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THE McCOY, NEVADA GEOTHERMAL PROSPECT
An Interim Case History
PART I (Text)
by Arthur L. Lange
Paper deLivered at the Fiftieth ArmuaZ Meeting of the Society of ExpZoration Geophysicists~
Houston~ Texas~ 17 November 1980.
AMAX Exploration Inc. Geothermal Branch 7100 W. 44th Avenue Wheat Ridge, Colorado 80033
Author's Note
Passages in brackets [ ] and footnotes were deleted during the presentation, in order to meet the time requirements
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THE McCOY, NEVADA GEOTHERMAL PROSPECT
An Interim Case History (Abstract, revised)
by Arthur L. Lange, AMAX Exploration Inc., Geothermal Branch
The McCoy prospect is located in Lander and Churchill Counties of central Nevada, approximately 65km NW of Austin. Exploration is being conducted by AMAX Exploration, Inc. partially supported by the Department of Energy's Industry-Coupled Program. The property has been consolidated with O'Brien Resources as a Federal geothermal unit. O'Brien is also participating in the exploration program.
The geothermal area occupies the junction of the Augusta and Clan Alpine Mountains and the New Pass Range, made up of Paleozoic through Tertiary sediments, and igneous rocks. Cenozoic volcanics range in age from 36 to 16 million years. The sequence has been subjected to multiple folding and . thrusting, and transected by basin-range faulting since Late Miocene time. Mercury has been sporadically produced from the McCoy Mine near the center of the prospect. One water sample was obtained from a well at the McCoy mine (390 C, 1065 TDS). The water chemistry forecasts a minimum equilibration temperature of 1860C but with a rather high deduced cold water fraction.
Temperature surveys made in 40 gradient holes and 5 existing holes resulted in a N/S-trending thermal anomaly 20km long and 3 to 5km wide. The heatf10w pattern, based on thermal conductivities from cuttings, exhibits three principal centers measuring 15 (over the mine) to 22HFU (near the southern end of the anomaly).
A complete Bouguer gravity map produced from 340 stations reveals two gravity low zones forming an upright V, intersecting at the region of highest heat flow. These zones coincide with two aeromagnetic low trends.
A passive seismic survey conducted during 31 days over 22 stations detected 36 microearthquakes. Of those locatable, a cluster of 3 occurred at the McCoy mine, 3 more at the southern end of the property and 3 to the west. A P-wave delay correlates with the highest heatflow in the southeast, where Poisson's ratio reaches 0.35. An advance is mapped in the vicinity of the McCoy mine, where a Poisson's ratio of .15 occurs (typical of hot, dry rock or the dry steam reservoir at The Geysers, California).
An extensive self-potential survey resulted in a negative anomaly of 50 to 90 millivolts corresponding approximately to the thermal feature. Localized negatives appear over the McCoy mine and the thermal high to the south.
A magnetotelluric survey of 38 stations r~~~il~ (in one-dimensional inversions) a resistive section to 10 or more km depth in the vicinity of the McCoy mine. The southern region appears conductive above one kilometer, while at depth, an extended conductive zone ascends to 3km
i i
depth in the vicinity of the highest heat flow, SP negative peak and the P-wave, delay zone. The MT soundings are supplemented in the upper zones by an electromagnetic survey conducted by Lawrence Berkeley Laboratories.
Two deep gradient holes (to 600 and 750 meters) in the zones of highest heat flow encountered warm aquifers at about 500m, whose waters permeated the wells to their bottoms. An interpretation of the southeastern anomalies suggests that a deep reservoir (and possibly a heat source) communicates with the lOOoe aquifer of the test well via a westward-dipping ' fault zone along the west side of a horst block that prevents migration of the fluids eastward.
iii
INTRODUCTION
The McCoy geothermal prospect was discovered during an AMAX
reconnaissance program in 1977 and [first described in a preliminary
report by H. Olson at the 1979 Geothermal Resource~ Council meeting]
(Olson, et ~., 1979. Located in Lander and Churchill Counties, Nevada,
approximately 65km NW of Austin (SLIDE lL), it is essentially a "blind"
hydrothermal system having no surface discharge of hot waters. It differs
from most Great Basin prospects in its occurrence within a range rather
than the valley or its margin. Its potential wa~ recognized from elevated
temperatures measured in mineral drill holes, one water sample from a
well, and the existence of mercury mines. The property has been
consolidated with O'Brien Resources as a Federal geothermal unit.
Exploration has been partially supported by the Department of Energy's
Industry-coupled program. I would like to thank the DOE, O'Brien, and
AMAX Exploration, Inc. for the opportunity to present the results thus
far. The talk will review the analysis of a variety of geophysical
results in the context of known and inferred geology. We shall proceed by
examining a Landsat image, heat flow, gravity and magnetics, seismic
properties and electrical effects.
1
LOCATION AND DESCRIPTION
The geothermal area occupies the junction of the Augu ~ta and Clan
Alpine Mountains and the New Pass Range, which together separate Di xi e
Valley on the west from Antelope Valley on the east (SLIDES 2R, 3L).
Elevations of the prospect range from about l200m just west of the gap
called Hole in the Wall to 2l62m on McCoy Peak, [resulting in a variety of ,
vegetation from that of desert brush to juniper and pinon pine fo rest ] .
Mercury has been produced sporadically from cinnabar deposits at the
McCoy and Wild~orse mines (SLIDE 4L). Sodium bicarbonate wate~ pumped
from a 390C aquifer at 58m in a well at the McCoy mine yielded an
equilibration temperature of l86oC, [after applying an 85% deduced co ld
water mixing model to the silica geothermometer].
2
LANDSAT FEATURES
On this portion of a Landsat image (SLIDE 5R), produced by Eureka
Resources, we recognize the principal topographic features of Dixie
Valley, Antelope Valley, and the three intersecting ranges: [Clan Alpine,
New Pass and the Augusta Mountains] . . Highway 50 from Austin is visible at
New Pass. The McCoy mine lies immediately east of an evident zone of
alteration. In addition to four sets of lineations recognized by
geologists, Cenozoic volcanic centers are present. The Fish Creek caldera
lies north of the prospect, . and a clearly discernible ring feature
surrounds the McCoy mine, cautiously described as an "incipient caldera."
