Low.Temperature Alteration of the Extrusive Seouence, Troodos Ophiolite, Cvprus

Canodian Mineralogist Vol. 23, pp. 431441 (198s)

LOW.TEMPERATUREALTERATION OF THEEXTRUSIVE SEOUENCE, TROODOSOPHIOLITE, CVPRUS

KATHRYN M. GILLIS AND PAUL T. ROBINSON Departmentof Geologltand Centrefor Marine Geology,Dalhousie University, Hatifax, Nova ScotiaB3H 3Js

ABSTRAcT tdt aprdsla formationde Ia cro0teoc6anique. En dehors deszones trbs localis€es d'alt6ration hydrothermale, les laves The lavas of the Troodos ophiolite have been altered du Troodosn'ont pas6t6 sujettes d un m6tamorphisme primarily by low+emperatureinteraction with seawaterand rdpandu;d'ailleurs, en dessous de la zonesup€rieure oxy' by localized hydrothermal upwelling. An uppermost zone dde, on ne trouve aucunevariation systdmatiquedans of highly oxidized lavas that contain abundant calcite and I'intensit6ou le caractdrede I'alt6ration avec la profondeur. smectiteis believedto be due to seafloor weathering.In Lesassemblages demin6raux secondaires ddpendraient des a few sections,where thick layers of umber overlie the vol- variationslocales en perm6abilit6,type d'unit6 de refroi- canic pile, the oxidized zone is absent, suggestingthat the dissement,temp6rature, rapport volumique eaulsolide et crust was sealedoff from circulating seawater.Below the compositioninitiale de la lave. oxidized zone, the rocks are less pervasively altered and (Traduit par la R6daction) locally contain fresh glass.Assemblages of ana'lcime,natrolite, phillipsite, chabaziteand gmelinite are common in the Mots-cl6s:alt|ration, bassetemp€rature, massif ophioliti- pillow lavas of this sequencewhereas assemblages of clino- que ptilolite, deTroodos, Chypre, cro0te ocfunique, permdabi- mordenite and celadonite are primarily associat- lit6, argiles,zdolites, oxydation. ed with the massiveflows. A basal sequenceof more siliceouslava has abundant secondarysilica. Alteration in thesezones is believedto reflect seawater-rock interaction INTRODUCTION during or shortly after crustal accretion. Outside the very localized zonesof hydrothermal alteration, the Troodos la- Previousstudies, summarized in Honnorez(1981), vashave not beenpervasively metamorphosed, and below have shown that alteration in the upper oceaniccrust the upper oxidized zone there are no systematicvariations is largely the result of seawater-basaltinteraction. in the intensity or characterof alteration with depth. The The distribution and paragenesisof secondaryminer- observed assemblagesof secondary minerals can be ex- als have been shown to dependupon such factors plained by local variations in permeability, type of cool- as lithology, fluid composition, temperatureand age ing unit, temperature, waterlrock ratio and initial lava composition. (Nt et al, 1985,Staudigel et al. 1981,Natland & Ma- honey 1978).The shallow depth of most Deep Sea Keywords: alteration, low-temperature,Troodos ophiolite, Drilling Project holes and the restricted lateral con- Cyprus, ocean crust, permeability, clays, zeolites, oxi- trol over each site have limited our understanding dation. of the processof seawaterpenetration into the crust and the extent of hydrothermal circulation and metamorphismat depth. SouvernE Ophiolites provide an alternative for studying in situ oceaniccrust. Unfortunately, the transportation Les laves du massifophiolitique de Troodos ont princ! and emplacementhistory of many ophiolites have palement 6td altereespar un dchangeavec de I'eau de mer made it difficult to study their original pattern of d bassetempdrature et, localement,par jaillissementde flui- deshydrothermaux. Une zone supdrieurede coul6esforte- alteration. Previousstudents of the Troodos ophio- ment oxyd&s, enrichiesen calcite et smectite,repr6enterait lite concluded that the extrusive sequencehad been une zone de lessivagemarin. Oir d'6paissescouches d'ombre metamorphosed,resulting in the developmentof two recouwent I'empilement volcaniquedans quelquessections, distinct facies interpreted in terms of axis and off- la zone oxyd6eest absente,ce qui indiquerait que la cro0te axis volcanism(Gass & Smewing1973, Smewing el 6tait isol6ede I'eau de mer en circulation. En dessousde al. 1975).In this paper, we show that the extrusive la zone oxyd€e,les rochessont moins complbtementalt6- rocks of the Troodos ophiolite have undergonethe rdeset contiennentm€me, ici et li, du verre frais. Desassem- sametype of low-temperature alteration as that ob- blages d analcime, natrolite, phillipsite, chabasite et servedin the modern oceaniccrusu also, we discuss gmelinite sont courants dans les laves i coussins de cette physical sdquence,et les assemblagesi clinoptilotite, mordeniteet the conditions that control the distribution c€ladonite sont surtout associ6saux coul6esmassives. Une and paragenesisof the secondaryminerals. A more s6quenceinfdrieure de lave plus siliceusecontient beaucoup detailed study of the physicochemicalconditions ac- de silice secondaire. L'alt6ration de ces zones refl6terait tive during the different stagesof alteration is still l'interaction desroches avec de l'eau de mer pendant ou underway. 431

