Sedimentary Ore Deposits
Sedimentary processes form ore either through the selective removal of nonmetallic components or by concentration of metallic minerals. Rock at the earth's surface is subjected to weathering and leaching, the process that turns rock into soil. Aluminum resists being leached, and bauxite, the ore of aluminum, is actually an aluminum-rich soil. Bauxite forms in the humid tropics from intense and prolonged weathering of aluminum-bearing rocks. The concentration of heavy metallic grains forms placer ores. Placer gold, for example, accumulates along streams were currents are too weak to carry the heavy flakes of gold but strong enough to winnow away ordinary rock fragments. Some entire beds of marine sedimentary rocks contain enough metal to be considered ore. Examples include sedimentary beds rich in iron, manganese, and even lead, zinc, and copper. For some, hydrothermal fluids issued from submarine hot springs may have been involved. Others may simply have been deposited directly from metal-rich ocean water.