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Geochemistry and Stable Isotopes of Surface Waterand Groundwater in the Continental Pit in Butte,Montana, USAAmber McGivernMontana Tech of the University of Montana

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Recommended CitationMcGivern, Amber, "Geochemistry and Stable Isotopes of Surface Water and Groundwater in the Continental Pit in Butte, Montana,USA" (2014). Graduate Theses & Non-Theses. Paper 3.

Geochemistry and Stable Isotopes of Surface Water and Groundwater in the Continental Pit in 

Butte, Montana, USA 

AmberL.McGivern

ANon‐thesisResearchPapersubmittedaspartialrequirementsfor:

MasterofScienceDegreeGeosciences:HydrogeologicalEngineeringOption

DepartmentofGeologicalEngineeringMontanaTechoftheUniversityofMontana

Butte,Montana

July17,2014

ResearchCommittee:

Dr.ChrisGammons(Chair)

Dr.GlennShaw(MontanaTech,Dept.ofGeologicalEngineering)

Dr.GaryIcopini(MontanaBureauofMinesandGeology)

 

1

Abstract

TheContinentalporphyryCu‐Momine,located2kmeastofthefamousBerkeleyPitlakeof Butte, Montana, contains two small lakes that vary in size depending on miningactivity.IncontrasttotheacidicBerkeleyPitlake,theContinentalPitwatershavenear‐neutral pH and relatively low metal concentrations. The main reason is geological:whereastheBerkeleyPitminedhighly‐alteredgraniterichinpyritewithnoneutralizingpotential, the Continental Pit is mining weakly‐altered granite with lower pyriteconcentrationsandupto1‐2%hydrothermalcalcite.

The purpose of this studywas to gather and interpret information that bears on thechemistryofsurfacewaterandgroundwaterintheactiveContinentalPit. Pre‐existingchemistrydata fromsamplingof theContinentalPitwerecompiled fromtheMontanaBureau of Mines and Geology and Montana Department of Environmental Qualityrecords. In addition, in March of 2013, newwater samples were collected from themine’smaindewateringwell,theSarsfieldwell,andanearbyacidicseep(PavilionSeep)and analyzed for trace metals and several stable isotopes, including D and 18O ofwater,13C of dissolved inorganic carbon, and34S of dissolved sulfate. InDecember2013, several soil samples were collected from the shore of the frozen pit lake andsurroundingarea.ThesoilsampleswereanalyzedusingX‐raydiffractiontodeterminemineralcontent.

BasedonVisualMinteqmodeling,waterintheContinentalPit lakeisnearequilibriumwithanumberofcarbonate,sulfate,andmolybdateminerals,includingcalcite,dolomite,rhodochrosite (MnCO3), brochantite (CuSO4·3Cu(OH)2), malachite (Cu2CO3(OH)2),hydrozincite (Zn5(CO3)2(OH)6), gypsum, and powellite (CaMoO4). The fact that thesemineralsareclosetoequilibriumsuggeststhattheyarepresentontheweatheredminewalls and/or in the sediment of the surface water ponds. X‐Ray Diffraction (XRD)analysisof thepond“beach”sample failed toshowanydiscretemetal‐bearingphases.Oneofthesoilsamplescollectedhigherinthemine,nearanareaofactiveweatheringofchalcocite‐richore,containedover50%chalcanthite(CuSO4·5H2O).Thiswater‐solublecoppersaltiseasilydissolvedinwater,andisprobablyamajorsourceofcoppertothepond and underlying groundwater system. However, concentrations of copper in thelatter are probably controlled by other, less‐soluble minerals, such as brochantite ormalachite.

Although the acidity of thePavilion Seep is high (~11meq/L), the flow ismuch lessthan the SarsfieldWell at the current time. Thus, the pH, major andminor elementchemistry in the Continental Pit lakes are buffered by calcite and other carbonateminerals. FortheContinentalPitwaterstobecomeacidic, the influxofacidicseepage(e.g.,PavilionSeep)wouldneedtoincreasesubstantiallyoveritspresentvolume.

Keywords:neutralminedrainage,tracemetals,pitlake,geochemistry,mineralsolubility

2

Introduction

JusteastofthefamousBerkeleyPitlakeinButte,Montana(DavisandAshenberg,1989),

there is a lesser known active open pitmine, the Continental Pit. The Continental Pit

containsa largebut low‐gradeCu‐Moporphyrydeposit (Czehura,2006). Twosurface

waterbodiesexistontheproperty,herecalledtheNorthPondandtheSouthPond(Fig.

1).Presently,theelevationsoftheselakesaremaintainedatalowlevelbytheSarsfield

pumpingwell,locatednearthenorthshoreoftheNorthPond.Pumpingceasedin2001‐

2003 after the mine temporarily shut down in 2000 due to unfavorable economics.

Dewatering resumed in 2003when themine reopened, and continues to the present

day,ata typical rateof800 to1200 litersperminute (S.Czehura,pers. comm.March

2013).

Figure1.GoogleEarthimageshowingthelocationoftheNorthandSouthPondsintheContinentalPit,withthenearbyBerkeleyPitlakeforcomparison.ThePavilionSeep(notshown)islocatedonthesoutheastwall

oftheContinentalPit.ImagetakenJuly16,2002.

