Canodian Mineralogist
Vol. 23, pp. 431441 (198s)
LOW.TEMPERATURE
ALTERATION
OF THEEXTRUSIVE
SEOUENCE,
TROODOS
OPHIOLITE,
CVPRUS
KATHRYN M. GILLIS AND PAUL T. ROBINSON
Departmentof Geologltand Centrefor Marine Geology,DalhousieUniversity,Hatifax, Nova ScotiaB3H 3Js
ABSTRAcT
The lavas of the Troodos ophiolite have been altered
primarily by low+emperatureinteraction with seawaterand
by localized hydrothermal upwelling. An uppermost zone
of highly oxidized lavas that contain abundant calcite and
smectiteis believedto be due to seafloor weathering.In
a few sections,where thick layers of umber overlie the volcanic pile, the oxidized zone is absent, suggestingthat the
crust was sealedoff from circulating seawater.Below the
oxidized zone, the rocks are less pervasively altered and
locally contain fresh glass.Assemblagesof ana'lcime,natrolite, phillipsite, chabaziteand gmelinite are common in the
pillow lavas of this sequencewhereasassemblages
of clinoptilolite, mordenite and celadonite are primarily associated with the massiveflows. A basal sequenceof more
siliceouslava has abundant secondarysilica. Alteration in
thesezonesis believedto reflect seawater-rock interaction
during or shortly after crustal accretion. Outside the very
localizedzonesof hydrothermal alteration, the Troodos lavashavenot beenpervasivelymetamorphosed,and below
the upper oxidized zone there are no systematicvariations
in the intensity or characterof alteration with depth. The
observed assemblagesof secondary minerals can be explained by local variations in permeability, type of cooling unit, temperature, waterlrock ratio and initial lava
composition.
Keywords: alteration, low-temperature,Troodos ophiolite,
Cyprus, oceancrust, permeability, clays, zeolites, oxidation.
SouvernE
Les lavesdu massifophiolitique de Troodos ont princ!
palement 6td altereespar un dchangeavec de I'eau de mer
d bassetempdratureet, localement,par jaillissementde fluideshydrothermaux. Une zone supdrieurede coul6esfortement oxyd&s, enrichiesen calciteet smectite,repr6enterait
une zonedelessivagemarin. Oir d'6paissescouchesd'ombre
recouwent I'empilementvolcaniquedansquelquessections,
la zone oxyd6eest absente,ce qui indiquerait que la cro0te
6tait isol6ede I'eau de mer en circulation. En dessousde
la zone oxyd€e,les rochessont moins complbtementalt6rdeset contiennentm€me,ici et li, du verrefrais. Desassemblages d analcime, natrolite, phillipsite, chabasite et
gmelinite sont courants dans les laves i coussinsde cette
sdquence,et les assemblages
i clinoptilotite, mordeniteet
c€ladonitesont surtout associ6saux coul6esmassives.Une
s6quenceinfdrieure de lave plus siliceusecontient beaucoup
de silice secondaire. L'alt6ration de ces zones refl6terait
l'interaction desrochesavecde l'eau de mer pendant ou
tdt aprdsla formationdeIa cro0teoc6anique.
En dehors
deszonestrbslocalis€es
d'alt6rationhydrothermale,
leslaves
du Troodosn'ont pas6t6sujettesd un m6tamorphisme
rdpandu;d'ailleurs,endessous
dela zonesup€rieure
oxy'
dde, on ne trouve aucunevariation systdmatique
dans
I'intensit6ou le caractdre
deI'alt6rationavecla profondeur.
Lesassemblages
demin6rauxsecondaires
ddpendraient
des
variationslocalesen perm6abilit6,typed'unit6 de refroidissement,
temp6rature,
rapportvolumiqueeaulsolide
et
composition
initialede la lave.
(Traduit par la R6daction)
Mots-cl6s:alt|ration, bassetemp€rature,
massifophiolitiquedeTroodos,Chypre,cro0teocfunique,
permdabilit6, argiles,zdolites,oxydation.
INTRODUCTION
Previousstudies,summarizedin Honnorez(1981),
haveshown that alteration in the upper oceaniccrust
is largely the result of seawater-basaltinteraction.
The distribution and paragenesisof secondaryminerals have been shown to dependupon such factors
as lithology, fluid composition, temperatureand age
(Nt et al, 1985,Staudigelet al. 1981,Natland & Mahoney 1978).The shallow depth of most Deep Sea
Drilling Project holes and the restricted lateral control over each site have limited our understanding
of the processof seawaterpenetrationinto the crust
and the extent of hydrothermal circulation and
metamorphismat depth.
