945

The Canadian M iner alngi st Vol. 31,pp. 945-956(1993)

THEFORMATION OFATACAMITE DURING WEATHERING OF SULFIDES ON THEMODERN SEAFLOOR*

MARKD. HANNINGTON GeologicalSurvey of Canala" 601 Booth Street,Ottawa. Ontario KIA 0E8

ABsrRAcr

Atacamite tCu2C(OtI)lJ is a Qommonsecondary mineral in the oxidized portions of black smokerdeposits on the modem mid-oceanridges. Weathered s€afloor sulfides from the TAG HytlrothermalField (Mid-Atlantic fudge) contain abundantFe oxyhytlroxides,jarosite, and secondarycopper minerals. All threepo,lymorphs of CurCl(OFI)r,atacamite, paratacamite and the rare mineral botallackite,have been identlfied in samplesrecovered from the seafloorgossans at TAG. Atacamiteforms when cuprousc!:loride somplexesand Cu2* ions are releasedfrom sulfides during corrosionby acidic pore-fluids. The copperions migrate in solution througb the gossansand reprecipitateas basic cupric salts in contact with the surroundingseawater. The atacamiteand paratacamileoccur as colloform masses,crystalline aggregates,or disseminationswithin amorphousFe oxyhy- droxides,goethite, jarosite, and secondaryCu sulfides. Botallackite is presentas sparsecorroded crystals. Gibbs free-energy data for atacamiteand pamtacamiteindicate that Cu2Cl(OH)3is the most stable salt of copper in the seafloor weathering profile, and solubility calculationsshow that it is only sparingly soluble in seawater.Observations at TAG suggestthat the atac4mitein gossansoverlying the depositsmay have been presenton the seafloorfor as long as 20,000 years and possibly 40,000-50,000yeaxs. Confary to recent suggestionsin the literature, atacamitedoes not form on the modem seafloorunder hydrothermalconditions. The susceptibilityof atacamiteto hydration and dissolutionin fresh water accountsfor its absencein most surficial environmentsand precludes its long-termpreservation in the geologicalrecord.

Key'vtords:atacamiig, paratacamite, botallackite, Fe oxyhydroxides,seafloor weathering, sulfide gossans,TAG hydrothermal fiel4 Mid-Atlantic Ridge.

SoMtilans

L'atacamite tCuzCl(OII):l est un rnin6ral secondairer6pandu dans les partiesoxyd6es des gisementsassoci6s aux fumeurs noin le long des dorsalesmddio-oc6aniques modernes. L'altdration des sulfuresdes fonds marins du champhydrothermal de TAG (dorsalem6dio-Atlantique) sontient une abondanced'oxydes de fer, de jarosite, et des min6rauxsecondaires du cuivre, Les trois polymorphesde Cu2Cl(OH)3,atacamite, pamtacamite, et memebotallackite, espOcerare, ont 6t6 identifi6s dansles dchantillonspr6lev6s des chapeauxde fer. L'atacamitese forme quandles complexeschlorurds de cuiwe (cupreux)et les ions Cu2+sont libdr6s dessulfures pendant leur corrosionpar une phasefluide tr caractdreacide. l,es ions de cuiwe se propagenten solution d travers des chapeauxde fer et forment un pr6cipitd de sels basiquescupriques au contact avec I'eau de mer ambiante.L'atacamite et Ia paratacamitese prdsententen massescolloformes, en agr6gatscristallins, ou en diss6minationsau sein de massesd'oxydes amorphesde fer, de goethite, de jarosite, et de sulfures secondairesde cuivre. La botallackite se pr6sentesous forrne de crisaux 6parscorrod6s. L,es donndes sur l'6nergie libre de formation de I'atacamite et de la para- tacamiiemontrent que Cu2Cl(OH),est la forme la plus stablede cuivre dansle milieu sous-marinprds des zonesd'alt6ration. Des calculs de solubilitd d6monfientque cos phasessont trbs peu solublesdans I'eau de mer. Les observations] TAG font penserque I'atacamitedes chapeaux de fer qui recouwentles gisementspourraient avoir 6t6 formdsil y a au moins 20,000ans, et peut€tre m6me40,000-50,000 ans. Contrairernentd une opinion 6miser6cemment daas la littdrature,l'atacamite n'est pa$ pr6cipit6edirectement sous conditions hydrothermales dans les fonds oc6aniquesmodemes. Sa susceptibilitda Cfiehy&at6e et dissoutedans I'eau fralche rend comptede son absencedans la plupart desmiJieux de surface,et expliqueson absencedans les rochesplus anciennes. (Traduit par la R6daction)

Mots-cl6s: atacamite,paratacamite, botallackite, oxydes de fer, lessivagesous-marin, chapeau de fer, sulfures,champ hydrothermalde TAG, dorsalem€dio-Atlantique.

* GeologrcalSurvey of Canadacontribution number 44492.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 946

