Geology Reference
In-Depth Information
rocks situated throughout the Klamath Mountain
province. Within the Klamaths, mining has focused on
what is called the Josephine ophiolite. This is a belt of
distinctive rocks that are part of the larger Western
Klamath terrane forming an arcuate outcrop from
southwest to the northeast across the province. What
was once crust beneath a deep ocean floor, ophiolites
have been raised up above sea level by continental
collision to make the mineral deposits in them accessi-
ble. The Josephine ophiolite closely resembles mineral-
rich seafloor deposits elsewhere in Cyprus, Turkey,
Newfoundland, and Italy that are referred to collective-
ly as "Cyprus type" massive, iron-copper sulfide depos-
its.
The Josephine ophiolite is believed to have
developed originally in a backarc spreading environ-
ment, where the sea floor is pulled apart on the
continental side of an island archipelago between the
volcanic chain and the mainland. With the spreading
process, tensional step faults form parallel to the axis
of spreading. Sea water begins to circulate by convec-
tion along the faults and through porous undersea
pillow basalts to emerge along the axis itself as sea
floor hydrothermal vents. During cooling, minerals
dissolved in the super-heated sea water precipitate in
faults as well as within the volcanics and sediments to
create metalliferous muds. In this way a host of ele-
ments as iron, copper, cobalt, zinc, nickel, gold, and
silver, with lesser amounts of other minerals accumu-
late along the deep ocean spreading ridge to form ore
bodies near the base of the ophiolite sequence. Chrome
and nickel precipitate in pods associated with periodite
in the upper mantle.
In
another mineralization process, smaller
amounts of ores are precipitated when mineral-laden,
hydrothermal fluids from a cooling pluton invade and
enrich the enclosing rock. Typical intrusions with
associated gold and silver can be found in the Chetco
complex of western Josephine County and in the
Ashland pluton in southern Jackson County.
Equisetum or horsetails and ginkgo grew as early as
Jurassic in southwestern Oregon.
during the early Cretaceous and which later rotated 45
to 50 degrees clockwise to its present position. It is
worthwhile to note that foundation or basement rocks
north of the lineament are of oceanic crustal composi-
tion, while basement rocks south of the lineament are
of continental crustal composition.
Gold
The search for gold stimulated the develop-
ment of southern Oregon which, in turn, led to the
discovery of other economically viable minerals as
silver, copper, and zinc from the mining regions of the
Klamath Mountains. Early miners examined river sands
and gravels for placer gold which had been eroded
from lode deposits. The discovery of placer gold along
the Illinois River in 1850 by a party of miners, most of
whom were from Illinois, set the stage for the gold
rush of 1851 in southern Oregon. In December of that
year thousands of men arrived in Jacksonville following
reports of gold at this campsite along a wagon route to
Mining and Mineral Industry
Long exploited for their mineral wealth, the
Klamath Mountains contain rich deposits of gold,
silver, copper, nickel, and chromite, as well as smaller
amounts of other minerals. The presence of precious
metals in southwestern Oregon relates primarily to
mineralization as part of the sea floor spreading
processes and secondarily to the intrusion of plutonic
