JOURNAL OF GEOPHYSICAL
RESEARCH,
VOL. 96, NO. B12, PAGES 19,733-19,748, NOVEMBER
10, 1991
Fluid Inclusions
andHydrothermal
Alterationon the Dixie Valley Fault, Nevada
W.T. PARRY, D. HœDDERLY-SMITH,
AND R. L. BRUHN
Departmentof GeologyandGeophysics,
Universityof Utah,SaltLakeCity
Footwallrocksof the 1954 rupturesegmentof the Dixie Valley fault showextensivehydrothermal
alterationrelatedto fluids that were presenton the fault duringtectonicevents. Hydrothermalalterationof
granifichostrocksconsistsof temporallyand spafiallyoverlappingmineralassemblages.
An early,biotite-
feldspar
assemblage
isfollowed
bylaterFe-chlorite
andepidote.
Bothchlorite
andepidote
arereplaced
by
hydrothermal
sericiteandcross-cut
by calcite-hematite
andquartz-calcite
veins. Biotiteis partiallyreplacedby
prehnite. The latest hydrothermalminerals are stilbite, laumontite,kaolinitc, alunite, smectite,illitc, and
pervasive
replacement
of rockunitswithfinegrainedquartz,chalcedony,
andopal. Secondary
fluid inclusions
trappedin healedmicrofractures
in igneousquartzincludetypeI inclusionsthat containa moderatesalinity
aqueousliquidandvapor,type1I inclusionsthatcontaina moderatesalinityaqueous
liquid andCO2; typeI11
inclusionsthat showcutecftcmeltingtemperatures
b•low the NaC1-H20 eutecticand containsubstantial
CaC12,
andtypeIV inclusions
contaiv2ng
halite
andother
daughter
minerals.
Microthermometfic
measurements
on theseinclusionsyield variablecompositions
and tiomogenization
temperatures.Salinifiesof type I
inclusions
varyfrom 0.1 to 12.9 wt % NaC1with themodein theinterval0 to 1%. Salinifiesof type11CO2
beatinginclusionsrangefrom 0.62 to 6.81 wt % NaC1relativeto H20, and salinitiesof type Ill inclusions
with low cuteericmeltingtemperatures
are 12.9 to 25.3 NaC1equivalentwt %. Salinitiesof halite-beating
inclusionsare 30.1 to 39.2 wt % NaC1. IIomogenizationtemperatures
spanthe range 120ø to 400øC. The
processes
of isochemicalcoolingwith upwarddisplacement
of the footwall, mixing of cool low-salinitywater
with hottercomponents,
andmixing of cool,evaporitebrine with hottercomponents
couldbe responsible
for
variablefluid inclusioncompositions,
homogenization
temperatures,
and densities.The P-T path of the fault
fluids is establishedby mineralequilibriaand fluid inclusioncharacteristics.The path includesa lithostafic
fluid pressure
at 305øCand 1570bars.Alongwith coolingandescapeof CO2 fromfluids,the fluid P-T path
probablyapproaches
hydrostaticpressureconditionsat lower temperatures.Hydrothermalalterationproduct
minerals,fluid temperatures,pressures,and compositionsin the footwall of the Dixie Valley fault constrain
minimumfault age to 20 to 25 Ma, displacement
to 6 km with about3 km of pre-10 to 13 Ma and 3 km of
post-10Ma uplift. Fluid compositionsand P-T data suggestthe following mechanismfor raptureinitiation
and arrest. Rupturesmay be initiated as a result of high fluid pressures,then openingof dilatant fractures
causesdrasticdecreasein fluid pressure,separationof steamand CO2. The drasticreductionin fluid bulk
modulusthat accompaniesvolatile phase separationpermitspropagationof the raptureseven though fluid
pressure
is reduced.h• areaswherefluid pressurereductionis not accompanied
by phaseseparation,
fracturesare
arrestedby dilatanthardening.
INTRODUCTION
Evidence of hydrothermal fluid circulation and chemical
interaction of fluids with fault zone rocks is evident in fault zones
Hydrothermalfluidsplay a significantrole in the mechanical
exhumedby erosion [Sibson,1981; Sibsonet al., 1988; Parry et
stabilityof faults. Increasedfluid pressure
reducesthe frictional
al., 1988] Microscopic to megascopicfractures filled with
shearstrength
of an existingfaultwherefailureis governed
by precipitatedminerals are common, and fluid inclusionstrapped
effectivestress[HubbertandRubey,1959].Alterationof thefault
within these minerals represent samples of the pore fluids.
rocktomineralassemblages
suchasmuscovite
withlowerfrictional
Characteristics of fluid inclusions together with associated
or flow strengthdecreases
stability[Janeckeand Evans, 1988;
hydrothermalalterationproductmineralsmay be usedto infer the
KirbyandKronenberg,
1987]. Increased
porevolumeproduced
physical state of the fault zone at depth, estimate constitutive
duringdynamic
rupturingwithaccompanying
reduction
in fluid
propertiesof fault zonematerialssuchas fluid bulk moduluswhich
controlfault stability[Parry and Bruhn,1990], constrainthe fluid
RoeloftsandRudnicki,1985], andhealingandsealingof cracks pressure[Parry and Bruhn, 1986; Parry et al., 1988; Parry and
pressure[Ismail and Murrell, 1976;Rudnickiand Chen, 1988;
withprecipitated
minerals
increase
stability.
Faultstabilityis also Bruhn, 1990], andestimatefault displacement
usingpressureand
relatedto the temperatureandpressureof the fault zoneand the temperatureconstraints[Parry andBruhn, 1987].
constitutive
properties
of faultzonerockandfluids. New pore
The Dixie Valley fault systemis part of a 300-km-longactive
volumeproduced
by dilatancy
coupled
withlowrockpermeabilityseismic belt in central Nevada [Wallace, 1984] with individual
canresultin decompression
of thefluidandmaystabilize
rupture.
segmen
,t,
5 capable
of producing
magnitude
7 earthquakes.
Exhumed
Separation
of thefluid intogasandliquidphases
resultsin drastic footwall rocks on segmentsof this fault display the effects of
reduction in fluid bulk modulus that diminishes the dilatant
hydrothermalalterationandprovidean opportunityfor observation
hardening
effectandmaypermitrupture
propagation
[Rudnicki
and of fluid characteristicsin fluid inclusionsand alterationproduct
Chen,1988;Parry andBruhn, 1990].
Copyright
1991bytheAmerican
Geophysical
Union.
Papernumber91JB01965.
0148-0227/91/91JB-01965505.00
mineralogynearseismogenic
depthson the fault.
The objectives of this study are to characterize and map
hydrothermalalterationassemblages
on the footwall of the 1954
rupturesegmentof the Dixie Valley fault Nevada,to determinethe
characteristicsof fault pore fluids using fluid inclusions and
19,733
19,734
PARRYETAL.: DIXIE VALLEYFAULT,NEVADA
alterationmineralogy, and to relate thesecharacteristicsto fault
behaviorandhistory.We havechosenthe 1954rupturesegmentfor
studybecausethe footwall exposesTertiary agegraniticrocksin
which alumina-silicatemineralsprovide an excellentrecordof
chemicalinteractionof fluidsandbecausethe ageof the alteration
mustpostdatetheemplacement
of theigneoushostrock.
THE DIXIE VALLEY FAULT
DixieValleyis a grabensystem
consisting
of aninnergraben
withvalleyfill 2 to 3.2 km deepanda shallower
outergraben
containing
150to 1500m of fill [Meister,1967;Thompson
and
Burke,1973;OkayaandThompson,
1985;Anderson
et al., 1983].
Faultdisplacement
hastakenplacealonga normalfaultatthebaseof
the Stillwaterrange(therangefrontfault) andalonga zoneof
normalfaultsa fewlorneastof therangefront(thepiedmont
fault
zone)[BellandKatzer,1987,1990].Smallscarps
of thePiedmont
fault
zone
shown
onFigure
1mark
theinnter
graben.
The Dixie Valley faultin thewestemBasinandRangeprovince
Thetopographic
lowin DixieValleyis dominated
by a large
of Nevadalies in an areaof highheatflow, lateCenozoicvolcanic
playaandtheHumboldtsaltmarshoccupies
46 square
miles(119
activity,andrecentseismicactivity. Historicseismicity
hasresulted
km2)ofDixieValley
atanelevation
of3365feet(1026m)
[Bateman
in surfacerupturingevents(fromnorthto south)at Pleasant
Valley and Hess, 1978]. The brine is 28.7 to 38.7 wt % salt.
(1915), FairviewPeak-DixieValley (1954), Wonder(1903), Cedar
Mountains(1932), ExcelsiorMountain (1934), MammothLakes
EXPERIMENTAL PROCEDURES
(1980), andOwensValley (1972) [WallaceandWhitney,1984].
The Dixie Valley fault systemformstheeastern
marginof the
Our studiesfocuson hydrothermal
alterationandfluid inclusion
Stillwater Mountainsshown in Figure 1, and close spatial characteristics
in footwallgraniticandvolcanicrocksof the 1954
correlationbetweenthe 1954 andolder fault scarpsindicatesthat rupture
segment
of theDixieValley rangefrontfault.Samples
were
repeatedsurface-rupturing
earthquakes
havecreatedthe structural collectedfrom outcropof the intrusiveandvolcanicrocksin the
reliefbetweentheStillwatermountains
andDixie Valley. Footwall footwallof theDixieValleyfaultatlocalities
shownin Figures
2a,
rocksexposedin the Stillwaterrangeshownin Figure 1 include 2b,and2c.Hydrothermal
alteration
mineralassemblages,
tectonic
Mesozoicsedimentary
rocksintrudedby the Jurassicgabbroic textures, and fluid inclusions were studied in 183 thin sectionsof
Humboldt lopolith [Willden and Speed, 1974] a multiphase rockscollected
duringthecourseof mappingfootwallalteration.