Ophiolites provide an alternative for studying in
situ oceaniccrust. Unfortunately, the transportation
and emplacementhistory of many ophiolites have
made it difficult to study their original pattern of
alteration. Previousstudentsof the Troodos ophiolite concluded that the extrusive sequencehad been
metamorphosed,resulting in the developmentof two
distinct facies interpreted in terms of axis and offaxis volcanism(Gass& Smewing1973,Smewingel
al. 1975).In this paper, we show that the extrusive
rocks of the Troodos ophiolite have undergonethe
sametype of low-temperature alteration as that observedin the modern oceaniccrusu also, we discuss
the physical conditions that control the distribution
and paragenesisof the secondaryminerals.A more
detailed study of the physicochemicalconditions active during the different stagesof alteration is still
underway.
431
432
THE CANADIAN MINERALOGIST
Recentstudiesof the Troodos Massif of Cyprus
have confirmed that it is a fragment of oceanic
lithosphere,as suggestedby Moores & Vine (1971),
although it is now believed to have originated in a
subduction-zoneenvironment rather than at a midocean ridge (Moores et al. 1984, Robinson el a/.
1983,Schminckeet al:1983, Mcculloch & Cameron
1983). The excellent exposure and relatively undeformed nature of this complex provide a unique
opportunity to study the processesof alteration
through a completesectionof oceaniclithosphere.
Early workers divided the extrusive sequenceinto
the Upper Pillow Lavas (UPL) and Lower Pillow Lavas (LPL) on the basisof color, abundanceof olivine phenocrysts,assemblages
of secondaryminerals
and the location of the sulfide orebodies@ear1960,
Gass1960).Gass& Smewing(1973)divided the sequenceinlo an axis sequence,consistingof the Lower
Pillow Lavas and the SheetedDykes, and a younger, off-axis sequenc€,representedby the Upper Pillow Lavas, based on a supposed metamorphic
boundary within the complex.
MALod,JNDA
Robinsonet al. (1983)haveoutlined two geochemical suites for the northern flank of the Troodos
ophiolite on the basisof glasscompositions:1) a lower andesite- dacite - rhyodaciteassemblageof arc
tholeiite affinity and 2) an upper picritic basalt - andesiticbasalt assemblagewith a depletedarc tholeiite
affinity. The stratigraphic level of this new geochemical boundary is variable within the extrusive sequencein relation to the old UPL/LPL boundary
(Frg. l). A third suiteof highly depletedbasaltshaving "boninitic" affinities has beenrecognizedon the
southern flank of Troodos (J.M. Mehegan, pers.
comm. 1984).
Detailed studiesof stratigraphicallycontrolled sections betweenthe villagesof Malounda and Margi
(Fie. 2) have shown that the extrusive sequencehas
not beenpervasivelymetamorphosed.Freshvolcanic
glassis preservedsporadically throughout the extrusive sequence,efiending downward to the top of the
Basal Group (9090sheeteddykes). Furthermore, the
distribution of the secondaryminerals within the extrusive sequenceindicatesthat thereis neithera sys-
KAMBIA
;.'..j.t
MARGI
UPL
-tpt
-
HIGH MsOHIGH SiOtSUTTE
eac
THOLEIITE
SU'TE
UPL
LPL
HIGH MgO.
H|GH SiO,'
SUITE
ARC
THOLEIITE
SUITE
s
o
c
o
ffi
HIAH MgOHIGH SiO2
SU'IE
ARC
THOLEIITE
SUITE
%
ffi
upper
Zone
Middte zone
Lower
zone
FIo. l. Schematicrepresentationof the geology of the study areas,outlining the stratigraphicposition of the old Upper Pillow Lavallower Pillow Lava boundaryand
the new geochemicalsuites, Upper zone: aphyric to olivine-phyric basaltsand picrites;middle zone:aphyricto slightly olivine-clinopyroxene-phyricbasaltto andesitic basalt; lower zone: aphyric to slightly clinopyroxene-plagioclase-phyric
andesites- dacites - rhyodacites.
LOW.TEMPERATURE ALTERATION
IN THE TROODOS OPHIOLITE. CYPRUS
Ftc. 2. Geologicalmap of the,northeasternportion 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
depth nor a metamorphicboundary within the lavas.
Localized zones of high-temperature hydrothermal
alteration underlie the rhassive-sulfideorebodiesIInternational Crustal Research Drilling Croup
(ICRDG) 19841,but theseare spatially restrictedand
have little overall effect on the lavas. The presence
of an oxidized zone in the upper 20 - ZO0m of the
extrusivesequenceand the relationshipsamongthe
ass€mblages
of secondaryminerals,type of cooling
unit and host Iithology both suggestthat alteration
waslargely due to circulation of low-temperatureseawater during and after crustal accretion. Regional
reconnaissancehas shown that the study area is
representativeofthe alteration in the northerrt flank
of the complex.
the basisof color, type of cooling unit and mineralogy (Mehegan& Robinson 1984,Schminckeet ol.
