-- ------------------ -

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

- -- ---- --------- - - - ----------

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

- ----.. - -- ------------------------

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 ~