Economic
Geology
Vol. 92, 1997,pp. 578-600
Secondary
PreciousMetal Enrichmentby Steam-Heated
Fluidsin the Crofoot-Lewis
Hot SpringGold-Silver
DepositandRelationto Palcoclimate
SHANEW. EBERTk*
KeyCentrefor Strategic
MineralDeposits,
Department
of Geology
andGeophysics,
University
of YVestern
Australia,
Nedlands, Western Australia 6907, Australia
AND ROBERT O. RYE
U.S.Geological
Survey,P.O. Box25046,Mail Stop963,DenverFederalCenter,Denver,Colorado80225
Abstract
The Crofoot-Lewis
depositis an adularia-sericite-type
(low-sulfidation)
epithermal
Au-Agdeposit,
whose
well-preserved
palcosurface
includes
abundant
opalinesinters,
widespread
andintense
silicification,
bedded
hydrothermal
eruption
breccias,
anda largezoneof acidsulfatealteration.
Radiogenic
isotope
agesindicate
thatthe system
wasrelatively
long-lived,
withhydrothermal
activitystarting
around4 Ma andextending,
at
leastintermittently,
for the next3 m.y.
Field evidence indicates that the surficial zone of acid sulfate alteration formed in a steam-heated environ-
mentwithinan activegeothermal
system.
A dropin thewatertableenableddescending
acidsulfatewaters
to leachAu andAgfromzonesof low-grade
disseminated
mineralization,
resulting
in theredistribution
and
concentration
of Au andAgintoore-grade
concentrations.
Thesezonesof secondary
Au-Agenrichment
are
associated
withopal+ alunite+ kaolinitc+ montmorillonite
_ hematiteandweredeposited
in openspace
fractures
at, andwithina fewtensof metersbelow,the palcowater
table.
The stableisotopesystematics
of aluniteandkaolinitcin the steam-heated
environment
are relatively
complex,
duetovariations
in theresidence
timeof aqueous
SO4thatformedfromtheoxidation
of H2Sprior
to precipitation
of alunite,andthesusceptibility
of fine-grained
kaolinires
to hydrogen
isotope
exchange
with
laterwaters.Mostof thealunites
areenriched
in 34Srelative
to earlysulfide
minerals,
reflecting
partialS
isotopeexchange
betweenaqueous
SO4and H2S.Abouthalf of the alunitesgivereasonable
calculated
z•SOso4_ou
temperatures
fora steam-heated
environment
indicating
O isotope
equilibrium
between
aqueous
SO4andwater.The6DH2o
values
of thehydrothermal
fluidsvariedby almost60 perrailoverthelifeof the
meteoric
water-dominated
system,
suggesting
significant
climatechanges.
Mineralization
isbelieved
tohaveresulted
fromlarge-scale
convection
ofmeteoric
watercontrolled
largely
bybasinandrangefractures
anda highgeothermal
gradient
withH2SforAucomplexing
derived
fromorganic
matterin basinsediments.
A wet climateresultedin the formation
of a largeinlandlakewhichprovided
abundant
recharge
waterfor the hydrothermal
system.
A fluctuating
watertablecontrolled
by changing
climaticconditions
enabledsteam-heated
acidsulfatefluidsto overprint
lowergrademineralization
resulting
in ore-grade
precious
metalenrichment.
Introduction
amajorrolein thegenesis
ofvarious
metal,industrial
mineral,
and
strategic
fuel
deposits
(Cronin
et
al.,
1983),
but
only
THE Crofoot-Lewismine is an adularia-sericite(Heald et al.,
recently
hasa linkbeensuggested
between
palcoclimate
and
1987)or low-sulfidation
(Hedenquist,
1987)typeepithermal epithermal
systems
(BergerandHenley,1989).The Great
Au-Agdeposit,locatedin northwestern
Nevada,nearthe Basinof thewesternUnitedStatesissusceptible
to theaccuabandoned
townof Sulphur(Fig.1). Badiometric
agesshow mulation
of largevolumes
of surface
waterduringperiods
of
that the hydrothermal
system
wasyoung(4.0-0.7 Ma) and abundant
precipitation,
andlargeinlandlakeshaveperiodiconsequently
manyfeatures
of thepalcosurface
remain,in- callyformedwithinthe GreatBasinat leastasfar backas
cludingsinter,beddedhydrothermal
eruption
breccias,
and late Tertiarytimes(Forester,1991).Theseinlandlakesexsurficial
acidsulfatealteration.
CurrentAu-Ag(Hg) mining pandedandcontracted
according
to palcoclimatic
conditions
by HycroftResources
andDevelopment
Incorporated
com- (Smith,1984;Forester,1991),reflectingmajorchanges
in
mencedin 1987,with provenandprobablereserves,
asof the regionalwatertable.Changes
in the levelof the water
October1994,of 57.8milliontons(Mr) grading
0.65g/t Au tablecanhavea majoreffectonthe surficial
environment
of
and containing
31,185kg Au. Miningis by openpit and activeepithermal
systems,
resulting
in the overprinting
of
cyanide
heap-leach
methods.
Currentannualproduction
is differentalteration
assemblages
(Simmons,
1991).
approximately
2,835kg (100,000oz) Au, 8,500kg (300,000 In thispaperwedescribe
indetailtheacidsulfate
alteration
oz) Ag,and13,600kg Hg.
andusefieldrelations,
combined
with lightstableisotope
It haslongbeenrecognized
thatpalcoclimate
hasplayed data,K-Ardating,palcoclimate
records,
andgeologic
constraints,
topropose
thatfluctuations
in thewatertableduring
theformation
of theCrofoot-Lewis
deposit
enabled
acidsul• Corresponding
author:
email,[email protected]
to overprint
weaklymineralized
near-surface
* Presentaddress:
Hot SpringGoldCorporation,
4650SierraMadre#813, fate alteration
Reno, Nevada 89502.
silicification,
resulting
in theleaching,
redistribution,
andsec0361-0128/97/1936/578-2356.00
578
CROFOOT-LEWIS
HOTSPRING
Au-Ag
DEPOSIT,
NV
119'30'
579
Tertiaryrocksare abundantin the surrounding
ranges,
118'15'
and includevolcanic,voleanielastie,intrusive,and fresh-water
Sleeper
41'15'
-tog Ranch
>z
Sulphur
(abandoned)
o
ø
e.>
HUMBOLDT
COUNTY
ß-r'
PERSHING
COUNTY
...
I I ;rofoot/Lewis
Mine
the basin 12 km north of the Crofoot-Lewismine (Garside
et al., 1988). The late Cenozoic (Miocene-Holocene) basin
5•' Rosebud
Gerlach
Irnlay
Florida
Canyon
40'30'
5•'Standard'-
Wind
5•'
Mountain
o 5 Io 15
sedimentary
rocks(Willden,1964).Igneousrocksare commonandrangein composition
fromolivinebasaltto rhyolite.
Sedimentary
rocksareabundant
in the downdropped
basins
that formedduringthe development
of basin-and-range
structure
(Stewart,1980).The sedimentary
rockswithinthe
BlackRockDesertbasinadjacent
to theoredeposit
arenot
exposed.
A general
description
oftheserocksisbasedondrill
cuttings
froma 2,417-m-deep
oil exploration
well drilledin
5•'
Trinity
F,•
Rochester
Nevada
sediments
andsedimentary
rocksin the BlackRockDesert
basincontaintracesof oil andgas(Garsideet al., 1988)and
areinterpretedto be 2,160m thick.From 1,600to 1,775m,
therocksarepredominantly
poorlyconsolidated
tounconsolidatedsiltstone.
From1,775to 2,160m, thecuttings
aretuffs,
clays,shale,sandstone,
conglomerate,
marl,andlimestone,
locallycontaining
tracesof pyrite.From2,160m to thebottom of the hole (2,417 m) volcanicrockswere encountered.
Unconsolidated
sedimentary
materialof Plioceneand
youngerage(<6 Ma) formswidespread
alluvialfan,stream
terrace,floodplain,valleyflat,andplayadeposits
thatgrade
from coarsegrainednearthe rangefrontsto fine silt and
clayin thevalleys.
Largeinlandlakesoccupied
manyof the
present-day
valleysof Nevadaduringthe Pleistocene,
and
manyof the presentlyexposed
playaandshorelinedeposits
originated
in theselakes(Morrison,1964).
Deposit
geology
A seriesof subparallel
basin-and-range-type
normalfaults
offsetrocktypesandcontrolmuchof thenear-surface
mineralization
andalteration
in thedistrict(Figs.3, 4). Thesefaults
north(N 15ø-30øE) anddip55øto 85ø
FIG. 1. Locationmapof the Crofoot-Lewis
mine,HumboldtCounty, strikeapproximately
Nevada,
showing
adjacent
precious
metaldeposits.
to the west.Northwest-andwest-striking
transferor relay
faultslocallycause
nilnoroffsets
in orezonesandalongnorthondary
enrichment
of Au andAg.In addition
we propose
a strikingfaults.
Theoldestrockunitin theSulphur
district
istheMesozoic
geneticmodelfor the formation
of younghot springgold
deposits
in theGreatBasin,
whichhasimportant
implicationsAuldLangSyncGroup,whichis presentbelowthe deposit
for exploration,
andto our knowledge,
hasnot beenpre- (Fig.4) andcropsout5 km eastof the minesite.Here the
unit consists
of laminatedslateand siltstone.
In places,the
viouslyproposed.
unitishighlycontorted
andfolded,andgraphiteis abundant
Geology
alongbedding-foliation
planes.
overlyingthe Auld LangSyncGroupis
A moredetailed
description
ofthegeology,
mineralization, Unconformably
group(informally
namedby
andsecondary
Au enrichment
at the Crofoot-Lewis
mineis the TertiaryKammaMountains
of predominantly
eastgivenin Ebertet al. (1996),andonlya brief summary
is Wallace,1987).Thisgroupconsists
dippingfelsicvolcanic,
volcaniclastic,
andpyroelastic
rocks.
presentedhere.
Rhyolitelavaflows(locallydomes),
withwell-developed
flow
Regional
geology
banding,arecommon
in the sequence.
Minordaciticto anThe regional
geology
surrounding
the Sulphur
districtis desiticrocks,andrhyoliticflowbreccias
andtuffaceous
units
dominated
by late Cenozoicbasin-and-range
extensionaloccurlocally.
faulting.The majorrocktypesin the regionhavebeen The KammaMountains
groupis overlainby the Sulphur
mapped
anddescribed
byWillden(1964)andJohnson
(1977) group(informally
namedby Wallace,1987),whichconsists
andaresummarized
in Figure2. Triassic
to Jurassic
rocksof of Pliocene
to Pleistocene
sediments
andsedimentary
rocks.
theAuldLangSyncGroup(BurkeandSilberling,
1974)are The Sulphurgroupis dividedintothe LowerSulphurgroup,
widespread
through
theregion.
TheAuldLangSyncGroup theCamelconglomerate,
theCrofootbreccia,
andsinterdeisa thicksequence
ofvariably
metamorphosed
metasedimen-posits.
TheLowerSulphur
groupconsists
offine-grained
silttary rocks,consisting
of phyllite,slate,metasiltstone,
fine- stones,tuffs,sandstones,
and minorpebbleconglomerates.
grained
quartzite,
andlocalhornfels
andispartof theJungo TheCamelconglomerate
isa poorlysortedmatrix-supported
Mesozoic
allochthonous
terrane(e.g.,Oldow,1984).
conglomerate
with clastsup to 25 cmin size,averaging
1.5
580
EBERTAND RYE
Qal
Qal
Hot Spdng
Black
Rock
Desert
t
Qal
Crofoot/Lewis
Valley
Mine
Oal
.:•
40ø45 '
Qal
Qal
Oa,
119000 '
118030 '
Quaternary
•
Undivided
alluvium
includes
gravel,
alluvial
fan,
landslide,
playa,
dune
and
stream
deposits
Quaternary-
Tertiary?
[-• Pliocene-Pleistocene
basalt
and
Tertiary
_-• Sedimentary
felsic
volcanics
and •[.:• Rhyolite
porpydtic
rhyolite
rocks • Undivided
and
sedimentary
rocks'"• Basalt
andesite
Cretaceous
['• Granodiodte
TriassicJurassic?
