Canodian MineralogistYol.26, pp. 567-587 (1988)

MINERALOGY. COMPOSITION, AND FLUID-INCLUSION MICROTHERMOMETRY OFSEAFLOOR HYDROTHERMAL DEPOSITS IN THE SOUTHERN TROUGH OF GUAYMAS

BASIN. GULF OF CALIFORNIA

JAN M. PETER AND STEYEN D. SCOTTMarine Geology Research Group, Department of Geology, University of Toronto, Toronto, Ontario MSS lAI

AgSTRACT

Numerous hydrothermal spires and rnounds composedof carbonates, sulfates, silicates, metal @e, Zn, Cu, Fe,Pb) sulfides and iron oxides occur at 2ffi0 m water depthin the sediment-filled Southern Trough of Guaymas Ba-sin, Gulf of California. Predominant minerals include cal-cite, barite, amorphous silica, stevensite, pyrrhotite andmarcasite. Major components (>190) from bulk chemical analyses are Fe, Si, Ca, Ba, CO2, S and SO3 (and, fors96e samples, Pb, Zn, Al, Na and Mg). The Guaymashydrothermal deposits therefore are fundamentally differentfrom sulfide-rich deposits at sediment-free spreading ax-es. Microthermometric measurements of aqueous, two-phase inclusions in calcite from chimneys indicate trappingtemperatures from about 150 to 3l5oC, and salinities of3.7 to 8.1 (mean 5.4) equiv. fl.qo NaCl. Trapping tem-p€ratlres are in good agreement wittr temperatures of vent-ing fluids measured lr sita by thermocouple probe. Fluid-inclusion salinities cannot be reconciled with salinities meas-ured from presently venting hydrothermal fluid and sug-gest that there are temporal variations in the chemistry ofthe vent fluid. Differences in salinity and homogenizationtemperatures for primary and secondary inclusions of in-dividual chimneys require that there has been mixing of hothydrothermal fluid and cold ambient seawater within chim-neys and spires. Mineral assemblages and compositions ofindividual minerals can best be explained by mixing of end-member hydrothermal fluid with seawater. Most mineralsare precipitated at the vent site largely in response todecreasing temperature induced by mixing with seawater.

Keywords: Guaymas Basin, hydrothermal precipitates,mineralogy, geochemistry, fluid inclusions, Gulf ofCalifornia.

SoMMAIRE

De nombreuses chemindes hydrothermales et des amon-cdlements de carbonales, sulfates, silicates, sulfur* (de Fe,Zn, Cu, Fe et Pb) et oxydes de fer se trouvent i une pro-fondeur d'eau de 2000 m dans la d6pression du Sud, si-tufe dans le bassin de Guaymas (golfe de la Californie).Les esp6ces min6rales les plus importantes sont calcite, bary-tine, silice amorphe, stevcnsite, pyrrhotite et marcasite. Lescomposants principaux (>lVo), d,apr€s les resultarsd'analyses chimiques globales, sont Fe, Si, Ca, Ba, CO2,S et SO3 (et, dans certains cas, Pb, Zn, N, Na et Mg).Sont aussi pr6sents, en concentrations moins importantes(<l9o): Zn,Pb, Na, Cu, Ag, As, Cd, Mn, Sb, Se et Sr.Des mesures microthermom6triques d'inclusions aqueusesi deux phases dans la calcite des chemin€es r6vdlent destempdratures de pi6geage entre environ 150 et 315.C, etdes salinit6s entr e 3 .7 et 8 .F/o (en poids; 5 .4go , en moyen-

ne) en 6quivalents de NaCl. Les temperatures de pi€geagesont en bon accord avec les temp€ratures des dmissions defluides des 6vents, mesur€es in situ par thernocouple. Tou-tefois, les salinites des inclusions fluides diffdrent sensible-ment des valeurs typiques des fluides 6mis des events ac-tuels, et indiquent donc des variations s6culaires dans Iacomposition du fluide hydrothermal. En conclusion, les dif-fdrences en salinitd et en temperatures d'homogdndisationdes inclusions fluides primaires et secondaires dans une seulecheminde indiquent la prdsence d'un m€lange de fluidehydrothermal avec l'eau de mer ambiante et plus froide,d l'int6rieur des chemindes. La meilleure explication des as-semblages min6ralogiques et de la composition des espd-ces mindrales implique un m6lange d'un fluide hydrother-mal non-dilu6 avec I'eau de mer. La plupart des min6rauxont 6td prdcipit6s i l'orifice, surtout d cause d'un refroi-dissement de la solution dt au m6lange.

(Traduit par la R6daction)

Mots-cl6s: prdcipit6s hydrothermaux, mindralogie, g€ochi-mie, inclusions fluides, bassin de Guaymas, golfe de laCalifornie.

IxrnooucrroN

Guaymas Basin is a hydrothermally active sea-floor-spreading segrnent (Lonsdale et al. 1980) rnthecentral Gulf of California (Fig. l). The basin con-sists of two northeast-trending grabens termed theNorthern and Southern Troughs, each about 40 and20 km long, respectively, and 3 to 4 km wide. Thesewere identified by Lonsdale & Becker (1985) to bea pair of en echelon, axial rift valleys that overlapat a non-transform offset (Lonsdale 1985). The basinfloor is covered with biogenic and terrigenous sedi-ments (Calvert 1966) at least 500 m thick (Lonsdale& Lawver 1980). Sedimentation rates of l-2 m/10@yrs (Calvert 1966) are nnusually high. The ter-rigenous component is derived predominantly fromTertiary volcanics of the Mexican mainland. Theabundant diatom remains in the central Gulf are aproduct of the area's prolific plankton blooms (Cal-vert 1966) which, together with the low oxygen con-centration in the bottom water and reducing condi-tions in the sediment, are responsible for the 3-490total organic carbon content of the sediments (Gold-haber 1974). Ocean-plate accretion occurs throughdyke and sill intrusions into the unconsolidated muds@insele et al. 1980, Einsele 1982, 1985, 1986).

The first evidence for hydrothermal activity withinGuaymas Basin was from heat-flow measurements

567

GUAYMASBASIN

568 THE CANADIAN MINERALOCIST

Ftc. l. Map of Gulf of California showing location ofGuavmas Basin.

Frc.2. Map of part of Southern Trough of GuaymasBasin showing location of hydrothermal depositsmapped by deep-tow side-scan sonar, and dive sites from1982 ALVIN expedition (modified from Scott 1985).

which locally exceeded 1.2 Wm-2 in the sediments(Lawver et al. 1975, Lawver & Williams 1979' Wil-harns et ql. 199). In 1977, ferromanganese-encrustedsulfide and talc deposits were discoverd during sub-mersible dives in the Northern Trough (Lonsdale1978, Lonsdale et ol. 1980). In addition, 3He valuesin the Guaymas Basin were found to be 65-7AVohigher than atmospheric levels, indicating input frommantle soruces (Lupton 1979). During Leg 64 of theDeep Sea Drilling Project (DSDP), three sites weredrilled in the Guaymas Basin (sites 477/4774 andsite 481 in the Southern and Northern Troughs,respectivel9, and one (site 478) in the basin betweenthe troughs (Frg. 2). Cores recovered hemipelagicsediments which were hydrothermally altered andintruded by basalt and dolerite sills of apparentlyvery limited lateral extent (Einsele et ol. 1980, Ein'sele 1985). Subsequently, inAugust 1980, extensivehydrothermal deposits were mapped by deep-towside-scan sonar and photography (Lonsdale 1980).Samples of these hydrothermal precipitates were col-lected by dredge in 1980, and their mineralogy wasdescribed by Koski et al. (1985).

In January 1982, a series of nine dives by the sub-mersible ALVIN in which Scott participated was con-ducted in the Southern Trough within the area ofvery high heat flow (Lonsdale & Becker 1985).Numerous hydrothermal mounds, chimneys, andspfues, some actively venting hydrothermal fluids,were mapped, and samples of hydrothermal fluidsand chimney and mound material were collected. InAugust 1985, the site was revisited by ALVIN withPeter participating. This paper describes the miner-alogy, bulk composition, and fluid inclusions of sam-ples recovered during the 1982 submersible dives. Asubsequent paper (Peter, Shanks & Scott, in prep.)deals with the stable isotope geochemistry of thesesamples.

DescnprloN oF Ttm DEPoSITS

Beneath the floor of the Southern Trough arezones of anomalously shallow acoustic penetrationwhere intrusive heating has induced unusually shal-low lithification with abrupt boundaries that areassumed to delineate the limits of the sills. It is onlyover these areas of high conductive heat flow thathydrothermal deposits and plumes have been locatedwithin a 2 x 6 km area centered on 27o03'N,'l1lo23'W (Lonsdale & Becker 1985). Figure 2 showsthe dive sites and the locations of most of the morethan 130 hydrothermal deposits which Lonsdale &Becker (1985) noted in five main fields. Individualdeposits are generally elongate northeast-southwestand cluster in lines parallel to faults (Fig. 2) thoughtto provide the main plumbing system that suppliesthe hydrothermal fluids to the sediments (Lonsdale& Becker 1985). The deposits have two end-membermorphologies: mounds, and chimneys or spires. Gra-

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HYDROTHERMAL DEPOSITS. CUAYMAS BASIN 569

dations between these were observed. The exposedportions of the mounds are conservatively estimatedto constitute some 5ffi,M tonnes of mineral precipi-tates (Peter 1986).

