Geology Reference
In-Depth Information
dredges, and the installation of gold-saving devices at
private sand and gravel plants have increased the
amount of gold recovered. The use of a process called
heap-leaching, where sodium cyanide or other chemi-
cals are employed to extract the gold from piles of low-
grade ores, has brought vigorous protests from environ-
mentalists. Large open pits, created in the extraction
process along with lagoons which contain toxic materi-
als would eventually be abandoned.
Mining ceased in 1928, although operations sporatically
continued for several years after that. There was even
limited gold and copper output from the Iron Dyke in
the 1980s.
Mercury
Mercury ore or cinnabar in north-central
Oregon occurs in the Clarno Formation where it is
associated with intrusive volcanic plugs and adjacent
faults. The Horse Heaven district in Jefferson County
is responsible for the highest production. Discovered in
1933, the Horse Heaven Mine operated from 1934 to
1944 and from 1955 to 1958 when the reserves here
were exhausted. During this period the mine yielded
17,214 flasks of quicksilver ranking among the top 5 in
the state. The Bonanza Mine in Douglas County is the
state's largest producer, while three other top mines in
the southeastern corner of the state averaged 15,000
flasks each. No other Oregon mines have produced
more than one thousand flasks each.
Copper
Deposits of copper are often found with gold
and silver in the Wallowa Mountains and along the
Snake River canyon. Copper prospecting began in the
late 1800s, and the bulk of the copper came from the
Iron Dyke Mine near Homestead on the Idaho border.
Copper here occurs in sheared, faulted zones within
Permian and Triassic volcanic and sedimentary rocks.
The mineralization or emplacement by hydrothermal
fluids along fractures in the host rock probably came
later in the Jurassic period. Copper occurs chiefly as
chalcopyrite with lesser amounts of other minerals
present.
Thundereggs
Of Oregon's gem-like minerals, the thunder-
egg, a spherical nodule or geode, is the most famous,
and in 1965 it was designated as the official state rock
by the legislature. In actuality, the thunderegg is not a
rock but a structure created in rhyolite, tuff, or perlitic
The Iron Dyke was discovered in 1897 al-
though serious production didn't begin until 1915. A
"rush" to Homestead occurred in 1890 with the promise
of major copper discoveries, however, output from
most of the deposits remained small. From 1900 to
1965 approximately S3 million dollars worth of copper
was extracted. The peak years were from 1913 to 1920.
The Iron Dyke Mine included
a main tunnel, eight levels
from four adits, and a 650
foot vertical shaft. The mine
is no longer in production.
