23
Thc Canalian M brcralogist
Vol. 34" pp. 23-28(1996)
THEFIRSTREPORTOF COBALTPENTLANDITE
FROMA MID-Af,LANTIC
DEPOSIT
RIDGEHYDROTHERMAL
NADEZ{DAN. MOZGOVA
Russia
IGEM P'/.S,Snromonetny
35,Moscow,109017,
SERGEYG. KRASNOV
WllOkeangeologiya,Angliysl<yProspect1, St.Petersburg,109121,Russia
BORIS N. BATTIYEV
PMGRE,Sevmargeologiya
Associatio4 Pobedy,24, Inmonosov, 189510,Russia
YLIRY S. BORODAEV
Departlnentof Geology,MoscowStateUniversity,Moscow, 119899,Russia
ANDREYV. EFMOV
IGEM P,AS,Staromnnetnt35,Moscow,109017,Russin
WADIMIR F. MARKOV
PMGRE,Sevmnrgeologiya
Associntion,Pobedy,24, Inmorwsov, 189510,Russin
TAMARAV. STEPANOYA
Wllokeangeologiya, AngliyslcyProspectl, St.Petersburg,109121,Russia
ABSTRACT
We reportthe first occurreuceof cobaltpentlanditein oceanicdepositsat 14'45'\ Mid-Atlantic Ridge (MAR). The mineral
occursas small (ess than 20 Um),round to irregular grainsand aggregatesassociateiwitl chalcopyrite,bomite, isocubanite,
chalcocite,digenite,with minor pyrite and marcasite,and with rare covellite and Au-bearingzincian copper.On the basis of
twenty-twoanalyses,
the compositional
range(in wt To)is: Co 42.9-53.9,Ni7.2-12,4,Fe 3.4-5.5,Cu 0.7-9.4andS 3L.G329.
Seventeenof the twenty-rwocompositionsplot bet'weenMenSsandMe15S13.
The narrow rangeof sulfur contentscorresponds
closely with thoseof syntheticpentlan&te(44.2-47.9 at.%o).T\ecobalt contentof a pentlandite-groupmineral dependson the
mineral assemblage
with which it is associatedKeywords:cobaltpentlandite,Mid-Atlantic Ridge, electron-microprobedata.
SomaLnr
Nous ddcrivonsle premier exemplede cobalt pentlandited'un gisementoc6anique,situ6 e 14'45'N sur la cr6te m6dioatlantique. h sulfire se pr{sente sous forme de petits (40 pm) grains ronds ou irrdguliers et en agr6gats,associ6si
chalcopyrite,bomite, isocubanite,chalcocite,digenite,avecpyrite et marcasiteaccessoires,et covellite et cuivre zincifdre et
pond6raux:
aurifbrerares.Nous avonseffectu6vingt-deux analyses,qui documententles intervallessuivants,en pourcentages
Co 42.\53,9,Ni7 ,2-12.4,Fe 3.4-5.5,C! 0,74.4, et S 31.G-32.9.Dix-septdesvingt-deuxcompositions
se situententreles
stoechiomdtries
MeeSret Me15S13.
L'intervalle restreintde teneursen soufrecorrespond6toitement ! celles de la pentlandite
synth6tique(44,2-47,9Vo,en termesatomiques).I,a teneuren cobaltdu mineral du groupede la pentlandited6pendde la nature
desmin6rauxassoci6s.
(Iraduit par la R6daction)
Mots-cl6s:cobaltepentlandite,crdtem6dio-atlantique,donn6esde microsonde6lectronique.
