One method of finding gold that is not all that well known is called drift mining. They weren’t all that uncommon during the early mining days, but the gold deposits had to be the right type to warrant this type of mining.
This type of mining involves tunneling horizontally into a hillside, but it is very different from a standard hard rock mine that is following a vein. In contrast, a drift mine for gold is working a layer of auriferous gravel in an alluvial deposit.
In the United States this method was most commonly done at coal mines. Drift mining is done for a variety of minerals that form in seams. Both coal and gold provide challenges when using this extraction method.
Miners would establish a drift mine at a location where exposed gravel benches were present on the side of a mountain or hill. Surface diggings would indicate that a layer of gravel layer going horizontally could produced good gold. If the gravels were rich enough, they would punch a tunnel into the hill and pull gravels out to the surface for further processing.
But digging in this proved quite dangerous because the cemented gravels were generally much looser than typical country rock. Tunnels needed to be shored up properly, and cave-ins were not uncommon. Many miners died in these early drift mining operations.
As with an underground operation, the gold-bearing ores needed to be brought to the surface for processing. Ancient gravels would be cemented and generally required crushing before they could be run through a sluice system
. Each operation was a little different, but it was generally noted that gold recoveries needed to be quite high to make drift mining profitable.
Drift mining was quite common during the early days of the Klondike Gold Rush
and several other rich placers in the frozen north. The permafrost that is present at depths in Alaska and the Yukon allowed miners to tunnel underground and follow seams of rich gold-bearing gravels.
A lot of drift mining also took place at Nome
, where beach placers were actually buried at depth and sat on top of false bedrock clay layers. Miners would extract the gravels from depth and then wait to process in spring when water was again plentiful.
The abundant tertiary gravels throughout California provided many opportunities for productive drift mining. However, if water was available, they soon found that hydraulic mining
was more productive and a much faster way to release gravels locked deep inside of the mountains. Powerful hydraulic monitors would literally was away the hillside. Environmentally it was 100x worse than drift mining, but from a profitability standpoint it was almost always more desirable.
Due to the dangers, we rarely see drift mining done in modern gold mining operations today. Commercial operators now rely on open-pit operations to tunnel down into a deposit rather than go underground, thus reducing the risk of cave-ins.NEXT: The Pre-Columbian Gold Museum in Costa Rica