Lessons from land to sea
A good amount of our exploration of the seafloor and understanding of its hydrothermal
processes and products stems from our deep knowledge, largely gained from industry,
of these same processes that formed ores mined on land. The geological record
is replete with massive accumulations of base and precious metal sulfides in
submarine volcanic and volcano-sedimentary terrains, so-called volcanogenic
massive sulfide (VMS) deposits (or volcanogenic hydrothermal massive sulfide,
or VHMS deposits as the Australians prefer to call them). Typically, these terrains
are found in extensional basins above subduction zones, behind or within island
arcs where volcanoes produce lavas and other products ranging in composition
from basalt to rhyolite.
Some of the ore deposits are huge. Windy Craggy in northwestern British Columbia
contains about 300 million metric tons of ore-grade sulfides, and Kidd Creek
in northeastern Ontario contains about 170 million metric tons. The Iberian
Pyrite Belt of southern Spain and Portugal contains 85 known VMS deposits totaling
1,765 million metric tons.
These numbers
are big, and studies of these deposits and others like them have provided extensive
knowledge on how and where VMS deposits form although it took some time
for some of this research to be accepted. In 1847, the French scientist Elie
de Beaumont surmised that what we now call VMS deposits were formed when minerals
precipitated from volcanic and metalliferous emanations; but his
work went unrecognized. In the early 1900s, Japanese scientists understood correctly
that their Miocene-age Kuroko VMS deposits were formed by hot springs on the
seafloor; but nobody in the West read their papers written in Japanese.
This image of a black
smoker on the East Pacific Rise first appeared in the November 1979 issue of
National Geographic magazine and introduced many people to black smokers.
Photo courtesy of Dudley B. Foster, Woods Hole Oceanographic Institution.
Finally, in the West in the late 1950s, a young Dick Stanton declared that massive
sulfide ores formed in island arcs. Those of Bathurst, New Brunswick, he said,
were simply sulphide metamorphic rocks formed by springs and
fumaroles in the seaboard areas of the adjacent volcanic terrain. Some
vilified Stanton for this, at the time, heretical opinion, which countered the
prevailing view that such deposits were formed by massive replacement of their
volcanic host rocks. The discovery of black smokers forming massive sulfides
on the modern seafloor would seem to still such opposition for all time, but
what goes around comes around. Modern research shows that replacement is in
fact an important process.
From the geological record, we know that large VMS deposits have several features
in common. For many, it is clear that they form in an extensional tectonic setting,
although compression commonly follows. Thick continental crust typically underpins
the volcanic rocks adjacent to the magma that are the immediate host rocks of
the deposit; for example, 30 to 40 kilometers of continental crust sit beneath
a veneer of arc volcanics that contain the famous and extensively studied Kuroko
deposits.
A heat engine of either magma or very hot rock is required to generate
the hydrothermal fluid. Metals and other elements are leached from rocks in
a high-temperature reaction zone by heated seawater and the much altered, buoyant
fluid rises rapidly. A metal-rich fluid that is generated by reaction with basal
passes through and further reacts with sediments, as it does at Guaymas Basin,
before discharging onto the seafloor.
On land, the heat engine, in the form of sill-like bodies, can be identified
in some ancient VMS terrains such as Matagami (Bell River Complex) and Noranda
(Flavrian Intrusion), both of Archean age in Quebec. Careful studies of ancient
terrains also reveal local structures at the site of deposition that serve to
focus the fluid flow onto or just beneath the seafloor. We also know something
of the nature of the ore-forming fluid as it is preserved in fluid inclusions:
it is usually acid, moderately saline and generally less than 350 degrees Celsius.
And we know a lot about the alteration of the rocks beneath the ore deposits.
The optimum conditions for the formation of VMS deposits, as ascertained from
land studies, are best met in an island-arc or back-arc setting. This fact is
one reason why a significant amount of research focuses on exploring the seabed
of modern arcs for hydrothermal activity. What other tectonic terrains should
we explore?
We have good examples on the modern seafloor of the Kuroko- and Cyprus-type
VMS deposits in volcanic rocks; but not for the Besshi- and Sedex-types, in
which sediments, rather than volcanics, play a predominant role. Perhaps this
lack of evidence is not surprising, especially for the Sedex-type. These deposits
probably formed from relatively low-temperature, highly saline and therefore
dense fluids that ponded in depressions. Such fluids would not generate a plume
in the seawater column, a signal relatively easy to detect for deposits forming
on sediment-starved mid-ocean and back-arc ridges. Another reason for their
apparent absence is that we simply havent looked for them, at least not
hard enough.
Seemingly, everywhere that marine scientists look for hydrothermal activity
on mid-ocean ridges and back-arcs even in some unexpected places
they find it. Hydrothermal sites are now known in all the worlds oceans
except where no one has explored, such as the High Arctic. Some mid-ocean sites
that are heavily sedimented such as Middle Valley on the Juan de Fuca
Ridge, Escanaba Trough on the Gorda Ridge, Guaymas Basin, and various deeps
of the Red Sea, such as the Atlantis II have some similarity to Besshi
and Sedex settings but lack at least one key feature. Perhaps the time is ripe
to extend our seafloor investigations to those settings that more closely mimic
these missing ore types.