24
TT{E CANADIAN MINERALOGIST
INTRoDUSrtoN
Gwm.ALDrscnprroN
OF TID HYDROTHMMAL FTN,:D
Cobaltpentlandite(Co,Ni,Fe)eSs
wasfirst described
by Kuovo et al. (1959) from several localities in
Finland. Subsequently,it was reported from various
localities world-rvide (Stumpfl & Clark 1964, Peftuk
et al. 1969,Haris & Nickel 1972,Lindahl 1973).The
cobalt contents of cobalt-bearingpentlandite range
from lessthan 10 at.Eota morc than 80 aLTowithin the
(Co,Ni,Fe)sites.As notedby Harris & Nickel (1972),
all compositionsof cobalt pentlanditefall within the
solid-solutionfield of syntletic pentlandile.Guidelines
fq1 naming mbmbersof a solid-solution series were
recentlypublishedby Nickel (1992).In a partial solidsolution serieswhere the known compositionsexteud
beyond the 50Vo mark, for example pentlandite
(Ni,Fe)eSs,the compositionof which centerson Ni:Fe
= 1:1,andwherethe endmembersarenot knpwn, only
one nameshouldbe given to the compositiohalrange.
For the colalt-bearing pentlandite, the name cobalt
pentlanditeis used for compositionswhere Co is the
major componentinthe (Co,FeNi) sites,andcobaltian
pentlandite is used for the other compositions of
cobalt-bearingpentlandite.
Variations in Ni/Fe ratio of synthetic and natural
pentlanditesin relation to assemblagesof associated
minerals have been discussedby several authors.
Lindahl (1973) indicated that tle cobalt content of
cobalt-bearingpentlanditereflects the sulfide assemblage in which it occurs. Where the mineral occurs
with linnaeite or cobalt arsenides,the Co content is
more than 8O x.Vo in the (CoNi,Fe) sites; where it
is associatedwith pyrrhotite - chalcopyrite- cubanite
- sphaleriteassemblages,
the Co contentrangesfrom
50 to 80 at.7o;in Co-Ni oresdominatedby pyrrhotitepydte - chalcopyrite,the Co conlentrangesfrom 10 to
55 at.Vo.The Co content of normal pentlandite is
generallybelow 10 at.7o.
Cobalt-bearingminerals have not been reported
from massivesulfide depositson the oceanfloor, even
thoughup to 0.5 wt.VoCo hasbeenreportedin sulfide
aggregatesfrom a near-axis seamountat 12'50'N
East Pacific Rise (Hekinian & Fouquet 1985). In a
comparison of massive sulfide deposits from
sedimented oceanic ridges with ancient sedimenthostedBesshi-typedeposits,Zieretberg et al. (1993)
noied that cobalt mineralsare commonin Besshi-type
deposits,but not in occurrenceson the present-day
seafloor. Pentlandite has been reported in stongly
alteredgabbrofrom the CarlsbergRidge in the Indian
Ocean@ozanova& Baturin 1971,RozanovaLW2)
and from the GordaRidge in the Pacific Ocean(Hart
et al. l9X)), but no dataon cobalt contentwere given.
In this paper, we describethe first find of cobalt
pentlandite in an ocean-floor deposig at 14o45'N,
Mid-Atlantic Ridge (MAR). The samples were
obtainedby a TV-contolled hydraulic grab samplerat
a depthof about3000 m.
The hydrothermalfleld within the MAR rift valley
was discoveredin late 1993 during cruise 7 of RA/
"Professor Logatchev" organized by the Sevmorgeologiya Association, St. Petersburg@atuev et al.
1994, Krasnov et al. 1995a).The field lies within an
uplifted block of the rift valley, adjacentto its eastem
wall (Fig. 1a).llltramafic rocks (peridotite,pyroxenite
and serpentinite) along with gabbro are prominent
within the block. Basalt is less common and totally
absent within ttre area of hydrothermal activity
w
w
n
a
w
EI6
05'
.'5000
55'
50'
45'
n'
!4'J5't't
44fi'w
Frc. la. Location and morphology of the l4'45'N
Mid-Atlantic Ridge (MAR) hydmthermal field- 1: riftvalley floor, 2: volcanic zones,3: crests of linear highs
flanking the rift valley, 4: cress of linear highs on the
rift-valley slopes,5: transversedislocations,6: position of
areain Fig. lb.