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Recentstudies of the Troodos Massif of Cyprus Robinson et al. (1983)have outlined two geochem- have confirmed that it is a fragment of oceanic ical suites for the northern flank of the Troodos lithosphere,as suggestedby Moores & Vine (1971), ophiolite on the basisof glasscompositions: 1) a low- although it is now believed to have originated in a er andesite- dacite - rhyodaciteassemblage of arc subduction-zone environment rather than at a mid- tholeiite affinity and 2) an upper picritic basalt - an- ocean ridge (Moores et al. 1984, Robinson el a/. desitic basalt assemblagewith a depletedarc tholeiite 1983,Schmincke et al:1983, Mcculloch & Cameron affinity. The stratigraphic level of this new geochem- 1983). The excellent exposure and relatively un- ical boundary is variable within the extrusive se- deformed nature of this complex provide a unique quencein relation to the old UPL/LPL boundary opportunity to study the processesof alteration (Frg. l). A third suiteof highly depletedbasalts hav- through a completesection of oceaniclithosphere. ing "boninitic" affinities has beenrecognized on the Early workers divided the extrusive sequenceinto southern flank of Troodos (J.M. Mehegan, pers. the Upper Pillow Lavas (UPL) and Lower Pillow La- comm. 1984). vas (LPL) on the basisof color, abundanceof oli- Detailed studiesof stratigraphically controlled sec- vine phenocrysts,assemblages of secondaryminerals tions betweenthe villagesof Malounda and Margi and the location of the sulfide orebodies@ear 1960, (Fie. 2) have shown that the extrusive sequencehas Gass1960). Gass & Smewing(1973) divided the se- not beenpervasively metamorphosed. Fresh volcanic quenceinlo an axis sequence,consisting of the Lower glassis preservedsporadically throughout the extru- Pillow Lavas and the SheetedDykes, and a youn- sive sequence,efiending downward to the top of the ger, off-axis sequenc€,represented by the Upper Pil- Basal Group (9090sheeted dykes). Furthermore, the low Lavas, based on a supposed metamorphic distribution of the secondaryminerals within the ex- boundary within the complex. trusive sequenceindicates that thereis neithera sys-

MALod,JNDA KAMBIA MARGI HIGH MsO- HIGH SiOt ;.'..j.t - -tptUPL - SUTTE eac THOLEIITE SU'TE HIGH MgO. H|GH SiO,' UPL SUITE LPL ARC THOLEIITE SUITE

o c upperZone o ffi

Middte zone HIAH MgO- % HIGH SiO2 SU'IE Lowerzone ARC ffi THOLEIITE SUITE

FIo. l. Schematicrepresentation of the geology of the study areas,outlining the strati- graphicposition of the old Upper Pillow Lavallower Pillow Lava boundaryand the new geochemicalsuites, Upper zone: aphyric to olivine-phyric basalts and picrites;middle zone:aphyric to slightly olivine-clinopyroxene-phyricbasalt to andesitic basalt; lower zone: aphyric to slightly clinopyroxene-plagioclase-phyric andesites- dacites - rhyodacites.

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Ftc. 2. Geological map of the,northeastern portion of the extrusivesequence. The Malounda, Kambia and Margi study are:Nare outlined. Note that the old Upper Pillow Lavallower Pfuow Lava boundary is rtro*n. r'or a cffiarison' of this old boundary with the new geochemicalboundary for each study area, seeFigure l.

tematic increase in the intensity of alteration with the basisof color, type of cooling unit and mineral- depth nor a metamorphic boundary within the lavas. ogy (Mehegan& Robinson 1984,Schmincke et ol. Localized zones of high-temperature hydrothermal 1983). These units have been grouped into three alteration underlie the rhassive-sulfideorebodies IIn- major stratigraphic sequencesreferred to as the up- ternational Crustal Research Drilling Croup per, middle and lower zones. Although the same (ICRDG) 19841,but theseare spatially restrictedand generalsequence is found throughout the area, these have little overall effect on the lavas. The presence zonesvary greatly in thickness, lithology and later- of an oxidized zone in the upper 20 - ZO0m of the al extent. extrusivesequence and the relationshipsamong the The basal 500 - 600 m of the extrusive sequence ass€mblages of secondaryminerals, type of cooling contains aphyric to sparsely clinopyroxene- and unit and host Iithology both suggestthat alteration plagioclase-phyricmassive and sheetflows, pillow was largely due to circulation of low-temperaturesea- lavas and hyaloclastites of andesitic to dacitic com- water during and after crustal accretion. Regional position that comprise the arc tholeiite suite of reconnaissance has shown that the study area is Robinson et al. (1983). Dykes are present through- representative ofthe alteration in the northerrt flank out this zone and increasein abundance toward its of the complex. base. This lower zone is overlain by approximately 5fi) m of greento greyishgreen pillow lavasand mas- RncroNar Grorocy sive flows. locally separated by thin layers of hyaloclastite. These aphyric to slightly olivine- and Recentstudies ofthe lavason the northern flank clinopyroxene-phyricunits havea basaltto basaltic of Troodos haveshown that the extrusivesequence andesitecomposition and belong to the high MgO is composedof numerouslithologic units, ranging - high SiO, suireof Robinson et al. (1983).Dykes, from 50 to 200 m in thickness,that are defined on sills and coarse-grainedbasaltic to doleritic bodies

