in the Pilbara region of Western Australia [Barley, 1992] and in the Barberton greenstone belt of South Africa [de Ronde et al., 1994], and many other examples ...
Physical and Chemical Processes of Seafloor Mineralization at Mid-Ocean Ridges Mark D. Hannington and Ian R. Jonasson Geological Survey of Canada, Ottawa, Canada
Peter M. Herzig and Sven Petersen Technical University Bergakadernie Freiberg, Germany
1. INTRODUCTION Seafioor hydrothermal activity at mid-ocean ridges has a major impact on the chemistry of the oceans [Edmond et al., 1979] and has been responsible for extensive alteration of modern oceanic crust [Alt, this volume]. Massive sulfide deposits, together with metalliferous sediments and hydrothermally altered rocks, are an important record of this process. A fossil record of seafloor hydrothermal activity is also contained in much older deposits now preserved on land. Ancient massive sulfides as old as 3.5 billion years are known in the Pilbara region of Western Australia [Barley, 1992] and in the Barberton greenstone belt of South Africa [de Ronde et al., 1994], and many other examples in Archean and younger rocks can be cited [Franklin et al., 1981]. Detailed studies of the mineralogy and chemistry of these ancient deposits indicate that the physical and chemical processes of seafloor mineralization in the early oceans were fundamentally the same as those observed on the modern mid-ocean ridges, and direct comparisons can be made between the formation of modern and ancient sulfide deposits. In addition, because seafloor hydrothermal systems have been active since very early in the history of the Earth, the study of fossil sulfide deposits may be
Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions Geophysical Monograph 91 Copyright 1995 by the American Geophysical Union
particularly relevant to arguments concerning the origins of 1992]. life [Corliss et al., 1981; Corliss, 1990; The following is a review of the different styles of mineralization found at seafloor hydrothermal vents and an introduction to some of the physical and chemical processes which control sulfide deposition. Several descriptive models are presented which outline the development of vent complexes from the earliest stages of hydrothermal activity following new eruptions to the growth of large sulfide deposits and their eventual destruction by seafloor weathering. Many of these processes are described in greater detail in other papers in the volume, and we have attempted to introduce the main concepts to make these articles more accessible to the reader outside the field of study. In an effort to generalize observations about the many styles of hydrothermal venting, we have drawn on examples from a wide range of vent sites in different volcanic and tectonic settings and at different stages of development (Figure 1). Although we depict the evolution of a vent field and the growth of large deposits as a thermally intensifying process, these examples encompass many deposits with considerably more complex venting histories. Whereas lowtemperature diffuse venting and white smoker chimneys may be the first to appear in some vent fields [e.g., Axial Seamount, Juan de Fuca Ridge: Hannington and Scott, 1988], high-temperature black smokers may form early in the venting history at other sites [e.g., 9°50 N, East Pacific Rise: Haymon eta!., 19931. The variable size, shape, and bulk composition of
sulfide deposits that have grown at these sites and some of the
factors which contribute to these differences are discussed.
115
116 SEAFLOOR MINERALIZATION AT MID-OCEAN RIDGES
(Z)
NORTH
Explorer Middle Valley›; Endeavour Axial Searnount' Cleft Segment/ • S. Juan de Fuca Escanaba Trough
Okinawa Trough
AMERICA%°
"I/Broken Spur TAG* • SnakePit
Guaymas -21°N —1
Mariana Trough
Lucky Strike
13°N 11N2,. 9°N /
Manus Basin N. Fiji Basin •
17°S
iaos
Lau Basin
20°S 21°S
//Galapagos
$
II
%I
,t
a
Fig. 1. Locations of modern seafloor hydrothermal vents and mineral occurrences discussed in the text [after
Hannington et al., 1994a].
