The Coyote Hills are located about 46 kilometers north-northwest
of Lakeview, Oregon, within the Basin and Range physiographic province.
These hills represent a complex volcanic center of bimodal
calc-alkaline igneous activity.
The oldest rocks recorded in the Tertiary succession are horn-blende-
bearing andesite and aphanitic basalt flows, laharic breccias,
conglomerates, tuffaceous sandstones, and lithic wackes of the late
Eocene to early Oligocene Lower Andesite formation. During middle
to early late Oligocene time, voluminous eruptions of predominately
basaltic andesite formed a large shield volcano. This unit, the
Upper Basalt formation, was followed, after a short hiatus, by the
Coyote Hills rhyolite of late Oligocene to early Miocene age. The
Coyote Hills rhyolite represents a complex spectrum of multi-phase
silicic volcanism and comagmatic near-surface plutonism. Magma
compositions varied from dacite to rhyolite and include lava flows,
volcanic plugs, a flow dome complex, and a hypabyssal quartz monzonite
intrusion. Volcanic activity that post-dates formation of the
bimodal Coyote Hills complex culminated with the lower slopes of
the shield volcano onlapped by the middle Miocene Steens Basalt, the
late middle to early late Miocene Plush tuff, and the late Miocene
to early Pliocene Upper basalt.
A prominent northwest and northeast-trending fault and fracture
system formed after emplacement of the Coyote Hills rhyolite, and as
early as late Oligocene to early Miocene in time. Basin and Range
faults post-date the Steens Basalt and have caused some minor displacement
of the younger rocks.
Penecontemporaneous with silicic volcanism of the Coyote Hills
rhyolite was a period of hydrothermal activity. Fluids ascended
favorable structures, altered the surrounding country rocks, and
deposited minor quantities of epithermal gold-silver-copper-mercury-molybdenum(?)-
lead(?), and zinc(?) in structurally controlled quartz-pyrite
veinlets and as disseminations. Because of the association
of mineralization with silicic volcanics in time and space, it is
concluded that the two processes were genetically related.
The hydrothermal system in the Coyote Hills is related to the
late stages of silicic volcanism. Evidence for a genetic relation
includes the close association of rock type, and chemical and mineral
zonations within the district. Geological and geochemical evidence
that includes rock type and alteration patterns, and mineral and
trace element zonations, collectively suggest that only the highest
level of the hydrothermal system has been exposed. It is entirely
possible that a large vein or disseminated-type deposit containing
both base and precious metals is present at depth. / Graduation date: 1981
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/36904 |
| Date | 02 February 1981 |
| Creators | Thomas, Thomas Holbeck |
| Contributors | Field, Cyrus W. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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