l.rrnooucrrotv few studieshave addressed the physicaland chemical conditionsunder which it forms. This paper examines In its type locality, the AtacamaDesert in the high the mineralogy, chemistry, and paragenesisof ata- Andes of northern Chile, the basic cupric chloride, camite in samples recovered from the TAG atacamite[Cu2C(OH)3], occurs in the oxidizedzones HydrothermalField on the Mid-Atlantic Ridge of exposedporphyry copperdeposits (e.9., Bandy (26'N). The sampleswere collectedby dredgingof a 1938).The rapid descentof the water table through large sulfide moundin 1985and during submersible these depositsand the onset of a sustained,hyperarid operationswith ALVIN in 1986and 1990. climate in the Miocene have resultedin the Dreser- vation of abundantsecondary copper sulfide. iulfate, OccrinnsNcpoF ATACAMTTErNTm and chloride minerals within a thick weathering Tac HynnorneRMALFEI-o profile (Mortimer 1973,Alpers & Brimhall 1988, 1989).The AtacamaDesert is now one of the driest The TAG hydrothermalfield comprisesthree large placeson tfie surfaceof the earth. In stark contrastto sulfide mounds.one active and two inactive.The this environment,atacamite also is commonlyfound activemound occurs on the floor ofthe rift valley and as a secondarymineral in the weatheredportions of is a large,steep-sided deposit measuring 200-250 m in massivesulfide depositson tfie modernseafloor. This diameter and 2040 m in height, with a vigorous paradoxowes itselfto the unique physical and chemi- central black smokercomplex and an apron of oxidiz- cal behaviorof atacamitein naturalenvironments. ing sulfide talus and metalliferousoxide sediments Atacamite was first reportedfrom the modern (Rona er al. 1986,Thompson et al. 1988,Metz et al. seafloorby Bonatti et al. (1976). They noted the 1988).Two additionalinactive sulfide depositsoccur occurrenceof secondaryatacamite in disseminated on the east wall of the rift valley and are currently and stockwork-typeveinlets of chalcopyriteexposed undergoingextensive erosion and mass-wasting (Rona in basaltic crust uplifted along fracture zones on the et al. 1990,1993).The largestsulfide depositshave Mid-Atlantic Ridge. A similar occunencewas noted been exposedto seawaterpossibly for as long as by Scott et al. (1982). The atacamitein botl cases 40,000-50,000years (Lalou et al. 1990),and parts are occurswith other secondaryCu oxide, Cu sulfide and now coveredby extensivedeposits of Fe oxy- Fe oxide minerals,and was interpretedto be a product hydroxides(up to severaltens of centimetersthick). of seafloorweathering of the chalcopyriteveins. Thesegossans resemble the weatheredcaps on many Atacamite has since been describedin weatheredsul- ancientsulfide deposits now exposedon land,and are fides from most of the known black smoker deoosits apparentlya productof similarproces$es of oxidation forming on the mid-oceanridges, and it is especially takingplace on the seafloor(e.g.,Henig et al. l99l). abundantat fossil hydrothermal sites, where sulfides Pyrite, marcasite,and chalcopyritehave been haveexperienced a long history of submarineweather- altered extensivelyto secondarymineral assemblages ing (e.g.,Hekinian & Fouquet1985, Alt et al. 1987, consistingmainly of amorphousFe oxyhydroxides, Alt 1988a,b, Embley et al. 1988,Scott e/ al. 1990, togetherwith abundantsecondary Cu sulfides,jarosite, Yankoet al. 1991).Despite the numerousdocumented atacamite,and tracesof native copperand native gold. occurrencesof atacamiteon the modern oceanfloor, The oxidizine sulfide mound containsacidic pore-

Ftc. 1. Bottom photographstaken from the submersibleALVIN showing the weatheredand oxidized surface of the TAG Mound. The greenishcolor of the outcrop is due to the presenceof abundantatacamite, for which this part of the deposit hasbeen labeled the "greencliffs". Approximatefield of view is 3 metersin photo (a) and 1 meterin photo (b).

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 TI{E FORMATION OF ATACAMITE ON TIIE SEAFLOOR

Frc. 2. a) Large block of massivesuLfrde from the TAG Mound, with an encrustationof emerald-greenatacamite. b) Crust of colloform atacamitewith central cavity lined by a thin film of amorphousFe oxyhydroxides.c) Cryptocrystalline,collo- form massesof atacamite.d) Euhedral rhombic prisms of atacamiteon the corroded surface of massivechalcopyrite. e) Intergrowthof atacamiteand paxatacamite(dipyramids) dusted with Fe oxyhydroxides.f) Large, blocky crystals ofbotallackite.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 948 THE CANADIAN MINERALOGIST

fluids, having pH values between3.6 and 5.5, masseson the outer surfacesof weatheredsulfide producedby the reaction of exposedpyrite and chal- blocks, (2) as crystalline aggregatesthat fill open copyrite with seawater(Hannington et al. L990). spacesin massiveFe-oxide crusts, and (3) as dissemi- Measurementsof pH and redox potential indicate that nated crystals intergrown with fine-grained Fe oxy- conditions within the oxidation profile are similar to hydroxidesor that fill open spaceswithin corroded thoseencountered in areasof acid mine-drainage.This sulfides.Atacamite is most commonin the wholly oxi- modified seawateris sufficiently acid and oxidized to dized remnantsof massivesulfide blocks and locally becomesaturated with atacamiteduring weatheringof constitutesup to 50 vol.Voof. the gossanousmaterial. the exposedsulfides. Thompson er a/. (1988)first The presenceof abundantbrightly colored atacamite noted the occurrenceof atacamite on the weathered in the gossansis a useful indicatorof weatheredCu surfacesof the active TAG mound.and its distribution sulfides in the substrate.The widespreadcrusts of on oxidized surfacesof the deposit was widely noted atacamitecoating weatheredsulfide surfaceson talus during subsequentcruises (Lisitsyn et al. 1989, 1990, slopesat the edgeof the active mound has led a num- Rona et al. 1990,Tivey et al. 1990).lfanningXan et al. ber of observersto describethese steep scarpsas (1988, 1991)briefly describedthe mineralogyand "green cliffs" (Fig. 1). Atacamitealso is a common chemistryofthe Fe-oxidegossans and the relationship detrital constituentof metallileroussediments that sur- of atacamiteto the secondarysulfide assemblages. round the TAG mound (Hanningtonet al. l99l). Samplesof gossanousmaterial were collectedfrom Thesesediments are composedof the weathered all three depositsby dredgingand using the sub- debris depositedon the flanls of the mound by mass- mersible. Cu chlorides in most of the samplesare wasting. intergrown with amorphousFe oxyhydroxides,lesser Nodular crusts,collofom mas$es,and blisters of amountsof goethiteand lepidocrocite,jarosite, and atacamiteon the surfacesof large, oxidized sulfide secondaryCu sulfides.Most of the atacamiteoccurs as blocks (Fig. 2a) are apparentlyprecipitated from Cu2* (l) patchy (1-2 cm thick) encrustationsor colloforrn ions leacheddurins corrosionof the sulfidesbv sea-

FIc. 3. SEM photographsof cryptocrystalline and coarseraacamite. a) Cryptocrystalline atacamitein colloform masses. b) SheathJike massesof atacamite. c) Slender prismatic crystals or needles of atacamite as overgrowths of crypto- crystalline globular masses.d) Slender prismatic crystals of atacamitewith blunt terminations.Photographs by L.K. Radburn(Geological Survey ofCanada).