While the focus is on deposits at volcanic-dominated midocean ridges, a number of references are made to sedimenthosted deposits and the impact of their host rocks on the formation of massive sulfides at the seafloor. As well, the similarities between modern and ancient deposits allow many direct comparisons, particularly with Cyprus-type massive sulfides which are considered to have formed in similar midocean ridge settings [Koski, 1987]. Most ridge-crest hydrothermal systems have in common a series of high-temperature chemical reactions between heated seawater and oceanic crust that contribute to the formation of an evolved, end-member hydrothermal fluid of uniform bulk composition (i.e., acidic, reducing, and sulfur- and metal-rich). However, a number of processes such as phase separation are responsible for large differences in the chemistry of hydrothermal fluids venting at the seafloor [Von Damm, 1990; Von Damm, this volume], and these phenomenon may have a major impact on the mineralogy and chemistry of sulfide deposits at different sites. The major controls on sulfide deposition (mixing, cooling, pH changes, and oxidation) are substantially the same everywhere, and the chemical parameters which influence the stabilities of different minerals at seafloor hydrothermal vents are outlined below.
2. STYLES OF VENTING AND THE GROWTH OF SULFIDE DEPOSITS 2.1. The Onset of Hydrothermal Activity and Focusing of Hydrothermal Upflow Among the least understood processes of seafloor mineralization is the manner in which hydrothermal vents are first established on the seafloor. Regardless of how a hydrothermal system evolves, it must start out by displacing a large volume of cold seawater that occupies void spaces within the crust (Figure 2). The initial flushing of the system may be very rapid, especially on medium- and fast-spreading ridges that are undergoing extension at up to tens of centimeters per year. Where there are frequent intrusions of magma close to the seafloor and fissure-fed eruptions along the axial rift, hydrothermal activity may begin with the rapid release of a large volume of hydrothermal fluid or "megaplume". Such events have been attributed to fracturing and the release of hot fluids stored in a hydrothermal reservoir at depth [Baker et al., 1987, 19891, the gradual build up of temperature and buoyancy forces beneath a mineralized cap [Cann and Strens, 19891, and more recently to the injection of dikes along the rift [Baker,
HANNINGTON ET AL. 117
100 M
0.
Acf DEBRIS APRON & METALUFEROUS SEDIMENT, °es,
GRADATIONAL CONTACT SILICIFIED STOCKWORK FE-RICH CHLORITE (MG-RICH AT MARGIN)
LIMIT OF INTENSE HYDROTHERMAL ALTERATION
ILLITE-SMECTITE OUTER ZONE
S°4 —. H2S
HIGH PERMEABILITY
1 KM
LOW PERMEABILITY 350-400°C • •• • ••••••• .•••••••••••• • •••••••••••••
METAL LEACHING S ROOK H 2 S
HIGH-TEMPERATURE END-MEMBER
OOOOO • • • • • • • • • • OOOOO OOOOOO • • • • • • • • • • • • • • • OOOOOOO • • • • • • • • • • • • • • • • • • • • • • • • • • • • OOOOOO
' • " " OOOO OOOOO • •"• ••• •• •"• •• •• ••• •• EPIDOTE-QUARTZ •••••••• • • •;.•.. SEMI-CONFORMABLE ALTERATION • • • • • • 4. ••• • 0. 00 •••••• • ....... . ........ .
.
• • 4 • • • • • • • • • • • • • • • • •••• • •••••••••••••••••••••
•••••••••••••••••••• •••••••
••••"•••••••••
400°C HIGH-TEMPERATURE REACTION ZONE
Fig. 2. Schematic diagram showing the principal components of a seafloor sulfide deposit and associated hydrothermal system [sketch of TAG Mound modified from Rona, 1992]. Arrows indicate fluid flow paths for seawater (open) and hydrothermal fluids (shaded). During hydrothermal circulation of seawater, SO4 is precipitated as anhydrite or reduced to H2S. Reduced sulfur in the hydrothermal fluids is derived mainly from leaching of the rocks. (SP = sphalerite, PY = pyrite, CP = chalcopyrite).
1994; Embley and Chadwick, 1994]. Cathles [1993] has argued that catastrophic displacements of super-heated seawater and hydrothermal fluid may result simply from the migration of a cracking front into a cooling subvolcanic intrusion. Recent observations at new hydrothermal fields on the Juan de Fuca Ridge [CoAxial and Cleft Segments: Embley et al., 1993; Baker, 1994] and the East Pacific Rise [9°50' N EPR:
Hayman et al., 1993] suggest that hydrothermal activity is directly linked to discrete volcanic eruptions. The megaplumes on the CoAxial and Cleft Segments are now understood to have been triggered by dike emplacement [Baker, 1994; Embley and Chadwick, 1994]. At CoAxial, these events were monitored acoustically, and the seismic signals were consistent with multiple lateral injections of magma along a 60-km segment of the ridge axis [Embley at al., 1993, 19941.
