lead isotope systematics of sulfide minerals in the middle valley
TRANSCRIPT
Lead isotope systematics of sulfide minerals in the MiddleValley hydrothermal system, northern Juan de Fuca Ridge
Brian L. Cousens and John BlenkinsopOttawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive,Ottawa, Ontario, K1S 5B6, Canada ([email protected]; [email protected])
James M. FranklinGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada ([email protected])Currently at Franklin Geosciences, 24 Commanche Drive, Ottawa, Ontario, K2E6E9, Canada
[1] The sources of metals for the sediment-hosted massive sulfide deposits currently forming at the sediment-
filled Middle Valley segment of the northern Juan de Fuca Ridge have been investigated using Pb isotopes.
Leg 139 of the Ocean Drilling Program drilled three sites inMiddle Valley at which basaltic rocks, sediments,
and sulfide mineralization were recovered. At site 856, massive sulfides composed of pyrite, pyrrhotite,
minor chalcopyrite, and sphalerite have isotopic compositions intermediate between Juan de Fuca basalts and
Middle Valley turbiditic sediments, with the exception of two samples that are relatively nonradiogenic.
Primary pyrrhotite-dominated and secondary pyrite-dominated sulfides overlap in isotopic composition,
indicating that both high- and low-temperature hydrothermal fluids have interacted with the sediment pile.
Drilling at sites 857 and 858 intersected hydrothermally altered basaltic sill-sediment complexes containing
disseminated aggregates and veins of secondary hydrothermal sulfides. The sulfide minerals have highly
variable, continental crust-like Pb ratios with Stacey-Kramers model ages ranging from zero to 1.5 Ga
(206Pb/204Pb � 16.2). Some massive sulfide samples from site 856 also include an ‘‘old’’ sedimentary Pb
component. The source of this old Pb is most likely Proterozoic detrital sulfides from turbidites within
Middle Valley, even though the dominant source of detritus is thought to be theMesozoic accreted terranes of
Vancouver Island. The relative abundances of Cu and Zn in Middle Valley massive sulfides do not correlate
with Pb isotopic composition, probably due to similar Cu/Zn in basalt and sediment components.
Components: 8053 words, 6 figures, 4 tables.
Keywords: Hydrothermal vents; Juan de Fuca Ridge; Pb isotopes; sulfides; Basalts; sediments.
Index Terms: 1040 Geochemistry: Isotopic composition/chemistry; 3015 Marine Geology and Geophysics: Heat flow
(benthic) and hydrothermal processes; 3035 Marine Geology and Geophysics: Midocean ridge processes; 8424 Volcanology:
Hydrothermal systems (8135).
Received 17 October 2001; Revised 28 January 2002; Accepted 30 January 2002; Published 23 May 2002.
Cousens, B. L., J. Blenkinsop, and J. M. Franklin, Lead isotope systematics of sulfide minerals in the Middle Valley
hydrothermal system, northern Juan de Fuca Ridge, Geochem. Geophys. Geosyst., 3(5), 10.1029/2001GC000257, 2002.
1. Introduction
[2] Middle Valley, a segment of the northern Juan
de Fuca Ridge, is a sediment-filled rift that hosts
several significant massive sulfide deposits formed
as a result of ongoing hydrothermal activity [Crane
et al., 1985; Davis et al., 1992; Goodfellow and
Blaise, 1988; Ocean Drilling Program Leg 139
Geochemistry Geophysics Geosystems
Geochemistry Geophysics GeosystemsAN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Published by AGU and the Geochemical Society
G3G3Article
Volume 3, Number 5
23 May 2002
10.1029/2001GC000257
ISSN: 1525-2027
Copyright 2002 by the American Geophysical Union. 1 of 16
Scientific Drilling Party, 1992]. At spreading cen-
ters, deep-circulating seawater is heated and reacts
with oceanic crust to strip metals of economic
interest (Cu, Zn, Ag, Au, and Pb). The metal-rich
solutions precipitate sulfide minerals when they
come into contact with cold seawater at or near
the seafloor [e.g., Humphris and Thompson, 1978;
LeHuray et al., 1988; Seyfried and Janecky, 1985;
Seyfried and Mottl, 1982]. If a thick sedimentary
pile is present within the spreading center, metals
will also be stripped from the sediments [Koski et
al., 1985; Von Damm et al., 1985]. Because the Pb
isotopic compositions of mantle-derived basalts
and exogenous sediments at a spreading center
can be very different, Pb isotope data may be used
as a tracer of the source of metals in a seafloor
massive sulfide deposit [Fouquet and Marcoux,
1995; German et al., 1995; Hegner and Tatsumoto,
1987; LeHuray et al., 1988]. This information can
then be used to compare metal sources of copper-
rich mineralization versus zinc-rich mineralization
both within individual deposits and between differ-
ent deposits or to investigate lateral or vertical
zonations within individual deposits that may
reflect waxing and waning of hydrothermal activ-
ity. In the following, we present Pb isotopic data
from massive sulfides from the near-surface depos-
its, secondary sulfide minerals precipitated in basal-
tic sills and flows, unaltered surface sediments and
basalts, and altered sediments from within a sill-
sediment complex at Middle Valley, in order to
trace the source of metals in the sulfides.
2. Geologic Setting
[3] Middle Valley is one of three spreading center
segments that form the north end of the Juan de
Fuca Ridge (Figure 1) [Barr and Chase, 1974;
Karsten et al., 1986]. Middle Valley is a deep
extensional rift that is filled by Pleistocene to
Recent turbiditic sediments derived from the con-
tinental margin of western Canada and the north-
western United States [Davis et al., 1992]. The
relatively impermeable sediment cover over the
spreading center serves to trap hydrothermal fluids
and allows for chemical interaction between the
fluids and sediments. Sulfide deposits have formed
where these fluids have exited on the seafloor.