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 THE FORMATION OF ATACAMITE ON TIIE SEAFLOOR 949

FIG. 4. SEM photographsshowing dominant habits of crystals of atacamile, paratacamite,and botallackite. a) Prismatic crystalsof atacamitewith corrodedchalcopyrite (see also Fig. 2d). b) Prismaticcrystals of atacamitewith well-developed twin. c) Intergrowth of atacamiteand paratacamite(dipyramids). d) Conoded,blocky crystalsof botallackite.Photographs by L.K. Radburn(Geological Survey of Canada).

water. In a numberof samples,atacamite occurs along Individual euhedrarange in size from a fe"r/tens fractures leading to the outer surfacesof the blocks, of micrometersup to about 2 mm, but are typically and these fractures appearto have been paths for less than 500 pm in size @g. 2d). Paratacamitecom- migrating Cu ions. T\e blisters of atacamiteare com- monly occurstogether with atacamiteand forms monly hollow, and someinteriors are lined by characteristichexagonal dipyramids up to 2 mm in euhedralcrystals of atacamiteor a thin fikn of amor- size(Fig. 2e).Botallackite (monoclinic) was identified phousFe oxyhydroxides(Frg. 2b). The more massive in one sample,lining the hollow interior of an atu nodular crustsof atacamiteconsist mainly of anhedral camite crust, where it forms blocky crystals up to masses(Fig. 2c). In somecases, crystals of atacamite 2 mm across(Fig. 20. The crystal structuresof the have grown directly on the exposedsurfaces of three polymorphshave been describedin detail by primary chalcopyriteand infill pore spaceswithin the Wells (1949),Frondel (1950), Fleet (1975) and corrodedsulfides (Fig. 2d). Hawthome(1985). Figures 3 and4 showthe common habits of atacamiteand its polymorphsin the TAG Mtr\ERALocyero Ctmasrny or AracaurrB gossans.In detail, the colloform massesof atacamite consist of cryptocrystalline material (Fig. 3a) and All three naturally occurringpolymorphs of sheath-likeaggregates @g. 3b). Theseare commonly Cu2Cl(OH), have been identified in the gossansfrom overgrownby characteristicslender, prismatic crystals TAG using single-crystal X-ray diffractometry. (Figs. 3c,d). Figure 4a showstypical fine-grained Atacamiteis most common,followed by paratacamite crystalline atacamite,with well-developed{010} and rare botallackite.The crystalline habits of ata- forms, presentin cavities and pore spaceswithin the camite (orthorhombic)observed in the gossansinclude oxidizing sulfides.Some of thesecrystals are twinned slenderprismatic crystalsand equant,rhombic prisms (Fig. ab). Atacamite and paratacamiteare usually that exhibitthe {110}, {011} and {010} forms. complexly intergrown (Fig. 4c), but can be distin-

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 950 THE CANADIAN MINERALOGIST

FIc. 5. Photomicrographsof atacamitetextures in reflectedand lransmittedlight. a) Fine-grainedatacamite within desiccation cracksin massiveFe oxyhydroxides(transmitted 1igh0. b) Colloform encrustationof atacamitecoated by a film of amor- phousFe oxyhydroxides(reflected lighi). c) Colloform massesof atacamitethat appearto have nucleatedon fine-grained Fe oxyhydroxideparticles and bacterialfilaments (transmiuedligh!. d) Colloform atacamiteovergrown by coarsecrystals (transmittedlight). e) Microscopic,biogenic filaments trapped within atacamite(transmitted light). Individual filamenls axe typically <20 pm in length and only a few micrometersin thickness.f) Microscopic fluid inclusions (<2 pm) trapped on growth planeswithin crystalline atacamite(transmitted light). The inclusionsmay representhealed etch-pits on former crystalfaces that were exposedto dissolutionby seawater.Individual inclusionsare too small to be useful for micro- thermometricmeasurements. Black patchesare inclusionsof amorphousFe oxyhydroxides.Scale bars are in micrometers.

guishedby their opticalproperties (see below) and in In transmitted light, atacamiteis bright to dark X-ray powder patterns.Rare crystalsof botallackite emerald-green,with strong pale to yellow-green observedin the scanningelectron microscope are pleochroism.Paratacamite also is dark green in trans- strongly etchedand corroded (Fig. d), and therefore mittedlight, but is not pleochroic.Botallackite was not do not appearto have been in equilibrium with the observedin thin section,but has a slight blue-green pore fluids in the gossan. tint in ordinary light (Fig. 20. In thin section,crys-