The 25-kilometer-diameter ring encompasses the juncture of the three
ranges and portions of Antelope and Dixie Valleys. A pervasive, evidently
deep, lineament crosses the ring in a northwestward orientation through
the McCoy mine and appears to control the main drainage of the feature,
emptying via Hole in the Wall into Dixie Valley, at a point due east of
the SunEdco geothermal discovery well.
3
GEOLOGY
During Pennsylvanian and Permian time, sediments were deposited in a
rapidly subsiding eugeosyncline (SLIDES 6L, 7R). These strata, consisting
of chert, volcanics, clastics, and limestone of the Havallah sequence are
shown in blue on the simplified geologiC map and E-W cross-section through
the "McCoy mine, as protrayed by H.D. Pilkington (1979).
The basal Triassic unit (TRc) (in green) is a detritus composed of
conglomerate, siltstone, sandstone and minor tuff resting unconformably on
the Paleozoics. This is overlain by mainly carbonate and sandstone rocks
of Triassic age (also in green).
The Tertiary rocks (in red) are primarily volcanics that accumulated
in lenses upon an erosion surface between 36 and 16 million years ago.
These are interlayered with sediments, and broken by basin-range faulting.
A Quaternary fossil travertine mound (Qt) about 10m thick and over
2km in area overlies the Tertiary rocks northwest of the McCoy mine. In
addition extensive bleaching and silicification have affected the rocks in
the central portion of the prospect.
Our detailed understanding of the McCoy geology is being considerably
expanded by Jos. Moore and Mike Adams of the University of Utah Research
Institute whose contribution I gratefully acknowledge.
4
TEMPERATURE AND HEAT FLOW
Temperature surveys made in 40 gradient holes and 5 existing holes
resulted in a N/S-trending thermal anomaly 20km long and 3 to 5km wide
(SLIDE 8L). The computed heatflow, based on thermal conductivities from
cuttings, produces an anomaly containing three principle centers measuring
between 15 (over the mine) and 23HFU (near the southern end of the
anomaly).
The plot of temperature at 100m (extrapolated where necessary)
essentially duplicates the pattern of heatflow (SLIDE 9L). Two deeper
gradient holes (750 and 600m) were drilled this year to test the
per~istence of the shallow gradients to depth. Along Line S, approx E/W
depth isotherms have been constructed by linear extrapolation from the
measured gradients, based on the assumption of conductive heatflow.
Comparison with the accompanying geological section shows that the
isotherms (disregarding the central zone of recharge) appear to assume the
form of the updomed geologic block of primarily carbonates, which, if
anything, would be expected to depress isotherms, because of their higher
thermal conductivity.
The 600m gradient well intercepted a 600 aquifer at 175m, whose
discharge affected the well to its bottom. We, therefore, cannot validate
any of the isotherms below the 600 zone.
Line A, running north-south (SLIDE lOR), passes through the McCoy
mine workings and the southern deep gradient well, where it bends
southwestward towards Edwards Creek Valley. Temperature highs occur:
a) at the McCoy mine, b) near the head of the drainage 4km north of the
mine, c) near the midpoint, and d) over the south well. [To the southwest
the isotherms drop off into alluvium of Edwards Creek Valley].
5
The deduced isotherms of Line C, through the southern well (SLIDE
llR), are inclined with the bedding on the west and even pull up on the
upthrown block at the left edge. ' To the east they plunge steeply at the
zone of faulting associated with the eastern margin of the ring. Note
that the horst blocks on the east interrupt the sequence of inclined
Triassic sediments and, evidently, the thermal anomaly as well.
In our southern drill , hole the high near-surface temperature gradient. o 0 of over 400 C/km asymptoted at 475m where an aquifer of 100 water was
intercepted in the Triassic conglomerate. As in the northern well, this
fluid permeated the column to the bottom resulting in an apparent
temperature inversion; and again the test for a continuing conductive
thermal regime is inconclusive.
A very similar isotherm profile was found by Chapman, Kilty and Mase
(1978) at Red Hill Hot Spring, Monroe, Utah (SLIDE 12L). There the hot
water ascends along a steeply dipping fault zone on the east to discharge
at the surface and thence drain westward as underflow in sediments of the
adjoining basin . In our case, the fluid may ascend from Paleozoic rocks
along the west edge of the horst barrier to discharge into ' the basal
conglomerate without breaking out onto the surface. Thence it may drain
using the avenue of the conglomerate passing the south well in its downdip
course southwestward towards the valley.
6
GRAVITY
A gravity survey of 340 stations was conducted by Microgeophysics
Corporation and AMAX (SLIDES l3L, l4R). The complete Bouguer map reveals
two gravity low zones forming an upright V, intersecting near the southern
thermal anomaly and merging with lows of the Edwards Creek Valley. Fred
Berkman has prepared two-layer depth profiles using a USGS automatic
inversion program. On Line B we can recognize the less dense volcanic
fill (2.lgm/cm3) near the center of the ring, where the denser
carbonates (2.8) plot out at depths down to one kilometer. The dense
rocks correctly plot to the surface over the upraised block at the McCoy
mine, and again dip under the volcanics and thin alluvium of Antelope
Valley to the east.
On Line C (SLIDE l5R) the volcanic cover thins as we move eastward
across the drill site and, at a point 2km east of the well, a mass of low
density material coincides with a mapped subsidence block. It appears to
be much deeper than the geologically deduced graben, but what i ts
relat ionship to the geothermal system may be, we are still not certain.
Gravity contributed to our understanding of subsurface geology at
intermediate depths, but revealed no deep heat source or reservoir.
7
/
MAGNETICS
An aeromagnetic survey was flown at 2440m (8000ft) by Geometrix
(SLIDE l6L). Like the gravity survey it yielded a V-shaped low whose
juncture falls about over the southern thermal anomaly, and extends from
there southward into Edwards Creek Valley.