Jungo
:/..:• Phyllite,
slate,
fine
grained
quartzite,
homfels
(Auld
Lang
Syne
Group?)
•- Terrane
Permian-
Triassic?
N
[]•Undivided
volcanic
and
sedimentary
rocks
tBlack
Rock
"[-•Intermediated
10
km
tobasic
volcanic
rocks
(Happy
Creek
Group)Terrane
I 8miles
' ,
Permian
y
Interpreted
position
ofMesozoic
Terraneboundary
(thrust
fault)
FIG.2. Generalized
regional
geology
of south-central
Humboldt
andnorth-central
Pershing
Counties.
Geology
from
Willden(1964)andJohnson
(1977).
cm.Theelasts
in thisunitarederivedfromtheAuldLang ehaleedonie
silica
interbedded
withsilieified
CamelconglomSyneGrouportheKammaMountains
group,andlithifieationerate.Sinterbedsrangefrom30 cm to severalmetersin
isa productofhydrothermal
alteration.
Crofootbreeeiarefers
thickness and occur interbedded with Crofoot breeeia and
tohydrothermal
eruption
deposits
thatarepervasively
altered Camelconglomerate,
havinga maximum
cumulative
thickandabundant
nearthesurface
attheCrofoot-Lewis
deposit.nessof about45 m. Individual
sinterbedsarefiatlyingand
Thesinterdeposits
consist
of bedsof massive
opaline
and extendforhundreds
of meters.
Thesinters
typically
contain
CROFOOT-LEYVIS
HOTSPRING
Au-AgDEPOSIT,
NV
581
t
Oiatamaceous
Bay area /
.......
, ..",•
Lewis
Pit
i' '_T_L--..'--
i'•-•_.. "1/1^ ^
•
Qa
\
/
•
Pit
/
i
^
•
^
Kamma Mountains
/
/ GroupVolcanics
ii
i
x
1:
,4
I
i
^ iI
Gap
Proposed
SouthCentral
Qal
I
^
,• A
I__-
I
Qal
Brimstone
Pit
•
I•
•
A'
• Location
ofsections
• f Normal
fault
^
i
- dashed
where
inferred
<
i
•
Alluvium
•
I:::::::::1
Conglomerate/Croloot
Bmccia
•
;.?...;•
Basal
acid
leach
altered
Croloot
Breccia
i
I
•
i--.r--.rl
OpaI-K-teldspar
altered
Camel
Qal
^ • '• ! I++1
Conglomerate
•
4 ,•
•
/
Illlte-smecltte
altered
Camel
g ,• < 4 •,'•
•
\
\
*`*`\ ....
,,%
x\
/
/
•11ver uamel
xxX
,•,\ •.
Hill
-,x,•..
\ •.
'•
,,.. - ,,
,
a
..........
oa•
-'.<.%.;
/ limitofknown
Approximate
surlace
and
subsurface
alteration
•--.• ...
^
.•
•.
Conglomerate
(partially
overprinted
by
blanket
leach
locally)
•
Mordenite-type
alteration
(overprinting
sinter
and
opal-K-feldspar
altered
Camel
Conglomerate
+ Crotoot
Bmccia
\. /^/ ^ ,•
,.
r=l
alteration
I• I Mordenite
(altering
talus/alluvium)
•, a < 4 • Weakly
silicifled
and
argillized
Camel
x•
I-'- I Conglomerate
+Crofoot
Bmccia
I(2•1Ore
zone/open
pit
\
,,•
\^
;--;--.-•......
,
900m
,
'
ooo.
"
•...
FIG. 3. Simplified
mapof the Sulphurdistrictshowing
structure,
hydrothermal
alteration,
ore zones,andlimitof
hydrothermal
alteration.
abundant
opalizedaquaticreedcaststhat are oftenin an
HydrothermalAlteration
uprightorgrowth
position.
Thepresence
ofopalized
aquatic Introduction
plantmaterialandthefiat-lying
andextensive
natureofthese
deposits
indicate
thattheyareshallow-water
lacustrine
depos- Figures3 and4 summarize
the distribution
of hydrotherits,probably
analogous
to the subaqueous
"poolsinter"de- malalteration
in theSulphur
district.
Hydrothermally
altered
scribedat BuckskinMountain,Nevada(Vikre, 1985).
rocksareextensive
in the district,covering
an areagreater
582
A
A
A'
West
East East
Albert
Oal Fault /f-16oom
Boneyard
OalFault
5000 ft-
Oal
4000
ft-
Central
Fault,/
Fault
* * •- > <'
oOoO • •
•'
oø • • •
• v•
•'
•
•
• • -1300
m
3500 ft_
roo
-•
Alluvium
,•-'• Kamma
Mountains
Group
•'• Auld
Lung
Sync
Group
B A
A'
West
EastEast
Bone'
ard
Albert
Q•I Fault
m
•" .• -16oo
Y
Fault
Central
Fault
O,a,
4L
....•,'•
•
....' ":::::
i:i:i,
i:!:i:i:iii:i:.
!! •<-1450
m
.+
I--
-///-\4--//T-?'17
t---I f
.' '. I V*11 ....
:F
\''''''
i
'++v^••ed•
-1300m
.
Y• + •
i !
>
'
•<Grøu•
p•c• ^
I
3000 ft-
F[o.4. Cross
section
through
theCrofoot-Lewis
deposit
showing
(A)rocktypesand(B)hydrothermal
alteration.
than3 by7 kmatornearthesurface
andextending
todepths propylitic
alteration
ispoorly
defined.
Themineralogy
ofthe
greater
than600m,thelimitof deepdrilling.
Therearefive propylitic
zoneis quartz+ K feldspar
+ chlorite+ calcite
maintypesofhydrothermal
alteration
in theSulphur
district: + illitc + marcasite
+ pyrite+ leucoxene
_+siderite.
(1) propylitic,(2) interstratified
illite-smectite
andillitc, (3)
opal-Kfeldspar,
(4) montmorillonite
+ mordenite
(zeolite), Opal-Kfeldsparalteration
alteration
covers
a largenear-surface
area
and(5) acidsulfate.
Briefdescriptions
of propylitic,
opal-K Opal-Kfeldspar
feldspar,illite-smectite,
and montmorillonite
+ mordenite (Fig.4), rangingfromabout60 to 213m thick.Thisalteration
alteration,
followed
bya detailed
description
of acidsulfate ischaracterized
byintense
silicification
consisting
ofopaland
alteration,
aregivenbelow.
chalcedony,
withbetween
15and50percent
K feldspar
and
minor
amounts
of
marcasite
+
pyrite
_
stibnite
_
leucoxene
Propylitic
alteration
and/or
rutfie.Potassium
feldspar
inthiszonereplaces
volcanic
Propyliticalterationoccursin the KammaMountains clasts
andfineparticles
in the matrixandis 4xtremel¾
finegroup,mainlylateralto, andlocallywithin,zonesof illite- grained,
withindividual
crystals
lessthan0.1/.tinih size.
smectite or illitc alteration. The extent and distribution of Opal-K
feldspar
alteration
contains
pervasive
low-grade
Au-
CROFOOT-LEWIS
HOTSPRING
Au-Ag
DEPOSIT,
NV
583
Agvalues.Minorveinsof bandedchalcedony
_+calciteoccur termed"blanketacidsulfatealteration,"
andassteeplydipwithinopal-Kfeldspar
alteration.
pingveinsandzonesthatextendbeneaththe blanketzone
A blackto darkbrown,opaque(somethinedgesarered- intotheunderlying
opal-Kfeldspar
alteration,
termed"acid
dish-brown to dark brown) bitumen, with low reflectance sulfate
veins"(Figs.5 and6B).
(lessthanquartzor calcite)andbluefluorescence
occursin
opal-Kfeldspar
alteration
andquartzandcalciteveins.The Blanketacidsulfatealteration
Blanketacidsulfatealteration
is verticallyzoned,with an
term"bitumen"is hereusedto describe
solidor liquidorganiematerial
thathasmigrated
fromitssoume
to formsolid upperzoneof intensely
leachedmaterialconsisting
of residhydrocarbon
(Parnell,1993).Bitumen
isgenerally
notvisible ualquartzandcristobalite,
termed"blanketacidleach,"and
maeroseopieally,
although
a fewerustiform-eolloform
banded a thinlowerzoneof intenseopalization,
termed"basalacid
chalcedony
veinscontaindarkbitumen-rich
bandsthat are leach"(Fig. 5). Blanketacidleachvariesin thickness
from
dearlyvisiblein handsamples.
Veinsamples
fromnearthe zeroto 100m andconsists
mainlyofa barren,bleached
white,
present
surface
to over425m in depthcontain
traceamounts skeletal
residueof whitepowderyquartz+ cristobalite,
with
to over10percentbitumen,averaging
between
2 and4 per- lesseramounts
of opal-A,alunite,natroalunite,
kaolinitc,
nacentbitumen.Bitumentypically
occursasdisseminations
in tive sulfur,gypsum,
cinnabar,andjarosite(Fig. 6C). This
chalcedony,
opal,quartz,orcalcite,
asvugfillings,
andlocally zoneispoorlyconsolidated
andhasa highporosity;
however,
as discrete bitumen-rich bands in eolloform or erustiform
despitethe intenseleaching,
originalrocktextures
arewell
bandedveins.A sampleof coarse-grained,
darkbrownto preserved
in manyareas.The intensity
of acidsulfatealterblackcalcite
contains
anestimated
20percent
bitumen
along ationgenerally
increases
towardmajorfaultsandextends
to
cleavage
surfaces
anddisseminated
withincalciterhombs
and greaterdepthsalongsomeof thesestructures.
Scattered
remmostlikelygetsits colorfromthe abundance
of bitumen.
nantsof unleached
opal-Kfeldsparalterationoccurwithin
blanketacidleach(Fig.5), indicating
thatblanketacidleach
Illite~smectite alteration
alteration
hasoverprinted
opal-Kfeldsparalteration.
An inBelowthenear-surface
opal-Kfeldspar
alteration,
theSul- tervalofhematite-stained
alluvium,
ranging
froma fewcentifur groupsediments
areintensely
argillized
to interlayeredmeters to 7 m in thickness,is common at the interface of
illite-smeetite
+ quartz+ pyrite_+ehlorite_+illitc_+calcite acidleachalteration
andoverlying
alluvium.
+_kaolinitc
_+pyrrhotite
(Fig4).Thisillite-smeetite
alteration Withtheexception
ofcinnabar,
sulfides
donotoccurwithin
ispervasive
throughout
thesediments
andextends
morethan blanket acid sulfate alteration. Cinnabar coats native sulfur
600 m belowthe surface.An envelopeof illitc alteration crystals
or occurs
disseminated
in intenseacid-leached
mate(<5% interstratified
smeetite)
+ quartz+ pyrite_+ehlorite rial.Nativesulfuroccurs
in beds,lenses,
andfracture
fillings
+ calciteis developed
around
steeply
dippingsilieified
fault within the blanket acid leach zone, and in someareas,native
zones.This envelopegradesoutwardawayfrom the fault sulfurfillsopenspacefractures
alongfaultzonesbelowthis
zoneinto illite-smeetite
alteration
with increasing
smeetite zone.Smallhorizontal
bedsandoval-shaped
elongate
pods
composition.
Calciteveinsandveinletslocallycut the illite- of fine-grained
alunite+ kaolinitc+ silica_+montmorillonite
smeetitealteration,andsmall(30-50/•m) rhombsof calcite occurlocallyin theupperpartof theblanketacidleachzone
aredisseminated
in theillite-smeetite
+ quartzmatrix.
(Fig.5). Theseappearto be remnantmudpoolswhichare
Montmorillonite
+ moralehire alteration
common at the surface in areas of steam-heated acid sulfate
springs
in geothermal
systems
(e.g.,Waiotapu,
NewZealand;
Montmorillonite
+ mordenite
alteration
occurs
mainlyin Hedenquist
andBrowne,1989).Remnantmudpoolsare esthenorthwest
Sulphur
district
(Fig.3),whereit partially
over- peciallyabundant
in the upperportionof the Boneyard
pit
printsopal-Kfeldspar
alteration
andsinterdeposits,
andalso wheretheyoccurashorizontal
bedsandlenses
offine-grained
altersunconsolidated
or poorlyconsolidated
alluvium.
