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By Claire Eamer
In July 1898, in the thick of the Klondike Gold Rush, a San Francisco newspaper reported that miner Daniel Sanderson was pretty sure he’d found the source of the gold that peppers the creeks and hillsides of the Klondike valley. The mother lode, he said, was a deposit of quartz, thick with gold, nearly a thousand feet above the bed of French Gulch, near El Dorado Creek, one of the richest creeks in the Klondike.
Fast-forward more than a century to 2009, when Yukon placer miner Shawn Ryan’s discoveries near the confluence of the White and Yukon rivers sparked a modern-day gold rush and reports in major media that the mother lode might finally have been found.
University of Alberta professor Duane Froese tends to come down on the “not” side of the discussion. Froese, the 2013 winner of the prestigious Hutchison Medal, recently talked about the origins of Klondike gold to a lunch-time audience at Yukon College in Whitehorse.
A lot of gold has come out of the Klondike – between 14 and 15 million ounces, according to official reports. About half of that came from Bonanza Creek and another quarter from Hunker Creek. Most is fine gold that has to be washed out of the gravel. The largest reported nugget was 72 ounces, but that was an exception.
In over a century of searching, no one has found the mother lode, the concentration of gold that fed all the creeks snaking through the region. It’s placer gold, says Froese, and that means there might well be no mother lode at all.
“You don’t necessarily need economic lode deposits to have placers.”
Placers are accumulations of heavy minerals, concentrated by mechanical transportation. The minerals erode out bedrock and are carried along by a natural mechanical force, such as streamflow, eventually settling into pockets and layers when the force peters out. Any heavy mineral can accumulate this way, but the ones of greatest economic interest have traditionally been diamonds and gold.
Decoding the Clues to the Past
All it takes to create a placer deposit is the right conditions, and Froese has been studying those conditions in the Klondike for two decades now. He started off, in the mid-1990s, mapping the Klondike goldfields for the Geological Survey of Canada, but slowly his interests shifted.
“I gradually drifted toward permafrost because I kept running into permafrost.”
The ancient permafrost that chilled and frustrated the old-time Klondike miners led Froese to the study of climate change. Now the Canada Research Chair in Northern Environmental Change, he spends more and more time thinking about long-term climate records. And the Klondike is a great place to think about them.
We live in an ice age, Froese says, that began about 3 million years ago. Great ice sheets have advanced across much of the northern hemisphere and then retreated in a series of glacial and interglacial periods, such as the current interglacial that began about 12,000 years ago.
Ice sheets covered much of Canada again and again, scraping the land down to its bones and then, as they retreated, covering it with sediment ground from the rocks themselves. That makes the geological record difficult to unravel. But those ice sheets rarely made it to the Klondike or to much of northern and western Yukon. Lying in the rain shadow of the high coastal mountains, the land didn’t receive enough moisture for snow and ice accumulate.
Still, it was cold. The layers of sediment that built up, undisturbed by ice sheets, froze – and, if they lay more than a few metres below the surface, they stayed frozen. Permafrost.
Temperature changes at different levels in the soil, combined with the thawing and freezing of the surface layers, caused cracks to form. When snow and meltwater filled them and froze again, the cracks expanded, creating great wedges of ice that penetrated deep into the sediments. Some – hundreds of thousands of years old – still exist in the cold Klondike sediments. Others have left V-shaped scars cutting through the layers of soil, which can be dated.
Both the permafrost and the ice wedges have helped Froese develop a climate history of the Klondike. Along with frozen sediment, they also contain the frozen and preserved remains of ancient worlds: pollen, bits of plant and wood, bones, even the intact nests of ground squirrels and the hair and soft tissues of long-dead mammals.
Ice isn’t the only clue Froese follows. The Klondike also preserves evidence of its past in its stratigraphy, magnetism, and layers of volcanic ash.
Stratigraphy – the painstaking charting of layers of rock and sediment across landscapes – is one of the geologist’s oldest tools. Stratigraphy provides a before-and-after sequence that lets the investigator determine the order in which the geology was created, if not necessarily the absolute age of each layer.
Magnetism helps date those layers. From time to time, Earth’s magnetic field swaps ends, and the giant magnet based in the planet’s molten core points in the opposite direction. At the surface, all the bits of mineral, sediment, and other matter influenced by magnetism line up neatly in accordance with the new magnetic direction. As they solidify into rock, the magnetic record is frozen in time. The last time the magnetic field swapped ends was about 780,000 years ago – creating a convenient time signal in the layers of sediment and frozen muck piled on top of the Klondike’s bedrock.
Volcanic ash tells an even more detailed story. Every volcanic eruption carries an individual chemical signature that can be identified and dated. The Klondike lies downwind of the active volcanoes of the Pacific Northwest, and volcanic ash has settled over the land many times, leaving visible layers in both the sediments and the ice.
All of these clues add to the story, says Froese: “Collectively, this allows us to get a time model of what happened in the Klondike.”
But What About the Gold?
So the Klondike holds plenty of geological clues to its past climate, but what about the gold? And what does climate have to do with gold?
Plenty, says Froese. To create placer gold, you first need weathering and erosion. There’s not much of that in a dry, cold climate, but the Klondike wasn’t always dry and cold. Three million years ago, it was warm and wet, with rainforests and wide, braided rivers. Trees and other plants dug their roots into the rock and helped break it up. Rain fell, percolated through soil and rock, and flowed away in streams and rivers that carried off the lighter, more fragile minerals, turning them into soil and leaving behind tough, resistant quartz and heavy, non-reactive metals like gold.
“Most of gold seems to be getting released at this warmer and wetter time,” Froese says.
The famous White Channel gravel, where most of the gold in the Klondike has been found, is 80 percent quartz, he says, and it’s more than 2.7 million years old. The first signs of glaciation – the scars of ancient ice wedges – appear in the layers above the White Channel gravel, indicating a major climate shift.
The colder climate dramatically reduced the erosion of bedrock and essentially stopped the processes that created the White Channel’s placer gold. Subsequent layers, collectively called the Klondike Gravels, covered the gold-bearing layer.
Eventually, new streams formed and cut new valleys deep into the sediments, right down to the old White Channel gravel. One of them was Bonanza Creek, where, in 1896, Skookum Jim, Dawson Charlie, and George Carmack spotted the gold that triggered the Klondike Gold Rush.
So – no mother lode. Just a millennia-long process that removed most of the material from bedrock, leaving behind the gleaming white quartz and glittering gold.
However, judging by more than a century of experience, that won’t stop people from looking for the mother lode.
Claire Eamer is a Yukon-based science writer who writes popularscience articles and books for both kids and adults, as well as writing and editing major scientific reports for international science-based organizations. Her latest book is Before the World Was Ready: Stories of Daring Genius in Science, published by Annick Press earlier this fall.