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