[4] Two areas of hydrothermal venting and sulfide
deposition at Middle Valley are known (Figure 1).
The northernmost is termed the Area of Active
Venting (AAV), located along normal fault struc-
tures, which includes several hydrothermal
mounds with anhydrite chimneys venting 184�–274�C fluids [Goodfellow and Franklin, 1993].
Hydrothermal activity also occurs 3 km south of
the AAV, close to an uplifted circular mound
termed Bent Hill [Goodfellow and Franklin,
1993]. The Bent Hill massive sulfide mound
(BHMS: sites 856G,H; 1035A,D,G) and Ore Drill-
ing Program massive sulfide mound (ODPMS: site
1035H) (deposit terminology of Fouquet et al.
[1998]) are located 100 and 450 m south of Bent
Hill, respectively.
[5] At Bent Hill, massive sulfides include pyrite,
pyrrhotite, magnetite, sphalerite, and Fe-Cu sul-
fides [Davis et al., 1992; Fouquet et al., 1998;
Goodfellow and Franklin, 1993]. An original
high-temperature assemblage of pyrrhotite, wurt-
zite, and isocubanite has been altered to pyrite,
marcasite, sphalerite, and iron oxides by reaction
with later, cooler hydrothermal fluids [Duckworth
et al., 1994; Goodfellow and Franklin, 1993;
Krasnov et al., 1994]. Hydrothermal mounds in
the AAV consist of hydrothermally altered hemi-
pelagic and turbiditic sediments capped by anhy-
drite-pyrite chimneys [Davis et al., 1992;
Goodfellow and Franklin, 1993]. Within the sedi-
ments are massive sulfide layers composed of fine-
grained pyrrhotite, chalcopyrite, pyrite, sphalerite,
galena, and anhydrite [Goodfellow and Franklin,
1993].
[6] Pre-ODP drilling surveys of the Middle Valley
sulfide deposits included dredge sampling, push
core sampling from DSRV Alvin and short drill
coring (maximum depth 5 m). In 1991, igneous
rocks, sediments, and sulfide mineralization were
recovered at three sites in Middle Valley during
ODP Leg 139 [Davis et al., 1992; Stakes and
Franklin, 1994]. Holes 856G and 856H intersected
the BHMS south of Bent Hill. Holes 857C and
857D, drilled south of the AAV, sampled a highly
altered sill-sediment complex in an area of high
heat flow and hydrothermal recharge. Sill margins
are intensely altered to chlorite and are cut by veins
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of chlorite, sulfide minerals, quartz, zeolites, and
epidote [Davis et al., 1992; Stakes and Franklin,
1994; Stakes and Schiffman, 1999]. Sulfides also
commonly fill vesicles or occur as ‘‘porphyro-
blasts’’ replacing mesostasis or, less commonly,
crystals in the basalts. The dominant sulfide min-
erals are pyrite and pyrrhotite and in some veins
chalcopyrite and sphalerite are common. Holes
858F and 858G drilled the AAV and intersected
sediments underlain by altered extrusive basalt
flows that once formed a topographic high. Secon-
dary sulfides are less abundant than in the igneous
rocks from site 857 but include veins of pyrite plus
chlorite, veins of chalcopyrite, sphalerite, quartz,
zeolite, and epidote, and ovoid pyrite ‘‘porphyro-
blasts’’ which are commonly overprinted by a
mixture of pyrite, chalcopyrite, pyrrhotite, and
sphalerite [Davis et al., 1992; Stakes and Franklin,
1994; Stakes and Schiffman, 1999]. In 1996, ODP
Leg 169 included further drilling at the BHMS (site
856H and sites 1035A-G) and sampling of the
ODPMS (site 1035H) deposit located south of the
BHMS [Bjerkgard et al., 2000; Fouquet et al.,
1998].
3. Analytical Techniques
[7] Massive sulfide samples from DSRV Alvin
push cores from the AAV and Bent Hill areas
[Turner et al., 1991], short drill cores, and dredge
hauls from Bent Hill, and drill cores from ODP
Holes 856G and 856H were dissolved in 8N
Figure 1. Location of Middle Valley, with sample sites, at the north end of the Juan de Fuca Ridge [Bjerkgard et al.,2000]. ODP Leg 139 and 169 drill sites are shown by red and yellow stars, respectively. Holes 856G,H and 1035A-Gdrilled the BHMS deposit, whereas hole 1035H drilled the ODPMS deposit (see text). DSRVALVIN push core sitesare indicated by blue diamonds [Turner et al., 1991]. Two short (5 m) drill core samples (MVNBD-2A,-5C) from theBent Hill deposit are located close to ALVIN 2253 core sites. Not shown are dredge hauls across ODP site 856 thatrecovered sulfides and unaltered hemipelagic sediments.
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HNO3, dried, and redissolved in HBr for Pb
separation and isotopic analysis. Basalt samples
from dredge hauls from Middle Valley, samples of
surface hemipelagic sediments, and altered inter-
sill sediments from site 857 were crushed in an
agate mortar. The powders were acid-washed in
3N HNO3 and 2N HCl to remove alteration
minerals, and the residues were processed for Pb
separation.
[8] Basaltic rocks from sites 857 and 858 are
extensively altered and include secondary sulfides
[Davis et al., 1992; Stakes and Franklin, 1994;
Stakes and Schiffman, 1999]. Sulfide minerals were
leached from the rock powders with warm 3N
HNO3 for a period of two hours, and the leachate
was dried and processed for Pb isotope analysis.
