r53
The Canadian Mineralogist
V o l . 3 3 , p p . 1 5 3 - 1 6 3( 1 9 9 5 )
BEHAVIOR
OFTHEPLATINUM.GROUP
ELEMENTS
DURINGDIFFERENTIATION
OFTHENORTHMOUNTAINBASALT,NOVASCOTIA
JOHND. GREENOUGH
Depanment of Geological Sciences, University of British Columbia, Okanagan University College,
3333 College Wa-,-,Kelowna, British Columbia VIV 1V7
BRIAN J. FRYER*
Centre for Eanh Resources Research, Depaftment of Earth Sciences, Memorial Universitv of New'foundlancl,
St.John's, Newfoundland, AIB 3Xs
ABSTRACT
Differentiated rocks in thick flows ofthe Jurassic North Mountain Basalt, Nova Scotia, display evidence for fractionation
of noble metals (Au, Pd, ft, Rh, Ru, and Ir). Meter-thick layers of mafic pegmatile and vesicular basalt high in the flbws are
enriched in Au and Pd but depleted in Pt, Rh, Ru and Ir relative to undifferentiated basalt. Mineral precipitation (e.9., chromite
removal) cannot explain Au and Pd enrichment and Pt depletion in the mafic pegmatites.The fraclionation pattem may reflect
the early movement and concentration of chloride-bearing hydrothermal solutions associatedwith rising plumes of vesicles
only months after extrusion. On average, basalts with orthopyroxene show Pd and Au depletions and Pt, Rh, Ru and Ir
enrichments relative to undifferentiated basalt, but there are large differences in concentration of individual noble metals
between samples. This intersample variability, resulting in Pt/Pd fractionation, could reflect noble-metal-bearing microinclusions in orthopyroxene. "Rhyolite" bands derived from the mafic pegmatites, possibly through silicate liquid immiscibility, contain the lowest concentrationsof noble metals in the flows. Some "rhyolite" samples show Ir enrichment, polentially
reflecting selective carrying capacity for magmatic fluids segregatedfrom the "rhyolite" or, if the "rhyolites" formed through
silicate liquid immiscibility, preferential partitioning of Ir into the siliceous liquid. These findings, and previous studies of
other Mesozoic mafic rocks from the eastern U.S., indicate that the incompatible behavior of Pd and Au can result in
substantial increasesin concentration of these noble metals in the upper reachesof thick flows and intrusions.
Keywords: tholeiitic basalt, Jurassic, eastem North America, "rhyolite", platinum-group elements, differentiation, volatile
phase,liquid immiscibility, Nonh Mountain Basalt Formation, Nova Scotia.
SonanaatnB
Les rochesdiffdrenci6es
descoul6esbasalriques
d'dge
6paisses
de la formationde North Mountain,en Nouvelle-Ecosse,
jurassique,ont subi les effetsd'un fractionnement
en metauxnobles(Au, Pd, Pt, Rh, Ru et Ir). Les niveauxde pegmatite
mafiqueet de basaltev6siculaire,donl l'6paisseurest e l'6chelledu mbtreet la position est dansla partie sup6rieuredes
primitives.Le fractioncoul6es,sontenrichisen Au et Pd, et appauvrisen ft, Rh, Ru et Ir par rapponaux rochesbasaltiques
nementde min6raux,par exemplela chromite,ne pourraitexpliquerl'enrichissement
de
en Au et Pd des 6chantillons
pegmatitemafique,et leur appauvrissement
pounaientr6sulterdu mouvementpr6coceet de la
en Pt. Parcontre,cestendances
concentration
de saumuresassoci6es
avecI'ascensionde bulles d peinequelquesmois aprbsI'extrusion.En moyenne,les
dchantillons
de basaltetr orthopyroxBne
sontappauvrisenPd et Au, et enrichisen ft, Rh, Ru et Ir, parrappo( aux 6chantillons
de basalteprimitii maisnousdocumentons
s6lectives
desdiffdrencesimportantes
d'un 6chantillonh l'autre.Cettevariabilit6,
qui estd l'origine d'une s6paration
de Pt et Pd,pounaitr6sulterde micro-inclusions
conlenantla suitede m6tauxnoblesdans
par immiscibilit6du liquidesiliceux,
I'orthopyroxbne.
Des niveaux"rhyolitiques"d6riv6sde la pegmatitemafique,peut-Ctre
contiennent
lesplusfaiblesconcentrations
desm6tauxnobles.Certains6chantillons
"rhyolitiques"tdmoignentd'un enrichissements6lectifen h, et doncpeut-Otre
d'une capacit6s6lectivede transfertdansune phasefluide magmatique
ou, dansle cas
d'une originepar immiscibilit6,d'une r6partitionpr6f6rentiellede l'iridium dansle liquide siliceux.Cesr6sultats,et ceux
d'6tudescomparables
incompatiblede
de suitesmafiquesm6sozoiques
de I'est desEtats-Unis,montrentquele comportement
Pd et Au pourraitcauserun enrichissement
de coul6es6paisses
subtantielde cesm6tauxnoblesdansles partiessup6rieures
et de massifsintrusifs.
(Traduitparla Rddaction)
Mots-cl6s:basaltethol6iitique,jurassique,est de I'Am6riquedu Nord, "rhyolite", 6l6mentsdu groupedu platine,diff6renciation,phasevolatile,immiscibilit6liquide,Formationdebasaltes
de NorthMountain,Nouvelle-Ecosse.
*Presentaddress:
Dean'sOffice,Facultyof Science,Universityof Windsor,Windsor,OntarioN9B 3P4.
t54
T}IE CANADIAN MINERALOGIST
INTRoDUcrroN
The North Mountain Basalt sequenceborders
shoresof the Bay of Fundy in Atlantic Canadaand
providesan exampleof continentalflood-basaltvolcanismassociated
with Jurassicrifting betweennorthern Africa and eastemNorth America (Wark & Clarke
1980;Fig. 1). The basaltshaveattractedconsiderable
attentionas a naturallaboratoryfor igneousprocesses
during a major rifting event.Severalauthorsused
them to study crustal assimilation(Dostal & Dupuy
1984, Greenoughet al. 1989).Othersused this
episodeof volcanismto placeconstraintson the time
of rifting as well as the absolute age of the
Triassic-Jurassic
boundary(Hodych & Dunning
1992).