As in the case of gravity, analysis of the magnetics has primarily
mapped the Paleozoic surface but has provided no apparent clues to the
existence of a geothermal reservoir or heat source.
8
PASSIVE SEISMIC SURVEYS
Microgeophysics Corporation conducted a 31-day survey, recording both
microearthquakes and te1eseisms (SLIDE 17R). ~-wave delays, which have
been used to map magma chambers under volcanos and zones of partial melt
beneath geothermal areas (Iyer, 1980), used arrival times from each of 9
distant earthquakes to produce the map representing P-wave delay
anomalies. Travel-time residuals are depicted in terms of depth to a
hypothetical boundary separating an upper slow medium from a lower one of
higher velocity. [This is merely an artifice for convenient
representation of delays; the retarding medium might actually be a deeply I
buried zone overlain by higher velocity material].
An evident zone of slow veloc~ty material (in red) underl ies the New
Pass Range, as recognized from delays of 2 events on three stations. Two
microearthquakes of around 3.Skm depth occurred over this medium. High
velocity zones (in blue) appear to underlie a block immed i ately south of
the McCoy mine, as well as a large area covered by rhyolite flows to the
west.
Microearthquakes
Microearthquakes are commonly associated with geothermal areas (Ward,
1972). Of 36 events recorded during the McCoy survey (SLIDE l8L), sixteen
fell within the boundaries of our map, including a cluster of 3 near the
McCoy mine area, 3 at the northern extremity of Edwards Creek Valley, and
3 south of Hole in the Wall.
9
Poisson's ratio, shown by the dashed line in the profile (SLIDE 19R),
is a rock property relating compressional wave velocity to that of the
shear wave. Roughly, the higher the ratio of P to S velocity, the higher
the Poisson's ratio.
Ratios in excess of 0.35 computed from local events are observed in
the valleys (green) and probably relate to saturated alluvium. A high
appears over the limestones of the Augusta Mountains to the north and also.
in the vicinity of our north well on Line B, where it accompanies a
locally high P-wave velocity (solid curve). As we know from the well, the
Au~usta Mountains limestone contains water. A ratio of less than 0.15
(red) is seen over the central volcanic fill region and" can be followed
eastward to the McCoy mine horst block, also a region of high P-velocity.
This value indicates dry, very competent material and is equivalent to
that seen over the Geysers by Gupta, et al. (1980).
Along Line C, (SLIDE 20R) Poisson's ratio (dashed) is about average
everywhere except over the horst blocks to the east, where it exceeds
0.35. Immediately west of the uplifted block P-wave delays (solid)
reaching 210 milliseconds occupy the zone of high heatflow at the well.
To the west, the P-arrivals come in early as we ascend the uplifted blocks
of McCoy Peak.
Profile A (SLIDE 21R) ties together the two thermal zones: on the
north, the high velocities but low Poisson ' s ratio over the thermal high
at the McCoy mine; and on the south, the cons iderable P-wave slowi ng in a
region where a normal Poisson's ratio gradually increases towards Edwards
Creek Va l ley.
10
SELF POTENTIAL
The principal self potential effects that one may expect to find in a
geothermal area are 1) thermoelectric, in which electric potential
differences result from temperature gradients: 2) electrokinetic, or
streaming potential, due to fluid movement through pores or cracks; and 3)
electrochemical, resulting from oxidation and reduction in groundwater
around mineralized conductors (Corwin & Hoover, 1979). Commonly, in a
geothermal area, circulating hot and cold fluids, high temperature
gradients, alteration and mineralization are all present, resulting in
compound self potential anomalies difficult to resolve.
A self potential survey was performed by Microgeophysics Corporation
along the dotted lines of the map (SLIDES 22L, 23R). Station spacing
varied between 50 and 200m, depending on voltage gradients, and 1000m
wires were used. The east-west lines were run along section lines and
tied with 4 north-south lines.
Positive anomalies are shown (on the left) by the orange to red
colors and negatives by green, blue to grey. The area of volcanic fill in
the southwest appears generally positive. As we cross the deep structure
underlying Hole in the Wall wash the potentials go increasingly negative
on the Augusta Mountain block probably due to hydraulic gradients.
The most striking SP effect is the hour-glass negative corresponding
to the thermal anomaly shown on the right, leading me to believe that the
thermeolectric effect plays a significant role at McCoy.
I have chosen to display the SP profiles along with those of
temperature (SLIDE 24R). Note the overall shapes of the two anomal ies.
Immediately west of the well, a normal fault that abu t s vo lcanics against
11
Triassic rocks bounds the thermal anomaly and results in a conspicuous
dipolar SP expression. On the east limb of the dome a series of faults
give rise to a step-wise SP profile and the dipolar feature seen. At the
McCoy mine a sharp negative of 75mv overlies the most fractured zone of
the dome, as well as a microearthquake focus, leading me to suspect that
the zone is permeable. The negative spike very well may represent a
mineralization anomaly superimposed on the much broader thermal effect.
The same anomaly appears on the north-south Line A (SLIDE 25R).
Additional SP negative peaks or dipolar anomalies occupy the zone around
the mine. Southward the SP remains rather uniformly negative until the
southern heatflow high is encountered where again a strong negative
appears. This spike is evident on Line C (SLIDE 26R) where it enhances
the broader negative over the thermal zone. The microearthquakes project
onto this region of the line-~made up of fractured blocks of
Paleozoics--where we earlier deduced that ascending hot water might
produce the steep isotherms. Note the similarity of the shapes of the two
anomalies (though one is upside-down).* I be~ieve that in general this
southern SP anomaly reflects not only the elevated temperatures measured,
but as well a conduit, dipping steeply westward to a deeper thermal
system.
*The SP response has the asymmetrical character of a positive anomaly modelled by Zablocki (1976) for a 450 westward dipping steam condGit at Kilauea. Though, normally, ascending fluids seem to generate positive electrokinetic anomalies, I am assured by R. Corwin, that in certain environments, negatives are possible. Alternatively, the fractures might also serve as a recharge zone near-surface, or alternate upward and downward circulation along the fault plane.
