Economic Geology Vol. 92, 1997,pp. 20-28
The Roleof Mercury-Organic Interactions in the Hydrothermal Transport of Mercury JEREMY B. FEIN* ANDA. E. WILLIAMS-JONES McGillUniversity, Department of EarthandPlanetary Sciences, 3450University Street,Montreal,Quebec, CanadaH3A2A7 Abstract
Despitethecommon association bet•veen mercury deposits andliquidhydrocarbons, theeffects of mercury organic interactions on mercury transport havereceived littleattention. In thisstudy,we estimate theextent of aqueous mercurycomplexation, andthe partitioning of mercuryamongaqueous liquid,aqueous vapor, andanorgmfie phase, quantifying therelativeimportance of eachphasein mercury transport. Thecalculations
suggest thatsignificant mercury transport in theaqueous liquid,predominantly asHg•aq), is onlypossible underrelatively oxidizing andalkaline conditions. Thefrequent occurrence ofliquidhydrocarbons in mercurydepositing hydrothermal systems, however,impliesthat conditions are reducingandthat aqueous liquid transport of mercuryis relatively unimportant. High concentrations of mercuryalsocanbe dissolved in aqueous vapor.However, boilingrarelyoccurs at thedepths atwhichtheoresolutions originate, andthus,the presence ofa vaporphaseisunlikely duringmercury transport. Extrapolation oflow-temperature experimental measurements of mercurysolubilities in organic phases enables quantitative estimates of mercury concentrationsin liquidhydrocarbons underhydrothermal conditions. Our calculations suggest that extremely high concentrations of mercurycandissolve in the organic phaseandthatorganic phasetransport maycontrol mercurymobilities in manymercuryore-forming hydrothermal systems. Introduction
phur Banksgeothermalsystem,California,and concluded wereunimportant tomercury transport ALTHOUGH theassociation of deposits ofmercury withpetro- thatthesecompounds because conditions were too reducing. It is possible, however, letonhasbeenlongrecognized (e.g.,DicksonandTunell, that under oxidizing conditions these compounds play a more 1968;Barneset al., 1973;White,1981;Peabody andEinaudi, significant role, and indeed, in some near-surface environ1992),little consideration hasbeengivento the possibility a significant component ofthetotal thatorganic molecules contribute to thehydrothermal trans- ments,theycanrepresent mercury in solution (e.g., Sehintu et al., 1989; Murphy et al., portof mercury. Mostworkers attributemercurymobilityin 1994).
hydrothermal systems eitherto thepresence of Hgø in an thatorganic phasetransport of mercury maybe aqueous gasphase(e.g.,White,1981;Varekamp andBuseck, Evidence important in forming economic concentrations of this metal 1984),orto Hgøand/or bisulfide complexes in aqueous solu- isprovided by reports of high concentrations of mercury (up tions(e.g.,Barneset al., 1967;Varekamp andBuseck, 1984; to 21 ppm; Bailey et al., 1961) in petroleum associated with WellsandGhiorso, 1988).Giventhatmercurydeposits pre-
geothermal systems. Moreover,expericipitateatcomparatively lowtemperatures, i.e.,between100ø mercury-depositing mental studies show that mercury solubility in alkanes ishigh, and200øC,it is possible thatmercuryformsstableaqueous andincreases withincreasing temperature. For example, uscomplexes withearboxylate ligands, dissolves asmethyl-mer- ingthe experimentally basedsolubility equation of Okouehi curycompounds, or ispartitioned intoanimmiscible organic andSasaki (1981),we estimate mercury solubilities in octane phase.In thisstudy,we consider eachof thesepossibilities.of 2.6 ppm at 25øCand 105 ppm at 100øC.Thesevalues Carboxylates havebeenmeasured in highconcentrations areat leastan orderof magnitude greaterthanthe highest in hydrothermal fluidsfromavarietyof geologic settings. For measured aqueous concentrations of mercuryin activegeoexample,MaeGowanand Surdam(1988)reportedacetate thermalsystems (White,1981). andoxalateconcentrations of 10,000and500 ppm,respec- In thispaper,we havedeveloped a comprehensive speeiatively,in formation watersfromthe SanJuaquin sedimentarytionmodelfor typicalmercuryore-forming fluidswhichinbasin(although it is stillunclearwhetherthesehighconcen- eludes a largenumberof inorganic aqueous species andthe trationsare representative of typicaldiagenetic fluids,e.g., following organic species: aqueous Hg acetate, Hg oxalate (to Kharakaet al., 1993). Martens (1990) determinedthat fluids represent mono-and difunetional earboxylates) andmethylventingfromthe Guyamas basincontainacetateandpropio- mercury.Althoughdisequilibrium biological processes can hateat concentrations rangingup to 65 and15 ppm,respec- affectmercury speeiation through methylation inlow-tempertively.Carboxylates formhighlystableaqueous complexesatureenvironments, weconsider onlyabiotieequilibrium syswith a varietyof metalcationsand havebeen invokedto tems.Ourmodelalsoaccounts forthepartitioning ofmercury explain mass transport of leadandzincin Mississippi Valley- amongaqueous liquid,aqueous vapor,and an immiscible type ore-depositing systems(e.g., Giordanoand Barnes, organic phaserepresented by C5-C•0alkanes. Thisstudyis 1981).Mercuryformsstable, water-soluble methylated com- thefirstto quantif•v theroleof aqueous earboxylate eomplexpounds in naturalwaters. WellsandGhiorso (1988)evaluated ationandorganicphasetransport in mercurymobilization. the contribution of methyl-mercury compounds in the Sul- Our modelprovidesa realisticevaluation of the behavior of mercuryin an organic-bearing hydrothermal system and the conditions requiredto transport mercuryor* Present address: University of NotreDame,CivilEngineering andGeo- establishes logicalSciences, NotreDame,Indiana46556-0767. ganieally in concentrations sufficient to formoredeposits. 0361-0128/97/1901/9,0-956.00
