Dredge No. 4 National Historic Site of Canada
Techniques of Placer Mining:
For many of us prospecting suggests an image of a person crouched by a stream, pan in hand, washing gravel in a vain or successful attempt to find gold. This picture has tended to leave the impression that gold is found in the stream itself or in the adjacent surface gravels. While this picture has a certain limited validity, it bears repeating that in the Yukon gold was primarily found on bedrock, overlain by a surface layer of decayed organic material called muck, and an intermediate layer of permanently frozen gravel. It was this fact that prevented the early miners from uncovering the rich pay streaks and explains why they confined themselves to working the surface gravels of bars and banks of various streams with primitive appliances such as the pan and rocker. Some method had to be devised to reach bedrock before mining per se could be established. This was accomplished at Forty Mile in 1887 with the introduction of wood burning, a technique that entailed thawing an area of frozen ground (six-by-four feet or 1.8 x 1.2 m) with wood fires, removing the thawed gravels, and repeating the process until bedrock had been reached. Once on bedrock, the miner tunneled laterally (drifting), all the while removing the pay gravels to a dump on the surface. The pay gravels thus removed were dumped in a location separate from the gravels removed in shafting because the latter contained little if any gold, and the object was to sluice only those gravels that were gold-bearing. While drifting, the miner would pan occasionally in order to ensure that his drift had bottomed in pay dirt. If it did not, he had the option of extending the drift, digging a new shaft, or, as a last resort, abandoning the claim. Whatever the outcome, the work was time-consuming, arduous and risky.
With the coming of spring, drift-work was discontinued. There were two reasons for this. Shafts and drifts were generally untimbered since permafrost, coupled with cold winter temperatures, eliminated the need for shoring. This was the prime reason for drifting in winter. Consequently, cave-ins were a real problem in warmer weather as the permafrost melted. Moreover, sluicing, the next stage in the process, required water in quantity and this was available only during the spring run-off. The pay gravel accumulated during the winter was run through a sluice, a series of inclined boxes with riffles on the bottom, where water separated the gold from the gravel. The gold fell to the bottom because of its high specific gravity and was trapped by the riffles. The rest of the gravel was carried away by the water. Every so often, depending on the richness of the ground, the riffles were removed, the water shut off, and the pay dirt was panned. This was called the clean-up.
Self-dumper in action in the goldfields© Parks Canada / Canadian Imperial Bank of Commerce / Woodside, H.J. / 41/62During the gold rush period (1897-1900) steam thawing and the self-dumper were introduced. Steam thawing was far more efficient than wood burning, and more economical over the long-term as well, although capital was required to buy boilers, pipe, hose and points. With this method the gravels were thawed by driving steel pipes called points into the frozen ground and forcing steam through them. The self-dumper was a great improvement over the old method of hauling dirt out of the shaft with a windlass. It enabled the miners who used it to transport the pay dirt directly from the shaft to the sluice, thus eliminating the need to move the dump to the sluice by shovel or wheelbarrow.
The great attraction of shaft and drift mining, apart from steam thawing and the use of self-dumpers which were used on a small scale prior to 1900, was that it required relatively little capital. So long as a miner had a large enough grub-stake to sustain him until clean-up, and enough money to buy wood for thawing and sluice boxes where needed, all that was required was hard work, long hours and a willingness to accept harsh living conditions. This is why this method was so widely used during the gold rush. Unfortunately for the small operator, the extensive application of shaft and drift mining during the rush, coupled with the high-grading principle on which it was based, resulted in the rapid depletion of the rich pay gravels on which shaft and drift mining depended for success. The problem was not that the gold field was exhausted; indeed, quite the opposite was true. What had happened was that ever-increasing amounts of low-grade gravel would have to be handled in order to yield an adequate return. For this, machinery, and the capital to acquire and operate it - not hand labour - would be needed. As a consequence a variety of new techniques based on the open-cut method of mining were introduced.
Open-cut mining was initially confined to those areas where bedrock was less than 6 m (20 feet) deep and where the depth of the muck and barren gravel was "inconsiderable." However, the great virtue of this method was that its potential for development was practically unlimited: as machinery improved, it could be used on almost any placer deposit-creek, bench or hill - regardless of depth. As originally practiced on creek claims with little or no grade, the muck and barren gravels were removed by steam or horse-powered scrapers, thawing being done where necessary. The pay gravels were then thawed by steam or exposure to the sun and transported, either by shovel, wheelbarrow or self-dumper, to the sluice. Scrapers, and subsequently steam shovels, were used on some claims to remove not only the muck and non-paying gravel but the pay gravels as well. A later development which grew out of the wish of some creek operators to work their claims on the hydraulic principle was the mechanical elevator, a device which furnished sufficient grade for both sluicing and the disposal of tailings (waste material).
Hydraulicking© Parks Canada / Anita John's Collection / Y.T.-23On creeks where adequate grade and sufficient water were available, the muck and "barren" gravels could be stripped by water under pressure. This was later extended to include the removal of all material between the surface and bedrock with jets of water under pressure (ground sluicing), the tailings being disposed in one dump, the pay gravels in another. Hydraulicking, which also required water under pressure, was employed on the bench and hill claims. Technically, the only difference between ground sluicing and hydraulicking was that the pay gravels uncovered by the former had to be transported to a sluice whereas the latter not only disintegrated but delivered the pay gravels to the sluice boxes.
Until 1905 most observers believed that the Klondike's low grade gravels could best be worked by the hydraulic system. After that date it became apparent that another form of open-cut work, dredging, held the key to the future exploitation of the gold field. Dredges, although complex-looking behemoths to the untrained eye, operated on a relatively simple principle and consisted of four basic components: a barge for flotation; a series of steel buckets which excavated the gravel in front of the barge and delivered it to a housing built on the barge; the housing itself, where the gravels were disintegrated with water and the gold recovered, and finally, a conveyor or stacker which disgorged the barren gravels behind the barge.
As was the case with all but the earliest forms of open-cut mining, dredging required water. For this reason the dredges were shut down once the dredge ponds froze over. This limitation was more than offset, however, by keeping the dredges employed 24 hours a day during the dredging season. Stripping and thawing were required on the creeks, but not on the river beds, which had been naturally thawed by stream action. This is why the Klondike River proved to be such good dredging ground and explains why the Canadian Klondike Mining Company, which operated there, was successful. Dredging was facilitated by the absence of large boulders in the Klondike and enjoyed a real advantage over hydraulicking since it required less water.
The introduction of keystone drills for prospecting early in the century and the discovery by 1920 that cold water was a more efficient thawing agent than steam completed the development of dredging technology. After that, improvements were realized through organizational changes and modified operating procedures. A high level of operating efficiency was achieved by the Yukon Consolidated Gold Corporation after 1933; ground that had shown good prospects was stripped in the first year by hydraulic monitors, thawed with cold water points in the second and dredged a year or two later.