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IABLE 1. SELECTED ELECTIOtr.Pf,OBE DArl Otr COI'TOSITIOTS 08 CLAY UItrEIALS

oelEda oolqdb angoo FDg Od snooo gBgof sEgog snooh

S102 s5.89 5\ .22 43.06 47.88 115.42 3?.X9 38.?4 45.61 2.76 5.07 1t.05 8.52 7.15 9.99 11.52 12.45 ilag, 22.31 15.91 .13.85 9.39 7.97 28.?8 12.05 l.'ll UB0 0.04 0.24 - 0.06 t{aO 5.15 ',t,10 16.68 18.26 21.20 7. 94 20.26 23.8',1 Ca0 0.63 0.35 O.72 0.97 0.90 2.38 t," tra2o o:5u "=t' r2o 5'.57 glos r]oe : 9'?o3 0.06 "1oa IOEAI 82.39 91.?4 88.51 er]ro 82.64 8?.30 e,r]:e sr]zo

Catl.oo Proporilgna o! tbo basta of 20 0

6.00 ST 7.21 7.35 5.94 6.46 6.37 5.6',t.'199 5.53 0.51 0.81 1.80 1.35 '1.18 1.94 1.93 Fe 2.93 1.80 1.60 1.06 0. 93 3.65 r.44 0.12

Ug 1.21 1.44 3.43 3.67 4.43 1.80 4.31 4.68 0.11 0.05 0.11 0.1s 0.1! 0.39 0.30 0.30 !la r 1.12 I .57 0.511 u.1,

. Total 1ro! s:prosaed a9 Feo; - lot dot9ot€d.

a' b grouldbass ooladoltte (K0s82r303' ro382t389)t o yellorlsb 8te€! EroqodBasa soeotlt€ (Kor82r108)t d yetlorlsb gresD SrourdEaao sEeotlte' blddlo of o (fOs82slO8); € oraDSe-broro groulahaaa sDsottts' oeBtle qf o ((0:82!108), f orao8q-bror! 6Bootttor vsa{o1s Ltal.!8 (K0:82r522)r 8, h ona[8€-btor! sleoilt€ ln otlvl.le pboaooryst (EO:82:008).

are present at this level, but are less abundant than SiO2suite. Intrusive bodies are absent at this strati- in the underlying sequence.Theupper zone at the top graphic level. of the sectionis approximately 20 to 300m thick and METHODS consistsof aphyric to olivine-phyric pillow basalts and picrites that are also part of the high MgO - high Samplesrepresenting the different assemblagesof

A ce ladonite

smeclite

O smectite in olivino o smectite in glass

?6'o ;< a ^ 4 " ^l^*t"i^: a 55p a -4.o "li" O N aA a Y a ilil 30 ,f:T^^'^i{^f^^ ?. r i :-.,1^^

Feo/ (FeotMso) FIc.3. Composition of clay minerals: wt. Vo K2O versusFeO/(FeO+MgO).

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secondaryminerals and intensitiesof alteration were TABLE2. SELECTEDELECTRON-PROBE DATA ON COMPOSITIONOF collected from stratigraphically controlled sections, CARBONATES chiefly within the Akaki River canyon near Malounda, the PediaeosRiver canyon near Kambia Si 02 0,06 0.07 0.01 and the Margi area. The stratigraphic relations in Alzoa 0.05 0.02 theseareas were defined F€ot- tlrs by Schminckeet al. (1983), Mtro 3.111 0.07 0.06 J.M. Mehegan(pers. comm. 1984)and Bailey(1984), Uao 0.82 0.57 0.56 respectively. Ca0 ,o.50 57.39 61.93 tr"20 0.02 0.03 0.03 Hand-picked secondaryminerals Keo 0.01 were identified Iotal 58,32 62-.63 on a Philips X-ray diffractometer (CuKo radiation). .63.11 Oriented smear-slidesof clays were scannedfrom 2" to 4O"20 at lo per minute. Representativesamples 1 fraotu.o fllllaa oalclto, fC82-153 2 grourdBaa8 oalolte, C!-1..12.I5 B of the different groups of clays were treated with 3 oaloLto 1! veslot6, CY-l.I2..15 s ethyleneglycol and rerun from 2" to l5o, The re- maining secondary minerals were scanned from 4o to 60o X) at lo per minute. clay minerals, zeolites,silica, carbonatesand iron The chemical analyses were obtained using oxides. electron-microprobe microanalyzers at Memorial University, St. John's, Newfoundland, the Smith- CIay minerals sonian Institute, Washington,D.C. and Dalhousie University, Halifax, Nova Scotia. Mineral para- Clay minerals, by far the most abundant of the genesiswas determinedfrom the polishedthin sec- secondaryphases in the volcanic pile, replacethe tions used for microprobe analysis. primary mineralsand groundmassmaterial and line or fill vesicles,vugs and veins. SEcoNDARyMnnnals Smectite,the most widespreadclay mi4eral, was identified by its broadlasal peak at 16 A and the The assemblagesof secondaryminerals consist of subsequentshift to 18A upon treatmentwith ethy-

Ca+Mg

A phillipsite a clinoptilolite- heulandite A mordsnite o natrolite o analc im e

Ftc.4. Triangular plot of the proportion of alkalis tNa - (Ca + MC) - Kl in the zeo_ lites. Data are from samplesfrom the Akaki River section.