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 TIIE FORMAT1ONOF ATACAMITE ON T}TE SEAFLOOR 951

talline forms of atacamiteare commonly observed Pana6nNEsrs within late-stagecavities in the massiveFe oxides, filling open spacesbetween colloform bandsof Fe Exposedsurfaces of sulfidedeposits on the modern oxides,or in desiccationcracks that form during the mid-oceanridges undergorapid oxidation owing to incipient crystallizationof goethitefrom amorphous the infiltration of cold seawaterinto the poroussulfide Fe oxyhydroxides(Fig. 5a). More massivecolloform matrix. Seawaterpenetrates into cracksand along atacamiteon the surfacesof oxidized sulfides shows grain boundariesin the sulfidesand reactswith the concenric growth-zonestypical of open-spacefilling exposedsurfaces of mineralsto producestrongly acid (Figs. 5b,c). Discretebotryoidal forms, rimmed pore-fluids.Acid conditionsare generatedby a series by crystalline atacamite,appear to have nucleatedon of sulfide-oxidationreactions, likely dominatedby: particlesof Fe oxidestrrg. 5d). Most of the atacamite FeSr+lD O, + HrO = Fe2++ 2SO]-+2H+ samplesthat were collectedare dustedwith fine inclu- FeS,+ l4Fe3++ 8HrO = l5Fe2++ 2Soo2-+ 16H+ sions of oxide particles (seeFig. 2e). In most pub- lished experimentalwork on thesephases, the 2Fe2*+ 1,12Or+ 5H2O= 2Fe(OH)3+ 4H+ presenceof such foreign nuclei appearsto have asproposed by Nordstrom(1982). encouragedcrystal growth. Fossilizedremnants Cuprouschloride complexesand Cu2*ions are of filamentous bacteria are presentthroughout the releasedby the dissolutionof chalcopyriteand massiveFe oxide gossanand also are locally pre- secondaryCu sulfidesin the acidic, oxidizing solu- servedwithin atacamite(Fig. 5e). The filamentous tions (e.9.,Rose 1976). The dissolvedions migrate threadsclosely resemblethe bacteriafound in low- outward through the gossansand are reprecipitatedas temperatureiron-rich depositselsewhere on the basiccopper salts upon contactwith the surrounding seafloor(e.g., Juniper & Fouquet1988), and they may seawater.The atacamitethat fills veidets or fractures have played an important role in the oxidation of sul- in the gossanslikely delineatesthe conduits through fides and the releaseof Cu2+ions to seawater which seawaterwas able to penetratethe massive (cf. Thiobacillusferrooxidans: Alt 1988b,Holm 1987, sulfides.Similar veinlets and fracture-fillings of Hannington& Jonasson1992). Crystalline forms of atacamitehave been describedby Alt et al. (1987) atacamitealso show distinct zoning, commonlywith in gossanousmaterial from oxidedeposits on the East abundantmicroscopic fluid inclusions along the Pacific Rise. The precipitation of the atacamiteas growthplanes (Fig. 50. Theseinclusions are generally crustson the outer surfacesof the Fe-oxidegossans too small to be usedfor microthermometricmeasure- usually representsthe last stagein the completehydra- ments. tion of exposedsulfides. Results of electron-microprobeanalyses of The equilibria controlling the speciationof copper atacamitefrom the TAG gossans(Table 1) indicate in seawaterare well known, and the stability relations averagecompositions of 56.2tlt.Vo Cu arrdl7.l wt.Vo of atacamiteand other basic coppersalts have been Cl. Two analysesof atacamitefrom sample2190-6-l examinedby Barton & Bethke (1960),Bianchi & correspondmost closelyto the stoichiometriccompo- Longhi (1973),Rose (1976) and, most recently,by sitionof Cu2Cl(OH)3.However, most of the atacamite Woods& Garrels(1986). Stability diagrams based on is slightly depletedin Cu, suggestingprobable hydra- early determinationsof the Gibbs free energiesfor the tion ofthe outersurface. basiccopper salts indicated that tenoriteshould be the stableproduct of seafloorweathering of Cu sulfides (e.9.,Bianchi & Longhi 1973,Rose 1976).However, the free-energydata for atacamiteand the cupric oxidespublished recently by Woods& Garrels(1986) TABt:e1, C'IIBMICALcoMposlnoN oF ATAcAMrEreoIurnE reo vouNxf indicate that atacamiteis the more likely stablephase at the pH and Eh of normal seawater(Frg. 6). Because 21E!&1-3 2163{"1-3 219SGr 219o4t 2lal{-2 Id€f of the high Cl-/SOf- ratio of seawater,brochantite also Poia (l) (2) (1) @) (1) c%(oElcl is less favoredthan atacamite.The low pH of the pore P& tuE 1.60 0.r3 la4 0.60 0.43 fluids in contactwith oxidizing sulfidesmay havepre- CUO 69.61 6936 66.05 74,p 7232 ,o.q P@ 0.m 0.00 000 025 0.m ventedthe formation of malachite.which otherwise ABzO 0.(r/ 0.m 0!0 0.0t 0.m might be the more stablecopper salt in seawater ggo 0.04 0.00 0.00 0.6 0.m sro o01 0.17 0.m 020 0.10 (Fig. 6b). The free-energydata reported by Woods& Rbp o't 0o6 0.00 0.8 0.01 M2O 0.m 0!0 0.m 0.00 0.m Garrels(1986) indicate that atacamiteand malachite K2O 0.00 0.9) 0.m 0.01 ,. 0.m g 17 17.80 1635 fi.Q nJ3 r660 are of nearly equal stability in surfaceseawater equi- sq 0.15 08 0.03 0.16 005 libratedwith the atmosphere(i.e., containingCO2 at an 91.08 a constantpartial pressureof 10-3'5bar: Fig. 6b). However,because their stabilitiesare so close,the pre- 'Em nlrropobe oalyu obuin€d on i0divid@l grd6 6ing a C@ Sx-50 niqlFote ferred speciesdepends critically on the local p(CO), (15 &V a@lsadng wluge l0 nA b@n ol@t, and 2& cmltog dne p6 elda, etth p@ del ad mi@I rErd{&. which may be a function of temperature,depth, and

