997

The Canadian MineralogiuVol. 31, pp. 997-1024 (1993)

MINERALOGY AND GEOCHEMISTRY OF ACTIVE AND INACTIVE CHIMNEYSAND MASSIVE SULFIDE, MIDDLE VALLEY, NORTHERN JUAN DE FUCA RIDGE:

AN EVOLVING HYDROTHERMAL SVSTEMX

DOREENE. AMES" JAMES M. FRANKLIN aNI MARKD. HANNINGTON

Geological Survey of Canadn, 601 Booth Street, Ottawa, Ontario KIA 088

AssrRAsr

Hydrothermal deposits occur a[ a water depth of 2460 m in Middle Valley, a sediment-fllled railed rift at 48'n' N htitude,on the nordrern Juan de Fuca Ridge. Sites of focused discharge are concentrated above a deep-seated system of fractures paxal-lel to the regional north-south trend of rift-propagating faults. The Area of Active Venting (AA$ and Bent Hill area are hoststo moderate-temperature (<276oC) vents. Active chimneys are sulfate-rich and sulfide-poor, consisting of a thick outer wall ofanhydrile and a thin clay-rich inner wall with very fine (20 pm) disseminated sulfides. The inner wall contains saponite,pyrrhotite, chalcopyrite, isocubanite, high-iron sphalerite (35-46 mole 7o FeS), pydte, marcasite, galena and arsenopyrite. Theactive chimneys have low concentrations ofthe base metals but ahigh CulZn value. The fluids discharging through thesechimneys have characteristically low a(O2) and a(S2), and locally have deposited significant amounts of hydrocarbon.Hydrothermal mounds associated with active venting are produced by the collapse of chimneys and by mineral precipitationwithin the pile of sulfide-sulfate talus (i.e., by inflation). The active mounds are composed of anhydrite, amorphous silic4saponite, serpentine, and minor chalcopyrite, low-iron sphalerite (<15 mole 7o FeS), lepidocrocite, pyrite, marcasite, galenaand barite. Inactive chimneys on or adjacent to the mounds are predominantly barite-rich, with lesser amorphous silic4 hydm-carbons, clay and marcasite. In contrast to the active chimneys, the Bent Hill deposits are sulfide-rich and have higher ZnlCuvalues. Two older massive sulfide deposits in the Bent Hill area contain pyrite, lepidocrocite, marcasite, moderately iron-richsphalerite (24-33 mole 7o FeS), chalcopyrite, covellite, galena, silica and barite. Pb-As-Sb sulfosalts occur locally within low-temperature, silica-rich crusts on the massive sulfides. The mineralogy and geochemistry of the active chimneys suggest thatthe present stage of hydrothermal venting at Midclle Valley is botl metal- and sulfur-depleted and is likely the product of themoderate-temperature interaction of hydrothermally modified seawater with local sediments. The massive sulfides at Bent Hillare apparently a product of an earlier hydrothermal event dominated by metal- and sulfur-enriched fluids, possibly derivedfrom the high+emperature interaction of circulating seawater with basaltic crust in the basement. Recent drilling by the ODPhas identified a large pyrite - pyrrhotite - magnetite deposit beneath the sulfide outcrop at Bent Hill. The present-day mineral-ogy of this older massive sulfide deposit may be the product of extensive reaction with the cool, sulfur-depleted fluids that arecurently venting at Middle Valley.

Keywords: massive sulfide, Middle Valley, Juan de Fuca Ridge, chemical composition, mineralogy, mineral chemistry,seafl oor, hydrothermal, anhydrite.

Sotvnuens

Nous documentons la pr6sence de zones d'activit6 hydrothermale i une profondeur de2,460 m le long de Middle Valley,rift avortd rempli de sddiments b une latitude de 48o27'N, dans la partie nord de la ride de Juan de Fuca. Les sites de d6chargefocalis6e sont surtout situds au dessus d'une zone de fractures profondes, parallble i la direction r6gionale des failles servant dpropager la distension. Deux zones, dites "Area of Active Venting" et "Bent Hill", sont les sites d'6vents e temp6ratuemoyenne (<n6'C). lrs chemin6es actives sont riches en sulfates et paulres en sulfires, et sont faites d'une 6paisse paroiexteme d'anhydrite et d'une mince paroi interne riche en argiles, contenant des sulfures diss6min6s i granulom6trie trbs fine(20 pm). La paroi hterne contient saponite, pyrrhotite, chalcopyrite, isocubanite, sphal6rite h teneur 6lev6e en fer (35-46Vo deFeS, base molaire), pyrite, marcasite, galdne et arsenopyrite. ks chemin6es actives font preuve de faibles teneurs en mdtauxde base et d'un rapport CuZn 6lev6. La phase fluide d6charg6e de ces chemin6es, typiquement d a(O) et a(S) faibles, auraitlocalement produit des quantit6s appr6ciables d'hydrocarbures. Les amoncellements hydrothermaux associ6s aur( zonesd'activit6 rdsultent de I'affaissement des chemin6es et de la pr6cipitation de min6raux i I'int6rieur des talus de sulfure - sulfate(c'est-i-dire, par ir:flation). Ies amoncellements actifs contiennent anhydrite, silice amorphe, saponite, et serpentine, avec,comme accessoires, chalcopyrite, sphal6rite i faible teneur en fer (1l5Vo FeS), ldpidocrocite, pyrite, marcasite, galdne etbarite. Les chemindes inactives sur ou pGs de ces amoncellements contiennent surtout de la barite, avec des quantit6s moindresde silice amorphe, d'hydrocarbures, d'argiles et de marcasite. En contraste avec les chemin6es actives, les gisements de Bent

* Geological Survey of Canada contribution number 42592.

998 THE SANIATIIAN MINERAL96I5T

Hill sont riches en sulhres, et possbdent un rapport ZnlCu plus 6levd. Deux gltes plus anciens pras de Bent Hill contiennentpyrite, l6pidocrocite, marcasite, sphal6rite i teneur moyenne en fer (24-33Vo FeS), chalcopyrite, covellite, galdne, silice etbarite. Nous avons trouvd des sulfosels i Pb--As-Sb dans des incrustations siliceuses form6es sur les sulfrres massifs e faibletemp6rature. La min6ralogie et la gdochimie des chemin6es actives font penser que le stade actuel d'activit6 hydrothermale iMiddle Valley est i la fois pauwe en mdtaux et en soufre, et r6sulterait de I'interaction d temp6rature moyenne de I'eau de mermodifi6e et des s6diments enfouis. lrs sulfures massifs de Bent }Iill r6sulteraient d'un 6v6nement antdrieur, of pr6dominaientdes fluides enrichis en m6taux et en sulfures, possiblement ddriv6s de I'interaction A temp6rature 61ev6e de I'eau de mer en cir-culation et de la cro0te basaltique du socle. Des forages r6cents (programme ODP) ont permis d'identifier un gisement impor-tant a pyrit€ + plrrhotite + magn6tite en dessous de l'affleurement de sulfures A Bent Hill. L'assemblage min6ralogique actuelde ce gisement ancien de sulfures massifs pourrait bien 6ue le r6sultat d'une r6action imFortarte avec les fluides appauwis ensoufre et plus froids qui 6manent pr6sentement des 6vents d Middle Valley.

(Iraduit par la R6daction)

Mots-cl6s: sulfures massifs, Middle Valley, ride de Juan de Fuca, composition chimique, mindralogie, composition desmindraux, fonds ocdaniques, hydrothermal, anhydrite.

, t \escA,uaea/

- Mord""tro T'a,tororm

fnouoH

EXPLORER(?";

THE JUAN DE FUCA RIDGE SYSTEM

Flc. l. l,ocation map of Mddle Valley, northem Juan de Fuca Ridge.

CHIMNEYS AND MASSIVE SI]LFIDE. MIDDLE VALLEY 999

INTRoDuc"iloN

Much of the recent research on massive sulfidedeposits on the modern seafloor has been restricted tosediment-starved ridges in the eastern Pacific and mid-

Atlantic. However, the first drscovery of active depo-sition of base metals on the seafloor was made in sedi-ment-covered basins in the Red Sea, a continental rift(Degens & Ross 1969). The importance of massivesulfide deposits in a sediment-covered oceanic spread-

?!I

$r,r'

v858

Fis.

\

q l

HILL856

II

I II

I$

,1,

0

LEGEND

./ Fault scarp

,t Fault; side-scan sonarr|\

qb Acoustic reflector

Sr ODP drill site

r' Bahyrptry, contour interval 10m

Ftc. 2. Location of the Area of Active Venting (AAV) and Bent Hill hydrothermal areas, situated west of the eastern boundaryfault within Mddle Valley. Fault traces and ODP drill sites are shown. The diagram was compiled from bathymetric datafrom Currie et al, (1,985), 'rith acoustic reflectors and fault traces superimposed [from SeaMARC 1A imagery: Frankliner a/. (id press) and SeaMARC 1l imagery: Dais et al, (1992)1. Locations of Figures 3 and 4 are indicated.

1000 THE CANADIAN MINERALOGIST

ing axis was realized with the discovery of sulfidemineralization in the Guaymas Basin, Gulf ofCalifornia (Lonsdale et al. 1980).

Mapping of mid-ocean ridges off the westem conti-nental margin of North America has revealed a num-ber of sediment-covered basins filled with turbiditicmaterial shed off the adjacent continental margin.Investigations of sediment-covered rifts along thenorthern Juan de Fuca and southem Gorda Ridge seg-ments led to the discovery of sulfide deposits, atEscanaba Trough (Morton et al. 1987) and at MiddleValley (Davis et al. 1987) (Fig. l). The recovery ofsulfide samples in dredges and sediment cores fromMiddle Valley prompted an ALVIN dive series in1990 (Franklin et al. 1991) during which scientistsobserved, photographed, and collected actively dis-charging chimneys. Samples of moderate-temperature

(184 to 276'C) vent fluids, sulfide and sulfate chim-neys, massive sulfide, hydrothermally altered andunaltered sediment, and vent-specific benthic faunawere recovered.

This contribution documents the composition of thesurficial massive sulfide deposits, hydrothermalmounds, inactive and active chimneys, and barite-richcrusts at Middle Valley and considers their relation-ship to the evolution of the hydrothermal system.These results are compared with data from hydro-thermal sites on other sedimented ridges and with theresults of recent drilling by the ODP beneath thdactive vents at Middle Valley. The investigation ofsediment-hosted volcanogenic massive sulfidedeposits is important in that many of the world'slargest massive sulfide deposits are hosted either at thecontact between volcanic and sedimentary sequences

A

ox/

J

E

tr

"q8a

Blology Sltes

Acffve Chlmneys

3 - 10 cm'Worm Holes"Mounds

Inactive Chlmneys

Rubble

Sample SlteSlope ',,,,

Ftc. 3A. Deailed geological map of the Area of Active Venting (AAV), with location ofrock samoles studied.

CHIMNEYS AND MASSIVE SIJLFIDE. MIDDI-E VALLEY 1001

or within the sedimentary basin-filI in a volcanicallyactive terrane.

Gsorocv AND HYDRoT}IERMAL Acrrvrrver Mrools VALI-sv

Middle Valley is a l5-km-wide, sediment-coveredaxial rift within the Juan de Fuca Ridge system; it islocated 200 km off the coa$t of Vancouver Island(Frg. 1).Two sediment-covered spreading centers withactive hydrothermal systems on the Juan de FucaRidge include Middle Valley and Escanaba TroughGig. 1), both sites of sulfide deposition.

Middle Valley was recently an active mid-oceanspreading center until approximately 200,000 yearsago, when spreading jumped west to the now-active,sediment-buried, intermediate-rate spreading center in

West Valley (60 mm/yr) (Davis & Lister 197'7).Middle Valley is still undergoing minqr readjustment,as indicated by the presence of fissures and faultswithin the sediment fill in the valley, which controlsthe position of the present-day hydrothermal activity(Frg. 2). Seismic profiles across the valley indicate aseries of downstepping block-faults, with the deepestpart of the valley along the axis. The surface expres-sion of the faults indicates that most are parallel to theregional north-south trend of the rift-propagatingregional faults, but several oblique NW-SE structuresalso are present (Johnson et al.1990).

Middle Valley is f i l led with turbidit ic andhemipelagic sediment derived from the continentalmargin during the Pleistocene, dominantly fromQueen Charlotte Sound. Two known sites ofhydrothermal activity, the Area of Active Venting

CHIMNEYTYPE ALVIN DIVE TRACKAnhydriteBarlie

Detailed studyarea

F)c. 3B. Dive tracls 2251,2252,2254,2255 of the ALVIN submersible in the Area ofActive Venting, showing the extent of exploration.

@

a22512252225422:55

tw2 THE CANADIAN MINERALOGIST

0 Metre 200

Sul0de outcropAcou€dc refleclorSample slleAc{ve chbnney

NO FAUNA

25t-1-.9259-14?253-14

Ftc. 4A. Geology of the Bent Hill area, with acousticallyreflectant areas outlining known massive sulfide deposits.Location of rock samples studied is shown. A-A': loca-tion of cross-section.