1983). These units have been grouped into three
major stratigraphic sequencesreferred to as the upper, middle and lower zones. Although the same
generalsequenceis found throughout the area,these
zonesvary greatly in thickness, lithology and lateral extent.
The basal 500 - 600 m of the extrusive sequence
contains aphyric to sparsely clinopyroxene- and
plagioclase-phyricmassiveand sheetflows, pillow
lavas and hyaloclastitesof andesitic to dacitic composition that comprise the arc tholeiite suite of
Robinson et al. (1983). Dykes are present throughout this zone and increasein abundancetoward its
base. This lower zone is overlain by approximately
5fi) m of greento greyishgreenpillow lavasand masRncroNar Grorocy
sive flows. locally separated by thin layers of
hyaloclastite. Theseaphyric to slightly olivine- and
Recentstudiesofthe lavason the northern flank clinopyroxene-phyricunits havea basaltto basaltic
of Troodos haveshownthat the extrusivesequence andesitecompositionand 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-grainedbasalticto doleritic bodies
434
THE CANADIAN MINERALOGIST
IABLE
S102
ilag,
UB0
t{aO
Ca0
tra2o
r2o
9'?o3
IOEAI
1.
SELECTED ELECTIOtr.Pf,OBE DArl
Fe
Ug
CLAY UItrEIALS
oolqdb
angoo
FDg Od
snooo
gBgof
sEgog
snooh
s 5. 8 9
2.76
22.31
0.04
5.15
0.63
5\ .22
5.07
15 . 9 1
43.06
1t.05
. 13 . 8 5
47.88
8.52
9 . 39
0.24
18.26
0. 9 7
115.42
7.15
7. 9 7
2 1. 2 0
0.90
3 ? .X 9
9.99
28.?8
0.06
7. 94
2.38
o:5u
38.?4
11.52
12.05
45.61
12.45
l.'ll
20.26
t,"
23.8',1
8?.30
e,r]:e
5'.57
0. 0 6
82.39
',t,10
0.35
16.68
O.72
glos
r]oe
91.?4
Catl.oo
ST
Otr COI'TOSITIOTS 08
oelEda
:
"1oa
e r ] ro
88.51
Proporilgna
o!
tbo
82.64
basta
of
"=t'
sr]zo
20 0
3.65
5.53
1.94
r.44
6.00
1.93
0.12
1.80
0.39
4.31
0.30
4.68
0.30
7.21
0.51
2.93
7.35
0.81
1. 8 0
5.94
1. 8 0
1.60
6.46
1.35
1. 0 6
6.37
'1.18
0. 93
5
' , .t 6
. '91 9
1.21
0.11
1.44
0.05
3. 4 3
0.11
3.67
0.1s
4.43
0.1!
1. 1 2
I .57
0.511
u.1,
!la
r
.
Total
1ro!
s:prosaed
a9 Feo;
-
lot
dot9ot€d.
o yellorlsb
a' b grouldbass
(K0s82r303'
ro382t389)t
EroqodBasa
ooladoltte
8te€!
(Kor82r108)t
gresD SrourdEaao sEeotlte'
blddlo
of o
d yetlorlsb
soeotlt€
f
(fOs82slO8); € oraDSe-broro groulahaaa sDsottts'
oeBtle qf o ((0:82!108),
h ona[8€-btor!
(K0:82r522)r
sleoilt€
8,
orao8q-bror!
6Bootttor
vsa{o1s Ltal.!8
pboaooryst
(EO:82:008).
ln otlvl.le
are present at this level, but are lessabundant than SiO2suite. Intrusive bodies are absent at this stratiin the underlying sequence.Theupper zoneat the top graphic level.
of the sectionis approximately 20 to 300m thick and
METHODS
consistsof aphyric to olivine-phyric pillow basalts
of
Samplesrepresentingthe different assemblages
and picrites that are also part of the high MgO high
A
celadonite
smeclite
?6'o
;<
O
smectite in olivino
o
smectite in glass
-4.o
N
Y
30
a
a
55p
O
4 " ^l^*t"i^:
a ^
aA
a
a
"li"
ilil
,f:T^^'^i{^
?. r i :-.,1^^
Feo/ (Feot Mso)
FIc.3. Composition of clay minerals: wt. Vo K2O versusFeO/(FeO+MgO).
LOW.TEMPERATURE ALTERATION IN THE TROODOS OPHIOLITE, CYPRUS
secondaryminerals and intensitiesof alteration were
collected from stratigraphically controlled sections,
chiefly within the Akaki River canyon near
Malounda, the PediaeosRiver canyon near Kambia
and the Margi area. The stratigraphic relations in
theseareaswere defined by Schminckeet al. (1983),
J.M. Mehegan(pers.comm. 1984)and Bailey(1984),
respectively.