Oligocene
granodiorite-quartz
monzonite-granite
intrusion
emplaced Alteration
mineralogy
andtectonic
textures
weredetermined
by a
at 28 Ma [Speedand Armstrong,1971], and a smallCretaceous combination
of petragraphic
microscope
andX ray diffraction
graniticintrusion.Theserocksare overlainby Tertiaryvolcanic techniques.
Clayminerals
wereidentified
by X raydiffraction
of
rocks.
oriented
smears
of claysizematerialfollowing
vaporglycolarian
at
40 ø
o
IO
20
I..... K•
•
Dixie Comstock Mine
• Stillwater
Range
DIXIE MEADOWS
HOT
SPRINGS
c
%
Tertiary
granific
rocks
Tertiaryvolcanie
rocks
Jurassle
garoieroeks
Jurassie/Triassie
Sedimentaryrocks
Fig.1.Index
mapofNevada
showing
theStillwater
Mountain
Range
andgeneralized
geologic
mapofaportion
oftheStillwater
Mountain
range
modified
fromPage[1965]Therange
frontfaultis shown
asa heavy
line.A, B,andC aresections
oftheDixie
Valleyfaultandfootwallshownin detailin Figure2.
PARRYET AL.: DIXm VALLEY FAULT, NEVADA
60oc and heating to 250ø and 550øC. The relative ages of
alteration assemblagesis established from crosscutting and
replacement
relationships.
Identity, density, and homogenizationtemperaturesof fluid
inclusioncontentsweredeterminedby observation
of phasechanges
in doublypolishedplateson a Fluid Incorporatedheating-freezing
microscope
stageandprocedures
outlinedby Roedder[1984],Parry
and Bruhn [1986], Parry [1986] and Parry et al. [1988].
19,735
were not measured; second, inclusions near one another with
apparentlysimilarliquid-vaporratioswerecheckedto insuresimilar
homogenization
temperatures.
FAULT ROCK PETROGRAPHY AND ALTERATION
Four separateplutonsin the intrusivecomplexshownin Figures
1 and2 wererecognizedin thefootwallof the 1954rupturesegment
Temperature
calibration
forthemicroscope
stage
wasaccomplished
duringourfieldinvestigations,
butthecontacts
between
these
usingsynthetic
fluidinclusions
ofknown
composition
[Sterner
and plutons
werenotmapped.
Theyare,fromsouthto north,granite
Bodnar,
1984].Phase
changes
thatwereused
tocharacterize
fluid (40% plagioclase,
33% K-feldspar,
25%quartz,
2% biotite)
inclusion
fluidsweretheCO2triplepointtemperature;
melting
of outcropping
in thevicinityof A in Figure1; granodiorite
(62%
clathrate
[Collins,1979;Bozzoet al., 1975];meltingof ice, plagioclase,
12%K-feldspar,
19%quartz,
7%biotite)
outcropping
hydrohalite,
andhalite;CO2liquid-vapor
homogenization;
and in thevicinityof B in Figure1;biotitequartz
monzonite
(26%
overall
homogenization
offluidinclusion
contents.
plagioclase,
34%K-feldspar,
27%biotite,8%hornblende,
9%
Fluidinclusion
salinities
wereestimated
fromicemelting quartz)
outcropping
in thevicinityof C in Figure1; anda small
temperatures,
fromclathrate
melting
temperatures,
andfromhalite granite
togranodiorite
stock
nearAlameda
Canyon
(DonFigure
1)
melting
temperatures
asappropriate.
Salinities
offluids
ininclusions
thatconsists
of 33%quartz,
43%plagioclase,
17%K-feldspar,
and
withnodetectable
CO2werecalculated
usingfreezing
point 7%biotite.
TheK-Arageofbiotite
fromthebiotite
quartzmonzonite
depression
estimated
fromice meltingtemperatures
andthe at locality108,Figure2c, is 28+2Ma (Table1) [Speedand
regression
equation
of Potteret al. [1978]. Salinities
for fluid Armstrong,
1971].Thegranite
isyounger
thanthegranodiorite,
but
inclusions
onthehaliteliquidus
werecalculated
usingtheequationtherelative
ageof thegranodiorite
withrespect
tothebiotitequartz
andcoefficients
ofSterner
etal.[1988].Salinity
offluidinclusions
monzonite
isnotknown
duetothelackofoutcrop.
TheK-Arageof
showing
eutectic
temperatures
belowtheNaC1-H20
eutectic
were muscovite
fromtheAlameda
Canyon
plutonat D onFigure1 is
estimated
usingtheequations
andcoefficients
fortotalsalinity
of 78.4+2.9Ma (Table1). Additional
smaller
intrusive
rocksarealso
Oakeset al. [1990].
presentat maplocalitiesshownin Figure2 andincludean altered
Thegreatest
source
of errorin fluidinclusion
measurements
is a quartzlatiteporphyry
dikeat37, a dioritedikeat 17,a quartzdiorite
consequence
of postentrapment
changes
in secondary
inclusionsdikeat 65 anda dioritedikeat 97. Thesesmallerbodiesaretoo
described
asneckedinclusions
by Roedder[1984]andtermed smalltomapatthescale
of Figure2.
maturation
by Bodnaret al. [1985]. Thesechanges
resultfrom
Faultbrecciaandcataclasitedeveloped
within the granitic
dissolution
andreprecipitation
of hostmineralsurrounding
the protolithdirectlyoverliethefootwallgraniticrocks.A pervasive
set
trappedfluid. The initialfluid inclusion
maybe irregularlyshaped of closelyspaced
extension
fractures
is concentrated
in thefootwall
but in timenecksdownto formmanysmallerinclusions
with more immediatelyadjacentto the fault. Spacingbetweenthesesteeply
regularshapes.
If neckingtakesplaceafterseparation
of a vapor eastdippingextension
fracturesis only a few centimeters
andthese
phase, then the result is varying liquid to vapor ratios. All fracturesarepresentthroughout
thebrecciaandwithinthefootwall
secondaryfluid inclusionshave undergonenecking,but generally, for several meters to tens of meters. Thin discontinuousshear
theneckingtakesplacebeforeseparation
of a vaporphaseor other planessubparallelto the main fault plane also extend into the
daughter
phases[Roedder,1984]. We haveavoidedmeasurementsfootwallandcoexistwiththepenetrative
fractureset.
of fluid inclusionswith liquid-vaporratios affectedby necking
Cataclasite
is thedominantfaultrock. Cataclastic
texturesinclude
usingthe followingprocedures:first, fluid inclusionsin close pervasivefracturingof largevolumesof rockandveins. Cataclastic
proximityto one anotherwith widelyvaryingliquid to vaporratios veins are thin tabular and branchingbodies of fine cataclasite
TABLE 1. Potassium-Argon
Age Data
Map Sample
Description
Dated
Mineral K, wt% 40Ar*/40Ar
Total,%
No
108 IXL Pluton fineto coarseplag
K-feldspar,quartz
biotite
6.91
69
40Ar*,
Age,
x10-10mo!/g
Ma_-h3
3.40
28+2.0
30041'27"N
118ø08'02"W
D
DV87-59
very coarsemuscovite
+quartzveinin granite
muscovite
8.91
82.1
89.7
12.37
78.4+2.9
18
DV87-7
sericite
completely
replacingfeldspars
in granite
muscovite
7.72
26.5
35.3
3.37
25.0•-_
1
20
DV87-5b
nearquantitative
replacement
of
feldspares
in granite
muscovite
7.07
54.6
2.69
21.8+0.9
Analyses
byGeochron
Laboratories.
)•[•=4.962xl
0-10/yr.Le+Lff=0.58
lxl 0-10/yr.
40K/K=
1.193xl
0-4g/g.
40Ar*=radiogenic
Ar.
19,736
PARRYETAL.: DIXm VALLEYFAULT,NEVADA
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/////
56
5
i4
52
51
• Creek
.5
Ikm
i
••
Tertiary
volcanic
rocks
•!• Least
altered
granitic
rocks
I I ¸uaternary
alluvium
.5
I
I
Ikm
I
Chlorite and epidote alteration
EXPLANATION
.
0
''• Sericite
alteration
IT• Zeolite
alteration
%
Dixie ValleyFault
•23 Samplelocality
Fig.2. Geologic,
alteration,
andsample
location
mapofportions
oftheDixieValleyfault.Temporally
andspatially
overlaping
hydrothermal
alteration
mineral
assemblages
infootwall
rocks
areshown.
(a)Coyote
Canyon
area,Co)BoxCanyon
area,and(c)
IXL Canyonarea.
PARRYETAL.:DIXIE VALLEYFAULT,NEVADA
.5
Ikm
i
laumontite,stilbite,andclay mineralshave 'alsobeensuperimposed
on earlier alterationmineralogieswith tracesof the earlierminerals
remainingin therock.
The earliest assemblageof hydrothermalminerals includes a
pervasive,partialto completereplacement
of igneousplagioclase
by
K-feldspar,leavingpatchyremnantsof twinnedplagioclasecrosscut
by K-feldspar veins visible in thin sections. Hydrothermal Kfeldsparalso occursas selvageson quartzveins and as quartz+Kfeldspar veins. Later veins of albite also crosscutplagioclase.
HydrothermalK-feldsparwasnot mappedbut is widely distributed
alongthe fault footwall. Hydrothermalbiotite alsoforms early in
the sequence, though the relative age of biotite cannot be
unequivocallytied to the K-feldsparreplacementof plagioclase.
