Ore deposits (contact metamorphism)
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Contact metasomatism forms new minerals through reactions between intrusive rocks and escaping gases from magma chambers. Important requirements include a magma source of ore ingredients and intrusion into reactive host rocks. Metals like Fe, Cu, Zn, and W can be deposited through this process. Hydrothermal deposits are formed when hot, mineral-laden waters circulate through fractures, leaching and redepositing metals. Sedimentary deposits can form through evaporation, biochemical processes, or mechanical concentration of minerals in placer deposits.
Ore deposits (contact metamorphism)
- 1. Contact Metasomatism • Contact metasomatism is a process of formation of new mineral by reaction between the contact rock and the escaping high temperature gaseous emanation with other important materials from the magma chamber. • For the deposit of this type, the magma must contain the ingredients of mineral deposit and must be intruded at depth at the contact of reactive rocks. • PROCESS: 1. Recrystallization and recombination of rock mineral in the contact zones, e.g. limestone and dolomite convert to marble, shale to hornfels and sandstone to quartzite. 2. The temperature for contact metasomatism may possibly ranging from 400 to 800⁰ C, or even higher. 3. The composition, size, shape, form and depth of formation of the intrusive body play important role in formation of contact metasomatism.
- 3. • Important sources of ore deposit are Fe, Cu, Zn, W, and other metals. • Barytes occurs in limestone of vempalle formation (cuddaph), Andhra pradesh.
- 4. Hydrothermal deposit • Hydrothermal mineral deposits are those in which hot, mineral laden water (hydrothermal solution) serves as a concentrating, transporting, and depositing agent. • The solutions are thought to arise in most cases from the action of deeply circulating water heated by magma. Other sources of heating that may be involved include energy released by radioactive decay or by faulting of the Earth's crust. • Conditions necessary for the formation of hydrothermal ore deposits include: a) Presence of hot water to dissolve and transport minerals b) Presence of interconnected openings in the rock to allow the solutions to move c) Availability of sites for the deposits, and d) Chemical reaction that will result in deposition
- 5. Hydrothermal Deposits Hot water and gases circulate through fractures in crust Metal ions leached from rock at depth are concentrated and redeposited Gold, zinc, lead, copper N. Lindsley-Griffin, 1998 Woods Hole Oceanographic Institution Hot water and sulfide particles issuing from a black smoker, East Pacific Rise Sulfide minerals deposited here Mineral Deposits
- 6. Hydrothermal deposits in ophiolites (on-land fragments of ocean lithosphere) Veins are deposited along fractures in basalts of oceanic crust - Divergent margins, oceanic rift valleys Ores are transported by subduction and plate movement, emplaced on land by terrane accretion in ophiolites - Convergent margins, active continental margins Mineral Deposits Houghton Mifflin, Dolgoff, 1998; N. Lindsley-Griffin, 1999
- 7. Hydrothermal ore deposits are forming today in the Imperial Valley of California - a graben formed by rifting along the northern end of the East Pacific Rise which runs up Gulf of CA. Metallic ions are leached from the sediments under the graben by hot fluids resulting from volcanism. Hot brines deposit siliceous scale containing 20% copper and 8% silver on the insides of pipes in drilled wells. Fig. B16.1, p. 494 Mineral Deposits N. Lindsley-Griffin, 1999
- 8. Hydrothermal deposits forming today in the Red Sea: Brines remain pooled in the deep graben because they are denser than sea water. This hydrothermal deposit is called a stratabound deposit, because the minerals are precipitated as layers interbedded with sediments. Mineral Deposits N. Lindsley-Griffin, 1999 Fig. B16.2, p. 495
- 9. Metamorphic deposit • Metamorphic processes profoundly alter pre-existing mineral deposits and form new ones. • The chief agencies involved are heat, pressure, time, and various solutions. • The materials acted upon are either earlier formed mineral deposits or rocks. • Valuable nonmetallic mineral deposits are formed from rocks chiefly by the crystallization and the combination of rock making minerals. • Several kinds of nonmetallic mineral deposits are formed as a result of regional metamorphism. • The source materials are rock constitutions that have undergone recrystallization or re-combination, or both. • Rarely, water or carbon dioxide has been added, but other new constitutions are not introduced as they are in contact metasomatism deposits.
- 11. • The enclosing rocks are wholly or in part metamorphosed; it is the rock metamorphism that has given rise to the deposits. • The chief deposits thus formed are asbestos, graphite, talc, soapstone, andalusite-kyanite-sillimanite, dumortieritea, garnet, and possibly some emery.