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 952 TI{E CANADIAN MINERALOGIST

a) Garrels (1986) uoted that under experimentalcondi- tions, the rate of crystallization (which is strpngly dependenton the concentrationof dissolvedcopper and the ratio of Cl- to II+ in solution) is apparenflyan important control on the particular speciesthat forms. They found that paratacamiteis the usual product in strongly acid solutions,whereas atacamite precipitates

o under more alkaline conditions(see also Sharkey& I Lewin 1971).Pollard et aI. (1989) confirmedthe + resultsof Sharkey& Lewin (1971)and notedthat * -rs BFOCI{ANIT'E the first phaseto crystallizein all experimentswas eJq.s9 botallackite, wtfch rapidly recrystalllzedto atacandte or paratacamite,'dependingon the goncentratiotrof Cuz+(aq). Metastablebotallackite was found to recrystallize to paratacamiteat low conggntrations of Cu2+(e.g., tens of ppm in solution) and to atacamite at intermediateconcentrations (e.9., hundredsof ppm log aj. * log a--e Cu), with atacamiterecrystallizing to paratacamite at evenhigher concentrations.On this basis,Pollard e/ (1989) paratacamiteis the more b) al, suggestedthat stable trimorph at ambient temperaturesand that crystallizationof metastableatbcamitd and botallackite is largely a result of kinetic iontrols. The presence of high concentrationsof Cl- in the experiments(1e., closeto thoseof typical seawater:0.5 M NaCl) tended to inhibit the recrystallizationof atacamiteto parata- carnite.This factor may accountfor the abundanceof atacamiteover paratacamitein seafloorgossans. Although botallackite has been reported at a number of localities where seawateris in direct contact with Cu minerals(e.9., rn old mine workings that extend below sea-levelin the Botallack mine. Cornwalll Pollard et al. 1989),its metastablebehavior accounts for its scarcity in nature. SEM photographsGig. 4d) of botallackitefrom the TAG gossansshow clear €-8.4€ evidenceof etching and confirm that this phaseis not tog p"q stable. Under acid, oxidizing conditions,Cu+ and Cu2* are paratacamite(atacamite), FrG. 6. Stability fields of tenorite, carried in seawatersolutions dominantly as cuprous brochantite, and malachite in seawaterat 25'C. The (CuCl; CuCl32) and Cu2+ approximate composition of seawaterare shown by chloride complexes and the star (modified after Woods& Garrels1986). ions, respectively(e.g., Rose 1976).Figure 7 shows the stability fields of the basic coppersalts, copper oxides,native copper,and chalcocite,and the solu- bility of copper in the acidic pore-fluids. The copper salts are quite soluble in the acid solutionsthat pene- local biological activity (e.9., Bianchi & Longhi trate into the oxidizing portions of the sulfide mound, 1973).For the most part, a lower p(COz)in the cold The releaseof Cu2+and the stability of CuCl2- bottom-watersof the deep ocean(i.e., undersaturated and CuCl]- complexesunder theseconditions allow in calcium carbonate)compared to avgragesurface- solubilitiesof the Cu-bearingphases in excessof waterswill inhibit the formation of malachite. 1,000ppm. The solubility of copperis highestat a low The free energies of formation for atacamite and pH and decreasessharply at slightly alkaline pH, paratacamitereported by Barton & Bethke (1960) resulting in the precipitation of atacamitewhere the and by Woods & Garrels (1986) imply only trivial oxidizing solutionscome into contactwith ambient differences in the predicted fields of stability of the seawater.Becauso of the high pH and high concen- two minerals.Apart from their thermodynamicproper- tration of Cl- in seawater,atacamite is only slightly ties, the kinetics of formation (nucleationand growth) soluble(i.e., =10 ppb Cu) undernonnal conditions on play an importart role in the crystallizationof the vari- the seafloor. This fact suggeststhat atacamitecrusts ous basic copper salts (Pollard et al. 1989).Woods & forming on the surfacosof the gossansshould persist

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 TTIE FORMAfiON OF ATACAMITE ON TIIE SEAFLOOR 953