25
COBALT PENTI-ANDITEFROM TIIE MID-ATI.ANTIC RJDGE
N
14"45.3',
45.2'.
45.0'
44"58.8'W
[.F], E,
[fs
[F|+ lTlu @o lT'lz EB
l-ofe [ilI,o
Ftc. lb. Geologicalmap. l: nafic and ulhamafic rocks, 2: rocks talus, 3: carbonatesediments,4: sulfide mounds,5: sulfide
sediments, 6: low-temperature hydrothermal deposits, 7: black smokers, 8: shimnering waters, 9: sample sites;
l0: hydrothermalbenthoscommunities.
@ig. 1b).Ttvelve sulfidemoundsup to 20 p high wgls small (lessthan 20 pm) round to irregulm grains and
mapped.The largestmound is about 200 x 100 m in aggregates associated with chalcopyrite, bornite,
plan. Two black smokerswere observedon its top, but isocubanite, chalcocite, digenite; minor pyrite and
no chimneysappearon the photographs,and chimney marcasiteare present,as are rare covellite and Aufragmentsarerare amongthe sulfide samples(Krasnov bearing zincian copper.Cobalt pentlanditecommonly
et. aL I995b). The position of the sampling sites is occurs in bomite veinlets or is partly rimmed by
shownin Figure lb.
bornite and digenite in a chalcopyrite matrix. Thin
veinletsofchalcocite cut cobaltpentlanditeandbomile
OccunnmtceANDAssocr,mox
(Fig. 2). Under reflected light, cobalt pentlandite is
oF CoBALThNu,axorre
white, without pleochroismor anisotropism.Compared
to chalc-opyrite,
its reflectanceandhardnessarehigher.
Cobalt pentlanditeoccursin sample9 (Fig. lb) as The grainsaretoo smallfor quantitativemeasurements.
26
TI{E CANADIAN MINERALOGIST
TAEIJ Z RESI'LIII OF Er.ECTRCN-MICRCFROBB ANALYSBS OF
@BALT PBIN.ANDITF" 14'45N, TdIIFAII.ANIIC RIDGE
S
coMFsC!
|
2
3
4
5
6
7
8
9
r0
t1
12
13
t4
rt
16
u
18
19
m
2L
2
Flc. 2. Back-scatteredelectron image of two inclusions of
cobaltpentlandite(grey)in aveinlet ofbornite (ight $ey)
within a matrix of chalcopyrite.Thin veinletsof chalcocite
(white) cut the chalcopyriteandthe veinlet ofbornite. The
scalebar at the bottom of the image is l0 p"rn.The compositionalprofile wasmeasuredalongthe travene shown.
429
442
&t
62
4&6
8,1
499
92
505
51.3
J1.4
'tt
51.6
5L0
t24
523
526
52"6
52.6
$.1
53A
53.9
Tho @lyd€l
5J
5.1
5J
43
43
4.0
43
4:t
4.0
4.0
43
46
4!
42
4A
3J
45
3J
39
4.0
4.0
3,4
9.6
9.1
9.1
9.1
roj
9.6
rcn
12-4
9.1
8J
92
rM
10'
9A
l0.l
96
103
9.1
99
8.4
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9.1
wlb
B@l@ O6€d@ U atoN)
T@l
9.4 nS 99
:l
&1 f2,2
63 !L6 93
yl.4
5.4 9l
39 324lm.0
43 94 9J
3.0 32J lmo
03 3aJ lmJ
93
3,4 fX
L4 !2i
fi2
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2;l n2
1.6 3a4 lm.6
lE 9,.4 1m,.4
L5 325 100.6
l:l 329 101J
2.4 3a8 mm
09 32.6folj
)-l 3f.6 9.f
r.4 3231m.1
23 t2-4 [email protected]
2.4 32.1 f03
2i ni
nL2
(oor.%l'ur.eFeo.?6cq.rir.*Su.ot
(Oo.edlfr.rFoo.ac0re)p.d"..