During the mine‐closure period of 2000‐2003, samples of groundwater from the

Sarsfield Well and/or samples of surface water from the North Pond (Fig. 2A) were

collectedmonthlybytheMontanaBureauofMinesandGeology(MBMG). Since2006,

theactiveminehasbeencollectingwatersamplestwiceayearfromseverallocationson

theirproperty,includingtheNorthandSouthPonds,theSarsfieldWell(sampledinstead

of theNorthPondwhen the latter isdryor frozen), and thePavilionSeep.Thewater

qualityresultsaresent totheMontanaDepartmentofEnvironmentalQuality(MDEQ),

andarepublicallyavailableuponrequest.ThePavilionSeepisarelativelysmallvolume

of acidic groundwater that enters the Continental Pit from a bench on the southeast

Sarsfield Well 

3

highwall. Atthetimeofthiswriting,PavilionSeepwateriscollectedandpumpedtoa

siteoutsideofthepitwhereitismixedwithSarsfieldWellwaterandusedinthemill.

Becausetheactiveminingcompanyintendstocontinueopen‐pitminingfordecades,no

formalplansforclosureoftheContinentalPitexist.Thefinalclosurescenariomaywell

notincludeapitlake.Nonetheless,itisinterestingtolookatthedatainhandtopredict

thewaterqualityofahypotheticallakeformedfromfloodingoftheContinentalPit,and

tocomparethisresulttothenearbyBerkeleyPitlake(Fig.2B).

Figure2.Close‐upphotographscomparingtheappearanceofwaterintheNorthPondoftheContinentalPit(left,withasubmersiblepump)andtheBerkeleyPitlake(right).PhotoscourtesyofNickTucci,MBMG.

BerkeleyPitBackground

TheBerkeleyPitbeganfillingwithwaterin1983,whenminingoperationsceased,and

shortly thereafter became a Superfund site (Gammons and Duaime, 2006; Metesh,

2006). The pit lake is a part of the Clark ForkBasin Superfund Complex, the largest

Superfund complex in theUnited States (Moore and Luoma, 1990; US‐EPA, 1994). Its

location at theheadwaters of SilverBowCreek,whicheventually flows into theClark

Fork River, makes it an important water body to monitor. The geochemistry of the

Berkeley Pit lake has been investigated by many agencies and academic groups for

decades(DavisandAshenberg,1989;Pellicorietal.,2005;GammonsandDuaime,2006;

GammonsandTucci,2013). SamplesarecurrentlycollectedbiannuallybytheMBMG,

and water quality results posted on the internet at the Bureau’s Groundwater

InformationCentersite(GWIC,2013). BasedontheUS‐EPA’sRecordofDecision(US‐

EPA, 1994), a water treatment plant will need to begin treating Berkeley Pit water

before the elevation of the lake reaches 5410’ above sea level, the so‐called “critical

level”. Currently, the critical level is projected to be reached sometime near 2023

(Pitwatch, 2014).With the exception of the bottom of the active Continental Pit, the

critical levelof5410’represents the lowestelevation intheButteSummitValley. The

4

groundwaterdividethatseparatesthepitlakeandsurroundingfloodedmineworkings

fromSilverBowCreekisshowninFigure3.

Figure3.Groundwaterflowmapwithundergroundworkingsofvariouslevelsshown.Thedashedanddottedlinerepresentsagroundwaterdivide(Duaimeetal.,2004;Gammons,etal.,2009).

Geology

The deposits in Butte are Cu‐Moporphyry style deposits cut by rich Cu‐Pb‐Zn‐Mn‐Ag

lode veins (Houston and Dilles, 2013). These deposits are hosted in Butte Quartz

Monzonite,partof theBoulderBatholith,which is approximately76millionyearsold

(Rusketal.,2008).TheBerkeleyPitwasminedforchalcocite‐richsupergeneoresand

zonesofdeeper,closely‐spacedveins thatwere leftbehindby theundergroundmines

(Czehura, 2006). The main ore minerals of the Continental Pit are chalcopyrite and

molybdenite(Czehura,2006).ThisdepositislocatedinaportionofthePittsmontDome

thathasbeenupliftedbytheContinentalFault(Fig.4).TheContinentalFaultisanormal

faultwithover1000mofdisplacement(Rusketal.,2008).

5

Figure4.GeologiccrosssectionoftheBerkeleyandContinentalPits,showinglocationoftheAnacondaandPittsmontDomes(Czehura,2006).

Methods

WaterchemistrydatafortheNorthPondfrom2001to2003weredownloadedfromthe

MBMG’s Groundwater Information Center website (GWIC, 2013). In June of 2003,

shortly beforedewatering resumed, a vertical profile of fieldparameters in theNorth

PondwascollectedbyMBMGfromaboat,andwatersamplesweretakenatnear‐surface

andat a depthof 11m. Additional data for theNorthPond, theSouthPond, and the

Pavilion Seep collected between 2006 and 2012were provided byMDEQ. All of the

water quality analyses were conducted by certified labs following strict quality

assurance protocols. In March 2013, the author sampled the discharges from the

SarsfieldWellandthePavilionSeep.Samplesweresubmittedforchemicalanalysis,as

well as determination of 18O and D of water, 34S of dissolved sulfate, and 13C of

dissolved inorganic carbon. In December 2013, soil sampleswere collected from the

shores of the North Pond and bottom of the Continental Pit. These samples, which

contained secondary copper salts, were ground to a fine powder using amortar and

pestleandisopropylalcoholandthenanalyzedbyX‐RayDiffraction(XRD)attheDept.

ofMetallurgicalEngineering,MontanaTech.

