Economic Geology
Vol. 60, 1965, pp. 1411-1421
HYDROTHERMAL
ALTERATION
IN GS-3 AND GS-4
DRILL HOLES, MAIN TERRACE, STEAMBOAT
SPRINGS, NEVADA •
ROBERT
SCHOEN
AND
DONALD
E. WHITE
ABSTRACT
Hot spring waters at SteamboatSprings;Nevada,depositsmall
amountsof mercury,antimony,gold,and silverand hydrothermally
alter
the rocksthroughwhichthey flow. Two patternsof alterationcan be
recognized
in the granodiorite
bedrockunderlyingthe Main Terrace:an
ill-defined
patternof alterationrelatedto depth,anda moredistinctpattern consisting
of zonesof variousmineralassemblages
envelopingfractures. The alterationmineral assemblages
relatedto fracturesresemble
the alterationmineral assemblages
in granodioriteof the Butte, Montana,
coppermining district.
Alteration relatedto depth includesthe replacementof someplagioclaseby hydrothermalK-feldsparin the upper300 feet of the springsystem, and the formation of albite and calcite from calcic plagioclaseat
depthsgreaterthan about200 feet.
Near fractures,hornblendeis the mostreactivemineral and is replaced
by randommixed-layerillite-montmorillonite
and iron-richchlorite. Bio-
tite alters pseudomorphically
to chlorite. Calcic plagioclasealters to
mixed-layer illite-montmorillonite.Plutonic K-feldspar and quartz are
generally unaffected.
The mostintensealterationis immediatelyadjacentto fractureswhere
all feldspars
andpreviously
formedchlorite,mixed-layerillite-montmorillonite, and calciteare replacedby illitc (with lessthan 10 percentinterlayeredmontmorillonite),quartz,andpyrite.
The K+/H + ratio in the deep high-temperaturewaters suggeststhat
the waters are in equilibrium with K-mica as determinedby Hemley's
reaction studies. Therefore, the present-dayspring waters may still be
producingthe observed
hydrothermalalteration. This hypothesis,
coupled
with the proven ability of thesewaters to depositmetals, indicatethat
the springwatersare similar to someore-formingfluids.
STEAMBOAT
SPRINGS,
NEVADA,is an area of hot springactivityin southern
WashoeCountynear the California-Nevada
boundary(Fig. 1). Because
mercury,antimony,gold, and silver are being depositedby the hot waters,
the springshavelong beenof interestto economic
geologists
(7, 2, 15, p.
110-113). The possibilitythat the thermalwaterswill be usefulin characterizing ancientore-formingsolutionsis enhancedby the nearnessof the
springsystemto the celebratedgold-silverdepositsof the ComstockLode
and to epithermalmercurydeposits(13).
A summaryof pastgeologicstudiesin the area was givenby Branno&
and others(2), and White and others(16). The alterationmineralogyin
Publication authorized by the Director, U.S. GeologicalSurvey.
1411
1412
R.SCHOEN
ANDD.E.WHITE
N
,•
EXPLANATION
NEVADA
st#me,oat
sm•n•s
I----'--"1
•
Z
GS-3
':
FIG.1. Generalized
geologic
map'of
Steamboat
Springs
thermal
area,Washoe
County,
Nevada.(Fromdetailed
map,
D.'E.Whiteandothers,
1964.)
corefromfourdrillholes
at Steamboat
Springs
wasdescribed
bySigvaldason
andWhite(11,12). Thepresent
report
describes
thealteration
mineralogy
in the'corefromtwomoredrillholes
in theMainTerrace,
themostactive
partof thearea,andconsiders
brieflytherelationships
between
thealteration
and the presentspringwaters.