In contrastto other flood-basaltsequences,
which
can be thousandsof metersin thickness.the North
MountainBasaltsequence
is merelya few hundred
merersthick (White 1989,Papeziket al. 1988).
However,two of the lava flows are amongstthe
largestknown individualflows on earth,with dimensionsof -0.2 km x 100km x 230 km (Greenough&
Papezik1987).As a resultof theirthickness,thesetwo
flows underwentsubstantialdifferentiation.Bowen
( I 9 I 7) recognized
the utility of studyingtheserocksto
help identify igneousprocesses
that aremaskedwhere
coolingis slower,as in gabbroicintrusions.
These
basaltspresbrveevidenceof crystal settling,silicate
liquid immiscibilityandvolatilemovementduringthe
differentiation of the mafic magma(Papeziket al.
1988,Greenough
& Dostal1992a,b).
I
The mechanismscontrollingdifferentiationin the
North MountainBasalt sequencemay not be well
understood.However,the cooling regimefor lava
as a result,the time
flows can be closelyconstrained;
when variousprocesses
of differentiationoccurredis
reasonablywell known (Greenough& Dostal 1992b,.
that
"Rhyolite" bandswithin the basaltsdemonstrate
substantialdifferentiationoccurred(Greenough&
Dostal1992a).The basaltsthusprovidean oppormniry
to study the effectsof magmadifferentiationon elementslike the platinum-group
elements(PGE),whose
behaviorduring magmaevolutionis not well understood(Crocket1981,Briigmannet al. 1987,Fryer &
sf mafic
G r e e n o u g h1 9 9 2 ) .S i m i l a ro u t p o u r i n g o
that differentiation
magmain the U.S. demonstrate
mechanismscan enrich specificPGE more than
ten-fold(Gofffried& Froelich1988).
In this paper,we reporton the PGE concentrations
in variousrock typesfrom the North MountainBasalt
Thesedata allow us to commenton the
sequence.
effectsof differentiationon PGEconcentrations.
GEoI-ocv AND SAMPLINc
The Jurassic(202 ! I Ma: Hodych& Dunning
1992)North MountainBasaltFormationconsistsof a
150-to 2O0-m-thicklower flow separated
from an
upperflow of similar thicknessby a 50-m-thick
middle unit of thin flows. The upperflow is locally
overlainby 75 m of thin flows (Papeziket al. 1988,
Greenough
et al. 1989).The two thick flows aretracefor over 230 km
able and isotopicallydistinguishable
r u o n r HM o U N T A t N
\
c
,!,
(t
c o E Eo u
0,
ot
a
CAP d,OR
t cHlt{Atl|PAs
ORAT{D
llANAr{
ITLAND
Flc. l. Map showingthe distributionof North MountainBasaltaroundthe Bay of Fundy.
NorthMountainBasaltalsooccursin theChinampas
exploratoryoil well (x).
155
BEHAVIOROF T}IE PGE.NORTHMOUNTAINBASALT
along the Bay of Fundy (Fig. 1), but they cannorbe
distinguishedon the basisof major- or trace-element
g e o c h e m i s t r ya l o n e ( J o n e s& M o s s m a n1 9 8 8 ,
Greenoughet al. 1989).The rocks consistof high-Ti
quartz-nonnative
tholeiite(Weigand& Ragland1970)
representative
of the Early JurassiceasternNorth
Americandoleriteprovince (Dostal & Dupuy 1984,
Papeziket al. 1988).
The lower portionsof thetwo thick flows tendto be
massive,columnarjointed, and have accumulated
orthopyroxene
in the Digby area(Papeziket al. 1988).
The percentages
of orthopyroxenedecreaseto the
TABLE 1. NOBLE METAL CONCD.ITRATIONS IN NORTH MOUNTAIN BASALT
$nple#
Rk aypeI
udrF
Lmtiod
EF-r(C)
Pemlite
I
E Fary
EF-I(D
Pegualite
I
E, Fcry
EF-2(C)
Fe@aite
t
E Fery
Noble Maals
Au
3.9
3.3
5.1
Pd
8.5
t6
lt
R
t.9
3.5
1.3
Rh
0.r9
0.25
0.21
fu
Ld
Ld.
n.d.
0.06
h
0.@
0.05
$daphile md ChalcophileElmts
Cr
n.d.
nd"
&d.
M
n.d.
&&
!.dG!
176
138
156
EF-3(C)
P€gEalite
t
E F6ry
EF-3(F]
Rhyolite
I
E Fcry
EFJ
Bsslt
I
E Fary
FP-3
Easlt
I
Frcepon
3.9
ll
0.98
0.t6
od.
ud.
o.24
o.42
t.3
0.01
&d.
0.05
2.7
1,5
4.6
o.a
D.d.
0.06
4.7
9.0
6.r
0.36
ud.
0.t2
rd"
4
t67
56
24
93
t9
Ld.
253
258
3l
94
1"4(C)
Pqedtr
I
Fntport
5.6
l.?
L5
0.10
od.
0.t0
u&
!d.
r00
&mplelt F?-9(C) r?-9(Fl IUCKr
iICIOB MCKIF'GltCKtT
MCI(3B rttct€Fc
RL d
Pg@tite R$yolite Bselt
P€gmtite Rbyolile P€g@aiteP€gmrireBbyotire
udrtr333333333333
Iedod
Fr€€pon tr'rc€pon llcKay IL McI&y tL lttcltay E McI&y g. McIQyH. McIGyIL
FP-4(n
Rbyotre
I
Frrcport
1.2
0.r8
0.34
0.6
0.17
t.2
ud.
ud.