FIc. 48. Cross section from A to A' and detailed plan-viewof the Bent Hill active chimney and associated sulfidemound. The samples used in this study are indicated.

ABUNDANTNORMATATTACHED FAUNA

t.=!l-.,iJl

ffiIt

lXjl "*"",,00"

ffiffiW

Normalsdlmeni

Sulffde lalus h sedlment

Anhydrtu

X Rod(sample

NORMAL FAUNA

TunlcatBg spong€,colonhl hydrozoans NORMAL FAUNA

Sponges,anenomer,funlcat€,polychaeio,bunolfis

SN

', /n

AA

M2448

24W

2452

2454

2456

2458

2460

?2'5s-.1-1

B

M

CIIIMNEYS AND MASSIVE SI]'LFIDE. MIDDLE VALLEY 1003

(AAV) and Bent Hill, are situated on the eastern sideof the valley 5 km and 3 km v7ss1 of the easternboundary fault, respectively, at a water depth of2460 m Grg. 2). The two sites of active hydrothermalventing were identified from geological @ranklin eraI. 1987) and geophysical @avis et al. 1987) observa-tions. The more northerly, Area of Active Venting(AAV), is defined by anomalously high acousticreflectance (Figs. 2, 3), and has over 20 vent sites. Thezones of high reflectance represent aprons ofhydrothermally indurated sediment surrounding thevent sites (Franklin et al. 1991). Another zone ofhighreflectance, south of Bent HiU (Ftg. 2), is a prominentmound of sulfide, with a single moderate-temperature(264"C) vent. A very large pyritic massive sulfidedeposit (at least 150 x 150 X 96 m) has been docu-mented (Davis ar al.1992) about 100 m south ofBentHill and 230 m north of the active vent @gs. 2, 4).

Drilling and seismic data reveal that Middle Valleycontains over 1000 m of turbiditic and hemipelagicsediment, and a variety of intrusive and extrusiverocks. The sediments are well lithified at a depth of400 m in the center ofthe basin, and at a depth of 175m below the AAV (see velocity - density - porositydata: Davis et al. 1992). The composition of the base-ment at Middle Valley is unknown, but ODP drillingintersected a variety of sills and extrusive rocks,including normal MORB-type basalt at the basin mar-gins along the eastern boundary fault (site 855, Fig.2),about 30 basaltic sills below 500 mbsf near the centerofthe valley (site 857, Fig. 2) and ooevolved" andesiticbasalt 250 mbsf under the AAV (site 858, Fig. 2). Thevolcanic rocls beneath the AAV form a paleobathy-metric high, possibly a seamount or a structurallyuplifted block. On Bent Hill, turbiditic sediment-fillexceeds 960 mbsf, and a series of young, relativelyfresh picritic sills has intruded the sediment to a depthof only 60 mbsf (Stakes et al. I99L).

Area of Active Venting

The AAV is an area of anomalously high heat-flow(i.e., 7L W m-2: Davis et al. 1987) and is the site of atleast 20 active vents with moderate temperaturesbetween 184' and 276'C (Franklin et al. l99L). T\event field is 800 m by 350 m, roughly rhombohedraland elongate along a north-south axis (Fig. 3). Tracesof faults (Fig. 2), evident on side-scan sonar maps,extend away from the southwestern and northeasterncorners of the acoustic reflector. Both faults are longi-tudinal, and probably correspond to a major buriedfault that is evident in seismic profiles (Davis et al.1992).

the AAV. Distinctive topographic highs within theAAV are low hydrothermal mounds with active andinactive chimneys, 1 to 6 m high, located near theirsummits. The area of high acoustic reflectance con-tains two groups of mounds. The northern groupincludes Heineken Hollow, East Hill, and Central Site,and the southern group includes Dead Dog Mound,Chowder Hill, and Inspired Mounds (Fig. 3A). DeadDog Mound is representative of the mound and chim-ney morphology of active vents in Middle Valley(Figs. 3, 5A, B). The mound is 10 to 15 m high by20 to 3O m wide, with steep sides sloping at about 30o.A 5- to 6-m-high conical chimney structure is perchedon top of the mound, and vents hydrothermal fluids at276'C. Blocks of chimney debris, 3-20 cm wide, forrnan apron several meters wide around the active vents.The sides of the mounds have many large displacedslabs of indurated sediment, typically 1 to 7 m wide,that attest to i:rflation of the hydrothermal mound. Anextensive zore2 to 3 m in diameter of white anhydriteenvelops the base of the chimney, with local patchesof bacterial mats on the surrounding mound. Deadtube-worms are attached to the base of the chimney,but otherwise the top of the mound is devoid of fauna.Numerous crabs inhabit the fla*s of the mound, andsmall clusters of tube worms and clams live at its base.Vent communities are very dispersed, but the largeclusters of tube worms that typify other ridge-crestvent areas are only poorly represented near the MiddleValley vents. Most chimneys observed during the1990 dive series are similar in size and overall sulfideabundance to the Dead Dog site; howevero the chim-ney sampled at Chowder Hill in 1991 (dive 2468) isabout twice as large, contains more sulfide, andis perched on a mound 50 m wide by 15 m high'

Flow from the active vents throughout the AAV isvigorous; clear fluid and, in some cases, grey "smoke"emalate from the tops, and less well-focused clearfluid is discharging around the base of most chimneys.The vent-fluid temperatures range from L84o to276C.T\ey have a pH of 5.3, high Ca and CO2 con-tents and low contents of base metals in comparisonwith typical ridge-crest hydrothermal fluids (VonDamm & Bischoff 1987).

Inactive, barite-bearing chimneys are not located onmoundso are not specifically close to active vents, andsit on a flat, moderately indurated substrate. They areoorootless" in that they are underlain by sediment(Figs. 5C, D). At Heineken Hollow, barite crustsimpregnated with silica, biological matter and alteredsediment (Figs. 5E, F) surround a 3-m-deep collapsedpit. A low-temperature active vent occurs in the centerof the pit.

The AAV is characterized by active chimneys withanhydrite, inactive chimneys with barite, hydrothermal Bent Hillmounds, and chimney rubble, but lacks any surfaceexpression of massive sulfide (Fig. 3A). Numerous Bent Hill is one of several mounds projecting frominictive barite-rich shimneys were found throughout the sediment-covered floor of Middle Valley. It is an

1004 TTIE CANADIAN MINERALOGIST

FIGS. 5A-D. A. Actively venting anhydrite chimneys on the top of a mound in the AAV. The chimneys are approximately I mhigh. B. Plan view of the central orifice of the Dead Dog mound active chimney. The top of the chimney is to the right.Note the white anhydrite outer zone (upper left and bottom) and the inner (dark grey) sulfide - Mg-bearing smectite lining.Scale card is 6.5 cm. C. Rootless inactive barite-bitumen chimney on the flat sediment seafloor in the Area of ActiveVenting. D. Cross-section of the top of a inactive chimney. Note the black bitumen and the semicircular oattern of srowthofthe brown barite.

uplifted block, approximately circular, 450 m indiameter and 60 m high, composed of turbiditic sedi-ment and minor sul{ide mud" sand and massive sulfrdealong the southern flank (Kappel & Franklin 1989,Joides 1991). Its western margin is very steep, coveredby talus blocks composed of indurated sediment, andis probably a fault @9. 4A). Several picritic sills wereintersected in ODP hole 856A and B; the uplift ofBent Hill may have occurred during their intrusion.Erosion of part of the sedimentary section, and thefreshness of the talus blocks, indicate that uplift haspostdated most of the sedimentation in the area, aswell as the time of formation of a sulfide deposit to thesouth (ODP site 856C-ID (Davis et a\.1992).

In the Bent Hill area, one active chimney only wasobserved, but several extensive ma$sive sulfide our-crops are present (Fig. aA). Massive sulfide mineral-ization is observed at two sites 230 m apart. A sulfideoutcrop at the base of Bent Hill (ODP site 856,

Fig. 4A), was found through extensive cnmera andvideo studies, ALVIN observations, and piston coring;it is visible on side-scan sonar images @rankln et al.L987,1991, Joides 1991). The sulfide deposit forms adistinct hill 35 m high and has a drill-indicated thick-ness of at least 96 m. This 100-m-long exposure isconsidered to be part of a more extensive sediment-covered deposit ofsulfide possibly up to 250 x 450 min plan view, as shown by the acoustic reflection out-line (Fig.4A).

The only known actively venting ehimnsy at BentHill (264"C) is associated with a second sulfidedeposit 230 m south of the ODP drilling site (856,Figs. 2,4A., B,5G, H). The active chimney is perchedon the lower tier of a fwo-tiered mound structure. at anelevation of 15 m (Fig. 4B), witl the highest tier at28 m composed of sulfide outcrop and sulfide talus.The chimney is surrounded by an anhydrite apron andis quite similar morphologically to the active chimney

CHIMNEYS AND MASSIVE SULFIDE. MIDDLE VALLEY

FrGs. 5E-H. E. Clear 184oC fluid venting from the depression at Heineken Hollow. Note the shimmering water at arow. Whiteanhydrite crusts and bacterial mats are near the vent, and barite-rich crusts are broken into slabs. F. Barite-dch crust atHeineken Hollow is very porous, with yellowish brown barite crystals as open-space fillings. Grey silica veinlets cross-cutthe sample (arrow). G. Sulfide mound south of the Bent Hill active chimney. Note the angular blocks of sulfide and theoxidation effects on the outer margin of the outcrop. H. Porous massive sulfide talus from Bent Hin Q253-l-1, Fig. 3B)composed dominantly of pyrite, marcasite, sphalerite and chalcopyrite, with an outer oxidized rim.

1005

sites at the AAV. Extensive weathering of the sulfldeoutcrop, the composition of the hydrothermal fluid(deduced by mineral assemblages, discussed below),and t}te large size of the Bent Hill sulfide deposit sug-gest that it may be sigaificantly older than the surficialhydrothermal deposits in the AAV and the active ventat Bent Hill.

Samnqc AND ANALYTTcAL METHoDS

The samples described in this paper were collectedduring a series of ALVIN dives (2251-2255) in 1990and a subsequent dive (2468) in 1991. Samplelocations are shown on Figures 3A and 48, and thosestudied are listed in Table 1. Exploration of the AAVwas much more extensive (five ALVIN dives:Fig. 38) than in the Bent Hill area (ALVIN dive2253). The hydrothermal material collected by themechanical arm of ALVIN was subsamoled and

vacuum-impregnated with "petro epoxy". Sixty-onepolished thin sections were prepared for petrographicexamination by reflected and transmitted light. ACambridge 5-200 scanning electron microscope witha Link Analytical AN10000 integrated energy-disper-sion X-ray analyzer atiachment was used for identifi-cation of f ine-grained (<2-20 pm) minerals.Quantitative mineral analyses were obtained with aCameca Camebax wavelength-dispersion electronmicroprobe. The accelerating voltage was 20 kV, witha beam cunent of 30 nA and a beam-spot diameter of5 pm. Bulk compositions were determined on air driedsamples ground to -200 mesh. The powders wereanalyzed at the Geological Survey of Canada laborato-ries by inductively coupled plasma (ICP-ES), atomicabsorption spectrometry (AAS), combustion and wet-chemical methods. Gold was determined by instru-mental neutron-activation analysis (II'{AA) utilizingthe facilities at the Royal Military College, Kingston,

r006 TIIE CANADIAN MINERALOGIST

AREA OF ACIIVE VENTINGACTIVE CHIMNEYSmpls Slts

INACNVE CHIMNEYSsnple Sne

n54"23-1 east ol DDM2*3-2 tM2255-61 euth ot EH

Sampls

226241826241C22'52"5"1C25+2+1

251-1-lp51"2-2251-2-h2,'51-3''.22251-3-32256-3-l

+1ffi4-22&-1

TABLE 1. SAMPLE USTAND €TTE LOCATIONS FROM MIDDLE VALLEY Area of Active Venting: active anlrydrite chimneys

Nine active ghimnsys were sampled from the AAVfrom Dead Dog Mound, Heineken Hollow, East Hill,Central Site, Chowder Hill and Inspired Mound.Actively venting chimneys from the AAV typicallyhave a 2-mm-wide clay-rich inner lining with finelydisseminated sulfides, and a thick (>10 cm) outer wallcomposed primarily of anhydrite (Figs. 5A, B, 64, B,C). In order of abundance, the sulfides present in theclay-rich inner wall are chalcopyrite, pyrrhotite, spha-lerite, isocubanite, pyrite, marcasite, galena andarsenopyrite (<LUVo modal abundance combined).Pyrrhotite, chalcopyrite, and trace isocubanite arecharacteristic of the high-temperature vents. Gypsum(and bassanite) are locally abundant in some chimneysampleso but these are secondary products formed bythe hydration of anhydrite following sample recovery.