HYDROTHERMAL
ALTERATION
IN DRILL HOLES
1413
The patternof rock alterationobservedat SteamboatSpringsis similar
to that reportedfrom Butte, Montana, and other famousmining districts,
and we considerit probablethat the present-daywaters are still producing
this alteration. Therefore, in spite of the fact that the metal depositsat
SteamboatSpringsare not rich enoughto be classedas ore (2), the presentday thermal waters may be similar to someore-formingsolutions.
The basementrocks of the thermal area consistof a granodioritepluton
of late Mesozoicage intrudedinto metasediments
and metavolcanics
of probable early Mesozoicage. Tertiary and Quaternaryvolcanicrockswere extruded on the eroded surface of this basement, and andesite dikes were
intruded into the granodiorite. Alluvium and silicioussinter were deposited
intermittentlyduring and after the last volcaniceruptions. The generalized
geologyand the locationsof drill holes, springs,and fissure systemsare
presentedin Figure 1. The drill holesdescribedin this report, GS-3 and
GS-4, intersectonly granodioritebedrock,below the surficial alluvium and
sinter.
METHODS
The mineralogyof the rockswas determinedby X-ray diffraction,microscopicstudy of thin sections,and chemicaland spectrographicanalyses.
X-ray mineralogicanalysiswas performedon 5 gram samplesground dry
for 15 minutesand packedin standard2 cm aluminummounts. Clay minerals were concentrated
by centrifugingan aqueoussuspension
and making
two successive
withdrawalswith a pipette of a small amount of suspension
containingthe lessthan 2 • size material (3).
The <2 • >0.5 • fraction
was then sedimentedon a glassslide for X-ray diffraction analysis
Significantmineralogicvariationsin the drill core were determinedprimarily by X-ray methods. X-ray diffractogramsof three samplesof fresh
granodioritefrom outsidethe hot spring area were the basis for a semiquantitativeevaluationof changesin mineralogyeffectedby alteration.
A strongX-ray peak, mostnearly free from interference,was studiedfor
eachmineral. The maximumpeak heightfor a mineral, on diffractograms
of the three fresh granodiorites,was equatedwith the averagevolume percentageof the mineral reported by modal analyses. The maximum peak
heightwas usedso that any reductioncouldbe related directlywith degree
of alteration. A further checkon the peak heightof quartz was obtainedby
calibratingagainstknown artificial and natural rock standards. The amount
of quartz determinedagainstknown standardswas 2 to 4 percent less than
that reportedby modal analyses.
Comparisonof the maximum X-ray peak height in the fresh granodiorite
with the peak height in altered granodioritegave a proportion that was
assumedto be linearly relatedto the percentageof the mineral presentin the
alteredrock. We are aware that this assumptionis not strictly valid and for
this reason calculated abundances are considered to be no more than semi-
quantitative. The volumepercentage
increaseof all the alterationphaseswas
considered
to dqualthe volumepercentage
decrease
of all the primary phases
1414
R. SCHOBN .4ND D. B. I/FHITB
andthe distribution
of alteration
phases
wasagainconsidered
to be linearly
related to peak heights.
AlthoughmodalandX-ray analyses
of the threesamples
of freshgranodioriteindicatedthat quartzwas a relativelyconstant25 percent,X-ray
resultsfor quartzfrom piecesof almostfresh drill core rangedfrom 20 to
30 vercentbasedon the knownstandards. Two samplesGS-3-513 and GS-3-
685 gavevaluesof 16 and 18 percentquartz,respectively.As thin sections
showthat quartzis not replacedduringhydrothermal
alteration,this slight
variationin quartz valuesis probablyrelatedto the difficultyof selecting
representativerock chips from small piecesof diamonddrill core. For this
reason,X-ray peaksof quartz from alteredrockswere interpretedas signifying25 percentof the rockif theyfell withinthe limitsof 16 to 30 percent
notedfor the fairly freshgranodiorite. Only whenthe quartzpeaksexceeded
theselimits was a greaterpercentageof the rock consideredto be quartz.