Hydrothermal biotite occurs as thin veins, as replacement of
hornblendeand as a replacementof igneousbiotite. Hydrothermal
biotite
96
19,737
is more
abundant
in more marie rocks where biotite
vein
networksoccurin plagioclaseandpyroxene. Hydrothermalbiotite
is notvolumetricallyabundantanywherein thefault footwall.
The dominantalterationmineralogyon the Dixie Valley fault
footwall is Fe-chloriteand epidoteassociatedwith local areasof
calcite,hematite,sericite,andprehnite.Chloritereplaceshornblende
andbiotiteandoccursin veinswith actinolite,greenbiotite,calcite,
quartz,or epidote. Epidoteforms a selvageto quartz+epidoteveins
that reach thicknesses of several centimeters.
Cataclasite and breccia
consistingof epidote and chlorite form the facesof some of the
facetedspurs.
Hydrothermalsericite (dioctahedralwhite mica) replacesall
previouslyformed minerals except quartz. In the most intense
sericiticalteration,feldspars
havebeennearlycompletelyreplaced
by sericite and fine-grainedquartz. The resultingaltered rock
consistsof up to 58% sericite,relict igneousquartz, and fine-
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///////•
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///////////
///////
grained hydrothermal quartz. Biotite, hornblende,chlorite, and
epidotearecompletely
replaced
by sericite.
With decreasing
intensity
of alteration sericite occursas disseminatedgrains within the
feldsparsand as isolatedveins. X ray diffractionof clay-sized
•///////
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///////
////////
separatesfrom sericitizedrocks showsboth kaolinireand smectite
///////
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•//////?
•//////
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/
//////
//////
//////
associated with the sericite. However, thin section examination
shows the kaolinire and smectite are later than the sericite.
Prehniteoccurslocallyasfibrous,radiatingmasseswith typical
bow-tieextinctionwhichpartiallyreplaces
biotite.
//////////////
Latesthydrothermalmineralsare laumontite,stilbite,kaolinire,
•r•-7
5
smectite,and very fine grainedillite that occuron the frontsof
Fig. 2. (continued)
facetedspursat exposures
of the 1954 rupturetraceof the range
frontfault (Figure2c). Silicificationof bothvolcanicandplutonic
consisting
of angularfragments
of quartzandfeldspar
occasionally rocksis the latestalterationevent.Chalcedony,kaolinire,alunite,
with a matrixof sericiteor chlorite. No sheardisplacement
of the smectite,and opal occur in volcanicsboth north and southof the
fracturewallsis apparent
in thinsectioneventhoughthefractures 1954rupturesegment.
Deepexplorationwellsin northernDixie Valley crossedtheDixie
arecrowdedwith angularfragments
of quartzandfeldsparandthe
Valley
fault. Well dataindicatethat volcanicand alluvial material is
quartz may be stretchedand recrystallized. We infer that the
cataclasitewas emplacedas a suspension
of fine rock particles alteredto albite, chlorite,illite, epidote,and clay. Plagioclaseis
alteredto albite,illite, epidote,calcite,andclay, andhornblendeis
possiblyduringhydraulicfracturing.
Temporallyand spatiallyoverlappinghydrothermalalteration alteredto biotite, chlorite,magnetite,epidote,and calcite. In the
///
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//////
/////
mineralassemblages
occurasa narrowbandnearthe 1954rupture meta sediments, illite forms from andalusite, and biotite alters to
in the fault footwall. Geometric associationof the alteration with the
fault and associationwith tectonic texturesindicate alteration mineral
vermiculite and calcite. Quartz, laumontite,and adularia veins are
observed[Bell et aI., 1980].
assemblages
arerelatedto tectoniceventsontheDixie Valleyfault.
From earliestto latest,theseassemblages
are biotite+K-feldspar,
JOINT AND VEIN SYSTEMS
chlorite+epidote,
sericite+kaolinite+
smectite,
pretmite+laumontite+
stilbite+kaolinite+smectite.The spatial distributionof areas
Mineral assemblages
precipitatedin veins andon joint surfaces
dominated
by sericite,chlorite+epidote,
andzeoliteassemblages
is occurin severalsetsof variablyorientedfracturesthatformedover
shownin Figure 2. The sericiticalterationis superimposed
on an extendedperiod of time. The youngestmineralassemblages
chlorite and epidote,tracesof which remain in the rock. Prehnite, consist of prehnite-laumontite-stilbite-clay,and calcite-filled
19,738
PARRYETAL.: DIXIE VALLEYFAULT,NEVADA
FLUID INCLUSION PETROGRAPHY
MICROCRACKS
N38øW
OLDER
VEINS
PRE-
I 0 MA
REGIONAL
EXTENSION
DIRECTION
Healed microcracksin both igneousquartz and vein quartz
containthinplanartrailsof numerous
secondary
fluidinclusions
that
oftencrosscutboundaries
betweenadjacentquartzgrains.In least
alteredrocksthereare a few trainsof small(<lgrn) fluid inclusions,
butmostquartzgrainsnearthefaultshowmanyintersecting
planar
arraysof secondary
fluidinclusions.
Whilenumerous
intersections
of fluid inclusiontrainsare apparent,it is not possibleto determinea
fluid inclusionchronologyfrom crosscutting
relationships.Necking
is a common phenomena. Fluid inclusionsappear as irregular,
immatureto very regularmatureinclusionswith negativecrystal
shapes.Nearby fluid inclusionsin linear arrays that arematureand
displaynegativecrystaloutlinesoften have similarliquid to vapor
N = 27
ratios sugggestingthat neckingprecededseparationof a vapor
OPEN
FRACTURES
phase.
The abundanceof secondaryfluid inclusionslargerthan lttm
can be correlatedwith proximityto the mappedtraceof the Dixie
Valley fault. Quartz grainsnearestthe fault trace containup to
2x105 fluidinclusions
per cubicmillimeter.Fluidinclusion
abundance
decreases
to3xl04fluidinclusions
percubic
millimeter
SLIP
DIRECTION
Ci'cle = 209•
/
within 6 m of the fault at CoyoteCanyonand400 m of thefault at
IXL canyon.Here thebroaderdistribution
is dueto thepresence
of
severalsubsidiaryfaults.
Arraysof secondary
fluid inclusionsaresubparallel
to cataclastic
veins with a secondset a[ high anglesto cataclasticveins. The
orientationsof fluid inclpsiontrails that define partly healed
• REGIONAL
EXTENSION
DIRECTION
(S68øE)
microfractures
weremeasuTed
on orienteds•nple• of graniticrock
using
auniversal
stage
(Figure
4).Poles
tothese
trails
areshown
in
Fig.3. (a)Rosediagram
of strikedirections
of openfractures
ontheDixie Figure 4 contouredin terms of standarddeviations(o) from a
Valleyfault,Nevada.(b)Rosediagram
of strikedirections
ofhydrothermal uniform
distribution
ofpoints.
Threeareas
ofhig•pointdensity
are
veins.
apparent.
Themostprøm'ment
crackorientation,
pointdensity
of 810o,represents
nearlyverticalplanesstrikingN38øWabout15ø
veinlets. This assemblage
is concentrated
in northto NE trending
fracturesthat parallel the Dixie Valley fault trace (Figure 3a).
Sericite-clayandoldermineralassemblages
areconcentrated
in NW
to WNW striking veins that strike at a high angle to the
contemporaryfault zone and in a secondaryset of NNE to NE
strikingveins(Figure3b). These oldermineralassemblages
form
bands of alteration
several tens to hundreds of meters wide that
eitherparalleltheDixie Valley fault zoneor cutintothefootwallat a
high angleto the fault zone (Figures1 and 2). Two samplesof
hydrothermal sericite were dated using the K-At method; one
yieldedan ageof 21.8 Ma andtheother25 Ma (Table 1), indicating
thatthisalterationoccurredduringlateOligoceneto earlyMiocene.
The sericite-clay alteration assemblageis younger than the
chlorite-epidote and biotite-K-feldspar assemblagesbased on
crosscutting
relationshipsseenin the field andin thin section.The
epidote-chlorite assemblageis best preservedfrom Little Box
Canyon to Willow Canyon (Figure 2), but remnants of this
assemblage
extendalongalmosttheentirelengthof thefaultzonein
the studyarea. In most localesalong the rangefront, the sericiteclayassemblage
overprints
theepidote-chlorite
mineralization.
Near
SheepCanyon,in the southernpart of the studyarea (Figure 2),
sericite-claymineralizationextendsabout 1 km into the footwall
along a closely spacedset of WNW to west strikingjoints and
veins.
Orientationsof mineral-filledveinssuggestthatan earlystageof
activityon the Dixie Valley fault with an ENE to WSW extension
directionis associated
with mineral-filledextensionveinsof quartz,
epidote,quartz-sericite,
clayminerals,andhematite.Fractureswith
dustingsof euhedralquartz,stilbite, chlorite,calcite,epidote,and
hematiteareparallelto present-day
extension
fractures.
Fig. 4. Equal-areastereographic
plot of polesto fluid inclusiontrails,lower
hemisphere.Contouredin termsof standarddeviationsfrom a uniform
distributionof points [Kamb, 1959] with the contourintervalof 2o
indicatedby widthof ruledlines. A uniformdistribution
is represented
by
3o, so the unruledarea representing
2•-4• represents
the areaof the
diagramin which the countingcircle containeda distributionof points
equivalent
to 1• on eithersideof thedensityof a uniformdistribution
of
points.