Clay minerals show micrometric concentric zoningwithin the inner wall (Fig. 6B). Individual bulbouslobes average 45 pm in cross-section (Fig. 6D). Thelayers of clay radiate out toward the fluid channelway,with colloform bands of alternating clay-rich andthinner sulfide-rich bands (Fig. 68). Clay casts ofdissolved lath-shaped anhydrite occur at the contactbefween the inner and outer walls of chimney samFles,within fossil channelways in the outer anhydrite zoneof chimneys and within the mounds @gs. 6C, E). Theclay minerals, identified by X-ray diffraction ()GD),electron-microprobe analyses and transmission elec-tron microscopy (TEM) studies, include a variety ofsmectite-group minerals, dominated by saponite, withless abundant cobalt-bearing smectite, Fe-rich saponiteand Fe-rich smectite (Percival & Ames 1993) (Fig,6B). Chlorite and Fe-rich smectite form the clay-richlining of the highest-temperature vent on ChowderHill Gercival & Ames 1993).

Chalcopyrite is the dominant sulfide phase in thecore of the active chimneys and tlpicaly occurs alonggrowth zones between lobes of Mg-bearing smectite(Fig. 6B). These growth zones suggest fluctuatingchemical or physical conditions (or both), and areindicative of a series of fluid pulses. Chalcopyrite andminor isocubanite are commonly intimately inter-grown with sphalerite, and these phases are interstitialto pyrrhotite plates. Within tle active shimnsyso thschalcopyrite-isocubanite is very fine-grained, typical-\y 2-5 trtm, but at East Hill the chimney containscoarser chalcopyrite, with individual grains up to0.2 mm. Sphalerite and chalcopyrite occur together inthe highest-temperature chimneys, and are mutuallyintergrown. Sphalerite grains in the chimney samplesare opaque owing to a high iron content (see micro-probe data below). Pyrrhotite occurs as blades or laths,typically as a network of randomly oriented, inter-penetrating crystals in open spaces or disseminatedwithin Mg-bearing smectite. The pyrrhotite blades are3 pm to 2 mm in length, but tt?ically attain 100 pm,

sito

CHCH

DDM

DDMHHHHEH

east 0t DDMIMIMCH

BAFITE CBUSTSamplo gne

1-2.1 HH251-2-3 HH

BENT HILL ABEAACNVE CHIMNEYSaffple Slte

2253.1-2 SOUTH

MASSIVE SULFIDESanpls Stte

pssl-1 souTH253-1-3 AOUTH253.14 SOUTH2253.1-5 SOUTH2253-2-1 oDP 856

uuMEueas uog Moum, nFEHen9ren HOW! HEW HU, NO Separde Crumneys,lM.lmplFd [rourds, two separsls chlme]€, CH-Chowder Hlll, SOUTH-$€ Figs.4A,B

Ontario, following the procedures outl ined byHannington & Gorton (1991).

Mnnnalocy aNp PanacsxEsts

Surficial deposits sampled by ALVIN at MiddleValley include active anhydrite-dominant chimneys,extinct barite- and hydrocarbon-bearing chimneys,barite-rich crusts, sulfide talus in sediment and mas-sive sulfide outcrop. Samples studied are listed inTable l, with their mineral assemblages summarizedin Table 2, and textural characteristics are illustratedin Fieures 5-9.

TABLE 2. MINERALOGY OF MIDDLE VALLEY FIYDRO]HERMAL SAMPLES

MINERAL ACTIVE INACTIVE BARITE MOUND SULPHIDECHIMNEY CHIMNEY CRUST DEPOST

elrydritogvpaunpyrhotitef'yfit0ru6llesphaledlechal@pylttob@banltegq9na@rcpyrltebaltebitumgn@rphoua sm&clayohloritoslphosalts6reltrtetron oxld6Pore spa6

mmr

Mm

mq

;tr

r toUmlr

I

i,v

,o

trto mrlo mrtr

: " o

m

: to$

----

: ;- m- u

, lM "

m l o M MM M

m

MMMv

Im

A-abundad (>40 vol. 7d, M-mlor (5-40 o/.) cmlM (1.5olo), tr-tra@ (<t7o),

CHIMNEYS AND MASSTVE SULFIDE. MIDDLE VALLEY 1007

Frc. 6. Scanning electron micrographs of active chimneys at Middle Valley. A. Back-scattered electron @SE) image of a sec-tion across an active chimney. The central orifice is on the upper left. At the contact between the anhydrite-rich (Anh) outerzone and the sulfide-rich lining (bright grains) is a narrow grey zone with saponite (Mg, Si) casts of dissolved anhydritelaths. Sample 2251-l-1, B. Close-up BSE image of the inner lining of the active chimneys, with the fluid pathway to theleft. Note the alternating colloform growth of the radiating saponite and fine grain-size of the sulfides. Sample 2251'2'3.C. Note the replacement of anhydrite by Mg-rich smectite (lvlg, Si) within fluid pathways in the anhydrite-rich outer zone.Sample 2251-2-3. D. Secondary electron image of a plan view of the inner lining of the Dead Dog active chimney. Fony-pm Mg-rich smectite (saponite) lobes with finely disseminated chalcopyrite. Sample 2251-1-1. E. BSE close-up image ofa saponite-sulfide band within the anhydrite-rich outer campace. The saponile forrns casts of anhydrite . Sarnple 2251'2-3,F. Secondary electron image ofeuhedral pyrrhotite (Po) crystals with inclusions ofgalena (ga). Sample 2251-1-1.

1008

ftc. 7. Scanning electron micrographs of inactive chimneys and barite crusts at Middle Valley. A. BSE image of radiatingbarite crystals (Brt) coated with amorphous silica (Si). Sample 22522-1a. B. Hydmcarbons @lack, HC) form betweenbarite (Brt), as inclusions in barite crystals and as secondary inclusions of liquid hydrocarbon. Sample 2255-3-2a.C. Secondary electron image of a fragment from tle barite-rich crust at Heineken Hollow. Note the dense interlockingtexture. Sample 2251-2-1'. D. BSE image of arcuate narrow filaments or tubes forming the nucleus for rosettes of barite@n). Sample 2251-2-1. E. BSE image of veinlets of amorphous silica (Si) that cross-cut the barite-rich altered sediment.In the silica-rich veinlets are foraminifera casts and rectangular rods of silica. Sample 2251-2-1. F. BSE image ofcolloform marcasite (Ms) contemporaneous with barite forming within and between tabular crystals of barite. Sample22511-1a3.

CHIMNEYS AND MASSIVE ST]LFIDE" MIDDLE VALLEY 1009

with aspect ratios (length:width) of 5:1. The highertemperature (n6C) vent at Chowder Hill contains thecoarsest pyrrhotite. Trace amounts of galena occur inthe active chimneys as 2-5 pm inclusions withinhexagonal pyrrhotite (Fig. 6F). Isolated 20-100 pmcrystals of galena also occur along fractures in anhy-drite and are slightly more abundant (l7o modal abun-dance) in the East Hill chimney. Minor chalcopyritemay occur locally along fractures in galena. Traceamounts of arsenopyrite occur in the Mg-bearingsmectite-sulfide inner lining of the active chimney atDead Dog Mound. Grains of arsenopynte are 5 pm indiameter and occur adjacent to pyrrhotite plates and asa fine, ragged open-space growth on pyrrhotite plates.Rare framboidal or cubic grains of pyrite also arepresent.

The porosity in the sulfate-rich outer zone is high,from 25 to 65Vo. dnhydrite is present as L mm to 3mm euhedral tablets and in rare rosettes (Figs. 6A, C).Trace amounts of barite are disseminated within theouter zone as 25 pm radiating rosettes or 70 pmblocky grains in cross-section. Channehvays for thefluid transect 1trs anhydrite zone and are lined withclays, sphalerite and chalcopyrite.

Area of Active Venting: inactive chimneys

Although the AAV hosts numerous vents, there aremore inactive than active chimneys. Inactive chimneysare most abundant on the east side of the soutlern partof the field and between Central Site and East Hill(Frg. 3A).Two inactive barite-rich shimneys from eastof Dead Dog Mound and one inactive chimney southof East Hill were studied. A complete barite chimney(2255-3-2,2A) and altered mud from its base wererecovered (Figs. 3A, 5C). The inactive chimneys arecomposed predominantly of barite and subequalamounts of amorphous silica, and minor clay and mar-casite. They have less pore space (about 20Vo) than theactive chimneys, and are commonly impregnated withhydrocarbons identified as hydrothermal bitumen(Simoneit et al. 1992). The inactive chimneys are con-centrically zoned, with thin barite bands alternatingwith mixtures of bitumen and clay minerals (Ftg. 5D).

Barite is the dominant phase and occurs as rosettes0.7 mm in length and as isolated grains between clayaggregates. Clay minerals occur in open spaces andbefween barite crystals in the inactive chimneys assubrounded, anhedral 1.2 mm blebs and sphericalradiating crystals in 4-mm aggregates. Amorphoussilica forms colloform layers and globular coatings onthe earlier-formed minerals and infills the pore spacebetween the clays and barite (Ftg. 7A).The silica coat-ing is typically 0.02 mm thick. Veinlets of silica alsocross-cut barite. Globules of hydrothermal bitumen, aslarge as 1 mm, occur in fractures between barite, andtrapped inclusions of liquid hydrocarbon are found inthe barite (Fig. 7B). Square cavities in barite, filled

with bitumen, are progtessively smaller, from 100 pmto l0 pm, from the core to the rim of a crystal' Muchof the trapped bitumen has squeezed out along grainboundaries as a result of decompression during ffans-port of the samples to the sea surface.

Area of Active Venting: barite crust

Deposits of barite adjacent to the low-temperatureactive vent in Heineken Hollow form a yellowish cruston top of the hemipelagic sediment @gs. 5E, D. Thecrust consists principally of a porous network of laceybarite filaments with 50Vo open space, 1070 amor-phous silica, trace marcasite, and 5-l0%o altered sedi-ment @ig. 7C). Euhedral barite partly infills the openspaces. Filamentous rods, 3 mm in length, are thenucleus for radiating rosettes of barite (Fig. 7D). Thecore of the barite filaments is commonly brown inplane-polarized light, and contains hemipelagic siltand clays. Some barite has grown on biologicalmaterial. Rare spheres of barite occur in the openspaces between blocky grains of barite. Amorphoussilica occurs as broken rods, semicircular, anhedraland bulbous accumulations and veins. The silicalocally contains casts of foraminifera (Fig. 7E) andapparent tnbular structures that may be the fossilizedremains of vent-related worrn tubes. Trace amounts ofcolloform marcasite occur in the core of barite rosettesand form triangular growths between crystals of barite

Gtg.7D.

Area of Active Venrtng: active nnund;

The hydrothermal mounds that host active chim-neys consist of toppled chimney material andhydrothermally altered sediment in a matrix of prima-

ry hydrothermal precipitates. Much of the sedimentaryand chimney material is replaced by secondary claysand infilled with interstitial silica and sulfide. Thesemounds are the sites of considerable hydrothermalreplacement and overprinting (Fig. 8). The primarycompositions of the earlier hydrothermal products aretherefore difficult to ascertain. Three hydrothermalmounds were studied: Dead Dog Mound, ChowderHill and Central Site.

Hand specimens of altered chimney debris consistof a very porous, interlocking network of yellowishgreen needles of clay minerals that are pseudomorphicafter anhydrite. The secondary clays also form casts as0.1 to 0.2 mm bulbous overgrowths on the original2- to 4-mm-long laths of anhydrite (Fig. 8A). Theoriginal clay-sulfide assemblage, probably from theinner walls of active chimneyso has retained its origi-nal colloform texture, but is more dense owing to laterdeposition of sulfides, with up to 30Vo chalcopyriteand sphalerite as <10 tr"rm grains. Rare laths of alteredpynhotite contain l-pm inclusions of galena.

Silica" Mg-bearing smectite, pyrite, marcasite, gale-

l 0 l0 TIIE CANADIAN MINERALOGIST

Ftc. 8. Textural features within hydrotherrnal mounds at Middle Valley. A. Pseudomorphic replacement of anhydrite laths byMg-rich smectite. Mg-rich smectite also occun as fine bulbous rims on the laths as casts of anhydrite. Transmitted light,sample 2'X24-1c. B. BSE image of secondary silica (Si) and pyrite @y) overgro*ths on fine lath-shaped voids. Note thedissolution features in the pynte. Sample 2252-41b. C. l,ow-temperature phases with barite, colloform marcasite andpyrite rimmed by amorphous silica. BSE image, sample 2252-4-lb. D. Partial to total dissolution of barite (Brt). Casrs ofsilica retain barite morphology. BSE image, sample2252-4-lb,

na and barite are the principal precipitates filling openspaces within the mound. Secondary amorphous silicacomprises up to 25Vo of the interstit ial moundmaterial, commonly cementing the mound rubble.Spheres of amorphous silica form a coating over thelower-temperature phases, such as colloform marca-site, barite and hydrothermal Mg-bearing smectite. Upto L5Vo pyrite is present as anhedral and rare euhedralcrystals and as partially corroded grains (Fig. 8B).Colloform marcasite is present locally (Fig. 8C).Minor colloform bands oi interlayered ilyrite, lariteand marcasite are observed; in most cases, marcasite ispseudomorphic after pyrite. Relict barite may occur inthe late silica, but more commonly the barite is com-pletely dissolved, leaving only casts or lath-shapedvoids lined by silica (Fig. 8D). Radiating laths ofbarite are commp4ly rimmed by pyrite and subse-quently by 0.1-mm amorphous silica.