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lene glycol. Severalvarieties of smectite have been Carbonqtes distinguishedon the basisof habit, color and composition. The compositionranges from Mg-rich to Carbonatesare pervasivethroughout the upper Fe-rich with varying K content (Table I, Fig. 3). zone of the extrusive sequenceand are locally abun- Orange-brownto brown smectiteis generallyMg- dant in the middle and lower zones.Calcite forms rich, whereasyellowish green smectiteis Fe-rich, irregular patchesand stringers in the groundmassof although some brown varietiesin the groundmass most lithologic units. It is relatively pure, but may areFe-rich and form oxidizedhalos around vesicles. contain minor amounts (< l9o) of Si, Mg, Fe, Na Orange-brown, Mg-rich smectite,Fe hydroxides and and K. A few crystals in narrow, cross-cutting veins calcite replaceolivine in the upper part of the sec- have up to 390 Mn (Table 2). Assemblagesof cal- tion. In the middle and lower zonesa similar smec- cite, smectite and Fe hydroxides replace olivine tite partly replacesplagioclase. Smectite that replaces phenocrystsin the basaltsof the upper zone. Iron- primary minerals is very finely crystalline, whereas stained, drusy and botryoidal calcite commonly fills that filling voids is usually fibrous to platy and radi- cavitiesin theserocks. Many veinshave undergone ates out from the wall of the cavity. Smectite that severalstages of calcitegrowth, asindicated by suc- replacesgroundmass material may be fibrous, radi- cessivelayers of euhedral crystals that €uecommon- ating, massiveor globular ly mantled by smectite or celadonite (or both). Celadonite,identified by its basal peak at 10 A Aragonite was identified in one occurrence near and its distinctivebluish greencolor, replacesinter- the base of the sequence. stitial groundmass-materialand partially fills voids, where it occrus €lsa fibrous, late-stagealteration Zeolites product. Chemically, celadoniteis distinguishedfrom smectiteon the basis of FeO/(FeO + MgO) and K2O Zeolites form distinctive assemblageswith other content (Fig. 3, Table 1). The generalparagenetic secondary minerals throughout the extrusive se- sequenceof theseclay mineralsis: Mg-rich, K-poor quence. They have been identified petrographical- smectite-Fe-rich smectite with variable K con- ly, on the basisof their XRD pattern and on the ba- tent*celadonite. sis of their Ca, Na and K contents@ig. 4, Table 3). Palygorskitewqs recognizedon the basis of the Clear, rvell-formedcrystals of analcime,accom- 10.4,6.4,and 4.5 A peaks,which remainunchanged paniedby sparse,radiating aggregatesof pink phil- upon glycolation and heating (c/. Natland & Ma- lipsite, commonly line fractures and pillow margins, honey 1978).This papyraceousto fibrous mineral line veins and locally replace groundmass material is restricted to late-stage,vertical veins in the upper of both massiveflows and pillows. phillipsite, an al- 500 m of the sequence,where it is commonly as- terationproduct of freshglass, is the earliestofthese sociated with calcite. two phases.Radiating prisms of natrolite, commonly associatedwith analcime,occur chiefly in vesiclesand vugs. Hexagonalclusters of pinkish orangegmelinite, 1ABLE 3. SELBCTED ELBCTl0ll-PB0BE DATA 0!l COUP03ITI0[ 0r commonly associated with chabazite, are locally zE0Llr8S abundant in the upper pillowed units. ol1! oll.Io Yellowish greenrosettes of clinoptilolite line the edgesof veins and fracture surfacesin massiveand SlO2 70.,t6 68.35 6'l.02 1102 0 .03 sheet flows and commonly occur as alteration I o .89 12.58 11 .'t7 products glassin hyaloclastite-richzones. $laeu 0.19 0.12 of fresh Uo0 0.05 Fibrous mordenite is typically associatedwith clino- Mao 0.06 0.13 Ca0 3.40 3.46 1.71 ptilolite in vesiclesand along pillow margins. Clino- tra2o 0.96 1.1? 11.68 its Al/Si r20 0.09 1.04 I .00 0.02 ptilolite is distineuishedfromheulandite by 1otal 86.48 86.60 86.93 95.9'l ratio (Mumpton 1960)and by the constancyof the Catlo! Proporiloo! 020peak upon heating@oles 1972). Trace amounts phase mineral, which may st 20.49 ei.gs 29.63 2.45 of a more calcic of this AI 3.72 u,u' 6.1{ 1.10 be heulandite, are present in a few carbonate veins Fo 0.05 0.04 l{a 0.01 (Fig. 4, Table 3). 0.03 0.09 pink, prismatic laumontite fills vesicles 1.06 rla" 0 .82 I White to Ia 0.511 0,99 4.40 0.e1 of somepillows cut by dykesat the baseof the lower 0 0.03 0.58 .56 zone. 48.00 ?2.00 't2.00 7.00 Silica I loiel lro! 6aprosa€d as Fooi - aot dsteoted. a Bordellte 1a v€aloLe of naaslYe llor (fcr82349?)t b' Secondaryjasper, chalcedony,opal-C, opal-CT o11lopt1lpllto-boulardLt6 (KG:82:466)i d analolB€ (BCr82!076). and clear colorless quartz form chiefly in the voids

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LPL :