Hand-picked secondarymineralswere identified
on a Philips X-ray diffractometer (CuKo radiation).
Oriented smear-slidesof clayswere scannedfrom 2"
to 4O"20 at lo per minute. Representativesamples
of the different groups of clays were treated with
ethyleneglycol and rerun from 2" to l5o, The remaining secondaryminerals were scannedfrom 4o
to 60o X) at lo per minute.
The chemical analyses were obtained using
electron-microprobe microanalyzers at Memorial
University, St. John's, Newfoundland, the Smithsonian Institute, Washington,D.C. and Dalhousie
University, Halifax, Nova Scotia. Mineral paragenesiswas determinedfrom the polishedthin sections used for microprobe analysis.
SEcoNDARyMnnnals
The assemblages
of secondarymineralsconsistof
TABLE2.
435
SELECTED
ELECTRON-PROBE
DATAONCOMPOSITION
OF
CARBONATES
Si 02
Alzoa
F€otMtro
Uao
0,06
tlrs
3.111
0.82
Ca0
,o.50
tr"20
Keo
0.02
0.01
Iotal
.63.11
0.07
0.05
0.01
0. 0 2
0.07
0.57
57.39
0.03
0.06
0.56
61.93
0.03
58,32
62-.63
1 fraotu.o
fllllaa
oalclto,
fC82-153
2 grourdBaa8
oalolte,
C!-1..12.I5
B
1! veslot6,
3 oaloLto
CY-l.I2..15
s
clay minerals, zeolites,silica, carbonatesand iron
oxides.
CIay minerals
Clay minerals, by far the most abundant of the
secondaryphasesin the volcanic pile, replacethe
primary mineralsand groundmassmaterial and line
or fill vesicles,vugs and veins.
Smectite,the most widespreadclay mi4eral, was
identified by its broadlasal peak at 16 A and the
subsequentshift to 18A upon treatmentwith ethy-
Ca+Mg
A
phillipsite
a
c l i n o p t i l o l i t e- h e u l a n d i t e
A
m o r d s ni t e
o
n a t r ol i t e
o a n a l ci me
Na
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.
436
THE CANADIAN MINERALOGIST
lene glycol. Severalvarieties of smectite have been
distinguishedon the basisof habit, color and composition. The compositionrangesfrom Mg-rich to
Fe-rich with varying K content (Table I, Fig. 3).
Orange-brownto brown smectiteis generallyMgrich, whereasyellowish green smectiteis Fe-rich,
although some brown varietiesin the groundmass
areFe-richand form oxidizedhalosaroundvesicles.
Orange-brown,Mg-rich smectite,Fe hydroxidesand
calcite replaceolivine in the upper part of the section. In the middle and lower zonesa similar smectite partly replacesplagioclase.Smectitethat replaces
primary minerals is very finely crystalline, whereas
that filling voids is usually fibrous to platy and radiates out from the wall of the cavity. Smectite that
replacesgroundmassmaterial may be fibrous, radiating, massiveor globular
Celadonite,identified by its basal peak at 10 A
and its distinctivebluish greencolor, replacesinterstitial groundmass-materialand partially fills voids,
where it occrus €lsa fibrous, late-stagealteration
product. Chemically,celadoniteis distinguishedfrom
smectiteon the basisof FeO/(FeO + MgO) and K2O
content (Fig. 3, Table 1). The generalparagenetic
sequenceof theseclay mineralsis: Mg-rich, K-poor
smectite-Fe-rich smectite with variable K content*celadonite.
Palygorskitewqs recognizedon the basis of the
10.4,6.4,and 4.5 A peaks,which remainunchanged
upon glycolation and heating (c/. Natland & Mahoney 1978).This papyraceousto fibrous mineral
is restricted to late-stage,vertical veins in the upper
500 m of the sequence,where it is commonly associated with calcite.
1ABLE 3.
SELBCTED
SlO2
1102
$laeu
Uo0
Mao
Ca0
tra2o
r20
1 ot a l
st
70.,t6
0.03
I o.89
0.19
0. 0 5
0.06
3. 4 0
0.96
0.09
86.48
20.49
3.72
0.05
0. 0 1
0.03
1. 0 6
0.511
0.03
AI
Fo
l{a
Ia
loiel
lro!
