SEDIMENTATION.ppt
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The document discusses the formation and types of placer deposits, primarily focusing on the Witwatersrand gold deposits in South Africa, which yield significant amounts of gold and are classified as fossil placers. It outlines the geological processes involved in placer deposit formation, sedimentation patterns, and the associated resource extraction methods, particularly for gold, uranium, and various minerals such as titanium and zirconium. Additionally, it highlights the historical context of mining in the Witwatersrand region and the complexities of sedimentary geology and tectonics affecting these deposits.
SEDIMENTATION.ppt
- 1. Ore Deposit Formed By Clastic Sedimentation
- 2. INTRODUCTION Weathering: In general weathering processes acting upon rocks contributes to the overall mineral resource may gives in five ways: Produce new minerals in site. Cause the redistribution of valuable elements. Released fine debris and chemical complexes. Released hard rocks into softer or useable rocks. Released of resistant minerals like gold.
- 3. TYPES OF PLACER DEPOSITS • Eluvial - Formal site of destruction • Deluvial - Formal down hill • Proluvial - Accumulation formal foot of a slope • Alluvial - Placer carried out by rivers of streams • Laterial of beach - Formed along the shores of lakes seas, etc. • Aeolian - By action of wind • Glacial - By activities of glacier • Fossils placer/paleo placer - Those placer deposits buried under younger rocks.
- 4. PLACER DEPOSITS Hails in 1976 he defined placers as the surficial mineral deposits formed by mechanical concentration, commonly by allurial but also by marine, aeoline, etc.of heavy minerals particles such as gold from weathered debris.
- 5. WITWATERSTRAND GOLD AND U DEPOSITS IN REPLUBLIC SOUTH OF AFRICA District lies in South Africa between Johannesburg and Welkom shown in fig. 1
- 6. • “The Rand” is well known because of gold deposits. • Yield 35 millions Kg of gold value at nearly $60 billion • It is a fossils placer types of deposits. • Most of the debris were transported from North, North West and South-West. • Viljoen Saagar in 1970 showed that types of sedimentations. • Later Kappel and Saager in 1974 studies of Isotypes • Presence of isotopes contribute debris to the witwatersrand basins.
- 7. STATIGRAPHY: The basemsent is Archean precambrian rocks The basement was unwrapped to steeply diapiric domes represented by B in Fig 2
- 8. • It is made up of granite, granitic gneiss and greenstone Terrances. • The domain group lie next to the basement. • Made up of district of quartz carbonate gold veins and younger potassic granite bodies with accessories minerals, uraninite. • Witwatersrand super group overlie unconformably to basement and domain group • Made up of sequence of thin conglomerate and thick lava flows • It’s divided into two – lower and upper unit or west rand group and central rand group. • Lower west rand group of witwatersrand consist of shales, quarzites, grits and conglomerate.
- 9. • Upper younger central rand group is 90% of quarizite grits and rare shales. • More gold bearing conglomerate. • Tilting and erosion of Witwatersrand supergroung and followed by igneous activity formed ventersdrop supergroup. • Erosion and subsidence ventersdrop formed Transvaals supergroup. • It is made up of thick sequence of clastic and dolomite limestones. • Bushveld age of igneous activities marked the end of transvaal supergroup. • Lastly, Karroo Supergroup was formed.
- 11. TECTONIC SETTING • Large synclinorium of 400 km long and 150 km wide elongated north Easternly direction. • Frame of synclinorium was formed as a part of complex fabric of regional and distinct scale interference folding. • Folding produce domes. • Coincide of of anticlinal axes and intersected with synclinal axes. • Broad witwatersrand synclinal basin filled by shallow broad intracratonic freshwater lake. • Bottom accept the witwatersrand sediments. • Synchlinorium is complicated. • Several mine areas has abandoned.
