Origin of Mineral Deposits

1. A mineral depositis a volume of rock enriched in one or more minerals. In this sense amineral refers to a useful material, a definition that is different from the way we defined amineral earlier in this unit. Mineral deposits can be classified on the basis of the mechanism responsible for concentratingthe valuable substance. Examples Include: Origin of Mineral Deposits 1) Hydrothermal Mineral Deposits Reference:Pages 585 - 596 Chapter 21 2) Magmatic Mineral Deposits 3) Sedimentary Mineral Deposits 4) Placer Mineral Deposits 5) Residual Mineral Deposits

2. 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 depthswhich naturally heat the water because of the geothermal gradient. (30EC per km depth) Such hot water can dissolve valuable minerals as it passes through alarge volume of rock. As the hot water moves into cooler areas of the crust, the dissolvedsubstances are precipitated from the hot water solution. Minerals are concentrated by hot fluidsflowing through fractures and pore spaces in rocks. Origin of Mineral Deposits 1)Hydrothermal Mineral Deposits

3. If the cooling takes place rapidlyin open fractures or upon reaching cool surface waters, thenprecipitation will take place over a limited area, resulting in a higher concentration of minerals than was originally present in the rocks. Ore minerals depositing in veins Cooler water moving toward surface Hot water moving at depths Origin of Mineral Deposits 1)Hydrothermal Mineral Deposits Examples: 1) Massive sulfide deposits 2) Vein deposits 3) Stratabound mineral deposits

4. Magmatic process such as partial melting, fractional crystallization, and crystal settling in amagma chamber can concentrate ore minerals containing valuable metals by takingelements andconcentrating them in minerals that separate from the magma. Minerals are concentrated within a body of igneous rockby magmatic processes likecrystal settling. Magma Chamber As minerals crystallize from a magma body, heavy minerals maysink to the bottom of the magma chamber. Crystal Settling Origin of Mineral Deposits 2)Magmatic Mineral Deposits

5. These mineral depositsform as a result of chemical sedimentation, where minerals are precipitated directly out of water. Minerals are concentrated by chemical precipitationfrom lake or sea water. 1) Evaporite Deposits- Evaporation of lake water or sea water results in the loss ofwater and thus concentrates dissolved minerals in the remaining water. When thewater becomes saturated with dissolved minerals, they precipitate from thewater. Deposits of halite (table salt) andgypsum (used in plaster and wall board),result from this process. Origin of Mineral Deposits 3)Sedimentary Mineral Deposits Examples Include:

6. These mineral depositsform as a result of chemical sedimentation, where minerals are precipitated directly out of water. Minerals are concentrated by chemical precipitationfrom lake or sea water. 2) Iron Formations- These deposits are of iron rich minerals that were deposited in marine environments during theProterozoic. 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 itis today. Origin of Mineral Deposits 3)Sedimentary Mineral Deposits Examples Include:

7. When the velocity of the water slows, minerals with a higherdensity 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. 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 1849began when someone discovered rich placer deposits of gold in streams. Minerals are concentrated by flowing surface waters depositing high density minerals eitherin streams or along coastlines. Origin of Mineral Deposits 4)Placer Mineral Deposits

8. Placer deposits occur in any areawhere current velocity is low, such as; 1) between ripple marks 2) behind rock bars PlacerDeposit PlacerDeposit PlacerDeposit Stream Direction Stream Direction Stream Direction 3) on the inside of . meandering streams 4) in holes on the bottom . of a stream PlacerDeposit Origin of Mineral Deposits 4)Placer Mineral Deposits

9. 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. 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. Minerals are concentrated by chemical weatheringprocesses. Origin of Mineral Deposits 5) Residual Mineral Deposits

10. In addition, an existing mineral deposit can be turned in to a more highly concentratedmineral deposit by weathering in a process called secondary enrichment. Origin of Mineral Deposits 5) Residual Mineral Deposits

11. Sample Problem Distinguish between hydrothermal and depositional methods of the formation of economic mineral deposits. Answer: Hydrothermal - hot solutions react with rocks in which they pass and as a result the hot solutions becomes concentrated with metals in solution. When the hot solution enters a cooler environment, the metals precipitate from the solution and form metallic mineral deposits. Ex. gold in quartz. Depositional - form by a process of sedimentation. Mineral deposits form as a result of chemical precipitation, evaporation, and density deposits in water environments. Ex. gypsum and halite deposits or gold accumulating in water environments as placer deposits.