This alunite+ kaolinitc+ opal-C+ opal-CT_+montmorillonite
alteration
isrestricted
tothenearsurface,
typically
extendingup to 0.9 m thickand15 m in length(Fig.6D). Thesebeds
nodeeperthan50 m belowthebaseof alluvium.
Thisassem- aretypically
surrounded
above,below,andlaterallyby opablageistypically
oxidized
andconsists
mainlyof montmoril- lizedor kaolinized
hydrothermal
eruptionbreeeia.
lonite, the zeolite mordenite(Na2,K.•,Ca)[A12Si100•4]
ß7H•O,
Basal acid leach alteration
and chalcedony
or quartz.Lesser,but locallysignificant,
Basal acid leach alteration occurs as a subhorizontal unit
amounts
of interlayered
kaolinite-smeetite
andkaolinitcare
present.Minoramounts
of jarositeandaluniteoccurwithin atthebaseofblanketacidleachalteration
(Fig.5). Basalacid
narrowfractures
cuttingmordenite-type
alteration,
arevery leachalteration
rangesfromlessthani m to over12 m thick
finegrained,
andappeartobelate.Wheremontmorillonite
+ andcommonly
contains
tracesto ore-grade
Au (averaging
up
mordenite
alteration
hasoverprinted
earlieropal-Kfeldspar to 0.6 g/t) andrelatively
littleAg.Basalacidleachalteration
alteration,
mareasite
andpyritehavebeenoxidized.
hasoverprinted
earlieropal-Kfeldsparalteration,as evidencedby remnantpatchesof opal-Kfeldsparalteration
Acidsulfatealteration
within this zone. Basal acid leach alteration is characterized
Acidsulfate
alteration
isa typeofintense
advaneed-argillie
by denseopalization,
normally
whiteor grayin color,but
alteration
containing
alunite+ kaolinitc
+ quartz(Hemley maybe a varietyof colorsincluding
redwherehematiteis
andJones,1964).Acidsulfatealteration
is extensive
andwell abundant
in the matrix(Fig.6G).Typically,
bothmatrixand
preserved
at the Crofoot-Lewis
mine,andoccursbothasa elasts
arealteredcompletely
to opal-A,opal-CT,or chalcedwidespread
blanket
thatcapsa largeportionof thedeposit, ony,withthe originalbrecciaor conglomerate
textures
pre-
584
EBERTAND RYE
Remnant
podofopal-K-feldspar
alteration
withmoderateto low
Nativesulfurin mounds
AuandAg(0.3g/tAu,4.2g/tAg)
Remnant
mudpools
andalong
fractures
CentralFault
noAuorAg
Blanketacidleach
variable
AuandlowAg
•
(0.3
g/t
Au,
<0.3
g/t
Ag)
ß ,,....•
Basalacidleach
Acid-sulfateveins/zones
high
AuandAg(1.4g/tAu,
28.2g/tAg)
Pods
orfragments
ofopal-
K-feldspar
altered
rock
which
arecoated
by,orcompletely
altered
to,white
opal
moderateto low
AuandAg(0.3
g/tAu,4.2g/tAg)
Blanketacid-sulfatealteration
[•
Acid-sulfate
alteration•
Blanket
acid-leach:
residual
quarlz
+cristobalite
+ alunite,
natroalunite,
kaolinite,
opal,
gypsum
cinnabar,
hematite,
andjarosite
Basal
acid-leach:
massive
opal
+alunite,
kaolinire,
hematite
Acid-sulfateveins/zones
E• Acid-sulfate
vein:
alunite
+kaolinite
+opal
+ natroalunite,
hematite,
goethite,
jarosite
__
•
Opal-K-feldspar
altertion:
Opal
+chalcedony
+ K-feldspar
+ pyrite
+ marcasite
+stibnite
I
10rn
I
FIC.5. Schematic
vertical
section
through
theupper
alteration
zones
attheCrofoot-Lewis
mine,
illustrating
thevertical
zonation
andprecious
metaldistribution
withinacidsulfate
alteration.
Average
Au-Ag
grades
in parenthesis.
served.Locally,alunite+ kaolinite+ hematitearedominant m, andall of theveinsnarrow
withdepth.Mostacidsulfate
in the matrix.Basalacidleachalteration,
although
wide- veins do not extend more than 100 m below the base of the
spread,
isnotalways
present
andistypically
poorly
developedblanketacidleachzone,although
a fewveinsextendmore
or absentin the highlyfractured
rocksoverlying
zonesof than200mbelowthiszone.These
veinsshow
a crudespatial
mineralized
acidsulfate
veins(Fig.5).
andtemporal
zoningfromearlyopal(commonly
opal-CT)to
Acidsulfateveins
alunite + kaolinitc to late montmorillonite. Veins with white
opalselvages
andalunite+ kaolinire
interiors
occurlocally
zonesof acidsulfatealteration,
white
At thebaseof theblanketacidsulfate
alteration,
steeply (Fig.5). In pervasive
dippingveinsandzonesof opal+ alunite+ kaolinitc+ opalcommonly
coatsfragments
thataresurrounded
byalumontmorillonite
___
natroalunite
_+hematite_+goethite_+ nite + kaolinire
+ montmorillonite,
andlateveinsof pink
jarosite
___
antimony
oxides
_+pyrite(atdepth)fillopen-spacemontmorillonite
locally
cutthesezones.
In places,
acidsulfate
fractures
andbrecciated
zones
in theunderlying
opal-Kfeld- veinsfill thecenterof earlierformedquartzor chalcedony
sparalteration.There is a continuous
transitionfrom the
blanket acid leach zone (and basalacid leach zone where
veins.
At deeperlevels(>70 m depth)someof the acidsulfate
developed)
to the underlying
acidsulfateveins,sincethe veinscontain
fine-grained
pyriteandlocally
traces
of marcaveins extend from the base of the blanket acid leach zone site.The pyriteisveryfinegrained
(<0.1 mm)or occurs
as
but do notcutit (Fig.5). Acidsulfateveinsasthickas2.4 small cubes that are disseminated or form bands within or
m exist,but mostrangefroma fewmillimeters
to about0.5 adjacent
toalunite,
natroalunite,
opal,and/orquartz,
andka-
CROFOOT-LE•VIS
HOTSPRING
Au-Ag
DEPOSIT,
NV
585
olinitc.Severalof thesepyriteshaveanomalous
As,Ni, and overprinted,
andoxidized,
by near-surface
montmorillonitemordenite
alteration.
Oxidation
of thisorestyleis indepenlocally,Co.
Somesteeplydippingacidsulfateveinscontainsections dentof structure,
andlocallyis beddingcontrolled
andis
withpreserved
horizontal
bedsandlaminations
of veryfine referredto as"horizontally
controlled."
Oxidizedorein this
grainedmixturesof opal + alunite+ kaolinitc(Fig. 6E). zonecontains
coinparable
gradesasthe underlying
unoxiSome of these horizontal laminations contain structures indicdizedmineralized
opal-Kfeldspar
alteration,
andthereareno
of precious
metalremobilization
or enrichment.
ativeof softsediment
deformation,
aswall-rockfragmentsindications
havefalleninto veinsand deformedthe laminations
(Fig.
6F). Thesetextures
indicatethattheveinsformedasopen- Mineralizedacidsulfatealteration
spacefillingsandwerefilledfrointhebottomup. Thissug- The majority
of oreminedto dateconsists
of steeply
dipgests
thatmuchoftheopal+ alunite+ kaolinitc
+_montmo- pingacidsulfate
veinsandzones
thatfillopen-space
fractures
rillonitein theseveinsprecipitated
fromsolution.
However, belowtheblanketacidleachzone(Figs.5 and7).TheLewis,
pitsareexamples
of this
alteration
of preexisting
minerals
suchasfeldspars
to alunite Gap,SouthCentral,andBrimstone
Areasofmineralized
acidsulfate
alteror kaolinitc
is apparent
in theveinselvages
andin partially typeofmineralization.
overprinted
podsof opal-Kfeldsparalterationthat occur ationhaveformedwithinopal-Kfeldspar
alteration
thathas
within blanket acid leach alteration, basal acid leach alter- beenintensely
fractured,
predominantly
in the hanging
wall
ation, and mineralized acid sulfateveins or zones.
of the Centralfaultwithina well-defined
grabenstructure,
Alunite froin the blanket acid leach zone and the acid
whereoreis continuous
for at least3 kin. Zonesof pervasive
consistmainlyof white opaline
sulfate
veinsistypically
whiteandgenerally
veryfinegrained; acidsulfatemineralization
in someof the deeperacidsulfateveins,alunitehasa light (+ chalcedony)
fragments
thataresurrounded
by alunite+
pinkcolor.Alunitecrystals
arelathor rhombshaped,
or less kaolinitc+ opal-chalcedony
+ montmorillonite.
The white
commonly,
cubicor anhedral.
Mostalunitesamples
contain fragments
aretypically
opalcoated
and/oropalized
fragments
grainsthatrangein sizefrom< 1 to about20/•m; however, of earlieropal-Kfeldsparalteration.
Hematiteandgoethite
themajority
ofgrains
areapproximately
thesamesize.Within +_jarositegivethezoneitsredand/oryellowhues.Pervasive
the blanketacidleachzone,alunitelathsandrhombsrange acidsulfatemineralization
givesway at depthto narrow,
fromi to 18/•min length,averaging
5/•m. In theacidsulfate steeplydipping,mineralized
acidsulfateveins.
veins,aluniterangesfroinlessthan i to 15 /•m in length, Figure7 showsa wall frointhe southfaceof the South
andthe deepest
veinsampleis alsothe coarsest.
AssociatedCentralpit andis anexample
of themineralized
acidsulfate
kaolinRe
crystals
are typicallyveryfine grained,predomi- alteration
currently
beingminedat the deposit.
In thisarea
nantlyi/•m or less.Scanning
electronmicroscope
analysesthebulkoftheorebody
consists
ofnumerous,
steeply
dipping,
revealthatseveral
purealunites
(K endmember)andnatroa- mineralized
acidsulfateveinswhichare widelyspaced
but
lunites(Na end member)occur.However,manyalunites haverelatively
highgrades,
making
thembulkmineable.
Indicontainsignificant
amounts
of Na, andmanynatroalunitesvidualacidsulfateveinshavevaluesup to 63 g/tAu,withan
containsignificant
amounts
of K. Mostalunitesandnatroa- average
grade(20veinsamples)
of6.4g/tAu.TheAgcontents
lunitescontain
nodetectable
Fe or Ca,although,
in a fewof of theseveinsreachvaluesover157 g/t andare typically'
the deeperacidsulfateveins,aluniteor natroalunite
is en- severaltimeshigherthanthe Au content.Adjacentunoxirichedin Ca at theexpense
of K or Na.
dizedopal-Kfeldspar
alteration
averages
only0.27g/tAuand
3.43g/tAgthrough
thissection,
indicating
thattheacidsulMineralization
fateveinsaresubstantially
enriched
in Au andAg relativeto
Introduction
the surrounding
opal-Kfeldspar
alteredrock.The zoneasa
Several zones of economic mineralization have been dewholehasanaverage
gradeof 0.89g/tAu.In individual
acid
finedat the Crofoot-Lewis
mine,including
the Bayarea, sulfateveins,the Au-Agcontentis highestnearthe topand
withdepth.
Lewispit, SouthExtension
pit, Gappit, SouthCentralpit, decreases
Scanning
electronmicroscope
analyses
showsthat Au in
Boneyard
pit, andBrimstone
pit (Fig. 3). Mineralization
is
asmicron-size
electruingrains,
withinand
hosteddominantly
by Cainelconglomerate,
Crofootbreccia, thesezonesoccurs
Most
and KammaMountains
groupvolcanic
rocks.Unoxidized,adjacentto opal,alunite,kaolinitc,or montmorillonite.
of the electruin associated with acid sulfate mineralization
mineralized,
opal-Kfeldsparalteration
extends
overa large
31 percentAg.
areaandconstitutes
a hugeAu-Agresource(>116,000kg hasbetween27 and35 percentAg,averaging
(AgC1)andlesscommonly
Au; Ebert,1995),but is uneconomic
dueto its low-grade Silveralsooccursascerargyrite
(AgI)associated
withalunite,kaolinite,
orjarosite.