This samereasoningcannotbe appliedto the other primary mineralsin
the. rocks becausethese mineralsare all replacedto some degree by the
alteration. The accuracyof this method is undoubtedlylow; however, its
ability to pick up major and even minor changesin mineralogyhas been
verified in this study.
MINERALOGY
Fresh granodioritefrom just outsidethe springarea containsplagiodase
feldspar(oscillatoryzoningnear andesinewith rims of oligoclase),quartz.
orthoclase,biotite, hornblende,and chlorite in decreasingorder of abundance.
The minor chloriteindicatesthat eventhis granodioriteis not entirelyfresh.
Granodiorite within the spring area is altered in relation to its distance
from flow channelsand presumablyto the size and length of time eachflow
channelhas existed. In addition,there is an ill-definedpattern of alteration
relatedto depth. However, eventhe freshestgranodioritefrom the diamond
drill holes is devoid of hornblendeexcept sporadicallynear the surface in
GS-1 drill hole (11, p. Dl17). Hornblendeis clearly the most unstable
mineral in the hydrothermalenvironment.
Drill Hole GS-$.--The fissuresystemof the Main Terrace, along which
alterationis mostintense,dips to the east,away from GS-3 (Fig. 1; detailed
log in White andothers,1964,table3). Hence,the granodiorite
is relatively
fresh except near fractures. Abundancesof mineralscomputedfrom the
X-ray data are plottedsemiquantitatively
in Figure 2. Major variationsin
percentages
of the mineralsoccurat shearzonesand veins,whichare indicatedalongthe right sideof Figure 2. Samplingwasnot sufficiently
closely
spaced
to defineall zonesof intensealteration.
Alteration controlledby fracturescan be divided arbitrarily into a less
intensezone of argillization away from fractures and an intenselyaltered
zone of sericitizationadjacent to fractures. The intensity of alteration is
consideredhere to be the degree to which hydrogen metasomatismhas
affectedthe rock (6).
Thin sectionsshow that in the argillic zone biotite is first bleachedand
HYDROTHERMAL
ALTERATION
IN DRILL HOLES
1415
then is replacedby chlorite. The chlorite replacesthe bleachedbiotite
pseudomorphically
withoutany visiblechangein volme and with the same
crystallographic
orientation.The morecalciccoresof the zonedplagioclase
APPROXIMATE PERCENTAGES
APPROXIMATE
P[RCENTAGES
0
O• PRINCIPAL MINERALS
25
50
I
I
o•,.'•.•.• •,
2.2
•0 Mm
75
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M(•i•
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cmq4•niq
II•, K-f,d•, W/
OF PRINCIPAL
MINERALS
I9O
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Lw•e*
/•
•
/
16
25
50
75
I
I
I
19O
W•ler
' tible
Opaline
sinter
with•-cdstoMlite
increasing
w•th depth
Alluvium containing quartz, K-feldspar, plagk)-
ciasa
andillite,pyrite,
kaolinite,
andl•-cristohalite
139
105
172
141
149
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505
GS-3
GS-4
Fro. 2 (Left) Semiquantitative
plot of mineralogyfrom drill hole GS-3, Main
Terrace, SteamboatSprings. Straight lines arbitrarily drawn betweenpointsof
observation.
Ch = chlorite.
Fro. 3. (Right) Semiquantitative
plot of mineralogyfrom drill hole GS-4,
Main Terrace,Steamboat
Springs. Straightlinesarbitrarilydrawnbetweenpoints
of observation.
1416
R. SCHOEN AND D. E. WHITE
crystalsare replaced
by fine-grained
wormymixed-layerillite-montmorillonite
with low birefringenceand a low index of refractionof about 1.53 or less.
Quartz and orthoclase
are unalteredin the argilliczone.
