How Placer Mining Works
Placer mining means extracting valuable minerals from surface gravel deposits. It differs from hard-rock or quartz mining, which refers to underground mining below the level of solid bedrock.
Placer mining activities include mineral exploration, construction of the mine site, mine operation and reclamation, also known as de-commissioning.
Yukon placer mining does not use or release chemicals. It creates no toxic by-products. Instead, it uses water, motion and gravity to extract the minerals.
What is Placer Gold?
Placer gold eroded from hard rock gold sources. It was then carried downstream by streams and rivers. These pieces of gold can be found in the form of nuggets, flakes or grains.
When the currents in the streams or rivers slow down, they eventually are no longer strong enough to suspend the gold, which is heavy, in their waters. When this happens, the gold is deposited at the bottoms of stream and river valleys.
Over thousands of years, streams and rivers changed course, and valleys filled up with sediments, including placer gold deposits.
The heavy placer gold migrated downward through the sediments and is typically concentrated near the bottom of the gravel layers, just above solid bedrock. Often, the gold is covered by 50 feet or more of sediments.
Yukon placer gold deposits are typically found in creek and river valley bottoms, but they have also been found on some hillsides along the edges of valleys. These hillside deposits are known as "benches."
How Do Placer Miners Recover Placer Gold?
Early prospectors and turn-of-the-century gold miners used picks, shovels and hand tools to dig down through frozen soil and gravel layers, often working as deep as a hundred feet underground in narrow shafts and tunnels.
Modern miners use heavy equipment like bulldozers and excavators to move the layers of soil and gravel out of the way, in order to uncover the gold-bearing gravels. Sometimes water courses may need to be diverted while mining takes place, to allow the miner to get at the "pay dirt" underneath.
Placer gold is recovered when the gravels are "sluiced" or washed in flowing water in a sluice box or wash plant. The heaviest particles, including gold, settle to the bottom.
Many placer deposits are covered by frozen layers of sediments. Water is used to thaw these frozen materials and at the same time to wash the gold loose. The water carries the gold, sand and gravel in a slurry to the sluice box.
The sluicing process washes away many other fine materials from the gold particles, often resulting in high concentrations of sediment left suspended in the sluicing water. This water runs into settling ponds, where the sediments slowly sink or "settle" to the bottom. After enough sediment has settled to meet regulatory standards, the water can be discharged back into the stream.
Some mines recycle the water, using it over and over again for sluicing.
What Happens Inside the Sluice Box?
A typical sluice box has one or more metal troughs, or runs, with cross bars on the bottom, called riffles. As the water passes over these riffles, little whirlpools are created behind each riffle, and the heavier gold particles are able to fall to the bottom. Lighter particles are washed over the top of the riffles and out the end of the sluice box.
A layer of mats under the riffles provides spaces for the gold particles to become trapped within.