(C%.;MlaFeo.rch.aJm.sse.q
(o6.;NL.uF%.6coo.sJm.esa.o
(406.;lq.{&0.6c!o.ds.*S.s
(&6.;NL.eF%.&qh.nb.o&.o
(O06.6y'r!.rroo..Cq.Ile.r&.o
([email protected]*.nF".o
(Ooo.?r!.lllaFeo."Ctb,ctr*..S".s
(c06.FNt..!Foo.rch.d*.o&.e
(ooe.o!{lr.aFoo.oo0o.J..nfr.o
(Coo.a!{r.a&o.aGo.ao\o.dq.o
(O%.eln.c&o.ocqo.z)E.a,&.o
(coo.dl'n1.;Feo.ooso,rtlo.n&.e
(O0o.6el{1r.iF%.oGo.aJe.q$.o
(Oe.*l0t.aFoo,ecoo.ds.o9.o
([email protected]&.e
(Cot.,!0!a$o.sGo.ril"...S",*
C{h;NL.sF%.sOo.rJe.6q.o
(co?.olrl,;B%.r?C!!.dp.cSr.*
(Oo?.sML!Fo!.!d(\.g,)re.s&.o
(Co?.;ry!.a,Feo.dc0o.dp.*!r.o
@ qu@d h wd8h %,
Table 2. Given the amount of copper present, the
mineralcanbe considereda cupriancobaltpentlandite.
The microprobe analysesshow that some gtains are
homogeneous,whereas others are zoned, 'rpith the
CrumcerDlm
centralportion richer in Cu and Fe, and poorer in Co,
thanthe outerzone.Statisticalteament of the twentyThe average concenfration of eigftt elements in two data sets shows a positive correlation betwesn
sample 9 and severallarge (tens to hundredsof kg) concentrationsofFe andCu, anda negativecorrelation
bulk samplesis listed in Table 1.The cobaltpentlandile betweenthe concentrationof each of thesg elements
identified in sample9 was analyzedon a CAMEBAX and Co Clable 3). The sulfur contents Qn at.Vo)are
elecfton microprobeoperatedat 20 kV, 20 nA, using plotted on a frequency-disnibutiondiagram (Fig. 3).
the following standardsand X-ray lines: pure metals The results show a slight sffi to the metal"rich side
(CoKa, NiKoq FeKa, CuKcr)and FeS, (SKcr).Results of the ideal formula MesSs(47.O5 at.7o S). The
of the electron-microprobe analyses are listed in compositionsare within the range of sulfur contents
OF MASSIVB SUI.FIDBS FROM OTIIR
SITES ON TIIE OCEANFI,M,
s"qde lelity
14o4tN Fr&!
SsBlle 9
Awage d 7 6ilF16
ni
n3
&4
$.0
2-r0
ljl
0.05
0.or
92
524
0i5
3,6
9.0
4l.9
!72
10.1
435
310
9J
633
TAG Flel4 acttvead
MAnK Fbl4 ddrl
MARr Fbld, @
Ear krinr
rr.T16
sadgs
Rise
4
y
6m
w
65
m
a2
92
t4
363
m
Ll
0.m
n5
n3
A
L8
0r'3
1r2
&
16
0.6
004
A
741
51
0J
Co'eanrwdFeCqbasdfb@q!@dhvt%;aodpqled@d&ooe6ol@r@@qldedhpFr.
ht(ffialdddsebhfrlofr@€cil
Ndlgldet8ls
m-dd€ln#&
lte.o|r€rdlad@de,
A€agedds@d6
G95b). A@ltswordrddin&ifrtrtltedoaloSbglPrGEe@tuR@@d
ZrdPb
w
€*ab&Srdb!
dod.
abspdho spr&Mpy,
adOardeed{olmstrS,bt
COBALT PENTI.ANDITE
FROM T}IE MID-ATLANTIC
RIDGE
27
TABLB 3. CORRB.AIION@EFFISE{TS BE'TWBAN
@NSNTRA'NONSOF
EI,IMB{TS IN @BALT PB{TL{T{DITB,MID-ATLANNCRID(IE
Co
Co
NI
F9
c0
Nl
Fo
1
0.88
&s
-{'.68
-o.91
4.4
0.7t
1
0.6
0.@
I
s
BE€d@@ltsd22olEt@-DiqlFmbe
@lys
Rds
=032 tsP -
.