Lowel ARC Zone THOLEIITE SUITE a- Ftc.5. Distribution of secondaryminerals in the Akaki River section.Depth refers to the stratigaphic thicknessof the sequence.The small circlesrepresent dominantly pillowed units, and the horizontal lines rlprisent dominantly massive flow-units. The shaded area shows the limit of the oxidized zone in this region. solid fn;; indicate that phase the secondary is lbund throughout the unit, and dashedlines indicate that the secondaryphase is locally present. Upper' middle and lower zonesrefer to the lithologic groupingsoutlined in the text. upL/LpL indicatesthe old bouqdary outlined by Gass(1960) and others, and itre trigtr-vgO - high Sio2 suite,/arctholeiite suite indicatesthe position of the new geochemical boundary outlined by nouinJon A. 6}ai1, others: g gmelinite, gv gypr"-, u aragonite. "t

of massiveflows and dykes. Chalcedonyis common- below in terms_ofthe three major stratigraphicse- ly associatedwith celadonite in zones of relatively quences. intensealteration within the massiveflows. Clear, colorlesschalcedony and quartz fill vesicles,vugs and Upper zone ruurow veins, particularly in the lower andesite- da- - cite rhyodacitesuite. Minor amountsof chalcedo- Two styles of alteration characterizethe upper 20 ny, however,are also presentin the massiveflows - 200 m of the extrusivesequence. Along most of - of the high MgO high SiO, suite. Late-stage,fine- the northern flank of Troodos, the uppermostpil- grained chalcedony typically occurs in calcite veins lows are greento reddishgreen in color, highly oxi- throughout the entire extrusivesequence. White to dized and leached. The thick pillow-margins are al- bluish opal-C and opal-CT (classification after teredto smectitewith minor zeoliteand calcite,and Jones& Segdt 1971)form very thin layerson frac- somecalcareous sediment occurs in interpillow voids. ture and cooling-joint surfacesin massiveand sheer Olivine phenocrystshave been completely replaced flows. by orange-brownsmectite, Fe oxidesand calcite, and the groundmassmaterial is altered to calcite and vel- lowish green to orange-brown smectite. Vesicies, ALTERATIoNPanTgRNs AND Tm DISTRIBUTION veins and fractures are filled with assemblagesof ol'SEcoNpany MrNsRAts smectite,carbonate, zeolites and tracesofK-feldspar. Many vesicleshave oxidized halos containing radi- The distribution of secondaryminerals in the ex- ating to globular massesof orange-brownsmectite trusive sequencealong the Akaki River near Maloun- with minor calcite and yellowish green smectite.The da is summarizedin Figure 5. Note that different entire oxidized unit is cross-cutby late, vertical veins mineral assemblagesare common to different types of palygorskiteand calcite. e1 soeling units and to different stratigraphic zones. In the Kambia-Margi area, wherethe extrusivese- pattern The of low-temperature alteration in the quenceis overlain by relatively thick layers of um- Akaki River sectionand adjacentareas is discussed ber @e-Mn sediment),the upper oxidized zoneis ab-

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sent.Pillows in the upper 2-5 m of the sectionhave and Fe oxides.The groundmassis partly to complete- grey, highly alteredinteriors, but fresh glassyrinds ly alteredto yellowish greenand orange-brownsmec- are typically preserved.Interpillow space$contain tite. Vesiclesare typically rimmed by oxidation halos umberiferous sediment,with minor calciteand smec- consisting of orange-brown smectitein a feathery to tite. Olivine phenocrystsare completelyaltered to globular habit. Minor calcile, yellowish green smec- orange-brownsmectite and calcite,and the ground- tite and celadonite are locally associatedwith these mass is replaced by yellowish green smectiteand halos. traces of orange-brown smectite. Vesiclesare gener- The degree of alteration within the massive and ally open but lined with clay minerals. Below this sheetflows rs quite variable. Most flows are relatively level, pillow interiors and brecciasare fresher in ap- fresh in appeaxance,with minor surficial staining of pearancethan thoseat the top ofthe sequence,but celadoniteor hematite.Individual columnsof some fresh glassis lessabundant. Vesiclesare lined with flows display concentric, clay-rich zones within the clay minerals and partly filled with calcite and zeo- outer 1 to 3 cm. Assemblagesof smectite,celadonite, lite, and pillow rirns are generally altered to smec- calcite, clinoptilolite, mordenite and silica are re- tite. Interpillow spacesare filled v/ith smectite and stricted to flow margins, vesicles,vugs and joint sur- calcite,with minor umberiferoussediment and zeo- facesand are locally concentratedin zonesof intense lites. Where present,olivine phenocrystsare com- alteration. monly fresh, but may be replacedby orange-brown Vesiclesand vugs are lined with clay minerals and smectiteand calcite. Clinopyroxene is generallyfrac- partly filled with silica or calcite. Narrow, clay-lined tured and partly altered to colorless or pale green calcite stringers are present in most units and local- smectite. Interstitial pale brown to orange-brown ly contain assemblagesof clinoptilolite, mordenite, smectitecommonly replacesthe groundmass. silica and celadonite. Most clinopyroxeneand plagioclasephenocrysts Middle zone are partly altered to smectite and celadonite, although those in the centres of flows are typically The degreeof alteration in the aphyric to sparse- fresh. The groundmassalteration in the massiveand ly olivine- and clinopyroxene-phyricpillow lavas and sheet flows is similar to that in the pillow lavas. massiveflows of the middle zone is highly variable between,and within, the individual lithologic units. Lower zone Becausethe pillows and massiveflows have distinct assemblagesof zeolites,clays and carbonates,the al- The andesite- dacite- rhyodaciteassemblage of teration is discussedfor eachtype of cooling unit. the lower zone is composedchiefly of massiveand The pillow lavas it the middle zone are greyish sheetflows accompaniedby minor pillow lavasand greento greyishbrown in color. Radial fracturesin hyaloclastites.The patterns of alteration and the the pillows are commonly coatedwith assemblages mineral assemblagesin this sequenssarg similar fo of smectite,calcite, analcime,natrolite, phillipsite, those describedabove for the middle zone. Near the gmelinite and chabazite in decreasingabundance. baseof this zone in the Malounda area, cleax,secon- Similar assemblagesreplace glassy pillow-margins, dary silica is abundant in vesicles,vugs and narrow, and single zeolile phases,with or without calcite, cross-cuttingveins. Along the PedieousRiver near commonly fill clay-lined vesiclesand vugs. Adjacent Kambia, jasper commonly forms along flow tops and to massiveflows and intrusive bodies, celadoniteand in discontinuous,cro$s-cutting veins. Relatively fresh rare clinoptilolite may occur on fracture surfaces. hyaloclastite layers are abundant in both areas. Narrow, calcite veins locally cross-cutindividual pil- Dykes rapidly increasein abundance toward the lows. Late, subvertical, palygorskite - carbonate baseof this zone, and the lavasbecome harder and veins cross-cutor locally bend around the pillow mar- lighter in color. Minor laumontite and trace gins in the upper ponion of this zone. aragonite have been identified in vesicleswithin pil- Resistantvertical zones,composed of reddish grey lows betweenthe dykes. basalt fragments and calcite, occur throughout this part of the sequence.These zones are I - 2 m wide Dtscusslox and have an oxidation halo that extendsinto the sur- rounding rock up to a few metres.Schmincke el a/. The assemblagesof secondaryminerals and their (1983)suggested that they representsyn- to postvol- distribution indicate a complex and locally variable canic fault-zones. history of alteration for the Troodos lavas. However, The primary minerals are relatively fresh in the pil- from this study it hasbeen possible to gain somein- lows of this zone. Clinopyroxenephenocrysts may sight into the styles, controls and timing of the al- be Fe-stainedand clay-rimmed, and plagioclaseis teration processesin this fragment of oceaniccrust. commonly cloudy in appearance.Sparse olivine is Four types of alteration reported from studiesof the commonly altered to orange-brown smectite, calcite oceanic crust have been recognized in the Troodos