Hydrothermal mounds are generally 5-25 m highand 10-50 m across, are stgep-sided, and locally arecovered by a dense mat of tubeworms. The whitishcolor of these mounds contrasts with the browns andreds of sulfide-rich mounds typically found onsediment-starved ridge crests. Figure 3a shows thetop of a 6/pical mound photographed from ALVIN.The limited illumination prevented acquisition ofgood panoramic photographs of an entire mound.

Large angular blocks of talus are scattered aboutthe base of many mounds, and construction ofmounds from talus may be an important process.Many mounds also have one or several horizontal"eaves" (e.g., Fig. 3c) which protrude like skirtsaround their edges. Eaves are about l0 cm thick, 0.5to 2 m wide, have a flat upper surface, and gener-ally protrude laterally or curye slightly downward.Such eaves have also been described by Lonsdale &Becker (1985) who noted that these, as well as smallere:les on chimneys or spires, are hollow and containhot overpressured water which vents from a highlyporous undersurface (Fig. 3b,c). Figure 4c shows thesubhorizontal internal layering within one of theseeavgs.

Hydrothermal fluid from mounds escapes viachimney structures and through small fractures. Inmany instances, excavation of a mound withALVIN's manipulator arm initiated venting, indicat-ing that hot water is circulating inside. Close tomounds, the normal brownish hemipelagic and ter-rigenous sediment is overlain by distinctly reddishbrown ochrous sediment up to about I cm thick. Thisflocculant is presumably the oxidized hydrothermalfallout from vent fluids escaping into seawater.

Chimneys and spires are columnar structures @ig.3a,b) that vary greatly in height from a few cm to>30 m. They are either directly on hydrothermalmounds, on lithified sediment close to hydrothermalmounds, or in isolated clusters far from any mounds.Chimneys have a central orifice up to several cm indiameter (Fig. 4a,b); in contrast, spires lack a cen-tral orifice but contain numerous holes or intercon-necting pores (< I mm to I cm in diameter) throughwhich hydrothermal fluid escapes. Unless otherwiseindicated, the general term spire is used to refer botlrto chimneys and spires. Active and inactive spirescan be further subdivided into two groups:carbonate + sulfate-rich, and sulfide-rich. Most spiresencountered on submersible dives were inactive. Ofthose that were active, only a few were ventingsulfide-particle-laden fluid of the kind typified bythe "black smokers" at 21"N East Pacific Rise(EPR), and most were venting cloudy, greyish fluid.The higbest exit temperature measured for vent fluids

Ftc. 3. ALVIN bow photographs of chimneys, spires, andmounds: A) spires on a mound; B) pagoda-like chim-ney. ALVIN's manipulator arm has broken off a pieceof the eave and allowed hydrothermal fluid to effuse.C) Close-up of hydrothermal fluid escaping from theunderside of a chimney eave. Manipulator arm is inforeground.

in Guaymas Basin on either of the expeditions was359oC at a seafloor pressure of 200 bars (H. Jan-nasch, pers. comm.). Many spires are of the varietytypified in photographs and illustrations of the 2l oNEPR site (slender, tapering, with a central orifice),whereas others are delicate and castellate (Fig. 3b).Some spires display prominent eaves which projectup to several tens of cm from the wall. The close

570 T}IE CANADIAN MINERALOGIST

stacking of several eaves forms elaborate pagoda-like structures (Fig. 3b).

Many of the spires, and some mounds, are satu-rated with petroleum derived from thermal altera-tion of sedimen-tary organic material (Simoneit &Lonsdale 1982). Preliminary assessment does not rev-eal any relationship among sample type, tempera-ture of venting fluid, and degree of oil saturation.In rare instances, venting hydrothermal fluids wereseen to carry droplets of oil from the vent orifice.Oil stains impart a dark brown color to the normalmottled grey-white exterior of spires. Oxidation fur-ther imparts an orange-brown color that is morepronounced on inactive spires than active ones.

ANer,rncer, METHoDS

Spires were toppled and collected with ALVIN'smanipulator arm, whereas mound samples were

excavated with the manipulator. Such samplingprocedures are acceptable for spires, but mound sam-ples are not representative of the mound interior.About 100 kg of hydrothermal mineral precipitateswere collected, with the largest piece weighing about20 kg. The carcinogenic petroleum, as well as anyseawater remaining in the sample, were removed byplacing the samples in methanol to remove water,and then in methylene chloride to extract thepetroleum. The petroleum acted as a binder in manysamples and, when this was removed, some samplesfell apart.

The mineralogy of the samples was determined byX-ray diffractometry (XRD), reflested and transmit-ted light microscopy, and scanning electronmicroscopy. In all, 41 polished thin sections' 17polished specimens, and 32 fluid-inclusion sectionswere examined. Mineral separates for XRD wereobtained either by hand-picking or by separation in

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Frc. 4. A) Cross-sectional slab of chimney I l?3-6. Narrow concentric zones of metal sulfides (dark grey) occur in themiddle of the wall. Calcite (whire) lines the orifice. B) Cross-sectional slab 6f ghimnsy 1169-18. Chimney is com-posed predominantly of calcite with minor sulfides. C) Cross-sectional slab of chimney eave l17zl-5C. Undersideis lined with pyrrhotite plates. Another such zone of pyrrhotite is located about 0.75 cm from the underside. Whiteis predominantly calcite, and medium-grey upper surface is a coating of iron oxide. D) Undersurface of chimneyeave I 173-10 showing delicate pyrrhotite plates (dark) oriented perpendicular to the eave and calcite (white) gxowthsurface.

HYDROTHERMAL DEPOSIIS, GUAYMAS BASIN 571

a heary liquid. Several samples were thermallytreated and glycolated to check for the presence ofsmectite. Minerals were analyzed using an ETECautomated microprobe equipped with an energy-dispersion analyzv, Operating conditions and stan-dards are given in Peter (1986).

Fourteen representative samples encompassing allthree types of hydrothermal precipitates (mounds,active and inactive spires) were selected for bulkchemical analysis. The 50-g samples were ground byhand in an agate mortar or powdered in a,1 aluminamill to <-2M mesh. Silica contamination is esti-mated to be less than 0.390. Bulk chemical analyseswere done by X-Ray Assay Laboratories Ltd. of DonMills, Ontario, and involved a wide variety of tech-niques including X-ray fluorescenca $pectrometry,argon-plasma-emission spectrometry, flameless andgraphite furnace atomic absorption, neutron activa-tion, fire assay, and wet chemistry. Additional ana-lyses performed at the University of Toronto weredone by instrumental neutron activation (INAA).

Because of their friable nature, samples selectedfor fluid-inclusion study had to be impregnated witha slurry of fine-grained portland cement and waterunder vacuum. Doubly-polished thin sections, 60 to100 pm thick, were prepared from the impregnatedspecimens (following the method of Holland et al.1978). A Linkam TH600 programmable heating-cooling stage (Shepherd l98l) was used to makemicrothermometric measurements. Repeated calibra-1i6n uttlizing the known melting points of varioussubstances (McDonald & Spooner l98l) within thetemperature range of interest were performed priorto and during microthermometric work to correctfor drift. In order to overcome problems associatedwith stretching of fluid inclusions during homogeni-zation runs, inclusion sections were broken intomany smaller chips. All freezing work was done priorto heating, and each chip was heated only once.

DEscRIprrvE AND CHEMrcer MrNsRALocySamples for mineralogical study were selected to

encompas$ all deposit morphologies and to cover alarge areal extent. Table I lists the minerals identi-fied and their abundances. All analyses cited beloware by electron microprobe.

Sulfides

Pyrrhotite. Pyrrhotite is ubiquitous in all samplesas thin euhedral hexagonal plates from <0.01 to 2.5rnm across @gs. 5a,b). Plates radiate perpendicu-larly from growth surfaces into open spaces or cavi-ties. Where cavities are absent, pyrrhotite plates areintergrown in a random or boxwork fashion. platesare incipiently fractured and, in inactive spires andcertain mound samples, are partly to completely oxi-dized to lepidocrocite and crusts of goethite and

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amorphous iron oryhydroxide. Pyrrhotite plates alsoform coalescent aggregates and microcrystallineinclusions in other minerals such as anhydrite andcalcite. The inclusions delineate growth zones andare indicative of fluctuating physicochemical condi-tions of the hydrothermal fluid from which theminerals were precipitated. Analysa indicate that thepyrrhotite contains only Fe and S. The mole frac-tion of FeS in pyrrhotite varies from 0.933 to 0.960(mean : 0.947,lo : 0.08, N = 19). XRD pat-terns identified only so-called "hexagonal" plnrho-tite Kissin & Scott 1982). No attempt was made toidentify polytypes. Pyrrhotite in the Guaymas Basindeposits is stabilized relative to FeS, by the low aO2ofthe discharging reduced vent fluids (Scott 1985).

