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.

Pegmatites

Since the vast majority of the materials composing the Earth's surface are composed of the big 8 elements and the silicates that are made from them, the finding of most of the other elements and minerals occurs mainly because there are processes that concentrate these rarer substances in particular places. When you see examples of large gems and crystals, a word that keeps appearing is "pegmatite", because most large crystals of otherwise rare materials usually occur in pegmatites.

The word "pegmatite" is not mineral-specific, but refers to igneous rocks that form in the final stages of the crystallization of molten magma. You could visualize a pocket in a large pool of magma that might be the last to crystallize because of a lower melting temperature, and which has a different concentration because the materials have a different density or some other property that separated them. That pocket might have the luxury of a long time period free of disturbances so that it slowly crystallized into large crystals. That description would be enough to bring Minas Gerais, Brazil into the minds of many geologists and gemologists, since the remarkable pegmatites of that region have been the source of most of the world's large crystals of beryl, chrysoberyl , topaz , tourmaline and spodumene .

A pegmatite can generally be recognized by the large crystals present, and in fact the working definition of a pegmatite is a rock in which most of the material is made up of crystals at least a centimeter in diameter. Contributing to the formation of large crystals, other than a slow crystallization process, is the fact that in the melt the crystallization of early materials leaves pockets of hot water which collect mobile ions into concentrated solutions. The higher mobility of the ions in the water, compared to inclusion in the molten rock of the magma, allows them to form large crystals more rapidly.

Reference:
Geology.com, Pegmatite