DataforeachchemicalanalysisoftheNorthPondbetween2001and2003(n=17)were

input into the program Visual Minteq, v. 3.0 (a recent adaptation of the original

MINTEQA2programofAllisonetal.,1991),andsaturationindices(S.I.)wereestimated

foranumberofmineralsandamorphoussolidphases.Inthispaper,theS.I.valuesfor

6

eachsolidwereadjustedtoonemetal ionperformulaunit. Forexample,amongstthe

commonsecondaryCuminerals,theS.I.valueformalachite(Cu2CO3(OH)2)wasdivided

by twoandtheS.I.value forazurite(Cu3(CO3)2(OH)2)wasdividedby three. Thiswas

donesothatS.I.valuescouldbemoreequallycomparedbetweenasetofmineralsthat

contain a common element of interest. Formodeling purposes, dissolved Cu andMn

wereassumed tobe in the+2valence, andMoandUwereassumed tobe+6.Fewas

assumedtobe+2intheSarsfieldWellandPavilionSeep,and+3intheContinentalPit

lakewaters. These redox assignments are consistentwith the presence of > 5mg/L

dissolved oxygen throughout the water column of the north lake in June 2003 (see

below).

Results

Waterqualityresultsfrom2013samplingoftheSarsfieldWellwererelativelysimilarto

pastdata;mostofthemajorionswerepresentinquantitiesslightlylowerthaninpast

samples (Table 1). Cu, Mn, and Zn concentrations were slightly higher than past

averages, while the concentration of Fe was significantly higher than the average

concentration during mine closure and moderately higher than the average

concentration during mining activity (7.47 mg/L compared to 0.05 and 4.0 mg/L

respectively).

WhilesamplingtheSarsfieldWellat itsdischargepoint, thepHwasmeasuredat5.83.

Thesamplesat in theopenfora fewminutesandduring this time thepHrose to6.8.

ThiswaslikelyduetodegassingofdissolvedCO2.ByinputtingthefieldpHof5.83,the

alkalinity, and the temperature intoVisualMinteq, a CO2partial pressure of 0.16 atm

wasobtained. This ismuchhigher than theequilibriumpartialpressureofCO2 inair

(around0.0003atmat theelevationofButte),andexplainswhyCO2(g)wouldrapidly

diffuseoutofthesampleafterpumpingtothesurface. Forthepurposeofcalculating

saturationindices(below),thepHof5.83wasused.

ThechemicalcompositionsoftheNorthandSouthPondsintheContinentalPitarevery

similar(seeTable1).BothwaterbodieshaveapHnear7,withasignificantamountof

bicarbonate alkalinity (> 100mg/L as CaCO3). The concentrations ofmostmetals of

concern,suchasCuandZn,arequitelowwhencomparedtotheBerkeleyPitlake,but

are nonetheless well above regulatory standards for surface water. However, it is

emphasizedthatthesestandardsarenotrelevantaslongasthemineisinoperation.

7

Table1.ChemicalcompositionofselectedwaterbodiesofinterestinthevicinityoftheContinentalPit.Allsoluteconcentrationsareinmg/L