9,0
Hg-ORGANIC INTERACTION IN HYDROTHERMAL SYSTEMS
21
Ore-DepositingEnvironment In theUnitedStates, mainlyin theCalifornia CoastRange, hasbeenfromdeposits reMercurydeposits forminlow-temperature (typically 100ø- mostof the mercuryproduction orpaleohot springactivity(e.g.,Dickson and 200øC)hydrothermal systems in a widevarietyof geologic latedto present settings. Characteristic of manyof thesedeposits is a close Tunell,1968;Fisk,1968;White,1981).A featureof manyof istheirassociation withhighconcentrations of spatial association withpetroleum and/orsolidorganic matter. thesedeposits andhigherhydrocarbons. In someeases, the active The largestmercurydepositin the world(over275,000 methane hotsprings areactually developed in oilandgas t) is locatedat Almaden,Spain.It comprises strata-boundmercury-rich fields (e.g., the Cymrie oil fields). Significantly, the petroleum orebodies hostedby quartzite horizons intercalated withina up to 21 ppm Hg, whereas thickvolcanosedimentary sequence dominated by organic- fromthe Cymriefieldcontains rich blackshales. The mercuryoccursmainlyas cinnabar brinesin the samewellcontain0.2 ppm mercury(Baileyet and either is massive,fills a network of fine fractures,or is al.,1961).Heavyoilsandtarryhydrocarbons fromtheWilbur Springs district (which hosts several deposits associated with interstitial to thequartzgrains. Pyriteistheprincipal gangue are reportedto contain1 and500 ppm mineral.Otherphases associated withthecinnabar arepyr- activehot springs) (Barnes et al., 1973).Brinesfromthe same rhotite,sericite, kaolinire, andorganic matter.Thedeposit is Hg, respectively 1.5 ppb Hg (Barneset al., interpreted to haveformedduringsedimentation andearly districtcontainapproximately diagenesis fromhydrothermal fluidsassociated withshallow- 1973). Thelargestmercurydepositassociated withanactivegeomarinevolcanicactivity.Thesefluidsare thoughtto have istheSulphurBankdeposit, California, froIn leached mercury fromtheblackshales andsubsequently ex- thermalsystem (White,1981). pelledinto the oceanor circulated in underlying partially whichnearly5,000t Hg hasbeenrecovered described deposits, thereis a dear lithifiedporous sands. In bothcases, cinnabar precipitationAs with the previously betweenmercury andhydrocarbons. Thedeposits isconsidered to haveoccurred primarily in response to large association at SulphurBankarehostedby a highlyaltered(acidsulfate) dropsin temperature (e.g.,Saup&1990). (44,500yr; SimsandWhite, 1981)augite Next to Almaden,the Huancavelica quicksilver district, late Pleistocene flowandunderlying lacustrine sediments. TheminPeru,hasbeenthemostimportant mercury-producing region andesite occursin veinsandopenspaces betweenoriginal in theworld(over48,000t minedprimarily duringtheSpan- eralization of cinnabar, hydrocarbons, mamaish colonialperiod)and,like Almaden,comprises strata- jointblocksandconsists (Whiteand boundorebodies hostedmainlyby Cretaceous sandstone.site,pyrite,stibnite,dolomite,and eristobalite 1962).The cinnabar wasinterpreted by theseauCinnabar is theprincipalmercurymineralandis accompa-Roberson, fromgeothermal fluidssimilar nied by pyrite,arsenopyrite, realgar,orpiment,stibnite, thorsto havebeendeposited withsporadic cinnabar deposiquartz,calcite,barite,andminornativemercury. The bulk to thosewhichareassociated The currentfluids of thecinnabar fillsporesbetween quartzgrains. Locally, it tionin someof the presenthot springs. of justunder200øC,pH values replaces silicacement,andin someplaceshasreplaced the havea sourcetemperature 1,500ppmof dissolved detritalquartzgrains. Thehighest concentrations ofcinnabar in therange6.6to 7.5,approximately are in discordant fracture zones which erosscut the sandstone salts, 500ppmof sulfur,500ppmNH;, andover3,000ppm and controlled the infiltration of the ore fluids. In addition of dissolved CO2(White,1981).Onegeneticmodelfor the to sandstone,some mineralizationoccurswithin limestone deposits involvesleachingof mercuryfroIn sedimentary andthe transport of unitsof similarageandalsoin younger (Tertiary)felsielavas. rocks,heatedasa resultof volcanism, The limestone-hosted mineralization occurs either as tabular thismercuryto thevadose zonewhereit wasprecipitated as of the hydrothermal fluidwith bodies concordant withbedding orasdiscordant veins.In the cinnabardue to interaction voleanies, thecinnabar occurs exclusively asfracture fillings. groundwater(White, 1981).Wellsand Ghiorso(1988),on oftheoretical calculations andgeologic observations, Hydrocarbons areassociated with the oresin all threehost- thebasis thatthe mercury wastransported asa bisulfide comrocktypesandin somelocalities heavyblackoilsandtarsare suggest plex and probably was deposited as a result of oxidation. reportedto seepfromrockopenings (Yateset al., 1951).To concentration ofthegeothermal fluidspresently ourknowledge, the genesis of thesedeposits hasnotbeen Themercury discharging at SulphurBank,however, isonly0.001ppm.At