A push-core sample was coUected at the top of themeuad immediately adjacent to the Central Site activevent, where low-temperature (100"C) fluid was dis-charging (Turner et al. L993). The core containsmineral assemblages typical of relatively low-temper-ature venting; saponite, coarse-grained galena, chal-copyrite, sphalerite and pyrite. The ratio ofbase metalsulfides to iron sulfides is high (Turner et al. 1,993).Saponite, lizardite and a serpentine-smectite mixed-layer mineral form the matrix (Percival & Ames1993). The sphalerite crystals are honey-colored andzoned, with a slightly darker core. Chalcoplrite occursas isolated grains or intergrown with sphalerite.Galena is coarser and more abundant in the CentralSite mound than in any of the other hydrothermaldeposits at Middle Valley. Anhedral and corroded(100 pm) grains of galena are disseminated within thesaponite and serpentine-smectite mixture.

CHIMNEYS AND MASSTVE SI.JLFIDE, MIDDLE VALI-EY t0 l1

Ftc. 9. Textural features of sulfide outcrop at southern Bent Hill. A. kpidocrocite pseudomorphically replaced pynhotitelaths, and secondary marcasite (Ms) and pyrite @y) rim the lepidocrocite. Sphalerite (sph) and chalcopynte (cpy) are inter-stitial to the laths. Reflected-light photomicrograph of sample 2253-l-3. B. Marcasite (Ms) forms casts of pyrrhotite-lepidocrocite laths, with mineral growth along cleavage planes. Reflected-light photomicrograph of sample 2253-1-3o. C.The massive sulfide is denser owing to coarse sphalerite (Sph) and chalcopyrite (Cpy) infilling pore space betweenlepidocrocite laths. Chalcopyrite is partially altered to covellite (Co). ReflectedJight photomicrograph of sample2252-l-3o. D. BSE image showing amorphous silica (Si) in the interstices between lepidocrocite.laths, sphalerite, andchalcopyrite grains decreases the pore space. Sample 2253-l-1. E. The massive sulfide, denser owing to s6ar'seningsphalerite and chalcopyrite and cementation by amorphous silica and marcasite, is cross-cut by veins rich ip silica (Si) andgalena (Ga). BSE image of sample 2251-L-3. F. Pb-As-Sb sulfosalts (Ss) associated with silica occur at the outer oxidizedrim of the massive sulfide outcrop. The outer rim is cornposed of barite, marcasite, Fe-oxides (FeO) and amorphous silica.BSE image 2253-l-3o.

1012

Bent Hill: suffide outcrop

The sulfide samples from the south flank of BentHill differ significantly from tle active chimneys inthe AAV. Much of the sulfide outcrop consists ofmassive pynrhotite, lepidocrocite, pyrite, marcasite andsphalerite, with minor chalcopyrite, covellite, galena,silica and barite. Unlike the active chimneys in theAAV, where pyrrhotite is the dominant Fe-sulfide,samples from the sulfide outcrops at Bent Hill arecomposed of pyrrhotite-pyrite. The massive sulfidesare dense (<l1%o pore space), with Cu and Zn sulfidesand late amorphous silica. cementing early pyrrhotite.In addition, the sulfides show evidence of extensiveiecrystallization and coarsening, secondary replace-ment, and seafloor Weathering, consisteirt with theapparently older age of the deposits. These depositshave apparently been affected by multiple generationsof hydrothermal fluid.

Original massive pynhotite (up to 35 vo1.7a) fromthe sulfide outcrop comprises a network of coarse-grained (50 pm), interpenetrating blades or laths, com-monly with a rim of pyrite. The oxidized remnants ofpyrrhotite-rich sulfides (Fig. 9A.) consist of fibrouslepidocrocite. At the outer edge of the outcrop, thepyrrhotite has been completely destroyed, leavinglath-shaped voids rimmed by amorphous silica ormarcasite (Frg. 9B). Pyrite is preserved in the inter-stices of the remnant laths of pyrrhotite.

Coarse sphalerite (0.25 nm) occurs between thepyrrhotite laths and is generally opaque. Chalcopyriteoccws as isolated grains and as inclusions with deli-cate serrate edges within sphalerite (Fig. 9C)."Chalcopyrite disease" is common. Chalcopyrite inthe massive sulJide is coarser (0.1-0.3 mm) than thatprecipitated in the inner lining of all of the activechimneys and is more abundant (57o modal abun-dance). Covell ite occurs locally as a product ofsecondary replacement of chalcopyrite and formsrandomly oriented crystals adjacent to coarse spha-lerite and chalcopyrite and within sphalerite (Fig. 9C).Late amorphous silica (25-35Vo modal abundance)forms 10 pm colloform coatings on the sulfideminerals and cements the massive sulfides (Frg. 9D).Silica also occurs in late cross-cutting veinlets. Galenais spatially associated with late-stage silica within themassive sulfides and is commonly corroded (Frg. 9E).

Samples of the massive sulfide mound and asso-ciated sulfide talus are mineralogically zoned, with anouter rim of goethite, barite, galena, marcasite, amor-phous silica and Pb-As-Sb sulfosalts. This assem-blage is believed to have been associated with a stageof low-temperature venting that reworked the massivesulfide mound. The barite typically forms coarse, radi-ating overgrowths on the massive sulfides, togetherwith abundant colloform marcasite and coarse(50-100 pm), euhedral galena. Amorphous silicacommonly forms coatings 0.5-l mm thick on the

barite and locally contains fine, 1-5 pm inclusions ofsulfosalts (Fig. 9F).

Most of the mineral textures in the sulfide depositsat Bent Hill are secondary, and much of the originalpynhotite has been pseudomorphically replaced byiron oxides and sulfides. Extensive reworking is illus-trated by late veining, infilling of pore space by coarsesphalerite, chalcopyrite and abundant silica, partialand total replacement of pyrrhotite, corroded galena inthe massive sulfide and euhedral galena in silica veins,and the distribution of Pb-Sb-As sulfosalts on theouter rim of the massive sulfide samples. Oxidation ofchalcopyrite to covellite and local replacement ofpyrrhotite by pyrite and marcasite also suggest thatmore than one generation of fluid has percolatedthrough the sulfide deposit.

Bent Hill: active chimney

A single chimney venting fluid at 264oC occurs onthe southernmost sulfide outcrop (Figs. 44, B). Asample from the active chimney resembles those fromthe AAV, with a thick outer wall of anhydrite and athin inner wall of clay, and disseminated grains2-20 trtm across ofpyrrhotite, chalcopyrite, sphaleriteand trace barite (sample 2253-l-5, Fig. 4B). As in theAAV, the active chimney appears to be a product ofrelatively sulfur-depleted solutions. The presence ofan active vent of this type on top of the sulfide outcropsuggests that the massive sulfide deposits south ofBent Hill are currently being flushed by hydrothermalfluids similar to those ventins in the AAV.

MTNERAL "oior,.ro*.

Electron-microprobe analyses of silicate and sulfideminerals were obtained for the various hydrothermaldeposits in Middle Valley. The results for sphaleriteare shown on Figure 10; results of analyses ofpyrrhotite, isocubanite, chalcopyrite, galena and bariteare listed in Tables 3 to 7, respectively. Clay-mineral

TABLE 3. ELECTRON MICROPROBE DATA ON PYFRHOTITE FROM MIDDLEVATLEY

Ac{vg chlmngyDoadDogMound 61.85 0.01 0 0.02 0.08 37.18 99.14251-1"1ae10

Suhhldo otncopslts856 60.80 0.02 0 0.m 0.76 37.95 9956n 3.2-1dn-3

CHIMNEYS AND MASSIVE SI'LFIDE. MIDDLE VALLEY 1013

analyses from the active chimn"y, unJ frbm onemound are discussed in Percival & Ames (1993), andthe clay compositions obtained from the shallow sedi-ments close to active vents are listed in Tvrter et al.(1993).

Three distinct populations of sphalerite are ap-parent, based on iron contents (Fig. 10). Iron-richsphalerite (3547 mole Vo FeS) is associated withpyrrhotite (t isocubanite) in the inner lining of theactive chimneys from the AAV (e.g., Chowder Hill,Dead Dog Mound, Heineken Hollow). Low-iron spha-lerite (<15 mole Vo FeS) is associated with marcasiteand galena in the active mound carapace at the CentralSite and at East Hill. This assemblage is interpreted tohave formed at a relatively low temperature and maybe related to the current phase of venting or an earlier,lower-temperature phase. Pyrite-pyrrhotite assem-blages in sulfide talus south of Chowder Hill and in

MIDDLE VALLEY

the Inspired Mounds chimneyd'contain sphalerite withmoderate levels of iron, i.e.,24-33 mole Vo FeS. Thisassemblage resembles material ftom the sulfide out-crops at Bent Hill, which contains coexisting pyrite,marcasite, and lepidocrocite (formerly pyrrhotite).Sphalerite from the sulfide outcrops at Bent Hill alsocontains '24-33 mole 7o FeS. The laree variation insphalerite composition in different pirts of MiddleValley reflects the large temporal and spatial variabili-ty in fluid compositions. In most seaflooi deposits,iron-rich sphalerite typically forms at high tempera-tures and low a(Or) and a(S); sphalerite with loweriron contents forms at lower temperatures and fromrelatively more oxidized solutions (Hannington &scott 1989).

Pyrrhotite compositions were determined in sam-ples from Dead Dog Mound (AAV) and Bent Hill(Table 3). Pyrrhotite from the active vent on Dead

l--l sph-cpylffil sph-mc-cpy

ffi mosphqy-pyNI sph-pyElffil sph-ga-mc

@ sph-pon=57

5 10 15 20 25 90 35 40 45 50 55 60 65

Mole 7o FeS in Sphalerite ,Ftc. 10. Histogram of mole Vo FeS in sphalerite andFe-Zn sulfides from Middle Valley.

The associated sulfide phases are noted. Symbols: sph sphalerite, cpy chalcopyrite,mc marcasiG, py pyrite, po pynhotite, ga galena.

TABLE 5. ELECTRON MICBOPROBE DATA ON CTIALCOPYRIIE FROM MIDDLEv llEY

Pb Nl Co Mn Zn As S TotalTABLE 4. ELECTRON MICFOPROBE DATA ON ISOCUBANITE FFOM MIDDLEVALLEY

Pb Nl Co Mn 7n As S Tolal

Dftus verdLEChm Area-Mound252.+1c 30.93 92.37 0 0.02 0.02 0 0.47 0.10 33.54 97.45

Acuve chhnnsyDead Dog Mound451-1-1a 19.08

Sulphlde orncrcpSodh ot B€it Hll2253-'l-3o 19.66

4.58 0 0 0 0..10 122

4.8 0 0.09 0.07 0.05 1.72

Acdvo chlmneylFplred Mound

0.05 34.60 98.63 ?25542a 30.93

MoundCentral Sho2255-S1PTI 33.98

0.27 35.00 97.34 33.90

28.61 3.21 0 0.03 0.10 2.03 0.90 32.50 98.31

m.44 0 0.01 0.07 0 0.31 0 34.37 98.18s0.04 0 0.01 0 0.02 0.27 0 u.a sa.47

6mntmllore

1014 THE CANADIAN MINERALOGIST

Dog Mound contains 48.3-50.0 at.Vo Fe, and that fromthe sulfide outcrop at Bent Hill contains 47.347.9at.VoFe. The different compositions of pyrrhotite fromthese sites are consistent with bulk mineralogy (i.e.,pyrrhotite-dominant assemblages at AAV versuspyrrhotite-pyrite assemblages at Bent Hill) and theiron content of associated sphalerite.

Isocubanite in the high-temperature chimneys fromthe AAV and in the sulfide outcrop at Bent Hill isclose to stoichiometric CuFe2S3 Gable 4). It is com-monly in intimate association with pyrrhotite andchalcopyrite, and generally occurs together with high-iron sphalerite (e.g.,46 mole Vo FeS in sphalerite froma pyrrhotite - sphalerite - isocubanite chimney onDead Dog Mound). Textural relations among thesephases suggest that Cu-Fe sulfide was precipitated asan intermediate solid-solution (/ss), which has subse-quently exsolved chalcopyrite and isocubanite. Thehigh Zn(S) content in isocubanite may have been pre-sent in solid solution in the original high-temperature/ss (Scott 1983).

Chalcopyrite from the AAV is stoichiometricCuFeS2, with traces of Zn, Co and As (Table 5). Thehigh levels of Zn and Pb in sample 22554-2abkelyreflect the presence of microscopic inclusions of spha-lerite'and galena, which are common in chalcopyritefrom this site.

Galena from one sample in the AAV contains nofraces of As, Sbo Ag, Bi, or Cu (Table 6). However,qualitative analysis of the sulfosalt phase, present infiace amounts on the outer crusts of sulfide samplesfrom the Bent Hill, was possible with the use of asganniag electron microscope with energy-dispersionspecfrometry. It was found to contain Ag, As and Sb.