(0.27g/t Au and4.15g/t Ag) andrefractory
nature.Higher iodargyrite
grades
occurwithinthisunit,andthe Bayareacontains
a Mineralized basal acid leach
refractory
resource
of about7 Mt at a gradeof 1.20g/tAu.
A thickened
portionof mineralized
basaladd leachhosts
Economic
near-surface
Au-Agmineralization
occurs
in four
pit.Thisoreischaracterized
byCrofoot
styles
(Ebertetal.,1996);horizontally
controlled
near-surfaceorein theBoneyard
ore, mineralizedacidsulfatealteration,mineralizedbasalacid breeeia in which both elasts and matrix have been altered to
whiteopal+ alunite+ kaolinitc___
hematite.Texturesand
remnants
of mineralized
opal-Kfeldsparalteration
indicate
Horizontally
controlled
near-surface
ore
mineralized
basalacidleachhasoverprinted,
and altered,
Themajority
of orein theBayareacamefrommineralized mineralized
opal-Kfeldspar
alteration.
mineralized
basalacid
opal-Kfeldsparalterationandinterbedded
sinterthat was leachaccounts
for a smallportionof the oreat the Crofoot-
leach,andlatequartz-chalcedony
veins.
586
E•ERT,4iVD RYE
lm
i
.
G
.•
3
i
5
6
1
8
i
Di
CROFOOT-LEWIS
HOTSPRING
Au-Ag
DEPOSIT,
•/
587
B
West
C/L32 {O,IMa)
Remnantmudpools
C/,L
7
.• •
CIL
9(2.1
Ma)
/
C/L
,1{2.4
Ma}
1400 m
:
1300m
C/L
12
(1.2
Ms)C,L15
C/L16
C/L
28
C/L
10
• ..•
(1.$
, [•
•
Central
Fault
!
East
Blanket
add-leach
Opal-K-feldspar
alteration\•
.[•
ß Mineralized
acid-sulfate
[• Illite-srnectite
alteration
'•'•
veins/zones
• x sample
location (2.4
Ma)K-Ar
age
iC/L
7 Sample
number •
6{2.0
Ma)
HorizontaJ
laminations
100
m
F•c. 7. Simplified
east-west
cross
section
throughthe SouthCentralpit showing
alteration,
sample
locations,
andKAr ages.An enlargement
of anacidsnlfate
veincontaining
horizontal
laminations
is alsoshown.
Rocktypes(notshown)
consist
dominantly
of conglomerates
andbreccias.
1994)andweredifficult
to reconcile
withthe
Lewismine,consisting
of 2,250,000t grading0.55 g/t Au, pers.commun.,
withrelatively
lowAgvalues.
geology
of thedeposit
andthetimingof acidsulfate
alteraLatequartz-chalcedony
veins
Mineralizedquartz_ chalcedony
veinsandveinletscut
tion.Potassium-Ar
and4øAr-a"Ar
agedataaresummarized
in
Table 1.
Examination
ofopal-Kfeldspar
alteredbreccia
byscanning
andinfiltrateblanketacidleachalteration,
opal-Kfeldspar electron
microscope
revealed
thatK feldspar
in thebreccia
alteration,
andfilite+ quartz+ pyritealteration
in theBrim- matrixisstoichiometric
whereas
K feldspar
replacing
thevolstonearea.Theseveinlets
createlocalized
zonesof ore-grade canicbreccia
clasts
contains
significant
amounts
of CaOand
mineralizationwithin the blanket acid leach zone, a feature
Na20 whichincreasetowardthe centerof the dasts.Presum-
uniquetotheBrimstone
area.Mineralization
withintheblan- ably,the presence
of CaOandNa20in K feldspar
within
ket acidleachzonecouldaccount
for up to halfof the eco- thevolcanic
clastsis a resultof incomplete
replacement
of
nomicmineralization
withinthe Brimstone
pit, whichcon- magmatic
feldspar
byhydrothermal
K feldspar,
andthisintainsabout39 Mt at a gradeof 0.58g/t Au.A mineralizedcomplete
replacement
mayaccount
for the olderthanexstockwork
containingsimilarveinletsoccursbeneaththe pectedK-Aragesobtained
fromthebreccia
samples
(Ebert,
Brimstone
pit;however,
its distribution
andextentis poorly 1995).
defined.
To resolve
the dilemma,
laserprobe4øhr/3•hr
datingof
hydrothermal
K feldspar
withinthematrixandwithinclasts
Ageof Mineralization
ofopal-Kfeldspar
altered
breccias
wasperformed.
K feldspar
Ageof opal-Kfeldsparandillite-smectite
alteration
inthebreccia
matrix
yielded
4øhr/3•Ar
ages
of3.8+_0.09and
C/L77andC/L 78),whereas
K feldUntilrecently
theageofopal-Kfeldspar
andillite-smectite3.9 _ 0.3 Ma (samples
clasts
yieldedagesof 10.6+_0.17and
alteration
at theCrofoot-Lewis
minewasenigmatic.
Unpub- sparin the volcanic
C/L 79 andC/L 81).TheagesoblishedK-Aragesof K feldspar
obtained
fromopal-Kfeldspar 14.9_+1.6 Ma (samples
to represent
the age
alteredbrecciagavedatesranging
fromEarlyto LateMio- tainedfromthe matrixareinterpreted
alteration
whereas
theolderagesobtained
cene(samples
C/L 74, 75, and76; D. JohnandT. McKee, of opal-Kfeldspar
F•c. 6. A. Viewof the Crofoot-Lewis
depositfromthe valleyfloor,facingeasttowardthe pedimentandrange.B.
Viewof the southwallof the SouthCentralpit facingsouthwest.
Blanketacidleachalteration
(lightcolor)occurs
likea
blanketabovetheorezone(darkercolored
) thatextends
belowthebaseof thepit.Thebenches
are6 m high.TheSilver
Camelhill,a partially
exposed
sfiicified
faultzoneformsthe ridgein the background.
C. Blanketacidleachalteration,
southeast
wallof southern
mostextension
of the SouthCentralpit. Benches
are6 m high.D. Closeup of blanketacid
leachalteration
containing
horizontal
podscomposed
of fine-grained
alunite+ silica+ kaolinitc,
interpreted
asfossilized
mudpools.E. Horizontal
bedsandlaminations
composed
of alunite+ sfiica+ kaolinitc
froma largeverticalacidsulfate
vein.SouthCentralpit.F. Closeupofhorizontal
laminations
withina near-vertical
acidsulfate
vein.Notethesoftsedimenttypedeformation
createdby thetwowall-rock
fragments
(rightcenter)thatappearto havefallenintothevein.G. Basal
acidleach-altered
breccia.
Thematrixanddastsaredominantly
massive
opalwithminoraluniteandkaolinitc.
The dasts
appeardarkerdueto thepresence
of hematite.
H. Opal-Kfeldspar
alteredhydrothermal
eruption
breccia.
588
EBERT AND RYE
TABLE1. K-Arand4øAr-39Ar
AgeDatafor theCrofoot-Lewis
Deposit
Sample
no.
Mineral
C/L
C/L
C/L
C/L
C/L
C/L
1
Alunite
2
Alunite
6
Alunite
9
Alunite
12 Alunite
15 Ainuitc
Description
Blanket acid leach zone
Acid sulfatevein
Blanket acid leach zone
Blanket acid leach zone
Acid sulfhtevein
Acid sulfate vein
C/L 32 Jarosite Fracture
fillingin blanket
acid
K-Arage
(Ma)
2.4
0.9
2.0
2.1
1.2
1.5
+_ 0.1 •
+_0.6j
_+ 0.1 j
_+0.1•
+_0.13•
+_ 0.12•
0.7 _+0.22
leach zone
C/L 71 Illitc
Illitc+ quartz+ pyrite'altered 4.0_+0.2)
volcanic rock
4øAr•9Ar
age(Ma)
hydrothermal,
andmagmatic
steam(Ryeet al., 1992).In the
supergene
enviromnent
acidsulfate'alteration
resultsfrom
theacidwaters
thatformbytheweathering
ofsulfide-bearing
rocksthatare exposed
abovethe watertable(e.g.,Guilbert
and Park, 1986). In the steam-heatedenviromnent,acid solu-
tionsformby the oxidation
of H.2Sto sulfatein thevadose
zone;in thisenvironment
H.2Sis derivedfromboilingliquid
at depth(e.g.,Schoenet al., 1974).In somecasesthisH.2S
maybe associated
withburstsof magmatic
steam(e.g.,the
Cactus,CA, deposit,
Ryeet al., 1992).The endproducts
of
supergene
andsteam-heated
acidsulfatealteration
zonescan
be similar,but the distinctionbetweenthe two can be made
by isotope
studies
in the contextof time-space
frameworks
(Ryeet al.,1992).Magmatic-hydrothermal
acidsulfate
'alterationisassociated
withhigh-sulfidation-type
oredeposits
and
breccia
isformedin hydrothermal
systems
withacidmagmatic
comC/L 77 K {'eldsparBreccia
matrix
3.8+_0.092
ponents
in theirfluids(Ryeet al., 1992).In thisenviromnent
C/L 78 K t'eldsparBreccia
matrix
3.9+_0.32
extremely
acidconditions
areformedby thedisproportionaC/L 79 K {bidsparFelsicvolem•ic
breccia
clast
10.6_+0.172
SOsandthe presence
of HC1andother
C/L 81 K l'eldsparFelsicvolcanic
breccia
clast
14.9_+1.62 tion of magmatic
acids,subsequent
to magmatic
vaporsbeingabsorbed
by
• D. JohnandE. McKee,U.S.Geological
Survey,
MenloPark,Cali•bmia
(unpub- groundwater (e.g.,Holland,1965;Henleyand McNabb,
lished data)
1978;Stoffregen,
1987;Hedenquist
et al., 1994).The mag2I. KigaiandM. Karpenko,
Institute
of Geology
andOreDeposits,
Petrography,
matic
steam
environment
is
characterized
by the formation
Mineraloe,andGeochemistry,
Bussian
Academy
of Sciences,
Moscow
(unpublished
of veinsof almostpurecoarse-banded
alunitein extensional
data)
C/L 74 K feldsparOxidized
opal-K{'eldspar
breccia 9.3 +_0.3•
C/L 75 K feldsparOxidized
opal-Kfeldspar
breccia23•
C/L 76 K feldsparUnoxidized
opal-Kfeldspar
10.3+_0.31
faults. The alunites are believed to form froin the oxidation
of the SOs-rich
magmatic
fluidsby the lossof hydrogen
or
of atmospheric
oxygen.
The dominance
of
fromthe volcanic
clastsare thoughtto be somewhere
be- the entrainment
to resultfromthedecompression
tweenthe ageof hydrothermal
alteration
andthe ageof the SOsin thefluidsisbelieved
fluids(Rye,1993).
volcanicelasts.Temperatures
in thisnear-surface
environ- of magmatic
Field evidence indicates that the blanket acid leach zone
mentappearto havebeeninsufficient
to resetolderK felddeposit
formedin a steam-heated
envispar.An illitc sample(C/L 71) separated
froman intensely at theCrofoot-Lewis
rotancut.
Here
acid
sulfate
'alteration
occurs
dominantly
asa
illitc + quartz+ PYrite altered
zone
ave
a
K-At
age
of
4.0
4
g
horizontal
blanket
that
caps
the
system,
generally
lacks
sulwhichisconcordant
withthe øAr/aOAr
ages
oftheK feldspar
matrix.
fides,and is zonedvertically,from a leachedresidualcap
(blanket
acidleach)at thetop,to a narrowintervalofintense
Ageof acidsulfatealteration
opalization
(basalacidleach).Thelowerzoneof acidsulfate
is inferredto coincide
with the palcowater
table
AluniteK-Aragesvarybetween2.4 and0.9 Ma (Table1); alteration
et al., 1974).The deepest
extentof acid'alterthelocation
of several
alunites
andthejarosite
fromtheblan- (e.g.,Schoen
of underlying
veinsof opal-chalcedony
+ aluket acidleachzoneandunderlying
acidsulfateveins'along ationconsists
(Fig.5). Thiszonation
is
with available
K-Ar agesareshownin Figure7. In Figure7 nite+ kaolinitc_+montmorillonite
alunitesfromthe blanketzonegiveolderagesthanalunites common in steam-heated acid sulfate 'alteration zones in acsystems
suchasSteamboat
Springs,
Nevada
fromthe underlying
veins.Fieldrelations
(Fig.7) showthat tivegeothermal
et al., 1974),Roosevelt
Hot Springs,
Utah(Parryet
theveinsdonotcrosscut
theoverlying
blanketzonebutsim- (Schoen
basinatYellowstone
Park,
plyextenddownward
fromitsbase.Thisrelation,combined al.,1980),andin theNorrisGeyser
(Whiteet al., 1988).Withintheblanketacidleach
withstableisotope
datadiscussed
below,andprecious
metal Wyoming
of nativesulfur,remnantmudpoolssimidistribution,
suggests
thattheblanketacidleachzoneislikely zone,thepresence
systems,
andtheassociato be the sameageastheunderlying
veinsandnotolder,as lartothosethatoccurin geothermal
the K-Aragesindicate.Giventheuncertainty
of theanalysestion with cinnabar indicate that this alteration formed in a
steam-heated environment. In numerous localities at the Crothesesamples
couldall be aboutthe sameage.
