Canadian MineralogktYol.29, pp. 4s3460 (1991)

ABSTRAcT

One dredge haul from a young East Pacific Risesqrmount, located approximately 15 km off-axis,recovered exclusively hydrothermal material that includedmassive sulfide, crusts consisting of euhedral drusyatacamite, and reddish brown iron oxide crusts and mud.The massive sulfide is primarily quartz, pyrite, andsphalerite, with accessory chalcopyrite, covellite, digenite,and galena; the sulfide fraction contains up to 2 ppm Auand 150 ppm Ag. Conditions of formation were estimatedfrom fluid inclusions in quartz. Inclusion trappingtemperatures were 240 + 35oC at the ambient seafloorpressure of 26 MPa, and the fluid had 4.0 wt.9o equivalentNaCl. The massive sulfide is atypical because it containssilica as quartz rather than an amorphous variety,suggesting that the usual kinetic barrier to quartzprecipitation was overcome. Some of the quartz apparent-ly replaced earlier amorphous silica, but much of itprecipitated as primary quartz, entrapping fluid in-clusions. Active hot springs on the seafloor thatprecipitate quartz have not been directly observed;however, these massive sulfides and other quartz-bearingsamples from seafloor deposits suggest that subsurfacediffuse flow and conductive heatloss may overcome theunfavorable kinetics of quartz precipitation.

Keywords:, sulfide, massive sulfide, fluid inclusions, EastPacific Rise, seamount, hydrothermal, atacamite,gold, silver, seafloor hot springs.

SoMMAIRE

Une suite d'dchantillons pr6lev6s des flancs d'un jeuneguyot situe environ l5 km de I'axe de la creteEst-Pacifique consiste exclusivement des produits hydro-thermaux suivants: sulfures massifs, cro0tes d'atacamiteidiomorphe, cro0tes riches en oxyde de fer brunrougeatre, et boues. Les dchantillons de sulfures massifscontiennent surtout quartz, pynte et sphal6rite, avecchalcopyrite, covellite, dig6nite et galbne accessoires. Lafraction sulfur6e contient environ 2 ppm d'or er 150 ppmd'argent.. Nous avons 6valu6 les conditions de formationi partir des inclusions fluides dans le quartz. Lestemp6ratures de pi6geage sont de240 + 35"C d. la pressionambiante i cette profondeur, 26 MPa; la phase fluidecontient 4.090 (poids) de NaCl (6quivalent). La pr6sencede quartz plut6t que d'une forme amorphe de silice dansles 6chantillons de sulfures massifs les rend atypiques, etmontre que les facteurs qui entravent normalement lapr6cipitation du quanz ont 6t6 contres. Une partie du

MASSIVE SULFIDES WITH FLUID-INCLUSION-BEARING OUARTZFROM A YOUNG SEAMOUNT ON THE EAST PACIFIC RISE

DAVID A. VANKO, BARBARA J. MILBY AND SEAN W. HEINZQUITHDepartment of Geology, Georgia State University, Atlanta, Georgia 30303, U.S.A.

quartz remplace la silice amorphe pr6coce, mais une plusgrande partie semble primaire, tout comme les inclusionsfluides. Les 6vents hydrothermau( sous-marins quiproduisent du quartz n'ont pas encore 6t6 observ6sdirectement. Ces sulfures massifs et les autres 6chantillonsquartziferes de gites sous-marins font penser que le fluxdiffus d'une phase fluide hydrothermale sous la surfaceet une perte de chaleur par conduction pourraientexpliquer la nucl6ation rdussie du quartz d cet endroit.

Clraduit par la R6daction)

Mots-clds: sulfure, sulfures massifs, inclusions fluides,cr€te Est-Pacifique, guyot, hydrothermal, atacamite,or, argent, 6vents hydrotherrnarD( sous-marins,

INrRooucuoN

Seafloor sulfide deposits have been discoveredat many sites along the world's mid-ocean ridgesystem, as well as atop several deep-sea volcanoes(Rona 1988). Known sulfide deposits from sea-mounts are less abundant than from ridge crests,probably as a consequence of sparse sampling.Those from young se€rmounts near the East PacificRise (EPR) have been described and analyzed byAlt and co-workers (Nt et al. 1987, Alt l988a,b)and Hekinian & Fouquet (1985). A silica-sulfide-sulfate deposit from Axial Seamount on the Juande Fuca Ridge has been studied extensively byHannington & Scott (1988). Other examples arefrom a submarine volcanic arc se€rmount (Urabe etol. 1987) and from a Mediterranean seamount(Minniti & Bonavia 1984). Hydrothermal depositsdevoid of sulfides occur at Loihi seamount(Malahoff et al. 1982), Red seamount on the EPRat 2l"N (Alt 1988a), and other seamounts in thePacific and Mediterra\ean (e.9., Exon & Cronan1983, Vanko et al. 1984, Varnavas et al. 1988,Uchupi & Ballard 1989). The relative importanceof submarine hydrothermal activity associated withseamounts is unknown. However, in terms of netcrustal cooling, net amount of seawater reactedwith crustal rocks, and potential for creatingseafloor sulfide deposits, seamounts could play animportant role in global budgets (Lonsdale el a/.1982, Batiza 1982).