Barite from a 254oC anhydrite chimney near EastHill contains only 0.64 wt.7o SrO (Table 7). In inac-tive chimneys and barite-rich crusts from the AAV,barite contains 1.43-1.57 wt.7o SrO. Barite fromsulfide talus in the Bent Hill area is similar in compo-sition to barite from the inactive chimneys and barite-rich crusts in the AAV. Goodfellow & Blaise (1988)noted a wide range of Sr contents (up to ll.6 wt.VoSrO) in barite from shallow, 2-m-long piston cores inthe Bent Hill area. The variability of SrlBa ratios inbarite from Mddle Valley may reflect variable contri-butions of sediment-derived Sr during hydrothermalventing (e.9., Campbell et al. L992).

TABLE 6. ELECTRON MICROPROBE DATA ON GALENA FROM MIDDLE VALLEY

Csrtral Slte Mound, 225&6-1Pf1, n-12

MEAN S.D. MEAN S.D

TABLE 7. ELECTRON MICROPFOBE DATA ON BAFITE FROM MIDDLE VALI.EY

BaO Slo CaO MnO MgO FeO SOg Total

Ac{vo Chimy2255-618 66.15 0.64 0.17 0.12 0.05 0.05 32.57 99.75n-g

Badtg crust251-2-1A 66.66 l.t 0.56 0.10 0.03 0.02 31.64 100.58n4

Baito chlmoy4+A-1 63.89 r.43 0.O7 012 0.M 0.07 33.75 99.36F l5

sllphlde lalus253"1"1 69.52 1.81 0.05 0.09 0.06 0.11 33.76 99.40F l 0

GrocnBurrnv

Whole-rock and trace-element chemical data forMiddle Valley chimneys, barite crusts, massive sul-fides and altered sediment are listed in Tables 8 and 9.Active chimneys and barite-rich inactive chimneysand crusts have low metal conlents, owing to the abun-dance of non-sulfide phases (anhydrite, barite, silicqclays). Active chimneys in the AAV and Bent Hill aredominated by anhydrite, amorphous silica, and clays(98 w%o). Chemical analyses of the inactive chimneys

1ABLE 8, GEOCHEMISTRY OF ACTI1/E AND INACTIVE CHIMNEVS. BAFITE CFUgfAND ALTERED MUD IN THE AAV. MIDDLE VALLEY,

ACTIVE CHIMNEYS IN^CTIVE BARIIECHIMNEY CNUST

g[o BDM DDM lM HH EH CH @et ot llHDDM

Mt,T'

8Sl OlDDM

T.qc 268 2S

SstrFl€l2*r 1.1.1A 1"1"1

$n 7.)gtq 854 s.72T1O2 <tt.02 <002A/4 0.27 4.25Fo2OoT 0.52 059MnO 0.01 0.0two 4.12 3.01CaO 37.0 gg.1Na2O 0,49 0.i{6K2o 0.11 0.r2

8 212 21.6

Ar(ptb)g 4Ba{ppn}120 214A S 4 . 2Cu 1300 ll007a A0 ,|4OP b ? a € €As 1,2 t.4sb 03 0.3So 2 2.9B 9,3 8.4Be 0,6 0.6Co <5 <5Cr <10 d0M <t0 doSr 1700 1500v 4 €Y t 1v < x 4 t

642 t-2-3 l"Vz

286 735 621<0.02 <0.@ <t.020.05 0.34 0270.92 0.72 0.580.01 0,a2 0.01't.44 4.36 25440.0 375, 47.7o.ar a& 0.570.111 a.12 0.12

a.2 ,fu p.o

274

24&.1 6.9-2 1-2.1b

1,17 8,42 75.0<0.02 <0.m 0.140.02 0.7a 271353 4 |80.01 0r2 0.020,65 0.1i 0.8437.6 034 2370.45 0.45 1.14036 0.17 053

- 4.A0 0.3023.9 12.'14 3.61

gt30.438.354250.tg2501,213.410630.@1.01

lg ao 7 81 335 S 859650 100 170 370 387c rA 2.7t/"4 4 . e . e 1 2 5 4

'f800 t60o 1100 210 17 78 l5o16.{1 400 570 1500 2 !l 87700 50 130 7522 22 17 U7 0.7 1.8 I 8 33.4 56.7

3.3 0.2 0.5 2.2 6.5 3 S.415.6 2.9 43 11 29A 4.4 629.1 8.7 5.3 " 5.3 302 S.00.7 01 0.6 <0.6 0.70 1.2 1,7< 5 < 8 < 5 < 5 7 7 1 4<10 <10 <10 <'t0 <10 20 59<10 <10 <J0 <10 <10 {0 <101400 1700 1500 1700 s00 2300 s0< 6 < 5 < 5 < 5 9 4 4 9 02 2 < 5 < 5 < 5 < 5 1 9

"2i <20 <20 40 4A n 74

Pb 85.@ 0.56 Bt 0.02 0.03Ag 0.04 0.02 Cu 0.08 0.19As 0.02 0.02 Fo 0.14 0.10sb 0.05 0.03 s 13.40 0.11

Total W.44 CH-Chodds HU AfT-Ano@d.Analyg* dore bV Gslo!fial slngy ot Canada labordodE q@pt Au; by NsuuonArdvdon Anaiy€8, RMC, Kh€Fton.

TABLE 9. GEOCHEMISTRY OF ACTIVE CHIMNEY. SULFIDE DEPOSIT ANDALTERED MUD IN THE BENT HILLAFEA, MIDDLE VALLEY

CHIMNEYS AND MASSIVE SLTLFIDE. MIDDI-E VALLEY 1015

The bulk gold contents of active chimneys in theAAV are uniforrnly low, with a maximum of 80 ppbAu. Gold concentrations in these samples are dilutedby the abundant non-sulfide phases, and gold concen-trations within individual sulfides could be as much astwo orders of magnitude hrgher (i.e., ppm concentra-tions). Bulk gold contents in the barite-rich chimneymaterial and barite crusts are 335 and 80 ppb Au,respectively. Four sulfide-rich samples from the BentHill area average close to 1 ppm Au, which indicateslocally significant enrichment of gold. One sample ofaltered sediment at the base of the inactive chimnevfrom the AAV also is notably enriched in gold(859 ppb). Within a hydrothermal mound at rheCentral Site, the gold concentrations are very low(12-19 ppb). Gold is concentrated in low-temperaturesilica-rich crusts on the massive sulfide deposit (sam-ples 2253-L4 and 2253-L4a), in sul{ide talus and atthe contact between altered sediment and inactivEchimneys (samples 2255-3-2 and, 2255-3-2a). Goldis strongly depleted, and concentrations are lowest, inthe massive sulfide at depth (Franklin, unpubl. data);levels also are lower in the inactive barite-enrichedchimney than in the altered sediment at its base, andlower in the core of massive sulfide samples than inthe crust of tle same samples.

Altered muds adjacent to tle mound contain signi-ficant TiO2 and Zr. The barite crusts also containsignificant TiO, and measureable Zr, lsflesting theirgrowth through the replacement of sediments thatsurround the active mounds. In contrast, the activechimneys in the AAV and the sulfide outcrop at BentHill contain no TiO, and less than 5 ppm Z. The lowTiO, and Zr content of the sulfide outcrop at Bent Hillsuggests that this material did not form by the replace-ment of the surrounding sediment.

The bulk Sr concentrations in hydrothermal precipi-tates throughout the AAV are fairly uniform(1300-3300 ppm Sr), whereas Ba concentrations andSr/Ba ratios vary by several orders of magnitude. Thebulk Sr/Ba value of active chimneys in the AAV ismore than 100 times geater than in the inactive chim-neys or barite-rich crusts, which reflects the absenceof barite and the high Sr contents of anhydrite in theactive vents. Bulk Sr/Ba ratios in barite-rich sulfidesfrom Bent Hill also are much lower than in activevents from the AAV.

DrscussloN

Fault control on the deposits

The importance of fault control on the localizationof massive sulfide deposits is well documented in bothmodern deposits [Gorda Ridge: Rona & Clague(1989); Axial Seamount: Chase et al. 1985)l and, \nancient deposits, where three-dimensional relation-ships can be determined. Kuroko-type deposits'(Scott

ACTIVE MASSIVESULFIDECHIMNB'fSorih Talus Oulorop Oror O.nmpBsntHl[ Crust

SEDIMENT

Top olBern Hlll

Terp.oc 2u

Sample(zsgt 1-5 l-l 14 1.4a

(w1.7,)sloz 0.33 68.5noz <0.02 <0.02Alzoa o.02 - 1.nFo2O3T 3.53 1Ag 12.9 18.6'MnO 0 .01 - 0 .11MSO 0.65 0.16CaO 37.5 0.16Na2O 0.45 0.47 0.65 0.55.KzO 0.06 - 0.14c - - 0 . 2 0s 23.8 - 10.83

Au(ppb)Ba(ppm)ASCuZiPbAssb8eB

CrNISr

Zr

3500.31%140

10500

;

1.73<0.020.0041.5 0.1i'0.030.080.090.@ 0.33'0,'1702039.40

3890 3351"/o 47.e/"32 201800036000 <50'150

6.2 9.860.2 121 0 .1

Au; by Neutmn Adlvallon An€lysos, RMC, Kingslon. Padal analyses abo by NAA' - elsmgntal iorm (Fs, Na).

also reflect the major mineral phases, with up to38 wt%o Ba present as barite, 8.0 wtVo SiO, as amor-phous silica, and up to 4.3 wt%o C as secondary hydro-carbons. The high Mg and Al in a number of samplesreflect the local abundance of clay minerals. Despitelow metal contents, the presently active chimneysfrom the AAV and massive sulfides from Bent Hill areclearly distinct on the basis of bulk Cu:Zn ratios(Fig. 1). Sulfides from active chimneys in the AAVhave high average Cu:Zn ratios of about 5:1, whereassulfides from the older Bent Hill deposits have aCu:Znratio of less than l. The inactive chimnevs andbarite-rich crusts in the AAV also have low bu:Znratios. Pb(Cu+Zn) ratios in the Middle Valley chim-neys are generally higher than in massive sulfidesfrombare-ridge deposits (Fig. 11).

Significant levels of Ag, As, and Sb are present in afew of the low-temperature sulfide assemblages fromBent Hill, with maximum measured concentrations of140 ppm Ag, 120 ppm As, and 96 ppm Sb. One sam-ple of the low-temperature, SiOr-rich crust containingPb-As-Sb sulfosalts contains 850 ppm As and200 ppm Sb. Although bulk As concentrations are lowin the active chimneys, trace arsenopyrite was identi-fied in one samFle.

300?% 0.4'1vo13 365't -600 1860cbu

119 850952 2062.1 2

370

2101500

42.21 1

< 0 . 5 - 0 . 1 - 0 . 1< 5 < 5 < 5 < 5 9 < 5< 1 0 - < 1 0 - < 1 0< t 0 - < 1 0 - 1 31 7 0 0 - 2 7 0 - 6< 5 - < 5 - < 5< 5 - < 5 < 5< 2 0 - 4 0 - 4 0

58.50.82

9.270.10

37.73.01'2.970.200.09

0.78/"

4180

0.8<0.289.82.1i o

9872201202.130

u \

a:J-x t ?

1016

1978, 1980) and Cyprus deposits (Adamides 1980,Constantinou 1980) occur at the intersection oforthogonal set$ of fractures. The importance of syn-volcanic faults in controlling the location of basemetal deposits has also been emphasized in theCanadian Shield, e.g.,in the Sturgeon Lake caldera(Morton et al. 1990), the Noranda Camp (Gibson &Watkinson 1990, Knuckey et al. 1982) and the SnowLake massive sulfide deposits (Galley et al. 1990).

Clear fault-structures are associated with the AAVand Bent Hill areas (Fig. 2). These deposits are local-ized at apparent offsets in the trace of the faults, whichmay be a setting analogous to the orthogonal sets offractures, although no cross structures have beendefined. The fault that extends to the south of the

THE CANADIAN MINERALOGIST

MIDDLE VALLEYa Actlve ahlmnsy Inactlve ohlmngyt sulfldEouicrop

GUAYMAS BASIN

o Actlve chlmnoyA lnacttve chlmnEytr Mound

ESCANABATROUGHx Polym€talllc sulf,dg+ Pyrrhotlte-rlchsultldo# FragmErilal sulffda In sdlmgnl

* Polymelalllc sulfldg camonl

Zn

v Axlal seamounto East Pactflo RlsE, 13" Na EastPacf lcBlse,2l"NI Explorer RldgeA Juan ds Fuca RldgoI Galapagos Rtft

Flc. 11. Cu-Pb--Zn diagram showing modem sediment-hosted massive suLflde depositsand massive sulfide deposits on sediment-free ridges. Fields are marked for MiddleValley active chimneys and most Guaymas Basin samples. Data sources for the sedi-mented ridges: 1) Guaymas Basin @eter 1986);2) Escanaba Trough (Koski ar a/., inpress). Data sources for the bare ridges are): 1) Axiat Seamount ft{annington 1986),2) EPR, 13'N (Fouquet er a/. 1988), 3) EPR, 21'N @ischoff er a/ 1983)' 4) ExplorerRidge (Hannington 1986), 5) Juan de Fuca Ridge @ischoff er aI. 1983)' 6) Galapagos(Embley et al. 1,988).