Morcasite and pyrite. Secotdary marcasite, whichmay have formed stably (Murowchick & Barnes1986), is conunon in many mound samples as pseu-domorphs of pyrrhotite, anhydrite, and amorphoussilica (Fig. 5c). Colloform and framboidal marca-site were not observed. Pyrite is only a minor con-stituent within crusts of lithified sediments from theexterior of mounds, where it occurs as colloformspheres, possibly oolitic, averaging 0.01 mm indiameter (Fig. 5d), finely-dispersed framboids 0.01to 0.4 mm in diameter (Fig. 5d), anhedral to sub-hedral blebs, and rarely as euhedral cubes. Pyrite isconspicuously absent in spires. Framboidal pyriteconsists of spheroidal aggregates of submicrometerparticles, whereas pyrite spheres contain colloformgrowth layers.

Sphalerite and wurtzite. Other than pyrrhotite,zinc sulfide is the most common sulfide. Bothsphalerite and wurtzite are present in all sampletypes, but generally cannot be distinguished bymicroscopy because high iron contents render mostcrystals opaque. The general term zinc sulfide is usedwherever distinction is not possible, althoughsphalerite is estimated to predominate by far overwurtzite.

572 THE CANADIAN MINERALOGIST

Frc.5. Sulfide textures. A) Boxwork intergrowth of pyrrhotite plates (po) with minorsphalerite (sph) in calcite (calc) (sample ll73-15, reflected lighO. B) Hexagonalpyrrhotite plate (po) in calcite (calc). A smaller bleb of intergrown sphalerire-isocubanite and galena is at the right (sample 1l7z[-4, reflected light). C) Marca-site pseudomorphs of pyrrhotite plates in calcite (sample I 175-8' reflected light).D) Colloform spheres and framboids (inset) of pyrite in lithified mound sedlment(sample I175-8, reflected light). E) Hexagonal wurtzite crystals in calcite (samplell?3-1, plane-polarized transmitted ligh0. F) Skeletal galena in sphalerite withintergrowths of isocubanite and sphalerite at top left (sample 1173-6, reflectedlighO. G) Vermicular intergrofih of sphalerite (sph) and isocubanite (iso) (sam-ple 1173-6, reflected [ght). H) Crystallographically oriented exsolution lenses ofchalcopyrite (cpy) in isocubanite (iso) (sample 1173-6, reflected lieh0.

Wurtzite was identified by its hexagonal crystalhabit, anisotropy in transmitted light (Fig. 5e), andits XRD pattem. No color zoning is present. Crys-

tals are prisms 0.01 to 0.1 mm in diameter. Sphaleriteand wurtzite have different stoichiometries, and thephase transition is a function of land 4S2 (Scott &

Barnes 1972). Scott & Barnes (1972) placed Ihe tran-sition from sphalerite to wurtzite at low aS, values,within the pynhotite stability field at Z > 150"C.Wurtzite is thought to be unstable relative tosphalerite in inactive chimneys and mounds (Zieren-berg et al. 1984). Active spires at Guaymas Basincontain fluid at higher I and lower aS" thanmounds and inactive spires. Consequently, wurtziteis expected to occur mainly in active spires, andsphalerite to predominate in inactive spires andmounds. Indeed, wurtzite is generally observed onlyin the inner walls and undersides of eaves of activevents. XRD patterns of hexagonal-shaped zinc sul-fide crystals from inactive spire samples revealed onlysphalerite. Wurtzite crystals commonly containnumerous minute inclusions of isocubanite. Theseinclusions are so small and dispersed that the opti-cal properties of wurtzite are altered in reflected light.Locally, wurtzite is replaced by isocubanite, galena,or both.

Sphalerite occurs as anhedral grains, <0.01 to 0.2mm in diameter (Figs. 5a,f), that were deposited on,and locally replaced, pyrrhotite plates. In most sam-ples, sphalerite is associated with isocubanite andgalena, with which it displays mutually interpenetrat-ing boundaries. Sphalerite also commonly occurs asarborescent and plumose intergrowths withisocubanite (Frg. 5g). In mound and inactive spiresamples, sphalerite has been partly oxidized to ironoxide along rims and in fracture fillings.

Cu in most samples ranges from 0 to 0.9 wt.grr,but is generally absent (mean : 0.1, lo = 0.2, N: 44). Several higher values (2.3 to 9.9 wt.Vo, N :8) probably reflect fine-grained isocubanite inclu-sions. Mn contents range from about 0.2to 4.4 wt.ulo(mean : 1.5, lo : 1.4, N : 52) and are quite vari-able among samples. Mn presence is consistent withsolubility calculations of Bowers el a/. (1985) which

HYDROTHERMAL DEPOSITS. GUAYMAS BASIN 573

indicate that Guaymas Basin hydrothermal solutionsare saturated with respect to alabandite (MnS). Cdcontents are below the minimum detection limit(MDL) of about 0.1 wt.9o. Fe contents of sphaleriteand wurtzite are among the highest found in nature,generally ranging between 25 and 44 mole go FeS(Fig. O, but some are as high as 50-55 mole go. Onlyone of these latter analyses has concomitant high Cu,possibly from a subsurface inclusion of isocubanite.The implied range in aS2 estimated from the FeScontent of zinc sulfide coexisting with pyrrhotite(Scott & Barnes 1971, Hutchison & Scott 1983) isl0-r2 to t6-z:, and is probably symptomatic ofstrongly fluctuating or disequilibrium conditions.

Microprobe traverses across single crystals revealcomplex compositional zoning not evident by opti-cal microscopy because of crystal opacity. Such zon-ing is expected because the aS2 is unlikely to bebuffered so precisely as to give constant iron con-tents. Fe concentration in zinc sulfide is also verytemperature-sensitive (Scott & Barnes 1971), andtemperature fluctuations in the hydrothermal fluidcould equally explain the spread of the data inFigure 6.

Isocubqnite. Isocubanite's composition lies in thecentral portion of the Cu-Fe-S system (e.g.,Mikaiyama & Isawa 1970, Cabri 1973, Cabi et ol.1973, Crug & Scott 1974, Sugaki et al. 1915,Wig-gins & Craig 1980). Isocubanite is present in traceamounts in almost.all samples, and is a minor con-stifuent in several spire samples. It occurs as anhedralblebs, as plumose and arborescent intergrowths withsphalerite (Fig. 59) which probably result fromepitaxial precipitation of isocubanite on wurtzite, asreplacements of pyrrhotite and zinc sulfide alongfractures, and as numerous inclusions of < I pm inzinc sulfide ("chalcopyrite disease" of Barton 1978and Barton & Bethke 1987). Microprobe-detected

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574 THB CANADIAN MINERALOGIST

minor elements in isocubanite are Zn (mean = 2.3wt.Vo, lcr : 1.4, N = 32) and Mn (mean : 0.2w t . V o , 1 o = 0 . 1 , N : 1 0 ) .

The anhedral blebs of isocubanite commonly con-tain crystallographically oriented lensoid exsolutionlamellae of chalcopyrite (Fig. 5h). The lamellae resultfrom the breakdown of an initial phase to chal-copyrite and isocubanite during rapid cooling (Cabril9?3). Quenching from >2l0oC preserves the cubicform of isocubanite. Chalcopyrite also rims andreplacs isocubanite in some samples, penetrating themineral in irreeular fracture fillings or along its crys-tallographic axes @g. 5h).

Galena occurs as anhedral grains to subhedral crys-tals, as rare euhedral cubes within pynhotite, as crys-tals on pyrrhotite, and as a replacement of pyrrho-tite. Galena displays mutual boundaries withsphalerite, and locally replaced or filled fractures init. Atoll structures derived by selective replacementby calcite (Fig. 5f) are also present. Although chem-ical analyses of bulk samples indicate the presenceof Ag, no Ag in galena was detected in EDX spec-tra. Koski et al. (1985) examined dredged moundmaterial and also failed to identify a silver-bearingphase. They appealed to the presence of a Pb-As sul-fosalt to explain the high Ag values.

Galena. Galena is present in trace amounts in allsample types, and in some chimneys it comprises up Sulfatesto 5 vol.Vo. Grain size varies from <0.1 to 0.2 mm. Anhydrite. Anhydrite, intergrown with barite, is

Frc.7. Mineral textures of non-sulfide phases. A) Tabular anhydrite crystal. A pla-nar array of fluid inclusions defines a growth zone (sample I 173-5, plane-polarizedtransmitted light). B) Network of radiating acicular barite crystals (sarnple I175-7, plane-polarized transmitted light). C) Veinlets and patches of calcite (white)that cross-cut and replace earlier calcite (dark erey) (sample ll!d{, gross-polarizedtransmitted licht). D) Growth-zoned calcite crystals (sample 1169-lB, plane-polarized transmitted ligh0. E) Fine-erained pynhotite inclusions defining growthzones in calcite (sample ll7zt-?, cross-po1*i2sd llansmitted light). F) Bundles ofstevensite fibers intergrown with pyrrhotite (sample ll74-8A, incident light).