(46.247.9 at.Vo) fot synthetic pentlandite with
Fey'Ni= 1.0 quenchedfrom 600'C (Shewman& Clark [s.&
nl
1970). Comparisonof our data on cobalt pentlandile
(Fe
in
oceanproportions
+
Cu)
and
Ni
of
Co,
4.
Plot
of
Frc.
with data from several land-baseddeposits (Fre. a)
floor cobaltpentlanditeand someland-basedoccurrences.
revealsthat the ocean-floorcompositionsareclosestto
Symbols: full squares: this study; open triangles:
those found in the depositsin Finland and Norway,
Kongsfiell, Norway,pyrrhotite- chalcopyrite- cubanitewherecubaniteis one of the principal sulfide minerals.
sphalerite assemblage(Lindahl 1973); plus signs:
DscussroN
The nonstoichiometry of pentlandite has been
discussedby Hanis & Nickel (1972). Many authors
havenotedthat both naturaland syntheticpentlandites
as well as cobalt pentlandite are stghtly sulfirdeficient (Curlook & Pidgeon 1953, Stumpfl & Cla*
1964, Knop et al. L965, Shewman& Clark 1970).
However,Harris & Nickel statedthat the evidencefor
appreciabledeviation from a 9:8 metal:sulfru ratio is
not strong because the sulfur values were not
consideredsuffi cientlv accurate.
Varislahti, Finlan{ pyrrhotite - cubanite- chalcopyrite
assemblage(Kuovo e/ al, 1959): crosses:Outokumpu
ming, pinland pyrrhotite - chalcopyrite- pyrite assemblage (Kuovo et al, 1959); open squares: Canadian
deposits, associationof Co-sulfides, Co-arsenideswith
pyrite, marcasiteand other minerals (Stumpfl & Clark
1964,Ilanis & Nickel 1972); opn diamonds:Birtavarre,
Norway, pyrrhotite - chalcopyrite- cubanite- sphalerite
(Lindahl 1973).
assemblage
The data obtained for the ocean-floor cobalt
pentlandite(22 electron-microprobeanalysesobtained
under the sa:neanalytical conditions and partly from
the samezonedgrain) show a nalrow rangeof sulfur
content, compared to that recorded in synthetic
pentlandite.This finding correspondsto early observations and is consistentwith the suggestionof-a narrow
field of nonstoichiometryin this mineral. The change
4
of the oxidation stateofiron from Fez+to Fe3+,along
o
with a slightly increasingquantityof Fe*, may account
2
:
for the small range in sulfur content. Our data also
support the conclusion of Lindahl (1973) that the
cobalt content of the pentlandite-groupmineral in
continental deposits depends upon the associated
sulfides.
Comparedto otheroccurrencesof massivesulfidein
the Pasific andAtlantic oceans(Krasnovet aL.1995a),
the samplesexaminedin this studyarericher in Cu and
Au, with lower Zn and Cd, but they show comparable
3 (doniaTo)
Co contents. However, Bogdanov et al. (L995)
reporteda relative enrichmentof sulfidesfrom this site
Ftc. 3. Frequencydistribution of sulfur contents (alVo) n
in cobalt. The main feature of the 14"45'N site is the
and MeeSg
ocean-floor cobalt pentlandite. Me1g.S15
rocks. The presenceof
represent range of sulfur proportions in synthetic wide disnibution of ultramafic
Ni mineralsshouldbe expectedon the basisof elevated
pentlandite(after Shewman& Clark 1970).
