Origin of Mineral Deposits ¾ A mineral deposit is a volume of rock enriched in one or more minerals In this sense a mineral refers to a useful material, a definition that is different from the way we defined a mineral earlier in this unit ¾ Mineral deposits can be classified on the basis of the mechanism responsible for concentrating the valuable substance Examples Include: 1) Hydrothermal Mineral Deposits 2) Magmatic Mineral Deposits 3) Sedimentary Mineral Deposits 4) Placer Mineral Deposits 5) Residual Mineral Deposits Origin of Mineral Deposits 1) Hydrothermal Mineral Deposits ¾ Minerals are concentrated by hot fluids flowing through fractures and pore spaces in rocks ¾ Hydrothermal deposits are produced when groundwater circulates down to depths and heats up, either by coming near a hot igneous body or by circulating to great depths which naturally heat the water because of the geothermal gradient (300C per km depth) ¾ Such hot water can dissolve valuable minerals as it passes through a large volume of rock ¾ As the hot water moves into cooler areas of the crust, the dissolved substances are precipitated from the hot water solution Origin of Mineral Deposits 1) Hydrothermal Mineral Deposits ¾ If the cooling takes place rapidly in open fractures or upon reaching cool surface waters, then precipitation will take place over a limited area, resulting in a higher concentration of minerals than was originally present in the rocks Examples: Ore minerals depositing in veins Cooler water moving toward surface 1) Massive sulfide deposits 2) Vein deposits 3) Stratabound mineral deposits Hot water moving at depths Origin of Mineral Deposits 2) Magmatic Mineral Deposits ¾ Minerals are concentrated within a body of igneous rock by magmatic processes like crystal settling ¾ Magmatic process such as partial melting, fractional crystallization, and crystal settling in a magma chamber can concentrate ore minerals containing valuable metals by taking elements and concentrating them in minerals that separate from the magma ¾ As minerals crystallize from a magma body, heavy minerals may sink to the bottom of the magma chamber Magma Chamber Crystal Settling Origin of Mineral Deposits 3) Sedimentary Mineral Deposits ¾ Minerals are concentrated by chemical precipitation from lake or sea water ¾ These mineral deposits form as a result of chemical sedimentation, where minerals are precipitated directly out of water Examples Include: 1) Evaporite Deposits - Evaporation of lake water or sea water results in the loss of water and thus concentrates dissolved minerals in the remaining water When the water becomes saturated with dissolved minerals, they precipitate from the water Deposits of halite (table salt) and gypsum (used in plaster and wall board), result from this process Origin of Mineral Deposits 3) Sedimentary Mineral Deposits ¾ Minerals are concentrated by chemical precipitation from lake or sea water ¾ These mineral deposits form as a result of chemical sedimentation, where minerals are precipitated directly out of water Examples Include: 2) Iron Formations - These deposits are of iron rich minerals that were deposited in marine environments during the Proterozoic These formed as a result of iron-rich waters reacting with oxygen released by algae in the early stages of the evolution of life The composition of sea water must have been drastically different than it is today Origin of Mineral Deposits 4) Placer Mineral Deposits ¾ Minerals are concentrated by flowing surface waters depositing high density minerals either in streams or along coastlines ¾ When the velocity of the water slows, minerals with a higher density are deposited Heavy minerals like gold, diamond, and magnetite will be concentrated in areas where water current velocity is low The lighter minerals (quartz) are carried away ¾ Gold originally formed in hydrothermal veins, is eroded out of the veins and carried in streams where it was deposited in placer deposits The California gold rush in 1849 began when someone discovered rich placer deposits of gold in streams Origin of Mineral Deposits 4) Placer Mineral Deposits ¾ Placer deposits occur in any area where current velocity is low, such as; 1) between ripple marks Placer Deposit Stream Direction 2) behind rock bars Stream Direction Placer Deposit 3) on the inside of 4) in holes on the bottom meandering streams of a stream Placer Deposit Stream Direction Placer Deposit Origin of Mineral Deposits 5) Residual Mineral Deposits ¾ Minerals are concentrated by chemical weathering processes ¾ These deposits often form as a result of chemical weathering in warm tropical climates that receive high temperatures and high amounts of rainfall which produces highly leached soils rich in both iron and aluminium Chemical weathering tends to remove the soluble materials, leaving the less soluble residues ¾ Two common mineral deposits formed in this way are iron-rich Limonite and aluminium-rich Bauxite Bauxite is the world’s primary source of aluminium Origin of Mineral Deposits 5) Residual Mineral Deposits ¾ In addition, an existing mineral deposit can be turned in to a more highly concentrated mineral deposit by weathering in a process called secondary enrichment [...]... materials, leaving the less soluble residues ¾ Two common mineral deposits formed in this way are iron-rich Limonite and aluminium-rich Bauxite Bauxite is the world’s primary source of aluminium Origin of Mineral Deposits 5) Residual Mineral Deposits ¾ In addition, an existing mineral deposit can be turned in to a more highly concentrated mineral deposit by weathering in a process called secondary...Origin of Mineral Deposits 5) Residual Mineral Deposits ¾ Minerals are concentrated by chemical weathering processes ¾ These deposits often form as a result of chemical weathering in warm tropical climates that receive high temperatures and high amounts of rainfall which