investigated. andcurrentdischarge ratesit wouldtake The only other countries whichhaveproducedlarge thisconcentration amountsof mercuryare Russiaandthe UnitedStates.The at least40 m.y. to form the deposit(Barneset al., 1973), Russian deposits arerelatively poorlydescribed, andnoinfor- whichisat least1,000greaterthanitsactualage(seeabove). mationis available on the gradesor tonnages of individual Thisimpliesthateitherthe mercurycontentof the aqueous ratesmuchgreater deposits. Animportant featureof manyof thedeposits, how- fluidwasmuchhigherand/ordischarge or mercurywastransported by anever,isa closeassociation between mercury andeitherpetro- thancurrentlyobserved, by White leumor bitumen. Forexample, theChukotka deposit, which othermedium,i.e., the vaporphaseassuggested ishosted bylistwaenites (earbonitized andsilieified serpentin- (1981)or the organicphase. in the United ites)in tectonic contact withelasticmetasedimentary rocks, One of the beststudiedmercurydeposits the onlyone in whichthe comprises a stockwork of cinnabar-bearing chalcedony veins States,and to our knowledge, betweenpetroleumandmercuryhasbeenthe andbitumen-petroleum-chalcedony stringers (Bahkinet al., relationship ofdetailed study, istheCulver-Baer cinnabar deposit, 1971).TheDonbass deposit, whichoccurs in argillites adja- subject cent to a saltdome, alsoconsistsof bituminousand cinnabar- California (Peabody andEinaudi,1992).Thisdepositis lobearing veins(Belous et al.,1984).Pyrite,calcite, andquartz cated25 km fromthe SulphurBankdepositandproduced arecommonly reportedto occurwith cinnabar. 700t Hg between1872and1973.Mostof themineralization
22
FEINANDWILLIAMS-JONES
is hostedby silica-magnesite rocksin a largestrike-slip fault seemsreasonable fromthe closespatialassociation to consystem at themarginof theGeysers geothermal area.These eludethat mercuryandhydrocarbons wereintroduced torockshavebeenshownto represent alteration of serpentinite getherandmaywell havehada geneticrelationship. Yates bodies that occur as boudins within the fault zone. The merandThompson (1959)reportthat,withtheexception of one curyis presentascinnabarandwasdeposited in veinsand sample,noneof the host-rock lithologies sampled contain brecciascontainingquartz,magnesite, pyrite,chalcedony,significant quantities of hydrocarbons or bitumen. _ petroleton, + •nillerite,_ barite.In thecaseof thebrecThe onlyothermercuryprovinceof notein the United cias,thebulkof themineralization andpetroleum isconcen- Statesis the Corderodistrict,Nevada(4,200t Hg; Dickson tratedalongthehanging walls.Texturalrelationships, notably and Tunell, 1968). In contrastto the other districts,there is the occurrence of petroleum andcinnabar withinand/orbe- no reportedassociation betweenmercurymineralization and tweenquartzcrystals, indicate simultaneous deposition ofthe hydrocarbon occurrences. In otherrespects, however,the threephases andarestrong evidence ofa genetic relationshipsettingis quitesimilarto that of the Californian deposits, betweenmercuryandhydrocarbons (Peabody andEinaudi, i.e.,thecinnabar mineralization wasrelatedto Tertiaryfelsie 1992).Theseauthors haveproposed thatthe sourceof both volcanism, isassociated withquartz,chalcedony, barite,marthemercury andpetroleum isorganic-rich elastic sedimentaryeasite,andpyrite,andwasproduced byhydrothermal fluids rockswhichare knownto occurin the area.Theyinvokea in an acidsulfatehotsprings setting. modelin whichheatingof thesedimentary rocksby anH,20In summary, a reviewof the largestand/orbestdescribed CO.,fluidliberatespetroleumfromkerogenandmobilizes mercury deposits of theworldreveals thatmercury deposits mercurypresentin tracequantities in therock.Theirmodel typically format lmvtemperature, commonly in epithermal involves a physical, ratherthana chemical, linkbetweenthe environments,and in most easeshave a dose associationwith or organicmatter. mercuryandthe petroleum andexplains the concentrationhydrocarbons of thesecomponents alongthe hanging wallsof the breedas Thermodynamic Calculations asbeingdueto similardensity-driven transport paths.They propose thatthe mercurytransport wasexclusively via the Althoughseveralauthorshavemeasured the stabilityof gasphase,andthatbecause of greaterbuoyancy relativeto aqueous mercuryacetateandoxalate complexes at 25øC(see theaqueous fluid,thepetroleum andgasphases weretrapped Martelland Smith,1977,for a compilation), thereare no against the hanging wall. dataonthestability ofthesespecies atelevated temperatures. Outsideof California, the nextmostimportantmercury Ontheotherhand,high-temperature (upto$00øC)formation province intheUnitedStates istheTerlingua district inTexas constants areavailable foracetate complexes of Fe'2+,Zn'2+, whichproduced over5,000t between1899and1937(Yates andPb'2+(Hennetet al., 1988;PalmerandDrummond,1988; andThompson, 1959).Aswith the Californian deposits, a GiordanoandDrummond,1991)andoxalatecomplexes of distinctive featureof theTerlingua mercury mineralization is Pb'•+(Hennetet al., 1988).We usedthe one-termisoeouloma closespatialassociation with hydrocarbons. The mercury hie extrapolation technique of Gu et al. (1994)to estimate occursin a narrow30-kin-long belt andhasbeenexploited high-temperature formationconstants for the 1:1 and 1:2 in 10mines.However, two-thirds of theproduction wasfrom mercuryacetateand