ZnPb

southwestern corner of the acoustic anomaly asso-ciated with the AAV area (Fig. 2) was intersected inODP hole 857D. This fault has exceptional pennea-bility (down-flow rates of up to 10,000 liters perminute were measured), thus providing an adequatestructure for transporting hydrothermal f luid.However. the AAV site also is coincident with aburied volcanic edifice that appears from drilling (site858) and seismic data to be a seamount-like structure,perched on the paleo-edge of the deepest part of theMiddle Valley structure. This buried seamount is anatural bamier to stratal permeability, providing animpediment to lateral movement of fluid that istrapped in the turbidite-rich lower sedimentary sectionat Middle Valley. Laterally moving fluids are forced

CHIMNEYS AND MASSIVE SI]LFIDE,.MIDDI.E VALLEY 1.017

up along the margins and continue their ascent fromits peak. Hydrothermal fluids ascend to the seafloorthrough a combination of faults and diffirsional upflowthrough the sediments, as indicated by pore-watercompositions (-ydon et al.1990).

Growth of the maunds and chimneys

The surficial hydrothermal mounds in the AAVgrow through inflation by precipitation of hydro-thermal and alteration minerals, through mixing ofhydrothermal fluid and seawater, and from trapped,conductively cooled hydrothermal fluid beneath animpermeable cap of alhydrite. Large, angular slabs ofsediment on the edge of the mounds indicate thatmound growth displaced, as well as replaced, thesediments.

Preliminary analysis of heat-flow data obtainedalong a systematic ALVIN traverse from the easternedge of the acoustic reflector due west toward DeadDog Mound, with a one-meter temperature probe,indicates that the highest temperatures (18'C) arefound away from the mound, and cooler temperaturesprevail as the active vent site is approached (8'C at thebase of the mound; P. Johnson, pers. comm.). Thisfinding suggests that mounds are self-sealing as aresult of the formation of an anhydrite cap, hydro-thermal precipitates and the alteration of the sedi-ments. Penetration of seawater into the top of themound contributes to the formation of anhydrite, Mg-rich smectite (saponite) and serpentine-group mineralsin the outer crust (Percival & Ames l993,Tumer et al.L991,1993). The original pore-space in the sedimenton the flanks of the mound have been infilled bylo'trer-temperature secondary minerals such as amor-phous silica, saponite, barite, marcasite and galena.The saponite casts of anhydrite observed in themounds remain after dissolution of anhydrite. Thus,the upper layer of the mound may become sealed,thereby inhibiting the ingress of cold seawater andpossibly trapping hydrothermal fluid beneath.

The presently active chimneys and vent fluids(Lydon et al.1992) have low metal contents; however,ttre mounds below contain significant levels of basemetals (Turner et al. 1993). This finding suggestspossible depletion of the vent fluids by subseafloorprecipitation. Fluid inclusions measured in anhydritefrom the active vent at Bent Hill give temperatures ofhomogenization (265"C) that correlate with measuredtemperatures of the vent solutions (264"C) (Leitch1991), as is common in many active sulfate-rich vents(Kusakabe et al. 1982, LeBel & Oudin 1982). Thelack of abundant sulfide minerals in the active chim-neys indicates that the chimneys formed primarily byquenching in seawater of moderate-temperaturehydrothermal fluid with a low metal content. Seawaterentrainment resulted in the formation of abundantsaponite forrning the inner wall of the chimneys.

The active chimneys are barium-poor, whereas theinactive chimneys and crusts at Middle Valley arebarium-rich. Similar observations have been made atmany areas of venting, including Axial Seamount,where lower-temperature vents are barite-rich @eelyet al. 1990). In the inactive barite-rich chimneys atMiddle Valley, temperatures of homogenization are170'C in primary inclusions in barite and 110'C insecondary hydrocarbon-rich inclusions (Leitch 1991).In high-temperature vent systemso barite remains insolution, and crystallizes in the water column (Feely eral. 1990). However, barite accumulates in lower-temperature systems typical of the waning stages ofchimney gowth.

Hydrothermal sediment and precipitates in theCentral Site mound formed as a result of collapse ofactive chimneys, remobilization of constituents withinthe insulated mound, and direct precipitation from thehydrothermal fluid. Textural features such as corrodedgalena and pyrite, zoned sphalerite and recrystallizedsulfides suggest that dissolution, remobilization andprecipitation of the metals are important processes inthe forrnation of mounds. Erosional processes, such asmass wasting of the chimneys, also are an importantpart of the development of the mounds. Turner et al.(1993) clearly documented chimney fragments in themound talus. Bacterial mats on the mounds may pro-vide a reducing sulir-rich subsffate, which favors theprecipitation of metal sulfides (Jonasson & Walker1987). The presence of bacterial mats also might aid inthe development of the constructional mounds.

Implications of the sulfide assemblages andmineral compositions

In general, the variety and nature of sulfide assem-blages from Middle Valley resemble those associatedwith hydrothermal activity in sediment-hosted ventfields at Escanaba Trough and at Guaymas Basin(Koski er al. L988, Peter & Scott 1988). Observationsat Bent Hill suggest that the mineralization here mostclosely resembles the sulfide deposits at EscanabaTrough, where massive sulfides outcrop along themargins of several uplifted sediment-capped hills. Thesulfides at Escanaba consist of massivepyrite-pynhotite assemblages, together with low-tem-perature polymetall ic sulfide and barite crusts.Limited, active venting at temperatures less than220oC occurs from sulfide chimneys on the massivesulfide mounds (Koski er al. 1988, Koski et al., inpress). Mineralization in the AAV is most similar tothat due to the presently active hydrothermal vents inthe Guaymas Basin. Mineralization is hosted at bothsites in sulfate-dominant chimneys that vent greyish,moderate-temperature fluids and contain only minorsulfides (a.g., Koski et al.1985, Peter& Scott 1988).

Three distinct mineral assemblages are present atMiddle Valley. In the AAV, moderate-temperature

1018 THE CANADIAN MINERAIOGIST

N - 1 2acl

(276"C) active vents are composed of ananhydrite-smectite assemblage in which pyrrhotite isthe dominant sulfide phase. Lower-temperature vents,inactive chimneys, and hydrothermal crusts consistdominantly of barite, amorphous silica, and clays, withpyrite and pyrrhotite present in varying amounts. Theinactive chimneys are notable for the presence ofabundant bitumen. The sulfide deposits in the BentHill area comprise a pyrite-pyrrhotite assemblage withabundant copper- and zinc-bearing phases. These rela-tions reflect a wide range of fluid compositions, fromrelatively high-temperature (ca. >300'C), metal-richand sulfur-bearing fluids that formed the sulfidedeposits in the Bent Hill area to lower-temperature(<300"C), metal-depleted and sulfur-poor fluids thatare currently venting from chimneys in the AAV. Theactive vents in the AAV appear to tap fluids derivedfrom the low-temperature interaction of seawater withthe sediment filI in Middle Valley. They have a char-acteristically high pH (5.1-5.8) and are locallycharged with bitumens derived from the thermalcracking of organic carbon in the sediments. In con-

o,o

250 350TEMPERATURE ('C)

Frc. 12. T--a(S) diagram showing the relative compositions of sphalerite in hydrothermalprecipitates from Middle Valley (Area of Active Venting, Bent Hill), Guaymas Basin,and 21oN, East Pacific Rise. Average mole 7o FeS contents only are shown; data axefrom Zerenberg et al. (1984), Peter & Scott (1988), and this study. Contours of mole7o FeS are based on data from Toulmin & Barton (1964), Scott & Barnes (1971), andCzamanske (1974). Sphalerite from the Area of Active Venting and Guaymas Basinformed at low a(S) conditions; sphalerite from Bent Hill and the Escanaba Troughformed at intermediate values of a(S), close to the pyrite-pyrrhotite boundary.

300

trast, the mas$ive sulfides at Bent Hill appear to be theproduct of either an earlier stage of the presenthydrothermal event related to high-temperature inter-action of circulating seawater with basaltic crust in thebasement, or a completely different, unrelatedhydrotherrnal system.

The broad range ofFeS contents in sphalerite fromthe Middle Valley sulfides (8-60 mole Vo FeS) reflectsthe wide range of conditions of sulfide precipitation.The presence of high-iron sphalerite from chimneys inthe AAV suggests a fluid composition buffered to lowa(S) (i.e., in equilibrium with pyrrhotite: Fig 12)'Although no sphalerite data exist for the inactivechimneys, the presence of abundant hydrocarbonsOitumen) also attests to a low a(Or) and a(S) in thefluids. Similar conditions of precipitation are sug-gested by the composition of sphalerite from depositsin other ssdimented envitonments (e.g.,22-55 mole VoFeS in Guaymas Basin chimneys: Peter & Scott 1988).Sphalerite from these deposits typically is more iron-rich than sphalerite from bare-ridge deposits. Forexample, the sphalerite recovered from sites at

CHIMNEYS AND MASSIVE S{ILFIDE. MIDDLE VALLEY 1019

Explorer Ridge and Axial Seamount on the Juan deFuca Ridge and from TAG and Snakepit on the Mid-Atlantic Ridge contains less than 30 and usually lessthatr 10 mole Vo FeS (Hanningto\ et aL 1991). Themost Fe-rich compositions of sphalerite in bare-ridgedeposits are associated with high-temperature fluids inpyrrhotite-rich black smoker chimneys (e.g., 2LoN,EPR: Fig. 12). The high iron contents in sphaleritefrom the AAV at Middle Valley and the GuaymasBasin reflect the strong buffering of fluid composi-tions [e.9., low a(Or), low a(S), and low Fe and Sconcentrationsl by local interaction with organic-matter-rich sediments. Pb and Sr isotopic data alsoindicate a strong component of a sedimentary sourcecomponent (Goodfellow & Franklin, in press). Theiron contents of sphalerite from the southern Bent Hillmassive sulfide deposit are much lower than thosefrom the AAV active chimneys, reflecting precipita-tion from a fluid with a significantly higher a(S) (1e.,

A,_l=:=_r-*

BENT HIU MAGNET]TE

HrSl - 1s'2

:tl;;H,silF::::\- \. .PYRRHOTITE

. . . . \

*' n"#Fnffi,*a ' '\.\

close to pyrite-pyrrhotite). Sphalerite from sulfidedeposits at Bent Hill, is similar in composition tosphalerite from Escanaba Trough (averaging close to27 mole Vo FeS:. Hoyt L992), which suggests a fluidsource intermediate between sediments and basalt.

Similar trends arc observed for the compositions ofpyrrhotite from different deposits. Pynhotite composi-tions ranging from 48.3 to 50.0 at.Vo Fe were found inthe pyrrhotite -r sphalerite t isocubanite assemblagesfrom moderate-temperature chimneys at Dead DogMound (AAV). These compositions indicate a lowa(S) (e.g., Toulmin & Barton 1964) and are con-sistent with the high iron content of coexisting spha-lerite @g. 12). Fyrrhotite compositions in both sulfideoutcrops at Bent Hill range from 47.3 to 47.9 at.Vo Feand are consistent with a higher a(S). Similar compo-sitions of pyrrhotite were found in massive sulfidefrom the Escanaba Trough deposits (averaghg 47.3at.Vo Fe: Hoyt 1992).

Frc. 13. A. pH-a(O) diagram showing the fields of stabilityfor the principal Fe-sutfldes and oxides at 300'C and totalreduced sulfur concentrations of 1f2 M and 10-3 M H2S.Sulf ides from Bent Hil l comprise a dominantlypyrite-pyrrhotite assemblage that formed at relativelyhigh a(Ot from sulfur-rich hydrothermal fluids. Activechimneys in the AAV formed at low a(O) and containpyrrhotite as the dominant Fe-sulfide. At low concentra-tions of total sulfur (a.g., 10-3 M H2S), the stability fieldsof the principal Fe-sulfides collapse, and magnetite willform at the expense of pyrite and pyrrhotite. Drillingbeneath the Bent Hill deposits suggests that massive sul-fides at depth have been affected by a recent flux of low-sulfur fluids similar to those presently venting in theAAV. This has resulted in significant recrystallization ofthe sulfides and replacement by late-stage magnetite.Equilibrium constarts are calculated from Barton (1984)and Bowers et al. (1984). B. The solubility of chalcopy-rite [CuCf] and sphalerite lZncl2"l as a function of pHand a(O) at 300'C and 10-2 M H2S. The solubility ofchalcopyrite at low a(O2) is significantly greater than thatof sphalerite in the same solutions, suggesting that ventfluids in the AAV may be enriched in copper reladve tozinc. Such fluids may effectively leach copper from sub-surface sediments. This could account for the depletionin copper in subseafloor sediments in the AAV (seeFig. l4). Equilibrium constants are calculated fromBarton (1984), Bowers et al. (1984), Crerar & Barnes(1976), and Ruaya & Seward (1986).