HYDROTHERMAL DEPOSITS, GUAYMAS BASIN ) / )

predominantly in active spire and mound samplesand is absent, or is present only in trace amounts, ininactive spires. This observation is consistent withthe retrograde solubility of anhydrite, whereby anhy-drite precipitated at > l40oc in seawater is redis-solved at lower temperatures (Blount & Dicksonl%9). Anhydrite occurs as aggregates of radiatingacicular crystals, 0.5 mm long, and as elongate tabu-lar crystals 0.2 to 0.3 mm long (Fig. 7a). It commonlycontains fine-grained sulfide inclusions which deline-ate growth zones, but some anhydrite is inclusion-free. Typically, cores of anhydrite are clouded withinclusions, and rims are clear. Many acicular anhy-drite crystals are partly to completely pseudomor-phosed by fine-grained calcite.

Barite. Barite is present in trace to major amountsin all sample types, and some inactive spires andmounds contain up to 30 vol.Vo. Barite occurspredominantly in the outer portions of spires assmall, tabular, euhedral crystals with wedgeJike ter-minations. Crystals are <0.01 to 0.3 mm and aver-age 0.2 mm in length. Rarely, large anhedral baritegrains poikilitically enclose slender prisms of anhy-drite and rhombs of calcite. In many samples baritehas formed after anhydrite and sulfides, occurringin cavities as clusters of small needle-like crystals per-pendicular to the substrate (Fig. 7b). Sr is ubiqui-tous in minor amounts (mean : 1.2 wt.Vo, lo :0.6, N : 7), and Ca was found in only one sample(0.2 wt.9o). No other elements were detected.

As pointed out by Koski et al. (1985), the presenceof barite and anhydrite as intergrowths with pynho-tite in some chimneys must repre$ent a disequilibriumassemblage. They should, however, precipitate uponmixing of the vent fluids with sulfate-enriched sea-water. Sulfur isotope analyses of Guaymas Basinbarite and anhydrite range from 20.7 to26.4/ss, a\dindicate that the source of sulfur in these mineralsis seawater sulfate that has been reduced in the ventfluid or source area (Peter et al. 1984. For baritefrom Guaymas Basin dredge samples,.Koski et al.(1985) obtained temperatures of formation of 208and 2l5oC from oxygen isotopic analyses. These are80 to 100oC lower than temperatures determinedfrom fluid inclusions in calcite which accompaniedsulfide deposition (see below), and must reflect mix-ing of hydrothermal fluid with cold seawater.

CarbonatesCalcite is very abundant in sptes, approaching l(F

vol.9o in some samples, but is not common in moundsamples. It is a principal constructional componentof spires; crystals typically line orifices and have theirlong axes normal to the direction of fluid flow. Cal-cite displays two habits: l) anhedral grains to sub-hedral interlocking crystals 0.01 to 3.0 mm indiameter, and 2) acicular and commonly radiatingcrystals 0.5 to 0.8 mm by 0.05 mm which are pseu-

domorphs of anhydrite. Some crystals of type-l con-tain dendritic sulfide inclusions or poikiliticallyenclose discrete pyrrhotite plates (Fig. 7e). Type-Zcrystals invariably contain disseminated fine-grainedsulfide inclusions which locally envelope the pseu-domorph. The numerous sulfide inclusions imparta cloudy, dark grey-brown color to the crystals. Mostcalcite pseudomorphs appear to be an agglomera-tion of smaller calcrte grains, although a few pseu-domorphs are optically continous and appear to besingle crystals. The small crystallite size of theagglomerations is presumably due to rapid chemi-cal replacement. Calcite commonly displays growthzoning (Fig. 7d), with individual zones defined byplanar arrays of extremely fine-grained pyrrhotiteinclusions. Such zoning indicates a dynamic changein vent-fluid chemistry or physical parameters (orboth) during mineral precipitation. Rarely, calcitecrystals are dissected by veinlets of calcite (Fig. 7c).Precipitation of calcite could result either from adecrease in temperature by mixing with seawater, orby degassing of CO2 from the vent fluid. Consider-ing the high COz content of the vent fluids (VonDamm et ql, 1985) and the retrograde solubility ofcalcite (Ellis 1963), the latter mechanism seems morelikely.

Partial microprobe analyses of calcite gave 0.3 to2.870 Mn (mean : 1.4 wt.9o, Lo : 0.9, N : 7),and Fe from <MDL to 0.2 wt.9o (mean =0.04 wt.Vo. lo : 0.07, N = 7). Mg was not detected.Cathode luminescence showd no compositional zon-ing but did produce a bright red-orange color indica-tive of Mn (Nickel 1978). For calcite from chimneys,6r3Cpou ranges from -9.6 to - 14 (N = 40; Peteret al. 1986). This suggests that CO2 in the ventfluids was derived mainly by mixing equal amountsof carbon from two sources: oxidized organic mat-ter, and dissolved marine carbonate. Aragonite is arare constituent of spires and has not been found inmound samples. Crystals are typically corroded orembayed. Aragonite's rarity is explained by its trans-formation to calcite in the presence of seawater (e.g.,Cooke & Kepkay 1980a,b).

SilicatesStevensite. Several greyish to drab olive-green sam-

ples from mounds (e.9., ll10-5) contain a rnineralwhose colloform habit is suggestive of depositionfrom a lower temperature fluid, as is consistent withthe lower temperatures within mounds. Anothersample, a piece from a chimney actively venting blacksmoke (1174-8A), contains a fibrous mineral.Individual fibers are extremely delicate, slender (0.1mm), and typically several cm long. Fibers occur inradiating aggregates intergrown with fine-grained sul-fides predominated by pyrrhotite (Fig. 7f).

Samples 1170-5 and ll74-8& are composedpredominantly of Mg, Na, and Si. The XRD charac-

576 THE CANADIAN MINERALOGIST

teristics of thermally treated and glycolated samples,together with chemical analyses, correspond tostevensite (Brindley 1980, Brindley et al. 1977), atri-octahedral 2:l smectite with the ideal formulaNaa;3Mg2.sSi4Or0(OfD2 (Hathaway 1979). Al sub-stitutes for Si in the tetrahedral layer, and Na-Mnsubstitutes for Mg in the octahedral layer. PossibleFe substitution is uncertain because the samples con-tain iron sulfide contaminants.

Amorphous silico. Amorphous silica is abundantboth in spire and mound samples, though it is presentin greater amounts in mounds. Th^e XRD pattern hasa diffuse peak at about 3.8-4.1 A characteristic of

opal A (Jones & Segnit 1971, 1975). Silica displaysthree habits: colloform layers, spheres or globules,and fibers or filaments @igs. 8a,b,c). Layers of sil-ica approximately 0.02 mm thick can coat all otherprimary minerals (Fig. 8c). In some places, multiplegrowth layering is evident. Uniformly-sized silicaspheres, 0.03 to 0.1 mm in diameter, occur in clusters(Fig. 8c) which are best trapped and preserved inpores and cavities. The fibers or filaments are about0.1 mm in diameter (Fig. 8a), are bent, and someare branched. Locally, the fibers coalesce and havea mesh-like appearance. Juniper & Fouquet (1988)have interpreted such structures in other modern

Frc.8. Amorphous silica, oxidatron, and biological textures. A) Rods of amorphoussilica composed of smaller silica spheres. This texture results from the coatingof filamentous bacteria by amorphous silica (sample 1175-7, plane-polarized trans-mitted ligh0. B) Calcite crystals pseudomorphed by amorphous silica (sample l169-lB, plane-polarized transmitted 1igh0. C) Amorphous silica spheres which coatpyrrhotite, sphalerite, and isocubanite (sample ll72-2&3, plane-polarized trans-mitted light). D) Rims of goethite Goe) on pyrrhotite plates (po) (sample 1172-2A3, reflected light). E) Fine-grained sulfides on bacterial filaments included incalcite (sample 1169-18, plane-polarized transmitted lichO. F) Close-up of sul-fides encrusting bacterial filaments in calcite (calc) (sample 1169-14l, plane-polarized transmitted light).

}IYDROTHERMAL DEPOSITS. GUAYMAS BASIN 577

seafloor-vent deposits to be branching microbial fila-ments coated by amorphous silica. The silica is abi-ogenic and filaments merely serve as a substrate fordeposition. Other fibers of silica having no appar-ent relation to microbial filaments rarely replaceanhydrite, barite, calcite, and even worm tubes.

Conductive cooling has been suggested by Janecky& Seyfried (1984) and Tivey & Delaney (1986) as thedominant cause of deposition of amorphous silicaa 2loN, Galapagos Rift, and the Endeavor Segment.If the temperature of hydrothermal fluid decreasessolely as a consequence of mixing wilh seawater, thesolubility of silica is never exceeded due to the dilu-tion effect (but is approached near 100'C) even ifno other silicates such as stevensite have precipitated(Chen & Marshall 1982). Hydrothermal fluid cool-ing solely by conduction could begin to precipitateamorphous silica at about 250oC, and a combina-tion of conductive cooling and mixing with seawaterwould result in precipitation below 250'C. Bowerset al. (1985) have modelled the conductive coolingof Guaymas Basin vent fluid and concluded that sucha process can explain the observed chemistry. Con-ductive cooling therefore may explain the greaterabundance of silica in mounds than in spires.

