Figure 3.30Many deposits of ore minerals are found in veins formed by hydrothermal solutions. (a) Groundwater percolating through fractured rock dissolves metal oxides and sulfides. When heated by a magmatic intrusion, it rises, precipitating metallic ores in the rock fractures. (b) This quartz vein deposit (about 1 cm thick) in Oatman, Arizona, which contains gold and silver ores, was formed by such a process.