Tens of thousands of years ago, the Northern Hemisphere was covered by continental glaciers. These glaciers acted like bulldozers, pushing dirt and rocks (called till) across the land and dumping them kilometres away from their original location.
Since then, trees and other vegetation have grown on top of this till, leaving a mystery for those who look for and study mineral deposits: How to tell if there is a hidden deposit buried metres below.
“Voisey’s Bay, for instance, was found because a small rusty outcrop was left behind after the glaciers had moved through,” said Dr. Derek Wilton of Memorial University’s Department of Earth Sciences. “From that small remnant, Archean Resources used geophysical techniques to track the mineralization down over a hill where it was buried beneath 20 metres of till. You would never have known it was there except for this little thing at the top of the hill.”
Since most of the mineral deposits at the surface have been found, looking for new deposits is particularly difficult because they are buried beneath the till. In searching for these hidden deposits of economic minerals, researchers have turned to other, more common, minerals that are often associated with ore minerals.
“When you are looking for diamonds, you will often find other robust minerals, like garnets, occur with the diamonds,” said Dr. Wilton. “If you can identify such ‘indicator’ minerals in the till, and you can figure out the direction from which the glacier travelled, you may be able to track your way back to where they were picked up. That’s where you will find the diamond deposit.”
This type of investigation also works for other types of deposits such as gold, platinum and base metals.
But finding indicator minerals is laborious and tedious work. It begins with samples of more than 40 kilograms of till, sifted down to a few hundred grams, and then separated further using heavy liquids to sort out the dense minerals. Those are then viewed under a microscope so that individual grains can be picked out from the thousands that remain. Often the scientist is only looking for one particular type of mineral, ignoring any other minerals and hence potential discoveries.
“What we’ve been doing is working to automate that process,” said Dr. Wilton. “I’ve been working with Altius Minerals of St. John’s on this in Labrador and also with Vale on the Voisey’s Bay Deposit, with some funding from the Newfoundland and Labrador Research & Development Corporation.
“With a particular piece of equipment in the Bruneau Centre, the mineral liberation analyzer scanning electron microscope (MLA-SEM), we’ve been able to take a much smaller 10 kilogram sample, sieve it down and mount the remainder in an epoxy puck to be analyzed by the machine. It can analyze up to 20,000 particles and give me a full range of what minerals are there and in what amounts.”
He’s been using the method to look for new hidden mineral deposits in Labrador, as well as examining known deposits and their minerals to determine which of those minerals might be robust enough use as indicators in future regional exploration.
But he’s literally just “scratching the surface” in terms of what could be done. Dr. Wilton is using the equipment in conjunction with others to test for contamination in industrial sites, places like the old asbestos mine site in Baie Verte, and has been speaking with archaeologists to see if the method could be used to help in their searches.
“They have the same problem we do, looking for stuff that is buried,” he said. “The question is, can you use this technology to detect whether humans have had an impact on a particular landscape. Is there something in the soil that humans have changed? If so, that would be a good place to look. The exciting thing is we have no idea where this research will take us.”