Part Seven: Ancient rivers with a gold lining …
As the story goes, the early gold rush stampeders found all the creeks staked when they arrived. The Sourdoughs already there jokingly told the newcomers to go to the top of the hills to find gold. One of those hills became known as Cheechako Hill just below the junction of Bonanza and Eldorado Creeks. Within the gravels near the top of the hills was one of the best pay streaks in the Klondike.
These high level gravels were deposited by a river system three to five million years ago. Due to the abundance of white quartz pebbles and cobbles within it, they became known as the White Channel Gravels.
About 250 million years ago in the area that would become the Klondike, volcanic activity deposited light coloured quartz rich (siliceous) rocks. These were later metamorphosed by heat and pressure into what is now called the Klondike Schist. Schist is a metamorphic rock that has a layered look. The metamorphism remobilized the original minerals into a linear texture.
Fast forward to between 20 and 50 million years ago. The climate was five to ten degrees Celsius warmer than it is today. It was relatively dry as well. This allowed the Klondike Schist to break down quicker than it does today. The area resembled a relatively flat dry plain with a few isolated hills. You can get a feel for what it looked like by driving along the Top of the World Highway. The tops of all the ridges are approximately the same height. Just imagine that the creek valleys are not there.
There were some higher points, where the rocks were more resistant. Quite often many ridges would radiate from these higher areas. In the Klondike these features were named domes. King Solomon Dome at the top of Hunker, Upper Bonanza, Quartz, Dominion, and Sulphur Creeks is a great example.
About five million years ago a couple of significant events started. The climate in the Klondike area had starting cooling but was still probably two to three degrees warmer than today and was getting wetter. There also was accelerated uplifting of the ground. This uplift was from tectonic activity from plate collisions in the Gulf of Alaska area. This uplift was responsible for the St. Elias Mountains in southwest Yukon as well.
The cooling wetter climate resulted in streams and rivers developing. One of the features of tectonic uplift is accelerated erosion. The watercourses started to erode faster to try and maintain their original configuration.
The lower levels of the White Channel Gravels have been dated as forming about five million years ago. The increased amount of eroded material could not all be carried away by the creeks and rivers. Braided rivers formed that had several shifting channels and lots of gravel bars. A good example of this today would be the Donjek River and many others in the Kluane area. The ample supply of eroded material from the mountains starts dropping out in the river bed when they reach the lower sloping ground. You can see this where the rivers cross the Alaska Highway.
In the Klondike some braided river systems developed. The schist that had been broken down by the earlier warmer climate started to erode away quickly. Most of the fine softer material from the decomposed rock was washed away leaving the harder quartz rich rock to accumulate.
The White Channel Gravels can be over 40 metres thick and have been historically divided into two main units. The most visible difference between the two is that the upper gravels are stained a yellowish tint from surficial soils.
Where did all the gold come from? There are some gold bearing quartz veins present in the Klondike Schist today. They are not large or high grade gold deposits but they do exist. The best known of these is the Lone Star. Gold bearing quartz veins occur on the ridge between Upper Bonanza and Eldorado Creeks. It was actually mined in the early 1900’s for a short period but was not profitable.
The amount of placer gold taken from the Klondike area is estimated to between 400 and 800 million grams (12 to 25 million ounces). A hard rock deposit today with that amount of contained gold would be a large world-class mine.
Modern sub-microscopic investigations have proven that gold from the quartz veins is identical to that found in nearby placers. There is however, a significant amount of gold recovered in the creeks that is not the same as the gold in the veins. The Klondike Schist has fine grained gold disseminated throughout that is not related to quartz veins. The theory is that the placer has come from weathering and concentration of large volumes of eroded schist as well as the quartz veins.
The richest portions of the White Channel Gravels are along the bedrock contact and one or two metres above it. The schist trapped gold because it was rough with lots of projections and cracks for it to concentrate. Gold is heavy and it works its way downwards. Studies in the Klondike, California, and New Zealand have shown that placer gold usually has not travelled horizontally very far from its original source, maybe in the order of tens of metres. This explains why placers just downstream of known hard rock gold areas are usually rich. There were many good claims around Grand Forks, just downstream of the Lone Star.
The earliest modern glaciation, about 2.5 million years ago triggered the end of the White Channel Gravels. Although the Klondike escaped being covered by ice it was cold and no water was flowing. The White Channel Gravels were preserved.
In the warmer periods between subsequent glacial events, water started flowing again. New creeks started eroding the landscape. They first cut down through the White Channel Gravels concentrating the gold further still. This second concentration process is why the richest ground is in the modern creeks.
The whole story of the White Channel Gravels and how it got its gold is not finished. Geologists with the Yukon Geological Survey (YGS) and the mining industry continue to investigate the mysteries of the area. The YGS recently discovered that a gold bearing gravel bed near Hunker Creek may be twice as old as the White Channel Gravels.
If you want to read more on the White Channel Gravels look at R.G. McConnell’s 1907 Geological Survey of Canada “Report on Gold Values in the Klondike High Level Gravels”. It is a very informative and readable report with a great map. You can find it at: http://publications.gc.ca/collections/collection_2017/rncan-nrcan/M41-1-3-1907-eng.pdf .
Next time you are in Dawson City, and the season is right, spend a day to check out the White Channel Gravels. Drive the Bonanza Creek-Upper Bonanza-King Solomon Dome-Hunker Creek loop. The Government of Yukon has a number of excellent interpretative panels along the way. The White Channel Gravels can be seen in many locations: above the Callison Industrial area outside of Dawson, three kilometres up the Bonanza Creek Road, further upstream between Claim 33 and Grand Forks, and the lower sections of Hunker Creek. Be sure to stop at Dredge #4 National Historic Site, Discovery Claim National Historic Site, and the Klondike Visitors Association Free Claim#6 while you are out in the goldfields.