- 13. SEDIMENTATION • Goldfield constitute only a small fraction of Witwatersrand sedimentations. • Goldfield develop on Rand. • The Rand fan shaped auriferous conglomerate systems interleaved into the arenite thickness of the supergroup. • These are six major goldfield mining areas in the district. • Each presenting a wedge of sediment entry into the basin. • Central Rand group consist series of sedimentation cycles that range in thickness from 30 to 600 meters averaging 250 meters. • Defined by basal unconformities mark by lag gravels • It composed of major sand, minor conglomerate and include siltstones, sandstones, etc.
- 14. In Witwatersrand gold and Uranium placers are not extracted occurs in at least four geologic sites in the goldfields as given below along with figure 5.
- 15. • 1. In the fan head – In the sandy near the base of fluvial pebbles supported conglomerates was filled by sand sized heavy minerals grains from the sediments flux. • 2. In mid-fan – In pyrite trough cross bedded sands in erosiion deposition with Golds, Uraninite or foresets, in bottom set spoons and sours. • 3. In upper-fan base – in sheets of cross bedded sands by winowing of quartz grains, leaving thin layers of heavy minerals as lag deposits. • 4. In the fan base – In carbonaceous layers on unconformeties in scour pools and in algal mats which acted both as mechanical riffle trops and as chemical trops.
- 16. • The ore bearing conglomerate strata in the Witwatersrand area are know as reefs or bankets. • The strata are not contineous as in a layer cake, but fans are qualitatively similar • The origin of the Witwatersrand conglomrate and quartzite was based on gold origin. • During depositions several environments have been included, marines, shorelines, large ddeltas enclosed basins, etc. • The Witwatersrand rocks have been mildly metamorphosed to greenschist faciest. • Except presence of quartz pebbles dynamics sedimentations. • Stretched during metamorphism. • Sphericity and roundnes has been retained • Space between pebbles are occupied by locally abundant pyrite, zircon, rutile, etc.
- 17. SOME OF THE ORE DEPOSITS ARE: Silica"Sand & Gravel" Sedimentary deposits are formed through the erosion, transportation, and redeposition of minerals that can survive the rigors of transportation. The most common is silica, which forms a number of materials, including silica sand, sand and gravel, and flint. The precursor is igneous quartz (e.g., in granite), and then the sedimentary deposit may undergo metamorphism and recementing to produce quartzite. Sand and gravel for construction use is extremely common, and production is more dependent on local markets than availability
- 18. Examples include the midwestern United States; Badgeley Island, Ontario, Canada; Cheshire in northwest England; certain areas of Belgium and the Netherlands; Cape Flattery Island, Queensland, Australia; and Sarawak, Malaysia. In many cases, the use of local sand is based on price rather than quality. The United States is the largest producer of industrial sand, accounting for more than one quarter of world production. Production of flint is much more restricted, based largely on the chalk deposits of southern England and northern France. . Certain areas are, however, noted for producing industrial sand that is sufficiently pure to be used in the manufacture of glass, ceramics, sodium silicate, and the like.
- 20. Clays: Several clays composed mainly of kaolinite are of sedimentary origin. Premier deposits of ball clay, the carbon content of which indicates that it was deposited in swampy conditions, occur in the Kentucky–Tennessee area of the United States, Devon in southwest England, and the Czech Republic. Flint clay, as produced commercially in the United States, China, Australia, and Argentina, is generally derived from the weathering of soil and deposition in shallow basins. Fire clay or refractory kaolin is a kaolinite material common in many parts of the world, particularly in association with coal deposits.
- 21. A 400-km belt of kaolinite-rich rocks extends from Aiken, South Carolina, to Eufaula, Alabama, and includes areas supplying high- and medium-quality kaolin and refractory kaolin. Another belt of kaolin, bauxite, and bauxitic and kaolinitic clays extends from western Tennessee into northeastern Mississippi. Other areas include southwest England and over the English Channel into France (kaolin and ball clay); various parts of the Czech Republic (kaolin and ball clay); Spain; the Amazon Basin in Brazil (bauxite, kaolin); Japan (kaolin, refractory clay, roseki, and toseki); and Queensland, Australia (bauxite, kaolin).