12
MAGNETOTELLURICS and ELECTROMAGNETICS
A tensor MT survey was run by Terraphysics during February 1980
(SLIDE 27L). [The 3 magnetic field components were detected by a
cryogenic magnetometer at base stations that included also two orthogonal
electric dipoles. One or two satellite stations, consisting of dipole
pairs, were telemetered back to the base and used as remote reference
signals for noise reduction. The remotes were seldom more than 2km
distant from the base.] Frequencies between .01 and 10hz were recorded.
Terraphysics applied the well . known Bostick 1-0. inversion, as a first
approximation. Here we view resistivity as deduced at5km depth.
The principal conductive anomaly (in red) appears on 5 or 5 stations
beneath the southern thermal zone, and is bounded on the east and
northwest blocks (blue to grey). A second conductive anomaly appears to
the west in the vicinity of Hole in the Wall.
A profile of the 10 inversion of the T mode, displays resistivity e
vs. a linear depth scale on Line B (SLIDE 28R). Here we see a conductive
zone at 3km in the vicinity of Hole in the Wall. Eastward we encounter a
very resistive zone under the thermal anomaly. The increasing thickness
of volcanics and alluvium toward Antelope Valley are detected.
Microearthquakes fall in the gaps of highly skewed zones.
Lawrence Berkeley Laboratories operated their EM-50 electromagnetic
system over port ions of Lines A and B, and they have kindly permitted me
to show some preliminary results. In thi~comparison, the resistiv i ties
computed from the two surveys tend to merge; both detected the
wedge-shaped volcanics to the east as lower resistivities. The MT highs
over the dome are confirmed by the EM at least wi thin the upper kilometer.
13
The prevalence of extreme resistivity changes is seen again along
Line A (SLIDE 29R) where the T and T modes diverged systematically. e m.
Over the central resistive block, electrical strike has rotated from
north-south to east-west, so that three-dimensional modelling is called
for. Her~ again, the EM profile confirms the resi~tive nature of the
rotated block, and reveals a veneer of conductive alteration. In the
vicinity of the south hole, the MT below lkm appears conductive, reaching
less than lli.. m at 7km. In the southernmost sounding, the EM began to
sense this deeper conductive zone.
Line C shows conformity in the short periods between T and T . e m
responses shown here in pseudosection (SLIDE 30R); however, the apparent
deep contact in the T pseudosection is "not evident" in the T , e m suggesting the presence of a conductive narrow dike-like feature. The
corresponding 10 inversion of T (SLIDE 31Rl) emphasizes the vertical e conductor and an associated deep conductor. Microearthquakes project onto
the line in the contact zone.
A resistivity log of the drill hole provides a comparison for
confirmatory evidence for the MT and EM results. Resistivities oscillate
between about 32 and 5.00J\.m in the log. MT resistivity averages 24 . .!1m,
while the EM ranges from 9 to 1500!\ m .
M. Wilt of LBL assisted me in generating several 20 models of the MT
data on Line C using a modified version of the Vozoff-Swift-Madden
program. By introducing a conductive dike along a contact and a
conductive zone on the west side at depth (approximately what we see in
the 10 inversion) we are able to reproduce the gross features of the two
pseudosections, although not the exact resistivities and dimensions.
14
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SYNTHESIS
The various geophysical surveys have yielded distributions of rock
properties resulting in what I call an "anoma1y sandwich". I shall
concentrate on the area of the known thermal anomaly shown on the heat
flow map (SLIDE 32L). The northern portion of the anomaly is an upraised
block made up of dense Triassic and Paleozoic sediments, altered and
possibly silicified. It exhibits a P-wave advance and Poisson's ratio
ranging from very high northwest of the McCoy mine to very low over the
- remainder. A self-potential negative occurs over electrically resistive
mate~ia1. Thus, the block ~eems to be hot and dry on the south and east,
but contains water to the northwest, as our dri11ho1e confirmed. We see
no underlying deep conductor in the MT sections that might correspond to a
liquid-dominated reservoir or magmatic heat source.
The southern portion of the thermal anomaly is underlain by a similar
arching of Paleozoic and Triassic rocks where two gravity and magnetic
troughs converge. The Poisson's ratio increases eastward from normal to
0.35 and a large P-wave delay underlies the southeast quadrant. The
thermal high exhibits a negative self potential and an MT-conductive
medium below 3 or 4km bounded by a resistive block on the east. High
concentrations of mercury -were reported from two wells by J. Moore of
UURI. Along the resistivity boundary, [1.5km east of the drill site,]
there appears a narrow conductive feature, aligned with a self-potential
negative peak, microearthquake foci, and plunging isotherms.
In cross-section along Line C (SLIDE 33R), my interpretation is the
following: In the MT and P-~vave delay profiles we see a deep reservoir at
3 or more kilometers, possibly heated by an intrusive and delivering hot
15
-". ---- --_ ._--- ----- - - --------------------
fluids to fault conduits bounding a horst block • . These encounter the
Triassic conglomerate where they become mixed with meteoric water en t ering
the permeable zone from the surface. The mixture drains southwestward
following the conglomerate, passing through our drill hole at a
f . 0 temperature 0 100 c.
avenues exist downdip.
The water returns to the system by whatever
Occasional seismicity insures that the path of
ascending fluid remains fractured and open.
[In summary, seismic, electrical and temperature surveys contributed
valuable insight into the nature of the geothermal anomaly at McCoy.
Gravity and magnetics helped our understanding of the ©eology. In the
future, dipole-dipole resistivity lines are planned to supplement the EM
and MT results, while additional intermediate-depth t emperature holes are
being schedvled to test the interpretations presented here.]
16
REFERENCES
CHAP~1AN, D.S., K.T. KILTY & C.W. MASE (1978). Temperatures and their dependence on groundwater flow in shallow geothermal systems. GeothermaL Resources Transactions 2 (2): 79-82.
CORWIN, R.F. & D.B. HOOVER (1979). The self potential method in geothermal exploration. Geophysics 44 (2): 226-245.