-35

N

d

o

pH

s=-= jBr : :

1020 TI{E CANADIAN MINERALOGIST

The mineralogical data suggest that massive sul-fides at Bent Hill formed from hydrothermal fluidssimilar to those cunently venting in the EscanabaTrough. Total concentrations of dissolved sulfur werelikely between 10-2 and 10-3 M H2S, and seem to havebeen intenhediate between bare-ridge hydrothermalfluids, like those at 21'N EPR, and the fluids associat-ed with hydrothermal activity in the Guaymas Basin(e.9., Campbell et al. 1992).Yent fluids responsiblefor mineralization at Bent Hill would have plottedclose to the pyrite-pyrrhotite boundary in pH-a(O2)space @igs. 13A, B). In contrast, sphalerite composi-tions for the active chimneys in the AAV indicateequilibration at low a(O) and a(S), and fluidsresponsible for present-day mineralization in the AAVplot well within the pyrrhotite field.

Recent drilling at Site 856 during kg 139 of theODP intersected 95 m of massive sulfide beneath theBent Hill deposit. This deposit is presumed to haveformed at the same time as the sulfide outcrop belowthe active chimney at Bent Hill, owing to the similartextural, mineralogical and geochemical characteris-tics of the massive sulfide deposits. However, thesulfides in Hole 856G comprise zones of pyrite,pyrrhotite and a mixed pyrite-pyrrhotite assemblagethat has undergone significant recrystallization andlate-stage replacement of sulfides by magnetite @aviset aI. L992). The growth of magnetite from pyrite andpyrrhotite reflects the re-equilibration of the sulfideswith a late-stage, sulfur-poor fluid similar to that cur-rently venting in the AAV. At low concentrations oftotal sulfur (e.g., Lf M H2S), rhe stability fields forthe principal Fe-sulfides will diminish, and magnetitewill begin to form at the expense of pyrite andpyrrhotite (e.9., Fig. 13A).

Implications of data on bulk compositions

Base metal contents in the Middle Valley sulfidesare low in the active chimneys, as are the ZnlCu andZnlPb ratios (generally <1 and (10, respectively).The lack of base metals in the AAV is likely due to thevery low init ial contents of base metals in thehydrothermal fluids. At the observed temperature andhigh pH of active vents in the AAV, the solubility ofall base metals is low (Franklin et al. l98l), and nomajor subseafloor accumulation of metals wasencountered beneath the AAV during drilling at site858 @avis et al. 1992). As well, pore fluids recoveredfrom Site 857 @g. 2) at depths in excess of 425 m arevery similar in major-element composition to theactively venting fluids in the AAV site (Davis el aL1992). These fluids are in equilibrium with their hostclastic and hemipelagic sedimentary rocks and mayrepresent metamorphic fl uids.

Examination of the trace-element geochemistry ofsediments (Davis er al. 1992) in the Site 857 drillholes, south of the AAV, shows that copper is vir-

."rt'

if

0 500 '1000 150(

ZnlOu x 100

Frc. 14. The variation inZtlCtt ratio with depth inhemipelagic sediment from ODP Site 857 illustrates asignificant depletion of copper below 430 m. Activelyventing chimneys at the AAV are copper-rich (relative tolevels of zinc).

tually absent from the rocks in equilibrium with thevent-type pore fluids at depth, whereas zinc is onlyslightly depleted (Frg. 14).This finding indicates thatthe hydrothermal fluids, although metal-depleted over-all, may carry significantly more copper than zinc.This is supported by the unusually low ZnlAt ratio ofprecipitates in the active chimneys. Figure l3B showsthat the solubility of chalcopyrite in fluids from theAAV may be more than an order of magnitude greaterthan that of sphalerite. Thus selective leaching of hacemetals from the sediments at site 857, south of theAAV, could account for the significant depletion ofcopper relative to zinc (Fig. l4).

If compared to ancient massive sulfide deposits (seeFranklin et al. L98l), the sulfide portion ofthe activechimneys from Middle Valley places them in theZn-Pb{u group of deposits, more typical of felsic- orsediment-dominated terranes. Some important infor-mation regarding the distribution ofbase and preciousmetals is evident on examination of a correlationmafiix for the data from the active chimneys. Silver iscorrelated with lead (0.99) and barium (0.84), but notwith copper (0.48). Franklin et al. (1981) showed thatin zinc-copper deposits, silver is strongly correlatedwith copper, but in Zn-Pb{u deposits, it correlateswith Pb. Samples from the active vents thu$ seem tobe much more similar to the latter class of volcano-genic massive sulfide deposit. The close correlation ofsilver with barium may indicate that silver hasremained conservative in the vent fluids, and hascoprecipitated with barite and galena in the relativelylow-temperature parts of the chimneys. The high uni-form Sr contents of hydrothermal precipitates at

ooG

6 /ooB.9ocogo

CHIMNEYS AND MASSryE SULFIDE. MIDDLE VALLEY t02l

Middle Valley reflects a large contribution of sedi-ment-derived Sr, possibly from dissolution of car-bonate in planktonic foraminifera and carbonatenodules during hydrothermal venting. These data areconsistent with Pb and Sr isotopic data (Goodfellow &Franklin, in press), which indicates that the fluidsresponsible for these active vents have probably equi-librated with sedimentary material.

In contrast to the active chimneys, the Bent Hillmassive sulfide deposits have hrgh ZnlCu anLd ZnIPbratios, at surface (2253-l-3, Table 9) and at depth(Davis e/ al. 1992). Although trace ,uaounts ofgalenaand Pb-Sb-As sulfosalts occur on the outer upperedge of the massive sulfide deposit, these mineralphases are volumetrically insignificant. The massivesulfide deposits at Bent Hill lie in the Zn-{u group ofdeposits as defined by Franklin et al. (1981). The basemetal content reflects predominantly basaltic source-rocks for tle massive sulfide deposits. Strontium iso-topic data, intermediate between a value typical ofmid-ocean-ridge basalts and hemipelagic sediments,reflect both basaltic and sedimentary reservoirs for thehydrothermal fluid that formed the sulfides at BentHill (Goodfellow et al.1990).

Remobilization, dissolution and reprecipitation ofboth base and precious metals in the sulfide depositare evident in the observed textures, the presence ofgalena, gold and Pb-Sb-As sulfosalts concentrated onthe upper fringe of the deposit and the lack of theseminerals at depth Qavis et al. 1992), and variations inbase metal ratios at depth (Davis et al. 1992). T\eseall indicate a metal zonation typical of many ancientmassive sulfide deposits (Franklin et al. L982,Knuckey et al. 1982). Late-stage hydrothermal fluidsremobilized and redistributed metals in the massivesulfide deposit at Bent Hill.

ColclustoNs

Our studies indicate that, at Middle Valley, twocompletely different types of deposit are defined bytheir contrasting assemblages of minerals, geochem-istry and source compositions. Mineralization at theAAV and in the active vent at Bent Hill is sulfur-poorand formed from fluids indicative of seawater-sedi-ment interaction. The older sulfide deposits at BentHill formed from fluids with a higher a(Sr) and a(Or)and a much greater degree of fluid-basalt interaction.

Indicators of a significant contribution of sedimen-tary material to the venting hydrothermal fluids are:low total levels of base metals and high levels of Pbcompared to bare-ridge deposits, a bulk enrichment inSr, a characteristic pyrrhotite-dominant sulfide assem-blage, Pb and Sr isotopic data (Goodfellow &Franklin, in press), high-pH fluids with low a(S) andlow a(O) based on mineral chemistry and associatedassemblageso and the selective leaching of copperfrom the sediment below site 857.

Indicators of a significant basaltic component to thehydrothermal fluid that precipitated the Bent Hill mas-sive sulfide deposits includes: high copper and zinccontent, low Pb and Sr isotopic values (Goodfellow &Franklin, in press), and abundant primary pyrite +pyrrhotite. Late-stage hydrothermal fluids have affect-ed the original sulfide assemblage at Bent Hill, as indi-cated by the recrystallization of sulfides, the formationof magnetite beneath Bent Hill, and the dissolution ofzinc and gold from the ODP sulfides at depth (Davis eral. 1.992) and precipitation near the mound surface,resulting in a zone-refined deposit typical of manyancient deposits on land (Franklin et al. 1981,,Knuckey et al.1982, Sangster 1972).

The two hydrothermal events, represented by theactive vents (AAV and Bent Hill) and the massive sul-fide deposits at Bent Hill, may represent a continuumproces$ of crustal seqling within a recently active riftenvironment or two independent events with a coinci-dental spatial relationship.

ACKNOWLEDGEMENTS

The authors thank 1) the ATLANTIS tr and DSRVALVIN crews for their efforts and help in obtainingthe samples, and 2) the scientists, M. Black, W.Goodfellow, P. Johnson, K. Juniper, R. McDuff, E.Southward, B. Taylor and R. Zierenberg. This confi-bution benefitted from the efforts of many staff at theGSC: L.K. Radburn provided excellent SEMmicrophotographs, G.D. Pringle wrote computer pro-grams to calculate mineral formulae, J.A.R. Stirlinggave excellent advice for the analysis of such fine-grained material by electron microprobe. We are par-ticularly grateful to R. Lancaster and K. Nyguen fortheir drafting expertise. R. Koski, I.R. Jonasson andM. Zentek clarified many aspects of the manuscript intheir careful reviews.

RxFERENCES

Aoavross, (1980): The form and environment of formationof the Kalavasos ore deposits, Cyprus. /r Ophiolites: InfOphiolite Symp. Proc. (Cyprus, 1977; A. Panayiotou,ed.). Cyprw Minisry Agriculrure Nat Resources, Geol.Survey Dep.,1 17-178.

BARroN, P.8., JR. (1984): Redox reactions in hydrothennalfluids. 1n Fluid-Mineral Equilibria in HydrothermalSystems (R.W. Henley, A.H. Truesdell & P.B. Barton,Jr., eds.). Rev. Econ. GeoL.l,99-114.

Brscnorr, J.L., RosENsauBn, R.J., ARUscAvAcE, P.J.,BAEDEcKER, P.A. & Cnocr, J.G. (1983): Seafloor mas-sive sulfide deposits from 2loN, East Pacific Rise; Juande Fuca Ridge; and Galapagos Rift: bulk chemical com-position and economic implications. Econ. GeoL TE,I711,-1720,

Bowrns, T.S., JAcKsoN, K.J. & HELGESoN, H.C. (1984):

1022 THE CANADIAN MINERALOGIST

Equilibriwn Activity Diagrams for Coexisting Minerals Foueunr, Y., AucLAR, G., CAMBoN, P. & Erounmeu, J.anlAqueous Solutiow at Pressares and.Temperatures to (1988): Geological sening and mineralogical and geo-5 kb and 600'C. Springer-Verlag, New York. chemical investigations on sulfide deposits near l3oN on

the East Pacific Rise. Martne Geol. U,145-178.Cer"pnruL A.C., GrnMaN, C.R., Pelrvren, M.R. & EDMoND,

J.M. (1992): Chemistry of hydrothermal fluids from theEscanaba Trough, Gorda Ridge. U.S. GeoI. Surv,, BulI.(in press).

Cnass, R.L., DBlemv, J.R., KARSTE\, J.L., Jom{soN, H.P.,JuNrpeR, S.K., LusroN, J.E., Scorr, S.D., Tuvrrcllrrn,V., HAnfi\4oND, S.R. & McDrm, R.E. (CASM ResearchGroup) (1985): Hydrotherrnal vents on an axis seamountof the Juan de FucaRidge. Naare 317 ,21,2-21,4.

CoNsralmNou, G. (1980): Metallogenesis associated withthe Troodos ophiolite. /n Ophiolites: Int. Ophiolite Symp.Proc. (Cyprus,19771" A. Panayiotou, ed.). CyprusMinistry Apiculture Nat. Resources, GeoI. Survey Dep.,663-674.

CneruiR, D.A. & BARNES, H.L. (1976): Ore solution chem-istry. V. Solubilities of chalcopyrite and chalcociteassemblages in hydrothermal solutions at 200 to 350'C.Econ Geol.7l,772-794.

Cunrus, R.G, Devrs, E.E., RDDtrroucu, R.P. & S.cwyBn, B.S.(1985): Juan de Fuca Ridge Atlas): preliminary Seabeambathymetry, Geol. Surv. Can., Open-File Rep.1143.

CZAMANsKE, G.K. (1974): The FeS content of sphaleritealong the chalcopyrite - pyrite - bornite sulfir fugacitybrffer. Econ. Geol.69. 1328-1334.

DAvIs, E.E., GooDFELLow, W.D., BoRNHoLD, B.D.,Aosmap, J., BIAISE, 8., VnLNcnn, H. & LnCurmer"r,G.M. (1987): Massive sulfides in a sedimented rift valley,northern Juan de Fuca Ridge, Earth Planet. Sci. left,82,49-6t.

- & LlslER, C.R.B. (1977): Tectonic structures on theJuan de Fuca fudge. Geol. Soc, Arn. Bull. E8,346-363.