Biological phases

In addition to the silica fibers interpreted to haveformed on filamentous bacterial structures, somesamples contain remnants of filamentous bacterialmats; submersible observations have shown thar

similar mats commonly encrust sulfides at GuaymasBasin. Aggregates of filaments with very fine sulfidecrystals precipitated on them are preserved in cal-cite (Figs. 8e,f). Single filaments are 0.5 to 1 pm indiameter and up to 150 pm long. That these wereentrapped at >200oC is indicated from fluid-inclusion data from the surrounding calcite. Well-preserved filaments are rare, but biogenic materialclearly has played an important role in the miner-alizing process by serving as a substrate for silica-sulfide deposition.

Minersl distribution

Table I compares the mineralogy of the differentsample types. There are distinct differences betweenspire and mound samples, and even between activeand inactive spires: marcasite is present only inmounds, wurtzite and chalcopyrite are present onlyin spires, anhydrite is generally absent in mounds,aragonite is present only in active spires, and amor-phous silica and barite are far more abundant inmounds than in spires.

Zonation of most of the spire samples is poor orabsent, probably because the many small anastomos-ing hydrothermal discharge pathways precluded anyconsistent thermal gradient. The few chimneys thatdo display concentric mineralogical zoning abouttheir orifice (Figs. 4a,b) have a core of massive chal-copyrite/isocubanite surrounded by a massive zincsulfide a galena annulus, an exterior zone of dis-seminated zinc sul{ide in a calcite-sulfate matrix. and

tite

sulfide

tlme ------'

TET,PERATURE (OC)

Frc.9. Generalized paragenetic sequence of mineralization in Guaymas Basin sam-ples.

578 THE CANADIAN MINERALOGIST

TAELE Z DESCFUPIIOII Ar.lD MT.IERALOOY OF SAMPTES SE ECIED rcR BL{.J( CfiETTFAL AML\/SS AND Ftt D r\rcLucilotll0oRoIlEFillOriEIITr

SAMPLE DESCRIPTION MINEFALOOY

1168€ trom mlldly dtrre (G50rC) mqmd, lan to llght grey aggregdeol whne crlr8tah ard cby; woll lndurated.

'l 't 69- bssal aross s€dbn of an 80 orn tall lnadlvo chlmney; llght gray.1Al

1 I 69- I B mld-sodlon ot an 80 crn tall chlmn€y.

l'f 70-3 -15 crn tall dlvo epllg wlth a mushroom-ehqed €€0/€.279"ClkrH renlod from undeolde d are.

Inacilve (?) mound; medlum green-brown; vory trlable.

cro€s-Eedlon ol amall 15 cm tall lndhg splre bdthout o€nttalorlflao, hd many smaller hol€6; grey to Uad<.

c.Gs€€atlon ol an Inacilve cplre; no contial oritlco, hlt manyemaller hole; aulflde.rlcfu darlc grey to black

lnaatlve (?) mord; medlum broryn.

oonrpclle eample ctl an 85 cm tall Inactho splre; no oenlraloritlco, hn many smaller hoba; sulflderlah; gr€y to bhd(

Inadlve (?) mdnd; groonl8h.

plece d oave lrom sphe actlvoly yentlng 291qC tluH lrom theundanHe ol the eavs.

amall, 7 cm dlam€ler lnacilve chlmney; 2 om dlanroterorlllce.

cros*eec{on d an eave cf a 17 m tall Sre vonllng 291€fluld lrom un&r the eave; hlgh epedfb grwfty.

afc8-sodlon talen abod half way up a 50 cm tall h6lre splre;well lrdurated, creamy whlte, tlna-gralned sample wllh red ando€rEe stalnlng.

sph, ho, py, chy, dlalom dobrb,lron oldde.

po, py, tph. bo, gal, amorpltouselllca, anhy, barlte.

cab, amophon d[ca, anhy,barlte, po, ZnS, ho, gal,amgonlle, barlto.

oalc, anhy, po, ZnS, lso, py, Sal.aragonfre, barlte.

sample rlch ln cby and stevanslto.

po, 6ph, bo, gal, cpy, aalc, anhy,amorphora ellba, bsdte.

po, sph. gal, cpy, barlte, calc,amophous slllca.

po, sph, barlte, amorphoue elllca,clat.

po, sph, cpy, bo, gal, oalc, barlle,anhn amorphoue ellha

po, oph, barlte, derr€nslte.

calc, po, ballto, anhy, ZnS, lso,gal.

calc, ZnS, gal, po, lso, cpy, lronoldde.

po, rph, gal, lso, py, calc, barlle,anhy, amorphoua elllca

po, sph, lso, barlte, anhy,amorphor.rs alllca, lron oxlde.

devensile, po.

po, py, lso, sph, marcadte,barllo, anhy, amorphore elllca

calc, amorptl(xls sfllca po, ZnS,bo, gal, py, lton oxHe.

po, sph, bo, barlte, anhy,amophoue sllha, lron oxlde.

po,6ph, lso, calc, badte,amorphoue eillca.

1 1 70-5

r 1 7 0 - 1 0

1 ' t 70- t I

1170-24

1172-2

1172-S

I I 73-1

I I 73-6

1175-14

1 1 7 4 - S

1174-84 2 m tall rplre wlth eave acllvely nontlng bbd( Emok6 (tomp.unknorn); dense aggregde ol bundles ot aclcular gr€on{reyc4,stab lntergrown wlth mlnot pynhotlto.

1175-8 mound rlth a crusl of onnge-nrown lron oxlde: hlgh s@flcgravltt.

1J75-10 emall chlmney wlth pynhotne-llned orltlca.

1'177-1 sdre acllvely yentlng 312cC lluld from a cenlral odllce; rcrydenee, hlgh epedth grat lty 6€amy whtte wlth orange bowndalnlng on surtace.

1177-6 mound sllh lron oxlde crud.

Mlneral abbrevldlone: eph aphalerite, loo lsodrba.rhe, py pyrlle, po pynhotite, gal gelsna, anhy anhydrlte,ZnS dnc sultlde, calc caldte, cpy chalcopylte.

(sl.%)

hhOls

ssaB*'

go24t203mr,tol.lagKqSrMn'no2

P206@2LOI

IOIA

(pPm)rsBAJBIqto

et oNbNIRbsbSosf!T9uZt

0.3 1.32 2.220.0023 0.23 0.86

0.00055 0.06 0.2A0.00030 0.13 0.4

nA 3.7i 0.84|\A o.4t 0.10

o.ool t t.5 1.806t .o 3 t .1 50 .07 .14 0 . r I 3 .120.33 21.2 0.051.48 0.61 28.30l\n 1.17 2.51t\n 0.23 0.34l.,A 0.438 0.05rtM 0.5,0 O.tll{A 0.10 0.i3t{A 0.02 0.1 1o.t 17.5 0.3lllA 6.39 14.6

98.73 104.0

<o.5 1A.8 10.5nA 67.0 24.OnA 0.043 0..rnA 0.0 0..r< 1 1 4 1/ t t , l A l , | A|{A <10 <10M <lo <.101 . 0 M 6l \ | A 2 0 2 09 l \ n n Al'A <10 <toll|A 17.0 140.0nA 30.0 75.0t \ | A < 3 3l\lA 4.2 0.2l \ l A 3 . 2 l l AI\lA <lo <10M 5 0 2 0

31.8 12.69 1.A2 4.03 0.34 4.54 0.55 3.993.78 r.58 0.52 0.58 0.28 0.81 0.02 2.800./$ 0.23 0.05 0.08 O.o5 o.lE 0.03 a.122.13 0.9! 0.05 0.26 0.03 0.A2 0.01 0.02l{A a.a2 0.93 6.70 0.57 3.53 0.10 0.1}.lA 3.78 8.37 5.70 t0.8it 1.51 o.Oe ii.10

0.0e r3.3 35.4 37.9 48.8 2.13 12.1 0.3032.6 34.6 25.7 2.72 5.E4 8.7A 3o.O 20.5}.|A 0.28 r.00 0.28 0.70 0.02 0.49 0.87

0.14 7.O 0 13.5 0 37.6 0.039 15.8l{A 0.12 1.40 0.32 2.57 0.20 0.13 10.2l{A 1.37 1.35 1.14 0.9e 0.6.1 0.8.1 2.08}{A 0.13 0.32 0.04 0.15 <O.02 0.13 0.40l{A 0.305 0.977 0.1m 0.581 0.120 0.517 0.i l0

o.r8 0.41 0.02 0.70 0.008 0.88 0.02 0.13l{A 0.08 0.m o.l8 0.23 0.01 0.22 <0.01M 0.02 0.05 0.02 0.03 0.o2 0.05 0.02r.t 5.0 3.0 tt.o 0.1 30.8 0.3 3.4l{A 18.1 10.2 -1.46 12.A 10.4 2.08 21

