Hydrothermal Deposits
In Chapter 6 you learned that all magmas contain varying amounts of water. When magma slowly cools beneath the surface, some of the water will become incorporated into the crystalline structure of certain minerals, and some will remain with the magma during the final stages of crystallization. Water is also commonly present in the rocks surrounding the magma, which geologists refer to as country rocks. Naturally, the intense heat from the magma body raises the temperature of any groundwater that is present in the country rocks. The combination of these waters results in hot, mineral-rich fluids that transport ions and chemically react with rocks in a zone around an igneous intrusion. Minerals that crystallize from these highly enriched fluids form what are referred to as hydrothermal deposits. Because sulfur (S) is a highly mobile element common in groundwater and magmatic water, hydrothermal deposits typically contain valuable minerals where the sulfide ion (S2-) is bonded to metals such as copper (Cu), lead (Pb), and Zinc (Zn). Gold (Au) and silver (Ag) ores are also common.