GUPTA, H.K., T.L. LIN & R.W. WARD (1980). Investigation of the seismic wave velocities at The Geysers geothermal field, California. Geothermal Resources CounciL Transactions 4: 57-60.
IYER, H.r~. (1980). Magma Chambers and geothermal energy. Paper at 50th AnnuaL Meeting of Society of ExpLoration Geophysicists~ Houston: GT-6.
OLSON, H.J., F. DELLECHAIE, H.D. PILKINGTON & A.L. LANGE (1979). The McCoy geothermal prospect: A status report of a possible new discovery in 'Churchi 11 and Lander Counti es, Nevada. Geothermal Resources Council Transactions 3: 515-518.
PILKINGTON, H.D. (1979). Exploration Inc. (Ms.).
Geology of the McCoy area, Nevada. 19p, 1 fig., 7 pl.
AMAX
WARD, PETER L. (1972). Microearthquakes: Prospecting tool and possible hazard in the development of geothermal resources. Geothermics 1: 3-12.
ZABLOCKI, CHAS. J. (1976). Mapping thermal anomalies on an active volcano by the self-potential method, Kilauea, Hawaii. u.S. GeoLogicaL Survey~ Hawaiian VoLcano Observatory~ Report: p. 1299-1309.
CONTRACTOR REPORTS
MICROGEOPHYSICS CORP. (1980). McCoy~ Nevada Gravity Survey. Report prepared for AMAX Exploration Inc. 1 January 1980.
r~ICROGEOPHYSICS CORP. (1979). McCoy~ Nevada Microearthquake Survey . Report prepared for AMAX Exploration Inc. 8 October 1979.
MICROGEOPHYSICS CORP. (1980) . Self-potential Survey~ McCoy~ Nevada. Report prepared for AMAX Exploration Inc. 15 June 1979.
TERRAPHYSICS (1980). TelLuric-MagnetoteLLuric Survey at McCoy Prospect~ ChurchilL County ~ Nevada . Report prepared for AMAX Exploration Inc. ~1a rch 1980.
17
THE McCOY, NEVADA GEOTHERMAL PROSPECT
An I nterim Case History
PART II (Figures)
by Arthur L. Lange
Paper deZivered at the Fifieth AnnuaZ Meeting of the Society of ExpZoration Geophysicists~
Houston~ Texas~ 17 November 1980.
AMAX Exploration Inc. Geothermal Branch 7100 W. 44th Avenue Wheat Ridge, Colorado 80033
FIGURES
lL. Location of the McCoy prospect.
2R. Orientation map, showing principal features of the McCoy prospect.
3L. View northward from McCoy Peak.
4L. The McCoy mercury mine • .
5R Partial Landsat image showing the ring in center surrounding the McCoy prospect. Hole in the Wall wash drains the ring and empties into Dixie Valley on the west.
6L. Simplified geologic map, showing locations of McCoy and Wildhorse mines. PP, Permo-Pennsylvanian sediments; TRJ, Triassic-Jurassic conglomerates, carbonates and sandstones; T, Tertiary volcanics; Qal, Quaternary alluvium; Qt, Quaternary hot spring travertines. (after Pilkington, 1979).
7R. East-west geologiC profile through McCoy mine (See Slide 9L), with profile of temperature @ 100m and conductive isotherms.
8L. Heatf10w map showing thermal anomaly shaded, highest heatflows stippled and lowest, striped.
9L. Map of temperature at 100m showing thermal anomaly shaded, highest temperatures stippled, and lowest, striped.
lOR. Profile of temperatures and isotherms along Line A (N/S).
1lR. Profile of temperatures and isotherms with geologic section along Line C (E/W) (Ge~logy after Pilkington, 1979).
12L. Isothermal section at Red Hill Hot Spring, Utah, from Chapman, Kilty & Mase, 1978. Compare with Line C isotherms.
l3L. Complete Bouguer gravity map. Highs are stippled; lows, striped.
14R. Gravity profile, Line B, with automatic interpretation for densities 2.1 (checked) and 2.8gm/cm3 (striped) .
. 15R. Gravity profile, Line C, with automatic interpretation for densities 2.1 (checked) and 2.8gm/cm3 (striped).
16L. Residual aeromagnetic map. Highs are stippled; lows, striped.
17R. Map of P-wave delays from teleseisms. Largest delays are striped; advances, stippled.
18L. Map of Poisson's Ratios (highs, striped; lows, stippled) showing also locations of microearthquake foci and fault-plane solutions.
19R. Profiles of P-wave delays and advances and Poisson's ratios along Line B.
18
20R. Profiles of P-wave delays and advances and Poisson's ratios along Line C.
21R. Profiles of P-wave delays and advances and Poisson's ratios along Line A.
22L. Map of self-potential response. Negatives are striped; highs stippled.
23R. Refer to Figure 9L.
24R. Self-potential profile, Line B, compared with profile of temperature at 100m and isother~s. Microearthquake epicenters shown a~ stars.
2SR. Self-potential profile, Line A, compared with profile of temperature at 100m and isotherms. Microearthquake epicenters shown as stars.
26R. Self-potential profile, Line C, compared with profile of temperature at 100m and isotherms. Microearthquake epicenters shown as stars.
27L. Resistivity at Skm depth from 10 magnetotelluric i~version (Te mode). Conductive zones stippled; resistive striped.
28R. Magnetotelluric section (Te mode, 10 inversion) along Line B compared with available EM section and geology.
29R. MT section (Te mode, 10 inversi on) al ong Line A compared with available EM data.
30R. MT pseudosections (resistivity vs. period) along Line C.
31R. MT section (Te mode, 10 inversion) along Line C, compared with geologic section.
32L. Refer to Figure 8L~
33R. GeologiC section along Line C, showing deduced geothermal reservoir feeding conduit of ascending hot water along limb of horst block. Upon encountering the Triassic conglomerate, hot water (probably cooled by cold meteoric water from the surface) drains westward-downdip--to eventually return t o the deep system.