Carbonatesare pervasivethroughout the upper
zone of the extrusive sequenceand are locally abundant in the middle and lower zones.Calcite forms
irregular patchesand stringersin the groundmassof
most lithologic units. It is relatively pure, but may
contain minor amounts (< l9o) of Si, Mg, Fe, Na
and K. A few crystalsin narrow, cross-cuttingveins
have up to 390 Mn (Table 2). Assemblagesof calcite, smectite and Fe hydroxides replace olivine
phenocrystsin the basaltsof the upper zone. Ironstained, drusy and botryoidal calcite commonly fills
cavitiesin theserocks. Many veinshave undergone
severalstagesof calcitegrowth, asindicatedby successivelayers of euhedralcrystalsthat €uecommonly mantled by smectite or celadonite (or both).
Aragonite was identified in one occurrencenear
the base of the sequence.
Zeolites
Zeolites form distinctive assemblageswith other
secondary minerals throughout the extrusive sequence. They have been identified petrographically, on the basisof their XRD pattern and on the basis of their Ca, Na and K contents@ig. 4, Table 3).
Clear, rvell-formedcrystalsof analcime,accompaniedby sparse,radiating aggregatesof pink phillipsite, commonly line fractures and pillow margins,
line veins and locally replace groundmassmaterial
of both massiveflows and pillows. phillipsite, an alterationproduct of freshglass,is the earliestofthese
two phases.Radiating prisms of natrolite, commonly
associatedwith analcime,occur chiefly in vesiclesand
vugs. Hexagonalclustersof pinkish orangegmelinite,
0r
DATA 0!l COUP03ITI0[
ELBCTl0ll-PB0BE
commonly associated with chabazite, are locally
zE0Llr8S
abundant in the upper pillowed units.
oll.Io
ol1!
Yellowish greenrosettesof clinoptilolite line the
of veins and fracture surfacesin massiveand
edges
6'l.02
68.35
sheet flows and commonly occur as alteration
11. ' t 7
12.58
products of fresh glassin hyaloclastite-richzones.
0.12
Fibrous mordenite is typically associatedwith clino0.13
ptilolite in vesiclesand along pillow margins. Clino1.71
3.46
1 1 . 6 8 ptilolite
1.1?
is distineuishedfromheulandite by its Al/Si
0.02
I .00
1.04
86.93
86.60
9 5. 9 ' l
ratio (Mumpton 1960)and by the constancyof the
Catlo!
Proporiloo!
020peak upon heating@oles1972).Trace amounts
of a more calcic phaseof this mineral, which may
2.45
29.63
ei.gs
1 . 1 0 be heulandite, are present in a few carbonate veins
6.1{
u,u'
0. 0 4
(Fig. 4, Table 3).
0.09
White to pink, prismatic laumontite fills vesicles
I
0.82
rla"
0.e1 of somepillows cut by dykesat the baseof the lower
4.40
0,99
0.56
0.58
zone.
48.00
I
Carbonqtes
6aprosa€d
a Bordellte
1a v€aloLe
o11lopt1lpllto-boulardLt6
(BCr82!076).
't2.00
?2.00
as Fooi
of
-
aot
naaslYe llor
(KG:82:466)i
7.00
Silica
dsteoted.
(fcr82349?)t
d analolB€
b'
Secondaryjasper, chalcedony,opal-C, opal-CT
and clear colorless quartz form chiefly in the voids
LOW-TEMPERATURE ALTERATION
437
IN THE TROODOS OPHIOLITE. CYPRUS
,--"*r--"..$st$.g--SXg
:
v
:l
tt
tl
tl
UPL
gv
I
HIGH MgO
HIGH SiO2
SUITE
LPL
:
ARC
THOLEIITE
SUITE
Lowel
Zone
a-
Ftc.5. Distribution of secondarymineralsin the Akaki River section.Depth refers to the stratigaphic
thicknessof
the sequence.The small circlesrepresentdominantly pillowed units, and the horizontal lines rlprisent dominantly
massiveflow-units. The shaded area shows the limit of the oxidized zone in this region. solid fn;; indicate that
the secondaryphaseis lbund throughout the unit, and dashedlines indicate that the secondaryphaseis locally present.
Upper' middle and lower zonesrefer to the lithologic groupingsoutlined in the text. upL/LpL indicates
the old
bouqdary outlined by Gass(1960)and others, and itre trigtr-vgO - high Sio2 suite,/arctholeiite suite
indicatesthe
position of the new geochemicalboundary outlined by nouinJon A.
6}ai1, others: g gmelinite, gv gypr"-, u
aragonite.
"t
of massiveflows and dykes. Chalcedonyis commonly associatedwith celadonite in zones of relatively
intensealteration within the massiveflows. Clear,
colorlesschalcedonyand quartz fill vesicles,vugsand
ruurow veins, particularly in the lower andesite- dacite - rhyodacitesuite.Minor amountsof chalcedony, however,are also presentin the massiveflows
of the high MgO - high SiO, suite. Late-stage,finegrained chalcedony typically occurs in calcite veins
throughout the entire extrusivesequence.White to
bluish opal-C and opal-CT (classification after
Jones& Segdt 1971)form very thin layerson fracture and cooling-joint surfacesin massiveand sheer
flows.
below in terms_ofthe three major stratigraphicsequences.