118 SEAFLOOR MINERALIZATION AT MID-OCEAN RIDGES
The intrusion of the dikes close to the seafloor and the major activity therefore may accompany each major eruption, beeruption of lava were coincident with the displacement of a ginning with a short-lived episode of diffuse flow that within 3 large volume of hydrothermal fluid. Shortly after the eruption, to 5 years may lead to more focused venting and the formation widespread diffuse flow of low-temperature (60°C) fluids of associated sulfide deposits, or else decline completely emanated from fractures in the fresh lavas and between new leaving only ferruginous sediments. pillows [Butterfield and Massoth, 1994]. Similar venting was On fast-spreading ridges, however, the vent field on top of observed from new fissures cutting older lavas on the flanks of the most recent lava flow may be quite small and ephemeral in the new flows in areas where the newly intruded dike did not comparison to other large, mature systems (e.g., 21°N and 11°N EPR versus the TAG Hydrothermal Field). This situation erupt. Prior to this event, the area was devoid of vent fauna, but large "plumes" of bacteria were found in and around the most likely reflects the frequent disruption of the hydrothermal new vents. These hyperthermophillic bacteria may have been fields by volcanic and tectonic events along the eruptive living within the basaltic crust before the eruptions [Deming fissures [Auzende et al., 1994a, b]. Because the upflow zone is and Baross, 1993] and appear to have flourished during the relatively narrow, it is subject to perturbation by tectonic hydrothermal pulse. Tube worm colonization has also now activity which affects permeability within the neovolcanic been established at CoAxial (V. Tunnicliffe, personal zone. The bulk porosity and permeability in the upper 100-200 communication 1994). A similar diffuse vent field which m of crust on medium- to fast-spreading ridges is considerable formed after the 1986-1987 megaplume events on the Cleft and is further enhanced by the intense fracturing, faulting, and Segment persisted at least until 1991, and a series of high- lava drain-back in the neovolcanic zone [McClain et al., 1993; temperature (260-320°C) black smokers remain active at the Rohr, 1994]. Off-axis and on slower-spreading ridges, the north Cleft site [Koski et al., 1994]. Detailed mapping of this collapse and infilling of voids, and the presence of more deeply penetrating faults, as indicated by larger seismic site between 1988 and 1991 suggests that about 5 years elapsed before the low temperature vent field was sealed by events, may contribute to more focused hydrothermal dishydrothermal precipitates, allowing sub-seafloor temperatures charge. On slow-spreading ridges, seafloor spreading is to rise and fluid discharge to become focused into deeper brought about by a more gradual build up of melt, deeper fracturing and intrusion of magma, and more localized erupfractures present along the axial rift. At 9°50' N, a pre-existing high-temperature vent field was tive centers [Eberhart et al., 1988; Gente et al., 1991; Kong et buried by new lavas in 1991, destroying a community of al., 1992; Fouquet et al., 1993a]. For example, volcanism on Vestimentiferan tube worms and other fauna [Haymon et al., the Mid-Atlantic Ridge is dominated by broad volcanic ridges 1992, 19931. Temperatures close to 400°C were measured at a and punctuated by hundreds of discrete axis and off-axis number of vents within days of the eruption. A year later, pillow volcanoes [Smith and Cann, 1990, 1992]. This structure diffuse venting in the area was much reduced, and black contrasts sharply with the classic fissure-fed rift zones of the smoker discharge was occurring at more focused high- East Pacific Rise and the relatively shallow axial magma associated with fast-spreading ridges [McClain et al., 1985; temperature vents. The abundant bacterial floc observed in the water column at the time of the eruption was largely gone the Detrick, 1987]. As a result, the development of a sustained following year, but new megafauna had colonized the site hydrothermal cell is more likely to be a slow, thermally intenrapidly. Tube worms occupying the vents in 1992 had grown sifying process brought about by the progressive heating of to 30 cm in length, and giant tube worms, absent in 1992, were seawater in proximity to a subvolcanic intrusion. Although there appears to