Location pH SC1 Ca Mg Na K SO42‐ Cl‐ F‐ HCO3‐ NO3‐N PO4‐PDatafromthisstudy

SarsfieldWell 5.83 1940 396 32 36 5.3 1100 9.0 2.8 146 <0.01 <0.02

PavilionSeep 2.87 1853 249 73 28 6.0 1200 n/a n/a <1 n/a n/a

PreviousResults

CP‐Nsurface2 7.34 2050 433 37 38 10 1241 9.1 3.3 146 1.3 <0.5

CP‐Nsurface3 7.05 n/a 502 43 36 8 1310 8.9 4.2 123 0.6 0.04

CP‐N11m4 6.55 2140 477 46 41 11 1440 8.6 4.3 92 1.7 <0.5

CP‐Ssurface5 7.33 1860 397 38 32 6.1 1070 8.9 4.1 115 0.23 0.08

Pavilion6 3.31 2360 225 86 24 4.6 2060 5.3 4.7 <1 0.57 0.14

BPitsurface7 2.75 7340 475 534 74 9.6 8170 <50 30 <1 <5 <5

BPit210m7 2.53 8160 442 515 74 7.6 9090 <50 30 <1 <5 <5

Al As Cd Cu Fe Mn Mo Ni Si Sr U ZnDatafromthisstudy

SarsfieldWell 0.44 0.001 0.01 0.66 7.74 3.78 0.45 0.018 10.5 2.61 0.08 3.89

PavilionSeep 14.8 <.015 0.21 33.7 14.2 15 n/a 0.09 22 1.24 n/a 20.5

Previousresults

CP‐Nsurface2 0.98 0.001 0.05 0.43 0.05 3.1 0.69 0.031 5.5 4.07 0.29 3.14

CP‐Nsurface3 1.01 0.005 0.04 1.61 4.0 5.5 n/a 0.043 9.3 2.94 n/a 6.46

CP‐N11m4 0.18 <.005 0.17 3.71 0.04 5.5 0.55 0.039 5.4 5.27 0.22 8.92

CP‐Ssurface5 0.24 0.003 0.06 1.05 0.23 8.6 n/a 0.047 8.7 1.75 n/a 6.76

Pavilion6 41 0.02 0.43 58.6 76.7 22 n/a 0.15 28 0.66 n/a 46.2

BPitsurface7 275 0.135 2.26 79 564 261 <0.02 2.4 38 1.55 1.97 627

BPit210m7 268 0.097 2.27 153 1007 248 <0.02 1.6 36 1.43 1.45 6381SpecificconductanceinµS/cm;n/a=notanalyzed2ContinentalPitNorthPond,averageof17samplescollectedin2001‐2003(GWIC,2013)3Continental Pit North Pond or Sarsfield pumpingwell, average of 11 samples collected in 2005‐2012(MDEQ,2013)4Lakesamplecollectedatdepthof11m(GWIC,2013)5Continental Pit South Pond, average of 10 samples collected between 2005 and 2012 (MDEQ, 2013)6PavilionSeep,averageof11samplescollectedbetween2005and2012(MDEQ,2013)7BerkeleyPitlakesamplescollectedonNov.6,2007(GWIC,2013).

UnlikesomepitlakeswithneutraloralkalinepH,theconcentrationofarsenicinbothof

theContinentallakesisnotparticularlyhigh(<10g/L).ThisisnotduetoalackofAs

in theorebody, since theButtedeposits are locally rich in theCu‐As‐sulfideminerals

enargiteandtennantite(Meyeretal.,1968).ItispossiblethatAsisbeingadsorbedonto

secondaryFe‐oxy‐hydroxidemineralsonweatheredbedrocksurfaces.Incontrasttothe

Berkeley Pit, which has undetectable quantities of dissolved Mo (Table 1),

concentrationsofMointheNorthPondareelevated(inthe0.5to0.7mg/Lrange).This

differenceisgeological,astheBerkeleydepositproducednoMo,whereasmolybdenite

isanimportantoremineralattheContinentaldeposit(Czehura,2006).Between2006

and2012,thePavilionseephadanaveragepHof3.31,andconcentrationsofdissolved

8

metals andmetalloidsweremuchhigher than in theNorth and Southponds, but still

considerably lower than theBerkeleyPit lake (Table1). Compared tohistoricaldata

provided by the MDEQ, the Pavilion Seep sample collected in this study had lower

concentrationsofmostsoluteswiththeexceptionofCa,Na,andK.Verylittleisknown

about the hydrology of the Pavilion Seep, and it is possible thatwater being pumped

fromthatlocationmaybeamixtureofacidminedrainageandbackgroundgroundwater

withtherelativeproportionchangingatdifferenttimesoftheyear.

Figure 5 summarizes long‐term trends in the concentrations of Cd, Cu and Zn in the

NorthPondand/orSarsfieldWellbetween2001and2013.Duringtheperiodofmine

closure(2001‐2003),concentrationsofthesemetalssteadilyincreased.TheNorthPond

wasseasonallystratifiedduringthistime(Fig.5),withachemoclinesituatedatadepth

of4.2minJune2003.Asinglewatersamplecollectedat11mdepthhadslightlylower

pHandsignificantlyhigherconcentrationsofCd,CuandZncomparedtoanear‐surface

sample(Table1).Dissolvedoxygenconcentrationswere>5mg/Latalldepths,whichis

incontrasttotheBerkeleyPitlake,whichtypicallyhasundetectableDOconcentrations

belowthetop1‐3m(Pellicorietal.,2005).Althoughthemonitoringdatabecamemore

scatteredafterresumptionofminingoperationsin2003,averagevaluesformostwater

qualityparametersintheNorthPondfrom2001‐2003aresimilartothosefrom2006‐

2013,suggestingthatwaterqualitytrendshavebeenfairlystableinthelastdecade.

AsshowninFigure6,verticalchangesinphysicalparametersintheNorthPondinJune,

2003wererelativelysmall.Aweakchemoclineexistedatadepthof4.2m,withslightly

higherSCandlowerpHbelowthislayer.Temperatureremainedbetween8and9°Cat

alldepths.Dissolvedoxygenconcentrationswere>9mg/Labovethechemocline,but

droppedbelow6mg/Ltowardsthebottomofthelake(Fig.6).