- 12. Sublimation • Process by which solid material passes into gaseous state without first becoming liquid. • Sublimation is a process of mineral deposits associated with volcanism, thermal springs and fumaroles where volatilised matter is redeposited at lower temperature and pressure. • Sulphur and Borax of Puga area, Ladakh are examples of such deposit. • They are associated with thermal springs and fumaroles.
- 13. Surfacial oxidation and supergene enrichment • When ore deposits are exposed to the oxidation zone they are weathered and altered with the country rocks. • The surface waters oxidize many ore minerals and yield solvents that dissolve other minerals. • An orebody thus becomes oxidized and generally leached of many of its valuable materials down to the groundwater table, or to depth where oxidation cannot take place. • The effects oxidation may, however, extend far below the one of oxidation. • As the cold, dilute, leaching solutions trickle downwards, they may lose a part or all of their metallic content within the zone of oxidation to give rise to oxidized ore deposits. • The oxidized or near-surface part of an orebody is made colorful due to the oxidation of sulfides to oxides and sulfates.
- 14. Contd……… • As the down trickling solutions penetrate the water table, their metallic content may be precipitated in the form of secondary sulfides to give rise to a zone of secondary or supergene sulfide enrichment. • The lower, unaffected part of the ore body is called the hypogene zone. • In some places the supergene zone is absent and in rare cases the oxidized zone may be shallow or lacking (as in some glaciated areas undergoing rapid erosion). • Special conditions of time, climate, physiographic development and amenable ores are necessary for the process of oxidation and supergene enrichment to be effective. • Such ores occur in most of the non- glaciated land areas of the world.
- 15. Deposits in the zone of oxidation When the oxidised zone is well developed and the secondary minerals sufficiently concentrated, it is a highly profitable zone to mine as the processing is much cheaper and easier and the metals more concentrated. However, most oxidised zones have been mined because they formed outcrops of easily identifiable gossans. The most common minerals found in oxidised zones are: • Copper: malachite, azurite, chrysocolla • Gangue minerals: quartz (usually cryptocrystalline), baryte, calcite, aragonite • Iron: goethite, hematite • Lead: anglesite, cerussite • Manganese: pyrolusite, romanechite, rhodochrosite • Nickel: gaspeite, garnierite • Silver: native silver, chlorargyrite • Zinc: smithsonite
- 16. Deposits in the zone of supergene enrichment In the supergene zone metals are concntrated in a narrow band just below the water table. This is the richest part of an ore deposit but in many instances, is either only very thin or not developed at all. The most common minerals found in supergene zones are: • Copper: chalcocite, bornite • Lead: supergene galena • Nickel: violarite • Silver: acanthite, native silver • Zinc: supergene sphalerite, wurtzite
- 17. Sedimentary deposits form by evaporation and precipitation Anhydrite, gypsum, halite Evaporite Deposits at Bonneville Salt Flats, Utah N. Lindsley-Griffin, 1998 Mineral Deposits
- 18. Sedimentary deposits form by biochemical reactions in seawater Banded iron formations were precipitated by biochemical reactions in a low-oxygen atmosphere during the Precambrian N. Lindsley-Griffin, 1998 Mineral Deposits Banded Iron Deposit, Lake Superior
- 19. Mechanical Concentration Placer deposits: Heavy grains sorted by currents Deposited in rivers or beaches Previously weathered from bedrock source Gold, platinum, diamonds, chromite, Zirconium and Titanium Olivine beach placers, South Point, Hawaii N. Lindsley-Griffin, 1998 Mineral Deposits
- 20. Placers are deposited: Behind rock bars In rock holes Below waterfalls In point bars inside meander loops Downstream from a tributary Along beaches and behind undulations on the ocean floor. Mineral Deposits N. Lindsley-Griffin, 1999
- 21. Residual mineral deposits form by chemical weathering Soluble minerals are leached - dissolved by rain water and carried downward by infiltration, leaving behind less soluble minerals. Laterites are mined for iron and sometimes nickel. Mineral Deposits N. Lindsley-Griffin, 1999 Iron ore, Australia
- 22. Residual mineral deposits Bauxite is the main source of aluminum ore - found in laterites formed in tropical climates. Mineral Deposits N. Lindsley-Griffin, 1999 Bauxite (aluminum ore) Weipa, Australia Fig. 16.26, p. 499