be little or no evidence for a large magma discovered at the site in 1993 and had grown to more than 1.5 chamber at depth beneath the Mid-Atlantic Ridge [Detrick et 1994]. [Lutz et al., m in length The main difference between the events at 9°50' N and the al., 1990], a possible heat source is suggested by a mid-crustal, megaplumes on the Juan de Fuca was that a focused upflow seismic low-velocity zone consistent with a largely solid, but zone had already been established at 9°50' N during earlier still hot intrusion beneath the median valley [Kong et al., volcanic-hydrothermal cycles. On a fast-spreading ridge such 1992]. Larger but less frequent tectonic events than on fasteras the EPR, the cycle of dike injection, eruption, and spreading ridges may promote longer-lived and more stable hydrothermal discharge may repeat itself with each new structures for hydrothermal upflow, and more deeply eruption, perhaps as often as every 3-5 years [Haymon et al., penetrating upflow zones may be accommodated by a thicker, 1993]. On the CoAxial segment, detailed mapping of lava brittle zone and larger normal faults in the median valley [Toomey et al., 1988] flows suggests that the incremental process of ridge accretion Focused fluid discharge is enhanced at deeply fissured sites (i.e., dike injection) occurs at 10-year intervals [Chadwick et if the volcanic rocks become sealed by mineral precipitation in press], and each increment of spreading may have been al., punctuated by a megaplume. A new phase of hydrothermal [Lowell et al., 1993]. This impermeable barrier or "cap"
HANNINGTON ET AL.
Fig. 3. Diffuse
Bottom photographs of diffuse hydrothermal vents. The field of
venting
through fr actures in basalt
in
view is
about 2-3
m
119
in each case. (a)
the caldera of Axial Seamount [ from Hannington and Scott, 1988].
(b) at diffuse vents in fractured sheet flows in the caldera of Axial Seamount [from Embley et al., 1 99 0]. (c) Field of vestimentiferan tube worms growing in area of diffuse flow at the CASM site, Axial Seamount [from CASM II, 1985]. (d) Diffuse venting from a large, barite-rich chi m ney covered by worm tubes and associated vent fauna at Explorer Ridge [photograph from V. Tunnicliffe]. Amorphous silica and Fe-oxide chimneys growing
insulates
high-temperature
upflow
from
the
surrounding
et ai., 1990; Schultz et al., 1992). These shrt-term events may
seawater and can be ruptured along discrete faults or fissures
serve to create new open fissures for hydrothermal discharge
to allow for the focused release of hydrothermal fluid. In order
and at the same time promote mineral precipitation within the
to sustain focused discharge, mineral precipitation and sealing
subseafloor.
of fractures beneath the seafloor on fast-spreading ridges must keep pace with the intense fracturing associated with rapid
2.2. Diffuse Venting and White Smokers
spreading rates. Sealing of the volcanic pile may be facilitated by the many successive hydrothermal pulses that accompany
Diffuse venting typically occurs throughout the life of a
each new eruption. Although megaplumes appear to be an
hydrothermal system. It may be the earliest form of discharge
important part of this process on medium- and fast-spreading
in a new hydrothermal field but commonly occurs at the mar
ridges,
smaller-scale hydrothermal pulses related to local
ground movements (i.e., earthquake tremors)
and lasting
gins
of
existing
high-temperature
upflow
where
rising
hydrothermal fluids mix with cold seawater. Diffuse venting
several days also appear to be common [Delaney et ai., 1990;
also typically dominates the last stages of activity in a waning
1990a). These are partly reflected in the short -term
hydrothermal system as high-temperature upflow collapses
variability of temperature and discharge rates that have been
around a cooling subvolcanic intrusion. The heat and mass
noted in a number of vent fields [Johnson et af., 1988; McDuff
flux due to diffuse flow in some vent fields is substantial and
Fox,
SEAFLOOR MINERALIZATION AT MID-OCEAN RIDGES
120
A
•
B silica-sulfate walls
D lo�ool 000
porous, massive sulfides
I c� I
mineraHined cavIties
o
massive barite and 1b.'T�". fossil worm tubes
o
10
� em
Fig. 4. Cross-section of a white smoker chimney or spire from the CASM field of Axial Seamount. (a) The spire contains multiple fluid channelways (numbers, arrows) and is encrusted by fossilized worm tubes in the outer barite rich zone.
