Paz, and
in the Plomosa region surrounding Quartzsite. Other regions are
situated in southern Yavapai County in the Weaver district, in
northern Maricopa County known as San Domingo Wash, at Oracle in Pinal
County, at Greaterville and Quijotoa in Pima County, in Teviston
district in Cochise County, and 20 miles south of Yucca in Mohave
County.
Scattered
over Arizona are thousands of acres of gold-bearing material which
could produce millions of dollars if the proper facilities were at
hand. Some of these arid regions may yet be reached by pipe lines for
carrying water, but there are others where the ingenuity of engineers
will always be taxed to provide machines and methods of handling the
material in the dry way.
Character and method, of treatment.—The
areas of dry-placer ground are largest in Yuma and Pima counties,
Ariz., where much of the richer loose material has been worked by
Mexicans or gambucinos and by other miners in the usual crude
way with the batea and the Mexican dry-washing machine. These crude
methods required material of very high grade to make it worth while,
and only the best was treated. In most of the dry-placer localities the
loose material has been worked over many times until the conglomerate
bed or lime-cemented gravel (caliche, argo masa) has been
exposed. Only the softest spots could be worked by the hand machines by
digging out the cement, inclosing it in a rawhide bag, and beating it
between bowlders. The coarse gravel is removed and the pulverized
cement panned in large wooden bateas. The method of operation is,
however, exceedingly slow and laborious, and with the invention of the
Mexican "dry washer" more material could be handled, though the
preparation of the cement for "washing" was still the same. The gold in
the cemented placer gravels does not occur within the pebbles,
bowlders, and rock fragments, but on their surfaces, being practically
all included m the calcareous cement which unites them. In other
localities, chiefly in Yavapai and northern Maricopa counties, the
rainfall is depended upon to provide water enough to work the dry