49
FP-7A(C)
P€gEarite
I
Fre.port
6.6
u
3.2
0,21
s4
0.ll
ud.
t
7t
MCIST
Mn2FC
Pegaasite Rhyolite
tUcl(ayE
MctGyH-
FP-7A(D
Rfrybite
I
Fr€€gort
o.d"
0.6
0.41
0.08
a.d,
o.31
!.d.
!.d.
109
FP{BG)
R$yolite
I
Fntport
0.51
0.44
0.50
0.05
ed.
0.10
!.d.
nd8t
MIIVP
llcKlo
PegmatiteV6idfrr
tt{cKayE lllcXayE
NobleMelals
3.6
l0
t.z
0.16
Ld.
Ld.
AU
Pd
P.
Rh
R[
It
4.74
0.21
0.28
a.d.
ad.
ud
1.8
8.1
4.6
g.2S
0.18
a.d.
Sid€ropbileed Chdcophile tselrenb
u
G
nd.
rd.
l{
ll
LrL
34
or
lgl
170
65
Samplet
Rk typd
Unitf
Iedotf
PEZ-Z!
OPX ss.
t
E. Frry
NoUs M€trb
t6
Alr
Pd
7,7
R
14
Rh
0.57
Ru
0.27
Ir
0.40
PA,A4
Blsslt
I
E. Frry
ta
4.2
5.5
0.28
Ld.
0.06
P&-25
PeSmtire
I
E Frry
ta
3.9
0.45
0.05
Ld.
!.d.
Sidophile md Chalophile EI@UG
368
Cr
85
l0
Ni9048m
91
147
Cu
192
l.
J.
l0
0.t4
0.21
!.d.
24
Al
lU
Pn-26
Boslt
I
E Fary
1.9
4.7
L6
0.16
B.d.
0.06
4l
34
rtt
0.66
0.41
o.29
0.02
0.14
ud.
2.2
7,8
0.70
0.07
!.d.
rd.
2t26t634'
lnl0s&!t3136
2N
t45
PE2-33
OIX Bs.
I
E Fery
r.6
4.3
12
0,61
0.6
t.t2
528
98
8r
2.6
lt
0.73
0.29
od.
od"
l8:t
V&-Z
P'&l-t
OFX B&e OPx Be
3
I
Digby N. Digby N.
1.3
r.2
t.2
0.t3
0.15
0.14
396
107
t3l
t.3
t.4
4.2
0.14
0.34
0.15
124
93
50
l.l
1.6
0.53
0.0r
o.&
0.04
yt
PA-l?
Bslt
Z
Digbl N,
L2
0.55
t.4
0.t3
ad.
0.12
1.6
4.6
0.36
0.23
t.d.
n.4
L4
9.0
0.93
0.15
a.d.
ad.
214
w
BCI
Bsslt
r
Fln Is
MBt4
Beelt
L6
4.4
4.4
0.21
0.16
0.o2
L6
5.6
8.6
0.31
ud.
0.03
80
243
79?3263656
16l
t4
3
McKayE
3.1
1.9
0.58
0.ll
ud.
ud"
215
1,2
6.0
3,3
0.26
Ld.
0.04
36
CD7
CIi-2
OPX Bts. Basolt
|
Z
Capd'&
6imps
3.2
13
7.4
0,88
0.92
2.5
t.1
2,5
0.08
rd.
142
380
t01
l0l
l7t
58
PegmadE= mafic pegmatite,Rhyolite = riyolite bandin mafic pegmatite. Sampleswith the samesamplenumberas a pegmatite
(e.9. EF-l) are from that p€matite. Basalt = massivebasaltfrom the bonom of thick flows or baslt from the tops of flows.
vesicular = vesicularbasaltftom layea in the thick flows. OPX Bas. - basaltbearingan accumulationoforthopyroxene.
Units numbersare as follows: I = lower unit thick flov, 2 = Middle unit thin flows, 3 = Upper unit thick flow.
=
-see = Freeport,
!. Ferry F:st Ferry, increasingsamplenumben indicae increasingdepthsin the upper 40 m of the flow. Frcepofi
increasingsamplenumbec indicaE decreasingdepthsin Oe upper - 40 rn of the flow, McKay H. = McKay Head,
Greenoughand
DosEl (1992b)for depthsin the flow. The preciselocationsofP82 samplesappea.rin Fapeziker al. (1988). Digby tt. = Drgby
Neck Preciselocationsofsamples ECl, MHl4, aad CD7 arc given in Grecnoughet al. (1989). The stratigraphiclocationofsample
CH-2 is given in Greanoughand Papezik(1987).
Copper,Ni and Cr concentralionsin Ppm. Noble meEl conceofatio0sin ppb. Mgf, = M/(MC + 0.9 Fe) atomic. N.d. = not
detected.
156
THE CANADIAN MINERALOGIST
northeast,and tho mineral disappearsin the Parrsboro
area(Greenoughet al. 1989).Layering in the upper
50 m or so of the thick flows was describedin detail
by Greenough& Dostal (1992a,b). ln summary,the
lowest layers consist of approximately25-cm-thick
mafic pegmatitethat containsa thin (2 cm) "rhyolite"
band in the middle. The layering graduallychangesin
charactertoward the top of the flows; the mafic layers
becomehighly vesicularandnonpegsaatitic,andlayering disappearsentirely approximately5 m below the
flow top. Both the pegmatiticand vesicularlayers are
horizontally traceablefor tens of metersand are vertically separatedfrom one another by approximately
l30-cm-thick sectionsof basalt.All basaltsexperiencedzeolite-faciesmetamorphism(Aumento 1966),
but the Sr isotopicstudyof Jones& Mossman(1988)
demonstratesthat carefully selectedsamplesgive an
accuraterepresentationof the original geochemistry.