the 1:1 mercuryoxalatecomplex.No a singledeposit(Chisos). Cinnabar isthemainmercury phase dataareavailable at25øCforthe1:2mercury oxalate complex, andis concentrated in veins,brecciapipes,andstrata-boundandtherefore,it is not possible to consider this spedesat tabularbodies. Mostofthemineralization ishosted eitherby elevated temperature. Cretaceous limestone (DevilsRiverFormation) or anoverly- Formation of the mercuryoxalateandacetatecomplexes ingsequence ofintercalated shales andargiflaceous limestone canbe represented by the following reactions: (Grayson Formation andBoquillas Flags).Minormineralizationoccurs in veinsandbreccias withinTertiaryalkalicintruHg + Ik(C,O) ø, sions.Economically, the tabularbodiesare the mostimHg'2++ CH3COO-• Hg(CH:•COO) +, (2) portantandoccurat the DevilsRiver-Grayson Formation and contact, wheretheyoccupyspacecreatedby dissolution of the limestone. The brecciapipesarethe nextmostcommon Hg•+ + 2CH:3COO• Hg(CH:•COO)•z( (3) andrepresent collapse features alsocaused bylimestone dissolution. Individual pipesarefunnelshaped, upto 25 mwide We havemodifiedthesereactions to be isocoulombic by and70m high,andarefoundin theDevilsRiverFormation, addingthefollowing reactions, respectively: theGrayson Formation, andtheBoquillas Flags.Veinmineralizationis locallyimportantwithinthe Boquillas Flags.PyPb(C.•O4) ø• Pb'•+ + CzOi(4) rite,calcite,kaolinitc, andlesscommonly hematiteandbarite, Zn(CH:3COO) + • Zn•+ + CH3COO-, (5) are the mostcommonganguemineralsand are generally accompanied by variablequantities of bitumenandliquid Zn(CH3COO). ø,• Znz+ + 2CH3COO-. (6) hydrocarbons; in someminesthe hydrocarbon contentwas highenough to provideanenergysource duringexploitation. The resultingisocoulombic reactions are: According to Yatesand Thompson(1959),the deposits formedat temperatures below$00øCfromorthomagmatic Pb(C,204) ø + Hg•+ • Pb•'++ Hg(C•O4) ø, (7) fluidsin anepithermal-hot springs setting. Theseauthors do notdiscuss theroleof hydrocarbons in mineralization, butit Zn(CH:•COO) + + Hga+• Znø'++ Hg(CH3COO) +, (8)
Hg-ORGANIC INTERACTION IN HYDROTHERMAL SYSTEMS and
23
TABLE '2. TypicalComposition of a Mercury-Depositing Geothermal Fluid
Zn(CH3COO).ø2 + Hg•+ *",Zn=++ Hg(CH3COO).ø2. (9)
Component
Logmolality
The 25øCequilibrium constants usedfor reactions (1) to (3) Na -0.40 arethosefromKhokhlova et al. (1982)for the twomercury K -2.00 acetatespecies, andfromLisovaya et al. (1973)for the merCa - 1.00 C1 -0.35 curyoxalatecomplex. The formationconstants for the Zn F -3.80 acetatecomplexes were takenfrom Giordanoand DrumCO2 -0.80 mond(1991),andthe Pb oxalatecomplexstabilityconstant S -2.00 is fromHennetet al. (1988).The 25øCequilibrium constant Oxalate -2.25 Acetate -0.77 values forreactions (7)to (9)weredetermined bycombining the 25øCequilibriumconstant valuesfor reactions (1), (2), or (3) with thosefor reactions(4), (5), or (6). Equilibrium Valuesrepresent totalmolality foreachelementor compound; except for constants forreactions (7) through(9) (well-balanced isocou- oxalate and acetate, which are maximum concentrationsfound in oil field lombicreactions) at elevatedtemperature werecalculatedbrines (MacGowan and Surdam, 1988), the data are from White (1981) fromthe one-termisocoulombic extrapolation equation (Gu et al., 1994):
cinnabarare presented in Table2. It canbe seenthat the only eations present in solution in significant concentrations In K(T)= -- (/XGrxn(.25øC,]bar))/RT, (10) 0
are Na+, K+, and Ca'2+; Ci-, HCO.•, and F- are the most
rangefrom whereT refersto absolute temperature. Valuesfortheequi- importantanions.Typicaltotalsulfurcontents libriumconstants for reactions(4) to (6) areknownat elevated 10•- to 102-molal, andpH varies withinhalfa logunitof
temperatures, soby difference wewereableto calculate val- neutrality. The oxygen fugacity is moredifficultto constrain. uesforthehigh-temperature formation constants forthemer- However,the coexistence of cinnabarandpyritein all of curyacetateandoxalate species. Theresults of thesecalcula- thesedeposits maybeusedtoplacesomelimitsonf%. Figure tionsarepresented in Table1. The one-termisocoulombic1 illustrates theironoxideandironsulfidestability fieldsand extrapolation techniqueyieldsestimated stabilities for the the cinnabar and native mercury stability fields in terms of mercuryacetatecomplexes thataresimilartothosecalculatedfo:and pHat150øC andforZS•= 10'2-•.Afeature ofthe byShock andKoretsky (1993),whose values arealsocompiled diagramis that the stabilityfieldsof cinnabarand pyrite in Table1 for comparison. largelyoverlap,explaining the coexistence of theseminerals
in mostmercury deposits. Assuming thatZS .2 log unitslowerthan thatof Hgi•,0 o at logfo0 _ < -43. Betweenlogf% valuesof approximately -46 and-50, the threemercurysulfurcomplexesbecomeimportantcomponents of theaqueous phase, andatlogfo._, equalto -47, HgS(H2S).• isthedominant mercuryspedes. Undermorereducing conditions, Hg%•'again represents the onlyimportantmercuryspedesin solution. As Figure2 illustrates, the total mereuwconcentration in equilibrium with cinnabarpasses througha minimumwith respectto oxygen fugaeity; mercuryconcentrations arehighestwhenconditions areoxidizing andlowestattheintermedi-
Hg-ORGANIC INTERACTION IN HYDROTHERMAL SYSTEMS
ateoxygen tilgacities underwhichmercury sulfurspecies pre-
A
25
-1
dominate.