the seafloor. This fact suggeststhat atacamitecrusts strongly alteredto goethite,and it seemslikely that the forming on the surfacesof the gossansshould persist observedatacamite formed during weatheringof for long periods,provided that the depositsremain the sulfides following uplift and exposureto seawater submerged.In contrast,atacamite will dissolvereadily alongthe fault scarps(cl Bonattiet al.1976, Scotte/ in fresh water and thereforeis rare in subaerialenvi- al. L982).Mossman & Heffeman (1978) also observed ronmenB. atacanite in cores of metalliferoussediment from the Severalresearchers have suggestedthat atacamite Atlantis tr Deep in the Red Sea.Because thesp sedi- in seafloor sulfide depositswas precipitated directly mentswere depositedfrom hot sulfidic brines,they from high-temperaturehydrothermal fluids. For exam- suggestedthat the atacamitemight have formed from ple, the occrurenceof atacamitein veins or as fracture- interstitial fluids under hydrothermalconditions. fillings in somegossan sanrples from the TAG mound However, analysesof fluids from the Atlantis tr Deep led Lisitsyn et al. (1989) to suggestthat the minelal indicate that atacamitecould not have formed at the formed by hydrothermal venting. However, the low pH and Eh conditionsprevailing at the bottom of presenceof atacamitewithin the desiccationcracks the brine pool during accumulationof the metallifer- in massiveFe oxyhydroxidecrusts (e.S., Fig. 5a) indi- oussediments (e.9., temperatures of 61.5oC,pH = 5.5, catesihat it formed late in the oxidation the massivE Eh = 310 mV: Bischoff 1969,Hartrnann 1985). Rose sulfides,long after precipitaliopof hydrothermal (1976)also showed that extrapolationofthe solubility phaseshad ceased.Sharas'kin et al. (1989), who product for atacamiteto higher temperaturesusing observedatacamite within subseafloorstockworkilike availabledata on free energy and enthalpyindicates a veins in uplifted blocks of oceaniccrust on the Mid- dramatic increasein solubility with temperaturesuch Atlantic Ridge, interpreted the atacamite to have that the stability field of atacamitevirnrally disappears formed from hydrothermal fluids at temperatures at temperaturesmuch greaterthan 25oC.More of 220160'C (basedon fluid inclusionsin coexisting recently,secondary Cu sulfideshave been documented quartz).However, sulfidesin theseveins were within sedimentsof the Atlantis II Deep, suggesting that in situ oxidation of the primary sulfide assem- blage has occurred(e.9., Pottorf & Barnes 1983).The formation of secondarygypsum in a number of the cores during storagealso suggeststhat hydration of the assemblageof primary minerals in the sedi- 1@0ppm i \i, mentsmay have occurredsince the time of collection (e.g., Ziercnberg& Shanks1983). The presenceof atacamitein samplesof the Red Sea muds therefore 600 is best explained in terms of sulfide oxidation, as observedelsewhere on the seafloor,and likely results @ a{E from a late diageneticreaction within the sedimentsat ambientbottom temperatures. Eh(mV) 2. \- N I I I CoNcr-usroxs \------r Atacamite forms readily in modern seafloor envi- ronmentsthrough the oridation of Cu sulfide minerals -2@ in contactwith arnbientseawater. Free-energy data on the formation of atacamiteand paratacamitesuggest .400 that they are the stablecopper salts in deepsubmarine pH gossans.Atacamite is only slightly soluble in sea- water, and therefore should persist as componentsof minerals Ftc. 7. Eh-pH diagramfor the systemCu-O- HaS-Cl in sea- the assemblageof secondary following com- water at 25"C (>Cl = 0.5 m, >S : 0.03 m). Stability plete oxidation of the Cu sulfides.Radiometric dating fields are shown for the basic copper salts (paratacamite of severalatacamite-bearing gossan samples from the and atacamite),aopper oxides (tenorite and cuprite), TAG mound indicatesthat they may be as old as native copper,and chalcocite.The fields of the principal 20,000 years and possibly as old as 40,000-50,000 aqueouscopper species(Cu2r, CuCl2-, and CuClS-)in years Q-alouet al. L990).Atacamite also is presenton equilbrium with the solid phasesare shown for lCu = 1 the floor of the rift valley in sedimentsderived from ppm and 1000ppm (dashedlines). Chloride complexesof the mass-wastingof relict sulfide depositson the walls Cu2+are much weaker than the cuprous complexes valley at least 13,000years ago (Metz et al. CuCl; and CuCf, and thereforeare not considered.The of the approximate pH and Eh of seawaterare shown by the 1988). However, the susceptibility of atacamiteto star. Stability fields were calculatedusing standardGibbs hydration and dissolution in fresh water likely free energiesof formation listed in Appendix l. accourltsfor its absencein fossil submarinegossans

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 954 TIIE CANADIAN MINERALOGIST

Conditionssimilar to thoseobserved in modern Bamy, M.C. (1938):Mineralogy of threesulphate deposits seafloorgossans prevailed during the descentof the of northernChile. Am. Mineral.23, 669-760. water table throughexposed sulfide depositsin Brrurs, P.M. (1960):Thermodynamic the AtacamaDesert. Here, atacamite was precipitated BARroN,P.B., Jn. & propertiesof somesynthetic zinc and copperminerals. by supersaturationduring tlte evaporationof saline Am.J. Sci.258A,2l-34. groundwaters,and the atacamitehas sincebeen preservedin this environmentin the absenceof fresh BraNcHr,G. & LoNcut, P. (1973):Copper in sea-water, waterto dissolveit. potential- pH diagrams.Corrosion Sci.13, 853-864.

ACKNOWLEDGEMENTS BrscHoFF,J.L. (1969):Red Seageothermal brine deposits: their mineralogy,chemistry and genesis./n Hot Brines The officers and crew of the NOAA ship and RecentHeavy Metal Depositsin rhe Red [email protected]. Degens& D.A. Ross,eds.). Springer-Verlag, New York Researcher,the Atlantis 11and the ALVIN Group are (402-406). tlanked for their supportin obtainingsamples from the TAG HydrothermalField. The author is indebted BoNATTT,E., GusnsrsrN-HoNNoREz,B.-M. & HoNNoREz,J. to PeterRona Q.{OAA) and Geoff Thompson(WHOI) (1976):Copper-iron sulfide mineralizations from fhe equa- for providing the opporlunity to participate in this torial Mid-Atlantic Ridge.Econ. GeoI. 71, 1515-1525. work. Meg Tivey, SusanHumphris, and Margaret Sulanowska(WHOD kindly assistedin the handling, EMBr-sv,R.W., JoNassoN,I.R., PERFrr,M.R., Fnaurl-tN, curation, and descriptionof the samples.Special J.M., Trvsv, M.A., MelauoFF,A., SMtrH,M.F. & thanks are due to Robert Kelly (photography)and FRANcIS,T.J.G. (1988): Submersible investigation of systemon the GalapagosRidge: Laura Radbum(SEM imaging),who photographedthe an extinct hydrothermal rnounds,stockwork zone, and differentiated1avas. petrographic sulfide relationshipsdiscussed in this paper. Can.M ineral. 26, 517-539. X-ray diffractometry was carried out by A.C. Roberts, and elertron-microprobeanalyses were performed by Flper, M.E. (1975):The crystal structureof paratacamite, J. Stirling in the MineralogicalLaboratories of Cu2(OH)3C1.Acta Crystallogr.831, 183-187. the GSC. The paperhas benefittedfrom the helpful commentsand reviewsof J. Jambor,P. Herzig, FRoNDEL,C. (1950):On paratacamiteand somerelated M. McNeil, G. Thompson,D. Mossman,and C. copperchlorides. Mineral. Mag. 29,34-45. Alpers. This work was supportedin part by grant #0552188from the NATO ScientificAffairs Division HANNNcToN,M.D., HALL,G.E.M. & Varve, J. (1990):Acid pore from an oxidizing sulfide deposit on the and by a specialgrant from the Alexandervon fluids Mid-Atlantic Ridge: implicationsfor supergeneenrich- Humboldt Foundation. ment of gold on the seafloor.Geol. Soc.Am., Program Abstr.22.442.