In the sericiticzoneadjacentto fractures,
onlyillite (sericiteof Hemley
and Jones,1964, p. 542) quartz, and pyrite appearto be stable. Finegrainedillite with lessthan 10 percentinterlayeredmontmorilloniteand with
fairly high birefringence
is produced
from the mixed-layerillite-montmorilloniteof the argilliczone. Illite alsoreplacesall remainingplagioclase
and
some orthoclase;where carried to completionall orthoclaseis replaced.
Chlorite,an alterationproductitself,is replaced
pseudomorphically
by coarsegrainedwhite micathat is probablychemically
similarto the illite, and by
pyrite. Chalcedonic
silicaformsin the sericiticzone,presumably
from silica
that is releasedby alterationof silicatesin boththe argillicand sericiticzones.
The intensealterationof the sericiticzone overridesall lesserdegreesof
alterationincludingthat relatedto depth.
Most of the fracturesenclosedby sericitizedgranodioriteare filled with
calcite,chalcedony,
and quartz. Abovethe depthof 174 feet calciteis absent
from theseveins,evidentlybecauseof the increasingsolubilityof calcitewith
decreasing
temperature.
Away from fractures,in the argillic zonethat constitutes
mostof the rock
in GS-3 drill hole,certainmineralogic
changes
relatedto depthcanbe defined.
At depthsgreaterthan about200 feet the calciccoresof plagioclase
crystals
are sometimes
replacedby albitethat is crystallographically
parallelor nearly
parallelto the originalsodicrims. Patchesof calcitealsoare foundreplacing
the coresof calcicplagioclase
crystalsat depthsgreater than about200 feet.
Thesemineralsoccurin additionto, ratherthaninsteadof, mixed-layerillitemontmorillonite.
Abovea depthof about300 feet someplagiodaseis replacedby hydrothermal K-feldsparin a mannersimilar to, but muchlessextensivethan, that
describedby Sigvaldasonand White (12) in the upper part 9f GS-5 drill
hole. The hydrothermalK-feldspar is not distinguishedfrom original
orthodasein Figure 2.
As previouslystated,the intensealterationof the sericiticzonedestroys
the alterationmineralsof the argillic zoneas well as the alterationminerals
relatedto depth. However, the degreeof developmentof the sericiticzone
is itself related to depth, being somewhatweaker near the surface. For this
reason,completereplacementis not alwayseffectedin shallowsericiticzones.
X-ray diffractograms
of the <2 t• fractionshowthat the hydrothermal
clay
rangesfrom a well crystallized
illite (with lessthan 10 percentinterlayered
montmorillonite)
with a 10 angstromperiodicitythat is relativelyunaffected
by heatingto 550øC for 1 hour and glyclolation,
to a randommixed-layer
illite-montmorillonite
with up to 45 percentmontmorillonite
layers. The
mixed-layer illite-montmorilloniteyields X-ray reflectionssimilar to those
reportedby MacEwan (8, p. 167). This includesa reflectionat about30
angstroms
withoutintegralsub-orderreflections.This phenomenon
hasbeen
interpreted(14, p. 217; 8, p. 167) asdueto somestatistical
regularitywithin
a long sequence
of layers.
HYDROTHERMAL ALTERATION IN DRILL HOLES
1417
Away from zonesof intensealteration,mixed-layerillite-montmorillonite
with a largepercentage
of montmorillonite
layersis the abundantclaymineral.
As zonesof intensealterationare approached,the percentageof montmorillonitelayersdecreases. In the zonesof intensealterationillire with lessthan
10 percentinterlayeredmontmorillonite
is the only clay mineral. Theseconclusions,derivedfrom X-ray diffractograms
of highlyalteredand lessaltered
piecesof core, are corroboratedin thin sectionsby a general increaseof the
lowestindex of refractionof the mixed-layerillite-montmorillonite
with more
intense alteration.
Although the changefrom mixed-layer illite-montmorilloniteto nearly
pure illire seemsto be continuousin the samplesstudied, two apparently
distinctphasesare indicatedby X-ray diffraction in somesamples. Therefore, illite and mixed-layerilIite-montmorillonite
are representedby separate
fieldsin Figure 2, but the short-dashed
boundarybetweenthem indicatesour
uncertaintyof the nature of the difference.