Log fO 2=-43] •
The concentrations of mercuryacetate,oxalate,andthe methylmereuD:species arenegligible underthe conditions of thesecalculations. Although the concentration of mercury oxalateis approximately eightordersof magnitude greater
-2
TotalHg
thanthatfbrHg2+andthusrepresents relatively strong eomplexation, it is not importantin lnereurytransport (the log molalityis --
The Roleof Mercury-Organic Interactions in the Hydrothermal Transport of Mercury JEREMY B. FEIN* ANDA. E. WILLIAMS-JONES McGillUniversity, Department of EarthandPlanetary Sciences, 3450University Street,Montreal,Quebec, CanadaH3A2A7 Abstract
Despitethecommon association bet•veen mercury deposits andliquidhydrocarbons, theeffects of mercury organic interactions on mercury transport havereceived littleattention. In thisstudy,we estimate theextent of aqueous mercurycomplexation, andthe partitioning of mercuryamongaqueous liquid,aqueous vapor, andanorgmfie phase, quantifying therelativeimportance of eachphasein mercury transport. Thecalculations
suggest thatsignificant mercury transport in theaqueous liquid,predominantly asHg•aq), is onlypossible underrelatively oxidizing andalkaline conditions. Thefrequent occurrence ofliquidhydrocarbons in mercurydepositing hydrothermal systems, however,impliesthat conditions are reducingandthat aqueous liquid transport of mercuryis relatively unimportant. High concentrations of mercuryalsocanbe dissolved in aqueous vapor.However, boilingrarelyoccurs at thedepths atwhichtheoresolutions originate, andthus,the presence ofa vaporphaseisunlikely duringmercury transport. Extrapolation oflow-temperature experimental measurements of mercurysolubilities in organic phases enables quantitative estimates of mercury concentrationsin liquidhydrocarbons underhydrothermal conditions. Our calculations suggest that extremely high concentrations of mercurycandissolve in the organic phaseandthatorganic phasetransport maycontrol mercurymobilities in manymercuryore-forming hydrothermal systems. Introduction
phur Banksgeothermalsystem,California,and concluded wereunimportant tomercury transport ALTHOUGH theassociation of deposits ofmercury withpetro- thatthesecompounds because conditions were too reducing. It is possible, however, letonhasbeenlongrecognized (e.g.,DicksonandTunell, that under oxidizing conditions these compounds play a more 1968;Barneset al., 1973;White,1981;Peabody andEinaudi, significant role, and indeed, in some near-surface environ1992),little consideration hasbeengivento the possibility a significant component ofthetotal thatorganic molecules contribute to thehydrothermal trans- ments,theycanrepresent mercury in solution (e.g., Sehintu et al., 1989; Murphy et al., portof mercury. Mostworkers attributemercurymobilityin 1994).
hydrothermal systems eitherto thepresence of Hgø in an thatorganic phasetransport of mercury maybe aqueous gasphase(e.g.,White,1981;Varekamp andBuseck, Evidence important in forming economic concentrations of this metal 1984),orto Hgøand/or bisulfide complexes in aqueous solu- isprovided by reports of high concentrations of mercury (up tions(e.g.,Barneset al., 1967;Varekamp andBuseck, 1984; to 21 ppm; Bailey et al., 1961) in petroleum associated with WellsandGhiorso, 1988).Giventhatmercurydeposits pre-
geothermal systems. Moreover,expericipitateatcomparatively lowtemperatures, i.e.,between100ø mercury-depositing mental studies show that mercury solubility in alkanes ishigh, and200øC,it is possible thatmercuryformsstableaqueous andincreases withincreasing temperature. For example, uscomplexes withearboxylate ligands, dissolves asmethyl-mer- ingthe experimentally basedsolubility equation of Okouehi curycompounds, or ispartitioned intoanimmiscible organic andSasaki (1981),we estimate mercury solubilities in octane phase.In thisstudy,we consider eachof thesepossibilities.of 2.6 ppm at 25øCand 105 ppm at 100øC.Thesevalues Carboxylates havebeenmeasured in highconcentrations areat leastan orderof magnitude greaterthanthe highest in hydrothermal fluidsfromavarietyof geologic settings. For measured aqueous concentrations of mercuryin activegeoexample,MaeGowanand Surdam(1988)reportedacetate thermalsystems (White,1981). andoxalateconcentrations of 10,000and500 ppm,respec- In thispaper,we havedeveloped a comprehensive speeiatively,in formation watersfromthe SanJuaquin sedimentarytionmodelfor typicalmercuryore-forming fluidswhichinbasin(although it is stillunclearwhetherthesehighconcen- eludes a largenumberof inorganic aqueous species andthe trationsare representative of typicaldiagenetic fluids,e.g., following organic species: aqueous Hg acetate, Hg oxalate (to Kharakaet al., 1993). Martens (1990) determinedthat fluids represent mono-and difunetional earboxylates) andmethylventingfromthe Guyamas basincontainacetateandpropio- mercury.Althoughdisequilibrium biological processes can hateat concentrations rangingup to 65 and15 ppm,respec- affectmercury speeiation through methylation inlow-tempertively.Carboxylates formhighlystableaqueous complexesatureenvironments, weconsider onlyabiotieequilibrium syswith a varietyof metalcationsand havebeen invokedto tems.Ourmodelalsoaccounts forthepartitioning ofmercury explain mass transport of leadandzincin Mississippi Valley- amongaqueous liquid,aqueous vapor,and an immiscible type ore-depositing systems(e.g., Giordanoand Barnes, organic phaserepresented by C5-C•0alkanes. Thisstudyis 1981).Mercuryformsstable, water-soluble methylated com- thefirstto quantif•v theroleof aqueous earboxylate eomplexpounds in naturalwaters. WellsandGhiorso (1988)evaluated ationandorganicphasetransport in mercurymobilization. the contribution of methyl-mercury compounds in the Sul- Our modelprovidesa realisticevaluation of the behavior of mercuryin an organic-bearing hydrothermal system and the conditions requiredto transport mercuryor* Present address: University of NotreDame,CivilEngineering andGeo- establishes logicalSciences, NotreDame,Indiana46556-0767. ganieally in concentrations sufficient to formoredeposits. 0361-0128/97/1901/9,0-956.00
