Later, chemical bonds form which also create different densities of rocks. Gravity then does more sorting by pulling the more dense objects closer to the earth's core. Volcanoes are nature's method of naturally refining and sorting minerals. Precious minerals such as copper, silver, gold, and iron are in solution within the magma which is deep within the earth. In solution, these minerals are in such low concentrations that they cannot be mined until later if the magma were to harden.
Water now becomes very important in the smelting process within the earth.
Many of the most important mines in the early history of civilization relied upon copper for tools. The cracks, fissures and vents produced by the water were created by the cooling and condensing of the magma. The water bonds to the copper in solution, and then carries the precious mineral back up the cracks, fissures or vents where it then hardens. The mineral is now in a higher concentration than it was in the magma. Ancient miners did not understand this process within the earth, but they understood that if they dug a horizontal shaft into a vent or fissure at the right depth they would find copper.