The upper part of drill hole GS-3 passesthrough hot spring sinter and
arkosicalluvium. Some of these rocks were altered in the past by strong
sulfuricacid solutionspercolatingdown from the then-existingsurfaceto the
water tablein existenceat that time. The sulfuricacid was formedby oxidation of hydrogensulfidegasthat separatedat the water tablefrom the rising
springwaters. Similar acid conditionsexist locallytoday on the Main Terrace, particularlyadjacentto fissures,and an outstandingexamplehas been
described from GS-7 drill hole in the Silica Pit area I mile west of the Main
Terrace (12).
The silicioussinter depositsconsistentirely of amorphousopal at the
surfacebut increasingamountsof/•-cristobalite (high cristobalite)occurat
depth,formedby crystallizationof opal. The /•-cristobaliteis distinguished
by a strongX-ray peaknear4.10 angstroms. Arkosicalluviumconsisting
of
disintegratedgranodioriteand fragmentsof volcanicand metamorphicrocks
is composeddominantly of quartz, K-feldspar, and plagioclase.Where
leachedby acid the alluvium containsquartz, kaolinite,/•-cristobaliteand small
amountsof pyrite, illire and rarely alunite.
For the purposeof examiningthe alterationeffectof risingwaterson the
rocks,the near-surfaceacid leachedzone must be ignored.
Drill Hole GS-4.--The mineralogyof drill corefrom GS-4 is plottedon
Figure 3. The observedmineralsand the pattern of alterationis similar to
that of GS-3. However,becauseGS-4 intersectsmany closelyspacedfracturesrelatedto the easterlydippingmain zoneof fissuresthat controlup-flow
of thermalwaters (Fig. 1), the rock masshas beenmore pervasivelyaltered.
This is indicatedin Figure 3 by the virtual absenceof biotite and replacement
calciteand the relative abundanceof pyrite, hydrothermalquartz, and illite
with little interlayeredmontmorillonite. Original plagioclaseis relatively
rare in GS-4, andmuchchloritethat replacedbiotitehasin turn beenreplaced
pseudomorphically
by white mica.
An anomaloussituationis found near the bottom of GS-4. Intensely
alteredcorefrom a depthof 476 feet containingillite, quartz,and pyrite is
devoidof K-feldsparand yet containsabundantalbiticplagioclase.A similar
1418
R. SCHOENAND D. E. WHITE
situation
occurs
at a depthof 505feetalthough
K-feldspar
andchloriteare
notcompletely
replaced.Theretention
of plagioclase
in theintensely
altered
zoneof sericitization
hasnotbeennotedelsewhere
at Steamboat
Springs.
CEOCaE•STR¾
OF A•-TER.•0N
Twenty-five
rockanalyses
madeby standard
rapidmethods
(10) were
available
for studyof therocksfromGS-3andGS-4. Analyses
recalculated
to constantrock volumeshowthat K20, S, H20, and someCO2 are added
duringalteration,and SiOn,Na=O, and someCaO are lost. The lossesand
gainsof thesechemical
components
canbe relatedin detailto thechanges
in mineralogypreviouslydescribed.
'Hornblende,
the mineralmostsusceptible
to alteration,
is replaced
by
mixed-layer illite-montmorilloniteand iron-rich chlorite. Sodium and cal-
ciumprobably
first go into montmorillonite
and theninto solutionduring
alteration
andareremoved
fromtherock. Somecalcium
formscalciteduring
this initial stageof alteration.