108.83 t9.7,4 42.78 84.75 103.60 87,11 sa.3e

24.6 353.2 2i5.4 125.2 53.8 t5.t 17.7 IRACE}.lA 280.0 ll0 67.0 38.0 280.0 240.0 rto.ol\A 0.2& 0.270 0.035 0.093 0.350 o.tgo 0.041l{^ 2.s 0..r 0.3 d.1 1.7 <o.l 0.9<1 67 20 29 19 24 2 1104 n A t \ | A t { A t l | A n A | \ A t { Al\A <to do <t0 <10 <10 do <10l\A <to d0 10 <10 20 do <to' f l l \ A t 9 l A 2 9 | ' | A 4 < tl{A 10 t0 <.t0 <.to t0 <10 109 M I { A i A | \ | A M I { A t l | AM <10 <10 <10 <to <10 <10 <10},,A 490 l{A t/30 l'|A 40.0 38.0 4.OM 250 730 fio 590 120 0.0 85.0o 1 2 3 3 € 3 3 . 0 3l{A O.2 0.3 0.2 0.! 6.2 1.1 1.8r{A 2.6 !{A 1.2 I'lA o.rl l,|A Ml.|A d0 <10 <10 <to <t0 <10 <10t{A 80 280 <to 280 <to 260 30

12.4 3.78 2.100.43 1.60 0.06e0.16 0.4/t 0.015o.47 0.05 0.0028lt|A 6.10 t{Alt|A 5.10 1\A

11.0 0.49 27.241.4 5.91 29.80.28 0.'n 5.200.15 32.9 0.O00.56 t.38 9.70M 0.43 |.n

o.fi o,o2 l.|At{A 0.120 t{A

0.01 0.38 0.03t{A 4.Ot t{All|A 0.02 liAo.2 2t.8 0.2nA 22.1 t\A

'tol.E2

4.1 3.0210.0 l\|A0.140 M

8.2 M6 8 4l l n €

d 0 l { A<t0 l\lA<t 302 0 ! \ l A| t | A 1 7<10 M18.0 M80.0 M

3 t l | A2.1 l{Al ' l A M

<lo |1lA4 0 M

97.1MM|{A223l{Al.A1 7t{A44l\AnA|\notinMMM

HYDROTHERMAL DEPOSITS, GUAYMAS BASIN 579

Concentrations of Si, Al, Ca, Mg, Na, K, Fe, Mn, Ti, P along with Sr, Rb, Cr, Y, Nb, and Zr were determined bywavelength-dipersion X-ray fluorescence spectrometry on fused glass discs (delection limits of 0.01 wt 9o and l0 ppm,respectivelr. Concentrations of Fe, Zn, Cu, Pb, Ag, Cd, Co, Ge, Mo, Mn, and Ni in the ppm range were determinedby multi-element direct coupled argon plasma-emission spectrometry (detection limits of 1.0 ppm). CO2 was analyzedby wet-chemical methods. As, Sb, and Bi were determined by flameless atomic absorption utilizing a heated quartztube furnace, and Te and Se by graphite-furnace atomic absorption (detection limits of 0.1 ppm). U was measured byINAA. Ag was determined by fire assay (detection limits of 0.5 ppm). Au was analyzed by fre-assay preconcentrationfollowed by direct coupled plasma-emission spectrometry (detection limit, 2 ppb).

an outer rim of calcite t anhydrite t barite. Sirni-lar zonation (without calcite) has been observed inchimneys recovered from other hydrothermal ventsites (e.9., Oudin 1983, Haymon 1983). Certainchimneys display multiple episodes of such zoning,or have a layer of calcite lining the chal-copyrite/isocubanite zone. Figure 9 depicts theparagenetic sequence determined from mineral tex-tures.

BULK CoMposrrroNs

Locations of the analyzed samples are given inFigure 2, descriptions are in Table 2, and bulk chem-ical analyses are in Table 3. Totals are not all nearl@90. LOI values may not be accurate as they were

determined by igniling the samples in air. High valuesresult from oxidation of metals, and loss of S andvolatiles such as H2O, SO4, and CO2 during fusion.Redistribution of S values between sulfide and sul-fate may also have led to inaccurate totals as origi-nal results, given as total S, were divided betweensulfide and sulfate based on relative proportions ofthose minerals estimated by optical microscopy andBa and Ca analyses. Ba-rich samples (1172-2, ll72-3, ard ll74-3) gave particularly low totals. Ba deter-mined by both XRF and INAA gave similar results,so the SO3 determinations must be too low. Na, Kand Cl analyses are reported as salts as all Na andK can be accounted for with an appropriate amountof Cl (except for those samples containing steven-

580 THE CANADIAN MINERALOGIST

site). The presence of these salts in the samples can-not be avoided, as washing prior to analysis woulddissolve anhydrite.

In general, spires have significantly higher content$of Fe, Zn, Pb, Cu, S, Ca, CO2, Ag, As, Bi, Cd,Mn, Sn, and Te than do mounds, consistent with theobservation that mounds are partly constructed fromoxidized chimney talus and debris. The mounds havelower metal contents because they are formed atlower temperatures and thus are dominated by non-sulfide minerals (discussed below). The lower sulfurcontent of mounds reflects the fact that metal sul-fides in mounds are commonly oxidized. As well,anhydrite is dissolved in spire fragments, and thisis reflected in the lower Ca values for mounds.Mound samples are higher in both Ba and Si thanchimneys and spires, in agreement with barite andamorphous silica being paragenetically late and low-temperature phases.

FLun INcr-usrorrrs

Petrology

Petrology and microthermometry of aqueousinclusions were done for samples of six chimneys(both active and inactive) from different locationswithin the Southern Trough. Inclusions occur in alltranslucent minerals, but because calcite contains byfar the most inclusions and is the dominant mineralintimately associated with sulfides in most spires, itis the focus of this study. The opacity of zinc sul-fide precluded the study of fluid inclusions in thismaterial. Anhydrite was not studied because Le Bel& Oudin (1982) found that inclusions in this host giveartificially elevated salinities due to partial dissolu-tion of the inclusion walls on freezing. Inclusions inbarite were avoided because of their susceptibilityto stretching during heating runs (Bodnar & Bethke1984). Inclusions in amorphous silica are relativelyscarce, and this host is not related temporally to sul-fide precipitation.

Inclusions were classified as primary or secondaryaccording to the criteria of Roedder (1984). The dis-tinction between secondary and pseudosecondaryinclusions was difficult, but given the geologically"instantaneous" growth of a chimney, inclusionsclassed here as secondary are likely pseudosecondiry.Primary aqueous inclusions are I to )50 g.m, andhave a wide variety of cross-sectional shapes. Theinclusions are two-phase (aqueous fluid and a vaporbubble), contain no daughter minerals, and typicallyhave uniform phase ratios with the vapor bubblecomprising 5 to 20 vol.9o of the inclusion @ig. l0a).On this basis the inclusion fluids do not appear tohave undergone phase separation. Primary inclusionsare interpreted to contain the extant hydrothermalfluid during mineral gofih. Aqueous inclusions areaccompanied by liquid and gaseous hydrocarbon-bearing inclusions in certain samples. These inclu-sions are interpreted to have formed from hydrocar-bon that was carried in the hydrothermal fluid as animmiscible phase, and possibly as solvated forms(Peter et al. 1988, Peter & Scott 1987).

Welhan & Craig (1982) have measured high con-centrations of hydrocarbon (including thermogenicmethane) and Von Damm et al. (1985) have meas-ured appreciable amounts of H2S in Guaymas Basinvent fluids. Although CHo and H2S therefore wereanticipated in the fluid inlcusions, none was detected.

Secondary inclusions, which far outnumberprimary ones in all samples, occur as discontinuousplanar arrays along fractures and on healed cleavageplanes (Fig. [0b). Secondary inclusions are I to>70 pxr and are typically irregular. Inclusions alongcleavages are slender and needle-like (Fig. 10b).Generally, secondary inclusions are two-phase andconsist of an aqueous fluid with a vapor bubble.However, they do not always have uniform phaseratios and, in extreme cases, contain only vapor orliquid. This is most likely due to changes after trap-ping (e.g., necking). Secondary inclusions are inter-preted to have formed from hydrothermal fluid

Ftc. 10. Photomicrographs of aqueous inclusions. A) Primary, two-phase aqueous inclusions in calcite. B) Elongate,secondary, two-phase aqueous inclusions in calcite.

HYDROTHERMAL DEPOSITS. CUAYMAS BASIN 58r

along fractures during or after late stages of mineralgrowth.

Microthermometry

Repeated freezing and heating runs on severalselected inclusions indicate that freezing temperatureswere reproducible to within t 0.5oC, and homogeni-zation temperatures to within + I to 4.C. No evi-dence of clathrate melting was observed.