9,0
Hg-ORGANIC INTERACTION IN HYDROTHERMAL SYSTEMS
21
Ore-DepositingEnvironment In theUnitedStates, mainlyin theCalifornia CoastRange, hasbeenfromdeposits reMercurydeposits forminlow-temperature (typically 100ø- mostof the mercuryproduction orpaleohot springactivity(e.g.,Dickson and 200øC)hydrothermal systems in a widevarietyof geologic latedto present settings. Characteristic of manyof thesedeposits is a close Tunell,1968;Fisk,1968;White,1981).A featureof manyof istheirassociation withhighconcentrations of spatial association withpetroleum and/orsolidorganic matter. thesedeposits andhigherhydrocarbons. In someeases, the active The largestmercurydepositin the world(over275,000 methane hotsprings areactually developed in oilandgas t) is locatedat Almaden,Spain.It comprises strata-boundmercury-rich fields (e.g., the Cymrie oil fields). Significantly, the petroleum orebodies hostedby quartzite horizons intercalated withina up to 21 ppm Hg, whereas thickvolcanosedimentary sequence dominated by organic- fromthe Cymriefieldcontains rich blackshales. The mercuryoccursmainlyas cinnabar brinesin the samewellcontain0.2 ppm mercury(Baileyet and either is massive,fills a network of fine fractures,or is al.,1961).Heavyoilsandtarryhydrocarbons fromtheWilbur Springs district (which hosts several deposits associated with interstitial to thequartzgrains. Pyriteistheprincipal gangue are reportedto contain1 and500 ppm mineral.Otherphases associated withthecinnabar arepyr- activehot springs) (Barnes et al., 1973).Brinesfromthe same rhotite,sericite, kaolinire, andorganic matter.Thedeposit is Hg, respectively 1.5 ppb Hg (Barneset al., interpreted to haveformedduringsedimentation andearly districtcontainapproximately diagenesis fromhydrothermal fluidsassociated withshallow- 1973). Thelargestmercurydepositassociated withanactivegeomarinevolcanicactivity.Thesefluidsare thoughtto have istheSulphurBankdeposit, California, froIn leached mercury fromtheblackshales andsubsequently ex- thermalsystem (White,1981). pelledinto the oceanor circulated in underlying partially whichnearly5,000t Hg hasbeenrecovered described deposits, thereis a dear lithifiedporous sands. In bothcases, cinnabar precipitationAs with the previously betweenmercury andhydrocarbons. Thedeposits isconsidered to haveoccurred primarily in response to large association at SulphurBankarehostedby a highlyaltered(acidsulfate) dropsin temperature (e.g.,Saup&1990). (44,500yr; SimsandWhite, 1981)augite Next to Almaden,the Huancavelica quicksilver district, late Pleistocene flowandunderlying lacustrine sediments. TheminPeru,hasbeenthemostimportant mercury-producing region andesite occursin veinsandopenspaces betweenoriginal in theworld(over48,000t minedprimarily duringtheSpan- eralization of cinnabar, hydrocarbons, mamaish colonialperiod)and,like Almaden,comprises strata- jointblocksandconsists (Whiteand boundorebodies hostedmainlyby Cretaceous sandstone.site,pyrite,stibnite,dolomite,and eristobalite 1962).The cinnabar wasinterpreted by theseauCinnabar is theprincipalmercurymineralandis accompa-Roberson, fromgeothermal fluidssimilar nied by pyrite,arsenopyrite, realgar,orpiment,stibnite, thorsto havebeendeposited withsporadic cinnabar deposiquartz,calcite,barite,andminornativemercury. The bulk to thosewhichareassociated The currentfluids of thecinnabar fillsporesbetween quartzgrains. Locally, it tionin someof the presenthot springs. of justunder200øC,pH values replaces silicacement,andin someplaceshasreplaced the havea sourcetemperature 1,500ppmof dissolved detritalquartzgrains. Thehighest concentrations ofcinnabar in therange6.6to 7.5,approximately are in discordant fracture zones which erosscut the sandstone salts, 500ppmof sulfur,500ppmNH;, andover3,000ppm and controlled the infiltration of the ore fluids. In addition of dissolved CO2(White,1981).Onegeneticmodelfor the to sandstone,some mineralizationoccurswithin limestone deposits involvesleachingof mercuryfroIn sedimentary andthe transport of unitsof similarageandalsoin younger (Tertiary)felsielavas. rocks,heatedasa resultof volcanism, The limestone-hosted mineralization occurs either as tabular thismercuryto thevadose zonewhereit wasprecipitated as of the hydrothermal fluidwith bodies concordant withbedding orasdiscordant veins.In the cinnabardue to interaction voleanies, thecinnabar occurs exclusively asfracture fillings. groundwater(White, 1981).Wellsand Ghiorso(1988),on oftheoretical calculations andgeologic observations, Hydrocarbons areassociated with the oresin all threehost- thebasis thatthe mercury wastransported asa bisulfide comrocktypesandin somelocalities heavyblackoilsandtarsare suggest plex and probably was deposited as a result of oxidation. reportedto seepfromrockopenings (Yateset al., 1951).To concentration ofthegeothermal fluidspresently ourknowledge, the genesis of thesedeposits hasnotbeen Themercury discharging at SulphurBank,however, isonly0.001ppm.At