-, HERzrc,P.M., GnEconr, C. & B€leNcen,P. (1991): REFERENCES Mineralogy and chemistry of complex Fe-oxide gossans from the TAG HydrothermalField, Mid-Atlantic Ridge. ALpERs,C.N. & BruMHALL,G.H. (1988):Middle Miocene Geol. Assoc. Can. - Mineral. Assoc. Can., Program climatic changein the Atacama Desert, northem Chile: Abstr.16, 457. evidencefrom supergenemineralization at La Escondida. Geol.Soc. Arn. Bull.100, 1640-1,656. & JoNessoN,I.R. (1992): Fe and Mn oxides at seafloor hydrothermal vents. /n Biomineralization - & - (1989):Paleohydrologic evolution and geo- Processesof Iron and Manganese(H.C.W. Skinner chemical dynamicsof cumulative supergenemetal & R.W.Fitzpatrick, eds.). Catena Suppl.2l,35l-370. enrichmentat La Escondida,Alacama Desert, northem Chile. Eco n. Geol. 84. 229-25 5. -, THoMpsoN,G., RoNA,P.A. & Scor-f,S.D. (1988): Gold and native copper in supergenesulphides from the Alr, J.C.(1988a): The chemistryand sulfur isotope composi- Mid-AtlanticRidge. Nature 333, &-66. tion of rnassivesulfide and associateddeposits on Green Seamount,Eastem Pacific. Econ. Geol.83, 1026-1033. HARTMANN,M. (1985): Atlantis II Deep geothermalbrine system.Chemical processes between hydrothermal brines - (1988b):Hydrothermal oxide and nontronitedeposits andRed Sea deep waler, Marine Geol.64,15'7-177. on seamountsin the EastemPacifrc. Marine Geol.8l, 227-239. HAwTHoRNE,F.C. (1985):Refinement of the crystalstructure of botallackite.Mil, eral. Mag. 49,87 -89. - LoNSDALE,P., HAyMoN,R. & MrJEHLENnacus,K. (1987): Hydrothermalsulfide and oxide depositson HrrrNnN, R. & FoueuEr,Y. (1985):Volcanism and metallo- seamountsnear 21oN" East Pacific fuse. Geol.Soc. Am. genesisof axial and off-axial structureson the East BulI.98.157-168. PacificRise near 13"N. Econ. Geo|.80.221-249.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 THE FORMATION OF ATACAMITE ON THE SEAFLOOR 955

HELcEsoN,H,C., DrurNv, J.M.,Nnsnrm, H.W. & Brnn,D.K. RoBrE,R.A, Hnlrrncw.nv,B.S. & FlsHER,J.R. (1978): ' (1978):Summary and critiqueof the thermodynamic Thermodynapic propertiesof minerals and related propertiesof rock-formingminerals. An. J. Sci.278A, substancesat298.15 K and I bar pressure(105 pascals) and at higher temperatures.U.S. Geol. Surv., Bull. Hnnac, P.M., HANNrNcroN,M.D., Scom, S.D., Meu$rrs, 1452. G., RoNe,P.A. & TuorupsoN,c. (1991):Gold-rich sea-floorgossans in the Troodos ophiolite and on the RoNe, P.A., Bocoauov, Y.A., GuRvrcH,E.G., RrMsKr- Mid-AtlanticNdge. Econ.Geol.86, 1747-1755. Kons,crov, N.A., Secelpvrrcu, A.M. & HerwnqcroN, M.D. (1993):Relict hydrothermalzones at the TAG Houra,N.G. (1987):Biogenic influences on the geochemistry hydrothermalfield, Mid-Atlantic Ridge 26"N, 45'W. of certain femrginous sedimentsof hydrothermalorigin. J. Geophys.Re s. 98, 9715-97 30. Chem.Geo|.63.45-57 . - HaxnrrcroN, M.D. & THovrpsoN,G. (1990): JUNIpE&S.K. & Fououn-r,Y. (1988):Filamenrous iron-silica Evolution of hydrothermal mounds,TAG hydrothermal depositsfrom modem and ancienthydrothermal sites. field, Mid-Atlantic Ridge 26'N, 45"W. Trans.Am. Can.Mineral. 26. 859-869. Geophys.Union (Eos) 71, 1650(abstr.).

