Geology Reference
In-Depth Information
Mining and Mineral Industry
Uranium
Although deposits of lithium, antimony, gold,
mercury, copper, and uranium have been known for
several years in the Great Basin of Oregon, only
uranium and cinnabar, or mercury, have been mined
extensively. The Lakeview district in the southcentral
part of the state as well as the McDermitt district in
southern Malheur County have produced small
amounts of uranium. Nearly 400,000 pounds of urani-
um have been recovered from two deposits in volcanic
rocks just northwest of Lakeview. Most of the material
was from the White King Mine, with smaller amounts
from the Lucky Lass Mine where shipments in 1955,
shortly after discovery, were the first uranium ores
marketed from Oregon. A mill processing 210 tons per
day was constructed at Lakeview in 1958, operating
until 1965 when closure resulted after several years of
only minimal production. Located in the McDermitt
caldera, the Aurora and Bretz uranium prospects are
the largest in total output of any yet found in Tertiary
volcanic rocks of the United States. This potentially
economic source of uranium ore, found as uraninite
and coffinite, is associated with rhyolitic rocks and lake
sediments. Once the thick, flat-lying lavas, covered by
tuffaceous lake sediments, had been broken up by
faulting, hot waters, containing minerals, followed
fissures and cracks to deposit uranium and mercury in
veins.
Caught between large-scale moving crustal mass-
es, Oregon is being sheared by extensive faults
running northwest to southeast across the state.
Poker Jim Ridge has an 1,800 foot precipitous scarp.
Steens Mountain is a large horst where the block has
been fractured into several pieces. The northern most
portion tilted to the west as it rose giving that side the
very gentle slope of Smith Flat. Extending for over 60
miles in a north-south direction, the steep east escarp-
ment displays successive layers of Steens basalt in
cross-section.
The southern and eastern margins of Oregon
are cut by faults that run hundreds of miles across
several physiographic provinces. These large-scale
cracks, termed strike-slip faults, move laterally and are
parallel to the southeast by northwest Olympic-Wal-
lowa lineament. From south to north they are the
McLoughlin fault, the Eugene-Denio fault, the Broth-
ers fault, and the Vale fault. The faults are actually
complex fault zones of smaller overlapping faults, but
the trends are remarkably parallel. Faulting is most
severe to the south and minimal to negligable along
the short Vale fracture which runs northwest through
the Owyhee Uplands. These fault systems relate to a
shearing action where central and eastern Oregon are
caught between two large-scale moving blocks. Action
along the blocks is similar to movement of the San
Andreas fault in California with the eastern block
moving south and the western block moving north.
Mercury
Within the McDermitt caldera complex, the
Opalite Mining district, which includes the McDermitt
Mine, the Bretz Mine, and the Opalite Mine have had
a total output of 270,000 flasks of mercury, the richest
supply of mercury in the western hemisphere. Mercury
exploitation in the Opalite district began in 1917 with
the discovery of cinnabar by William Bretz who had
prospected in this region for a number of years. By
1925 tunnels had been driven 80 feet below the ore
body and the cinnabar brought to the surface where it
was processed in a large rotary furnace completed here
in 1926. Over the years production was sporatic,
stimulated in 1940 by the rising price of quicksilver
prior to and during World War II when large amounts
of mercury were used as ballast in submarines. The
opening of several new pits signalled renewed mining
as late as 1957 when the ore was sent to Salt Lake City
for processing.
Very small deposits of cinnabar, copper, and
gold along the eastern edge of the Steens and Pueblo
mountains are largely unproductive.