Some inclusions exhibited eutectic melting of bothNa and Ca hydrates. Sudden initiation of meltingin most inclusions occurred between -24"C and- l9oc, corresponding to the melting of hydroha-lite (NaCl.2HrO). Final melting of ice occurredbetween -4.6 and -1.3"C. Another cluster ofmeasurements around -49"C corresponds to theeutectic melting of CaClr.2HrO (antarcticite).MgCI2 must be negligible in the inclusions as evensmall amounts lower the eutectic melting point to< 52" C (Crawford et al, lW9). The hydrohalite melt-ing temperatures allow estimation of theCaClrlNaCl ratios in the inclusion fluids. Ratiosrange from 0 to 0.56:1, with the majority of inclu-sions giving a ratio of 0.12:1. Table 4 summarizesthe results of fluid-inclusion microthermometry forindividual chimneys.

Salinities (Fig. l l) were calculated from the tem-perature of final ice disappearance (freezing pointdepression) using the equation of Potter e/ al. (197 8).The means of primary and secondary fluid-inclusionsalinities are 5.3 (lo : 0.7) and 5.5 (lo. : l.l) equiv.fi.%o NaCl, respectively. All measured salinities arehigher than Pacific sea\ryater (3.5 equiv. wt.go NaCl),Guaymas Basin vent fluids (3.8 to 4.LVo see below),and fluid inclusions from 2l"N EPR (3.890 in wurt-zite; Styrt et ol. 1981,2.9t/o in anhydrite; Le Bel &Oudin 1982). Salinities are also higher than thoseobtained by Koski et al. (1985) from calcite inGuaymas Basin dredge samples (4.0 to 4.290). Ourvalues, however, fall within the salinity range of fluid

TABLE4. SUMI{AFYOFFLU|OIloU.Aot\,MCROTI{EFMqUETFyOATA

sAMpls tNcujatoN TRApptNorBtp.($). sAlNny€oulv.wr.%itacl)TY"E N 10 MEAN N 1o MEAN

t16t-1B pr lmart@tdat

I 1 7O-g pr lmary@day

117O-16 pr lmary8@daly

1173-1 pr lnary€@ldary

I |73.6 pr lmart@rdary

1 175-1 0 pr lmary@daty

19 0.49 5.54s 0.78 6.43

18 0.71 5.76t I 1 . 2 5 . 8

6 0.67 5.065 0.14 4,e2

: _ 5 . 3

29 0.62 6.146 0.99 4.95

5 0.18 4.12 0.21 1,1

4.7 216

17.1 2&t20.8 A

4.1 a733.0 24

125 m355 81

212 &312 AA

3037

21*12p,

la1 8

8

4 4.4 4.8 A.2 5,8 6 6.4 e.8 7.2 7.A Isu,mY Ga m.% Nso)

Ftc. I 1. Frequency distribution of fluid-inclusion salinitiesfor all primary and secondary aqueous two-phase inclu-sions.

inclusions in some ancient volcanogenic massive sul-fide deposits (e.g,, 3.5 to 7r/o in Kuroko deposits:Pisutha-Arnond & Ohmoto 1983, Bryndzia et al,1983, Foley 1986). Elevated salinities have also beenreported for inclusions in anhydrite from thesouthern Juan de Fuca Ridge (about 5070 greaterthan seawater,Brettet al.1985), and from vent fluidsof several eastern Pacific sites (Kim et al. 1984, VonDamm & Bischoff 1987, Michard et ol. 1984). Severalmechanisms for producing elevated salinities in ventfluids have been proposed (e.g., Bowers et ol. 1988),but none of these can be tested unambiguously atGuaymas Basin.

Figure 12 gives the frequency distribution of meas-ured homogenization temperatures. A pressure cor-rection (Potter L%7) of + 15oC, corresponding toa salinity of 5 equiv. fi.70 NaCl and the 2000-mwater column overlying the deposits, must be appliedto obtain true trapping temperatures. Table 4 listsmean trapping temperatures of populations of inclu-sions from individual chimneys. The distributionsboth of primary and secondary fluid inclusions showa pronounced negative skewness of -0.84 and-0.66, respectively, with trapping temperatures aslow as 125"C relative to a modal value of 26loc.Such skewed distributions ile best explained by themixing of hot, end-member hydrothermal fluid withcold seawater.

Comparison of microthermometrywith fluid chemistry

Fluid-inclusion measurements provide a uniqueopportunity for comparison with vent-fluid chemi-cal data obtained by VonDamm et ol. (1985). Fluid-inclusion trapping temperatures (Iable 4) are in goodagreement with vent-fluid temperatures measuredwith ALVIN's thermocouple probe. Salinities of ventfluids were calculated from the data of Von Dammet al. (1985) using the empirical relation between

6&ctDAFr f rnmnnvbo$tngt mua009MEAN65 ECI MEAN€3 E{Tm.% NaCl Wf,% NaCtld. &v..1,1 id dev.4.7N-32 N-78

'a prmrE onwtiol o, +15oO @r$pondhg to th€ 2cOOm mte, @lmn ryerMm tho

i8:ff l?rlTt * to obrajn rappltE r€nFE atu@ ror a s equtv. wL% !,tao iiutd

582 THE CANADIAN MINERALOGIST

s0

RE 2 5

! z oNc 1 5

t o

€EOO'U'I' I PRN'AfTn nlrna n&N.!!n €MEAN.2iS. UEANE9C,I d. dd..dt.4"C 1 dd dEvF34.ecN - 1 2 4 N - 9 9

IIO,OGNZATrc'{ IB,|PENAIUE fC)

Ftc. 12. Frequency distribution of homogenization temper-atures for all primary and secondary aqueous inclusions.

salinify (S) and chlorinity (Cl): S (/6s) : 0.03 + 1.805Cl (7oo) (Gerrard 1966). Calculated values of salin-ity range from 3.3 to 4.1 equiv. wt.9o NaCl, onlyslightly above normal seawater values (3.590 NaCl).Enigmatically, with the exception of sample 1175-l0 (Iable 4), fluid-inclusion salinities are significantlyhigher than that of seawater and cannot be recon-ciled with these hydrothermal fluids. The differenceis not a result of experimental error.

The end-member fluid chemistry of Von Dammet al. (1985) gave calculated values (fi.90) of 3.17for NaCl, 0.51 for CaCl2, and 0.27 for KCl. MgCl2is not expected in the fluid inclusions becauseundiluted vent fluid contains no appreciable Mg (VonDamm et ql. L985), The fluid chemistry used in thesecalculations may not be representative of the fluidstrapped in our inclusions, but it can be helpful inassessing melting phenomena observed in fluid inclu-sions. The fluid chemistry indicates that the clusterof hydrohalite melting measurements betwen -24

and - 19'C reflects eutectic melting in the systemHrO-KCl-NaCl at -22.9"C, which is slightly lowerthan the H2O-NaCl eutectic at -20.8oC. Suchmultiple-hydrate melting events axe difficult to detectin the inclusions because of the very small volumesof melt generated at the eutectic in low-salinity fluids.The CaCl2lNaCl ratio of 0.16:l calculated fromvent-fluid chemistry data is in good agreement withratios calculated above from fluid inclusions. Differ-ences between the fluid-inclusion and vent-fluid datasuch as the salinities, indicate that fluid-inclusionstudies of chimney minerals, particularly whenparagenetic sequences are known, have considera-ble potential for delineating complex temporal vari-ations in vent-fluid chemistries.

Mixing of hydrothermol fluid snd seowoterwithin chimneys

From the microthermometric data in Table 4 it is

apparent that: l) primary fluid-inclusion homogeni-zation temperatures are higher (by 16 to 37'C) thanthose for secondary inclusions in three of five sam-ples; and 2) for two of the three samples, primaryfluid-inclusion salinities are also concomitantlyhigher by 0.15 to 0.19 equiv. wt.9o NaCl than thosefor secondary inclusions. In the other samples, salin-ities of primary and secondary inclusions are indistin-guishable. Mann-Whitney U Mann & Whitney 1947)and Kolmogorov-Smirnov (Siegel I 956) two-tailed,non-paramelric tests indicate that homogenizationtemperatures and salinities obtained from primaryand secondary fluid inclusions are from the samepopulation in all the spire samples studied. Differ-ences may arise when cold, ambient seawater (2oC,3.5 equiv. wt. Vo NaCl) breaches a chimney wall andenters its interior. The sudden temperature drop maylocally fracture the calcite, and the mixture of dis-charging hydrothermal fluid and ambient seawateralong these fractures may become trapped as "secon-dary" inclusions. The term discharging is used hereto describe a fluid immediately at the sea-water/seafloor interface (in this case within a chim-ney, at its base). Such a fluid may have mixed previ-ously with circulating seawater, or a modifiedcomponent thereof, in the upper zone of the sub-surface.

