Metals may also be concentrated as late stage fluids left over when a magma body cools. As magma cools the silicate minerals crystallize. The lef over fluid often contains valuable metals. When this fluid crystallizes as quartzs veins gold, silver, and other metals may crystallize in the quartzs veins. Concentration by Weathering Processes and Rivers - Placer Deposits Ores may be weathered from a rock and then concentrated as material in streams or along the coast.
Some mining is done under ground.
The Homestake Gold Mine in the Black Hills of South Dakota is greater than 12,000 feet deep. Tunnels and shafts are driven to extract the ore. Common in diamond and gold mining where a vertical ore body that continues deep into Earth is being mined. The heavy gold is buried in gravel and in depressions in the stream bed. In order to find the gold it must be sifted from the gravel.