Geological Processes

• Igneous processes are processes by which rocks and minerals are created by the cooling and hardening of magma or molten lava. Many different igneous processes have produced exploitable mineral deposits.
• Hydrothermal deposition is when rocks form from magma cool and solidify within the Earth's crust. This produces pressurised water at high temperatures that dissolves minerals from the surrounding rocks. These solutions travel along fissures away from the igneous batholith, cooling as they do. As they cool, dissolved minerals crystallise and come out of solution. They do this in order of solubility, with the least soluble crystallising first.
• Hydrothermal deposition is an example of fractional crystallisation. It starts with a mixture of minerals that couldn't have been exploited, but once they become separated and deposited, they then later become exploitable. Possible mineral include: tin, copper, lead, silver, gold and arsenic.
• Metamorphic processes are when igneous processes and tectonic movements of crustal plates can alter existing rocks with high temperatures and pressure, producing semi-molten rock. High pressure and temperature can change limestone to marble, whilst extreme pressure can change mudstone to slate.
• Sedimentary processes are when minerals are caused to settle and build up to produce layers of deposited sediment. This causes cementation at the Earth's surface and within bodies of water, creating sedimentary rocks and minerals.
• Alluvial processes involve materials that were carried and separated by water. The ability of water to carry solids depends upon the velocity of the water and the density of the solids. Possible exploitable minerals include gold, diamonds, tin ore, gravel, sand or clay.
• Evaporites are when areas of water become isolated then the water may have evaporated leaving crystallised minerals such as halite (sodium chloride).
• Biological processes are those where living organisms form mineral deposits. These processes often concentrate minerals that can be deposited in sedimentary rocks. Limestone and chalk come from shells of marine organisms, coal comes from terrestrial vegetation and crude oil and natural gas come from marine organisms.
• Proterozoic marine sediments are iron ore deposits such as hematite and magnetite. They were formed when dissolved iron compounds became oxidised by the oxygen released by photosynthesis, producing insoluble iron oxide deposits. This occurred 2.5 billion years ago.
• Secondary enrichment is when many economically important metals can form minerals that are soluble or insoluble depending on the conditions, especially the availability of oxygen. They can be transported in solution, by moving water and then deposited as their oxidation state changes.