Similarities between the layering in the North
Mountain Basalt Formation and in the Hawaiian lava
lakes (Helz 1987, Helz et al. L989) suggestthat the
layers of mafic pegmatiteand vesicularbasalt represent segregationsof liquid siphonedinto thermally
induced horizontal cracks formed in tJte downwardgrowing, semirigiduppercrystallinemush(Greenough
& Dostal I992b). Thus the layers form from residual
interstitial liquid derivedfrom the crystalmush.
The "rhyolite" bands consist of a brownish, vesicular aphaniticrock andis called o'rhyolite"becauseit
containsin excessof 70VoSiO2.The bandsappeared
during a second stage of formation of horizontal
cracks,with interstitial liquid in the mafic pegmatites
forced into the crack by effervescenceand filter
pressing (Greenough& Dostal L992b).In the mafic
pegmatites,interstitial, polygonal patchesof microcrystalline Si-rich minerals (chemically equivalentto
and traceableinto the 'thyolites'l) and similar areasof
glass-like Fe-rich stilpnomelane(enveloping skeletal
Fe-Ti oxides) havebulk chemicalcompositions,relative proportionsand total modal percentagessuggesting that they once were Si-rich and Fe-rich glasses,
respectively(Greenough& Dostal 1992b).
Samplesused in the study come from the three
lower recogn:izedstratigraphicunits (Table I, Fig. 1).
They have been divided into five rock groups:
(OPX) basalts,mafic pegorthopyroxene-enriched
matites, "rhyolites'0,vesicular-layerbasalts,and
basalts.The single sampleof vesicular-layerbasalt
reflectsthe dfficulty in obtainingan unalteredsample.
5amFlescategorizedas basalt originate from the tops
of the thick flows, from thin flows, or from the massive (noncumulate)lower portions of some thick
flows.
ANALYTICAL METHODS
Samplesnot usedin previousstudies(samplenumbers prefixed with EF and FP) were powderedand
analyzedfor their major elementsusing atomic
absorptionspectrometry.Thesedata can be obtained
from tle Depository of UnpublishedData, Canada
Institute for Scientific and Technical Information"
National ResearchCouncil, Ottawa,OntarioKlA 0S2,
or by writing J.D.G. Concentrationsof Rb, Sr, Sc, V,
Cr, Ni, Cu, Zn and Ga were determinedusing X-ray
pellets,with precision
fluorescenceon pressed-powder
and accuracybetter than t5%ofor all elementsexcept
Cu (XIUVo).All other traceelements,exceptthe noble
metals, were determinedby inductively coupled
plasma- massspectrometryGCP-MS) following the
sodium peroxide sinter procedwe of.I-nngeich et al.
(1990). hecision and accuracyare better than4Vofor
theseelements.
Concentationsof the noble metals(Au, Pd, ft, Rh,
Ru, and Ir, Table 1) were determinedby ICP-MS on
L\Vo aquaregia solutionsafter preconcentrationof
10 g of rock powder by NiS fire assay.Typical
reagent-blankconcentrationsarc 0.77, 0.59, 0.57,
0.06,0.05and0.04ppb for Au, Pd, Pt, Rh, Ru andh,
respectively.Run-specific blank concentrationswere
subtractedfrom the raw data. Average "detection
limits" (3o abovebackground)are equivalentto 0.20,
0.15,0.08,0.01,0.10and0.02ppb (in a rock sample),
respectively.Jacksonet al. (1990) shouldbe consulted
for details of the procedure.Replicateanalysesof the
reference material SARM-7 have given (in ppb)
245 (Au), L4sz (Pd),2n (Rh),3923 @),380 (Ru)
atd77 (b), with relative standarddeviationsof 7Voor
betterfor all elements.All PGE valuesarewithin 107o
of the certified concentrations.Replicate analysesof
sampleMCKI indicate precision of. lU%oor better for
Au, Pd andPt and 307ofor Rh. Data on MCK 10 suggesta precisionof l50vo for h concentrationsnearthe
detectionlimit (0.02 ppb). Al1 analytical work
describedabovewas doneat The Memorial University
of Newfoundland.
Analytical methods and major- and trace-element
data (other than for the PGE and Au) for all samples
that do not begin with EF or FP appearin Greenough
& Papezik (1987), Papezik et al. (7988), and
Greenoughet al. (1989).
Rrsurrs
Variation diagrams plotted in terms of Mg#
[= Mg/(Mg + 0.9 Fe) atomic] illustrate the geochemical relationshipsamong rock types (Fig. 2);
averagecompositionsfor eachrock type appearin
Table 2. The OPX-enrichedbasaltstend to have the
highestMg# and Cr values,as well as the lowest concentrationsof alkali elements,high field-strength
elementsQIFSE)andrare-earthelements(REE)of the
entire suite. The basaltsand mafic pegmatites,taken
togetherwith the OPX-enrichedbasalts,define correlation lines with concentrationsof alkali elements,
HFSE and REZ increasingand that of "compatible"
t57
BEHAVIOR OF THE PGE, NORT}I MOUNTAIN BASALT
2.na
t
ttt
Sl O2
'
2.OO
r.5OTi oa
AR
o
60
r.oo
tltlg.g
Fi'
ii:J'di^..
ltt
e.50
KzO2l . o o
Yv
ara
-f{lerr_
r.oo
c.50
, i,
vL
r60
r40
t20
roo
.o
xJ
'!f{^^
zoo
r80
atr
ttl
,
'r
o
o t ^ -^!D^r.
tf
lrt
o.30
o.25
o.20
o.,o
trou
o.ro
30
25 Lo
ao
80
400
350
300
250
200
r50
roo
o.50
20
r5
40
v
Y
l'
tr
'.tlt**.
o{tf,a^...
Nb
20
lo
a
500
300
tt
400
olo
a
300
tr
200
A
roo
rlrr
0.20
(-^
.
- i .o - r rt li
.