investigated. andcurrentdischarge ratesit wouldtake The only other countries whichhaveproducedlarge thisconcentration amountsof mercuryare Russiaandthe UnitedStates.The at least40 m.y. to form the deposit(Barneset al., 1973), Russian deposits arerelatively poorlydescribed, andnoinfor- whichisat least1,000greaterthanitsactualage(seeabove). mationis available on the gradesor tonnages of individual Thisimpliesthateitherthe mercurycontentof the aqueous ratesmuchgreater deposits. Animportant featureof manyof thedeposits, how- fluidwasmuchhigherand/ordischarge or mercurywastransported by anever,isa closeassociation between mercury andeitherpetro- thancurrentlyobserved, by White leumor bitumen. Forexample, theChukotka deposit, which othermedium,i.e., the vaporphaseassuggested ishosted bylistwaenites (earbonitized andsilieified serpentin- (1981)or the organicphase. in the United ites)in tectonic contact withelasticmetasedimentary rocks, One of the beststudiedmercurydeposits the onlyone in whichthe comprises a stockwork of cinnabar-bearing chalcedony veins States,and to our knowledge, betweenpetroleumandmercuryhasbeenthe andbitumen-petroleum-chalcedony stringers (Bahkinet al., relationship ofdetailed study, istheCulver-Baer cinnabar deposit, 1971).TheDonbass deposit, whichoccurs in argillites adja- subject cent to a saltdome, alsoconsistsof bituminousand cinnabar- California (Peabody andEinaudi,1992).Thisdepositis lobearing veins(Belous et al.,1984).Pyrite,calcite, andquartz cated25 km fromthe SulphurBankdepositandproduced arecommonly reportedto occurwith cinnabar. 700t Hg between1872and1973.Mostof themineralization
22
FEINANDWILLIAMS-JONES
is hostedby silica-magnesite rocksin a largestrike-slip fault seemsreasonable fromthe closespatialassociation to consystem at themarginof theGeysers geothermal area.These eludethat mercuryandhydrocarbons wereintroduced torockshavebeenshownto represent alteration of serpentinite getherandmaywell havehada geneticrelationship. Yates bodies that occur as boudins within the fault zone. The merandThompson (1959)reportthat,withtheexception of one curyis presentascinnabarandwasdeposited in veinsand sample,noneof the host-rock lithologies sampled contain brecciascontainingquartz,magnesite, pyrite,chalcedony,significant quantities of hydrocarbons or bitumen. _ petroleton, + •nillerite,_ barite.In thecaseof thebrecThe onlyothermercuryprovinceof notein the United cias,thebulkof themineralization andpetroleum isconcen- Statesis the Corderodistrict,Nevada(4,200t Hg; Dickson tratedalongthehanging walls.Texturalrelationships, notably and Tunell, 1968). In contrastto the other districts,there is the occurrence of petroleum andcinnabar withinand/orbe- no reportedassociation betweenmercurymineralization and tweenquartzcrystals, indicate simultaneous deposition ofthe hydrocarbon occurrences. In otherrespects, however,the threephases andarestrong evidence ofa genetic relationshipsettingis quitesimilarto that of the Californian deposits, betweenmercuryandhydrocarbons (Peabody andEinaudi, i.e.,thecinnabar mineralization wasrelatedto Tertiaryfelsie 1992).Theseauthors haveproposed thatthe sourceof both volcanism, isassociated withquartz,chalcedony, barite,marthemercury andpetroleum isorganic-rich elastic sedimentaryeasite,andpyrite,andwasproduced byhydrothermal fluids rockswhichare knownto occurin the area.Theyinvokea in an acidsulfatehotsprings setting. modelin whichheatingof thesedimentary rocksby anH,20In summary, a reviewof the largestand/orbestdescribed CO.,fluidliberatespetroleumfromkerogenandmobilizes mercury deposits of theworldreveals thatmercury deposits mercurypresentin tracequantities in therock.Theirmodel typically format lmvtemperature, commonly in epithermal involves a physical, ratherthana chemical, linkbetweenthe environments,and in most easeshave a dose associationwith or organicmatter. mercuryandthe petroleum andexplains the concentrationhydrocarbons of thesecomponents alongthe hanging wallsof the breedas Thermodynamic Calculations asbeingdueto similardensity-driven transport paths.They propose thatthe mercurytransport wasexclusively via the Althoughseveralauthorshavemeasured the stabilityof gasphase,andthatbecause of greaterbuoyancy relativeto aqueous mercuryacetateandoxalate complexes at 25øC(see theaqueous fluid,thepetroleum andgasphases weretrapped Martelland Smith,1977,for a compilation), thereare no against the hanging wall. dataonthestability ofthesespecies atelevated temperatures. Outsideof California, the nextmostimportantmercury Ontheotherhand,high-temperature (upto$00øC)formation province intheUnitedStates istheTerlingua district inTexas constants areavailable foracetate complexes of Fe'2+,Zn'2+, whichproduced over5,000t between1899and1937(Yates andPb'2+(Hennetet al., 1988;PalmerandDrummond,1988; andThompson, 1959).Aswith the Californian deposits, a GiordanoandDrummond,1991)andoxalatecomplexes of distinctive featureof theTerlingua mercury mineralization is Pb'•+(Hennetet al., 1988).We usedthe one-termisoeouloma closespatialassociation with hydrocarbons. The mercury hie extrapolation technique of Gu et al. (1994)to estimate occursin a narrow30-kin-long belt andhasbeenexploited high-temperature formationconstants for the 1:1 and 1:2 in 10mines.However, two-thirds of theproduction wasfrom mercuryacetateand the 1:1 mercuryoxalatecomplex.No a singledeposit(Chisos). Cinnabar isthemainmercury phase dataareavailable at25øCforthe1:2mercury oxalate complex, andis concentrated in veins,brecciapipes,andstrata-boundandtherefore,it