Because the original goal of the study was to
determine the Pb isotopic composition of the sul-
fide-free basalts, the HNO3 attack was in some
cases longer than necessary to remove only the
sulfide component. Multiple leach experiments on
two samples show that some of the HNO3 leachates
include some Pb leached from the silicate phases in
the basalt. Because the secondary sulfides were
removed by acid-leaching, no attempt was made to
determine Pb concentrations in the leachates. Mass
spectrometer runs for the leachates yielded much
larger Pb signals than for the leached basalt resi-
dues, owing to the higher Pb content and cleaner
matrix of the leachate compared to the residue.
[9] All samples were processed for Pb isotopic
analysis using standard anion exchange techniques,
followed by thermal ionization mass spectrometry
(details by Cousens [1996b]). All Pb mass spec-
trometer data are corrected for fractionation using
NIST SRM981. The average ratios measured for
NIST SRM981 are 206Pb/204Pb=16.891±0.011,207Pb/204Pb=15.430±0.014, and 208Pb/204Pb=
36.505±0.048 (2s), based on 40 runs between
September 1992 and March 1996. The fractiona-
tion correction, is +0.13%/amu (based on the
values of Todt et al. [1984, 1996]). The Pb isotopic
compositions of surficial sediments and dredged/
cored massive sulfides, ODP massive sulfides,
secondary sulfides leached from basaltic sills and
lavas, and ODP turbidite sediments, are listed in
Tables 1, 2, 3, and 4, respectively. Isotopic data
from basaltic rocks are available upon request from
the first author.
4. Isotopic Results
4.1. Massive Sulfides
[10] The Pb isotopic compositions of massive sul-
fide recovered from BHMS Holes 856G and 856H,
Alvin push cores, short drill cores, and dredges of
chimneys and mineralized sediments are plotted in
Figure 2. Also shown are published Pb isotope data
from analyses of massive sulfides from BHMS
Holes 856G, 856H, 1035A to 1035G [Bjerkgard et
al., 2000; Stuart et al., 1999], and sulfides from
ODPMS Hole 1035H [Bjerkgard et al., 2000]. The
massive sulfide samples from this study generally
plot between the field of dredged basalts (B. Cou-
sens, unpublished data, 1997) and sediments from
Middle Valley and fall in the middle to high part of
the range of all sulfide samples from Middle Valley.
Two sulfide samples from Hole 856G/H (data set by
Stuart et al. [1999]) have significantly lower206Pb/204Pb than the majority of the sulfides and
plot to the left of the Middle Valley sulfide array.
Sample 856H-3R1 95–97 from this study plots just
to the left of the Stacey-Kramers average upper
continental crust evolution curve at a model age of
� 0.8 Ga and is distinctive. The Pb content of this
sample, 21 ppm, is virtually identical to that of
sample 856H-3R3 97–99 located just 2 m below
it, but these two samples have very different Pb
isotopic compositions.
4.2. Site 857 and 858 Basaltic Rocks
[11] The acid-washed basaltic rocks have a limited
range of Pb isotopic compositions. In plots of207Pb/204Pb or 208Pb/204Pb versus 206Pb/204Pb, the
sills and lava flows form a linear array extending
fromMiddle Valley basalts toward the compositions
of sulfides and sediments from Middle Valley
(Figure 3). In contrast, the sulfide-dominated
HNO3 leachates from the basaltic sills and lavas
are highly variable in composition. Many lie within
the field of massive sulfides fromMiddle Valley, but
a significant number are to the left of Juan de Fuca
mid-ocean ridge basalts (MORB) andMiddle Valley
surface sediments, toward the Stacey-Kramers
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curve. Some secondary sulfides lie close to the
Stacey-Kramers curve, with model ages ranging
from zero to nearly 1.5 Ga.
5. Discussion
5.1. Isotopic and Base Metal Systematicsof the Massive Sulfides
[12] Pb isotopes are sensitive indicators of the
sources of Pb in seafloor sulfide deposits. For the
Juan de Fuca and Explorer Ridge areas, bare-
rock hosted bulk sulfide Pb isotope ratios closely
match the isotopic composition of the local basalts
(Figure 2). Sulfides from the Explorer Ridge have
more radiogenic Pb and are underlain by enriched-
MORB basalts [Cousens et al., 1984; Fouquet and
Marcoux, 1995; Michael et al., 1994], whereas
sulfides from the southern Juan de Fuca Ridge
are less radiogenic and are underlain by normal,
depleted MORB [Hegner and Tatsumoto, 1987]. At
Middle Valley, the measured Pb isotopic composi-
Table 1. Pb Isotopic Composition of Surficial Sediments and Massive Sulfidesa
Lab Number Area Type 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb Pb, ppm
ALV 2251-1-1 AAV Sulphide 38.73 15.67 18.83 68ALV 2251-3-2 AAV Sulphide 38.67 15.64 18.90 130ALV 2251-3-2a AAV Sulphide 38.50 15.58 18.87 130ALV 2251-3-2b AAV Sulphide 38.53 15.59 18.87 130MVDR05-02 BHMS Sediment 38.85 15.65 19.05 13MVDR05-03 BHMS Sediment 38.89 15.66 19.06 13MVDR07-01 BHMS Sediment 39.11 15.73 19.08 97MVNBD-2A BHMS Sulphide 38.61 15.63 18.88 81MVNBD-5C BHMS Sulphide 38.59 15.62 18.88 190MVDR0201 BHMS Sulphide 38.53 15.59 18.88 1500MVDR0218 BHMS Fe-oxides 38.62 15.62 18.90 1300MVDR0603 BHMS Sulphide 38.47 15.59 18.84 5500MVDR0225 BHMS Sulphide 38.77 15.63 18.90 21MVDR0204 BHMS Sulphide 38.72 15.63 18.91 38ALV2253-1-1B BHMS Sulphide 38.43 15.57 18.83 17ALV2253-2-1 BHMS Sulphide 38.46 15.57 18.84 –ALV2253-1-3 BHMS Sulphide 38.49 15.57 18.86 150ALV2253-4-1A BHMS Sulphide 38.47 15.57 18.86 23
aUncertainty in 208Pb/204Pb is +/� 0.05, and 206Pb/204Pb and 207Pb/204Pb are +/� 0.02. Pb concentrations determined by ICP-ES at the
Geological Survey of Canada. Sample types are as follows: ALV are Alvin push cores, MVDR are dredges, MVNBD are short drill cores. AAV isArea of Active Venting, BHMS is Bent Hill massive sulfide. ‘‘– ’’ indicates no data available.