Chlorite,whichtypicallyoccursin pseudomorphic
replacement
of biotite,
is green and pleochroicand generallyexhibitsan anomalousbrown interferencecolor. However,interference
colorsfrom somechloriterangefrom
anomalousblue up to first order red. The chloriteflakeswith red interference
colorsare length-slow
andare interpreted
to be iron-rich(1). Presumably
potassium
and silicaand possiblysomemagnesium
are lost and water added
during the alterationof biotite to chlorite. The releasedpotassiumis
probablyfixednearbyin mixed-layer
illite-montrnorillonite
that is replacing
plagioclase
feldspar. Analysesindicatethat the illite is phengiticwith the
magnesiumpresumablyderivedfrom biotite and hornblende.
The alterationof plagioclase
feldsparto mixed-layerillite-montmorillonite
andcalcitefollowsthe oftenobserved
patternof greatestinstabilityfor calcic
plagioclase
followedin the mostintensealterationby the moresodicplagioclase. Silica,calcium,and somesodiumare releasedas productsof this
alteration.
The mostintensealterationobservedin GS-3 and GS-4 producesillire
from mixed-layerillite-montmorillonite
by the fixationof potassium
from the
hydrothermalfluid. K-feldspar also alters to illire in this environment.
The pseudomorphic
replacementof chlorite by coarse-grained
white mica
(sericite) releasesiron which combineswith sulfur from solutionto form
pyrite. This causesan inverserelationshipbetweenchlorite and pyrite.
Direct replacementof biotite by sericite,without an interveningstageas
chlorite, has not been demonstrated.
Silica is releasedto solutionduring the alterationof all the primary
silicateminerals
andchlorite. Analyses
indicate
thatintense
alterationslightly
reducesthe amountof SiO• in a rock,but figures2 and 3 showthat quartz
increases
considerably
in the intenselyalteredrocks. This paradoxcan be
explainedby the fact that about75 percentof the freshgranodioriteis composedof mineralsthat yield somesilicato solutionduring alteration. Much
of this silicaprecipitatesin the intenselyalteredrocksadjacentto veins,thus
HYDROTHERMAL
ALTERATION
IN DRILL HOLES
1419
explainingtheir higher quartz content. But somesilica escapesfrom the
rock either to precipitatein the veins or to be carried off in the rising
solutions. This lossof silicaexplainsthe lower SiO2 contentof rocksfrom
the most intenselyaltered zone.
The patternof alterationobservedin GS-3 and GS-4 is very similar to
that reportedfrom Butte, Montana (9). At Butte, mild alterationyieldsa
zone of montmorilloniteand chlorite,and intensealterationdestroysall
original and early hydrothermalmineralsand yields sericite,pyrite, and
quartz. However, at Butte thesetwo zonesare separatedby a zonein which
montmorillonite
and chlorite are converted to kaolinite.
In GS-3
and in
GS-4, no intermediate kaolinite zone is found.
Comparisonof the mineralogyof GS-3 and GS-4 with GS-1, 2, 5, and 7,
describedby Sigvaldason
and White (11, 12), indicatesgenerallygood
agreement.The agreementis especiallygood for GS-5, which is on the
Main Terrace425 feet north of GS-4. The kaolinitereportedat depthin
GS-1 on the Low Terracemay be equivalentto the intermediatekaolinitezone
of Butte. We donotyet understand
why kaoliniteis developed
soextensively
in the Low Terrace, apparentlyunrelatedto oxidationof H2S above the
watertable,butis absentin thedeepalterationof the Main andHigh Terraces.
The mineral assemblage
illite-quartz-pyrite,found in the sericiticzone
immediately
adjacentto fractures,is inferredto be in stableequilibriumwith
the deephydrothermalfluid that movedupwardalong the fractures. The
composition
of this fluid can be inferredfrom experimentaldata on aqueous
fluidsin equilibrium
with illite andcanbe compared
with present-day
hydrothermal fluids of the SteamboatSpringssystem.
