February 2013

Deep exploration

Wayne Goodfellow unravels the mysteries of SEDEX deposits

By Alexandra Lopez Pacheco

Wayne Goodfellow, a research scientist with the Geological Survey of Canada (GSC), vividly remembers the time he and his cousin thought they had discovered a major coal deposit near their family farm in New Brunswick. “It was on a coal seam in Pennsylvanian rocks exposed along the Miramichi River,” he says. “We were 12 and we thought we’d found the mother lode.”

Goodfellow went on to study geology at Mount Allison University and the University of New Brunswick. In 1976, he applied for a research position with the GSC, got the job and found himself responsible for a three-year project to undertake geochemical surveys of large areas of the Yukon. In 1985, he and his colleagues discovered a zinc-copper massive sulphide deposit on the seafloor not far from Vancouver Island, which turned out to be one of the largest of its kind in the world. In his CIM Distinguished Lecturer presentation, “Sedimentary-exhalative (SEDEX) Zn-Pb Deposits: current concepts on their geology, genesis and exploration,” Goodfellow examines the attributes of SEDEX deposits and suggests refinements to exploration methodology.

CIM: Why have you chosen to focus your research on SEDEX deposits?

Goodfellow: It is an important class of deposits. It’s a major source of zinc, lead, silver and copper. Canada has world-class deposits like the Sullivan deposit in B.C., which is no longer in production. I want to help develop an understanding of the fundamental controls for the formation of these deposits, and to apply this knowledge to the exploration of sedimentary basins in Canada.

CIM: Your research on SEDEX deposits spans decades. What did some of your earlier work involve?

Goodfellow: I had an opportunity, through GSC’s sea floor minerals program, to work on sulphide deposits that are actively forming along the Juan de Fuca Ridge off the coast of B.C. It was a great experience: I actually went down in a submersible to observe and sample deposits, and participated in the drilling of these deposits under the Ocean Drilling Program. This research on a modern system provided a lot of insight into the formation and exploration of SEDEX deposits in ancient sedimentary basins.

CIM: What makes understanding the formation of SEDEX deposits a challenge?

Goodfellow: SEDEX deposits only started to form about two billion years ago. And since their formation was dependent on the seafloor environment, it is important to understand ocean evolution through geological time.

During the Archean geological eon, oceans were very different from what they are today. They were dominated by ferrous compounds and were very low in sulphur. That composition impeded the buildup of hydrogen sulphide, or H2S, in the water column. H2S is essential for the precipitation of lead, zinc, copper and other metals that form SEDEX deposits. Even if those metals were discharged into an Archean ocean by vents in oceanic ridges – where the seawater had seeped into the oceanic crust and ­dissolved minerals – they would never precipitate and form sulphide deposits on the ancient seafloor. But around two billion years ago the oceans changed from enriched in ferrous iron to dominated by reduced sulphur, reflecting the buildup of atmospheric oxygen and seawater sulphate. Higher levels of sulphate promoted the bacterial reduction of sulphate to sulphide (H2S), which was essential to the precipitation of zinc, lead and other metals.

Another factor that led to the formation of SEDEX deposits is the stratification of ancient oceans. Oceans today are well-­circulated and ventilated, meaning there is oxygen throughout the water column. But over the last two billion years, the oceans were periodically stratified with an upper water column that was oxygenated and a lower water column that was depleted in oxygen and enriched in H2S.

CIM: What does all this mean from the perspective of exploration?

Goodfellow: If we understand which geological factors determine the amount of metals vented over time, and the origin and size of the deposits, we will be able to identify the best indicators for the exploration of SEDEX deposits. The geochemical vectors that are most effective in locating deposits, particularly those concealed at deep below the Earth’s crust, will hopefully lead to more discoveries.

Book Wayne Goodfellow as a Distinguished Lecturer or learn more about the CIM Distinguished Lecturers Program.

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