PARRY
ETAL.:DDClE
VALLEYFAULT,NEVADA
19,739
TABLE 2. Characteristics
of Major FluidInclusionGroups
Moderate
Property
Type II
Type 121I
CO2-Bearing
Low Eutectic
Type IV
Halite
Salinity
40-90%1
10-60%v
Phases at
room
60-90% H201
10-40% CO21
+CO2v
temperature
Aqueousphase
at low
80-90%1
6-20%v
freeze at -30 ø to-40øC
freeze at -60 ø -80øC
coarsens -40øC
meltingof last
solid (ice) 0ø to -11øC
temperature
H201
H2Ov
2-10% NaC1
visible melt -40øC
meltingof last
solid ice -26 ø to -9øC
Bubblebehavior
no change
CO2 melts
at low
-57.5 ø to -54.4øC
temperature
Gashydrates
clathrate melts
none
6.4 ø to 9.7øC
Heating
behavior
CO21-v 22.5ø-29.3OC(v)
CO21-v 25.4ø-29.4øC(1)
homogenization
to
homogenization
to 1 at 125ø-400øC
TmNaC1
homogenization
to liquid 129ø-283øC
160ø-356øC
Thl-v 275o-382oc
1 at 197ø-358øC
Number of
256
17
45
7
measurements
v, homogenizes
to vaporphase;1,homogenizes
to liquid phase.
clockwise
of the dominant
NW
to WNW
concentration
of older
Two types of high-salinity inclusions are present. A few
veins observedin outcrop. The next most prominent, 6-8{5, inclusionsconsistingof only vapor and liquid showedeutectic
well belowthe NaC1-H20eutectic(typeIlI). The
represents
planesN10øW andthethirdset,4-6 c5,represents
planes temperatures
strikingN72OE.
The K-Ar age of hydrothermalillire in the areais 21 to 25 Ma
(Table 1) corresponding
to theearlierepisodeof extensionwherethe
fourthtypeof fluidinclusions
containdaughter
minerals(typeIV).
Thesedaughter
mineralsincludeanisometric
saltprobablyhalite, a
small,high-reliefunidentified
salt,anda platy,birefringent
mineral
extension
direction
is WSW-ENE
consistent
with mode I
thatmaybe sericite.
microcracks hosting fluid inclusions striking N38ow. Fluid
inclusion fluids on this section of the Dixie Valley fault must ModerateSalinityInclusions(TypeI)
thereforebe associatedwith the early phaseof Basin and Range
Microthermometric
measurements
of typeI fluid inclusionshave
extension.
The orientations
of fluid
inclusion
trails
are
revealedwidediversityin chemicalcomposition
andhomogenization
subperpendicular
to theinferredOligoceneto earlyMioceneregional
stress field.
FLUID INCLUSION MEASUREMENTS
temperatures.
Salinities
in NaC1equivalent
wt % calculated
from
icemeltingtemperatures
areshownfor 265 fluidinclusions
in the
histogramof Figure 5. Salinity data form a highly skewed
distribution
with mostfrequentvaluesin thelow salinityrange0 to
Phasechangesobservedin fluid inclusionson the heating- 1% NaC1. Howeversignificantnumbersof highersalinityfluid
freezingmicroscopestagehave revealeda diverseassemblage
of inclusions were observed.
chemicallydistinctfluidson theDixie Valley fault.Fluid inclusions
7O
arecategorized
by theircontents
intoliquid+vapor,moderatesalinity
inclusions
(typeI); carbondioxidebearinginclusions
(typeII); low
60
eutectictemperatureinclusions(typellI); halitebearinginclusions
(type IV); and one-phase(liquid) inclusions. A few inclusions
containsolid phasesin additionto halite. The one-phase(liquid)
inclusionsare the youngest.The thermometricmeasurements
are
o 40
summarizedin Table 2. The majorityof fluid inclusions•neasured
consistof an aqueousphaseanda vaporbubbleusuallycomprising
O 3O
10 to 30 vol % (typeI). Ice is thelastphaseto melt on warnting.A
s•ond andpossiblyrelatedtypeof inclusioncomainsa largevapor
• 20
bubble(more than 60 vol %) and a small volume of low-salinity
liquid. These two typesof inclusionsare occasionallyin close
10
proximityto one anotherand may showsimilar homogenization
0
temperatures,the first to liquid and the secondto vapor. These
•
characteristics
couldbe consideredevidencefor boiling of fluids at
some stage of fault development. However, the widespread
occurrenceof necked inclusionssuggeststhat necking probably
0
DIXIE
VALLEY
FAULT
5
10
SALINITY
15
Nacl
EQUIVALENT
20
25
WT.%
accountsfor thisassociation.
CO2-beafinginclusions
(typeII) are
Fig.5. Histogram
of salinities
of TypeI (ruledlines)andType121I
(solid)
fluid inclusionscalculatedfrom ice melting temperaturesusing the
presenton somesectionsof the fault.
regression
equationof Potter et al. [ 1978].
19,740
PARRYETAL.:DIXIEVALLEYFAULT,NEVADA
COYOTE
TO
SHEEP
CANYON
-b
20
100
150
200
HOMOGENIZATION
250
300
350
400
TEMPERATURE
Fig. 7. Temperatureof homogenization
of fluid inclusionsplottedversus
salinity.Symbolsareopensquares,
low eutecticinclusions
(type111);open
circles,CO2 inclusions(typeII); solidcircles,moderatesalinityaqueous
inclusions(type I). Moderate salinity aqueousinclusions(type I) are
contoured
in termsof density(g/cm3)usingthe equations
andcoefficients
of Potter and Brown [1977]. Density contoursdo not apply to CO2
inclusions
shownasopencircles.Trendsin thediagramindicatedby arrows
anddiscussed
in the text are A, boiling;B, cool,low-salinitywatermixing;
C, coolevaporitemixing;andD, isochemical
coolingof thefaultfootwall.
lOO
150
200
250
HOMOGENIZATION
300
350
400
TEMPERATURE
BOX
and Figure 7 is contouredin terms of density. Becausefluids
trappedin inclusionsare constantdensitysystems,the fluidsmust
be trappedalongisochores
definedby thesedensityvalues.
CANYONS
C02-BearingFluid Inclusions(TypeII)
N--104
Secondary
fluid inclusionscontainingCO2 arepresentin several
areasof the Dixie Valley fault footwall. Theseinclusionscontaina
recognizablemeniscusseparatingliquid CO2, vaporCO2, andan
aqueousliquid at room temperature.Thermometricpropertiesof
theseinclusionsare shownin Table 3. The liquid CO2 phasethatis
frozenby supercoolingto below -90øC, meltsin the presenceof
CO2 vapor at temperaturesof-57.6øC to -54.4øC. The most
frequentmeltingtemperature
is within0.2øCof-56.6øC,thetriple
pointtemperature
of pureCO2. Lowermeltingtemperatures
could
o
be accountedfor by small amountsof methane. The melting
temperatures
above-56.6øCcannotbe accounted
for by anyknown
phaseequilibria,butthesetemperatures
arereproducible
andnotthe
Fig. 6. (a) Homogenizationtemperatures
of typeI aqueousfluid inclusions
from the Dixie Valley fault footwall. (b) Homogenizationtemperatures
of resultof experimentalerror.
Clathratemeltingtemperatures
of CO2-bearinginclusions
yield
fluid inclusions from the Coyote Canyon area of Figure 2a. (c)
Homogenizationtemperaturesof fluid inclusionsfrom the Box Canyons salinitiesof 0.62 to 6.8 wt % NaC1relativeto H20 (Table 3). These
area (Figures2b and 2c).
salinitiestogether
with thehomogenization
temperatures
areplotted
lOO
150
200
HOMOGENIZATION
250
300
350
400
TEMPERATURE
on Figure7 for comparison
withaqueous
inclusions,
butthedensity
contours
onFigure7 applyonlyto theaqueous
inclusions.
Homogenization of type I fluid inclusions occurred by
TheCO2 component
of theseinclusions
homogenized
to liquidat
disappearance
of the vapor bubble. The rangein homogenization
25.4o-29.5oc or to vaporat 20.5ø-29.3øC. Homogenization
to
temperatures
is 120ø to 400øC (Figme 6). The modeof Th valuesis
theliquidimpliesa greater
density
of CO2andentrapment
athigher
270ø- 280øC in the southernareanear Coyote and SheepCanyon
pressure.
(Figure2a) and 180ø-190øCin theBox Canyonsarea(Figme2b),
Overallhomogenization
of CO2 bearinginclusions
occurred
over
an area dominatedby chlorite and epidote alteration (Figure 6),
nearly as wide a range (197ø-371oc) as the moderatesalinity
althoughboth areashave homogenizationtemperaturesthat span
inclusions(Table2 andFigure7).
nearlythefull range.
Homogenizationtemperatureis plottedversussalinityfor typeI,
Low EutecticTemperatures
(TypeIII)
typeII, andtype11I fluid inclusionsin Figure7. The datapointson
Figure 7 indicate no apparent trends of cooling and dilution.