Coarse-grained
(upto4 ram)jarosite(sample
C/L 32)from foot-Lewis mine bedded mounds and lenses of native sulfur
vertical,elongate
vesicles,
a texturethatindicates
the
narrowfractures
cuttingtheblanketacidbachzoneyielded contain
a K-Arageof 0.7 + 0.2 Ma. Thiscoarse-grained
samplehas sulfurwasmolten,andtherefore,hada minimumtemperaisotopic
compositions
characteristic
of a steam-heated
origin ture around 113ø to 120øC(White et al., 1988). Combined,
precludesupergene
proandrepresents
theyoungest
documented
episode
of geother- thesetexturesand temperatures
cesses.
mal activity.
The presence
of magmatie
hydrothermal,
acidsulfatealterField Evidencefor a Steam-Heated
Origin
ation
can
be
ruled
out
based
on
the
geometry
of the add
for Acid Sulfate Alteration
sulfatealteration,
the generalabsence
of sulfideminerals
in
Acidsulfate
alteration
commonly
formsin fourgeologicallythis zone, and as discussedbelow, the lack of a detectable
distinctenvironments:
supergene,
steam-heated,
magmatic magmatie
component
(e.g.,Byeet al., 1992).Stableisotope
CROFOOT-LEWIS
HOTSPRING
Au-AgDEPOSIT,
NV
589
data,presented
below,
support
a steam-heated
originforacid beingthatthe acidfluidsweregenerated
bytheoxidation
of
sulfate
alteration
attheCrofoot-Lewis
deposit;
however,
mi- H2Sgasandnot sulfideminerals.
The mechanism
by which
noramounts
of supergene
aluniteprobably
occurin thedis- Au andAgaretransported
in theseacid,relatively
oxidizing,
trict.
low-temperature
fluidsis uncertain.
The abilityof the acid
sulfatefluidsto leachevenrelatively
insoluble
elements
is
Evidence for Fluctuations in the Water Table
displayed
by the largeareascontaining
90 to 100percent
The levelof the watertablehasa majorcontrolon the residual
silicathathavehadessentially
all othermajorand
distribution
of epithermal
mineralization
andalteration
as- minorelements
removed(Ebert,1995).We speculate
that
semblages
in thenear-surface
environment
(e.g.,Foleyet al., thiosulfate
andC1arelikelyligands
in thisenvironment
(e.g.,
1989;Simmons,
1991).At the Crofoot-Lewis
depositalter- Listovaet al., 1968; Mann, 1984; Webster and Mann, 1984;
ationassemblages
andsinterbedshavebeenusedto constrain Stoffregen,
1986;Webster,
1986,1987);however,
supporting
thelevelofthewatertableduring
theformation
ofthedeposit experimental
dataareneeded.The physical-chemical
envi(Ebert,1995).Thepresence
ofremnant
opal-K
feldspar
alter- ronmentin the steam-heated
environment
is highlyvariable
ation (an assemblage
that formsbelowthe water table; anddeposition
of Au andAgminerals
fromsuchfluidscould
Browne,1978;Henneberger
andBrowne,1988)withinblan- be triggered
by changing
fo2,temperature,
evaporation,
or
ket acidsulfatealteration
(anassemblage
thatformsabove mixing.
the water table; Schoenet al., 1974) demonstrates
that the
StableIsotopeStudy
watertabledropped
between
theperiodofearlyopal-Kfeldsparalteration
andlateracidsulfate
alteration.
OverprintingSampledescriptions
andanalytical
results
ofopal-Kfeldspar
alteration
byblanket
acidsulfate
alteration
Briefsample
descriptions
andstable
isotope
dataaresumindicates
thatthewatertabledropped
a minimum
of 18 m
marizedin Table2. Samplelocations
for several
of the sampies
are
shown
in
Figure
7.
The
seven
kaolinitc
samples
listed
Mineralized
stockwork
quartzveining
hasbeenexposed
in
in
Table
2
were
separated
from
mixtures
of
fine-grained
alurecentminingexcavations
andencountered
in twocoreholes nite + kaolinitc + silica and are assumed to have formed
in theproposed
Brimstone
pit. Thisstockwork
cutsblanket
with the alunite.Sixsamples
of pyriteacid sulfate alteration in the Brimstone area. The distribution eontemporaneously
mamasite
were
separated
from
samples
of
opal-K
feldspar
andextentof thisstockwork
ispoorlyconstrained;
however,
alteration,
and
one
sample
of
pyrite
was
separated
froma
thecoresamples
demonstrate
thatlatecrosscutting
quartz
sample
of
illite-smeetite
alteration.
In
addition,
two
pyrite
veiningextendsa minimumof 50 m abovethe baseof the
samples
(samples
C/L
13
and
C/L
17)
were
separated
from
blanket
acidsulfate
zone(Ebert,1995).Thisrequires
atleast
narrow
alunite-kaolinite-pyrite
veins
that
occur
relatively
a 50-mrisein the levelof the watertablefollowing
acid deepin the system.
A whole-rock
•34Svaluewasobtained
sulfate alteration.
duringacidsulfatealteration.
from an unaltered felsie volcanic rock from the Kamma
Shallow Enrichment Processes for Precious Metals
Mountains
group(sample
C/L 58)anda •34Svalueofpyrite
was obtained from interlaminated
slate and metasiltstone
Figure5 illustrates
the relationship
betweenrelatively
theAuldLangSyncGroup(sample
C/L 43).
high-grade
oxidized
acidsulfate
mineralization,
relatively
low- from
Three
samples
of
present-day
surface
and
subsurface
acidic
gradeunoxidized
opal-Kfeldspar
mineralization,
andbarren
waters
have
been
analyzed
for
aqueous
sulfur.
The
subsurface
blanketacidleachalteration.
The presence
of remnant
in anareawhichcontains
nooxidized
patches
of weaklymineralized
opal-Kfeldspar
alterationacidwaterwassampled
by oxidation
(at shallow
within barren blanket acid leach alteration is evidence that rockand mostlikelyoriginated
depths)
of
HsS
gas
that
is
presently
exsolving
from
a reservoir
blanket
acidleachalteration
overprinted
weakly
mineralized
of
warm
water
at
depth.
Exploration
drill
holes
have
encounopal-K
feldspar
alteration
andremoved
AuandAg.Themintered
warm
(up
to
40øC),
locally
acidic
(pH
to
3.9)
water
and
eralizedacidsulfateveinsandzoneslocallycontain
several
H2S
gas.
Standard
stable
isotope
techniques
were
used
and
timesmoreAu andAgthanadjacent
opal-Kfeldspar
alter- are summarizedin Ebert (1995).
ation,andsimplemass-balance
calculations
demonstrate
that
leaching
ofAufromtheoverlying
blanket
acid-leached
mateDiscussion
of IsotopicData
rial can account for most of the observed Au increase in the
Oxygen
and
hydrogen
isotope
datafor alunite,
jarosite,
underlying
acidsulfateore(Ebert,1995;Ebertet al., 1996). and kaolinitc
We propose
thata dropping
watertableenabled
acidsulfate
waters
to overprint
weaklymineralized
opal-Kfeldspar
alter- Figure8 shows
thefieldsfor thepossible
compositions
of
ation,resulting
in theleaching
of Au andAgasthesefluids supergene
aluniteandkaolinitc
(Ryeet al.,1992).Theshaded
percolated
downward
towardthewatertable.Theseacidsul- regionlabeledSASFis the fieldfor •D-•18Oso
4 valuesfor
fatefluidsprecipitated
AuandAg(dominantly
aselectrum,supergenealunitein the absenceof bacterialreductionof
AgC1,
andAgI),associated
withopal+ alunite+ kaolinitc
+ aqueous
sulfateor exchange
of aqueous
sulfate
withlowpH
montmorillonite,
in open-space
fractures
at, and mainly waters. The area between the dotted lines labeled SAOZ
within a few tens of meters below, the water table. This represents
thepossible
rangeof 6D-61sOon
values
in superprocess
resulted
in leaching
of precious
metals
froma large geneequilibrium
withmeteoricwaterbetween20øand80øC.
areaandconcentration
intorelatively
confined
zonesbelow Thesefieldsareforreference
onlyandmanynonsupergene
thewatertable;theyprobably
formedin a similarmanner
to alunites
haveisotopic
compositions
withinbothofthesefields
classical
supergene
enrichment
zones,the maindifference (Ryeet al.,1992).Mostof thealunites
in Figure8 cluster
in
590
EBERT AND RYE
CIlOFOOT-LEIVIS
HOTSPIllNGAu-AgDEPOSIT,
NV
591
592
EBEItT AND RYE
acid sulfate veins. Four alunites from the blanket acid leach
zonegivetemperatures
between40øand90øC.Suchtemperaturesare reasonable
for alunitesthat originatein the zone
abovethewatertable,wheretemperatures
•villgenerally
not
exceed100øC(Ryeet al., 1992).Fouralunitesamples
from
add sulfateveinshavetemperatures
that rangefrom60øto
180øC.Suchhighertemperatures
for alunitesin the add
sulfateveinsare consistent
with their greaterdepthin the
geothermal
system.
Isotopicdataof sulfateandsulfides
Figure
10A
isaglot
showing
the6•SOso
4vs.6'•4S
values
of
alunite,andthe 6 4Srangesfor sixsamples
of pyriteand
marcasite
from
opal-K
feldspar
alteration,
and
sulfatefrom
-20•
threesamples
ofpresent-day
acidwater.The634S
values
for
-30
-20
-10
0
10
20
30
earlysulfides
rangefrom -10.8 to -15.2 per mil andthese
values
approximate
the634S
values
of H2Sin thegeothermal
680
fluid,because
fraedonation
between
aqueous
H2Sandpyrite
lowtemperatures
(< 180øC)likelyto
FIC,.8. The6D,6•SOso4,
6•8OoH
values
of alunites
andjarosite,
6D,and issmallat therelatively
6]sOofassociated
kaolinires,
andcalculated
6D.2o and6•SO._,o
offluids
for havebeenattained
in thisalteration
zone(Ohmoto
andRye,
alunitesandjarositesampleC/L 32. Fluid compositions
calculated
from 1979;Ryeet al.,1992).Allbuttwoofthealunites
areenriched
equations
of Stoffregen
et al. (1994)andRyeandStoffregen
(1995).Lines in 34S
relative
to earlysulfides.
andfieldsareMWL: meteoric
•vaterlineofCraig(1961);PMW= primary
The
634S
value
of pyritesample
C/L 42 (-14.5%o)sepamagmatic
waterfieldof Taylor(1979);kaolinitc
line of SavinandEpstein
illite-smeetite
alteration
iswithin
(1970);SASF(shaded)
= supergene
aluniteSO4field;andSAOZ= super- ratedfromthewidespread
genealuniteOH zoneasdescribed
in Byeet al. (1992),
theinterpreted
634S
rangeof initialH.2S.