Upper zone
Two stylesof alteration characterizethe upper 20
- 200 m of the extrusivesequence.Along most of
the northern flank of Troodos, the uppermostpillows are greento reddishgreenin color, highly oxidized and leached. The thick pillow-margins are alteredto smectitewith minor zeoliteand calcite,and
somecalcareoussedimentoccursin interpillow voids.
Olivine phenocrystshave been completely replaced
by orange-brownsmectite,Fe oxidesand calcite, and
the groundmassmaterial is altered to calcite and vellowish green to orange-brown smectite. Vesicies,
ALTERATIoNPanTgRNsAND Tm DISTRIBUTION veins and fractures are filled with assemblagesof
ol'SEcoNpany MrNsRAts
smectite,carbonate,zeolitesand tracesofK-feldspar.
Many vesicleshave oxidized halos containing radiThe distribution of secondaryminerals in the ex- ating to globular massesof orange-brownsmectite
trusive sequencealong the Akaki River near Maloun- with minor calcite and yellowish greensmectite.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 extrusiveseThe pattern of low-temperature alteration in the quenceis overlain by relatively thick layers of umAkaki River sectionand adjacentareasis discussed ber @e-Mn sediment),the upper oxidized zoneis
ab-
438
THE CANADIAN MINERALOGIST
and Fe oxides.The groundmassis partly to completely alteredto yellowishgreenand orange-brownsmectite. Vesiclesare typically rimmed by oxidation halos
consistingof orange-brown smectitein a feathery to
globular habit. Minor calcile, yellowish greensmectite and celadonite are locally associatedwith these
halos.
The degreeof alteration within the massiveand
sheetflows rs quite variable. Most flows are relatively
fresh in appeaxance,with minor surficial staining of
celadoniteor hematite.Individual columnsof some
flows display concentric, clay-rich zoneswithin the
outer 1 to 3 cm. Assemblagesof smectite,celadonite,
calcite, clinoptilolite, mordenite and silica are restricted to flow margins, vesicles,vugs and joint surfacesand are locally concentratedin zonesof intense
alteration.
Vesiclesand vugs are lined with clay minerals and
partly filled with silica or calcite. Narrow, clay-lined
calcite stringers are present in most units and locally contain assemblagesof clinoptilolite, mordenite,
silica and celadonite.
Most clinopyroxeneand plagioclasephenocrysts
partly altered to smectite and celadonite,
are
Middle zone
although those in the centresof flows are typically
The degreeof alteration in the aphyric to sparse- fresh. The groundmassalteration in the massiveand
ly olivine- and clinopyroxene-phyricpillow lavasand 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
of
The andesite- dacite- rhyodaciteassemblage
of zeolites,claysand carbonates,the alassemblages
and
of
massive
chiefly
zone
is
composed
lower
unit.
the
of
cooling
is
for
each
type
teration discussed
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
in this sequenssarg similar fo
the pillows are commonly coatedwith assemblages mineral assemblages
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,seconreplaceglassypillow-margins, dary silica is abundant in vesicles,vugs and narrow,
Similar assemblages
and single zeolile phases,with or without calcite, cross-cuttingveins. Along the PedieousRiver near
commonly fill clay-lined vesiclesand vugs. Adjacent Kambia, jaspercommonly forms along flow tops and
to massiveflows and intrusive bodies, celadoniteand in discontinuous,cro$s-cuttingveins. Relativelyfresh
rare clinoptilolite may occur on fracture surfaces. hyaloclastite layers are abundant in both areas.
Dykes rapidly increasein abundancetoward the
Narrow, calcite veins locally cross-cutindividual pillows. Late, subvertical, palygorskite - carbonate baseof this zone, and the lavasbecomeharder and
veinscross-cutor locally bend around the pillow mar- lighter in color. Minor laumontite and trace
gins in the upper ponion of this zone.
aragonite have beenidentified in vesicleswithin pilResistantvertical zones,composedof reddishgrey lows betweenthe dykes.
basalt fragments and calcite, occur throughout this
Dtscusslox
part of the sequence.Thesezonesare I - 2 m wide
and have an oxidation halo that extendsinto the surof secondarymineralsand their
The assemblages
rounding rock up to a few metres.Schminckeel a/.