Fluid inclusionswith ice meltingtemperatures
between-10.1øC
aslow as Homogenization
temperature
andsalinitydefine thefluid density and -26.0øChad very low initial meltingtemperatures
PARRYETAL.: DIXIE VALLEYFAULT,NEVADA
19,741
TABLE3. Mierothermomctric
Properties
of CO2-Bearing
FluidInclusions
Tm
Map
12
DV87-1f
1
4
8
14
15
16
17
-56.5
-56.9
-56.8
-57.4
-56.4
-57.6
22
DV87-4
17
-56.8
18
-56.7
9.7
19
-57.2
20
18
43
D
ThCO2
CO2 L-V
Sample No. TmCO2 Clath
9.2
8.4
7.9
9.7
9.6
7.9
9.0
Volume
Th
Percent
%CO2 WNaC1
XNaC1
XH20
XCO2
30
29
27
30
25
39
40
1.63
3.19
4.14
0.62
0.82
4.14
2.03
0.005
0.010
0.012
0.002
0.002
0.012
0.006
0.931
0.942
0.933
0.934
0.939
0.902
0.909
0.065
0.049
0.055
0.064
0.058
0.086
0.085
371v
80
0.62
239D
17
3.95
0.001
0.012
0.726
0.921
0.272
0.068
28.0v
20.5v
26.5v
27.9v
29.3v
28.0v
27.1v
304
313
297
300
278
335
340
29.4
270v
22.5v
8.0
28.41
390
320
360
340
340
450
440
8
-56.4
11
12
13
-57.0
-57.5
8.5
8.5
8.2
26.61
26.21
29.01
312D
254D
197
42
25
9
3.10
3.10
3.67
0.008
0.008
0.011
0.810
0.890
0.947
0.182
0.102
0.042
5
6
7
8
-56.3
-56.3
-56.3
-56.3
8.5
8.5
8.1
8.6
27.2v
26.4v
27.5v
22.5v
312v
280
308
318
23
31
32
3.10
3.10
3.86
2.91
0.009
0.009
0.011
0.008
0.066
0.943
0.924
0.935
0.068
0.048
0.065
0.056
DV87-19
1
2
6
7
-56.4
-56.9
-57.0
-57.1
7.1
6.9
6.8
7.9
24.7v
24.0v
27.5v
28.0
80v?
210D
250D
261D
90
15
26
28
5.59
6.03
6.12
4.14
0.009
0.018
0.019
0.012
0.479
0.948
0.926
0.927
0.512
0.033
0.055
0.060
DV87-54a
1
2
3
4
5
6
7
-55.4
-55.1
-54.9
-54.4
-56.7
-56.5
-56.8
8.5
9.1
8.1
8.0
7.6
7.8
7.8
26.51
27.21
25.41
27.01
28.7v
28.4v
27.5v
240D
252D
280
268
324
320
303
21
25
31
27
34
36
30
3.00
1.83
3.76
3.95
4.69
4.32
4.32
0.009
0.005
0.010
0.011
0.014
0.013
0.013
0.906
0.895
0.856
0.881
0.910
0.907
0.924
0.085
0.100
0.134
0.108
0.076
0.080
0.063
DV87-55
1
3
5
-56.7
-56.5
8.3
7.7
7.9
30.0v
29.41
29.0v
234
208
244
14
10
21
3.38
4.51
4.14
0.010
0.014
0.012
0.950
0.939
0.908
0.039
0.047
0.082
360
670
88O
DV87-59
1
2
3
4
5
7
9
10
1!
12
-56.0
-56.0
-55.6
-56.6
-56.6
-56.7
-55.2
-56.8
-56.8
-55.6
7.9
9.2
8.9
8.4
8.4
6.4
6.7
7.4
6.9
8.2
27.5v
29.5v
28.9v
29.41
29.51
24.11
27.31
27.61
28.31
26.21
219
216
222
279
287
268D
248
259
305
294
10
10
11
33
32
25
18
19
32
39
4.14
1.63
2.22
3.19
3.19
6.81
6.29
5.05
5.94
3.57
0.013
0.005
0.007
0.009
0.009
0.020
0.019
0.015
0.017
0.009
0.958
0.963
0.961
0.866
0.866
0.875
0.908
0.909
0.857
0.824
0.030
0.032
0.033
0.126
0.125
0.105
0.073
0.076
0.126
0.167
280
310
300
1060
1200
DV87-Sa
DV87-7
307
P, bar
3.19
540
350
380
340
1480
1300
550
430
370
1250
1380
1570
1450
v,homogenizes
tovapor
phase;
1,homogenizes
toliquid
phase;
D,decrepitated.
Mapnumber
onFigures
1and2.
45øC andice crystalnucleation
temperatures
aslow as-60øCto -
presence
of liquidonly, estimated
salinityis only approximate
80øC. Thesecharacteristics
suggestthe presenceof a significant
CaC12 componentin the fluid. These fluid inclusionshave
salinitiesof 13.6-25.3wt % CaC12+NaC1(Figure5). In two cases,
fluid inclusionswere sufficientlylarge and visible to observe
because
of theunknownslopeof isopleths
in P-T space[Sterneret
hydrohaliteand ice melting temperatures.In thesetwo fluid
inclusions
theweightfractionNaC1relativeto CaC12was0.3 and
0.55. Homogenization
of liquidandvaporLnfluidinclusions
with
low eutectic temperaturesranged from 120ø to 321øC.
Homogenization
temperatures
andsalinities
of theseinclusions
are
temperatures
of 160ø-297øCfor inclusions
thathomogenize
by
vapordisappearance
are30.1 to 37.9 NaC1equivalent
wt %. In
somehalite-bearing
inclusions,however,halitewaspresentat the
temperatureof vapor bubble disappearance.Some of these
inclusions
decrepitated
beforeall halitehad dissolved.The few
halite-bearkng
inclusions
thathomogenized
by dissolution
of halite
yieldedsalinities
of 39.2to43 NaC1equivalent
wt %.
shownasopensquaresin Figure7.
al., 1988].
Fluidinclusions
with a halitedaughtersaltarethehighestsalLnity
fluid inclusions.
Salinities determined from halite melting
InclusionsContainingHalite (TypeIV)
Discussion
Severalfluid inclusionsthat contain a vapor bubble, a halite
Evidencethat fluid inclusionsand alterationmineral assemblages
crystaland liquid at 25øC were observed. Theseinclusions
represent
fluids andtheir Lnteraction
with rocksrelatedto faulting
homogenize
by disappearance
of thevaporbubbleorby dissolution
of the halite crystal. The homogenizationof liquid and vapor
occurredat a temperature
belowthehalitemeltingtemperature
in
includesoccurrenceof alterationbandsin the fault footwall, spatial
orientationof fluid inclusionstrails consistentwith the stresssystem
severalof theseinclusions.Becausehalite melting occurredin the
producing
thefaulting,andcorrelation
of fluid inclusionabundance
19,742
PARRYETAL.: DIXIE VALLEY FAULT,NEVADA
with proximity to the fault. All fluid inclusionsmeasuredand all
DixieValleyfaultfootwall.Minimumpressures
of entrapment
are
observedalterationmineralsarein thefootwallof theDixie Valley
fault,andall postdate
theintrusionof thegraniticrocksat 28 Ma so
thatthe fluid inclusionfluidsandalterationhavea maximumage
neartheinceptionof extension.
The diversity in salinity of fluid inclusionscould result from
mixing of low-salinity and high-salinity components. These
components
may includecool, low-salinitymeteoricwater,saline
waterformedin evaporitebasins,moderatesalinitythermalwater,
estimated
fromCO2 containing
fluid inclusions
andfrominclusions
containing
a halitecrystal.
The salinity, CO2 density,and homogenizationtemperaturesof
type1I fluid inclusionswereusedtogetherwith theCO2-H20-NaC1
phasediagramto estimateCO2 contentand minimum pressureof
entrapment
usingthe procedures
of Parry [1986]. Carbondioxide
contentrangesfrom 3 to 17 mol %, andestimatedpressures
on the
two-phaseboundarycurve for inclusioncompositionsvary from
anda high-temperature
boilingfluid with temperature
andsalinity 280 to 1570 bars. Thesepressuresrepresentminimumentrapment
variationscausedby steam separation. Lower temperatures pressuresof a homogenousfluid. Homogenousfluids couldhave
resultingfrom boiling would correspondto higher salinitiesas beentrappedat highertemperatures
andpressures
alongappropriate
shownby thetemperature-salinity
mixinglineslabeledA on Figure isochores
for eachfluid composition.
7. Mixing of low-temperature,
low-salinitymeteoricwaterwith
RoedderandBodnar[1980]andRoedder[ 1984]showedthatthe
boiling fluids at any stagein their evolutiontowardlower T and minimumpressure
of entrapment
of a homogenous
fluid canbe
highersalinityis labeledB onFigure7. Thepresent-day
Dixie estimated
fromhalite-bearing
fluidinclusions
byfirstassuming
the
Valley containsevaporitesin HumboldtSalt Marsh, and the fluidproperties
areadequately
represented
bytheNaC1-H20
system
extensiveevaporitesof CarsonandHumboldtSinkslie 30 km to the andthatNaC1solubilityis independent
of pressure.The bulk
west.
composition
of the fluid inclusionfluid and its densitywere
DixieMeadows
hotsprings
(Figure1) nearthe1954rupture estimated
fromthemelting
temperature
ofNaC1using
theregression
termination
consists
of 35 springs
andseeps
over4 square
miles(10 equation
of Sterneret al. [1988].Thecomposition
anddensity
of
km2) withwide
variations
intemperature
andsalinity.
Springs
the halite saturatedsolutionon the liquid-vaporcurveat the
emerge
fromalluvium
in thehanging
wallof theDixieValleyfault. temperature
of vapordisappearance
werecalculated
usingequations
Temperature
andsalinityareinversely
correlated
in thesespringsfrom Haas [1976]. At the temperatureof liquid-vapor
suggesting
thatdissolution
of evaporites
bycoldwaterandmixing homogenization,the inclusionvolume is the volume of saturated
with
lowsalinity
hotwater
accounts
forthevariation
intemperature
solution
ofknown
density
plus
thevolume
ofhalite.
Themass
of
and
salinity
[Bell
etal.,1980].