Table 2. Pb Isotopic Composition of Site 856 Massive Sulphidesa
Lab Number Hole Core Sec Top btm Depth 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb Pb, ppm
003R 01 856G 3R 1 61 64 �18.2 38.44 15.57 18.85 25856GR029092 856G 7R 2 90 92 �58.1 38.45 15.57 18.84 46003R 01 856H 3R 1 95 97 �23.4 37.22 15.56 17.35 213R 3 856H 3R 3 97 99 �25.7 38.46 15.58 18.84 20004 O1 856H 4R 1 84 86 �27.5 38.30 15.59 18.58 –004R 02 856H 4R 2 74 76 �28.8 38.46 15.59 18.81 –006R 01 856H 6R 1 71 73 �38.1 38.39 15.58 18.78 20008R 01 856H 8R 1 87 89 �48.9 38.59 15.62 18.87 130013R 01 856H 13R 1 49 50 �71.4 38.58 15.62 18.87 53856H15R246 856H 15R 2 4 6 �82.0 38.43 15.57 18.81 49016R 01 856H 16R 1 46 48 �85.7 38.49 15.59 18.84 52017R 01 856H 17R 1 29 31 �90.4 38.47 15.58 18.84 –
aCore depth on meters below sea floor. Uncertainty in 208Pb/204Pb is +/� 0.05 and 206Pb/204Pb and 207Pb/204Pb are +/� 0.02 and 207Pb/204Pb are
+/� 0.02. Pb concentrations by ICP-ES at the Geological Survey of Canada.
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tions of massive sulfide samples analyzed in this
study commonly plot between Middle Valley
basalts, within the Juan de Fuca MORB field, and
the field of surface turbiditic sediments from Mid-
dle Valley, indicating that the Pb in the sulfides is a
mixture of both mantle-derived and sediment-
derived Pb (as concluded by Fouquet and Marcoux
[1995], Goodfellow and Franklin [1993], and
Stuart et al. [1999]). Studies of the sulphur and
helium isotope ratios in the massive sulfides and
secondary sulfides in the basalts show that these
two elements also have igneous and sedimentary
sources [Duckworth et al., 1994; Stuart et al.,
1994; Zierenberg, 1994]. A mixing line (‘‘M’’ in
Figure 2) between these two components is shown
in Figure 2 and indicates that as much as 20% of
the Pb in the sulfides may be derived from a
sedimentary source. Most of the sulfides, however,
include less than 5% of a sedimentary Pb component
and thus most of the Pb in the sulfides has a basaltic
source, as concluded by others [Duckworth et al.,
1994; Krasnov et al., 1994; Stuart et al., 1999]. It is
Table 3. Pb Isotopic Composition of HNO3 Leachates of Site 856, 857, and 858 Basaltsa
Lab Number Hole Core Sec top btm Depth 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb
17 HNO3 856B 9X 1 4 8 �62.4 38.44 15.57 18.8520 HNO3 857C 59R 1 106 108 �472.2 38.49 15.58 18.8732 HNO3 857C 59R 4 111 113 �476.6 37.30 15.44 17.8033 HNO3 857C 60R 1 13 15 �480.9 38.45 15.57 18.8451 HNO3 857C 63R 1 69 71 �510.4 38.57 15.67 19.1653 HNO3 857C 64R 1 63 66 �520.0 38.49 15.58 18.8958 HNO3 857C 65R 1 15 17 �529.3 38.77 15.61 19.0461 HNO3 857C 66R 1 94 96 �539.3 36.98 15.39 17.3365 HNO3 857C 67R 1 73 75 �549.1 37.50 15.49 17.8269 HNO3 857C 68R 1 90 92 �558.9 38.00 15.53 18.3974 HNO3 857C 68R 2 28 30 �559.8 38.17 15.56 18.5985 HNO3 857D 2R 1 77 79 �590.4 38.39 15.57 18.8589 HNO3 857D 3R 2 67 69 �601.5 38.23 15.58 18.7193 HNO3 857D 4R 1 9 11 �609.0 38.39 15.56 18.8197 HNO3 857D 7R 1 16 18 �637.8 38.34 15.56 18.79101 HNO3 857D 8R 1 96 98 �648.3 37.79 15.59 18.01103 HNO3 857D 15R 1 58 60 �715.4 38.49 15.57 18.87105 HNO3 857D 18R 1 98 101 �744.6 38.47 15.57 18.86108 HNO3 857D 20R 1 64 67 �763.1 38.28 15.57 18.70110 HNO3 857D 21R 1 62 65 �772.8 38.47 15.58 18.86113 HNO3 857D 22R 1 52 55 �782.4 38.50 15.58 18.89114 HNO3 857D 23R 1 80 84 �792.3 38.43 15.57 18.85118 HNO3 857D 24R 2 64 66 �803.3 37.60 15.54 17.96122 HNO3 857D 25R 1 64 66 �811.4 37.55 15.55 17.98123 HNO3 857D 26R 1 16 19 �820.5 38.36 15.63 18.81125 HNO3 857D 27R 1 96 98 �830.6 38.42 15.56 18.83126 HNO3 857D 29R 1 145 148 �849.9 38.29 15.54 18.76128 HNO3 857D 32R 1 58 61 �878.8 38.01 15.64 18.28132 HNO3 857D 35R 1 27 29 �907.5 38.46 15.57 18.85132 HNO3L1 857D 35R 1 27 29 �907.5 38.35 15.54 18.80132 HNO3L2 857D 35R 1 27 29 �907.5 38.48 15.58 18.86138 HNO3 858F 25R 1 111 113 �250.0 38.55 15.60 18.90139 HNO3 858F 26R 1 42 44 �259.0 37.74 15.55 18.15139 HNO3L1 858F 26R 1 42 44 �259.0 37.56 15.56 17.93139 HNO3L2 858F 26R 1 42 44 �259.0 37.83 15.52 18.35150 HNO3 858F 29R 1 63 65 �287.9 37.24 15.52 17.65161 HNO3 858G 8R 1 36 38 �344.9 36.14 15.37 16.29170 HNO3 858G 16R 1 77 79 �423.7 38.17 15.60 18.51170 HNO3L1 858G 16R 1 77 79 �423.7 38.10 15.59 18.43170 HNO3L2 858G 16R 1 77 79 �423.7 38.16 15.57 18.60