Lessons from sea to land
Marine studies of the past two decades have confirmed much of what had been
hypothesized about VMS ores. They have also contributed to our fund of knowledge
about such deposits and may even provide some new guides for their exploration
on land.
Bob Ballard, now head of the Institute for Exploration in Mystic, Conn., and
Jean Francheteau, now at the University of Western Brittany in Brest, France,
pointed out a long time ago before the complexities of seafloor spreading
ridges were properly known that hydrothermalism on mid-ocean ridges was
more prevalent in the ridge axis shallowest region, the so-called bathymetric
minimum. This trend is understandable, given our current knowledge of
axial magmatism, in which seismic studies show that magma (the heat engine)
is closer to the seafloor and more prevalent along a ridge axis. The trend is
also consistent with the changing compositions of mid-ocean ridge basalts approaching
the bathymetric minimum and the main hydrothermal center. This change is recorded,
for example, in the basalts large ion lithophile elements, potassium,
barium and strontium. The bathymetric minimum is obvious on the modern seafloor,
but how might it be detected in ancient terrains on land? The progressive change
in basalt composition provides a clue that needs to be tested.
Without a doubt, the most important information to come from seafloor studies
lies in vent fluids. Hydrothermal geochemists such as the late John Edmond of
the Massachusetts Institute of Technology, his former student Karynne von Damm,
who is now at the University of New Hampshire, Jean-Luc Charlou of IFREMER (the
French Institute of Research and Exploitation of the Sea), Toshitaki Gamo of
Tokyo University and David Butterfield of NOAA and the University of Washington
perfected the technique of sampling vent fluids from submersibles. As a result,
we now have a huge collection of such data from many different sites in different
geological settings.
Armed with these data, we can much better understand the nature of fluid-rock
interactions and the causes of variability in the compositions of sulfide ores.
An earlier supposition was that, because hydrothermal solutions were produced
at mid-ocean ridges when heated seawater reacted with basalt of more or less
the same composition everywhere, all vented end-member fluids (i.e., uncontaminated
by entrained fresh seawater) should have the same composition. We now know otherwise:
end-member fluids have a wide range of compositions. Much of this variability
arises because initial heating of a hydrothermal system brings unique physical
and chemical conditions, because high-density and low-density saline phases
separate and each carry different dissolved constituents; and because a hydrothermal
system stabilizes as it matures.
A further complexity is that magmatic fluid rich in metals and volatiles might
also contribute to the hydrothermal circulation system, and thus to ore formation.
This metalliferous fluid was first identified and analyzed in a modern submarine
setting by Kaihui Yang and the author in a paper published in the Oct. 3, 1996,
Nature, although we were not able to demonstrate that this magmatic fluid actually
made it into the hydrothermal system. We described precipitates of ore metals
within the magmatic vapor bubbles of melt inclusions contained within phenocrysts
and in small vesicles of an andesite from the Manus back-arc basin. In the August
2002 Economic Geology, we showed that the metals in these precipitates change
progressively from more copper-rich to more zinc-rich with fractionation of
the lavas from basalt to rhyolite. Interestingly, ancient ores in basalts are
rich in copper and those in rhyolite are rich in zinc.
Other researchers, such as Yves Fouquet of IFREMER, as well as Cornel de Ronde
of the Institute of Geological and Nuclear Sciences in New Zealand, Peter Herzig
of Freiberg University in Germany and their colleagues, have provided isotopic
and chemical evidence from gases for the presence of magmatic fluids in marine
hydrothermal systems.
Giant ore deposits of more than 100 million metric tons require a superabundant
supply of metals. Simple leaching of volcanic rocks or sediments by heated seawater
cannot easily provide this supply within reasonable times. And observed volumes
of leached rock beneath ancient VMS deposits would not yield enough metals.
Perhaps a clue to the origin of giant VMS deposits and evidence for their presence
in a particular locality lies in the unequivocal but elusive evidence for the
release of magmatic fluid from shallow intrusions.
Many studies of hydrothermal plumes show that free venting of hydrothermal fluid
into seawater results in the loss of as much as 90 percent of the metals in
the form of particulates that are widely dispersed. Tim McConachy of the Commonwealth
Scientific and Industrial Research Organisation in Australia points out that
a typical plume from a black smoker discharging at 2 meters per second can produce
1 million metric tons of particulates in 10,000 years, while a plume discharging
at 200 meters per second can produce the same amount in 100 years. Also, we
know from isotopic dating by Claude Lalou of CNRS in Gif-sur-Yvette, France,
that seafloor hydrothermal systems can last for at least 100,000 years. However,
according to the model by Richard Feely of NOAAs Pacific Marine Environmental
Laboratory in Seattle, 75 percent of the particulates representing the coarsest
fraction that are most prone to settle near their source are too dispersed over
several square kilometers of the seafloor to form a sulfide ore. Fallout of
these particulates contaminates seafloor sediments, although the amount of metalliferous
sediment actually observed in volcanic terrains of the modern seafloor is surprisingly
small.