Halite-cemented
breccias
are
present
halite
crystal
isobtained
from
thedifference
insalinity
atthe
inthefaultin thevicinity
ofthehotsprings
area.Saline,
temperature
ofliquid-vapor
homogenization
and
atthe
temperature
gypsiferous
claysoccurin theTruckee
formation
of upperMiocene of NaC1melting,anditsvolumeis calculated
fromhalitedensity.
agein western
Nevada[Morrison,1964]. Mixingof cool,salty Thesecalculations
yielddensities
of 1.137and1.275g/cm3for
fluid withhigher-temperature,
lower-salinity
fluidsisillustrated
by inclusion•
homogenizing
at theNaC1
meltingtemperature
of 313ø
thetrendlabeled
C onFigure7. A fourthaltemative
issuggested
by and 356oc, respectively. Extrapolationof the density
continuousdisplacement
of the fault footwallto shallower,cooler
determinations
of Urusova[1975]at 350oc to higherpressures
depths.Thistrendis labeledD onFigure7.
indicatea minimumentrapment
pressure
of about1500bars,but
Geothermal
fluids
atdepth
inDixie
Valley
presently
contain
CO2.extrapolation
ofthe
MRK
equation
ofstate
using
the
coefficients
of
Fluid
inthedeep
wells
contains
312-396
mg/L
Na,37-54
mg/L
K, Bowers
and
Helgeson
[1983]
indicates
apressure
of800bars
at
and
CO2comprises
0.12-0.22
wt%ofthefluid
(0..09
mol%)ata 350øC.
Extrapolation
ofUrusova's
[1975]
density
measurements
temperature
of206ø-249øC
[Benoit,
1989].Highbicarbonate
in to 300oc
yields
apressure
estimate
of240bars
fortheinclusion
springs
emerging
fromigneous
rocks
nearmajor
faults
[Bohm,homogenizing
at313oc.
1984];
vast
travertine
deposits
atSou
hotsprings,
47kmnortheast
Wenext
assume
that
lithostatic
pressure
represents
areasonable
ofthe
1954
rupture
segment;
and
high
bicarbonate
(870-936
mg/L
at maximum
andhydrostatic
pressure
represents
a reasonable
Hyder
hotsprings,
42kmnortheast,
maybetheresult
ofCO2minimum
forfluid
pressure.
Second,
weinfer
athermal
gradient
so
leakage
from
adeeper
geothermal
reservoir.
that
pressure-temperature
gradients
maybe
shown
inFigure
8.The
Secondary
fluid
inclusions
ingranitic
rocks
often
show
final
icethermal
gradient
onthe
Dixie
Valley
fault
isprobably
greater
than
the
melting
temperatures
inthe
range
below
the
NaC1-H20
eutectic
30øC/km
gradient
that
ischaracteristic
ofthe
Basin
and
Range
[Konnerup-Madsen,
1977,
1979].
These
types
ofhigh-salinity
Province
[Sass
etal.,1981].
The
northern
end
ofthe
belt
ofhistoric
fluids
arecommon
ingranitic
rocks,
and
their
occurrence
inhealed
seismicity
in thecentral
Nevada
seismic
beltis anareaof
microfractures
intheDixieValley
fault
rocks
together
with
an conspicuously
high
heat
flow
called
the
Battle
Mountain
High
[Sass
intrusive
agenearthetimeofinception
ofBasin
andRange
etal.,1971]
where
thermal
gradients
are40øto50oC/km
[Sass
et
extension
suggests
they
may
have
been
important
fault
fluids.
al.1981
].
CaC12-NaC1brineshavebeenobserved
in retrograde
Ca-rich
Additionalindications
of elevatedgeothermal
gradients
include
amphibolite
whereretrograde
reactions
withmuscovite,
epidote,
and
intense
hydrothermal
alteration
along
the
range
front
fault,
fumaroles
chloriteas productmineralsare suggested
as a mechanismfor
concentrating
Na, K, andCa in fluids[Crawfordet al., 1979]. andhotwaterwithin30 m of thesurface,AsandHg in soilsof the
valleyneartherangefrontfault [JuncalandBell, 1981],andhot
TheseCa-richcompositions
aremostcommon
in metamorphosed
widelydistributed
in thehanging
wall of theDixieValley
carbonate
richsediments
[Roedder,1984,p. 351]. Hydrothermalsprings
calcium chloride brines are also known from continental rift
systems[Hardie, 1990].
fault. The Dixie Valley geothermalareais 47 km northeastof the
1954rupturesegmentwheregradients
of 75oC/krnandabovehave
beenmeasured
in drillholes[Belletal., 1980]dueto hydrothermal
convection.
These
observations
lendsupport
to a thermalgradient
in
FLUID TEMPERATURE AND PRESSURE
Dixie Valleythatexceeds
thenormalBasinandRangegradient.
Fluidinclusion
characteristics
andmineral
equilibria
maybeused Lithostatic
andhydrostatic
pressure
gradients
areshownonFigure8
to placeconstraints
on the fluid pressure
andtemperaturein the for thermalgradientsof 45ø and60øC/krn.
PARRYEl' AL.: DIXm VALLEYFAULT,NEVADA
19,743
quartz-rich
rocksrequirestemperatures
belowabout270øC.
2OO0
Pyrophyllite
hasnotbeenobserved
despite
numerous
fluidinclusion
homogenization
temperatures
thatexceed270øC.Thereasonfor
this is that fluids contain sufficient K + to stabilize muscovite
accordingto
"•
1000
3A12Si4010(OH)2+ 2K+ = 2KA13Si3010(OH)2+ 2H+.
(pyrophyllite)
(muscovite)
5OO
050
I
100
/I
I
1 •0
Thelow-temperature
stabilityof epidot½
in thepresence
of quartz
andanaqueous
solutionatpressures
between
500and3800barsis
represented
by reaction
(3) [BirdandHelgeson,
1981].Prelmite
is
Na(•l
200
:5O
400
(2)
4•0
TEMPERATURE, øC
Fig.8. Minimum
pressure
andtemperature
forentrapment
of CO2fluid stablerelative to laumontiteaboveabout220oc (reaction(4)) and
inclusions
(solidcircles),
lithostatic
andhydrostatic
pressure
gradients, stilbite is stable relative to laumontite below about 130ø-140øC at
mineralequilibria,
andboilingcurves.
K, kaolinitc;
P, pyrophyllite;
below600bars(reaction(5)):
Q,quanz;
W, water,
L, laumonfite;
Pr,prehnite;
S,sfilbite;
H, heulandite.pressures
Dataforreactions:
K+Q=P+WfromHemIcy
etal.[1980];L=Pr+K+3Q+5W
fromBirdandHelgeson
[1981];S=H+WandH=L+3Q+2W
fromChoet 2Ca2Al(A1Si3010(OH)2
+ A12Si205(OH)4
al. [1987];
andS=I+3Q+3W
fromLiou[1971].
Isochores
calculated
for1% (prehnite)
(kaolinitc)
NaC1solution
andvarious
homogenizafion
temperatures
fromdataof Potter
andBrown[1977]. Boilingcurvecalculation
useddataandequations
of
Haas [ 1976].
=2Ca2A13Si3012(OH)+ 2SIO2+ 3H20
(epidote)
(quartz)
(3)
A heatsourcefor anelevatedgradientis readilyavailable.The 2CaA12Si4012.4H20
earliest
phase
ofextension
before18-15Ma wassynchronous
witha (laumontite)
generally
southward
migrating
beltof intermediate
to silicic
+ A12Si205(OH)4
+ 3SIO2+ 5H20
volcanism[Sonderet a/.,1987;Wernickeet aI., 1987]. Major =Ca2Al(A1Si3010(OH)2
(prehnite)
(kaolinitc)
(quartz)
Cenozoic
igneous
activityin theDixieValleyareabeganwith
(4)
andesiticlava flows 35 Ma [Riehleet al., 1972]. Voluminous
= CaA12Si4012.4H20
+ 3SIO2+ 3H20 (5)
eruptions
ofquartz
latitetorhyolite
ashflowtuffsaredated
at32-22 CaA12Si7018.7H20
(laumonfite)
(quartz)
Ma [Riehleet al., 1972;Speed,1976;BurkeandMcKee,1979]. (stilbite)
Basaltflowscapthesequence
andhavebeendatedat 13-17Ma
[Nosker,
1981].Smalligneous
dikesandplutons
oflatitetodiorite Examinationof equilibria (1), (4), and(5) on Figure8 shows
alongwithfluidinclusion
properties
record
occur
along
theDixieValleyfaultzone.
TheDixieValleyfaultmay thatmineralassemblages
a systematicdecreasein both temperatureand pressure with
of thefootwallduringfaulting. Fluidtemperature
at
crustalongthe fault might allow for differentsubsidiaryfault displacement
depth
on
the
fault
exceeded
350øC
and
pressure
exceeded
1500
displacements
and spreadingof grabens[Thompson,1966;
bars. K-feldspar,biotite,epidote,chlorite,andmuscovite
werethe
Thompson
andBurke,1974;OkayaandThompson,
1985].
Thepressure
andtemperature
of entrapment
of typeI moderam earliestformedstablemineralphases.As temperatureandpressure
salinityinclusions
mustlie onisochores
deFmed
by thefluiddensity. decreased, kaolinitc was stabilized relative to muscovite and
belowabout270øC,prehnite
wasstabilized
relativeto
Fluid densityis calculatedfrom salinity and homogenization pyrophyllite
epidote
and
then
laumontite
was
stabilized
relative
to
prehnite
at
temperature
measurements
shownin Figure7. Representative
be rootedwithin magmaticintrusionandfurtherintrusioninto the
isochores
fordemities
of0.55to0.95g/cm
3 areshown
inFigure
8
about220oc. At 140oc, stilbite is stablerelative to laumonfiteand
mustbelowerthan600bars,otherwise
heulandite
corresponding
to densitiesof 1 wt % NaC1 solutionsthat thefluidpressure
would
occur.