aCore depth in meters below sea floor. Uncertainty in 208Pb/204Pb is +/� 0.05, and 206Pb/204Pb and 207Pb/204Pb are +/� 0.02. L1 and L2 are
consecutive weak leach steps (see text).
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interesting to note that a best-fit mixing line (‘‘BF’’
in Figure 2) through the densest part of the massive
sulfide array and the three sulfide samples with the
lowest 207Pb/204Pb (analyses from Stuart et al.
[1999]) infers an average basaltic source with a
higher 206Pb/204Pb than the ‘‘M’’ model line and
an average sedimentary source with a lower206Pb/204Pb than the ‘‘M’’ model line. This implies
that more radiogenic basalts are present at depth
within the Middle Valley hydrothermal system
and that the average sediment within the hydro-
thermal system has a lower 206Pb/204Pb than that
of surface sediments. If the ‘‘BF’’ mixing line
represents mixing between average basaltic and
sedimentary end-members, then two massive sul-
fides from this study and some of the massive
sulfides analyzed by Stuart et al. [1999] and
Bjerkgard et al. [2000] plot well to the left of
the mixing line, requiring that at least one more
isotopic component is contributing Pb to the
hydrothermal system.
[13] Whereas Pb isotopes are an unambiguous
indicator of the origin of Pb in massive sulfide
deposits, it is important to show that there is a
relationship between Pb and other base and pre-
cious metals. The BHMS deposit is mineralogi-
cally and chemically stratified, based on detailed
analysis of Hole 856H (Figure 4a) [Bjerkgard et
al., 2000; Krasnov et al., 1994]. An original high-
temperature (>300�C) pyrrhotite-wurtzite-isocu-
banite mineral assemblage has been variably
replaced by a lower temperature assemblage of
pyrite-sphalerite-iron oxides [Duckworth et al.,
1994; Goodfellow and Franklin, 1993]. It is
proposed that the early, high-temperature fluids
derived most of their metals from basaltic crust,
whereas later low-temperature fluids interacted to
a large degree with the sedimentary pile and may
have assimilated some metals from this source
[Goodfellow and Franklin, 1993; Krasnov et al.,
1994]. In this scenario, Pb in the original, high-
temperature sulfides should be isotopically similar
to basaltic crust, whereas Pb in the later secondary
phases should be a mixture of basalt and sediment
lead. Zone 1 (0–25 mbsf) and Zone 3 (45–75
mbsf) are dominated by the secondary mineral
assemblage, whereas zones 2 (25–45 mbsf) and
zone 4 (75–90 mbsf) retain a large percentage of
the original assemblage. Bulk sulfide samples
from the four zones show no correlation between
Pb isotope ratios and deposit zonation. Further,
primary and secondary Fe-sulfide minerals overlap
in isotopic composition. Pyrrhotite separates
[Stuart et al., 1999] and bulk sulfides dominated
by pyrrhotite always have 207Pb/204Pb > 15.55
indicating that the higher-temperature pyrrhotite
includes Pb from both basaltic and sedimentary
sources. Pyrite and pyrite-dominated massive
sulfides define the range of 207Pb/204Pb in Middle
Valley massive sulfides, indicating that its Pb can
be basalt-dominated or sediment-dominated. Pyrite
separates with the highest and lowest 207Pb/204Pb
are interpreted to be part of the primary, high-
temperature assemblage, whereas two pyrites inter-
preted to be secondary in origin have 207Pb/204Pb
< 15.55 (i.e., basalt-dominated) [Stuart et al.,
1999]. Thus both the high- and low-temperature
Fe-sulfide minerals include Pb derived from both
basaltic and sedimentary sources in variable but
similar proportions. Apparently, even high-temper-
ature hydrothermal fluids scavenged Pb from
Table 4. Pb Isotopic Composition of Site 857 Sediments and HNO3 Leachatesa
Lab Number Hole Core Sec top btm Depth 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb
S2 857D 3R 2 125 127 �602.0 38.42 15.57 18.83S12 857D 22R 1 17 19 �782.0 38.63 15.60 19.00S22 857D 37R 1 21 23 �918.2 38.53 15.58 18.95S2 L1 HNO3 857D 3R 2 125 127 �602.0 39.38 15.73 20.25S12 L1 HNO3 857D 22R 1 17 19 �782.0 39.49 15.68 20.13S12 L2 HNO3 857D 22R 1 17 19 �782.0 39.01 15.62 19.71S22 L1 HNO3 857D 37R 1 21 23 �918.2 38.69 15.66 19.30
aCore depth in meters below sea floor. Uncertainty in 208Pb/204Pb is +/� 0.05 and 206Pb/204Pb and 207Pb/204Pb are +/� 0.02. L1 and L2 are
consecutive weak leach steps (see text).