Based on the computed saturation indices (Fig. 7),water in theNorth Pond in 2001‐

2003wasclosetoequilibriumwithgypsum(CaSO4·2H2O)andfluorite(CaF2),aswellas

a number of carbonate minerals, including calcite, dolomite, smithsonite (ZnCO3),

hydrozincite(Zn5(CO3)2(OH)6),andrhodochrosite(MnCO3).Onaverage,thelakewaters

wereundersaturatedwithotavite(CdCO3).ItispossiblethatCdresidesasanimpurity

incalciteoranothercarbonatephase.AlargenumberofCu(II)mineralswerealsonear

equilibrium, including malachite (Cu2CO3(OH)2), azurite (Cu3(CO3)2(OH)2), antlerite

(Cu3(SO4)(OH)4), brochantite (Cu4SO4(OH)6), langite (Cu4SO4(OH)6·2H2O), andpossibly

atacamite(Cu2Cl(OH)3).TheSarsfieldWellsamplecollectedinMarch2013hadsimilar

9

results for the saturation indices of most minerals except carbonate minerals, which

wereconsistentlyundersaturatedrelativetotheNorthPondsamples(Fig.7).Themain

reasonforthisdifferenceisthattheSarsfieldWellhasalowerpHwhenfreshlypumped

duetoahighpartialpressureofdissolvedCO2.Ifthesaturationindexcalculationsare

repeated using the pH value of 6.8 obtained after the sample had sat in the open for

severalminutes,theS.I.valuesforcarbonatemineralsaremoresimilartothehistorical

valuesfortheNorthPond.

Figure5.ConcentrationsofCu,Cd,andZnintheNorthPondfrom2001through2003(GWIC,2013),from2006through2012(MDEQ,2013)and2013(thisstudy).SomeofthelatterdataarefromtheSarsfieldWell.TheelevatedconcentrationsfromMay13,2003werefromasamplecollectedatadepthof11metersinthe

NorthPond.

Figure6.Verticalprofilesinwatertemperature,specificconductance(SC),pH,anddissolvedoxygen(DO)

fortheNorthPondoftheContinentalPit,collectedonJune12,2003.

Temperature, °C

6 7 8 9 10

Dep

th,

m

0

2

4

6

8

10

12

SC, S/cm

1600 1800 2000 2200 2400

DO, mg/L

5 6 7 8 9 10

Dep

th,

m

0

2

4

6

8

10

12

pH

5 6 7 8 9 10

pH

DO

SC T°C

A. B.

10

Figure7.Saturationindicesofvariousmineralsbasedon17chemicalanalysesoftheNorthPondoftheContinentalPit.Rectanglesrepresentonestandarddeviation.Linesextendtomaximumandminimum

values.RedpointsrepresentdatafromtheSarsfieldWellcollectedin2013.

BoththeNorthandSouthPondshaveadistinctivegreenishbluecolor(Fig.1,Fig.2A),

whichcouldbepartlycausedbyprecipitationofoneormoreofthesesparinglysoluble

secondarycopperminerals.SamplesofprecipitatesfromaroundtheNorthPondandon

the floor and wall of the Continental Pit were obtained and analyzed using X‐ray

diffraction.Atthetimeofsampling,theNorthPondwasfrozenandsamplesfrombelow

the water line could not be obtained. A sample of a blue mineral obtained from a

chalcocite‐richpartoftheoreonthesideoftheContinentalPitwasdeterminedbyXRD

analysistobechalcanthite(Cu5SO4·5H2O).Itisunlikelythatchalcanthiteisstableinthe

pondwateritself,sinceitistoosoluble(saturationindicesarestronglynegative,andare

notshowninFig.7).Afterrainevents,thechalcanthiteonthegroundsurfaceprobably

dissolves and is flushed into the groundwater and/or themine ponds. Much of this

copperprobablyre‐precipitatesasother, lesssolubleCuminerals, suchasbrochantite

ormalachite. However, theexistenceof these lesssolubleminerals in thepondcould

notbetestedinthisstudy.

It issignificantthatmetalconcentrationsintheNorthPondappeartobecontrolledby

mineralsolubility limits,asthismeansthattheconcentrationsofelementssuchasCd,

Cu, Mn, and Zn cannot increase without a simultaneous decrease in pH and/or

11

bicarbonate concentration. Amongst the various silica polymorphs, the lake waters

were closest to equilibrium with chalcedony (micro‐crystalline quartz). Finally, it is

interesting that the lakewaterswere found to be near equilibrium or supersaturated

with several molybdate minerals, including CaMoO4, CdMoO4, CuMoO4, and ZnMoO4.

This is not surprising considering the abundance of molybdenite, an important ore

mineral,inthedeposit.

Acidity and Alkalinity  

In2013,theSarsfieldWellhadanalkalinityof146mg/LCaCO3,whichisequivalentto

2.39meq/Lofalkalinity.TheacidityofthePavilionSeepcollectedonthesamedaywas

calculatedtobe5.19meq/L,asshowninthetablebelow.Thesecalculationsassume

thatalldissolvedFeinthePavilionSeepwasferrous(Fe2+),andthateachmoleof

dissolvedAlcontains3molesofmetalacidityandthateachmoleofdissolvedFe,Cuor

Zncontains2molesofmetalacidity.UsingaveragesofdatafromMDEQ,theacidityof

thePavilionSeepis11.03meq/L,significantlyhigherthanthealkalinityoftheSarsfield

Well.AlthoughtheflowfromthePavilionSeepissmallrelativetotheflowfromthe

SarsfieldWell,itisnoteworthythattheacidityofthePavilionwateris2to5times

higherthanthealkalinityofthedewateringwell.Ifthevolumeofacidicwaterinthe

ContinentalPitweretoincreaseinthefuture,thetotalaciditycouldpossiblyexceedthe

acid‐bufferingcapacityofthenon‐acidicgroundwaterandsurfacewateronthemine

property.Thispossibilitycannotbeevaluatedfurtherwiththedataofthisstudyalone.