As
pressure
and
temperature
decreased,
CO2
homogenize
by vapordisappearance
atvarious
temperatures.
These
separated
so
that
zeolites
were
stabilized
relative
to
calcite
and
clay.
fluidswouldhavebeentrappedalongtheirrespective
isochores
at
pressures
andtemperatures
betweenlithostatic
andhydrostatic
REGIONAL IMPLICATIONS
pressures.
Theminimumpressures
andtemperatures
of entrapment
of type
TheStillwater
Rangeliesinpartof a regional
WNW trending
belt
II, CO2-bearing
inclusions
areshownon Figure8. Thesefluids,
of
mostly
silicic
volcanism
that
extended
across
southern
Nevada
too,wouldhavebeentrapped
onisochores
definedby thedensityof
thefluid. The widerangein minimumpressures
of entrapment
has from late Eocenethroughearly Miocene time. This belt was
by widespread
hydrothermal
alterationandNW to
beeninterpreted
tobetheresultofpressure
transients
accompanyingcharacterized
dilatantfracturing[ParryandBruhn,1990].The highestpressure WNW structural trends that included numerous dikes, caldera
andhigh-angle
faultandjointsystems
[Boden,1986].
values are near lithostatic pressure,and the lowest are near complexes,
In
some
locales,
NE
rending
faults
formed
as
subsidiary
features
hydrostaticpressure.
1987].
Additionalconstraints
areplacedonP-T conditions
by mineral [Rielheet aI., 1972;Boden,1986;Hudsonand Geissman,
rotation
equilibria.Equilibriaamongkaolinitc,pyrophyllite,
quartz,and Hudsonand Geissman[1987] notedthatcounterclockwise
of rocksin thecentralStillwaterRangeandClanAlpineMountains
wateraccordingto
occurredas the resultof movementon a complexarrayof strike-
A12Si205(OH)4
+ 2SIO2= A12Si4010(OH)2
+ H20
(kaolinitc)
(quartz) (pyrophyllite)
(1)
shownonFigure8 indicatethatthepresence
of kaolinitcin these
slip,oblique-slip,
anddip-slipfaultsandsuggested
thatrotation
was
accomplished
bymovement
onNW-striking,
right-lateral
faults.Li
et aI. [1990] seepalcomagnetic
evidence
forcounterclockwise
block
rotations elsewhere in northern Nevada.
19,744
PARRY
ETAt.: DD•E VALLEY
FAULT,NEVADA
Regionalextension
in thegreatBasindatesfromabout30 Ma and veinarrays
indicate
thatthese
features
probably
reflectOligocene
to
occurred
in twoepisodes.
The earlyepisode
of extension
in northern earlyMiocenedeformation
andhydrothermal
activity,which
Nevada was oriented
S68øW-N68øE. At about 10 Ma the occurred
priorto theonsetof NNE trending
Basin andRange
extension
direction
change4
byclockwise
rotation
tothepresent-daynormalfaulting[Stewartet al., 1977].
extension
directionof N65ow-s65oE [Zobacket al., 1981;Zoback
Thelaterepisode
of extension
beginning
at 10-13Ma [Eddington
andThompson,
1978;Eaton,1982]. Theearlyphaseof extension et al., 1987;Zobacket al., 1981]is indicated
by 10Ma oldbasalton
datedat23-15Ma based
onangular
unconformity
androtated
high the west flank of Job Peak that is tilted 10o west [Wallace and
anglefaults[Johnet al., 1989] wasaccommodated
byrotation
of Whitney,
1984].Basaltandandesite
capthe Stillwater
rangeatan
crustal
blocks,strikeslipmotiononnorthwest
trending
faults,and elevationof 2200m andalsoproduceseismic
reflections
in Dixie
normalfaultsthatpredatewidespread
basalticvolcanism
datedat 13 Valleyatanelevation
1280m belowsealevelfora totalofpost-!3to 17 Ma [Hudsonand Geissman,1987]. Remnants
of this NV½ !7 Madisplacement
onthefaultof 3.5km[Wallace
andWhitney,
strikingright-lateralfault set werepreservedin the interiorof the 1984]to2.2kmfarther
north[OkayaandThompson,
1985].
Stillwater
Range
nearWhiteRockCanyon
andCoyote
Canyon
and
Moderate
salinityfluidinclusions
weretrapped
at thepressure
mayberepresented
by thewesternfaultbranchin the"bend",within andtemperature
definedby theisochore
for the fluiddensity
thepresent-day
Dixie Valleyfaultzone(Figure9). Hydrothermalillustrated
on Figure8. Fluidinclusions
containing
CO2were
activity
andtectonic
displacement
on atleastpartof theDixieValley trapped
atminimum
pressures
ashighas1570barsata temperature
faultbegan duringthisearlystageof extension.K/Ar agesof of 305øC(Table3) shown
in Figure8 andcorresponding
to a
hydrothermal
sericiteon thefaultare21.8 to 25 Ma (Table1) and lithostatic
pressure
at a depthof over 6 km. Aqueousfluid
mineral-filled veins and fluid inclusion train orientations are inclusions
thathomogenize
at380o-400oc
thenif trapped
nodeeper
consistentwith the earlier extensiondirection.The orientationand than6 kmmusthavebeentrapped
ata thermal
gradient
of nearly
ageof hydrothermal
minerals
in theolder,NW toWNW trending 70øC/km.
b
N
()
8 km
•
I
0
4
I
Possible
Oligocene
to
Late Hicocene
fault
(NNWto NWtrending)
Trace
of
Q
uate
rna
rg
fault scarps
•
O km
I
I
Fault
developed
in Oligocene
to
Late Hiscane stress field
Fault developedin contemporarg
(•) stress
field
(Late
Hiocene
togecent)
Fig.9.(a)Mapviewoftherapture
trace
oftheDixie
Valley
fatfit.
Localities
referred
tointhetextare1,White
Rock
Canyon;
2,The
Bend;
and3,Coyote
Canyon.
(b)Mapviewofproposed
development
oftheDixieValley
faultfromsuperposition
offaults
developed
in contempomy
stress
fieldonO!igocene
tolateMiocene
faults.
PARRYETAL.: DIXm VALLEYFAULT,NEVADA
19,745
Supporting
geologicalevidencefor prebasaltdisplacement
is
evidenton the 1:125,000scalemap of Page [1965] and the
1:250,000scalemapof WilldenandSpeed[1974]. Volcanicrock
unitsonthese
mapsinclude
olderTertiarylatiteweldedtuffs0.6-1.8
kmthickthatareoverlain
byrhyolite
torhyodacite
weldedtuffs0.63.0km thickandrhyolitetorhyodacite.
Basaltonthewestflankof
sericite,andlaumontite-prehnite-clay.
Secondary
fluidinclusions
presentin healedfractures
in quartzassociated
with syntectonic
chlorite-epidote-sericite
alteration
contain3 to 32 mol% CO2with
4.5 to 17.3 wt % NaC1solution.The modeof homogenization
temperatures
is 285oc.Estimated
minimumentrapment
pressures
vary from 600 to 2950 bars. The modeof homogenization
JobPeak directlyoverliesthe older latitewelded tuffs. Rhyolite to
rhyodaciteandup to 3.0 km of rhyodaciteweldedtuffs are missing.
The olderlatite weldedtuffs areexposedalonga seriesof northto
NW striking faults. Farthernorth, the basalt is in contactwith
Jurassic volcanics' rhyolite to rhyodacite, up to 3.0 km of
rhyodaciteweldedtuffs, and0.6 to 1.8 km of latiteweldedtuffs are
missing.The olderrock unitsareexposedon northto NW trending
faults. The footwall of the Dixie Valley fault in the areaC to D of
Figure1 alsotrendsnorthto NW. HereTertiaryplutonshavebeen
unroofed,a Cretaceouspluton at D hasbeenunroofedandthe entire
sequenceof Tertiary volcanicrocksis missingexposingMesozoic
sedimentaryrocks.
Evidencesuggestthat the present-dayDixie Valley fault zone
beganto formduringOligoceneto earlyMiocenetime andmayhave
undergonetwo phasesof extension.First, the fact that sericite-clay
and older alterationmineral assemblagesoccur in a narrow band
along the present-dayfault trace suggeststhat a zone of intense
fracturingand hydrothermalalterationwith a geometryroughly
similarto thatof theQuaternaryfaultzoneformedduringOligocene
to early Miocene time. Second,palcofluid entrapmentpressures
estimatedfrom measurementsof high-temperaturefluid inclusions
indicate a minimum footwall uplift of 6 km. This is 3-4 km more
verticaldisplacementthanthe post-13-17ma displacementestimate
of 3.5-2.2 km [Wallace and Whitney,1984; Okaya and Thompson,
1985], suggestingthat the early phaseof vertical displacementwas
similarin magnitudeto thatof the laterphase. Notably, the post-13~
17 Ma displacementis estimatedfrom offset of basaltsthat were
extrudedover a fairly flat surface. A period of tectonicquiescence
musthave occurredbetween'theearly andlater phasesof extension
acrossthe DixieValley fault zone. Third, we note that the Dixie
Valley fault traceis highly sinuous,with two strongmaximain fault
temperaturesof secondaryinclusionsassociatedwith the later
laumontite-prehnite
assemblage
is 100øC,salinityrangesfxom2.0
to 16.0 NaC1equivalentwt %, andno CO2 wasdetected.