Lar-ou,C., THoMpsoN,G., Annolo, M., Bnrcnsr, E., -, KLrNKHavnrn,G., NELSEN,T.A., TREFRy,J.H. & DRUFFEL,E. & RoNe,P.A. (1990):Geochronology of EI"DERFIELD,H. (1986): Black smokers,massive sulfides TAG and Snakepithydrothermal fields, Mid-Atlantic and vent biota at the Mid-Atlantic Ridge.Nature 321, Ridge: witnessto a long and complexhydrothermal 33-37. fustory.Earth Planet. Sci. Lett.97,113-128. RosE,A.W. (1976):The effectof cuprouschloride com- LrsrTsvN,A.P., BocoaNov,Y,A., ZoNENSHAYN,L.P., plexes on the origin of red-bedcopper and related Kuz'urn, M.I. & SacalrvrcH, A.M. (1989): deposits.Econ. Geol. 71, 1036-1M8. Hydrotlermal phenomenain the Mid-Atlantic Ridge at Lat. 26"N (TAG hydrotherrnalfield). Int. Geol.Rev.3l, Scorr, R.B.,TEMILE, D.G. & PsnoN,P.R. (1982): The nature I r83-1198. of hydrothermalexchange between oceanic crust and seawaterat 26oN latitude, Mid-Atlantic Ridge. - SAGALEvTc!,A.M., CurnresHEv,G.A. & Snesurov, In The DynamicEnvironment of the Seafloor(K.A. N.L. (1990):Investigation of a hydrothermalvent in the Fanning& F.T. Manheim,eds.). Lexington Brooks, Atlantic Oceanfrom the Mir submersibles.Dokl. Acad. D.C.(3814r6). Sci. USS& Eanh Sci.Sect. 3ll, 248-252. Scorr, S.D.,CHess, R.L., HANNTNGToN,M.D., Mranaru P.J. Mrrz, S., TREFRY,J.H. & NmsrN, T.A. (1988):Hisrory and & McCoNacrv, T.F. (1990):Sulfide deposits, tectonics, geochemistryof a metalliferous sedimentcore from tlte and petrogenesisof Explorer Ridge, NortheastPacific '87 Mid-Atlantic Ridge crest at 26'N. Geochim"Cosmochim. Ocean.In Troodos (J. Malpas,E.M. Moores, Acta 52,2369-2378. A. Panayiotou& C. Xenophontos,eds.). Geological SurveyDepartrnent, Nicosi4 Cyprus(719-733). MoRrrMER,C. (1973):The Cenozoichistory ofthe sourhern AtacamaDesert, Chile. ./. Geol. Soc. London 129. Suanas'nN, A.Y., KerupnsrsKly,V.S., SusHcHEvsKAyA, 505-526. T.M. & Kxveznva, S.N. (1989):Hydrothermal parata- camite-quartzmineralization in metamafic rocks of the MossMAN,D.J. & IiSFFERNaN,K.J. (1978):On the possible oceaniccrust. Dokl. Acad. Sci. USSR,Earth Sci. Sect. primary precipitation of atacamiteand other metal 3M.213-215. chloridesin certainstratabound deposits. Chem. Geol.2l, l5 1-159. SHenrsv,J.B. & LswN, S.Z. (1971):Conditions governing the formation of atacamiteand paratacamite.Am. Mineral.56. 179-1,92. Nomsrrou, D.K. (1982):Aqueous pyrite oxidationand the consequentformation of secondaryminerals. In Acid THoMpsoN,G., HuMpHRrs,S.E., SCHRoEDER,B., Sulphate Weathering: Pedogeochemistry and Sularowsra, M. & RoNa,P.A. (1988):Active venc and Relationshipto Manipulationof Soil Materials(J.A. massivesulfides at 26'N (TAG) and 23'N (Snakepit)on Kitrick, D.S. Fanning& L.R. Hossner,eds.). Soil Sci. theMid-Atlantic Ridge. Can. Mineral. 26, 697 -7 11. Soc.Am. Press,Madison, Wisconsin (37-56). Trvrv, M.K., Hr.rwrrus, S.E.,THorvlpsoN, G., WARD,K.A., Polleno, A.M., THoMAs,R.G. & Wnrraus, P.A. (1989): HaNNrr.lcror,M.D. & RoNa,P.A. (1990):Mineral Synthesisand stabilitiesof the basic copper(II) chlorides precipitation at the TAG (26'N) and Snakepit(23"N) atacarnile,paratacamite, botallackite. Mineral. Mag. 53, hydrothermalsites, Mid-Atlantic Ridge. Trans.Am. 557-563. Geophys.Union (Eos) 71, 1650(abstr.).

PorroRF,R. & BARNES,H.L. (1983):Mineralogy, geochem- VaNro, D.A., Mrlny, B.J. & HErNzeurrH,S.W. (1991): istry, and ore genesisof hydrothermalsediments from Massivesulfides with fluid-inclusion-bearingquartz from the Atlantis II Deep, Red Sea. Econ. GeoI., Monogr. 5, a young seamounton the EastPacific Rise. Can.Mineral. 198-223. 29.453-460.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021 956 TIIE CANADIAN MINERAIOGIST

WAGMAN,D.D., Evexs, W.H., P.lnren, V.B., APPSNDD(I. AND SEX.ECIBD SCHUMM, T'{ruODYNAMIC DATA R.H., HALow, I., BAILEv,S.M., CuunNev,K.L. (1982): & Nurrell, R.L. The NBS tablesof chemical Minenl 6 Slsies Gol&Ymole) R€fe[aw thermodynamicproperties. J. Phys. Chem.Reference DatalL, Suppl.2,392p. 'lvoods Ataqmit€ C!(OI!6C12 -1341.8 &Gmli (1986) WEL6, A.F. (1949):The crystalstructure of atacamiteand PmtacmitE Cta(Ol0o(xz -131.8 BE!o!&Betbko(1960) the crystal chemistryof cupric compounds.Acta T66ite CUO -125.9 Woods&Gmls (1986) -'l&.0 (19E2) Crysrallogr.2, 175-180. Cbpdle crquo ws8sm etql. Chak@it€ C'uzS -85.6 WagIro elal. (1982) C\ro 0.0 Wooos, T.L. & GARRELS,R.M. (1986):Phase relations Natiw Cu of somecupric hydroxy minerals.Econ. Geol.8l, H4 b4) 0.0 1989-2007. a'@q) -131.3 Robieetql (198) soi2- taet -1Ms WagroetaL (1982) Zlrnmnrnc, R.A. & Suexrc, W.C., III (1983):Mineralogy OI'- Os) -157.3 Robiceral (197E) and geochemistryof epigeneticfeatures in metalliferous Hlo $) 47.2 HelgMetal (198) sediment,Atlantic II Deep, Red Sea.Econ. Geol.78, a](':h @t) -243.1 Ros(lg7o 57-72. CqCt3l fa4) -n6.9 R@(197O a?* @q) +65.5 Wags@dal(1982) N 1oq) +50.0 Wagruetal(l982)

ReceivedFebruary 9, 1993, revisedmanuscript accepted The followins activity coef6cientifrom Bi4chi ud longhi (193) wW us€din a[ catcElatiois:cl- (r)J7), whEg€d sp*is (1.13).mnovalent sFci6 (0.7) June14. 1993. md divalmt spci6 (0.12).

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/31/4/945/3446737/945.pdf by guest on 01 October 2021