Table2.PavilionSeepaciditycalculations.Concentration(mmol/L)=concentration(mg/L)/gramformulaweight of element, except H+, which was calculated using the average pH of the Pavilion Seep.meq/Lobtainedbydividingtheconcentration(mmol/L)bythechargeoftheelement.

Element

AverageConcentration

(mg/L)

2013Concentration

(mg/L)

AverageConcentration(mmol/L)

2013Concentration(mmol/L)

AverageAcidity(meq/L)

2013Acidity(meq/L)

Fe2+ 77 14.2 1.38 0.25 2.76 0.51Al3+ 41 14.8 1.52 0.55 4.56 1.65Cu2+ 59 33.7 0.93 0.53 1.86 1.06Zn2+ 46 20.5 0.70 0.31 1.41 0.63H+ ‐ ‐ 0.45 1.35 0.45 1.35

TotalAcidity= 11.03 5.19

12

 

Water, Sulfur, and Carbon Isotopes 

Resultsforstableisotopicanalysisofwater,dissolvedinorganiccarbon(DIC),and

sulfateareinTable3.

TABLE3.ResultsfromisotopicanalysisofsamplesfromtheSarsfieldWellandthePavilionSeep.

Location δ13C‐DIC δ34S‐Sulfate δ18O‐H2O δD‐H2O

SarsfieldWell ‐6.8 2.5 ‐17.8 ‐141

PavilionSeep ‐ 3.0 ‐17.7 ‐137.5

Theisotopiccompositionsofδ18OandδDintheSarsfieldWellandPavilionSeepwaters

aresimilartowaterinthefloodedmineshaftsofButte,andplotneartheintersectionof

theButteMeteoricWaterLine(MWL)andLocalEvaporationLine(LEL)(seeFig.8).As

discussedbyGammonsetal.(2006),theintersectionoftheMWLandLELmostlikely

representsaveragerechargewatersinButtethathavenotbeenaffectedbyevaporation.

Incontrast,theBerkeleyPitlakeandtheYankeeDoodletailingspondweremoderately

tohighlyevaporatedwhenthesamplesplottedinFigure8werecollected(seePellicori

etal.,2005).ThefurtheroutalongtheLELagivensampleplots,themorewaterwas

losttoevaporation.

Figure8.Isotopicdataforsamplescollectedinthisstudy(SarsfieldWell,PavilionSeep)plottedagainsttheButteMeteoricWaterLineandLocalEvaporationLine.DatafortheBerkeleyPit,thefloodedmineshafts,

andthetailingspondarefromGammonsetal.(2006)andGammonsetal.(2009)

‐170

‐150

‐130

‐110

‐90

‐70

‐50

‐20 ‐15 ‐10 ‐5 0

D‐w

ater, ‰

18O‐water, ‰

Berkeley Pit

Flooded Mine Shafts

Tailings Pond

Sarsfield Well

Pavilion Seep

13

ThefactthatthewaterintheSarsfieldWellissimilartothatofgroundwaterinthe

floodedmineshaftsandnottotheBerkeleyPitlakeortailingspondwaterisevidence

thatneithertheBerkeleyPitlakenorthetailingspondisleakingtoanysignificant

degreeintotheContinentalPitatthistime.Thewater‐isotopedataclearlyshowthatthe

waterfromtheSarsfieldWell(andfromthePavilionSeep)issimilartoaveragerecharge

watersinButte,andhasnotbeenevaporated.Monitoringthestableisotope

compositionofwaterintheSarsfieldWellovertimecouldbealow‐costmethodtoseeif

thissituationchangesinthefutureif,forexample,theactivemineexpandstowardsthe

BerkeleyPit.

The isotopic composition of sulfate in the Butte underground mine waters and the

BerkeleyPitlakeistheresultofthreeprocesses:oxidationofpyriteandothersulfides,

dissolutionofsulfateminerals,andbacterialsulfatereduction(Gammonsetal.,2009).

In Figure 9, S‐isotopic compositions of dissolvedH2S aremuchmore depleted in34S

compared to sulfate in the same mine waters due to the large fractionation factor

associated with bacterial sulfate reduction (Seal, 2003; Gammons et al., 2009).

Dissolved sulfate in the SarsfieldWell and Pavilion Seep has S‐isotopic compositions

similar to that of pyrite in the Butte deposit (Field et al., 2005). It is also similar to

sulfate in the Berkeley Pit lake (Pellicori et al., 2005). The Continental Pit shows no

evidenceofbacterialsulfatereduction,ashasbeenseeninthefloodedmineshaftsofthe

West Camp andOuterCamp (Gammons et al., 2009), so it is likely that the S‐isotopic

compositionsarefromoxidationofpyrite.