28
Ni contents of ulfamafic rocts, which are subject to
leaching by hydrothermal solutions. However, Ni has
limited mobility in chloride solutions, and is captured
from solutions by chlorite, serpentine and other
secondary magnesian minerals present in ocean-floor
rocks (Grichuk & Krasnov 1989). Therefore, Ni is
expected to have low mobility in oceanic hydrothermal
processes, and never becomes endched in massive
sulfides. This is evidently the reason for the formation
of cobalt pentlandite instead of pentlandite in the
samples studied.
Acln.towreDcmttn\Ts
The study was carried out in the context of a project
on Orogenesis in the Oceans, sponsored by the State
Scientific and Technical Program. We are most
gratefirl to Drs. Donald C. Harris, of the Geological
Survey of Canad4 and Robert F. Martin, for their
efforts to improve the quality of presentation of our
findings. In addition, we acknowledge tle helpfirl
comments of reviewers Roger Moss and Steven Scott.
Rmrnwcrs
BATUEv,8.N., KRorov, A.G., Manrov, V.F., Gnnxasmv,
G.A., Knesrov, S.c. & LrsnsyN,Yn.D. (1994):Massive
sulphide deposits discoveredand sampled at 14"45'N,
Mid-Atlantic ridge. BNDGE Newsletter6, 6-10.
Kroe, O., I.nRAril\4,
M.A. & Suraruo (1965): Chalcogenides
of the transition elements.IV. Pentlandite.a natural n
phase.Can.MbrcraLE,291-316.
KRAsNov,S.G., Gnm.resHsv, G.A., Srsper.lova,T.V. *o
rEN ornEps (1995b): Detailed studies of hydrothermal
fields in tlte North Atlartic. 1z HydrothermalVents and
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Geol.Soc.,Spec.PabL E7,43-64.
Ponosnnre,I.M. & Cnsmasmv, G.A. (1995a):
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Kuovo, O., Htrnra, M. & Vuonrranvm, Y. (1959):Anatural
cobalt analogueof pentlandite.A/la Mineral, U,897-9m.
LNDAnL, I. (L973): Cobalt pentlandite from Kongsfiell,
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Ntct<u,,E.H. (1992):Solid solutionsin mineralnomenclatue.
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Pnrnur, W., HARRrs,D.C. & SrEwARr, J.M. (1969):
Iangisite, a new mineral, and the rare minerals cobalt
pentlandite, siegenite, parkerite and bravoite from the
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norphic rocks of Rift zone of Arabian-Indian ridge.
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thermalfield at 14'45'N, Mid-Atlannckdge. DoH. A|<nd.
-357
(in Russ.).
Nauk Y3(3), 353
& Batunnq, G.N. (1971): On tle hydrothermalore
occurrences on the Indian ocean floor. Oceatnlagy |'l(6),
CuetooK, W. & PpcBor, L.M. (1953):The Co-Fe-S system.
Can Inst Mining Metall. 8u11.46, 297-301.
Gmcutx, D,V. & KRAsNov,S.G. (1989): Sourcesof orefonning elementsin modern hot springs of the ocean
floor. Litholog and Mineral Deposirs1, 80-88(in Russ.).
Ilanrus, D.C. & Nlcr<s., E.H. (1972): Pentlanditecompositions and associationsin some mineral deposits. Can
MincraL ll, 861-878.
HARr, R., Hotrs, J. & Pyr-a,D. (1990): Multistage hydrothermal systemsin the Blanco fracture zone. In Gorda
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Springer-Verlag,New York, N.Y. (51-57).
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Risenear13"N.Econ.Geol.80,221.249.
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relations in the Fe-Ni-S systemand noteson rhe monosulfide solid solution. Can.l. Earth Sci.7, 67-85.
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Received August 15, 1995, revised mana*cript accepted
November2i, 1995.