Thesource
of the
S for pyritein the illite-Slnectite
alterationzonewasmost
likelythe H2Sthatwasexsolved
fromboilingsolutions
which
a groupwith6D values
around-120 per mil and6•SOo]•flowedalongpermeable
zonesandcondensed
(alongwith
and6D-6•SOso4
values
between
about0 and10permil.Two CO2)into marginal,relatively
stagnant,
groundwater.This
alunitesfall outsidethisgroup,with 6D valuesnear -160 sourceagreeswithmodelsforthe formation
of interstratified
per mil. The 6D valuesfor jarositesare muchlowerthan illite-smeetite
alteration
by Bartonet al. (1977)at Creede,
thosefor alunite,with a rangefrom -183 to -145 per ]nil. andHedenquist
(1990)attheBroadlands
geothermal
system,
Supergene
kaolinires
orhalloysites
typically
have6Dand6•sO New Zealand.
values
closetothekaolinitc
line(Lawrence
andTaylor,1971; Figure10Bisa plotof alunite634S
vs.A•SOso•_on
values.
Marumoet al.,1982).Thekaolinires
allplotto theleftof the All of the alunites from the blanket add leach zone have
supergene
kaolinitc
linein a tightgroupwithanaverage
6D partially
exchanged
634S
values,
andthese
values
overlap
with,
of about-160 values
anda rangeof 6•sOvalues
of about0 but havea widerrangethan,thoseof the blanketalunites.
to 7 per mil (Fig. 8).
Mostof thealunites
withexchanged
634S
values
(leastnegative)
give
reasonable
A•SOso4_on
temperatures.
ThisindiAluniteA•SOso•_oH
temperatures
catesthat the residence
time of aqueous
sulfatein solution
The6•sOoi•
values
ofalunite
arecommonly
in equilibriumforsomeof thesamples
wassufficient
forequilibrium
oxygen
with the parentwater,andif the aqueous
SO4alsoreached isotope
andsignificant
sulfurisotope
exchange
betweenaqueoxygen
isotope
equilibrium
withtheparentwater,theisotopic
dataofalunitecanbeusedasa single-mineral
isotopic
geothermometer(Ryeet al., 1992;Stoffregen
et al., 1994).The
Vein
Blanket
average
uncertainty
of thesetemperatures
isprobably
about
2O
_+30øC.
Eightof the alunitesfromthe Crofoot-Lewis
mine
yield
18
o
o
A Oso4-oH
temperatures
thatrangefrom40 to 180C, con-
E
40
•
6o
v
sistent
witha steam-heated
origin.
TheA•SOso4_oi•
values
of
theotheralunites
yieldunrealistic
temperatures
forthisshallowenvironment
andareinconsistent
withthoseof thepresent mineralassemblages.
Similarvalueshavebeenobtained
foralunites
fromthesteam-heated
zones
in activegeothermal
systems
(Ryeet al., 1992)andaretypicalof thoseof steamheatedalunites
in whichoxygen
isotope
equilibrium
wasnot
obtainedfor the aqueous
sulfate.
Figure9 isa plotofdepthvs.calculated
A•SOso4_oi•
tem-
lOO
0 ....
5•) ....
11•0 ' ' '1•0 ....
200
Temperature
øC
peratures
foralunites
yielding
reasonable
formation
tempera- FIG.9. Depthvs.calculated
alunite
A]SOs%_oH
temperature
forblanket
andacidsulfate
vein(solidtriangles)
samples.
turesfromthe blanketacidleachzoneandthe underlying addleach(opensquares)
CROFOOT-LEWIS
HOTSPRING
Au-Ag
DEPOSIT,
NV
ism.Fluidinclusions
arerareandtypically
small(2-5
andidentification
andmeasurements
arehampered
bystrong
doublerefraction.
Onlyonereliable
homogenization
temperatureof87øCwasobtained.
Thesample
hasanextremely
low
didvalueof -183 permilduetothelargeD-H fractionation
between
jarositeandwater(RyeandStoffregen,
1995).The
calculated
didvaluefor the parentfluidis about-120 per
mil, consistent
withthe rangein composition
of the steamheatedalunitefluids.Thissamplehasa K-Ar ageof 0.7 _
0.2Ma andrepresents
theyoungest
knownepisode
of steam-
ß
ß
ß
ß
G/l_2
I
Pyrite/marcasite
I
heated alteration.
I
AcidwaterSO4
A
I
-17
b34S
40øC
Twosamples
of cryptocrystalline
jarosite(C/L 33 andC/L
34) consist
of lightbrownfracturecoatings
that occurin
oxidizedand unoxidized
opal-Kfeldsparalterationzones
whichhavenot beenacidleached.Bothof thesesamples
haveunexchanged
di34S
values
(- 11,- 13%,)andhavevery
smallor negative
A1SOso4_o,
values.
Thesejarosites
could
besupergene.
It isquitelikelythattheoxidation
of anyshallowsulfides
wouldoccursimultaneously
withthatof H2Sin
a shallow
geothermal
environment.
[]
180øC
Sourceof SulfurandHydrocarbons
ß
[] ß
[]
[]
Sulfides
relatedtoprimary
mineralization
andalteration
at
theCrofoot-Lewis
deposit
havelowdis4S
values,
ranging
from
ß
[]
ß
-10 Pyrite/ marcasiteI
-•5
593
-10.8 to -15.2 perrail.Thesevalues
areverydifferentfrom
thoseof thelocalTertiaryvolcanic
rocksor theunderlying
metasedimentary
AuldLangSyncGroup.Onewhole-rock
dis4S
analysis
fromunaltered
Tertiary
volcanic
rocksadjacent
to theCrofoot-Lewis
minehasa valueof 10permil.A single
sample
ofpyritefromtheAuldLangSyncGrouphasa dis4S
valueof -1.9 per mil. Assuming
a relatively
homogeneous
dis4S,
the AuldLangSyncGroupis not a likelysource
of
(•=Blanket)
B
Vein
-io
•534S
-•
sulfur for mineralization.
FIG.10. A. Alunite6•8Os04
vs.634S.
Therangeof 634S
values
forearly
Hydrocarbons
orotherorganic
material
aretypically
incorporated
into
hydrothermal
solutions
as
they
pass
through
sedA•8Oso4_o.vs.
634S
values
ofblanket
(opensquares)
andveinalunite
(sold
rocks(Hanor,1980).Dataondi34S
values
fromthe
triangles)
samples.
Dashed
linescorrespond
to calculated
A•SOso4_oH
tem- imentary
basinsediments
adjacent
to theCrofoot-Lewis
peratures
of 40øand180øC.
The634S
rangeof earlypyrite-marcasite
isalso lateCenozoic
shown.
deposit
arenotavailable.
However,
RiceandTuttle(1989)
investigated
thedis4S
values
oflatePleistocene
toearlyHoloceneagesediments
fromWalkerLake,in west-central
Neous sulfate and the fluid. The fact that some of the vein
vada.Sulfides
andorganic
sulfurin thesesediments
contain
alunitesamples
showa greaterdegreeof S isotope
exchangeisotopically
lightsulfur,
withdis4S
values
of sulfides
ranging
thansamples
fromthe blanketzoneis consistent
with the from
-41.5to+24.4
•errail,
averaging
-15.6
per
rail(total
highercalculated
aluniteA1SOso4_on
temperature
rangein of56samples),
anddisSvalues
oforganic
sulfur
ranging
from
theveinsamples.
Figure10Bemphasizes
thatindividual
sam- -26.5 to +8.1 perrailandaveraging
-4.4 perrail(totalof
plesof steam-heated
alunitein whichisotopicequilibrium 12 samples).
RiceandTuttle's(1989)results
showthatlow
wasnotattainedmaynotbe distinguishable
fromsupergene disaS
values
arecommon
in organic
materandsulfide
minerals
aluniteonthebasisof isotopic
dataalone(Ryeet al., 1992). withinlateCenozoic
basinsediment.
HaS,likehydrocarbons,
canbe derivedfromthe thermaldecomposition
of organic
Jarositeisotopic
data
materandhydrocarbons
in sedimentary
basins(Le Tran et
organicmatter
Darkbrown,relatively
coarsely
crystalline
jarosite(upto al., 1974).Givensufficientheat,degrading
pyrite-marcasite
and for acid mine water is also shown.B. The
4 mm) occursin a veinlet that cuts the blanket acid leach within the basin sedimentscould contribute CO2, H.2S,and
zone(sample
C/L32).Because
thereisnosulfide
orevidence volatile
hydrocarbons
tothegasphase
(Peabody
andEinaudi,
of weathered
sulfides
above,adjacent
to, or immediately
be- 1992). Alternatively
organicmatter and/orhydrocarbons
lowthissample,
anoriginbysupergene
oxidation
of sulfides couldbe incorporated
into the rechargefluidsand subseisunlikely.
Thisjarosite
hasanexchanged
di34S
valueof -6.4 quentlyhydrothermally
decomposed
(matured)alongthe
permil andyieldsa A18Oso4_oH
temperature
of 70øC(Rye flowpath.
Therearethreedominant
sedimentary
unitsin thedistrict,
andStoffregen,
1995),suggesting
a steam-heated
origin.In
thinsection,
thejarositecrystals
arezonedandaregenerally theAuldLangSyncGroup,bedswithinthe BlackRocktertransparent
withlightyellow-green
to deepbrownpleochro- rane,and the late Cenozoicbasinfill. Abundantgraphite
594
EBERT AND RYE
withinthe Auld LungSyncGroupindicates
that, at least quartzandcalcitesamples)
aswellasa largerangein surface
fromalunitesamples).
Thisrangeofvalues
locally,priorto ore deposition
theserocksweresubjectto water(calculated
of processes
including
variations
temperatures
abovethe oil stability
windowandare an un- couldreflectcombinations
likelysourcefor bitumen.The BlackRockterranecontains in theisotopic
composition
of thewatersovertime,modificawaters,boiling,and
igneous
andsedimentary
rocks,including
cherts,mudstones,tionsduringmixingof deepandshallow
sandstones,courseelasticrocks, and minor lacustrinelime-
water-rock interaction. Two of the calculated chloride waters
stones
(Oldow,1984;Russell,
1984).The degreeof maturity (fromquartz)havedidvaluesnear-118 perrail.Thisvalue
waterandis the best
of hydrocarbons
in thisunit is unknown.
The 2,160-m-thick fallswithinthe rangeof earlysurface
valuefor earlychloridewater.Two
sequence
oflateCenozoic
basinsediments
thatfillstheBlack estimateof an average
RockDesertbasinis considered
to be the mostlikelysource calciteandonequartzsamplehave6DH2ovaluesof fluids
of bitumenin the region.As previously
mentioned,
several rangingfrom -137 to -153 per rail.The paragenetic
relaofthelateCenozoic
basins
throughNevadacontain
showingstionsof thesetwosamples
aredifficultto demonstrate;
howof oil andgas,andanoil exploration
welldrilledin theBlack ever,oneof the veinscutsopal-Kfeldsparalteration
and
RockDesertbasinencountered
tracesofhydrocarbons
above onecutsillite-smectite
alteration,
therefore
bothsamples
are
a 2,150-mdepth(Garsideet el., 1988).Thus,available
evi- relativelylate. The 19 to 35 per rail differencein the
dencestrongly
indicates
that the late Cenozoic
basinsedi- 6DH•,O
valuebetweenthewaterfor thesesamples
andearly
mentsare the sourceof bothbitumenandisotopically
light chloridewater(-118%•) cannotbe accounted
forbyboiling
(Truesdellet el., 1977). These low valuesresultedfrom mixsulfurat the Crofoot-Lewis
deposit.
ing
of chloridewaterwith a low didvaluesurface
water,or
Characteristics of Fluids
alternatively,
the 6D valueof the chloridewatersdecreased
Isotopic
composition
of chloride
waters
significantly
duringthe laterstages
of the system.
A
sample
of
quartz
(+
chalcedony)
fromthe mineralized
The term"chloride
waters"refersto the deephydrotherquartz
stockwork
that
cuts
the
blanket
acid
leachzonein the
malfluidandis analogous
to the neutralpH chloridewaters
areahasa relatively
low6•sOvalueof4.1perrail
in activegeothermal
systems
(White,1969;HenleyandEllis, Brimstone
C/L 44). Thissampleformedverynearthe present
1983).The isotopic
compositions
of ohoride
waters(Fig.11) (sample
surface,
at
anestimated
temperature
between100øand120øC
havebeencalculated
from6•sovalues
of quartzandcalcite
(estimated
from
the
hydrostatic
boiling
pointversusdepth
usingfluid inclusion
homogenization
temperatures
(Ebert,
Thecalculated
rangeof 618OH20
values
forthiswater
1995)and6DH.,O
valuesdetermined
directlyfrominclusion curve).
to low 6DHzo
fluids.The quartzsamples
havenumerous,
but verysmall are -16.8 to -14.4 per rail, corresponding
did =
(mostly<2/.tin) fluidinclusions.
