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Minerals on Land, Minerals in the Sea
Steven Scott


Sidebars:
Chimneys from an ancient ocean

Biology and minerals


Like many people, I was introduced to black smokers and their chimneys by photographs published in the November 1979 issue of National Geographic magazine. These photos revealed a black smoker at 21 degrees north latitude on the East Pacific Rise. Three years later, I saw one live. Onboard the submersible Alvin, I was 2,000 meters deep at the Guaymas Basin in the Gulf of California.

I have spent my career as an ore deposits geologist studying hydrothermal systems on land — systems in which high-temperature, aqueous fluids deposited ores and minerals on ancient seafloors. I specialize in studying base and precious metal sulfide deposits in volcanic terrains. Seeing a live black smoker (in reality, it was gray), I was in awe of witnessing the process of hot water spewing metals out onto the seafloor in the form of grotesquely shaped chimneys and dense clouds of particulates in the water column. My Guaymas experience gave me a true concept of the scale of hydrothermal events, which can potentially form ores, and the realization that we can study an important ore-forming system in progress. In most geological studies, the geologist is like a medical pathologist, trying to figure out from faint clues how something had lived and how it died, often long after the event. Being able to watch modern seafloor hydrothermalism has empowered the geologist/pathologist to dissect a living system.

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 haven’t 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 world’s 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 NOAA’s 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 deposit’s 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 world’s 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 world’s 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 Verne’s 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.


Scott is the Norman B. Keevil Professor of Ore Genesis, Director of the Scotiabank Marine Geology Research Laboratory and Chair of the Department of Geology, all at the University of Toronto. He also holds a visiting professorship at the University of Western Brittany’s “European University Marine Institute” in Brest, France. E-mail him at scottsd@geology.utoronto.ca.

Chimneys from an ancient ocean

Earthquakes, underwater landslides and volcanic eruptions can at anytime wipe out an entire community of creatures living in a hydrothermal vent region. The same processes can crumble the drip-castle formations of the sulfide smoking chimneys, which many of these animals call home. Vent fauna have clearly evolved to accommodate such a hazard-filled and sunless lifestyle, but documenting this evolution is difficult. Most of the answers regarding life on the seafloor more than 250 million years ago have subducted along with the oceanic crust beneath the continents.

But occasionally, part of the oceanic crust has escaped such a fate. During the subduction process, oceanic crust can scrape along the continental edges, or in zones of convergence can be thrust to the surface. Remnants of the ancient hydrothermal processes are seen in examples of volcanogenic massive sulfide deposits and banded iron formations around the world. Still, the geological processes involved in saving swaths of oceanic crust on land also deform the material.

This long, vertical black smoker chimney was found in the Wutai Mountains, North China Craton. It is part of a 2.5-billion-year old black smoker chimney-hydrothermal seafloor vent system, discovered in the upper portions of a fragment of Archean oceanic crust. The oceanic crust is now part of the Central Orogenic belt. Photo courtesy of Jianghai Li.

Most of the oldest examples of massive sulfides are between 2.75 and 2.6 billion years old. “These Late Archean deposits occur in major greenstone belts, such as those of the Archean Superior Province of Canada,” says Mark Hannington of the Geological Survey of Canada in Ottawa. “Similar deposits undoubtedly formed before that time, but their host rocks are not as well preserved in the geologic record, so we know of only a few deposits older than 3 billion years. These include some small deposits in the Pilbara granite-greenstone terrane of Western Australia, which have been dated at 3.2 to 3.4 billion years.”

Now, researchers working in China have discovered a truly unusual occurrence, that of one narrow, straight and tall chimney, and about six mound structures around the central chimney, still standing after 2.5 billion years.

Like houses left untouched after a tornado has leveled everything around them, the chimney and its structures remain intact and are relics of the past environment that may hold clues about their historical inhabitants. Tim Kusky of Saint Louis University in Missouri, working with Jianghai Li of Peking University in Beijing, came across the chimneys in August while exploring China’s Wutai Mountains. The chimney sits about 500 kilometers south of and in the same orogenic belt as the Dongwanzi ophiolite, the squeezed remnants of an oceanic crust formation that got caught between parts of the North China craton as it crashed together (Geotimes, July 2001).

Kusky and Li came across the chimney while looking for podiform chromites, which are pieces of the asthenosphere, the once partially molten region of the mantle that lies beneath the lithosphere. “These types of rocks are only known to have formed at oceanic spreading centers,” Kusky says. “In the Zunhua and Dongwanzi ophiolite areas, we noticed that these podiform chromites were also associated with fragments of oceanic crust, banded iron formation, chert and massive sulfide — black smoker types of deposits. Further south in Wutai Mountain we found podiform chromites and a basaltic unit with very well preserved massive sulfides, including a preserved black smoker chimney at the top of the basalt section. The sulfide minerals are concentrated around faults, which acted as conduits for the sulfide-rich fluids to migrate along.”

Having an Archean black smoker chimney and ophiolites from 2.5 billion years ago in China is exciting for Kusky, who hopes to help fill the gaps in the story of life along hydrothermal vents. “Around this time, life changed from primitive prokaryotes to eukaryotes, with a cell nucleus,” he says. Perhaps hidden in the chimneys of the past are links to explain the hydrothermal community of the present.

But even more tantalizing is the idea that the development of heat-loving life around hydrothermal vents somehow helped in the evolution of photosynthetic organisms. Around 2.5 billion years ago, Earth’s surface began a dramatic shift from reducing environments to highly oxidizing conditions. “This may be when photosynthesis, the metabolic strategy common today, developed in sulfur bacteria,” Kusky suggests. If he and his colleagues identify the isotopic ratios that suggest life at that time existed on the sulfide chimney deposits, then they will search for fossil evidence. “Fossils that age are hard to find,” he says. “But this is so well preserved we’re likely to find some.”

Christina Reed

Read a related story in the July 2001 Geotimes, The oldest ophiolite.

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Biology and minerals

Biomineralization is an intriguing field of investigation relative to ore genesis. Recent research by people such as Grant Ferris and Chris Kennedy of the University of Toronto, Danielle Fortin of the University of Ottowa, and the author has demonstrated that certain bacteria such as Leptothrix and Gallionella precipitate ferrihydrite, an amorphous iron oxyhydroxide, and silica on their bodies. It is not difficult to imagine that such a process could be responsible for the formation of some of our iron ores. Also, Kim Juniper of the University of Quebec at Montreal has shown that a particular worm living at hydrothermal vents has thick coatings of iron sulfide on its body (pictured are bacteria coated with hydrated iron oxide).

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 Juniper’s worm, play an important role in precipitating metal sulfides.

Steven Scott

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