Figure I Generalized map of the Wilbur Mining ... - University of Utah

were thrust eastward over Mesozoic strata during
the Sevier orogeny and buried teneath andesites
from a nearby major volcanic center during the
mid-Tertiary. A major thrust fault, the Pavant
thrust, is exposed in the Pavant Range north of the
KGRA. Paleozoic rocks in the upper plate were
overturned during thrusting and can te seen exposed
above outcrops of Jurassic Navajo sandstone
(Crosby, 1959). Volcanism was renewed during the
Pliocene and Pleistocene with several rhyolitic
eruptions to the west in the Mineral Range and to
the northwest in the southern Sevier Desert.
The youngest volcanic feature in the area is a
Quaternary basaltic cinder cone three miles west of
the area currently teing explored by Union.
A large, shallow thennal gradient anomaly is known
to exist at Cove Fort. Shallow holes drilled by
Union and now available from UURI have tenperatures
of up to 119'F at a depth of 300 ft. The anonaly
appears to extend several miles to the north, v*iere
Hunt Energy drilled a geothermal test in 1978, and
to the northwest in Paleozoic strata in the upper
plate of the Pavant thrust. There have been
reports that deeper temperature holes drilled on
the extensions of the anonaly tecome isothermal
when the water table is encountered at a depth of
about 1,000 ft. Apparently a large horizontal flow
of warm water is taking place at the water table in
the carbonates.
Exploratory drilling at Cove Fort has found largescale,
secondary porosity and permeability in
dolomites to a depth of 5,221 ft. Porosity is
present in the form of fractures, breccia zones,
zones of dolomite sanding and caverns. There may
not yet te enough control to establish the continuity
of major fractures or the role of folding in
the creation of secondary porosity. Water appears
to descend rapidly to depth along some structures,
te heated, and then return to shallow depths along
other structures where it spreads out laterally in
permeable carbonates creating a large thermal
gradient anomaly. This is basically the model
proposed by Morris (1968) for Tintic. The structures
associated with water movement at Cove Port
appear to te similar to those found at Tintic and
it is likely that additional drilling and subsurface
geology at Cove Fort will further add to the
similarities tetween the two areas. The mciximura
tenperature found at Cove Fort to date is about
350'F in a contact metamorphic marble near the'
bottom of the 7,735 ft well. A weak flow of 43,000
Ibs/hr was produced frcm this zone during a flow
test. The temperatures and pressures associated
with ore deposition at Tintic are telieved to have
been greater than those yet found at Cove Fort
but were probably similar to those found in other
geothermal fields. The problem at Cove Fort
appears to te that the carbonates are too porous
and permeable,' allowing rapid convection and an
early cooling of the deep reservoir.
BEX3WAWE AND CARLIN
All of the productive wells drilled at the Beowawe
KGRA prior to 1974 were completed in an area of
fault intersections near the Beowawe Geysers.
These wells bottomed in either Tertiary basalts ard
andesites or underlying siliceous rocks of the
183
Edmiston
Western Assemblage assigned to the Valmy Formation
(Ordovician). In early 1974, Chevron drilled a
9,551 ft test which was designed to intersect the
Roterts Mountains thrust near its intersection with
the Malpais fault. This well, the Chevror>-ATR-Ginn
No.1-13, encountered two lost circulation zones
near TD and was believed to have bottomed in a
fault zone. A successful drill stem test was
conducted. Geologists familiar with Beowawe
have since debated whether the well intersected the
Malpais fault zone or fractures related to the
Roterts Mountains thrust. Some geologists have
also questioned the reservoir potential of the
underlying cartonates. The Carlin gold deposit,
located 28 miles northeast of Beowawe, can te used
as an exploration model to gain insight into the
geology of the geothermal reservoir at Beowawe.
At Carlin, microscopic disseminations of gold are
found in altered dolomites of the Roterts Mountains
Formation (Silurian) in a window of the
Roterts Mountains thrust. The Roterts Mountains
is variously included in either the Eastern
(Carbonate) Assanblage or the Transitional Assanblage
in the geologic literature of the region.
The geologic section above the ore deposit is
reconstructed in Figure 3 which is based on a
figure from Hausen and Kerr. Hausen and Kerr state
that penneable horizons in the Roterts Mountains
Formation provided a favorable environment for
mineralizing solutions which had travelled up high
angle normal faults ard which subsequently leached
calcite and deposited clay minerals and silica.
Noble and Radtke (1978) state that the temperature
of the fluids was 175*-225*C. Unconformably
overlying the ore horizon in the mine area is the
Popovich Formation (Devonian) which is a gray
fossiliferous, medium bedded, locally dolomitic and
silty limestone. Hydrothennal fluids invaded the
Popovich along its base and along joints and
fractures with subsequent calcite leaching and
silica deposition, but the fabric of the rock was
not totally, altered and it is not an important
ore host.
The Itoterts Mountains thrust separates the Popovich
from the overlying Vinini Fonnation of the Western
Assemblage which was thrust eastward into its
present position. Hausen and Kerr state that the
thrust is about 10 to 20 ft thick and consists of
bleached, brecciated and iron stained rock rubble.
Cartonate minerals have been leached from the fault
zone and frcm adjacent beds telow the thrust. They
conclude, and Noble concurs, that the thrust plane,
although not an important structure in the localization
of ore, did serve as a conduit for migrating
solutions over a long period of time. The Vinini
Fonnation at Carlin consists of siliceous shales.
In other areas the Vinini usually contains minor
quartzites and limestones. Following the main
period of hydrothennal activity there occurred a
later stage of acid leaching related to boiling
of the fluids as at Cove Ftirt arxJ Tintic (Noble
and Radtke, 1978). This resulted in the leaching
of dolomite as well as calcite, intense argillic
alteration, and precipitation of quartz in the
shallower part of the alteration zone. Details of
the origin of the Carlin deposit are given in a
paper by Radtlce, Rye and Dickson (in press). The
leaching of calcite at high temperatures has been