4s3

454 THE CAIIANIAN MINERALOGIST

The present paper gives results of petrographicand geochemical studies on hydrothermal rocksdredged from a young volcanic seamount near theEast Pacific Rise at 14"09'N. Rock types recoveredin a dredge haul of the seamount summit includedabundant e rthy, low-density reddish orange crustsand poorly consolidated sediments, several denserand harder pieces of massive sulfide, and rare crus$coated by euhedral crystals of atacamite. Thesesamples are described, and results of partialchemical, X-ray diffraction, and fluid inclusionanalyses are presented.

MerHoos

Samples were obtained by dredging after thevolcano was mapped using the Sea Beam multinar-row-beam echo-sounding system aboard the R/VThomas Woshington. Navigation was carried outby conventional transit satellites, which limitlocational uncertainties to roughly I km. Dredgepositioning was aided by using real-time Sea Beamdata in conjunction with the previously producedSea Beam map of the seamount.

The mineralogy of the samples was determined

by transmitted- and reflected-light microscopy andpowder X-ray diffraction. Fluid inclusions wereanalyzed by conventional heating-freezing techni-ques on a modified USGS-type gas-flow micro-scope stage. The accuracy of routine determina-tions of temperature is estimated to be + 0.loCduring the freezing runs and * 1.0"C during theheating runs.

Several bulk samples were analyzed for Au, Ag,Cu, Pb, Zn, and, Fe by Bondar-Clegg, Vancouver,B .C .

GEoLocrceL SsmlNc

The seamount at 14o09'N, 1M"22'W is locatedon crust about 150,000 years old l5 km west of theEPR (Fie. l). Lavas and hydrothermal depositsseem to be fresh and to have formed recently. Thevolcano is roughly 4 km wide and 7 km long at itsbase, and is 800 m high above the surrounding3000-m-deep sea floor. It has a broad flatcross-section, similar to that of many previouslydescribed young East Pacific volcanoes (Batua &Vanko 1983). It has no caldera at present, similarto roughly 2o-60t/o of those young deep-sea

N}.N'!NhH\) " ) Ezz

d_

1\\ \ \

Yo- t09ow 1060 1o4o lo2o

10/,o17W

FIc. I . Sea Beam map of three volcanoes located on the west flank of the East Pacific Rise (labeled EPR on theinset, after Macdonald et al. 1984). The rise axis at this latitude is at l04o l5'W. about 15 km east of theseamount dredge haul (shown as a stippled rectangle).

MASSIVE SULFIDES ON THE EAST PACIFIC RISE 455

volcanoes examined during Sea Beam or side-scansonar surveys (Batiza & Vanko 1983, Searle 1983,Fornari et al. 1987). Instead, the summit containsseveral high points separated by broad saddles.Three dredges of the summit recovered basaltic lavain all but one: the shallowest dredge recoveredseveral tens of kilograms of hydrothermal material,but no basalt. The hydrothermal material consistsof massive sulfide, atacamite crusts, and reddishorange crusts.

NATURE oF THE MessIw Sul-nross

The massive sulfide is gray, fine grained, andporous. The porosity, varying from about 5 to20t/0, is due to small millimeter-scale vugs andirregular tubes that are about I mm across andseveral millimeters long. Some of these tubes areIined by drusy (subhedral crystalline) quartz, andothers are filled completely with quartz. Theexterior surfaces of these samples are weathered toa rust-red color, probably owing to an assemblageof iron oxides and hydroxides, and several sampleshave accessorv atacamite in the weathered crust.

TABLE 1. I4ODAL COMPOSITIONS OF I.IASSIVE SULFIDES

M i n e r a l

quartzpyri techal copyr i teother opaquesspha l er i tevo i ds

N o t e : ' O t h e r o p a q u e s ' i n c l u d e s c o v e l l i t e ,d igeni te, and i ron oxy-hydroxide.

X-ray diffraction of the massive sulfides indi-cates quartz and pyrite as ubiquitous and dominantconstituents, but optical petrography indicates thepresence of sphalerite in variable proportions up to25t/0, and chalcopyrite present in accessoryamounts (< 30/o; Table l). Traces of covellite,digenite, and galena also occur.