The scarcity
of sufficientlyvaluesof --158to --131perrailfromthe relationship
86•so
+
10
(Craig,
1961).
largefluidinclusions
makesdetermination
of accurate
filling
temperatures
problematic
andis a possible
sourceof error Oxygen
isotope
shiftin deepthermalwaters
in the calculated
6•so,,ovaluesof the quartzfluids.The
Theamount
ofoxygen
isotope
shiftduetowater-rock
interstableisotope
dataonfluidsin Figure11recorda largecomaction
at
depth
in
the
deeply
circulating
geothermal
fluid
positional
rangein chloridewater (calculated
from vein
-80
•.•t/
Ran ofearly
.•'/
surface
and
'7
early
chlodde
fluids
-3-• !....
-lOO
•
•
•q- /
• /
-12o
'-/--- q-'
/
+1•'-
' -t•
•
3•.
......
,-x--f- • .....
,,
/ []
-14o
,
,'
,
-16o
ß
A,•/
/
/ __
........
I
'
i
I
(chloride
waters)canbe estimated
fromthe 6•SOH•o-6DH.•O
valueofwaterthatwasresponsible
forveinquartzdeposition.
Thewatersthatdeposited
quartzfallalonga lineparallelto
4 Calculated
alunitethe meteoricwaterline,but areenrichedabout4 per railin
fluids
(surface
water)
isattributed
to
[] Calculated
quartz •sO(Fig.11).This4 perrail•sOenrichment
oxygen
isotope
shift
due
to
water-rock
interaction
and
is
simifwl•itdtr
)(choride
0 Calculated
calcite lar to that observedin moderngeothermalareassuchas
fluids
(exchanged
Yellowstone
(Craiget el.,1956;Craig,1963;Truesdell
et el.,
chloride
water
or
•
'
a--
mixture)
1977).
fluids
(100øC)
The calcitefluidsare enrichedabout10 to 11 per rail
relativeto unexchanged
meteoric
water.CalcitesampleC/L
Calculated koalinite
ß Calculatedkoalinite
•
f,u•ds
(soøc)
68 is from a calcite vein that occurs within illite-smectite
alteration.
Fluid inclusions
fromthis samplehavevariable
liquidtovaporratiossuggestive
ofboiling(e.g.,Bodnar
et el.,
1985).Someinclusions
in thissamplehavepositivemelting
temperatures
(upto2.6øC)
whichmayresultfromtheformationof clathrate
duringfreezing,
indicating
thepresence
of
AA
• .• ,_, ,, ,'
- b- •
'
E•matedrangeof
.'-'.
I
latesurfaceand late
chlondefluids
'
I
CO2 or the formationof metestableice (Roedder,1984). This
'
samplemostlikelyformedfroma COs-rich,
steam-heated
ground
waterora mixtureofground
waterandchloride
water
8180
(e.g.,Hedenquist,
1990).CalcitesampleC/L 69 is froma
darkbrownto black,coarsely
bandedcalcite+_chalcedony
F]c..11. Fluid•DH20and•18OH20
values
fordeepandshallow
fluids
vein.
Fluid
inclusions
from
this
sampleare alsodominantly
calculated
fromalunite
6D and•SO values
and/¾SOso4_oH
temperatures,
liquidto vaporratios.The increase
andquartz
andcalcite
•SOvalues,
fluidinclusion
•Dn2ovalues,
andhomoge- twophasewithvariable
in •SOshiftin thesecalcitefluidsoverthatfor the quartz
nizationtemperatures.
-30
-20
-10
0
10
CROFOOT-LEWIS
HOTSPRING
Au-Ag
DEPOSIT,
NV
595
exchange
witha late,isotopically
lighthydrofluidsis possibly
dueto isotopic
enrichment
duringboiling bleto H isotope
(Truesdellet al., 1977)or duringinteraction
of CO2-rieh thermalfluid (O'NeilandKharaka,1976).Figure12 shows
from the veinsare slightly
steam-heated
waterswith the countryrock,as described
at that the 8D valuesof kaolinites
in D relativeto kaolinites
fromthe blanketzone.
the Broadlands-Ohaaki
geothermal
systems,
New Zealand depleted
(Simmonsand Christenson,1994).
Oneinterpretation
of thistrendisthatkaolinites
in theveins
reached
a greater
degree
ofisotopic
equilibrium
withanoverIsotopic
composition
of acidsulfatewaters
printing
fluidthankaolinites
in theblanket
zone.Thisdifferenee
would
be
consistent
with
higher
elevation
oftheblanked
Isotopic
compositions
of watersthatdeposited
alunitecan
zone
which
most
likely
resulted
in
lower
temperatures
or a
becalculated
fromtemperature
information
andthefraetionshorter
duration
of
contact
with
an
overprinting
hydrothermal
ationfraetors
of Stoffregen
et al. (1994).Figure11shows
the fluid.
calculated
isotopic
compositions
ofwaterin equilibrium
with
that H isotopeexchange
in kaolinitcrethosealunites
thatyieldreasonable
/X]SOso4_OH
tempera- The observation
quires
the
influx
of
fluids
having
a
substantially
lower
tures.The8D and8]SOvalues
of alunite
waterplotnearthe
8Dmo
value
than
that
of
the
primary
fluids,
and
along
with
meteoricwater line, and mosthave 6D valuesbetween -100
the
low
8D
value
of
alunite
C/L
1,
suggests
low
8Dn2o
v'alues
and -123 per mil, whichis doseto the valueof chloride
system.
waterandis consistent
withthe average
value(-120%o)in- for fluidslatein the life of the geothermal
Relation of Mineralization to Palcoclimate
terpretedto havebeentypicalof meteoric
waterin thisarea
backto the late Tertiary(Friedmanet al., 1993).Alunite
The Crofoot-Lewis
depositrecordslargefluctuations
in
waterswhichplottotherightofthemeteoric
waterlinecould
both
the
isotopic
composition
of
fluids
and
in
the
level
of
the
beduetoevaporation
in thesteam-heated
environment
(Ellis
paleowater
tableovertime.Thesechanges
canbecorrelated
and Mahon, 1977; Truesdell et al., 1977).
with
Plioeene
to
Pleistocene
palcoclimate
recordsfor the
The fluidfor onealunitehasa 8D valueof -158 per mil
western
United
States
and
are
therefore
interpreted
to reflect
andalongwiththecorresponding
8•SOH,2o
valueplotsnear
major
elimate
changes
over
the
life
of
the
system.
the meteoric
waterline (Fig.11).Thisalunitewascollected
Studies
of marineoxygen
isotope,
foraminiferal,
anddiafroma remnantmudpoolnearthe top of the blanketacid
tom records,combinedwith terrestri'altaxaand other indicaleachzoneand, basedon its positionin the system,it is
1991),suggest
thatfromapproximately
4.8
youngerthanthe otheralunitesamples(although
its K-Ar tors(Thompson,
to
2.4
Ma
relatively
high
rainfall
and
mild
temperatures
exageindicates
thatit is slightlyolder).The low 6D}•2o
value isted in the western United States. This wetter climate crefor thisalunitesamplecannotbe accounted
for by temperalargelakesthroughout
westernNorthAmerica
ture-related
fractionation
or evaporation,
andit wasalmost atedseveral
(Forester,
1991).
At
about
2.4
Ma,
the
firstlargecontinental
certainlydeposited
froman isotopically
lightsurface
water.
glaeiation
events
occurred
in
the
northern
hemisphere,
and
This isotopically
light surfacewater is consistent
with the
thesecoolertemperatures
andgreaterariditylastedfrom2.4
composition
of the lateehoridewaterspresented
above.
to about2.0 Ma (Thompson,
1991).From2.0 to about1.8
Ma,
warmer
and
moister
conditions
prevailed
throughwestPostdepositional
D exchange
in kaolinites
and•D
em NorthAmerica(Thompson,
1991).
of late-stage
fluids
Detailedstudies
of late Cenozoic
perennial
lakedeposits
Withoneexception
(aluniteC/L 1), allkaolinite
•D values fromSeades
Lake,California,
bySmith(1984)demonstrated
are41 to 50 per mil lowerthanthoseof associated
alunites sixdistinct
paleohydrologie
conditions
in theGreatBasinover
(Fig. 8). Basedon the fraetionation
factorsof alunite-H.20 the past3.2 m.y.:3.2 to 2.5 Ma (wet);2.5 to 2.0 Ma (dry);
(Stoffregen
et al.,1994)andkaolinite-H•20
(LiuandEpstein,
1984), there is little differencebetween•D valuesof alunite
andkaolinitewhichformedfromthe samefluidat tempera-
turesabove150øC.Since/X•SOso4_on
temperatures
calculatedfor aluniteindicatethatseveral
of the samples
formed
at temperatures
between
40øand180øC,
andthealuniteand
kaolinite
appearto havebeendeposited
eontemporaneously,
thegreaterthan40 per mil difference
in •D valuesbetween
coexisting
aluniteandkaolinitccannotbe explained
by temperature-related
fraetionation.
We suggest
that•D valuesof
thefine-grained
kaolinites
havebeenresetby exchange
with
late,isotopically
lightfluids.Experimental
studies
showthat
kaolinitcis susceptible
to low-temperature
H exchange
in
the absence
of significant
O isotopeexchange
(O'Neil and
Kharaka,1976;Kyserand Kerrieh,1991).The mechanism
involves
exchange
with OH siteandis distinctfromthe DH changes
thataccompany
reerystallization
or neoformation,
bothwhichareassociated
withchanges
in •]SOvalues.
50
150
• KaoJinite
Blanket
)
ß
200
-170
Kaolinite
Vein
'-1•0
'-1•0
'-140
6D
The kaolinitesamples
analyzed
in thisstudyareveryfine
FIC. 12. Depthvs.•SDfor kaolinitc
fromthe blanketacidleachzone
grained,
typically
lessthan1/am,andarethereforesuseepti- and acid sulfate veins.
596
EBERTAND RYE
2.0 to 1.3 Ma (intermediate);1.3 to 1.0 Ma (wet), 1.0 to 0.6
ationto overprint
opal-Kfeldspa•
alteration,
resulting
in secMa (intermediate);
0.6to 0.3Ma (dry).
ondaryAu-Ag-enriched
zonesthatare economically
mineThe variousepisodes
of alterationat the Crofoot-Lewisable.Duringthelaterpartofthisperiod,6D values
of fluids
depositbetween4 to 0.7 Ma andthe •D valuesof meteoric decreased
fromabout-120 to -158 per mil.
watercorrelate
wellwiththispalcoclimate
record.
A possible 3. 2.0 to 1.3 Ma: warmer, moister intermediate conditions
history
of the system
basedonthe combined
palcoclimateresulted
in a risein thelakelevelandwatertableallowing
studies
of Thompson
(1991)andSmith(1984)ispresentedthegeothermal
system
to overprint
thenear-surface
blanket
belowandisillustrated
schematically
in Figure13:
acidleach
alteration
(Fig.13C).Thehighwatertable,possibly
related
toglacial
meltwater,enabled
a lateepisode
ofminer1. 4.8 to 2.5or 2.4 Ma:wetconditions
andmildtempera- alizedquartzstockwork
veiningto reachthe near-surface,
tures created a lake in the Black Rock Desert basin. Geother- locally
overprinting
andmineralizing
blanket
acidleachaltermalactivity
beganduringthisinterval,
andfluiddischargeationin the Brimstone
areaandresetting
kaolinitc
to lower
•D values.
nearthemargin
of thelake,focused
alongrange-bounding
normalfaults,resulted
in widespread
near-surface
opal-K 4. 1.3 to 1.0 Ma: relatively
wet conditions
mayhaverefeldspar
alteration
andsinterdeposition
(Fig.13A).
sultedin a furtherrisein the water-table(at least50 m above
2. 2.5 or 2.4 to 2.0 Ma: lowertemperatures
and more thelowest
watertablelevelduringthesecond
episode).
aridconditions
associated
withglaciation
at highelevations 5. 1.0 to 0.6 Ma: intermediate
anddry conditions
again
resulted in a reduced size of the lake in the Black Rock
resulted in an increase in •D values of meteoric waters
Desertbasin(Fig.13B).Theresulting
drop(18m minimum) (-120%o)anda possible
dropin thelakelevelandthewater
in the water table enabled steam-heated acid sulfate alter- table(Fig.13D).Late(0.7Ma)steam-heated
jarosite
formed
4.0 to 2.5 Ma wet mildclimate
A
2.5 to 2.0 Ma coolermore arid climate
aD~-120
to-100
'Poolsinter'
interbedded Build
upofnear-
Talus
B
High water table
withsilicified
conglomeratessurfaceSilicification
debris
Widespread
ac•-surrate
I
alteration,/
••
.... •,•'I,•
'
High lake level
Drop in lake level
.