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Figure 2. Pb-Pb isotope plots of massive sulfides, surface sediments, and dredged basalts from Middle Valley (filledsymbols from this study; additional data from Leg 169 (open squares, circles) [Bjerkgard et al., 2000]; Leg 139(crosses) [Stuart et al., 1999]). Middle Valley basalt analyses are from dredge hauls 71–23 and 70–16 from the westflank of Middle Ridge, 30 km north of Bent Hill (locations from Barr and Chase [1974]), and dredge haul 90-MVDR-01 from the ‘‘Rubble Mounds,’’ 12 km south of Bent Hill (location from Goodfellow and Blaise [1988]).Also shown is field of Juan de Fuca/Gorda Ridge basalts (compiled by Cousens [1996a]), bare-rock sulfides from theJuan de Fuca Ridge [Fouquet and Marcoux, 1995; Hegner and Tatsumoto, 1987], and a model curve for uppercontinental crust evolution with ticks every 0.5 Ga [Stacey and Kramers, 1975]. Mixing line ‘‘M’’ is between averageMiddle Valley basalt and surface sediments, whereas line ‘‘BF’’ is a line through the dense cluster of massive sulfidesamples. The weight percentage of the sediment Pb component in the mix is indicated by tick marks, assuming Pbconcentrations of 0.5 and 20 ppm in basalt and sediment components, respectively.
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local sediments as the massive sulfide deposit
developed.
[14] The BHMS deposit is also zoned in terms of
base metals, controlled by the relative abundances
of sphalerite and chalcopyrite [Krasnov et
al., 1994]. Figure 4 presents the variation in207Pb/204Pb, Cu/Zn, and Pb/Zn with depth at Bent
Hill and site 856. The square symbols in Figures
4b and 4c are for massive sulfide samples for
which Pb isotope data are available. There is a
subtle tendency for samples from the shallowest
part of the deposit and from zone 3, with higher
Pb/Zn ratios, to have higher 207Pb/204Pb, although
zone 3 also includes high-Cu/Zn sulfides with
very low 207Pb/204Pb. However, there is no corre-
lation between Pb isotope ratios and either Pb
content (Tables 1 and 2) or Pb/Zn between indi-
vidual samples [see also Stuart et al., 1999].
There is also no correlation between Cu/Zn and207Pb/204Pb, which should correlate negatively if
Cu is derived primarily from high-temperature
fluids that have derived their base metals from
basaltic rocks. The lack of correlation between
isotopic and base metal ratios may reflect the
similarity in Cu and Zn contents, and thus Cu/Zn,
in typical Juan de Fuca MORB and average unal-
tered sediments from Middle Valley [Cousens et al.,
1995; Goodfellow and Peter, 1994]. In addition,
hydrothermal alteration has leached more Cu than
Zn from the sediments [Goodfellow and Peter,
1994] and thus a fluid which has interacted mostly
with sediment may have the same Cu/Zn as a fluid
that has interacted largely with basalt, but their Pb
isotope ratios would be very different. Thus the
Cu/Zn ratio may not be diagnostic of basaltic
Figure 3. The 207Pb/204Pb - 206Pb/204Pb plot for secondary sulfides in site 857 and 858 basalts and hydrothermallyaltered host basaltic sills and lavas. Also shown is field for altered turbidites from sites 856H, 1035A [Bjerkgard et al.,2000], and 857D (this study). Other data sources from Figure 2. NHRL, Northern Hemisphere Reference Line [Hart,1984].
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versus sedimentary sources, as is commonly
assumed.
5.2. Isotopic and Base Metal Systematicsof the Secondary Sulfides in Basalt
[15] Alteration of the basaltic rocks drilled during
Leg 139 commonly deposited secondary sulfides,
primarily pyrite in veins and vugs, accompanied by
pyrrhotite, chalcopyrite, and sphalerite in a few
samples [Davis et al., 1992; Stakes and Franklin,
1994]. Analysis of fluid inclusions suggests that
these pyrite-dominated secondary sulfides were
deposited from lower-temperature (<340�C) hydro-thermal fluids similar to those now venting at the
AAV [Peter et al., 1994]. Many of the secondary
sulfides leached from the basaltic sills and lavas at
sites 857 and 858 plot between the MORB and
surface sediment fields, within the field of massive
sulfides in Figure 3. However, one of the massive
sulfide samples from this study and nearly 50% of
the secondary sulfide analyses fall well outside the
range of possible mixtures between Juan de Fuca
MORB and analyzed surface sediments. The fan-
shaped array of Pb isotopic compositions reflects
mixing of Pb from basalt and sediment compo-
nents, but the sedimentary component (dominated
by turbidites) is clearly more isotopically heteroge-
neous than the analyzed surface sediments (domi-
nantly hemipelagic). The composition and inferred
model age of the sedimentary component in the
sulfides varies widely from sill to sill or lava to lava
at sites 857 and 858. Pb isotope ratios vary irreg-
ularly with depth in the sill and basalt lava com-
plexes, requiring that the Pb isotopic composition
of hydrothermal fluids has been highly variable
with time and that the sills and lava flows have
been selectively mineralized over the duration of
hydrothermal activity at these sites.