Accompanying this subsurface mixing are changesin temperature, composition (including salinity) andmineral $tabilities. If the microthermometric data forprimary inclusions accurately reflect the propertiesof discharging hydrothermal fluid, this fluid hasmixed with seawater within individual chimneys in

TA4T6. HYDROTHERIIALFLUIDBEAWATENMXNORAIIO6CAI'I'LATED FNOMRIjIDtNCUrSl(}tl MCFOIHEFNIOMEIFIC DATA

SAI'PI.E DATASET VCXIJME% VOLIJME% MXNO RATIOHYOFOTHEFII'AL ENTFAINED {IIYDROI}IEFT'AL

FLUID BOTTOMWATER FUJIDEEAWAI:N1

1 1 69-l B horEg !emp'8allnlty

1 I 70-3 norDg tompsallnlt,

1 1 70-15 hdtDg bnpsallnltt

1 1 73-3 hotrrogbtrpsallnlly

1 175"1 0 ndDg bmpsallnlly

EquadoB6€d: r(Th1) + (1-x)(Isy) - I1t2x(8!) { (1'xxssw}-$2

whe@ r - wluno ot ttydtothemel fiuH, (1-t) - wlmo ol anblant 8ffiisr, Tsw -tmpsrduc ot ambls[t sMbr h'o (?c br o!ay@ Baln)' Thl - m@hotrbgenladon tonpemtm of prtnaty tbld hdudm, Th2 - rem nomogsnhafotgmpentuc ol wldaty ttqd lndusbm h oC, Ssw - salhny ot anblont twderln €qutv. {t% NaCl (35 €q. rl% NaCl br G€ynas Bsln), q - t|M slniy otprlmary fiid hctrslom h eq. Yl% l,lao, ad q - mm slnliy ot @ndaty frublrclcbm ln €q. wL% NaCL'€lcotatod usl|E n€dhn valus: ail othoa €lculaled udry atlthnotb mm Yalug

-m Etoa rcre @la!dod; dd8 show€d m nbdtq.

8592

;,a490

90r 0 0

1 r5

' i.i16.7

1 5

. ;

l 0

1 00

HYDROTHERMAL DEPOSITS. GUAYMAS BASIN 583

a ratio calculated to be about 9-11.5:l (Table 5). Notsurprisingly, the amouirt of seawater lhat has mixedwith hydrothermal fluid at the site of mineral precipi-tation is relatively small.

Sample I169-1B is the only one in Table 5 which,on the basis of temperature data, displays very littleevidence for mixing. The sample is from the mid-section of a 0.8-m-high chimney that is taller and hasa thicker wall than all other chimneys sampled; con-sequently, almost no mixing of hydrothermal fluidand seawater would have occurred. Although chim-ney walls seem to be effective insulators, breachingby cracking, and dissolution by hydrothermal fluidsor seawater, must result in some mixing between sea-water and hydrothermal fluid (Haymon 1983).

DIscUssIoN

A model for the formation of spires ond moundsFigure 13 depicts a model for the growth of spires

and mounds. On initiation of venting into ambientseawater, the first minerals to precipitate are anhy-drite and stevensite, followed closely by pynhotite.The presence of stevensite is consistent with thepredicted occurrence of a Mg silicate in Janecky &Seyfried's (1984) mixing model for hydrothermalfluids and seawater. Also, Thornton & Seyfried(1987) found that by reacting an organic-rich, diato-maceous clay from the Gulf of California with sea-water at 350"C and 500 bars in an experimentdesigned to simulate conditions of the GuaymasBasin hydrothermal system, aqueous silica concen-

trations near amorphous silica saturation developedearly and promoted smectite precipitation. Theirresults suggested that smectite in altered GuaymasBasin sediments and hydrothermal precipitates ismore stable than chlorite due to elevated aqueoussilica levels associated with the dissolution of dia-toms. Bowers et al. (1985) calculated that mixing ofGuaymas Basin hydrothermal solutions with sea-water should result in the precipitation of talc,although the present study indicates that stevensiteis the stable Mg silicate. Precipitation of stevensiteprobably occurs over a large temperature range, asthe mineral is found both in mound and active-chimney samples. Guaymas Basin hydrothermal ventfluids are depleted in Mg (Von Damm et al. 1985),so Mg in stevensite is likely derived from ambientseawater, whereas the silica probably originates fromend-member hydrothermal fluid.

Seawater heated to supersaturation with respectto CaSOo precipitates anhydrite along with steven-site around the margins of the discharging fluid vent,thereby forming a crust or carapace that is imperme-able to seawater (Fig. l3a). Hydrothermal fluidescapes through fractures in this crust, and eventu-ally leads to the formation of juvenile spires. Veryyoung venting structures are composedpredominantly of a porous network of minerals, suchas anhydrite and stevensite, formed by the mixingof hydrothermal fluid with ambient seawater wherea central orifice has not yet formed by mineral dis-solution and reprecipitation. Throughout the hot,active life of the chimney or mound, these minerals

CHIi'NEY TATIJI'

fiPENUEAB!.EAfroFHoUsstuc*ROHW

"C[,ND SEDIMENI

TALT'9

Frc. 13. Generalized development of chimney and mound structures. See text forexplanation.

fl,@,t

584 THE CANADIAN MINERALOGIST

continue to build walls upwards and outwards at the hydrothermal fluids continue to flow through the sys-seawater interface (Fig. l3b). This framework pre- tem, the sediments adjacent to the fissure heat up,vents extensive mixing of hydrothermal fluid with and calcite and barite are precipitated as veinlets andambient seawater, and allows pyrrhotite, zinc sul- in the cap. Once formed, this cap serves as a physi-fide, isocubanite, and galena to precipitate intersti- cal and thermal barrier to mixing of hydrothermaltiallytoanhydriteandstevensite.Zincsulfidenucle- fluid and seawater. Cooling within the incipientates on, and partly replaces, pre-existing pyrrhotite. mounds is largely by conduction, leading to theIsocubanite replaces and is largely co-depositional precipitation of amorphous silica. Metal-rich fluidswith zinc sulfide, as seen by its arborescent and penetrating the mounds from below cannot rapidlyplumose intergrowths with zinc sulfide and its mix with seawater; sulfides are precipitated intersti-presence as extremely fine-grained inclusions in that tially and replace non-sulfide phases. The moundmineral. With decreasing temperature, chalcopyrite grows by the upward migration of the amorphousunmixes from isocubanite to form lensoid lamellae. silica ( + barite), resulting in the dissolution of theGalena is consanguineous with zinc sulfide and lower part of the siliceous crust and reprecipitationisocubanite,/chalcopyrite, and locally replaces both on top. Spires so weakened collapse, and the sedi-pyrrhotite and zinc sulfide. mentcontributestothe$owthofthemound.Figure

As high-temperature venting continues, calcite l3d depicts a mature, fully-developed mound withprecipitates both interstitially and as a replacement many of the features observed from ALYIN.phase (Fig. l3). With time, walls become thick andvoids and fractures are sealed, preventing the inward ACKNowLEDGEMENTSincursion of seawater. As mixing across chimney Samples used in this study were obtained duringwalls is inhibited, vent fluids become hotter, and sul- ni Filto 6 cruise to Guaymas Basin (p.F. Lonsdale,fides continue to precipitate internally and to replace

"f,i"f ,.i"r,tlitl with the submersible ALVIN and sup-

non-sulfides in external zones. In old chimneys, the ;;"*r.iRl V iutu. We thank shipboard scientisis,central orifice eventually becomes clogged by precipi- ifi. el_VfN group, and crew,rr".b"r, for technicaltation of high-temperature sulfides, resulting in a iuppo.t at ia. i.f.C. Spooner and T.J. Barrettspire. Fractures which develop in a chimney resu.lt pii"iA.o"uruable advice and critically reviewed thein the lateral escape of vent fluid. The buoyant fluid iirit uutt o.,, M.Sc. thesis, on which this paper isrises and, as it escapes, bathes the chimney surface U*"a. .;.fri, work has benedtted greatly from discus-immediately above the fracture, causing the precipi- ,i*, *itf, W.C. Snunn, B.R.t. Simoneit, M.p.tation of anhydrite and stevensite as an incipient eave CoJon, T.F. McConactry, Vt.p. Hannington, D.R.(Fig. l3c,d). With conductive cooling and decreas- il;r;; and L.T. gryndzia. Expert photographying temperature, amorphous silica coats all primary *d dr"ftil ,..i prouid"d by B. O'tjonouin andphases and replaces some minerals in the outer por- S. Sfr"otuglrespeciively. This study was funded bytions of the chimney or spire. The precipitation of ifr" N"trr"A Sciences and Engineering Researchminerals such as barite and amorphous silica, and Coun.ii oi CanaOa through grirrts to 3.D. Scott.to some degree calcite, which are largely insoluble Fi"*.iJrupport to peteiwis also provided by ain cold seawater, results in the growth of extremely Uil;;itv ;fioronto Open Fellowship and an H.V.large and complex structures (e.9., Fig. 3a,b). Dense E[sworth Graduate Scholarship in mineralogy.clusters of tube worms likely play a role in mineralprecipitation and chimney growth by insulating ventiluid from cold, ambient seawater, and by promot- REFERENCES

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THE CANADIAN MINERALOGIST

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Received October 6, 1987; revised manuscript acceptedJune 9, 1988,