(b)
Photograph of the interior of the spire showing porous network of dendritic sphalerite [from
Hannington and Scott, 1988].
may be an order of magnitude greater than that of focused
high temperature flow [Schultz et aI., 1992; Rona and Trivett, -
1992; Ginster et aI., 1994].
consisting of Fe-oxides, silica, barite, or marcasite [Tunnicliffe and Fontaine, 1987; Arquit, 1990; Milligan and Tunnicliffe,
1994]. The presence of such organisms can influence mineral
Periods of diffuse flow can sustain a vibrant biological
precipitation in the early stages of venting by reducing fluid
community (e.g., Figure 3a), but are not generally associated
flow velocities and providing insulation for new vents. Tube
with extensive mineralization because the low temperatures of
worms lining cracks in the basalt also provide an important
the fluids «10°C to 50°C) do not allow for the transport of
framework for mineral precipitation and, as a result, many
significant concentrations of dissolved metals and s ulfur. The
low-temperature chimneys
mineral precipitates associated with diffuse venting typically
fossilized remains of worm tubes
consist
of
amorphous
authigenic clays
and
Fe-oxyhydroxides,
silica
(Figure
3b).
Mn-oxides,
Small,
unstable
chimneys may form from the Fe-oxide and silica precipitates,
1988J. Sedentary chimneys precipitates
may
animals
be
(Figure
are
partly
living
at
progressively
4),
and
constructed
on
the
[Hannington and Scott,
the
the
surfaces
overgrown
by
preservation
of
the
mineral of
fossil
but these are generally so low in temperature «500C) as to be · . non-smokers. Large areas of the seafloor experiencing diffuse
organisms in this manner is an important record of the nature
flow may be covered by luxuriant fields of tube worms (Figure
massive sulfides [Haymon et aI., 1984; Oudin et aI., 1985].
of former hydrothermal activity in both modern and ancient
3c) up to hundreds of square meters in extent [Milligan and
For example, petrographic evidence exists for the preservation
Tunnicliffe, 1994]. Clusters of tube worms in the diffuse flow
of fossil polychaete tubes in sulfides from the TAG Hydrother
are typically anchored to fresh basalt or a mineral substrate.
mal Field, even though these organisms do not presently
.
HANNINGTON ET AL. 121
ANHYDRITE BARITE AMORPHOUS SILICA DENO RITIC SPHALERITE COLLOFORM SPHALERITE MARCASITE PYRITE WURTZITE CHALCOPYRITE PYRRHOTITE ISOCUBANITE
, .
Ei
Co c.;
rn
••■•••
9, 1,- --, -- 'cl
ve;
cS
CD CD CD CD CD C) VVVVVV
CD
c) un Co C,) r-
('4 CC'
CS, 6 NO
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CR CR 0-; CA un rn op CD
cS
t„
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CV
ci 47i CC C' on
vD
•••
cs n
c)
on
■
01 0? cs 6
Cs 'C' N
°°.
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en
6
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C).
CD
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r-
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un
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et
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et un
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un
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HANN1NGTON ET AL. 143
TABLE 2. Base and precious metals in modem seafloor hydrothermal systems Depth Dimension of Largest Deposit Cu Zn Pb Ag Au (N) (approximate) (m) (wt.%) (Ppm)
Refs.
Mid-Ocean Ridges
Southern Explorer Ridge Endeavour Ridge Axial Seamount Southern Juan de Fuca North Gorda Ridge 21°N, East Pacific Rise 14°N Seamount 13°N Seamount 13°N, East Pacific Rise 11°N, East Pacific Rise 18°S to 26°S Galapagos Rift TAG Mound, MAR Snalcepit, MAR
1800 2100 1500 2200 2700 2600 2500 2500 2600 2600 2600-2900 2700 3600 3400
250 m x 200 m up to 200 m small vent fields small vent fields small vent fields small vent fields small fields 800 iii x 200 m small vent fields small vent fields small vent fields 100mx 100m 250 m x 200 m up to 100 m
3.6 3.0 0.8 1.4
7.8 1.9 6.8 4.1 6.2 2.0
6.1 0.1 132 1.0 (66) 4.3