?a
erl
.
trooa
^^a.
la
.rt
250
200
r50
Cu
roo
0.40
Mg*
Mg*
A BASALT
I VESICULAF BASALT
'. OPX BASALT
Orsarrc PEGMATTTE
Y RHYOLITE
Ftc. 2. Variation diagrams showing selected major element (wt. 7o oxides, volatile free)
and trace elements (ppm) plotted against Mg# [Mg# = Mg/(Mg + 0.9 Fe) atomic].
OPX basalt: orthopyroxene-enrichedbasalt; see text for usageof "rhyolite".
elementsdecreasingas Mg# decreases.
The single
vesicular-layer
basaltis distinctivein havingthe highest TiO2 and P2O,concantrations
observed,and,REE
concentrationsapproachingthose of the "rhyolites".
The "rhyolite" samplesare characterizedby their high
SiO2,K2O, Zr andLa concentrations,
and low TiO2
andSr. Copperconcentrations
tendto increaseasMg#
decreases
in the mafic rocks.but thereis considerable
scatter; individual samples of "rhyolite" contain the
highest and amongst the lowest Cu values.
The distinction among rock types is not as apparent
in the noble metal data (c/ Figs. 2,3), partly reflecting
the lower precision at low concentrations (Table l).
N e v e r t h e l e s s ,m e a n s f o r m a n y e l e m e n t s ( 2 3 o f
30 pairs tested) between rock types are statistically
different at the 0.10 level of significance or better
r58
THE CANADIAN MINERALOGIST
a
a
4.2
a
al
Au
'
Oc)
.- 'r aO oA o
t.
-'t$^^
2,25
a
lt
.
qLoo-
aV
.ttv
-l_t
I2
I
^t
aA
a
A
_
l2
^oo
80
70
,
to
a
-
4
60
50 Rh
o
6
40
30
20
A
^^l
^-it
*{.}^rl^t'
ara
aGe^roo
--L!l
.^
lo
o..990
0.8
.ao
o.7
or..f6 0
R u o) . 6 0
t.2 0
o.
o.
o.40
,.40
o.20
,,20
opd
a
a
t4
2
4
o
o
,Y.r
o.30
,^),
o.ao
a
o
o.
A
vlLr
o.70
0.20
Mgs
o.
iAll"
0.40
0.60
Mg?
A BASALT
V RHYOLITE
I
O uarrc pEGMArrrE
OoPx BASALT
vEsrcuLAR BASALT
FIc. 3. Variation diagrams showing the noble metals (ppb) plotted against Mg#. OPX
basalt: orthopyroxene-enrichedbasalt; see text for usageof "rhyolite".
(Table 2, footnote). Gold (correlation coefficient,
r = *-0.69) and, to a much lesser extent, Pd (r = 4.34,
level of significance of correlation = 0.10) seem to
increase with decreasingMg# in the mafic rocks; the
"rhyolite" samples contain the lowest concentrations
of these elements (Fig. 3, Tables 1, 2). In contrast,
levels of Pt, Rh, Ru and Ir tend to decreasewith
decreasing Mg#. The concentrations of Ru and Ir
commonly fall below detection limits except in the
OPX-enriched basalts.
Average mantle-normalized patterns indicate substantial fractionation of the noble metals after extrusion (Fig. 4, averagedvalues given in Table 2). T\e
pattem representativeofbasalt is straight, with a slight
negative Ir anomaly. On average, the OPX-enriched
basalts show higher concentrationsof Ni, Ir, Rh and Pt
than the basalts,but the pattern flattens out between Pt
and Au. The pattern for mafic pegmatite displays the
opposite relationships, with average concentrations of
Ni to Pt below, and Pd and Au above, those in the
basalt. The vesicular-layerbasalt sample displays
similarities to both the basalts and mafic pegmatites.
The pattern representingthe average "rhyolite" shows
a positive Ir anomaly; concentrationsof the other
noble metals fall below those in the other rock tvpes.
DlscusstoN
I nterp retation of maj or- elementand
trace-elementdata
Although the samples come from several flows
and outcrops up to 200 km apart, the coherency of
major- and trace-elementdata (Fig. 2) point to 1) similar initial composition of the mafic magma, 2) similar
igneous geochemical processesat all localities, and
3) minimal effects of alteration. The first point is
emphasized by the incompatible trace elements
because small initial variations in concenfiation tend
to be enhancedduring differentiation. The wide range
of values of Mg# (0.70 - 0.20) and SiO2 contents
(53 - 75 wt.%o,volatile-free basis) emphasize the
extent ofdifferentiation. The processesresponsiblefor
differentiation are equivocal, as reviewed below.
159
BEHAVIOR OF TI{E PCE. NORTH MOUNTAIN BASALT
Mass-balancecalculations based on major elements
indicate that the mafic pegmatites can, to a first
approximation, be derived by crystal-liquid differentiation from the basaltic melt (Papezik et al. 1988).
However. similar calculations show that the most
TABLE2.