- Morr:,, M.J., Frssnn, A.T., Er N-. (1992): Proceed-ings of the Ocean Drilling hogram, Initial Reports 139:College Station, TX (Ocean Drilling Program), 1-1026.

DEcENs, B.T. & Ross, D., eds. (1969): Hot Brines andRecent Heavy Metal Deposits in the Red Sea; aGeochemical and Geological Account. Springer, NewYork.

EMBLEY, R.w., JoNAssoN, I.R., PERFIT, M.R., FnaNrr,n,J.M., Trvav, M.A., MelasoFF, A., SMITH, M.F. &FRANcrs, T.J.G. (1988): Submenible investigation of anextinct hydrothermal system on the Galapagos Ridge: sul-fide mounds, stockwork zone and differentiated lavas.C an. Mineral. 26. 5 17 -539.

FEELY, R.A., GETsELMAN, T.L., BAIGR, E.T., MAssom, G.J.& Hauuom, S.R. (1990): Distribution and compositionof hydrothermal plume particles from the ASHES VentField at Axial Volcano, Juan de Fuca Ridge. J, Geophys.Ras. 95, 12,855 - 12,87 3.

& Lrnon, J.W. (1990): Geological setting and

FneNrr,nq, J.M., Gooorsllow, W.D., AMFs, D.E., LYDoN,J.W., JoNassoN, I.R. & Devrs, E.E. (1991): MiddleValley, a major centre of hydrothermal activity in a sedi-mented ridge crest" northem luan de Fuca Ridge. Geol.Surv. Can, Cunent Activities Forum (Ottawa), ProgramAbstr.,20.

Bt,atse, 8., ANcml, C.D., Hanvrr-Kellv,F.L. & MACDoNAID, R. (1987): Geological map and dis-tribution of sulfide deposits in Middle Valley, northemJuan de Fuca Ridge. Am Geophys. Union. Trans. (Eos)68(44), 1545 (abstr.).

-, Jom{soN, H.P. & CunRIE, R. (in press): SeaMARC1A side-scan imagery of the Middle Valley area, northernJuan de Fuca Ridge. Geol. Surv. Can., Open File.

_- LyDoN, J.W. & SaNcsren, D. (1981): Volcanic-asso-ciated massive sulfide deposits. Econ Geol,,1Sth Anniv.Vo|.,485-627.

GelLEv, A.G., BArLEs, A.H., SYME, E.C., Blearpn, W.,Mecer, J.J. & GoRDoN, T.M. (1990): Geology and min-eral deposits of the Flin Flon and Thompson belts,Manitoba. Geol. Surv. Can., Open-File Rep.2165,

GrBsoN, H.L. & WerrrNsoN, D.H. (1990): Volcanogenicmassive sulphide deposits of the Noranda cauldron andshield volcano, Quebec. /n The Northwestern QuebecPolymetallic Belt (M. fuve, P. Verpaelst, Y. Gagnon,J.M. Lulin" G. Riverin & A. Simard, eds.). Can. Inst.Mining Metall., Spec. Vol. 43,ll9-1'32.

GooDF&Irw, W.D. & BulsE, B. (1988): Sulfide formationand hyclrothermal alteration of hemipelagic sediment inMiddle Valley, northern Juan de Fuca Ridge. Can.M ine r al., ?,6, 67 5 -696.

- & FRANrsb,l, J.M. (1993): Geology, rnineralogy andgeochemistry of sediment-hosted clastic massive sulfidein shallow cores, Middle Valley, northern Juan de FucaRidge, Econ. GeoI. I (in press).

origin of the Middle Valley sulfide deposits, northernJuan de Fuca Ridge. Trans. Am. Geophys. Union (Eos)71(43), 1565 (abstr.).

HANNINcToN, M.D. (1986): Geology, Mineralogy andGeochemistry of a Silica - Sulfate - Sulfide Deposit,Arial Seannunt, NE Pacific Ocean M.Sc. thesis, Univ.Toronto, Toronto, Ontario.

- & GonroN, M.P. (1991): Analysis of sulfides for goldand associated trace metals by direct neutron activationwith a low-flux reactor. Geostandards Newslett. 15, 145-t54.

-_ HERzrc, P., Scor:r, S., THoIi4?soN, G. & Role, P.(1991): Comparative mineralogy and geochemistry of

CHIMNEYS AND MASSIVE SULFIDE. MIDDLE VALLEY to23

gold-bearing sulfide deposits on the mid-ocean ridges.Martne Geol. l0l. 217 -248.

- & Scorr, S.D; (1989): Sulfidation equilibria as guidesto gold mineralization in volcanogenic massive sulfides:evidence from sulfide mineralogy and the composition ofsphalerite. Econ. Geol. 84, 1978-1995.

Hoyr, T.L. (1992): Mineral Chemistry of Massive Sulfideand Sulfate Samples ftom Escanaba Trough, SouthernGorda Ridge. M.Sc. thesis, Univ. California, Davis,Califomia.

JoHNsoN, H.P., Twnv, M.A. & FneNrr-rN, J.M. (1990):Tectonic control on hydrothermal systems on the northemJuan de Fuca Ridge): Middle Valley and the Endeavoursegmenf Trans. Am. Geophys, Union (Eos) 7l(43),1566(absf.).

JoIDES (1991): Leg 139: Sedimented Ridges. 1. ScienceOperator Report. JOIDES J. 17(3),8-12.

JoNAssoN, I.R. & Warxrn, D.A. (1987): Micro-organismsand their debris as substrates for base metal sulnde nucle-ation and accumulation in some Mid-Ocean Ridgedeposits. Trans. Am Geophys. Union (Eos) 68(44),1546(abstr.).

KAppEL, E.S. & FnaNrLrN, J.M. (1989): Relationshipsbetween geologic development of ridge crests and sulfidedeposits in the northeast Pacific Ocean. Econ. GeoL M,48s-505.

KNUcKEv, M.J., Corrasa, C.D.A. & Rrvrnn, G. (1982):Structure, metal zoning and alteration at the Millenbachdeposit, Noranda Quebec. Geol. Assoc. Can., Spec. Pap.25,255-295.

KosKI, R.A., BENNTNGER, L.M., ZrnnsNsERG, R.A. &JoNAssoN, I.R. (1993): Composition and growth historyof hydrothermal deposits in Escanaba Trough, southemGorda Ridge. U.S. GeoI. Sarv,, Bull.2022 (in press).

-, LoNsDArr, P.F., SHeNKs, W.C., m, BsRNDr, M.E. &HowE, S.S (1985): Mineralogy and geochemistry of asediment-hosted hydrothermal sulfide deposit from theSouthern Trough of Guaymas Basin, Gulf of Califomia.J. Geophys. Res. 90, 6695-6707.

- SHANKS, W.C., m, BonnsoN, W.A. & OscARsoN,R.L. (1988): The composition of massive sulfide depositsfrom the sediment-covered floor of the Escanaba Trough,Gorda Ridge: implications for depositional processes.C an- M tuu ra l. ?l;, 655 -67 3.

Kuserese, M., Cume, H. & Ornaoro, H. (1982): Stable iso-topes and fluid inclusion study of anhydrite from EastPacific Rise at 2loN, Geochem. "/. f6. 89-95.

LE BEL, L. & Oupnr, E. (1982): Fluid inclusion srudies ofdeep-sea hydrothermal sulfide deposits on the EastPacific Rise near 2loN. Chent Geol.37. 1,29-136.

LsncH, C.H.B. (1991): Preliminary studies of fluid inclu-sions in barite from the Middle Vallev sulfide mounds.

northem Juan de Fuca Ridge. Geol. Suw. Can., Pap.9l-ta,n-30.

LoNsDAr.iE, P.F., BrscroFF, J.L., BuRNs, v.M., KASTNER, M.& SwurNrv, R.E. (1980): A high{emperature hydrother-mal deposit on the seabed at a Gulf of Califomia spread-ing center. Earth Planet. Sci. Izx. 49,8-20.

LyDoN, J.W., Gooorrllow, W.D. & Aurs, D.E. (1990):Composition of subsurface hydrothermal plumes, MiddleValley, northern Juan de Fuca Ridge. Eighth IAGODMeet., Program Abstr., M73.

- - & GnEconn, D.C. (1,992): Chemical composi-tion of vent and pore fluids in an active hydrothermal dis-charge zone, Middle Valley. Geol. Sun. Can., MiruralsColloquium ( Ottawa), Pro gratn lbstr., 24.

MoRroN, J.L., Hor-uns, M.L. & KosKI, R.A. (1987):Volcanism and massive sulfide formation at a sedimentedspreading center, Escanaba Trough, Gorda Ridge, north-east Pacific Ocean. Geophys. Res. Lett.14,769-772.

- KosK, R.A., Norumm, W.R. & Ross, S.L. (1990):Distribution and composition of massive suLfide depositsat Escanaba Trough, southern Gorda Ridge. 1z GordaRidge: a Seafloor Spreading Center in the United States'Exclusive Economic Zone (G.R. McMurray, ed.).Springer-Verlag, New York (77-92).

PERcIvAL, J.P. & AMES, D.E. (1993): Clay nineralogy ofactive hydrothermal chimneys and associated mounds,Middle Valley, northern Juan de Fuca Ridge. Caz.Mineral.3l,957-971.

PErE& J.M. (7986): Genesis of Hydrothermal Vent Depositsin the Southern Trough of Guaymas Basin, Gulf ofCalifurnia: a Mineralogical and Geochemical Sndy.M.Sc. thesis, Univ. Toronto, Toronto, Ontario.

- & Scom, S.D. (1988): Mineralogy, composition, andfluid-inclusion microthermometry of seafloor hydrother-mal deposits in the southern trough of Guaymas Basin,Gulf of Califomia. Can. Mineral.26. 567-587 .

RoNA, P.A. & CLAGUE, D.A. (1989): Geologic consols ofhydrothermal discharge on the northertr Gorda Ridge.Geology 17,1097-1101.

RuAyA, J.R. & SEwARD, T.M. (1986): The stability ofchlorozinc (II) complexes in hydrothermal solutions up to350'C, Geochim. Cosmochim, Acta 50.651-661.

SANGSTER, D.F. (1972): hecambrian volcanogenic massivesulphide deposits in Canada: a review. Geol. Surv. Can.,Pap.7L22.

Scon, S.D. (1978): Structural control of the Kuroko depositsof the Hokuroko district, Japan. Min . Geol. ?3, 30 I -3 1 I .

- (1980): Geology and structural control ofKuroko-typemassive sulphide deposits. GeoL Assoc, Can., Spec. Pap,70.705-722.

LA',U TT{E CANADIAN MINERALOGIST

(1983): Chemical behaviour of sphaleri te andarsenopyrite in hydrothermal and metamorphic environ-ments. MineraL Mag. 47, 4n 435.

- & BanNes, H.L. (1971): Sphalerite geothermometryand geobarometry . Econ. Geol. 66, 653-669.

SrvoNur, B.R.T., Goonrnllow, w.D. & FRANKLTN, J.M.(1992): Hydrothermal petroleum at the seafloor andorganic matter alteration in sediments of Middle Valley,northern Juan de Fuca Ridge. Appl. Geochem.7,257-2&.

Srarrs, D.S., FnaN<u,1, J.M. & THE LEc 139 StmsoARoScmrunc Panrv (1991): Initial results from drilling inthe Middle Valley of the Juan de Fuca Ridge: hydrother-mal alteration of mafic sills in a sedimented ridge envi-ronment. Trans, Am. Geophys. Union (Eos) 72(44),553(abstr.).

Towvm, P., m & BARroN, P.8., JR. (1964): A thermody-namic study of pyrite and pyrrhotite. Geochim.Cosmochim. Acta 8. &l -67 l.

hemipelagic sediments in the hydrothermal areas ofMiddle Valley, northern Juan de Fuca Ridge: shallowcore data. Can. Mineral. 31,973-995.

._ LErcH, C.H.B, Aves, D.E., HoY, T., Flamnq, J.M.& Gooprrrrow, W.D. (1991): Character of hydrothermalmounds and adjacent altered sediments, active hydrother-mal areas, Middle Valley sedimented rift, northern Juande Fuca Ridge, northeastern Pacific : evidence fromALVIN push cores. GeoI. Surv. Can., Pap.9l-18,99-108.

Vor Dauu, K.L. & Blscuorr, J.L. (1987): Chemistry ofhydrothermal solutions from the southem Juan de FucaRidge. "L Geophys. Res. 92,11,334-11,346.

ZTERENBERc, R.A., SHANKS, W.C., III & Btscnorr, J.L.(1984): Massive sulfide deposits at 21oN, East PacificRise: chemical composition, stable isotopes, and phaseequilibria. Geol. Soc. Am" Bull. 95, 922-929.

TLRNSR, R.J.W., AMES, D.E., Flamsio{, J.M., GoonFsr-Low,W.D., Lsrrcn, C.H.B. & Hoy, T. (1993): Character of Received. August 7, 1992, revised manuscript accepted Marchactive hydrothermal mounds and nearby altered 5,1993.