The massive sulfide rocks appear to be a mosaicof fine-grained (of the order of 0.1 mm and less)anhedral quartz containing abundant embedded

61 58 65 52 65 5721 20 15 16 18 26< 1 2 < l 3 < 1 12 < 1 2 . 1 < 1 < l9 3 <1 25 I 47 7 6 1 7 5 8 1 3

Ftc. 2. Photomicrograph of domal lamination in massive sulfide, consisting of granular and colloform sulfide (mainlypyrite) and mosaic quartz. Field of view is 5 mm wide. Plane-parallel light.

456 THE CANADIAN MINERALOGIST

blebs and stringers of colloform pyrite, whichcommonly align into curved stringers or domalfeatures up to several cm long (Fig. 2). Quartz iscoarser and clearer in pockets and small veinletsthat measure up to about 2 mm in maximumdimension. These correspond to the quartz-filledvugs seen in hand specimens. Pyrite occurs withthis coarser quartz as cubes measuring about 0.5mm. Transparent isotropic sphalerite occurs asinterstitial grains up to 0.25 mm across, in stringersand thin colloform layers, and rarely in euhedralgrains 2 mm in diameter. Chalcopyrite forms smallinterstitial patches and irregular replacementswithin sphalerite; it is rimmed and in placescompletely replaced by covellite and digenite.Galena(?) forms tiny inclusions in sphalerite.

Practically all of the quartz grains exhibitundulose extinction. Furthermore, in some filledvugs, concentric Iayering and the development ofplumose growth-habits are common, indicatingthat the quartz here may be primary. Some mosaicquartz encloses faint spherulitic outlines, a textureinterpreted by Alt et al. (1987, their Fig. 4B) rorepresent quartz replacement of primary opalinesilica. Therefore, as for other examples from EastPacific seamottnts (Nt et aL l987), some early silicawas precipitated as opaline or amorphous silica.Such material later recrystallized, and vugs andveins were subsequently filled by primary quartz.

Quartz grains, particularly the primary wg-fill-ing quartz, contain sparse fluid inclusions thatconsist of liquid and a vapor bubble. The inclusionsare equant but have irregular walls. They areisolated inclusions, and are interpreted to representprimary trapping of fluids from which the quartzwas precipitated or which induced recrystallizationfrom pre-existing amorphous silica. Ice meltingtemperatures of 64 inclusions in six samples (Fig.3) gave an average of -2.4oC + 0.5.C (onestandard deviation), corresponding to 4.0 + 0.8wt.9o equivalent NaCl (Roedder 1984, p. 233). Thisvalue for the ice melting temperature is stightlyIower than that for seawater [-1.9"C, correspond-

155 2r0 h265

320 375

FIc. 4. Frequency histogram of measured temperaturesof homogenization of fluid inclusions (allhomogenizations to the liquid phase). The presumedtrapping temperatures of the fluids are about 25'Chigher than the homogenization temperatures becauseof the ambient 26 MPa pressure at the seafloor.

ingto 3.2 wt.9o NaCl equivalent (Weast 1973)1. Icemelting temperatures obtained from each of the sixsamples have identical average values.

Temperatures of homogenization of fluid in-clusions into the liquid phase [Th L-V (L),hereafter designated Thl vary from about 160 to350oC, but with a strong mode around 216"C (Fig.4). Because the massive sulfide was formed at theseafloor (ambient pressure of 26 MPa), a pressurecorrection ofabout 25"C + 5'C should be appliedto Th (Roedder 1984, p. 262), which gives a meantrapping temperature of the fluid of about 240'C+ 35"C (one standard deviation). Trappingtemperatures of inclusions in anhydrite and calcitein several other studies of seafloor sulfides agreewell with the temperatures determined throughactual measurements by thermocouple or fromcoexisting mineral chemistry (Le Bel & Oudin 1982,Kusakabe et al. 1982, Koski et al. 1985, Peter etal. 1986, Bfett et al. 1987). Thus, the 240octemperature indicated by the present study isconsidered to be a good approximation of thetemperature of quartz growth.

Five samples of the massive sulfide wereanalyzed. for selected trace and heavy elements(Table 2). If we assume that Au and Ag resideprimarily in sulfide phases, the bulk compositionscan be recalculated to quartz-free values. Sulfidephases contain from 0.2 to 2.3 ppm Au and from50 to 150 ppm AC. Cu and Zn values are elevatedbecause of the presence of chalcopyrite andsphalerite, respectively. The concentration of leadis low (0.02-0.03 wt.9o).

