Modern Placer Drift Mining – Chatanika Valley, Alaska
I had a mining engineer friend who, knowing I was interested in modern mining as well as mining history, invited me to see an operating placer drift mine operation in the Chatanika Valley. In placer drift mines, vertical or diagonal shafts are sunk into alluvial deposits (usually to bedrock where the gold accumulates) and horizontal tunnels (the drifts) are dug following the gold deposit.
At the mine I visited, the gold was about 200 feet underground in permanently frozen ground (permafrost). In this type of operation, the tunnels are worked only during the winter, to prevent thawing of the ground.
|Tunnel slanting down into mine
Heavily insulated doors at the entrance to the tunnel seal it off during the summer. A sloping shaft, large enough to drive down, was dug to the gold-bearing level. The tunnel was then flooded, allowing a thick layer of ice to form on the walls, reinforcing them. You can see a pneumatic hose on the upper right side of the tunnel.
The mine is ventilated. Air in the mine is continually exhausted to the outside through vertical shafts to prevent heat buildup. Machinery and human bodies can put out enough heat to raise the air temperature a few degrees.
|Gallery slowly collapsing
Even when it is -40° F outside, the temperature underground is about +28° F, so a small rise in air temperature is enough to thaw the frozen gravel. No cribbing or tunnels supports are used, and when the frozen gravels are allowed to thaw, the tunnels will start sagging and eventually collapse.
Horizontal drifts and galleries are blasted out of the frozen gravel. Pneumatic drills are used, and air pressure is also used to pump blasting powder into the holes. Pneumatically powered equipment (with the air pumping machinery above ground) is used wherever possible.
The loose gold-bearing gravel is scooped up by small front-end loaders and put in equally small dump trucks to be taken to the surface. The loaders and trucks were specially designed for underground work.
|Hauling gravel to the surface
The gold-bearing gravel is stockpiled until summer, when liquid water from nearby streams will gush through large sluices to separate the dense gold from the lighter gravels.
It was a fascinating trip and gave me lots of material for future drawings.