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Sea Floor Exploration: Nautilus Masters the Sea

Toronto-based Nautilus Minerals leads the charge in exploring the Pacific Rim of Fire for sea floor massive sulphide deposits

By Virginia Heffernan

On an autumn day in 1995, Roger Moss slipped his passport into his breast pocket, kissed his wife and baby son goodbye and embarked on a six week journey from Toronto to Lau, Papua New Guinea, where he would join other scientists aboard the RV Yokosuka, a Japanese research vessel.
 
The mission? To plumb the depths of the Bismarck Sea for "black smokers" that might serve as modern analogues to - and provide an exploration guide for - land-based volcanogenic massive sulphide (VMS) deposits.

Several days into the voyage, Moss found himself bent double in a tiny submersible, his calves stinging with pins and needles, as he scanned the ocean floor for vent sites. But the discomfort would be worth it. While traversing the side of a rocky ridge at depths of almost two kilometres below surface, Moss came across a tell-tale plume.

The eponymous "Rogers Ruins" now form the most northerly site of the Solwara 4 prospect, one of several high-grade massive sulphide systems being evaluated by Toronto-based Nautilus Minerals as potential mining opportunities.

"This was purely a research mission to see if we could observe VMS deposits as they were forming and apply that knowledge to land-based exploration." recalls Moss, a graduate student at the University of Toronto at the time. "I was skeptical about the possibility of them ever being mined."

But as land-based deposits become increasingly scarce, explorers are turning to the ocean bed as a source of metals to meet growing demand from developing economies such as China and India. Leading the charge is Nautilus, which has been exploring the Pacific Rim of Fire for sea floor massive sulphide (SMS) deposits since Papua New Guinea (PNG) became the first country in the world to grant commercial exploration licenses for such deposits in 1997.

Though Nautilus was a hard sell in the early days, when low metal prices were driving investors away from even the most conventional mineral exploration, the junior eventually convinced Placer Dome (now Barrick Gold) to invest in the sea floor venture through a farm-in agreement that allowed commercial exploration to begin in earnest in 2005.

Today Nautilus is preparing to start up the world's first sea floor copper gold mine in 2010, just off the coast of PNG. The company will use a sea floor mining tool to break up and suck the ore from the ocean floor and pump it though a steel pipe to a ship at surface, where the ore will be dewatered and loaded onto barges.

The first deposit slated for mining, Solwara 1, has inferred and indicated resources of 2.17 million tonnes averaging (at 4% copper cut-off grade) 7.2% copper, 0.6% zinc, 31 g/t silver and 6.2 g/t gold: small by land-based standards, but one of several high-grade deposits the company is continuously discovering.
 

This is not the first time explorers have looked to the ocean as a source of metal. In the 1970s, a consortia of private and government companies spent about $US1 billion to mine polymetallic nodules scattered around the ocean floor in international waters. But the venture failed for a variety of reasons – low metal prices, metallurgical challenges and the lack of a title system chief among them.

Research on SMS deposits began in the 1980s when geoscientists realized that the massive sulphides forming on the ocean floor were modern analogues of the VMS deposits currently being mined on land and, as such, may hold the secret to finding more land-based ore.

To test this theory, a team led by Ray Binns from CSIRO, Australia's national research organization, in collaboration with geologist Steve Scott from the University of Toronto began to study the seafloor vents. The Yokosuka cruise that Moss participated in was one of several expeditions they helped to organize.

The focus of most of the research to date has been on hydrothermally active vent fields that produce signatures that can be detected up to 10 kilometres away. This has led to the discovery of about 150 fossil and active sites mostly lying at depths of 1500-3500 m.

The challenge for commercial explorers is to find the inactive SMS deposits that have cooled enough to be mined.

With that in mind, Nautilus has developed a deep-ocean electromagnetic (EM) technique along with its new joint venture partner, Teck Cominco Ltd., and Vancouver-based Ocean Floor Geophysics. The new technology allows the partners to better target their drilling for faster, more cost-effective exploration.

"The system is spectacularly successful in finding copper-rich systems because copper is highly conductive," says Michael Johnston, vice-president of corporate development for Nautilus.

The exploration sequence at sea mirrors that of land-based projects: outline a large area of interest, and then progressively narrow down the target. The difference is that sea floor explorers know exactly where the deposits lie in the stratigraphic sequence, an advantage that saves both time and money.

"Our systems are generally outcropping," says Johnston. We know where that sea floor boundary is, and we drill it only when we think the system warrants drilling and want to know what the average grade of it is, not whether it is there or not."

When looking for SMS deposits, explorers first conduct a study of seafloor topography using bathymetric maps and other nautical information to find target structures. Then they run echosounder traverses to gather further detail on prospective topographic features. The next step is to go plume hunting, looking for increases in particulate material reflected by decreases in transmissivity and/or slight changes in temperature or pH.

At 1500 m below surface, ocean conditions are fairly uniform in terms of water temperature (2.5-2.6° C), acidity, turbidity and sediment concentrations. "It has the same general properties all around the planet, so what you are looking for are any slight changes in those normal conditions," says Johnston.

Geophysical surveys, both magnetic and electromagnetic, can help pinpoint the best targets. Finally, the vents are sampled by dredging, grab sampling and sediment coring. If the results are promising, core drilling follows.

"Improvements in seafloor exploration technology are continuously occurring," says Joanna Parr, project leader for the seabed minerals division of CSIRO Exploration and Mining. "This is a relatively new field of commercial endeavour so advancements are happening rapidly. Some of the techniques are unique to seafloor systems -  looking for active plume signatures in the water column, for example - but others are adaptations of well-tested land-based technology."

Considering that only about 3% of the ocean floor has been explored for SMS deposits, the potential for further discovery is enormous. But there remain barriers to exploration that may prevent the activity from becoming commonplace just yet.

Though Nautilus holds tenements and exploration licences off the shores of Papua New Guinea Fiji, Tonga, the Solomon Islands and New Zealand, many maritime states do not have legislation that would allow commercial exploration in their Exclusive Economic Zones (the area in which a coastal state has sovereign rights over all the economic resources of the sea, seabed and subsoil.)

Another barrier is the environmental impact. Environmentalists worry about the potential for disturbing unique ecosystems that surround the active vent sites, which sometimes lie in close proximity to the inactive sites that will be mined. There is also concern that the particulate matter stirred up by mining could clog the gills of fish, and the noise could disturb passing fish and whales.

These impacts, and how to mitigate them, will be addressed in the Environmental Impact Statement for Solwara 1 that will be submitted to PNG’s Department of Environment and Conservation by the end of the year. Nautilus is currently conducting detailed environmental monitoring of the project site.

Johnston believes that while sea floor exploration will never match what takes place on land, parts of the world that have a combination of hydrothermal activity and legislation to allow commercial exploration may become hotspots in the not-so-distant future.