Plate I.
Plate II.
Plate III .
PLATES .. Stacked, profiles of Line A.
Stacked profiles of Line B.
Stacked profiles of Line C.
19
( . " . " ' ~'
( -';;...' ' · ' i
NEVADA
~' McCOY
N
e DIXIE
VALLEY
Location of the McCoy prospect lL.
-AUSTIN
.1
-TONOPAH
eELKO
r ,
( ,
MAP ( o 2 4
km
.J L
2R. Orientation map, showing principal features of the McCoy prospect
......I M
.,.. E
~ :::I U ~ Q) E >, o u u
:E:
Q) .c:: I-
5R . Partial Landsat image showing the ring in center surrounding the McCoy prospect . Hole in the Wall wash drains the ring and empties into Dixie
I
Valle_yon the west
6L Simplified geologic map, showing locations of McCoy and Wildhorse mines . PP, PermoPennsylvanian sediments; TRJ 9 TriassicJurassic conglomerates, carbonates and sandstones; T, Tertiary volcanics; Qal, Quaternary alluvium; Qt, Quaternary hot spring travertines. (after Pilkington 9 1979)
( -- > . .. ...
I,e 50
. 'e
SL
8-8'-8 11
TEMP AT 100m AND
EXTRAPOLATED ISOTHERMS
e ....
(::~. 7R. East-west geologic profile through McCoy mine (See Sl i de 9L ) , with profile of temperature @ 100m and conductive isotherms
r
(
e'"
'--
b ~ 0 ,.... , ..... .....
I
390 55' +
390 45' +
I I , \ \ ,
"
SL. Heatflow map showing thermal anomaly shaded, highest heatflows stippled and lowest. stri ped
b (")
0 ,.... ..... ..... I
+
HEAT FLOW (HFU) • WELLS
o 2 4 , , , km
-;
.J
', .
.. ' .'. , ,
;.
"
,
~ ~ 0 )} ~. ,:~~~
0 ~ .... ..,0
,--
39° 5 5 ' + 4~/jJJ:J5
.........
. 18.'6
TEMP. AT 100m {t>, • WELLS
o 2 4 I I I
lrrn ~
-,
:3 ::1 :=j ~ :=1 -:::; " o·
'"
, '1.o,~
I I \ \
"-"'-"
6. //
I I \ \
39° 45' + 20:5~\ 19:;/1\
9L. Map of temperature at 100m showi ng therrila 1 anomaly shaded, highest temperatures stippled, and lowest, striped
/ ~
0 ~ ('t) .' .. :j . 0 ~ ,--,--
•
~~5B"
+
('
·c 601-
501-
40~
30~
201- __ -A"1"1"'~
~
~\) 1\
J )
)
f"'('
~ i
~ ffil'r,l'h. .J ,;,(
I. ~
~
~ /
".,(/
I
'\
j
1800~ A" -l---- 40 NORTH
.%\\ ... .. ... . ~ ~lt".;,(J'/:S)· .... ... ,; .•.....
~ij~)" 13i
--15
TEMPERATURE AT 100m AND EXTRAPOLATED ISOTHERMS
lOR. Profile of temperatures and isotherms along Line A (N/S)
\
3
TEMP AT 100m EXTRAPOLATED
ISOTHERMS
e -:z: .... a... w Q
z o
2 SL~ > w
3 Ld
11R. Profile of temperatures and isotherms with geologic section along Line C (E/W) (Geology after Pilkington, 1979)
---\
P
NW
400 o
300
~ .P
DISTANCE (m)
200 100
RED HILL SPRING -v()
o
- 20 500
E --- 40 I t-o... 60 w o
80
60°
57.1~ RH3
VERTICAL EXAGGERATION 2: I
72.1° RH2
70°
72 .~ 100-1 RH 4
12L. Isothermal section at Red Hil) Hot Spring, Utah, from Chapman, Kilty & Mase, 1978. Compare with Line C isotherms
~
SE
100
o t-... ....
•
MCCOY COMPLETE BOUGUER GRAVITY
(MILL/GALS)
o 2 4 I I
km
..-c'
13L. Compl~te Bouguer gravity map. Highs are . stippled; lows·,stri ped .... : .. .. .. ~: .. :: " ;'; . , .... ,.~ -<.t .. . . :. ~ ',:-' : '
.... " .' :-. -
~" o .. " .... ~
01 II 10
... -' -= ... :::; ~ -10
8
.... -' -= ...
-10 :3 iE
8"
- , ,ul ... 1 .2
Ii :!! :z: .... .... ... c::a
B-B!..B" , at 8' RESIDUAL GRAVITY I n_ L X i'\ ~ ~ _ i __
PROFILE I
(COMPLETE BOUGUER) SL~ AND DEPTH ANALYSIS
3
14R. Gra vity profile, Line B, with automatic interpretation for densities 2.1 (checked) and 2.8qm/cm3 (striped)
IBOOm
E :!
2 SL Q ... c(
3 > W ..J W
3
r ~. ~
"
01- 10
.,. -' ..., ... ::: · 10 E
t
C-C' RESIDUAL GRAVITY
PROFILE (COMPLETE BOUGUER) AND DEPTH ANALYSIS
(' T ~ "'U~ 1800m .~ • .' •.•• i», 01 nl'. ·n3...t..~-':. 'IIiIH.61 . • E
.>t
z o
-SL~ 2 c(
> W ..J 3 w
15R. Gravity profile, °Line C, w~th automati~ interpretation for densities 2. 1 (checked) and 2.8gm/cm (striped)
.,. ..... ..., -10 ~ .....
:i
1J800!!! ~' -, ~",
~~ ~ I -
111'''''1: ~
16L. L
Residual aeromagnetic map. Highs are stippled; lows, striped
;
. c' o ..... ------
"lliJ,~,~, RESIDUAL MAGNETIC
INTENSITY
(gammas]
1 1 A km N
o I
,.r--.., ~:,
P-WAVE DELAY , DEPTH TO EQUIVALENT HIGH
VELOCITY SURFACE (KMS) .. • SEISMOGRAPH STATION.