THB ArecanarrE CRUSTS

Several small (2-5 cm) pieces of iron oxide crustin the dredge haul contain, on one side only, acoarse drusy layer of atacamite crystals. The

25

20l5

10

;L100

25

20

15N

10

I

0r--4 - 3 - 2 - 1 0

Tu(rcr)Frc. 3. Frequency histogram of measured temperatures

of ice melting from fluid inclusions within quartz. Thestrong mode at -2.4"C corresponds to 4.0 wt.9o NaClequivalent, i.e., slightly more saline than seawater.

X = 2 t 6 . 4 " C

N = 6 4-X = -2 .4oC

MASSIVE SULFIDES ON THE EAST PACIFIC RISE

Pb Zn Fewt.% wt.% wt.%

3 . 4 0 . 0 3 4 . 6 9 . 0

2 . 2 0 . 0 2 3 . 2 8 . 9

0 . 0 2 2 . 3 1 0 . 4

0 . 0 2 7 . 0 8 . 1

457

IABLE 2. CHEMICAL DATA ON THE MASSIVE SULFIDES

Sampl e Au Ag(ppm) (ppm)

Cuwt.%

102-3 r .Z32 . 3

102-4 0.510 . 9

102-5 0.140 . 2

102-7 0.140 . 3

L02-8 3 . 2 0 . 0 3 6 . 4 8 . 6

Analyses for Au and Ag by f i re assay. Cu, Pb,and Zn were by atomic absorption, and Fe totalrvas determined t i t rametr ica l ' ly . A l I analyseswere performed by Bondar-Clegg Ltd.For Au and Ag, the upper value is the actualana l ys i s , and the va lue unde rnea th i s ca l cu -)ated quartz- f ree, us ing the modal data f romTable 1. I t thus represents the Au and Agcontent of the sul f ide a lone, assuming thatthe quartz is f ree of these e lements.

crystals measure I to 2 mm across, and exhibit the[10J, [0ll] and [010] forms. X-ray-diffractionpatterns show that an indeterminate amount of thehexagonal polymorph paratac€rmite also is present.In thin section, the crystals are transparent tocloudy, with zones of cloudiness forming aeuhedral pattern of concentric growth-zones. Nofluid inclusions were observed in the atacamite.

Atacamite also is present, as noted above, as anaccessory phase at the margins of some of themassive sulfide samples. There, it is earthy, occursin veinlets, and resembles atacamite described frommany other seafloor hydrothermal settings (e.9.,Mossman & Heffernan 1978, Bonatti et al. 1976,Nt et al. 1987).

THE IRON OXIDES

The majority of the dredged materials arereddish orange rocks and crusts that resemble theiron oxide - hydroxide materials observed andsampled from low-temperature hydrothermal fieldson seamounts (e.9., Lonsdale et al. 1982, Malahoffet al. 1982, Exon & Cronan 1983, Yanko et al.1984, Hekinian & Fouquet 1985, Alt 1988a).Samples are porous, low in density and earthy.Smear slides prepared from the earthy parts containoptically isotropic red translucent grains andabundant red translucent filaments. The filamentsare typically 2 to 4 pm wide and may attain lengths

7 9 . 8150

4 0 . 672

3 4 . B5 0

3 5 . 586

1 . 6

3 . 8

Fra. 5. Photomicrograph of red iron oxide crust materialin oil, showing the abundant rod- and filament-shapedsheaths of amorphous iron oxide, interpreted to bemolds of filamentous bacteria. Field of view is 75 pmwide, and a typical filament mold is I to 2 tm across.

of tens of pm: they exhibit curved and convolutedforms, commonly branching, ald less commonlyintertwined (Fie. 5). In size and shape, theyresemble the long branching filaments described,for example, by Alt (1988a) and Juniper & Fouquet(1988). These authors inferred that the filamentsprobably represent iron-encrusted filamentous bac-teria or the molds of such bacteria. Filamentousbacteria isolated from modern living bacterial matsin hydrothermal deposits on the Loihi seamountare generally a few pm across and tens to hundredsof pm long; nonbranching forms are dominant(Karl et al. 1988\.

DISCUSSION

The hydrothermal material dredged from thesummit of the volcano is part of a seafloorhot-spring deposit. It is unlikely that any of thesamples are from a sub-seafloor stockwork (Hon-

458 THE CANADIAN MINERALOGIST

norez et al. 1985) because there is no morphologicalevidence for tectonic exposure such as calderafaulting (Fig. l). The seamount deposits exploredwith submersibles near 12'N and 21 'N along theEast Pacific Rise and described, respectively, byHekinian & Fouquet (1985) and by Nt et al. (1987)are the closest known analogues to the presentdredged deposit. The atacamite crusts rueanalogous to those observed at the TAG Mid-Ar-lantic site (Thompson et ol. 1988) and East PacificRise seamounts (Alt et al, 1987), formed bydiagenetic oxidation and remobilization of copperwithin sulfide mounds.