(1983)suggested
that they representsyn-to postvol- distribution indicate a complex and locally variable
history of alteration for the Troodos lavas.However,
canic fault-zones.
from this study it hasbeenpossibleto gain someinThe primary mineralsare relatively fresh in the pillows of this zone. Clinopyroxenephenocrystsmay sight into the styles, controls and timing of the alin this fragment of oceaniccrust.
be Fe-stainedand clay-rimmed, and plagioclaseis teration processes
commonly cloudy in appearance.Sparseolivine is Four types of alteration reported from studiesof the
commonly altered to orange-brown smectite,calcite oceanic crust have been recognizedin the Troodos
sent.Pillows in the upper 2-5 m of the sectionhave
grey, highly alteredinteriors, but fresh glassyrinds
are typically preserved.Interpillow space$contain
umberiferoussediment,with minor calciteand smectite. Olivine phenocrystsare completelyaltered to
orange-brownsmectiteand calcite,and the groundmass is replaced by yellowish green smectiteand
tracesof orange-brownsmectite.Vesiclesare generally open but lined with clay minerals. Below this
level, pillow interiors and brecciasare fresher in appearancethan thoseat the top ofthe sequence,but
fresh glassis lessabundant. Vesiclesare lined with
clay minerals and partly filled with calcite and zeolite, and pillow rirns are generally altered to smectite. Interpillow spacesare filled v/ith smectite and
calcite,with minor umberiferoussedimentand zeolites. Where present,olivine phenocrystsare commonly fresh, but may be replacedby orange-brown
smectiteand calcite. Clinopyroxeneis generallyfractured and partly altered to colorless or pale green
smectite. Interstitial pale brown to orange-brown
smectitecommonly replacesthe groundmass.
LOW-TEMPERATURE ALTERATION IN THE TROODOS OPHIOLITE, CYPRUS
lavas: deuteric, hydrothermal, low-temperature interaction of seawaterwith newly accretedcrust, and
seafloor weathering.
All alteration that occursduring the cooling of the
lava from the temperatureof eruption to that of ambient seawateris ascribed to deuteric processes.
However, the effects of suchalteration are commonly difficult to distinguish from later seawater-rock
interaction, as the deuteric minerals may be overprinted by later assemblages.
High-temperaturedeuteric minerals have not been recognizsd, but we
believe that some of the alteration in the centresof
massiveflows and thick pillows, particularly the formation of low-K, high-Mg smectite,may be ascribed
to this process.However,plagioclasein theserocks
is commonly replacedby K-feldspar, which suggests
at least some penetrationby seawater.
Relatively high-temperature, hydrothermal alteration, well known in the Troodos lavas, occursin
narrow stockwork zonesand gossansand affects a
very small part of the extrusive sequence.This type
of alteration has beendescribedpreviously (ct, Constantinou & Govett 1973)and will be discussedin
more detail in a seriesof papersto be publishedin
the ICRDG Initial Reports.Henceit is not covered
here.
Most of the alteration observedin the Troodos extrusive sequenceis believedto be due to seawaterrock interaction during or after crustal accretion.
Distinctive mineral-assemblages formed by this
processoccur throughout the sequence.Pillow lavas
have assemblages
that include smectite,calcite, analcime, phillipsite, natrolite, gmelinite and chabazite,
whereasmassiveflows typically contain smectite,calcite, celadonite,clinoptilolite, mordenite, opal-C,
opal-CT, chalcedonyand quartz. Although exceptions do occur, the constancyof theseassemblages
throughout the extrusive sequencesuggeststhat the
inhslsaf physicalpropertiesof eachtype of cooling
unit, particularly grain size and permeability, were
important factors that controlled the distribution of
the secondaryphases.
Isotopic investigationshave mnfirmed that the circulating fluid in the Troodos massif was seawater
(Staudigelet al. 1984, Heaton & Sheppard 1977,
Spooner et al. 1977). The zeolite and silica assemblages present in the extrusive sequenceare stable
at temperatureslessthan 150'C (Miyashiro & Shido
1970).Many of the zeolitesin theseassemblages
are
stable with smectiteat temperatureslessthan 100"C
in the geothermal fields of Iceland @almason el a/.
1979).This indicatesthat low-temperatureconditions
were dominant during the alteration of the extrusive
sequence.The occurrenceof laumontite in pillow
screens at the top of the Basal Group in the
Malounda area indicates that higher temperatures
prevailed in these rocks than in the extrusive sequence.Palmasonetal. (1979)haveassigneda temperature range of 100-200'C for the stability of
439
laumontite in Iceland. Experimental work by Liou
(1971)sueeeststhat laumontite is stablein the temperature range of 150-300'C.
