Hydrothermal Ore Deposits
As magmas solidify, water and other gaseous constituents tend to be concentrated in the decreasing amount of molten rock. At some point these constituents may literally boil off, penetrate the surrounding rock, and condense to a hot, water-rich fluid. These igneous emanations are termed hydrothermal fluids. They are mobile and capable of dissolving metals from rock through which they pass. They tend to lose the metals they carry and form ore deposits when they encounter a favorable location. Favorable spots include rock fractures or openings along faults were hydrothermal fluids form veins. Other sites include sedimentary rocks like limestone or gypsum. Hydrothermal fluids can chemically react with these rocks to deposit ore. Alternatively, the hydrothermal fluid may simply mix with groundwater, causing the fluid's temperature and composition to change and ore to be deposited.
Hydrothermal fluids need not be igneous emanations, but groundwater heated by a nearby mass of magma. Hydrothermal waters may reach the surface in hot springs and geysers, as at Yellowstone National Park. Water sampling and drilling at locations similar to Yellowstone has shown that ore minerals are being deposited at depth.