Processes that Concentrate Mineral Resources
A number of processes occur in igneous rocks to provide accumulations of otherwise rare elements or minerals. Most minerals are made up of the eight abundant elements, and most surface minerals are silicates. Our capacity to find the "needle in the haystack" of the metals such as gold, silver, etc. comes from the fact that there are igneous processes which tend to concentrate them in one location.
The concentration of some of the metals comes from the fact that their ions are heavier and tend to sink to the bottom of the molten magma. Later in the process of magma cooling, water and some of the more volatile elements that are not included in the earlier crystallation processes become more concentrated in the remaining melt. In these enriched fluids, ion migration can contribute to the formation of large crystals, sometimes centimeters to meters in size. The resulting rocks that are formed from these large crystals are called pegmatites. Some minerals, like amblygonite, are found only in such pegmatites because they concentrate what would otherwise be trace constituents.
Concentration frequently occurs from hydrothermal solutions. The hydrothermal deposits are thought to arise when hot, metal-rich fluids in the late stages of magma solidification can migrate large distances through the surrounding rock. As the fluid moves along fractures or bedding planes, the metal ions can precipitate from the solution. Many of the productive deposits of gold, silver and mercury are formed as hydrothermal vein deposits.
Igneous rock concepts
Lutgens & Tarbuck