- 22. Volcanic ash deposited as part of a sedimentary sequence eventually forms sodium or calcium bentonite. Important bentonite deposits occur in the United States in the Wyoming–Montana region (sodium-based bentonite) and in the Mississippi– Texas region (calcium-based). Almost 40% of the world’s bentonite production is from these and some smaller deposits in the United States. More modest tonnages are produced in Mexico and Canada. In Europe, bentonite is mined on Milos Island in Greece, Turkey, Sardinia in Italy, Bavaria in Germany, southwestern England, Ukraine, and Spain.
- 24. Titanium: Titanium is found in many minerals. Ilmenite (FeTiO3) and rutile (TiO2) are the most important sources of titanium. Ilmenite provides about 90% of the titanium used every year. It is estimated that the resources of ilmenite in the world contain 1 billion tons of titanium dioxide. The estimated resources of rutile in the world contain about 230 million tons of titanium dioxide. Rutile and ilmenite are extracted from sands that may contain only a few percent by weight of these minerals. After the valuable minerals are separated, the remaining sands are returned to the deposit and the land recultivated.
- 25. In the United States, titanium-rich sands are mined in Florida and Virginia. Even though the United States mines and processes titanium and titanium dioxide, it still imports significant amounts of both. Metallic titanium is imported from Russia (36%), Japan (36%), Kazakhstan (25%), and other nations (3%). TiO2 pigment for paint is imported from Canada (33%), Germany (12%), France (8%), Spain (6%), and other nations (36%). Most titanium is used in its oxide form. TiO2 is a white pigment used in paint, varnishes and lacquers (49%), plastics (25%), paper (16%), and other products such as fabrics, printing inks, roofing granules, and special coated fabrics.
- 26. Titanium
- 27. Zirconium Minerals (Rare Earths): Placer and palaeo-placer mineral deposits are important sources of heavy minerals such as ilmenite, rutile, and zircon. The rare earth sources monazite and xenotime are invariably associated with the mineral sands deposits. Many titanium/zirconium/rare earth mineral deposits are Tertiary and Quaternary in age because this was a period of geological uplift that provided the correct conditions for accumulation, plus the fact that older examples have been destroyed.
- 28. Most placer deposits are in marine sand deposits along or near present coastlines, where they are concentrated by a combination of tidal action, longshore currents, waves, winds, and natural traps such as a cape. Most commercial placer deposits are recent beaches and dunes along coastlines, with some older deposits being stranded by land elevation or ocean withdrawal. Important areas include the east and west coasts of Australia, parts of Florida and Georgia in the southeastern United States, around Richards Bay in South Africa, Sierra Leone in Africa, the coastal areas of Tamil Nadu and Kerala states in southern India extending into eastern Sri Lanka, and the coastal areas of Brazil.
- 30. CONCLUSION • Another aspect of gold and uranium occurrence in Rand sediment concerns local post depositional re-distribution of those elements especially Uranium. Near the end of World War second, when interest in fissionable materials was gaining momentum, it was discovered that the frequently mentioned ‘Carbon’ of the Rand ores was actually a Uraninite – Carbon agregate earlier, but improperly called Thucholite (Davidson and Bowie 1951). Mineraloid whose very name describe its Thorium, Uranium, Carbon, Hydrogen and Oxygen composition.
- 31. • Serious mining in the Witwatersrand district began in 1886 when the ‘Gold Rush’ started this activity. Since that time, many mines have been developed along the 400kms of strike of the Witwatersrand conglomerate, Beds on the northernly and southwesternly sites of the Witwatersrand basin. Several mines are operating at depts of more than 300kms. The east Rand property mine operates at the dept of about 3.5kms. The western deeps mine near Carletonville, the deepest in the world, has reached nearly 4Kms. Even deeper mines are planned to reach further down along ore bearing strata, descent now locally more than 30Kms downdip.
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