lead isotope systematics of sulfide minerals in the middle valley

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

2 of 16

GeochemistryGeophysicsGeosystems G3G3

COUSENS ET AL.: LEAD ISOTOPE SYSTEMATICS 10.1029/2001GC000257

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.

3 of 16


COUSENS ET AL.: LEAD ISOTOPE SYSTEMATICS 10.1029/2001GC000257COUSENS ET AL.: LEAD ISOTOPE SYSTEMATICS 10.1029/2001GC000257

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

4 of 16



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.

5 of 16



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).

6 of 16



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).

7 of 16



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.

8 of 16



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].

9 of 16



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].

10 of 16



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].

11 of 16



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].

12 of 16



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.

13 of 16



[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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lead isotope systematics of sulfide minerals in the middle valley

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