is not possible to consider this spedesat tabularbodies. Mostofthemineralization ishosted eitherby elevated temperature. Cretaceous limestone (DevilsRiverFormation) or anoverly- Formation of the mercuryoxalateandacetatecomplexes ingsequence ofintercalated shales andargiflaceous limestone canbe represented by the following reactions: (Grayson Formation andBoquillas Flags).Minormineralizationoccurs in veinsandbreccias withinTertiaryalkalicintruHg + Ik(C,O) ø, sions.Economically, the tabularbodiesare the mostimHg'2++ CH3COO-• Hg(CH:•COO) +, (2) portantandoccurat the DevilsRiver-Grayson Formation and contact, wheretheyoccupyspacecreatedby dissolution of the limestone. The brecciapipesarethe nextmostcommon Hg•+ + 2CH:3COO• Hg(CH:•COO)•z( (3) andrepresent collapse features alsocaused bylimestone dissolution. Individual pipesarefunnelshaped, upto 25 mwide We havemodifiedthesereactions to be isocoulombic by and70m high,andarefoundin theDevilsRiverFormation, addingthefollowing reactions, respectively: theGrayson Formation, andtheBoquillas Flags.Veinmineralizationis locallyimportantwithinthe Boquillas Flags.PyPb(C.•O4) ø• Pb'•+ + CzOi(4) rite,calcite,kaolinitc, andlesscommonly hematiteandbarite, Zn(CH:3COO) + • Zn•+ + CH3COO-, (5) are the mostcommonganguemineralsand are generally accompanied by variablequantities of bitumenandliquid Zn(CH3COO). ø,• Znz+ + 2CH3COO-. (6) hydrocarbons; in someminesthe hydrocarbon contentwas highenough to provideanenergysource duringexploitation. The resultingisocoulombic reactions are: According to Yatesand Thompson(1959),the deposits formedat temperatures below$00øCfromorthomagmatic Pb(C,204) ø + Hg•+ • Pb•'++ Hg(C•O4) ø, (7) fluidsin anepithermal-hot springs setting. Theseauthors do notdiscuss theroleof hydrocarbons in mineralization, butit Zn(CH:•COO) + + Hga+• Znø'++ Hg(CH3COO) +, (8)
Hg-ORGANIC INTERACTION IN HYDROTHERMAL SYSTEMS and
23
TABLE '2. TypicalComposition of a Mercury-Depositing Geothermal Fluid
Zn(CH3COO).ø2 + Hg•+ *",Zn=++ Hg(CH3COO).ø2. (9)
Component
Logmolality
The 25øCequilibrium constants usedfor reactions (1) to (3) Na -0.40 arethosefromKhokhlova et al. (1982)for the twomercury K -2.00 acetatespecies, andfromLisovaya et al. (1973)for the merCa - 1.00 C1 -0.35 curyoxalatecomplex. The formationconstants for the Zn F -3.80 acetatecomplexes were takenfrom Giordanoand DrumCO2 -0.80 mond(1991),andthe Pb oxalatecomplexstabilityconstant S -2.00 is fromHennetet al. (1988).The 25øCequilibrium constant Oxalate -2.25 Acetate -0.77 values forreactions (7)to (9)weredetermined bycombining the 25øCequilibriumconstant valuesfor reactions (1), (2), or (3) with thosefor reactions(4), (5), or (6). Equilibrium Valuesrepresent totalmolality foreachelementor compound; except for constants forreactions (7) through(9) (well-balanced isocou- oxalate and acetate, which are maximum concentrationsfound in oil field lombicreactions) at elevatedtemperature werecalculatedbrines (MacGowan and Surdam, 1988), the data are from White (1981) fromthe one-termisocoulombic extrapolation equation (Gu et al., 1994):
cinnabarare presented in Table2. It canbe seenthat the only eations present in solution in significant concentrations In K(T)= -- (/XGrxn(.25øC,]bar))/RT, (10) 0
are Na+, K+, and Ca'2+; Ci-, HCO.•, and F- are the most
rangefrom whereT refersto absolute temperature. Valuesfortheequi- importantanions.Typicaltotalsulfurcontents libriumconstants for reactions(4) to (6) areknownat elevated 10•- to 102-molal, andpH varies withinhalfa logunitof
temperatures, soby difference wewereableto calculate val- neutrality. The oxygen fugacity is moredifficultto constrain. uesforthehigh-temperature formation constants forthemer- However,the coexistence of cinnabarandpyritein all of curyacetateandoxalate species. Theresults of thesecalcula- thesedeposits maybeusedtoplacesomelimitsonf%. Figure tionsarepresented in Table1. The one-termisocoulombic1 illustrates theironoxideandironsulfidestability fieldsand extrapolation techniqueyieldsestimated stabilities for the the cinnabar and native mercury stability fields in terms of mercuryacetatecomplexes thataresimilartothosecalculatedfo:and pHat150øC andforZS•= 10'2-•.Afeature ofthe byShock andKoretsky (1993),whose values arealsocompiled diagramis that the stabilityfieldsof cinnabarand pyrite in Table1 for comparison. largelyoverlap,explaining the coexistence of theseminerals
in mostmercury deposits. Assuming thatZS .2 log unitslowerthan thatof Hgi•,0 o at logfo0 _ < -43. Betweenlogf% valuesof approximately -46 and-50, the threemercurysulfurcomplexesbecomeimportantcomponents of theaqueous phase, andatlogfo._, equalto -47, HgS(H2S).• isthedominant mercuryspedes. Undermorereducing conditions, Hg%•'again represents the onlyimportantmercuryspedesin solution. As Figure2 illustrates, the total mereuwconcentration in equilibrium with cinnabarpasses througha minimumwith respectto oxygen fugaeity; mercuryconcentrations arehighestwhenconditions areoxidizing andlowestattheintermedi-
Hg-ORGANIC INTERACTION IN HYDROTHERMAL SYSTEMS
ateoxygen tilgacities underwhichmercury sulfurspecies pre-
A
25
-1
dominate.
Log fO 2=-43] •
The concentrations of mercuryacetate,oxalate,andthe methylmereuD:species arenegligible underthe conditions of thesecalculations. Although the concentration of mercury oxalateis approximately eightordersof magnitude greater
-2
TotalHg
thanthatfbrHg2+andthusrepresents relatively strong eomplexation, it is not importantin lnereurytransport (the log molalityis --