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lavas: deuteric, hydrothermal, low-temperature in- laumontite in Iceland. Experimental work by Liou teraction of seawaterwith newly accretedcrust, and (1971)sueeests that laumontite is stablein the tem- seafloor weathering. perature range of 150-300'C. All alteration that occursduring the cooling of the The depositionalhistory of the secondaryminer- lava from the temperatureof eruption to that of am- als in the volcanic pile is complex and locally varia- bient seawateris ascribed to deuteric processes. ble. The generalparagenetic sequence is as follows: However, the effects of such alteration are common- smectite-zeolites or celadonite-carbonate. It must ly difficult to distinguish from later seawater-rock be stressedthat this order of deposition may be interaction, as the deuteric minerals may be over- reversed, particularly the relationship between the printed by later assemblages.High-temperature deu- zeolite and clay minerals, and that more than one teric minerals have not been recognizsd, but we cycle of this sequenceis commonly present.Many believe that some of the alteration in the centresof variations in this pattern havebeen documented, sug- massiveflows and thick pillows, particularly the for- gestingthat the physicochemicalconditions changed mation of low-K, high-Mg smectite,may be ascribed with time. For example,the accessibilityof water to to this process.However, plagioclase in theserocks the cooling units was not continuous. Glassymar- is commonly replacedby K-feldspar, which suggests gins and hyaloclastitesmay be sealedoff at an early at least some penetrationby seawater. stage of alteration, preventing exchangeof cations Relatively high-temperature, hydrothermal alter- betweenthe seawaterand basalt. Later, when the al- ation, well known in the Troodos lavas, occursin teration is more advanced,these glassy zones break narrow stockwork zonesand gossansand affects a down very quickly, supplying a new sourceof ca- very small part of the extrusive sequence.This type tions to the system(Honnorez 1981). of alteration has beendescribed previously (ct, Con- Lateral and vertical variations in the intensity of stantinou & Govett 1973)and will be discussedin alteration are common within individual stratigraphic more detail in a seriesof papersto be publishedin units. For example,in units with an abundanceof the ICRDG Initial Reports.Hence it is not covered massiveflows, the cooling surfacesof one flow may here. be intensely altered, whereasa few metres beneath Most of the alteration observedin the Troodos ex- it, the margins of flows are only slightly stainedwith trusive sequenceis believedto be due to seawater- celadonite.This suggeslsthe presenceof preferred rock interaction during or after crustal accretion. paths of fluid mieration within the extrusivesequence Distinctive mineral-assemblages formed by this that are largely a function of permeability. processoccur throughout the sequence.Pillow lavas A much more comprehensivestudy on a regional have assemblagesthat include smectite,calcite, anal- scale is required to fully understand the role of cime, phillipsite, natrolite, gmelinite and chabazite, permeability in bringing seawaterto a depth greater whereasmassive flows typically contain smectite,cal- than the baseof the extrusivesequence. We know cite, celadonite,clinoptilolite, mordenite, opal-C, that seawaterpenetrates the SheetedDyke Complex opal-CT, chalcedonyand quartz. Although excep- and is eventually brought back up to the surface in tions do occur, the constancyof theseassemblages zones of upwelling (ICRDG 1984, Spooner e/ a/. throughout the extrusive sequencesuggests that the 1977, Heaton & Sheppard 1977). It is not clear, inhslsaf physicalproperties of eachtype of cooling however, if the downward migration of seawaterevi- unit, particularly grain size and permeability, were dent in the extrusive sequenceis sufficient to supply important factors that controlled the distribution of the zonesof upwelling above which the orebodies the secondaryphases. formed. Other channelways,such as the boundaries Isotopic investigationshave mnfirmed that the cir- betweentectonic blocks, may serveas large-scalecon- culating fluid in the Troodos massif was seawater duits for the downward flow of water. (Staudigelet al. 1984, Heaton & Sheppard 1977, The widespreadred to reddishgreen, highly oxi- Spooner et al. 1977). The zeolite and silica assem- dized zone at the top of the extrusive sequenceis be- blages present in the extrusive sequenceare stable Iieved to be the result of prolonged seafloor at temperaturesless than 150'C (Miyashiro & Shido weathering.The assemblagesof secondaryminerals 1970).Many of the zeolitesin theseassemblages are in this zone are similar to those recovered from the stable with smectiteat temperaturesless than 100"C upper levels of the oceanic crust wheie similar in the geothermal fields of Iceland @almason el a/. processeshave beeninvoked (i.e., DSDP Site 417, 1979).This indicatesthat low-temperatureconditions Donnelly et al. 1980). This oxidized zone is absent were dominant during the alteration of the extrusive only where synvolcanic umbers are relatively thick, sequence.The occurrenceof laumontite in pillow as in the Kambia-Margi and Kalavasos areas. We screens at the top of the Basal Group in the suggestthat the umbersthat were depositedin topo- Malounda area indicates that higher temperatures graphic lows on the seafloor formed an early, im- prevailed in these rocks than in the extrusive se- permeable layer that prevented seawater from quence.Palmasonet al. (1979)have assigned a tem- percolating downward into the upper part of the lava perature range of 100-200'C for the stability of pile. In areaswhere the umbers are absent, seawater

