mineral deposits

• Hydrothermal mineral deposits are associated with hot water circulation in the lithosphere. Heat sources may be magma or radioactive minerals. 
• Meteoric water is a source for hydrothermal water - rainfall and infiltrates the rocks. Seawater is also a source where it enter the rocks through cracks in the sea floor or is trapped in sediments. 
• Magma is also a source for hydrothermal deposits, it releases magmatic water during crystallisation. 
• Hydrothermal vein mineralisation is when hot waters around magmas dissolve metals from rocks, the hot water convects and rises to the surface where it cools and the minerals precipitate out of solution in cracks. 
• Insoluble minerals precipitate closer to the intrusion - metals are zoned according to solubility. 
• Mid ocean ridge black smokers ae sulphide mineral chimneys. Seawater percolates through the cracks in the sea floor, the water is heated to high temperatures and dissolves metals and volcanic gases. This water rises and the cold ocean water causes dissolved minerals to precipitate immediately. This forms chimneys of sulphide minerals like pyrite, silica and anhydrite.
• Pegmatites are coarse grained igneous rocks with large crystals. They are made or quartz and often contain rare minerals like lithium. 
• Pegmatites are formed with the crystallisation of anhydrous minerals, the remaining magma becomes more concentrated with water.. Rare earth elements are not incorporated in the solidifying crystals and are enriched in the water. The water reduces the viscosity of the magma so the crystal sizes are large. 
• Pegmatites are found along the edge and in veins around plutons. 
• Magmatic segregation deposits are called cumulates, formed by slow cooling of mafic of ultramafic intrusions - minerals with the highest melting points crystalise first (chromite, magnetite and olivine). These minerals are high density and sink, olivine is less dense and any convections keep it in circulation. If convections slackens then olivine will settle. Repeated magma injections can develop layers of the dense minerals. 
• Thick layers of cumulates need slow cooling of a sill. 
• Chilled margins represent the original magma composition, and will have more dense minerals due to them cooling before sinking. 
• Exogenic deposits are associated with sedimentary processes. 
• Aluminium is very abundant and difficult to extract due to it being reactive. Chemical weathering can concentrate and create deposits of bauxite from which aluminium can be extracted. 
• Jamaica has a climate that creates rapid chemical weathering of limestone. Bauxite is found in thin layers of volcanic ash within limestone. Bauxite Is insoluble so not affected by carbonation. As the limestone is dissolved by carbonation the bauxite is concentrated.