9 ~ 1 M
~ ~
'41:'$i;
1lllii) 390 55' +
.......... , ................ ~
................
" ~8 ~
~~
~
~ .-..'-"--.-...-...-..'-"
17R.
3 9° 4 5 ' + b ....
Map of P-wave delays from teleseisms. Largest delays are striped ; advances, stippled
o ,... .... ....
C---
/ /
/ / ~
.. < , , I ,
+' , y ,
t o CO)
o ,...-.... .....
'""
f".
; OISSON'S RATIO AND
MICROEARTHQUAKES
..,. SEISMOGRAPH STATION
39° 55' + 0 6.9 EPICENTER AND DEPTH (km)
, FAULT PLANE SOLUTIONS
18L.
~ 024 I I I
km
..,.
3 9° 4 5'+ o ~
Map of Poisson's Ratios (highs, striped; lows, stippled) showing also locations of microearthquake foci and fault-plane solutions
o ..... ... ...
~ ;,t\!.
t::.
0 S.1
..,. ..- ........-..-"- .'2.0
c-~!v
~ ..
.. .. 4(
1
(!)J • .1
tJAw~-1
+1 1
1 1
..,.' I ~k\.....-!-..-..-..-~~Y-
..,. II
II
II -j-o (")
o ..... ... ...
/")
B"
..-..-..-c'
OJ.,
OJ. S
' ( .
/"""'.
~ " 5t Q z
~ -lot .... ~ ... u z -c - I-
!;.l 'D5~
B~ WE
~ t z ~ .05 ... ~ ~ - ~ -c ..... ... Q .LLW,Y
. lo~
8-8'-8": POISSON'S RATIO '
AND P- WA VE DELA V
PROFILE SL
"")-. .J
A1r1l\ J
... I'
I '
''';t-l A III liB J~
.~~ 4~' _,~ 11 I I I I I I II-
- l \
\ l \ # \ , \ , \ l ~ w
--t .45
r IfJJ1
.35
T,,~ : • a
Z EASTI co
."
I'" ." C; ....
' -1.20
-f. 15
8' " Qt 'X' . ' , 1800m
Oofi 1Y.~~'Q"~i'D\~'>"~~ ' ,:;C; ~-r . ..
3
-. E ~ -
2 SL ~ t= c(
3 > III ..J III
19R. Profiles of P-wave delays and advances and Poisson's Ratios along Line B
-:,
( ...
• en co :z
;15 --
~ ;10~ w ~ w
"" :z - ,/
:: f-~ -.05
. en co :z co "" w ~ ,... c ..... .... co
f.05
~
.10 I-
I-
~ .15
l-
I:-
.20 I-
/~ /
C-C' POISSON'S RATIO
AND P-WAVE DELAY
PROFILE
\
, . \
\
v
v
20R. Profiles of P-wave delays and advances and Poisson's Ratios along Line C
.45
.20
.00
, ."
"" :z co u ... ." ... .... :z c :-~-
+ ." CI :z co u ... ~ -c .... ... CI
pt. ~ i I}
"\
/ I\~~ _ N~
15 ~ - .45
r- 1\ r- 1\ _ .40
101- r'\ P.R •
- .... ----... ...... " ."'--, ,.-- - 35 ---------------------=-~---__ . ' ~ . 6 .
05 - -----. l "~~I - * !-~-"." _. ' ---------------------------i~-:." "\ .' - - _ I T f - .30 --------------------------~r--~~_.I • -. .
1----------------------------1 -.... I , "' { , fA" / ~ ~ ~ ,_,' A
SOUTH • NORTH • - . •
- I l - .20 . 05 •.
f- I / .
I- I __ _."; .. ,, - .15
1/ ------.101- .' \ J 1/ HIGHLY SILICIFIED OR DRY - .10 -- .\
\ J 1/ . -- , V
.15 r- \'\ I V POISSON'S RATIO AND - .05
, J V " ~IY P-W A VE DELA V PROFILE
'\ :1 i-' ......... ...~ ..... A-A'-A"
21R . Profiles of P-wave delays and advances and Poi sson 's Rat ios a l~ng Li ne A
co ~
, c ... ~ :z co ."
~ co ....
r.--~ I
.......• G ... ,
. .. SELF POTENTIAL
(millivoltsl
• .1 .
.,.
................ SURVEY GRID
024 I I I
I[Ill ~ 39 0 45' +
1170 40'
:\ ......... . d>
.... ... .'
c.j~~ :
22L. Map of self-potential response. Negatives are striped; highs stippled
+ 1170 30'
~
g" .. ~"~-n t
rt 1.3
........ ~
c'
.......
~. '
.--.....
~
o '.;t o I"-yo-yo-
..-J>
39° 55' +
.'8.6
TEMP. AT 100m ee). • WELLS
024 I I I
Inn ~ 39° 45' +
23R. Refer to Figure 9L
") . ').o.()
.'
:3 =:1 =:j -=:1 :=1 -:::; ">
0' f\-
n J;
JJ~U1P5 .........
I I \
/).
\ "-
/ I \ \
"'-""
/" /'
20.5~ \/W~. / ~
~
o (f)
o I"-yo-
yo-
+
)
•
~5B"
0 i\
".e 50
40
30
20
~ , ~ ~~J'
1 irf f"!!"l-r-
8 -r I I I I I " ,
•• ~~'III r"r B
U
40--1- EAST WEST
...... .... c::o ~ :; ~ :Ii
'.e
20
o
8 I 'I ~ " , I L.IIw WEST ttl IV t i i ;",.,r,
8-B'-8" TEMP AT 1 OOm'
AND SELF POTENTIAL
PROFILES
24R. Self-potential profile, Line B, compared with profile of temperature at 100m and isotherms. Microearthquake epicenters shown as stars
·· ':!i:';{ ;f.~~~ • .... ~
• e: .... ..... c::o
8" EAST
-20
-40 ~
-60
-80
.... c:> ... :; .... :E
,-------.. :p
...
r 601-
50t-
40t-
30t- r-
201-,..... ......... ~
A-A'-A" TEMP AT 100m
AND SELF POTENTIAL
PROFILES
I11It\
~
.~ J
f\ I
I .,J ~,~ 1\
I /
~/
A' ..("),.
sotfjff
I \ \l;~ 25R.