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gained accessto the lava pile until the crust was ef- Ba!-sy, D. (1984): Volcanic Stratigraphy qnd Geo- fectively sealedoff by the deposition of pelagic sedi- chemistry of a Portion of the Troodos Ophiolite, ment$ of Campanian age. Cyprus. M.Sc. thesis, Dalhousie University, Hali- fax, N.S.

Suunaany BBan,L.M. (1960):The geology and mineral resources of the Akaki-Lythrodonda area. C\prus Geol. Surv. Four types of alteration have been recognized in Dep., Mem.3. the lavas of the Troodos ophiolite: deuteric, Borrs, J.R. (1972): Composition, optical properties, hydrothermal, low-temperatureseawater-basalt and cell dimensions, and the thermal stability of some seafloor weathering. Relatively high-temperature heulandite group zeolites. Amer. Mineral. 57, minerals typical of deuteric alteration have not been 1463-1493. identified in the Troodos lavas, but low-K smectite CoNstarcrrNou, (1973): may haveformed by this process.Hydrothermal al- G. & Gowrr, C.J.S. Geology, geochemistry, and genesis teration in the lavasis restricted of Cyprus sulfide deposits. to narrow stockwork Econ, Geol. 6E, 843-858. zonesand is extensivein the SheetedDyke Complex. Most of the alteration in the Troodos lavas is the Dor.INEu.y,T.W., TnovrsoN, G, & Sarrsguny, M,H. result of low-temperatureseawater-basalt interaction (1980): The chemistry of altered basalts at site 417, during and after crustal accretion. The assemblages Deep Sea Drilling Project Leg 51. /n Initial Reports of of secondary minerals and their distribution were the Deep Sea Drilling Project Legs 51, 52 & 53, Part 2 (T.W. Donnelly, J. Francheteau, W. Bryan, controlled by such factors permeability, as: type of P. Robinson, M. Flower, M. Salisbury, et al. eds.), cooling uuit, accessibilityof water and composition l3l9-1330. of the host rock. The highly oxidized zone in the upper 20-2O0m of the extrusivesequence indicates that Gass, I.G. (1960): The geology and mineral resources there was a period of prolonged seafloor weather- of the Dhali area. Cyprus Geol. Sum. Dep., Mem.4. ing prior to the deposition of the Campanian pelagic & SMrwrNc,J.D. (1973):Intrusion, extrusion sediments.This oxidized zone is absent in areas and metamorphismat constructiveplate margins: where syrnvolcanicumber was sufficiently thick to evidencefrom the Troodos Massif, Cyprus.Nature limit the percolation of seawaterinto the upper por- u2,26-29. tion of the volcanic pile. Hreror, T.H.E. & Ssrppeno,S.M.F. (1977): The alteration patterns in the extrusive sequence Hydro- gen and oxygen isotope evidencefor seawater- indicate that there is no systematicincrease in the hydrothermalalteration and oredeposition, Troodos degreeof alteration with depth and that no consis- ophiolite,Cyprus. .Iz VolcanicProcesses in Ore Gen- tent stratigraphicor metamorphicboundary can be esis.Geol. Soc.London, Spec. Publ.7,42-57. drawn in the volcanic pile on the basisof distribution of secondaryminerals. Hor.Nonsz,J. (1981):The agrngof the oceaniccrust at Iow temperature.In The Sea. 7. The Ocean Lithosphere(C. Emiliani,ed.). John Wiley & Sons, AcKNowLEDGEMENTS New York.