(lake)
late
change
inaD
from
~.o-120
to
-158lowwatertable
..
.
Relatrvely
..
(lake)
•.... ,'.-' !• ß !'';
:
•
• ""-• .....
:;".i:
o½/'
..
•,,.
:.:
C
Rise
in lake
Pervasive
day
0 -0
alteration
1.0 to 0.6 Ma warm moistclimate
2.0
to6D
1.0
warm
moist
climatecuts
Late
quartz
stockwork D
~Ma
-158
to -137
allotheralteration/
High
Water
table
zones
[ ••.
level
•
Drop in lake level
•
steam-heated
alteration
Low
w•ter
table
aD
~Minor
-137
to
-120
or
greater
End of lake?
I (lake)•
.... :'
ii•i•'
"':'"
(lake?)
.
i':
....... ;;.•
;-
'; .":•;.
/
I
Opal-K-feldspar
alteration
containing
low-grade
Au- Ag
r• Illite-smectite
alteration
-'• Blanket
acid-leach
alteration
I
1km
I
Mineralized
acid-sulfate
veins
Hydrothermal
eruption
breccia
Stockwork
quartz
veining
Boiling
conditions
Massive
opaline
sinter
deposits
Dominant
fluid
flow
pattern
FIG.13. Schematic
cross
section
through
theCrofoot-Lewis
deposit
showing
formation
through
timeandinterpreted
changes
in 6D values
(seetext).
CROFOOT-LEWIS
HOTSPRING
AuoAg
DEPOSIT,
NV
597
section
through
theSulphur
region
thatisconsistent
along
fractures
above
thewatertableintheblanket
acidleach rivecross
with all of the data available from the area.
zone.
Mostactive
geothermal
systems
andepithermal
oredepos6. 0.6to 0.3 Ma:dryconditions
caused
a furtherdropin
in timeandspace
withsubaerial
volcanism,
thewatertable.Geothermal
activityprobably
ceased
during itsareassociated
heatsource
isoftendemonstrated
orinferred
this time. Currentwarmwatersand H•S gasin the area andanigneous
(Henley,1985).Figure14, however,
is drawnwithoutan
probably
reflectconductive
heating.
Factors other than climatic conditions could account for
igneous
heatsource,
although
the modeldoesnotrequire
theabsence
of sucha heatsource.
TheCrofoot-Lewis
deposit
the observed
fluctuations
in thepaleDwater
table.However, andseveral
geothermal
systems
innorthern
Nevada
aresomeastheknownvariations
in paleDclimate
canexplain
boththe whatanomalous
in thattheylackobvious
coevalvolcanism
fluctuationsin the water table and the variable 6D values, (Crewdson,
1976;Blackwell,
1983).Manyauthors
consider
changes
in paleDclimate
are considered
to be the bestex- thehighgeothermal
gradient
in northern
Nevada
tobesuffiplaination
ofthe6D-timedataat theCrofoot-Lewis
deposit. cientto account
formostof thepresentgeothermal
systems
intheregion
(HoseandTaylor,1974;Sass
etal.,1979;Black-
Fluid Circulation Model
well, 1983; Buchanan,1990).
The regionsurrounding
the Sulphur
districtfallswithin
Thissection
proposes
a simpleconvection
modelfor the
the
Battle
Mountain
high,
an
area
of
anomalous
heatflowin
Crofoot-Lewis
system
in whichmeteoric
waterflowedalong
Basin
andRange
province
thathasa geothermal
extensional
fractures
in the basinandwasheatedmainlyby thenorthern
ofabout50øC/km
(Duffield
etal.,1994).Thevolumithehighthermalgradient
in theregion.Thismodelisconsis- gradient
of silicasinterattheCrofoot-Lewis
mineinditentwiththeSisotope
dataandtheapparent
longevity
ofthe nousdeposits
byfluidsfroma reservoir
witha temperature
Crofoot-Lewis
system,
but is admittedly
speculative,
mainly catedeposition
than210øC
(Fournier,
1985).Sincethedepthtothe
because
of a lackofgeophysical
dataonthebasin,thepaucity greater
transformation
nearcentralPershing
County
anduncertainty
of the radiometric
agedataontheCrofoot- brittle-ductile
1992),meteoric
waters
Lewisdeposit,
andthelackof supporting
hydrologic
model- isbetween12 and18km (Catchings,
should
be
able
to
circulate
easily
to
deeper
than
4
km
and
ing studies.Nevertheless,
it is importantto presentthis
gradient
to reach
model,because,
if correct,the potentialareathat canbe be heatedonlyby the highgeothermal
above210øCasshown
in Figure14.
targeted
forexploration
of younghotspringgolddeposits
in temperatures
theGreatBasinincreases
significantly.
Figure14isa specula- Seismic reflection and refraction data from northwest NeNW
CrofootJLewis
Black
Rock
Range
Mine
Black
Rock
Desert
•
•
SE
Kamma
Mountains
King
Lear
Federal
1-17 •
,vwvv•
,•vvvvvvvv
....................
•
..............................................................
vv
.•
"?•"•-'•-•__
,.,.......
-_
,,/• -•-,,-,•,•y
v v, •L
.....
...............................................................
•
ß
Geothermal
Depth
km
• 0
gradient50øC/km
--
0oc
--
100oc
--
30OOC
•Z;E; :::::::::::::::::::::"•7•;...
'•;;;2;;;;;.'
.........
;•.......'•7;;;.';;Z
..........7.';.';.';;;.';.'.';
;;;;.';;;Z;;:.
=======================
...........................................................
v
.......................
.':';7:2:..':;::.'E::2
'•.CC;:'
2.C';.'::.;:C.
,'.'.':.'.':.'.'.;'.';.'.;•C
C'CCCC
,';:::!v ,••. ...'
v
Paleozoic
clastlc
sedimentary,
'•
•'
?
carbonate
and
volcanic
rocks
?
2
-- 8
Master fault
--
.........
u•il•'tr
a•si
ti•'n
r•xi•ate•y
500 - 700øC (e.g. Catchings,1992)
B•tl•'-d
(."?)
•'pp
•.::• Late
Cenozoic
basin
sediments
Triassic
toJurassic
Auld
Lang
Syne
Group,
fine-grainedmetasedimentaryrocks
.•
ß
Talus-fanglomerate
deposits
Mesozoic
(+Paleozoic)
Black
Rock
terrane
•
Tertiary
Kamma
Mountains
Group,
felsic
volcanic
and
volcaniclastic rocks
rocks, and fine to course clastic rocks.
includingintermediate
volcanicand plutonic
10
--500øC
--12
--600øC
0
2
I
I
krn
Drawn to scale
Fluid
flow
path
Fla. 14. Simplified
interpretive
cross
section
through
theBlackRockDesert
andadjacent
ranges,
showing
interpreted
geology
andhydrothermal
fluidconvection.
Anigneous
heatsource
isnotshown
butcouldbepresent.
598
EBERT AND RYE
vadaindicatethat the majorrange-bounding
faultsflatten Thepaleoclimate
wasanimportant
factorin theformation
with depthandthat the intensityof faultingandfracturing of the Crofoot-Lewis
depositby controlling
the levelof the
ismuchgreaterwithinbasins
thanin adjacent
ranges
(Catch- palcowater
tableand the availability
of meteoricrecharge
ings,1992).Seismic
reflection,
gravity,
andgeologic
datafrom water.Duringperiodsof highsurface
wateravailability,
the
the Dixie valleyarea,Nevada,alsoshowthe rangesto be highlyfractured
areasbeneaththe basins
areinterpreted
to
composed
of a fairlysolidblock,whereas
theadjacent
basins be favorable sites for meteoric water convection. Meteoric
are highlyfaultedandfracturedto depthsof 15 to 20 km watersrecharged
alongfaultsthrough
thelateCenozoic
basin
(Okaya,1985).The simplified
structure
presented
in Figure sediments,
acquiring
Sandhydrocarbons.
Thesedeeplycircu14 illustrates
the highdensityof faultingandfracturing
in- latinggeothermal
fluidswere eventually
focusedand disferred to occur below the Black Rock Desert basin.
chargedalongbasin-and-range
faultsnearthe marginof a
The highdensityof faultingwithinthe basinsprovides lake.An igneous
heatsourceis possible
butnotapparent
at
pathways
forthedownward
migration
of meteoric
water(e.g., thesystem,
andit issuggested
thatthehighregional
geotherHoseandTaylor,1974).Fluidconvection
is favoredin frac- malgradient
wassufficient
to formtheoredeposit,
withfluids
turedrockswithhighpermeability
thatareexposed
to a geo- circulating
to minimumdepthsof 4 km.
thermalgradient(CrissandHofmeister,
1991).Convective
Acknowledgments
flowcanbecaused
bytheinverse
density
gradient
thatresults
Discussions
with DavidGroveshelpeddevelopthe confromthe thermalexpansion
of water,suchthathot fluidis
in thispaper,andhiseditingandreviewis
displaced
upwardby the inwardflowof cooler,denserfluid ceptsproposed
DavidJohnandTed McKeefromthe
(Crisset al., 1991;Bj0rlykke,1993).In Figure14 recharge greatlyappreciated.
Geological
Survey
performed
K-Aranalyses
on
forthesystem
isshown
to occurwithinthebasinwithupflow UnitedStates
samples
andoneillitcsample
fromtheCrofoot-Lewis
alongthe range-bounding
faultsat thebasinmargins
aspro- alumitc
available
for stableisotope
posedbyMaseandSaGs
(1980)fromtheirstudyof theBlack depositandmadethesesamples
Rock Desert area.
analyses;
theirsharing
ofdataandideasisgreatly
appreciated.
with BradWigglesworth,
KenJones,
AlainCotFluiddischarge,
andthelocalization
of geothermal
activity Discussions
contributed
to
andepithermal
mineralization
in northernNevada,is com- noir,andFrederickFelderhavesignificantly
in thispaper.We acknowledge
reviews
monlyrestricted
to therange-bounding
faultsalongthepedi- the ideaspresented
mentat the basinmargins.
Geophysical
studiesshowthat by ByronBerger,Phil Bethke,Jeff Doebrich,A1 Hofstra,
and Economic
manybasinsin northernNevadaare highlyasymmetrical,DavidJohn,Ed Mikucki,StuartSimmons,
refereesthat havecontributed
significantly
to imwiththedeepersideadjacent
to thedominant
or masterfault Geology
thismanuscript.
Several
of thegasextractions,
isotoandthe othersidemarkedby valleyward
tilt anda complex proving
andcountless
othertasks
weredonein thelaboseriesof antithetic
faults(Stewart,1971).We predictthatthe picanalyses,
at the U.S.G.S.
moresignificant
anddeeperpenetrating
masterfaultshavea ratoryby CarolGent,andher contributions
in Denverare greatlyappreciated.
significant
role in focusing
deepfluidupflow(andperhaps stableisotopelaboratory
andDevelopment
Inc. andGranges
Inc.
heat-flow
fromdepth),andthereforeexerta majorcontrol HycroftResources
financial
support
fortheproject.
Additional
support
onthe distribution
of epithermal
andgeothermal
systems
in provided
wasprovided
bytheKeyCentreforStrategic
MineralDeposthe basin-and-range
setting.
itsandanOverseas
Postgraduate
Research
Scholarship
at the
Summary
University
of WesternAustralia.
Radiogenic
isotopeagessuggest
that the Crofoot-LewisNovember
6, 1995;June26, 1997
system
waslong-lived,
with at leastintermittent
activityfor
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Aminoil USA, Inc., 1978, Geochemical Survey, Leach Hot Springs, Pershing County, Nevada: NV/LCH/AMN-1, 11 p.

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