AVERAGE COMPOSITIONOFROCK
1YPES IN THE NORTE MOT'NTAIN BASALT
oPx
Baslt
Mlnc
Pmtlt€
Vgiohr
B&lt
Riyoltte
s5
ALOr
Fqo,
MnO
MgO
CaO
Na?O
KrO
PrOr
Mei
13.13(0.55) 53.15(0.J4) 54.E6(1.68) 53.66
1.27(0.13)
0,920.r2)
1.78(0.16) 2.&
14.31(0.38) 12.754.45) 12.58(0.80)
11.27(0.50)
9.96(0./,
13.78(0.63) t4.53
0.17(0.02)
0.16(0.02) 0.21(0.05) 0.29
6.47(0.66)
9.52(t.U2) 4.E20.40)
5.14
9.66(0.54) 10.91(0.54) 7 . 9 10 . l J )
6.06
2.64(0.58)
1.92(0.13) 2.63(0.t7)
x.27
0.93(0.29)
0.63(0.08) 1.20(0.28) l.E4
0.15(0.02)
0.09(0.03) 0.22(0.05) 0.31
0.56
0.43
0.68
o.44
7t.32Q.47)
0.68 (0.15)
10.55(0.53)
7.37 (1.42)
0.11 (0.05)
1.73 (0.68)
2.20 p.m
2.92 (0.20)
2.98 (0.40)
0.14 (0.04)
o.34
Alkall ed alkalle @trh Delal9
Rb
27 (3'
l9 (4)
Cs
1.2 (0.3)
0.s8(0.20)
sr
190 (ll)
151 (8)
Ba
213 (38)
t74 (t9)
l8l 08)
u2 (€r)
Hlgh Fleld Strengll! Eloeft
rh
3.12 (0.39)
u
0.88 (0.34)
z'
107 (r8)
lrf
3.05 (o.sa
(2)
Nb
1r
Ta
2.1 (l.E)
Y
26.3 (4.t
2.32(0.23\
0.48(0.07)
83 (!3)
2.aQ3v
8 (2)
1.0 (0.2)
2X.4 (X.4)
3.990.01) 5.96
l.l0
r34 Q5)
255
3.66(0.E2) 6.32
l5 (3)
t9
3.1 cJ)
1.6
30.7 (6.8) 41.0
ll.4 (1.2)
23.2 (3.2)
18.6 (3.9) 26.1
40.0 Q.V
61.0
5.470.04)
2.6 (4.2)
5.42(0.88) 7.?E
1.49(0.1n
2.gj
5.39(1.00)
0.93(0.16) t.22
6.21(1.14)
L26 (0.23)
3.59(0.68)
0.5r (0.10)
3.18(0.54)
0.75(0.9?
0.61
32.9 Q.r)
69.9 (4.V
8.62 (0.40)
33.0 (1.9)
8.22 (0.65)
l.9r (0.50)
7.04 (0.69)
1.42 (0.10)
8.67 (O,72,
1.76 (0.It
5.25 (0.43)
0.76 (0.0o
4.59 (0.56)
0.67 (0.10)
38.5 (6.4)
35J (53)
lo C0
67 0l)
15 (10)
3
l5l (4n
74 (9)
19 (2)
E.97 (3.78)
28 Q4)
(7)
6
70 (17)
2
39 (11)
49
101 (14)
163
298
t03 Q2)
602 (6t
lt.3
0.4)
3r7 (69)
8.25 O.94)
32 (9)
ll.5 o0.9)
43.0 (4.t
chemicallyevolvedmafic rocks,the vesicularlayers,
showa poorfit; problemsariseowingto their elevated
concentrationsof alkalis (Greenough& Dostal
1992b).The calculatedrate of upper-crustgrowth on
the flow indicatesthat the first of thesevesicular
layers(depth-5 m) formedafteronly about6 montis
& Dostal1992a).The vesicular
ofcooling (Greenough
natureof theselayers and the fact that they formed
earlyled Greenough& Dostal(1992b)to proposethat
arein part a resultofthe
their chemicalcharacteristics
early upwardtransportof elementsin vesicleplumes
eitherasvolatilecomplexesdissolvedin the magmaor
in a separatefluid phase.Vesicleplumesare commonly preservednear the baseof mafic lava flows,
includingtheNorthMountainbasaltflows.
Mass-balancecalculationsinvolving primary
phasessuggestthat crystalfractionationis incapable
of producingthe "rhyolite" (Greenough& Dostal
1992a),which may thus resultfrom a Si-rich immisAlong
cible liquid derivedfrom the mafic pegmatites.
with petrographicdata alreadydiscussed,the mafic
similarto rocks
pegmatites
havebulk compositions
known to produceimmiscibleliquids, and the "rhyolite" resemblesknown Si-rich glassproducedby
immiscibility(Philpotts1982,Greenough& Dostal
Th, Zr, Hf andBa behavedanti
1992b).Nevertheless,
experimentsprethetically to what synthetic-system
Rare Earii El@enls
Pr
Nd
Sm
Eu
cd
Tb
Dy
HO
Tm
Yb
Lu
r3.9 (1.t
29.7 (3.2)
3.3E
rJ.3 (2.0
3.75(0.36)
1.20(0.12)
3.35
0.7r (0.0E)
3.86
o.n
2.28
0.3r
2.32(0.2?)
0.36(0.04)
Sldercphlle
Sc
v
cr
co
Ni
Mo
cu
zn
ca
and Chalcophlle
41.0 (4.3)
284 08).
122 Q4)
s4 (3)
19 07)
6 (3)
(50)
llt
73 (8)
(3)
l8
Noble Mebk
Au
Pd
Pr
Rh
Ru
k
2.6 (0.8)
5.1 (2.7)
4.7 (1.8)
0.23 (0.09)
0.17 0.09 (0.08)
rt.2 (t.2)
2.88(0.32)
0.90(0.09)
0.58(0.07)
1.96(0.23)
0.31(0.04)
Elemonts
46.6 (4.8)
246 (t9t
399 (69)
63 (9)
85 (25)
3Q,
tO6 (42)
66 (10)
t5 (2)
I.8 (0.7)
5.6 (4.5)
8.r (4.7)
0.4?(0.29)
0.46(0.27)
0.4s(0.32)
33.0
y5
3
52
6
20
3.6 (1.3)
9.r {3.4)
1.4 (1.0)
0.16(0.o7)
t.2
6.0
3.3
0.26
0.08(0.02)
0.04
l9l (108)
50 (13)
t8
6)
0.85 (0.42)
1.0 (t.4)
0.50 (0.30)
0.06 (0.07)
0.16
0.36 (0.4r)
l. Maj6 elemeniorid$ in w. 9o !@lculated b 100% volarile fte wilh bhl Fe I F.iq.
T@ elemoB in ppm scept noble mehls in ppd ercepl noble meuls in mb. The
sdard deviarim is given in b@ke6. N.d, = oot der*red, A bldk = not demined
or in lhe @ of &e sbdard daiadm rhm w
s2 mpl6.