At the temperatureof the rocksin the lower part of the drill holes(about
170ø C), illite wouldbe stablein a solutionwhosemolal ratio of K + to H* was
between108'4and 10•'• in the presenceof quartzand at a total pressureof
1,000 atmospheres.These valuesare extrapolatedfrom the experimental
data.Hemleygot from 200øC to 500øC (6 p. 548, and 5, p. 245). The
effect of the much lower pressurein the drill holes (from 10 to 25 atmospheres)is presumablyto movethe stabilityfield of illite to slightlylower
K*/H*
ratios.
An average
valuefor K* in watersfromdrillholesGS-3,4, and5 is 65 ppm
or a molalityof 1.67 X 10-8. Water at depth,with all CO2 in solution,has
about650 ppm total CO•.. Thermodynamic
calculations
basedon theoretical
valuesfor thedissociation
constants
of H2COaat hightemperature(4) indicate
a molalityof H* of 7.94 x 10-* for this waterat 170ø C. Thesecalculations
take into accountthe activitycoefficients
of the hydrogenand bicarbonateions
at 170ø C. The effect of sodiumnot balancedby chloride,which must be
balancedby bicarbonate,
is also includedin the calculations.Using these
valuesthe K*/H* molal ratio of the present-day
thermalwatersat depth
is 10TM. Thisvalueliesin thekaolinitestabilityfieldcloseto theextrapolated
boundarybetweenkaoliniteand illite. However,the effectof lowerpressm'e
wouldplacethis K+/H" valueslightlywithin the illite stabilityfield.
Similarly,the calculatedlga*/H* molal ratio in the present-daythermal
watersis 104'*,a value alsolying in the extrapolatedkaolinitestabilityfield
1420
R. SCHOEN AND D. E. WHITE
but closeto the boundarybetweenkaoliniteand montmorillonite
(6, p. 549).
The effect of lower pressurewould be to favor the formationof montmorillonite over kaolinite. However, montmorilloniteis not found closeto frac-
tures,whichmustmeanthat the Na+/K+ ratio in the hydrothermal
fluid is not
high enoughto make stablea sodium-richphase.
The fact that montmorillonite
becomes
a larger proportionof the mixedlayer clay with increasingdistancefrom the fractures, indicatesthat the
Na+/K + ratio of pore fluidsis increasingaway from fractures. This increase
in Na+/K + is causedby: a decreasein K + due to the formationof illite in the
sericiticzone, and an increasein Na+ due to the breakdownof plagioclase
feldspar. Orthoclase,the dominantpotassium-rich
mineralin the fresh rock,
doesnot break downuntil it is encroached
on by the sericiticzonewhere it
providesK + to the pore fluids, and results, perhaps,in a slightly higher
t(+/H + ratio than in the adjacentfracture.
Therefore, becauseillite appearsto be a stable phasein the most altered
rocks of drill holesGS-3 and GS-4, and becausethe water presentlyin the
spring systemhas a calculatedcompositionin which illite shouldbe stable,
we concludethat the present-dayspring waters are producingthe observed
hydrothermalalteration. This hypothesis,
coupledwith the provenability of
thesewaters to depositmetals,'indicatethat the spring waters are similar
to fluidsthat haveformedsomeore deposits.
ACKNOWLEDGMENTS
Theauthors
benefited
fromdiscussions
ofthecalculation
ofhydrogen
ion
concentration
at elevatedtemperatures
with: G. G. Ehrlich, I. Barnes,W. L.
Polzer, and J. J. Hemley. Critical review was graciouslyprovidedby ]. ].
Hemley and L. J. P. Muffler.
U.S. GEOLOGICAL
SURVEY,
MENLOPARK,CALIFORNIA,
Jan. 20, 1965
REFERENCES
1. Albee, A. L., 1962, Relationshipsbetweenthe mineral association,chemical compositionand
physical properties of the chlorite series: Am. Mineralogist, v. 47, p. 851-870.
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HYDROTHERMAL
ALTERATION
IN DRILL HOLES
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12..
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