The researchreported in this paper was supported ICRDG (1984):Are Troodosdeposits an EastPacific analog?Geotimes 29, 12-14. by DalhousieUniversity and the Natural Sciencesand Engineering Research Council of Canada (grant JoNrs,J.B. & Sncmn,E.R. (1971):The natureof opal. 47410 and CSP A4652 to P.T. Robinson). Discus- I. Nomenclatureand constituentphases. J. Geol, sions with J. Mehegan,D. Bailey and other mem- Soc.Aust,1E, 57-68. bersof the ICDRG werevery helpful, and we thank R. Jamieson,L. Horne and H. Staudigelfor criti- Lrou, J.G. (1971): P-T stabilities of laumontite, cally reviewingthe manuscript. We also thank Costas wairakite,lawsonite, and relatedminerals in the sys- Xenophontos, Andreas Panayiotou, tem CaAl2Si2Os-SiO2-H2O.J. Petrology 12, and George 379-411. Constantinou of the Cyprus Geological Survey Department for their assistanceand discwsionswhile McCurrocr, M.T. & CeurnoN,W.E. (1983):Nd-Sr in Cyprus. isotopicstudy of primitivelavas from the Troodos ophiolite, Cyprus:evidence for a subduction-related setting.Geology ll, 727-731. RSFERENcTS MSHEcAN,J.M. & RosrNsor,r,P.T. (1984):Regional Alr, J.C., LevBnNe,C. & MusHr-sNBacus,K. (1985): geochemistryand pyroxenecompositions of the Alteration of the upper oceaniccrust': D.S.D.p. Site Troodosophiolite lavas: the volcanicevolution of 504: mineralogy and processes.Initial Reports, Deep Troodos. Geol.Assoc. Can. - Mineral. Assoc.Can. Sea Drilling Project 83,217-249. ProgramAbstr. 9,88,

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Mryasnrno, A. & Snroo, F. (1970): Progressive RosrxsoN, P.T., Mar.solr, W.G., O'HeanN, T. & metamorphism in zeolite-assemblages.Lithos 3, Scsurxcrr, H.-U. (1983): Volvanic glass composi- 25t-260. tiorx of the Troodos ophiolite, Cyprus. Geologt ll, 400-4M. Moonrs, E.M., RosrNsoN,P.T., Merpas, J. & Scnuncrc, H.-U., RaurENscHLBrN,M., RonnvsoN,P.T. XsNopHoNoros,C. (1984): Model for the origin of & MsHrcAN, J.M. (1983): Troodos extrusive series the Troodos Massif, Cyprus, and other mideast of Cyprus: a comparison with oceanic crust. Geol- ophiolites. Geology 12, 500-503. ogy 11,405409.

& Vrr.re,F.J. (1971):The TroodosMassif, SurwrNc, J.D., SruoNran,K.O. & Gass,I.G. (1975): Metabasalts from the Troodos Massif, Cyprus: Cyprus and other ophiolites as oceanic crust: evalu- genetic implication deduced from petrography ation and implications. Roy. Soc. London, Phil. and geochemistry. Trans. 268,443-466. trace element Contr. Mineral. Petrol- ogy 57, 49-54.

MuurroN, F.A. (1960): Clinoptilolite redefrnd.. Amer. Spoot.tEn,E.T.C., Cnepver, H.J. & SrrcwrNc,J.D. Mineral. 45, 351-369. (1977): Strontium isotopic contamination and oxidation during ocean floor hydrothermal metamorphism of the ophiolitic rocks of the Troodos NarleNn, J.H. & ManoNEv, J.J. (1978): Alteration in Massif, Cyprus. Geochim. Cosmochim. Acta 41, igneousrocks at Deep SeaDrilling Project Sites458 873-890. and 459, Mariana fore-arc region: relationship to basement structure. .ln Initial Reports of the Deep Stauorcrr, H.,.Grrrrs, K.M., Ronnvsor.r,P.T. & GrssoN, SeaDrilling Project 60 (M. Lee & R. powell, eds.), I.L. (1984):Isotopic composition of somesecondary 769-788. phasesin the Troodos ophiolite, Cyprus. Geol. As- soc. Can. - Mineral. Assoc, Can. Program Abstr. 9, 108. Parvasot, G., AnuonssoN, S., Fnotnrssox, I.B., KrusnriratNsoorrlR, H., Saguurossou, K., Sr-rnerus MuEnr.Bt{sacus, K., RrcHemsoN, S.H. & soN,V., Srcrucnuvssor.t,8., Torr,ussox,J. & Knrsr- Hanr, S.R. (1981): Agents of low temperature (1979): rANssoN,O. The Iceland crust: evidence ocean crust alteratiot. Contr. Minerol. Petrologlt TT, from drillhole dala on structure and processes.In 150-157. Deep Drilling Resultsin the Atlantic Ocean: Ocean Crust, Maurice Ewing Series2 (M. Talwani, C.G. Harrison & D.E. Hayes, eds.).Amer. Geophys. Un- Received fune 7, 1984, revised manuscript accepted ion, Maurice Ewing Ser. 2, 43-65. February 19, 1985.

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