M8, = Mg/(Mg r 0.9
Fe) domic).
2. No, ot spls
is $e nunb6 analrzed for the nqjd oladsb, md of the @ clendts
and all Oe noble neEls. B@@ a fd t@ elen6B y@ not demined in all
wplq
n will be 16 for !h* eled6B. A fev l@ el@b
(e.9. Cr ed Ni) dd
the noble mesls (lable t) v@ not d@Ed i! ema spls
th8 the ma b b€sedon
fffi
d& poinb.
3. Se Tablc ! for cL lyps dginidonr.
4. Udng dE sd4t !8,
t@! lor Au, Pd, P|, Rb ad lr i! @h @k $oup (qepr dE
r*iobr.latd
bsslt) 6 dlf@t
fton @ eo$s d tlb 0. l0 t*cl of siglifi@
with
tha folloeing 7 q€ptiom od of 30 t&d: the m@s for lr bery@ baglt ed naic
pegnatiE, bqelt dd 'rhyoli@', dd mfic Fg@tib ed 'rhl,ollc'i tte n@3 for pd
bery@ basi! and OPx€riched baelt, od mafic peg@ib dd OPx$rtchid bastli
tha n@s tor Au begq ba3di dd OPx€nrich€d bsslt.
lr.l
J
F
z
trJ
(,
t
u
o.l
Y
o
o.ol
E
o.001
Ni
lr
A BASALT
Rh
Pt
Pd
Au
Cu
V RHYOLITE
I V E S I C U L A RB A S A L T
PEcMATTTE aoPx
luarrc
BASALT
Ftc. 4. Average mantle-normalized concentrations of the
noble metals in each of the major rock types in the North
Mountain Basalt sequence. Note that the pattem for the
vesicular-layer basalt is represented by a single sample.
The significance of differences between data points can
be evaluated with the standard deviation and T-test data
in Table 2. The values used for normalization are from
Barnes et a/. ( 1988). OPX basalt: onhopyroxene-enriched
basalt; see text for usage of "rhyolite".
160
THE CANADIAN MINERALOGIST
dict for silicate-silicateliquid immiscibiliry (Watson
1976,Ryerson& Hess 1978);althoughmuchof the
evidencesupportsimmiscibility, it is not conclusive
(Greenough
& Dostal 1992a).
Papeziket al. (1988) concludedon rhe basisof
major-element
mass-balance
calculationsand traceelementmodelingthat the evolvedmafic rocks in the
Digby areacan be formed from normal basaltsby
the fractionationof mineralspresentin the orthopyroxene-enriched
basalts.
an importantprocessby itself. Olivine doesnot occur
in the North MountainBasalt.Chromitefractionation
can help explainthe high Cr valuesin the OPXenrichedbasalts(Fig. 2), but chromiteis not an important phasein this suite. Furthermore,major-element
massbalancecoupledwith trace-element
modeling
showthat augiteremovalalonecan explainthe variation in Cr (Papeziket al. 1988).Anotherproblemwith
callingon chromitefractionationis that the level of Pt
alsois higher,on average,in the OPX-enriched
basalts
and lower in the mafic pegmatitesthan in the basalts
Thedistribution of noblemetalsin mafic rocks
(Table2). Normally,Pt follows the low-melting-point
PGEalongwith Au in evolvingmagmas(Barneser a/.
Averagechondrite-normalizedpatternsfor the 1988,Greenough
& Owen1992).
noble metalsin basalts,mafic pegmatitesand OPXThe resultsobtainedfor the North MountainBasalt
enrichedbasalts(Fig. 4, Table 2) showthat the high- sequence
aresimilarto the resultsof studiesof ft and
melting-pointnoblemetals(Ir, Ru andRh), as well as Pd in sheetsof Mesozoichigh-Ti quartz-normative
Pt, occurat higherconcentrations
in the OPX-enriched t h o l e i i t i c d i a b a s ei n t h e e a s t e r nU n i t e d S t a t e s
basaltsthan in the basalts.Pd and Au concentrations (Gottfried& Froelich1988,Talkingtonet al. 1988,
are similar or lower in the OPX-enrichedbasaltsthan Gottfriedet al. 1990).Chemicalsimilaritiesbetween
in the basalts.Relationshipsbetweenthe mafic peg- the North MountainBasaltandtheserockshavebeen
matitesand basaltsare the reverseof thosebetween emphasized
by Puffer & Hurtubise(1982)and Dostal
the OPX-enrichedbasaltsand basalts.Although the & Greenough(1992).The PGE studiesindicatethat
mafic pegmatiteand OPX-enriched
basaltpattemsare orthopyroxenecumulatesin the Mesozoicdiabase
complementary,
crystallizationof OPX-enriched sheetsshowPt enrichmentandPd depletionrelativeto
basalt is probably not directly related to mafic undifferentiated
diabase.In contrast,ferrogabbrohighpegmatiteformation becauseOPX-enrichedbasalts er in the sheetsandlocally "pegmatitic"in texturecan
do not occur in the northeast(e.g.,McKay Head). havePd enrichedup to l0 times(165ppb) that in the
Mechanismsproposedto explainchangesin the con- undifferentiated
canbe
basalt.Platinumconcentrations
centrationof the noble metalsin evolvingmagmas higher (up to 2.5 times) or even lower than in the
werereviewedby Crocket(1981),Naldrett(1981), contactbasalt.The Pd enrichmentsare accompanied
Barnese, al. (1985,1988),Briigmannet al. (1987), by elevatedAu and Cl concentrations.