Fluid pressureand temperaturewithin this segmentof the
Wasatchfault evolvedalonga path from lithostaficto hydrostatic
with continueddisplacement
of the footwallrelativeto thehanging
wall. Age constraints
areprovidedby a 17.6+0.6Ma K-Ar ageof
hydrothermal
sericitefrom a samplewith meanTh of 309øC, and
7.3 to 9.6 Ma fission track ages of apatite (closuretemperature
120ø_+15øC).
Temperature
andpressure
estimates
suggest
post-17.6
Ma fault displacement
is 11 km at a thermalgradientof 30øC/km
[Parry and Bruhn, 1987].
The majortectonicconsequence
of theporefluid andchemical
reactions observed on the Wasatch
fault is reduction of effective
stressby pore fluid pressure.Elevatedpressures
are enhancedby
chemicalreactionsthat seal the tectonicallyinducedcracksin the
rock. The mechanicalbehavior of the rock is also modified by
formationof chlorite and sericitewith epidotein the fault rock at
deeperlevels,andvein fillingsof zeolite,calcite,andclayminerals
at shallower
levels.
A contemporary
faultsystemthathasbeenexaminedin detaildue
to bonanzaeconomicmineralization,the Comstockfault at Virginia
City, Nevada,showsmany similaritiesto the Dixie Valley fault.
The Comstockfault is a majormineralizedcrustalstructure135km
southwestof Dixie Valley. This fault displacesMiocenevolcanic
rocks 762-1219 m [Vikre et al., 1988] and is mineralizedover 4.6
km of strikelengthandto 1000m belowthepresentsurface.PostMiocene displacementis less than 183 m, and the fault is not
presently
active.Fault-related
alteration
andfluidcharacteristics
date
from the earlier episodeof Basin and Rangeextensionwithout
superimposition
of youngerhydrothermal
events. Alterationwtfich
strike, one toward the WNW to NNW and the other NNE to NE
is associated
with preciousmetal veins and the Comstockfault
(Figure 9). Perhapsthis patternis partly inheritedfrom the earlier, includesstrongsilicificationadjacentto the Comstockfault with
Oligoceneto early Miocene phaseof extension,when deformation accompanying
sericite,chlorite,andpyrite. Near lode margins
wasconcentrated
on WNW strikingstrike-slipfaultsandsubsidiary quartz,sericite,chlorite,montmorillonite,
andpyriteoverliequartz+
normalandnormal-oblique-slip
faults,manyof whichtrendedNNE sulfidelodes.K-feldspar,albite,sericite,chlorite,andcalcitelocally
to NE, forming a rectangularfault patternwhen observedin map occur with sulfides.
view [Riehle et al., 1972; Hudson and Gelssmart, 1987; Boden,
Chlorite,calcite,epidote,albite,quartz,and pyrite comprisea
1986]. Presumably,rotationof extensionfrom N68øE to S65øE propylitic assemblage
with no closeassociationto the Comstock
about10 Ma enhancedmovementon NNE to NE trendingnormal fault. A younger albite, natrolite, stilbite assemblage,and an
faults,relativeto oldersetsof WNW to NNW trendingfaults. assemblage
of quartz,alunite,pyrophyllite,
andclayarealsonot
Nevertheless,the distributionof hydrothermalalterationminerals, faultrelated.
andtheinternal
structure
of theDixieValleyfaultzoneindicate
that
Fluidscirculated
atleast2000m belowtheMiocenepalcosurface
partof theoverallstructural
geometry
is inheritedfromOligocene at temperatures
from about250ø-300øC,withsalinities
of 1-6 %
andearlyMiocenedeformation
andhydrothermal
circulation.
N aC1andup to 2.1 wt % CO2. Fluidswith recognizableCO2
contain <3.2% NaC1, fluids with no CO2 contain 0.4-6.1% NaC1,
andfluid inclusionswith daughterNaC1andKC1contain23% NaC1
and 47% KC1.Boiling andmixing wereinvokedby Vikre [1989] to
Two well-studiedcontemporary
fault systemsin theGreatBasin accountfor variable solutioncompositions,but the CO2-bearing
provide valuable comparisons.The Wasatchfault in the eastern inclusionsweredeterminedto be laterthanthosewith no CO2.
Great Basin [Parry and Bruhn, 1986;Parry et a/.,1988] and the
The Wasatch,Dixie Valley, and Comstockfaults shareseveral
COMPARISONS WITH
CONTEMPORARY
Nevada Comstock fault in western Nevada
FAULTS
[Vikre, 1989] are
normalfaultswith displacement
historiesbeginningin theMiocene.
common
characteristics.
Fault
fluids
contain
disolved
salts and
CO2, homogenizationtemperaturemodesare 250ø-300øC. Fluid
Exhumation of the footwall at 'the southern end of the Salt Lake
pressureson the Wasatch and Dixie Valley faults varied between
segmentof the Wasatchfault has exposedhydrothermallyaltered lithostatic and hydrostaticpressures. Alteration minerals in the
chlorite,epidote,sericite,
graniticrocks. Vein filling and pervasivealterationmineralogy footwall includethe commonassemblages
includes,from earliestto latest,biotite-K-feldspar,chlorite-epidote- prehnite,laumontite,zeolite,silicification,andclay.
19,746
PARRYETAL.:DIXIEVALLEYFAULT,NEVADA
CONCLUSIONS
REFERENCES
Detailedmapping,petrographic,
andfluidinclusionworkon the Anderson,R. E., M. L. Zoback,and G. A. Thompson,Implicationsof
selected subsurface data on the structural form and evolution of some
Dixie Valley fault and comparison
with the Wasatchfault and
basinsin the northernBasin and Rangeprovince,Nevada and Utah,
Comstockfaultsuggests
a modelfor BasinandRangeseismogenic Geol. Soc. Am. Bull., 94, 1055-1072, 1983.
faults from which the relationshipof fault behaviorto fluid Bateman,R. L., andJ. W. Hess,Hydrologicinventoryand evaluationof
characteristics can be derived.
Nevada playas,Proj. Rep. 49, 137 pp, Water Resour.Cent., Desert
Res.Inst., Univ. of Nev., Las Vegas, 1978.
Hydrothermally
alteredfootwallrockshavebeenexhumedby
casestudy, northern
faultdisplacement
anderosionon theDixie ValleyFault,Nevada. Bell, E. J., et al., Geothermalreservoirassessment
Much of the footwall of the 1954 rupture segmentconsistsof
BasinandRangeprovince,northernDixie Valley, Nevada,report,223
pp., Mackay Miner. Res. Inst., Univ. of Nev., Reno, 1980.
Tertiary
agegranitic
rocks
thatprovide
alumino-silicate
parentBell,J. W. andT. Katzer,
Surficial
geology,
hydrology,
andlate
mineralsfor the alterationandconstrain
themaximumageof the
Quaternary
tectonics
of theIXL Canyon
area,Nevada,
Bull.Nev.Bur.
alteration to be less than 28 Ma.
MinesGeol.102,51 pp.,1987.
Overlappingepisodesof
hydrothermal
mineral
formation
wereproduced
asdisplacement
of Bell,J.W.,andT.Katzer,
Timing
oflateQuaternary
faulting
inthe1954
Dixie Valley earthquakearea,centralNevada,Geology,18, 622-625,
thefaultprogressed.
Thesemineralassemblages
arefromearliest
to
1990.
latest: hydrothermal biotite+K-feldspar, chlorite+epidote, Benoit, W. R., Carbonatescalingcharacteristicsin Dixie Valley, Nevada
sericite+kaolinite+smectite,
prelmite+laumontite+stilbite+clay,
and
geothermalwell bores,Geothermics,18, 41-48, 1989.
chalcedony,opal, smectite, alunite,andkaolinite. Micro- Bird, D. K., and H. C. Helgeson,Chemicalinteractionof aqueous
thermometric measurement of fluid inclusion characteristicsindicate
solutions
with epidote-feldspar
mineralassemblages
in geologicsystems,
II, Equilibriumconstraints
in metamorphic/geothermal
processes,
Am.J.
a diversefluid chemistry,temperature,
andpressure
of entrapment. $ci., 281,576-614, 1981.
Hydrothermalalterationmineralsandcharacteristics
of secondary Boden,D. R., Eruptivehistoryandstructuraldevelopmentof the Toquima
fluid inclusionstrappedin healedmicrofractures
in igneousquartz
calderacomplex,centralNevada, Geol. $oc. Am. Bull., 97, 61-74,
1986.
placeconstraints
onfluidpressure
andtemperature
duringfaulting.
Fluid inclusionsin hydrothermally
alteredfootwallrocksof the Bodnar R. $., T. $. Reynolds, and C. A. Kuehn, Fluid-inclusion
systematics
in epithermalsystems,in Geologyand Geochemistry
of
Dixie Valley fault, Nevada indicate that pore fluid pressure Epithermal Systems,Rev. Econ. Geol., vol. 2, editedby. B.R. Berger
fluctuatedfrom 1570 to 350 barsin the temperaturerange350ø200oc. Scatterin thepressure
estimates
at constant
temperature
is
andP.M. Bethke,pp. 73-97,.Societyof EconomicGeologists,
E1Paso,
Tex.,1985.
springwaterchemistry
in
interpreted
aspaleo-fluidpressure
transients
at depthsof up to 3-5 Bohm,B., Possiblerelationsbetweenanomalous
the StillwaterRange,andthe Dixie Valley geothermalsystem,northern
km on theDixie Valley fault.The pressure
transients
aregreatestin
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