Metalliferous sediments decimeters to meters thick are common in the vicinity
of ancient VMS deposits, which in some cases extend several kilometers along
the ore horizon good examples are the Japanese tetsusekiei (iron-quartz),
Noranda Contact Tuffs and Matagami Key Tuffite. The presence of such rocks is
a good exploration guide for nearby VMS deposits, but explorationists have tried
in vain to find geochemical clues in these rocks pointing to ore. Intuitively,
one would think that such vectors must exist and perhaps further research into
the modern seafloor metalliferous sediments will unlock this secret.
Much of the dissolved material may never reach the deposits chimney. Drilling
into sulfide mounds by the Ocean Drilling Program and observations of the interiors
of deposits incised by faults is revealing processes within the mound and beneath
the seafloor. The current seafloor work, because it catches this process in
action, is allowing us to quantify the process. A somewhat startling result
from these studies is that extensive replacement is occurring beneath the seafloor,
particularly replacement of sediments such as at Middle Valley; but also, according
to Fouquet, of ultramafics at the Rainbow site on the Mid-Atlantic Ridge. Mounds
grow by replacement beneath the seafloor and by internal inflation above the
seafloor. This process may be an answer to the enigma of how large VMS deposits
can form when free venting into the water column is known to be so very inefficient.
Land-based studies have implied that most VMS ores formed in depressions. But
the modern seafloor has shown otherwise, revealing that the ores form on high-standing
structures, such as ridges. Perhaps this difference is more apparent than real,
because deposits forming on mid-ocean ridges tend to be confined to the axial
graben, or depression, of what is otherwise a high-standing volcanic structure.
The same is true of deposits forming in the caldera of ridge-associated volcanoes
such as Axial Volcano at 46 degrees north on the East Pacific Rise, an area
that has been the focus of the NOAA-Canada NeMO Project. (NeMO, or New Millennium
Observatory, is a long-term observation of the biology, chemistry and geology
of an active volcanic portion of the mid-ocean ridge.) It is also true of the
large sulfide deposit forming in a graben on the flank of an off-axis volcano
to the east of the East Pacific Rise at 13 degrees north, described by Roger
Hekinian, formerly of IFREMER, and by Fouquet.
Can we mine ocean sulfides?
The largest of the known seafloor deposits could be an economic resource in
their own right. I have made this case in several articles over the past 15
years, most recently in the June 2001 issue of Geoscience Canada. Others who
have made similar cases are Ray Binns and David Decker of CSIRO Australia, in
a 1998 article in Scientific American Presents The Oceans, as well as Herzig
and Fouquet.
The seafloor hosts deposits of apparent size and grade that, if they were on
land, would definitely be targets for further evaluation. That they are under
several hundred to a few thousand meters of water is of no particular concern.
Deep marine technology is up to the challenge. The oil industry moved offshore
in the mid-20th century, despite huge resources remaining onshore and despite
the technology for marine recovery of oil not having been perfected. Today,
about one-third of the worlds oil production is from the offshore, and
that portion is growing. Oil exploration is extending beyond 3,000 meters water
depth, about the maximum depth at which potentially viable mineral deposits
have been found.
Will the mining industry attempt ocean recovery of polymetallic sulfides? The
industry was terribly disillusioned by the $650 million failure in the 1970s
and 1980s of manganese nodule mining (which interestingly is again in vogue
in Korea, Japan and China). Most companies are reluctant to try again.
Two small entrepreneurial companies, Nautilus Minerals Corp., based in Australia,
and Deep Sea Minerals, based in the United States, are betting that a market
will be out there for marine polymetallic sulfides. Nautilus has an exploration
license from the government of Papua New Guinea covering 2,458 square kilometers
of the Manus Basin and an application pending for an additional 580 square kilometers
further east around Lihir Island where Ladolam, one of the worlds richest
gold deposits, is being exploited. Deep Sea and their partner, Phelps Dodge,
have several applications pending.
More than 130 years ago, the character Captain Nemo in Jules Vernes Twenty
Thousand Leagues Under the Sea claimed, In the ocean depths, there exist
mines of zinc, iron, silver and gold which would be quite easy to exploit.
Perhaps, as with so many of his insights, Verne was right.
Chimneys
from an ancient ocean Read a related story in the July 2001 Geotimes, The
oldest ophiolite. |
Biology
and minerals Photo courtesy of Chris Kennedy, University of Toronto. The biomass of seafloor hydrothermal systems is immense.
In a 1998 paper in Advances in Marine Biology (vol. 34), Verena Tunnicliffe
of the University of Victoria identified 443 invertebrate species, most
of which were new to science. More have been discovered since. Biologists
will be occupied for a very long time to determine what role each creature
plays. Perhaps some, like Junipers worm, play an important role
in precipitating metal sulfides. |
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