TheseenrichGreenough
& Fryer(1990)andNaldrettet al. (1990). mentsdo not appearrelatedto thicknessof the diabase
Nucleationandsegregation
of an immisciblesulfide sheets.Similar enrichments(150-200ppb Pd) are
liquid (Hertogenet al. 1980,Hamlyn et al. 1985) observedin the York Havendiabasesheet(-550 m:
shouldeffectivelyremoveall the noblemetalsduring Gottfried et al. 1990)and Palisadessill (-300 m:
magmaevolution;measuredsulfide liquid - mafic Tafkingtonet al. 1988),whereasthe marginally
magmapartition coefficients (K) zre on the order of thinnerNorth MountainBasaltflows (-200 m) show
ld to 105(Peachet al. 1990,Sioneer al. 1990).Pd maximumPd concentrations
of <16 ppb. Maximum
and Au concentrationstend to increasebetweenthc Pd enrichmentoccursin narrow zones;thus there is
basaltsand mafic pegmatites(bulk Ka < l), arguing t h e p o s s i b i l i t yt h a t o u r s a m p l i n go f t h e N o r t h
againstremoval of sulfide liquid as a significant MountainBasaltmissedthe most stronglyenriched
process.Similarly, Cu is higher in the mafic peg- layers.
matitesandslightly lower in the OPX-enriched
basalts
Authorsof the paperson the easternU.S. rocks
thanin thebasalts(Table2,Fig.4). Thusthe behavior providedno mechanismfor the Pt enrichmentother
of Cu indicatesthat not more than 0,47oof fraction- than to associateit with orthopyroxene
accumulation.
ating phases(for a bulk D < 1) could be sulfide The enrichmentin Pd and Au in the ferrogabbrowas
becausethe Cu sulfideliquid - silicateliquid partition attributedto movementby "late- to post-magmatic
coefficient is high (-250, Rajamaniand Naldretr fluids and volatilesenrichedin Cl and S" (Gottfried
1978).
et al. 1990,Gottfried& Froelich1988).
The fractionationof oxidephases(Crocket& Chyi
The degreeof Pd enrichmentand Pd/Pt fraction1972, Agiorgitis & Wolf 1978),PcE-bearingalloys ation is not as extremein the North MountainBasalt
( B i r d & B a s s e t 1 9 8 0 , K e a y s 1 9 8 2 )o r o l i v i n e a s i n s o m eo f t h e e a s t e r nU . S . d i a b a s es h e e t s .
(Briigmannet al. 1987)hasbeenproposedas a means Nevertheless"
the abovemodelfor Pd and Au enrichof removingthe high-melting-pointPGE (i.e., Ru, Ir ment fits well with data availablefor the North
and Os) from evolvingmafic magmas.The formation MountainBasalt,exceptthat usingthe basalts,it is
of PGE alloys appearsassociated
with chromitepre- possibleto show that the 'olate-stage"
enrichment
(e.9.,
cipitation
Briigmannet al. 1987)andmay nor be probably occurred early after emplacement
BEHAVIOR OF T}IE PGE" NORTH MOT'NTAIN BASALT
t61
(Greenough& Dostal 1992b).As noted above,rising Thedistribution of the noblemetals
plumesof volatilesprobablyalteredthe major-element inthe "rhyolite" bands
geochemistry
of vesicle-richlayers
and trace-element
Regardlessof whetherthe "rhyolite" bandsformed
and may have had a similar though less extensive
effect on the layers of mafic pegmatite(Greenough& by crystalfractionationor silicateliquid immiscibility,
Dostal 1992a,b). Throughcomparisonwith Hawaiian Figure4 showsthat the average"rhyolitic" liquid was
lava lakes, we know that theselayers formed very stronglydepletedin most of the PGE and Au, but
enrichedin Ir. Figure3 illustratesthat the Ir anomaly
earlyin thecoolinghistoryof thelavaflow.
Although there is disagreementon whether only occursin a few "rhyolite" samples,whereit is
The high variabilitycould be relatedto a
hydrothermalmovementof the PGEdirectlyresultsin pronounced.
ore deposits,mostauthorswould agreethat suchsolu- samplingproblem causedby a "nugget effect".
of Cu are similarly variablein these
tions are capableof at least local movementof the Concentrations
metals(Barnes& Campbell1988,Wood et al. 19921. rocks,but thereis no correlationbetweenCu and lr,
As in the easternU.S. diabasesheets(Belkin 1989;, and thus no indicationthat the Ir residesin a sulfide
studiesof fluid inclusionsand halogen-bearing phase.Analogousrocks termed"granophyres"from
mineralsin intrusionssuch as the Bushveldand easternU.S.diabasesheetsalsocontainextremelylow
Stillwatercomplexessuggestthat the PGE can be levelsof Pd andPt (Gottfried& Froelich1988).
moved by Cl-rich fluids (Boudreatret al. 1986,
ACKNOWLEDGEMET.{TS
Ballhaus& Stumpfl 1986,Boudreau1988,Hamey &
Merkle 1990).Thermodynamic
calculationspredict
Dr. J. Hodych participatedin the sampling.Dr.
significant solubility of Pd in Cl-rich magmatichydrothermalsolutions(Sassani& Shock 1990). H. Longerich,Dr. S. Jackson,Ms. G. Andrewsand
Preliminaryexperimentalresultssuggestevenhigher Mr. W. Gosseacquiredthe analyticaldata.Diagrams
solubilitiesof Pd than predictedfrom calculations weredraftedby Ms. M. Halko. Ms. M. Wesnoskiand
(Hsuet al. l99l). Althoughthe theoreticalandexperi- Ms. C. Neid preparedthe tables.The constructive
mental work supportsthe possibility of moving the criticism of the reviewersand helpful commentsof
PGEwith Cl-rich fluids,it cannotasyet quantitatively R.F. Martin and AssociateEditor M. Zientek are
explainfractionationof the PGE or assessthe impor- greatly appreciated.This researchwas supportedby
tanceand effect of other species(e.g.,S, NH3 and individual operatinggrantsto the authorsfrom the
Councilof
andEngineeringResearch
NaturalSciences
othercations)on PGEsolubility.
Volatile complexingprovidesa tentativeexplana- Canada.
tion for Pd and Au enrichment in the mafic
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on nucleationkineticsandgrowth
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