14

Figure9.SulfurisotopecompositionofdissolvedsulfateintheSarsfieldWellandPavilionSeep,comparedtootherminewatersfromButte.DataforsulfateandsulfidefromtheWestCampExtractionWell(WCEW)and

theothermineshaftsaresummarizedinGammonsetal.(2009).DatafromtheBerkeleyPitarefromPellicorietal.(2005).DataforButtepyrite,anhydrite,andbaritearefromFieldetal.(2005).

Dissolved inorganic carbon (DIC) in the SarsfieldWell is a mix of dissolved CO2 and

HCO3‐ ion that is isotopicallyheavier thanotherminewaters in theButtedistrict (see

Fig.10). Inmostof the floodedmineshaftsofButte,DIChasaC‐isotopecomposition

that is close to equilibriumwithhydrothermal rhodochrosite in theMain Stage veins,

basedondataforthelattergiveninGarlickandEpstein(1966).DICintheOuterCamp

workings(e.g.,OrphanGirlandOrphanBoy)hasmuchlighter13C‐DIC,duetoinputsof

light CO2 from bacterial sulfate reduction (Gammons et al., 2009). The DIC in the

Sarsfield Well shows no evidence of input of biogenic CO2, and probably came from

dissolutionofcalciteintheContinentalorebody.CalciteintheContinentalPitoccursas

lateveinsanddisseminationsinalteredgranite,andcomprisesasmuchas1%ormore

oftherockvolume(NewbroughandGammons,2002;Lamsma,2012).Althoughitwas

notpossibletoanalyzeanyofthiscalciteforC‐isotopesinthisstudy,datainGarlickand

Epstein(1966)showthathydrothermalcalciteintheButteDistrictisabout3‰heavier

in13C than rhodochrosite. So, the fact that the SarsfieldWell has heavierDIC could

15

partly be explained by the abundance of calcite and scarcity of rhodochrosite in the

Continental deposit. Some isotopic enrichment of DICmay also have been caused by

lossofCO2duringsamplingorpreparationof theSarsfieldWellwater forDIC isotope

analysis. AsshownbyClarkandFritz (1997),dissolvedCO2 isabout10‰lighter in

13CcomparedtoHCO3‐ion.So,lossofCO2duringsamplingwouldhavecausedthe13C

oftheremainingDICtobecomeheavier.

Figure10.CarbonisotopecompositionofdissolvedinorganiccarbonintheSarsfieldWellcomparedtoother

minewatersfromButte(mineshaftdatafromGammonsetal.,2009).

Discussion

Thedatapresentedaboveshowastarkcontrastbetween thehighlyacidicandmetal‐

richBerkeleyPit lakeandthenearbyContinentalPit lakeswhichhaveneutralpHand

relatively lowmetal concentrations. Themain reason for thisdifference is geological.

ThetwoopenpitminesareseparatedbytheContinentalFault(Fig.1),a largenormal

fault with > 1 km of vertical displacement (Czehura, 2006; Rusk et al., 2008), down‐

dropped to the west. The Berkeley deposit is rich in pyrite (> 5 wt %) with highly

alteredgraniticbedrock(allfeldsparandmaficmineralsconvertedtomuscoviteand/or

clay + quartz) that has no acid‐neutralizing potential. In contrast, the Continental

deposit,whichrepresentsadeeperandmoreperipheralmineralassemblagewithinthe

ButteDistrict,containsweakly‐alteredgranitewallrock(freshfeldsparswithabundant

primaryandsecondarybiotite)withlowerpyritecontent(1to2wt%)andasmallbut

16

significant quantity of hydrothermal calcite. The latter occurs as thin veins and

disseminations throughout the ore body (Newbrough and Gammons, 2002; Lamsma

2012).ThinsectionsinFigure11,takenduringthestudyofNewbroughandGammons

(2002), compare the mineralogy of the pits. In addition, whereas the Berkeley Pit is

surroundedandunderlainby1000’sofkmofundergroundmineworkings(Duaimeet

al., 2004; Gammons et al., 2009), very few underground workings extended into the

ContinentalPitarea.Thus,thehydrologyofgroundwaterflowisdominatedbyopenor

backfilled/collapsed voids in the vicinity of the Berkeley Pit, as opposed to fractured

bedrockinthevicinityoftheContinentalPit.

Newbrough and Gammons (2002) characterized rock samples from the Berkeley and

Continental Pits by conducting static acid‐base accounting (ABA) tests. Three samples

werecollectedfromtheContinentalPit;twofromtheeastsideoftheContinentalFault

that areweakly potassic/phyllic altered, and one from thewest side of the fault that

represented phyllic alteration like many of the samples from the Berkeley Pit. The

results of the tests are in Figure 12. The 3:1 and 1:1 slopes represent neutralization

potential ratios (NPR), or the ratio of acid neutralizing potential to acid generating

potential(NP/AP).Itwasdeterminedthatwhilebothsamplesetsfromeachpithadacid

generatingpotential,theBerkeleyPitsampleswerefarmorelikelytogenerateacid.The

sample from the far west side of the Continental Pit behaved like the Berkeley Pit

samples,asitalsoshowedhighAPandalmostnoNP.

17

Figure11.ThinsectionsfromtheBerkeleyPit(AandB)andContinentalPit(CandD).qtz=quartz,biot=biotite,Kspar=potassiumfeldspar,plag=plagioclasefeldspar.