[16] Unfortunately, no base metal determinations
are available for secondary sulfides in sills from
sites 857 and 858. Base metal contents in some of
the secondary sulfide minerals have been measured
Figure 4. Variation in 207Pb/204Pb, Cu/Zn, and Pb/Zn (times 100) in ODP Holes 856G and 856H (additional datafrom Franklin, unpublished data, 1993, Krasnov et al. [1994], and Stuart et al. [1999]). Squares indicate samples forwhich Pb isotope data are available; crosses indicate samples lacking Pb isotope data. Zones 1–4 indicated by arrows[Krasnov et al., 1994].
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by electron microprobe, but Pb concentrations are
below detection limit in all minerals except sphaler-
ite (J. Franklin, unpublished data, 1993). However,
Cu, Zn, and Pb concentrations have been measured
on whole-rock samples [Stakes and Franklin,
1994], wherein Cu and Pb, and to a lesser extent
Zn, should be strongly partitioned into the secon-
dary sulfides. Basaltic sills with abundant secondary
sulfides have Cu, and less commonly Zn, contents
higher than average MORB or fresh West Valley
basalts [Cousens et al., 1995], reflecting concen-
trations of these elements in the secondary sulfides
(Figure 5). Of 59 samples analyzed for Pb by
inductively coupled plasma-mass spectrometry
(ICP-MS) and inductively coupled plasma-emission
spectroscopy (ICP-ES), twenty-five have Pb con-
centrations between 2 and 70 ppm, showing that a
significant fraction of new Pb has been added to the
sills in the form of secondary sulfides [ J. Franklin,
unpublished data, 1993; Stakes and Franklin,
1994]. Cu/Zn in the secondary sulfides is generally
high, comparable to Cu/Zn in surface and zone 1 to
3 massive sulfides from Bent Hill, and up to 4 times
the ratio in fresh basalts. Many of the samples with
high Cu also have lower 206Pb/204Pb than fresh
Middle Valley basalts, suggesting that some if not
all of the Cu may be derived from the ancient,
nonradiogenic component.
5.3. Variation in Pb Isotopic Compositionin the Sedimentary Component
[17] The massive sulfide samples and leached sec-
ondary sulfide minerals show that the Pb isotopic
Figure 5. The 206Pb/204Pb in secondary sulfides versus Cu and Zn contents in whole-rock basalt powders [Stakesand Franklin, 1994] from sites 857 and 858. Average Cu and Zn contents in fresh, unmineralized basalts from WestValley are shown for comparison [Cousens et al., 1995; Van Wagoner and Leybourne, 1991].
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composition of the nonbasaltic component ranges
from that of modern surface sediments to a con-
tinental crust-like component with a model age of
�1.5 Ga. This old component cannot be derived
from the mantle (via basalts) and cannot be derived
from deep Pacific seawater [von Blanckenburg et
al., 1996]. This leaves only the sediments within
Middle Valley as a source of the old component.
The inferred old age of this sedimentary component
is surprising, given that the vast majority of
the rocks being eroded from the west coast of
North America and transported as turbidites to the
Middle Valley area are Mesozoic in age or younger.
The sources of the turbiditic sediments to Middle
Valley are from the north, including detritus mostly
from Vancouver Island. Pb isotopic studies of
the Mesozoic rocks and sulfide mineral deposits
from central Vancouver Island show that these
rocks are much too radiogenic to be the old
sedimentary component seen in Middle Valley
sulfides (Figure 6) [Andrew et al., 1991; Andrew
and Godwin, 1989a; Andrew and Godwin, 1989c].
[18] Rocks of Proterozoic age are found in south-
eastern British Columbia, northeastern Washington,
and northern Idaho, along the eastern margin of the
Cordillera. Recent U-Pb studies of the Monashee
Complex, southern British Columbia, show that
some of these metamorphic rocks include zircon
grains that are as old as 2.3 Ga (J. Crowley, personal
Figure 6. Comparison of the Pb isotopic composition of massive sulfides from Middle Valley with those of majorBesshi-type mining districts [Andrew et al., 1991; Andrew and Godwin, 1989a; Andrew and Godwin, 1989b; Andrewand Godwin, 1989c; LeHuray, 1984; McCutcheon et al., 1993; Peter and Scott, 1997; Sato and Sasaki, 1976;Swinden and Thorpe, 1984] and other modern seafloor hydrothermal vent deposits [J. Peter, unpublished data, 1998;Hegner and Tatsumoto, 1987; LeHuray et al., 1988]. Stacey-Kramers average continental crust evolution curve isshown with crosses every 0.5 Ga. NHRL, Northern Hemisphere Reference Line [Hart, 1984].