The depositionalhistory of the secondaryminerals in the volcanic pile is complex and locally variable. The generalparageneticsequenceis as follows:
smectite-zeolites or celadonite-carbonate. It must
be stressedthat this order of deposition may be
reversed, particularly the relationship between the
zeolite and clay minerals, and that more than one
cycle of this sequenceis commonly present.Many
variations in this pattern havebeendocumented,suggestingthat the physicochemicalconditions changed
with time. For example,the accessibilityof water to
the cooling units was not continuous. Glassymargins and hyaloclastitesmay be sealedoff at an early
stage of alteration, preventing exchangeof cations
betweenthe seawaterand basalt. Later, when the alteration is more advanced,theseglassyzonesbreak
down very quickly, supplying a new sourceof cations to the system(Honnorez 1981).
Lateral and vertical variations in the intensity of
alteration are commonwithin individual stratigraphic
units. For example,in units with an abundanceof
massiveflows, the cooling surfacesof one flow may
be intensely altered, whereasa few metres beneath
it, the margins of flows are only slightly stainedwith
celadonite.This suggeslsthe presenceof preferred
paths of fluid mieration within the extrusivesequence
that are largely a function of permeability.
A much more comprehensivestudy on a regional
scale is required to fully understand the role of
permeability in bringing seawaterto a depth greater
than the baseof the extrusivesequence.We know
that seawaterpenetratesthe SheetedDyke Complex
and is eventually brought back up to the surface in
zones of upwelling (ICRDG 1984, Spooner e/ a/.
1977, Heaton & Sheppard 1977). It is not clear,
however,if the downward migration of seawaterevident in the extrusive sequenceis sufficient to supply
the zonesof upwelling above which the orebodies
formed. Other channelways,suchas the boundaries
betweentectonicblocks, may serveaslarge-scaleconduits for the downward flow of water.
The widespreadred to reddishgreen,highly oxidized zone at the top of the extrusivesequenceis beIieved to be the result of prolonged seafloor
of secondaryminerals
weathering.The assemblages
in this zone are similar to those recoveredfrom the
upper levels of the oceanic crust wheie similar
processeshave beeninvoked (i.e., DSDP Site 417,
Donnelly et al. 1980). This oxidized zone is absent
only where synvolcanic umbers are relatively thick,
as in the Kambia-Margi and Kalavasos areas. We
suggestthat the umbersthat were depositedin topographic lows on the seafloor formed an early, impermeable layer that prevented seawater from
percolatingdownward into the upper part of the lava
pile. In areaswherethe umbers are absent,seawater
MO
THE CANADIAN MINERALOGIST
gained accessto the lava pile until the crust was effectively sealedoff by the deposition of pelagic sediment$ of Campanian age.
Ba!-sy, D. (1984): Volcanic Stratigraphy qnd Geochemistry of a Portion of the Troodos Ophiolite,
Cyprus. M.Sc. thesis, Dalhousie University, Halifax, N.S.
Suunaany
BBan,L.M. (1960):The geology and mineral resources
of the Akaki-Lythrodonda area. C\prus Geol. Surv.
Dep., Mem.3.
Four types of alteration have been recognizedin
the lavas of the Troodos ophiolite: deuteric,
hydrothermal, low-temperatureseawater-basaltand
seafloor weathering. Relatively high-temperature
minerals typical of deuteric alteration havenot been
identified in the Troodos lavas,but low-K smectite
may haveformed by this process.Hydrothermal alteration in the lavasis restrictedto narrow stockwork
zonesand is extensivein the SheetedDyke Complex.
Most of the alteration in the Troodos lavas is the
result of low-temperatureseawater-basaltinteraction
during and after crustal accretion. The assemblages
of secondary minerals and their distribution were
controlled by such factors as: permeability, type of
cooling uuit, accessibilityof water and composition
of the host rock. The highly oxidized zone in the upper 20-2O0m of the extrusivesequenceindicatesthat
there was a period of prolonged seafloor weathering prior to the deposition of the Campanianpelagic
sediments.This oxidized zone is absent in areas
where syrnvolcanicumber was sufficiently thick to
limit the percolation of seawaterinto the upper portion of the volcanic pile.
The alteration patterns in the extrusive sequence
indicate that there is no systematicincreasein the
degreeof alteration with depth and that no consistent stratigraphicor metamorphicboundary can be
drawn in the volcanic pile on the basisof distribution of secondaryminerals.
AcKNowLEDGEMENTS
The researchreported in this paper was supported
by DalhousieUniversity and the Natural Sciencesand
Engineering Research Council of Canada (grant
47410 and CSP A4652 to P.T. Robinson). Discussions with J. Mehegan,D. Bailey and other membersof the ICDRG werevery helpful, and we thank
R. Jamieson,L. Horne and H. Staudigelfor critically reviewingthe manuscript. We alsothank Costas
Xenophontos, Andreas Panayiotou, and George
Constantinou of the Cyprus Geological Survey
Departmentfor their assistanceand discwsionswhile
in Cyprus.
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Received fune 7, 1984, revised manuscript accepted
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