Figure12.ResultsofABAtestingofsamplesfromtheBerkeleyPitandContinentalPit.BQMisButteQuartz

Monzonite.FromNewbroughandGammons(2002).

qtz

Kspar

plag

Kspar

plag sericite

biot

2° biot

2° biot

calcite

qtz

A B

C D

18

Newbrough andGammons (2002) alsoused long‐termhumidity cell tests to compare

leachatefromcrushedbedrockexposedintheBerkeleyandContinentalPits,andfound

very similar water chemistry results to the data presented in this paper. However,

NewbroughandGammonsobserved that calcitewasbeingdepleted from thecrushed

Continental Pit samples faster than pyritewas being oxidized. Extrapolation of these

resultssuggestedthattheContinentalPitleachatescouldbecomeacidicatafuturetime,

whentheneutralizingpotentialwascompletelydepleted. Dueto thesmallnumberof

samples in theNewbrough andGammons study, and the inherent difficulty of scaling

humiditycellteststofieldsettings,itisdifficulttosayiforwhenthesmallpondsinthe

ContinentalPitmightbecomeacidic.ThePavilionSeepshowsthatsomeacidicdrainage

already exists in the Continental Pit. Although the volume of this acidic water is

presentlymuchlessthanthepH‐neutral,alkalinewaterthatresidesinthepondsorthat

isbeingpumpedfromtheSarsfieldWell,theacidityoftheaciddrainageis2to5times

greater than thealkalinityof thenon‐acidicwaters.Over time, therelativevolumesof

acidicandnon‐acidicwaterscouldchangewithcontinuedweatheringoftheorebody.

ConclusionsandRecommendations

TheContinental Pit contains two small lakes that havenearneutral pHand relatively

low metal concentrations compared to the Berkeley Pit lake, which is highly acidic.

Miningactivityisexpectedtocontinuefordecades,andclosureplansfortheContinental

Pitmaynot includeapit lake.BasedonVisualMinteqmodeling, theexistingponds in

the Continental Pit are close to equilibrium saturation with a number of carbonate,

sulfate, andmolybdateminerals. Formetalswhose solubility is limited by carbonate

minerals (e.g., Zn, Cu), dissolved concentrations can only increase if pH or HCO3‐

concentrationsdecrease. Nocleartrend inthisdirection isapparent fromthepast10

years of water quality monitoring. Some acid mine drainage does exist within the

boundariesoftheContinentalPit,andit ispossiblethatthevolumeand/orseverityof

thisAMDcouldincreaseinthefuturewithcontinuedweatheringoftheorebody.This

scenariocannotbeevaluatedatthepresenttime.

Othersignificantfindingsfromthisstudyincludethefollowing:

TheSarsfieldWellhasalowerpHwhenfreshlysampledcomparedtowhenthe

waterhasbeenallowedtosit in theopenforseveralminutes. This isprobably

duetodegassingofdissolvedCO2.

19

The isotopiccompositionofwater in theSarsfieldWellshowsnoevidence that

theBerkeleyPitortailingspondiscurrentlyleakingwatertotheContinentalPit.

Basedon S‐isotopes, sulfate in the SarsfieldWell, aswell as the acidicPavilion

Seep,ismostlikelyderivedfromoxidationofpyriteandothersulfidemineralson

theminewalls.

The13CofdissolvedinorganiccarbonintheSarsfieldWellisquiteabitheavier

thanDICfromthefloodedmineshaftsofButte.Someofthisdifferencecouldbe

due to the relative amounts of calcite and rhodochrosite in the district, as

previous work has shown that hydrothermal calcite contains heavier carbon

comparedtorhodochrosite.

Somerecommendationsforfurtherworkincludethefollowing:

It is important tocontinue tomonitor thewaterqualityof thedewateringwell

and surface waters in the Continental Pit, to see if conditions change with

continuedminingandwithcontinuedweatheringoftheminewalls.

ModelingthesedatainaprogramsuchasPHREEQCcouldgivemoreinformation

aboutthewaterqualityofahypotheticalContinentalPitlake,includingchanges

in water chemistry that might occur over time in response to changes in the

relativevolumesofSarsfield‐likewaterandPavilion‐likewater.

Measuring the isotopic composition of calcite in theContinental Pitmight help

answerthequestionofwhethertheDICoftheSarsfieldWellisisotopicallyheavy

duetomineral‐buffering,orduetolossofCO2duringsampling.

MeasuringtheisotopiccompositionofwaterintheSarsfieldWellcouldbealow‐

cost method to test if leakage from the Berkeley Pit or tailings pond into the

expandingContinentalPitoccursinthefuture.

Acknowledgements

Iwouldliketothankvariouspeoplefortheirsupportwiththisproject;GaryIcopiniand

Glenn Shaw for being a part of my committee; Gary Wyss for assistance with XRD

analysis;MontanaResources for access to data and also permission to sample on the

property,andTedDuaimeforhelpingtosettheprojectup. Iappreciatethehelpfrom

NickTucciofMBMGwithsamplingandJamesCastroofMontanaDEQforhelptracking

downdata.SpecialthanksforthesupportandguidanceofmyadvisorChrisGammons.

20

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