Gold - Part II
Written for the KidsKnowIt Network by:
The second type of primary gold deposit is very similar to the first. The second type of primary gold deposit is called intrusion related. Here is how it works: Hot magma is sometimes forced up from deep inside the earth. As it rises from deep underground it forces its way up though the rock, sometimes following faults, sometimes just simply melting its way though. As the magma cools down it forms igneous rock. This type of rock formation is called an igneous intrusion. Sometimes as the magma forces it way to the surface it draws extremely hot water with it. Just like before, as the water cools, minerals fall out of the water and are deposited along they way. I can almost here you ask, if the water is right next to magma why doesn’t it just boil away? Magma is so much hotter than the boiling point of water. That’s true, but the water is under so much pressure that it stays liquid long after it should have boiled away.
Secondary deposits are called secondary because erosion and water carry the gold from a primary deposit and deposit the gold in a different location. Secondary deposits are divided into two types: Placer and Laterite deposits.
Placer deposits are formed when water crumbles the rock and sweeps the gold downstream. Gold is very heavy for its size so the water has to be moving pretty fast in order to carry the gold downstream. When the water slows down enough, the gold settles to the bottom of the stream. The best place to find gold in the water is where the river slows down - like behind a big boulder, or as the river bends. The most famous kind of gold mining is used to find placer gold. The method is called panning for gold. The idea behind panning for gold is pretty simple. You load up a high-sided metal or plastic pan with dirt you hope has gold in it. Fill the rest of the pan with water. You shake the pan in such a way that the heaviest stuff (gold) in your dirt falls to the bottom of the pan. As you continue to shake the pan you carefully dump the mud and water out of you pan until just the heaviest stuff remains at the bottom. You have to carefully pick out the rocks and make sure they are just rocks. Keep working until you have washed out all the dirt and are just left with gold. It sounds simple, but I have personally never tried it so I don’t know how easy it really is. I think it takes a lot of practice; otherwise you might end up washing the gold out of the bottom of the pan along with everything else.
Most of the large gold rushes, like the California Gold Rush and the Klondike Gold Rush, were initially people looking for and working these placer gold deposits. In the case of the California Gold Rush, the miners eventually followed the placer deposits back to the primary deposits and mined the primary deposits as well. The mining of placer deposits can be very profitable. The world’s largest producer of gold is South Africa. 40% of the gold that South Africa produces comes from the Witwatersrand Basin. However, during the gold rush in California 1849, the people who made the most money were the merchants who sold supplies to the miners.
Laterite deposits are also formed from erosion. Many of these laterite deposits are the leftovers after the main primary deposit has been almost entirely eroded away. Laterite deposits may also be flecks of gold so broken down and so small they have been carried away by weathering and worked into the dirt.