Ocean drilling provides look at mineral deposit as it forms

A senior public outfit recently intersected 94 metres of massive sulphides grading an average 6% zinc and 1.5% copper. Other intercepts were just as thick and just as rich. Ordinarily that would be market-moving news. But only an audience of lobsters, crabs and other assorted bottom feeders could truly appreciate the locale — the drill holes were collared under about 1.2 km of seawater.

The outfit in question is the Geological Survey of Canada. The intersections were cored during a recent segment of its Ocean Drilling Program in the Middle Valley, a sedimented oceanic rift at the northern extremity of the Juan de Fuca Ridge. The deep-ocean valley is about 200 km off the coast of southern British Columbia.

While the deposit may not be minable — how does one stake, let alone mine, the ocean floor? — the GSC is casting considerable light on the genesis of such epithermal orebodies. Millions of years ago seafloor hydrothermal vents spewed base-metal-rich massive sulphides that today are mined by Kidd Creek, Brunswick Mining and Smelting and many others.

“These systems are pumping away, belching black smoke into the water,” says Wayne Goodfellow, a research scientist with the GSC and a participant in the 2-month drilling exercise this past summer. “Basically, we’re observing an ore deposit forming right now.”

This is a pure massive sulphide deposit. It is not obscured by the “over-printing” effect of epochs of deformation, such as folding and faulting, and alteration that scar ancient deposits.

The GSC drilled 23 holes in the July-September program, focusing its efforts in three areas — the margins of the rift system where seawater begins its descent beneath the ocean floor, a known sulphide mound called Bent Hill, and “reaction zones” where seawater and high-level magma meet and the mineral-laden recirculation to surface begins.

For the interest of geologists, the multinational Deep Ocean Drilling Program off the British Columbia coast has led to some conclusions about ore genesis. Most important perhaps is a possible explanation of why massive sulphide deposits associated with sedimentary rocks are an order of magnitude larger, on average, than volcanic-hosted deposits. According to Goodfellow, a sedimentary sequence serves as a thermally insulating, impervious cap. This reduces conductive and advective heat loss and focuses hydrothermal fluid discharge to the seafloor.

Interestingly, when the GSC held a press conference in Victoria, B.C., after the program was completed, members of the environmental organization Greenpeace participated.

“They were very supportive of our ocean-drilling program,” Goodfellow said. Olav Svela is editor of The Northern Miner Magazine.