Metals in the Crust

Understanding how a metallic-ore deposit forms requires an understanding of the geological processes that concentrate metals. The behaviour of metals in the Earth's crust begins with identifying possible sources for the metals. The metal source may be vapour dissolved in an intruding molten rock beneath the Earth's surface, or it may be black shales heated and deformed to the point where their metals are released. However, the metals do not have economic value in a dispersed and disseminated state. 

It is critical to understand, down to the atomic level, the chemical conditions controlling the mobilization of metals. The mobility of metals combined with a clear path for their migration is the first step toward concentration of metals in a restricted space. The path may be outlined by small or large structural irregularities in the rock. Metal transport may be in response to chemical gradients as metal-bearing fluids seek a state of chemical equilibrium with the surrounding rocks. Mobility leading to spatial confinement and concentration is essential for an ore deposit to be economic. 

Copper-bearing sulphide, chalcopyrite (gold colour), in quartz vein with red feldspar. Click on the diagram to enlarge.After their journey, be it short or long, the metal-bearing fluids release their metals to form minerals, most commonly by bonding with other elements. For instance, metals bonding with oxygen are known as oxides and metals bonding with sulphur are known as sulphides. These minerals may occur freely in the host rock or in veins together with quartz or calcite. It is the deposition of these minerals that provides the copper, molybdenum, lead, zinc, gold, silver, and many other metals for our daily use. 

Mining copper and molybdenum from the Malanjkhand mine in central India.