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communication, 1997). These Proterozoic rocks are
eroded by the headwaters of the west-flowing
Fraser, Spokane, and Columbia Rivers. Whereas
some detritus from the Fraser River may be depos-
ited on the sea floor east ofMiddle Valley, sediments
from the Columbia River exit far south of Middle
Valley and are not thought to be transported to the
north. However, it is possible that sometime during
the Pleistocene this may not have been the case. The
nonradiogenic Pb isotope ratios are best explained if
the source of the Pb is a low U/Pb mineral in the
sediments, and we suggest that this mineral may be
detrital pyrite. Provided that transport and burial
times are rapid and pyrite is not exposed to highly
oxidizing conditions, it could be preserved within
the turbidites filling Middle Valley. This Proterozoic
pyrite would then by mobilized by hydrothermal
circulation around the sills injected into the sedi-
ments, then deposited (locally?) as a significant
component in secondary sulfides in the altered sills
and as a minor component in massive sulfide
minerals at higher levels in the hydrothermal sys-
tem. The great range in Pb isotopic composition of
the secondary sulfides reflects variable proportions
of ancient sediment Pb, modern sediment Pb, and
basalt Pb in hydrothermal fluids over time and
space, forming the roughly triangular data field for
secondary sulfides in Figure 3.
5.4. Comparison to Besshi-Type OreDeposits
[19] Middle Valley is one of three modern analogs
of Besshi-type massive sulfide deposits [Slack,
1993], which include strata-bound deposits within
clastic sedimentary rocks and intercalated basalt.
Many massive sulfide deposits have variable Pb
isotopic compositions depending on the basement
rocks from which the Pb was derived [Franklin et
al., 1981; Peter and Scott, 1997; Sato and Sasaki,
1976; Slack, 1993]. In particular, Besshi-type sul-
fide minerals commonly have Pb isotope ratios that
fall between those of the mantle and the continental
crust evolution curve, indicating that the Pb is a
mixture of Pb leached from volcanic rocks and
from clastic sediments [Slack, 1993]. Figure 6 is a
Pb-Pb plot showing the measured variability of
massive sulfide deposits from several mining dis-
tricts, including data from the two other known
modern analogs of Besshi-type deposits at Esca-
naba Trough and Guaymas Basin. The Pb isotopic
compositions of Middle Valley massive sulfide
samples span a large range compared to the Esca-
naba, Guaymas, and other individual massive sul-
fide deposits. The fields for mining districts (e.g.,
Japan, Vancouver Island, Newfoundland) are of the
same size as Middle Valley, even though they
include sulfide analyses from several deposits,
some with different ages [e.g., Sato and Sasaki,
1976]. Although it is reasonable to expect that the
clastic sediments filling Middle Valley would be
heterogeneous in Pb isotopic composition, and as a
result the component of Pb in hydrothermal fluids
derived from the sediment would be variable, the
heterogeneity observed at Middle Valley is large
compared to both modern and ancient Besshi-type
massive sulfide deposits.
[20] The observed heterogeneity in sulfide isotopic
composition at Middle Valley is larger than that
seen in other sediment-hosted massive sulfide
deposits. This may be due to a unique source
characteristic at Middle Valley but may also reflect
the relative youth of the deposit. As the hydro-
thermal system continues to wane, late-stage fluids
may remobilize or recrystallize the sulfides and
produce a final ore with a more homogenous Pb
isotopic composition.
6. Conclusions
[21] Fe-rich massive sulfides and gossans from the
Bent Hill deposit at Middle Valley exhibit a range
of Pb isotope ratios indicative of Pb derivation
from basaltic and sedimentary sources, to a max-
imum of 20% from the sedimentary source. Both
primary high-temperature, pyrrhotite-dominated,
and overprinting lower-temperature, pyrite-domi-
nated sulfides include a significant sedimentary
Pb component, indicating that hydrothermal fluids
have interacted with sediments in Middle Valley
throughout the thermal history of the Bent Hill
deposit. Pb isotopic ratios do not correlate with
base metal ratios, such as Cu/Zn, probably due to
the similarity in Cu/Zn in both basaltic and sedi-
mentary components.
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[22] Basaltic lavas and sills beneath the AAV
include secondary sulfides, dominantly pyrite,
deposited during hydrothermal alteration. Like the
massive sulfides, Pb in the secondary sulfides is a
mixture of basalt- and sediment-derived Pb, but the
sedimentary component is highly heterogeneous
compared to analyzed surface sediments. Stacey-
Kramers model ages range from zero age to Proter-
ozoic. Several BHMS massive sulfide samples,
which plot to the left of the best-fit mixing line
(Figure 2, ‘‘BF’’) through most sulfides from the
Bent Hill deposit, may also include a sedimentary
component with an old model age. Although the
source of sediments for Middle Valley is thought to
be largely from Mesozoic rocks from Vancouver
Island, the Pb isotope data imply that sediment
derived from weathering of older rocks (Protero-
zoic, Archean?) is being accumulated within the
rift. We suggest that the nonradiogenic Pb charac-
terizing this ancient component is derived from
detrital pyrite in the sediments that is mobilized
by hydrothermal fluids, then deposited in altered
sills or less commonly advected into the overlying
massive sulfide deposit. The secondary sulfides
appear to have high Cu/Zn, and appear to reflect
the relative ease with which Cu is removed from
basalt and sediment by hydrothermal fluids.
[23] The range of Pb isotope ratios in Middle
Valley sulfides is greater than in either modern or
ancient Besshi-type ore deposits. This may reflect
an anomalously heterogeneous sediment pile at
Middle Valley or late-stage Pb isotope homogeni-
zation of ancient deposits.
Acknowledgments
[24] Many thanks to Wayne Goodfellow, Rob Zierenberg, and
Debra Stakes for comments on earlier versions of the manu-
script and to two anonymous Geochemistry, Geophysics, Geo-
systems reviewers for their constructive reviews. Analytical
costs were defrayed by a research grant from the Geological
Survey of Canada. B.L.C. was supported by a NSERC Post-
doctoral Fellowship.
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