. ..... '::'::::::::: .. ::.::: : .. :.::.::,., .. : ':., ":'<: :S;:( : '~3;:;{:'}!' y .:) '. ~ ~ 3 :z:
to.... ... Cl
...... 5
~ ~
~ \l
\
\
't \ \1 ~ ~
~ ~~!)~ \I /I~ I
Serf-potential profile, Line A, compared with profile of t emperature at 100m and isotherms. Microearthquake epicenters shown as stars
~
A" NORTH
-1 ·20
-
- HO
\ .,.
-I ~
'('" C> -::. --'
-1 ,60 E
-
-;'80
-
Of-
WEST
C-C' TEMP AT 100m
AND SELF POTENTIAL·
PROFILES
\ \ \ \
\
- 5
.......
26R. Self-potential profil e , Line C, compared wi th profile of temperature at 100m and isotherms. Microearthquake epicenters shown as stars
C E AST
- ·20
-
- ·40
-iI: ;::: .-
- -60 == C> .-... en
-- ·80
-
. : .. '
;/ \
~ ~
..
"
:.:". \( .. " (
. '. '.' ! -'" ' f
,', '.' . . ~:~ 4.;
•. \ >.< .'i', '~'.~ . ,
re' ~I \,
•
MAGNETOTELLURIC
RESISTIVITIES <{).m) AT 5km DEPTH
( 1 D T e Mode Inversion)
• STATIONS
9 ~ t AA
,
27L. Resistivity at 5km depth from 1D magnetotelluric inversion (T mOde). Conductive zones stippled; resistive striped ~
'1
~~I 0"
•
. , .'
1-D I
. 1 I
!. i
., ... .. .
" .\. ,
' . ::". :~'II ' I "';
: , . , .
< .! . ..• ,
" .
~ ~ ~ ~ ,
WEST 8-8'-8" I
MAGNETOTELLURIC 1-D INVERSION WITH .
~ .
EM 10 dill :x: t.....
2 ~
8"
8 at 8' ,<= O~'+ .1l:~~'\~~nTo'¥)Xd~: ,t, ,,"=,, IBOOm
EM AND GEOLOGIC PROFILES E ::!
28R.
SL
:3
Magnetotelluric section (T mode, lD inversion) along Line B compared with available EMesection and geology
2 SL Z Q I<I:
3 > W ...J W
EI .... c
:x: t..... ... co
~ ' . "
~ "It' ~iyl
MAGNETOTELLURIC 1-D INVERSION WITH EM PROFILE
A-A'-A"
EM no dill
) ;:.::
800m
e ~
:c ID... IoU 2 g
11"
~
Ai A6 1/"
"HI no ~ '" u IVI U r M , D , D I &t M I q A 1 4 /\. H 1 x A 1 A 13M 1 3 B 1 3 1800 m _Iii - I 'I -f-, "",Ln .. _. I I I · I I I 1_ I
1D I
SOUTH
4 a
I ·· ··· i(:~til~W~AII III) L?J:/\ \1.4JJ~"AJF~ III I \ \ '+~ I ~ ; ? ~
D...
29R. MT section :(Te mode, 10 inversion) along Line A compared with available EM aata
IoU
6 g
8
10 NORTH
i'. ' :"
Te
Tm
-0 ..., ::zI
C>
= en "" n C> :z: CI
~
-0 ..., ::zI
C> CI
cn
"" on C> :z: CI
~
C-C' MAGNETOTELLURIC PSEUDOSECTION
30R. MT pseudosections (resistivity vs. · period) along Line C . . ._ .. • . : .. :.; .• ... ••.. .•..• ...... . . ' : , ' . ! .. . , '" ~ _ . . ' ~ , .... " 1 _ . .:v', .).~.. ' .. :;'~;.' . .
_ ~ ~ . . ' t o' .• •• '~.' __ .. "_'-: .• :~ .•. ~_':'-: . . " .,. .•.• :._ ..• ~ •..• _ •• : ...•.• ~ . ~ f , : . ,',: . .: . ... .. . .. .... . .. .. ' ., ,' ;-- "" ".- " '. .. . .. ...-.... i •. ' :: . . " : ' '.,,: ~: ."' : : " _". '. ', ~ .~ ' ... " -.-~
C C' A9 M9 89 1800rn
2
4
6
8
10
SL 2
C-C' MAGNETOTELLURIC 1-0 -INVERSION WITH
~ GEOLOGIC PROFILE 31R. MT'sec~ion (Te mode, 10 invers;on) along Line~, compared with .
. . .t .; ,. " .~ . _ geologlc sectlon . . ; . . . . ; . ~ . ' ~ . ' : . .... : ... ..'-'.". ~ .::~ .. ' .-. ;' :: ' ;4 ... ,"_: ~ ~ '. ~. . '
. .~ . ' .- . , " . '
r
( ,
L
, 0 ~ 0 "," , .... .... I
390 55' +
390 45' +
I I , \
' \ , , "
32L. Refer to Figure ' 8L
. -;:',
,
0 C")
0
"'" .... .... I
+
+
HEAT FLOW (HFU) • WELLS
-;
o , 4 .. ' 2 4 , , I
km . .:"" . .. ... ~'"..' .. \
, "
.J
0j ~ }t )
PEAK (-)
c C' "Ho 1800m
Res I -If-"Ho ' ~
SL---=I "Af 2
3 lPu
CONDUCTIVE ZONE DEPTH UNCERTAIN
33R. Geologic section along Line C, showing deduced geothermal reservoir feeding conduit of ascending hot water al ong limb of horst block . . Upon encountering the Triassic conglomerate, hot water (probably cooled by cold meteoric water from the surface) drains westward--downdip-to eventually return to the deep system.
z o
2 SL~ > w
3 ~