Elemental analyses of massive sulfide yieldvalues for Au, Ag, Cu, Zn, Pb, and Fe that aretypical for samples from many seafloor sulfidedeposits (Rona 1988, Hanningron & Scott 1989).The gold and silver values match fairly closely thoseof the primary Fe and Fe-Zn massive sulfide fromTAG (Hannin5ton et al. 1988), as well as massivesulfide from Axial Seamount and the SouthernExplorer Ridge (Hanningron & Scort 1989). coldand silver values exceed those in massive sulfidesfrom 2loN East Pacific Rise (e.g., Zierenberg etal. 1984). Levels of copper and zinc are lower thanin many seafloor samples (Rona 1988). Despite anassociation with atacamite, there is no dramaticsecondary (supergene) Au enrichment in thesulfides, as has been discovered at TAG (Han-nington et ol.1988).

An unusual aspect of the seamount massivesulfide samples is the abundance of quartz and thepresence of fluid inclusions within primary quaftz.Black smoker fluids are generally supersaturatedwith respect to quartz as well as amorphous silicaat seafloor presslues (e.g., Von Damm et al. 1985,Bowers et al. 1985, Michard et al. 1984), but quartzand, to a lesser extent, amorphous silica are rarewithin many black smoker chimneys. Black smokerfluids apparently rise nearly adiabarically from adeep reaction-zone where the fluids are in equi-librium with quartz at high temperature andpresswe. During rapid upwelling, quartz nucleationis suppressed, and dissolved silica ends up beingflushed out into the ocean. The presence ofamorphous silica or quartz with sulfides in moundsor chimneys attests to a process that allowsnucleation of silica to occur; one possibility is thatfluid encounters a regime of slower and morediffuse flow, perhaps combined with substantialconductive heat-loss (c/. Hannington & Scott 1988;their Fig. l7). Tivey & Delaney (1986) proposed asimilar scenario for portions of massive sulfidemounds that were sealed by late-stage precipitationof amorphous silica.

In contrast to black smoker chimneys that aredevoid of silica, the other end of what may be a

complete natural spectrum is the silica-rich, sulfide-free chimneys from the Galapagos spreading center(Herzig et ol. 1988). These structures, composed ofamorphous silica (opal-A), were precipitated at 32'to 42"C, based on oxygen isotopic values. Herziget al. (1988) suggested that fluids responsible forthese inactive chimneys were cooled from high-temperature Si-rich solutions by a combination ofconductive cooling and mixing with cold seawater.They hypothesized that associated sulfides mayhave been deposited in the subsurface.

The massive sulfides described in the presentpaper fall in the middle of the silica spectrum. Theyare similar to sulfides from other East Pacific Riseseamounts described by Alt et al. (1987) andHekinian & Fouquet (1985) that contain abundantamorphous silica (opal), as well as quartz that hasboth replaced opal and has crystallized directly invugs and fractures. The silica-sulfide-sulfate spirestrom Axial Seamount (Hannington & Scott 1988)similarly contain abundant amorphous silica. Altet al. (1987) estimated temperatures of quartzprecipitation on the basis of oxygen isotopic values;if we assume that the fluid was unshifted fromseawater (6180 = 0/oo), the quartz grew between230o and 250"C. If the fluid was shifted to a Dr8Oof +2.0o/oo then the corresponding temperaturerange was 280o to 320"C. These temperatureconstraints are in excellent agreement with ourrange of fluid-inclusion-based temperatures, 240"+ 35oC. We therefore concur with other workersthat moderate-temperature hot springs (in themid-200'C range) may form substantial silica-sul-fide deposits either within seafloor mounds orbeneath the seafloor.

AcKNoWLEDGEMENTS

This research project was supported by NSFgrants OCE-8710717 and OCE-8812316. Wethankthe Master and crew of the R/V ThomasWashington, the chief scientist (Rodey Batiza) andour shipboard colleagues (especially Jill Karsten,who dredged the sulfide deposit) for their help inobtaining the samples. The manuscript was readcritically and improved by W. Normark. We thanktwo anonymous referees for their helpful reviews,which resulted in substantial improvements in thepaper, and James Franklin for his ideas uponexamining the samples petrographically.

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Receiued August I, 1990, revised manuscript acceptedFebruary 25, 1991.