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KIMBERLITES

KIMBERLITES. Kimberley (South Africa ). Lecture contents: What are kimberlites – why are they important Where are they emplaced and when Structural model of a kimberlite: Facies Petrology of kimberlites: - Peridotite solidi - Petrologic classification & relationships with other magmas

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KIMBERLITES

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  1. KIMBERLITES Kimberley (South Africa)

  2. Lecture contents: What are kimberlites – why are they important Where are they emplaced and when Structural model of a kimberlite: Facies Petrology of kimberlites: - Peridotite solidi - Petrologic classification & relationships with other magmas Geodynamical aspects: - Thermal character of Eons - Tectonic setting of kimberlite-bearing areas 6. Kimberlites as natural mantle samplers 7. Kimberlite prospection 8. Kimberlite mining 9. Market of diamonds References.

  3. Part 1

  4. 1. What are kimberlites – why are they important: Kimberlites are a very rare type of magma with extremely deep origin (>150 km). Their key characters are high K, Mg and fluids (CO2) contents. Some kimberlites originate in diamondiferous part of the Earth’s mantle and carry diamonds to the Earth’s surface, which make these rocks economically important. Despite a large interest, kimberlites are still poorly understood rocks due to their ambiguous composition and difficulties in modeling their source and forming conditions following standard geological methodologies.

  5. 1. Kimberlites are the only source of diamonds excepted another unusual alkaline magma suite: Lamproites Lamproites: potassic-rich mafic to ultramafic alkaline rocks characterized by the presence of Ti-bearing minerals like Ti-phlogopite, K-Ti-richterite, wadeite (K, Zr silicate)

  6. 1. Chemical and mineralogical composition: -Ultramafic magmatic rocks (SiO2 25-35 wt%, MgO up to 38 wt%) - K2O/Na2O = 2 K2O up to 2 wt% (highly alkaline) - CO2 + H2O  8-15 wt% (high fluid content) - Inequigranular texture - textural components: • Macro- & Megacrystals • Fine matrix • Peridotite and Eclogite • xenoliths •  Diamonds

  7. 1. Chemical and mineralogical composition: 1, 2, 3, Lamp.: Mitchell, 1986, lamproite: Leucite Hills, USA MORB: Cmiral et al., 1998 *recalculated to 100% volatile free

  8. 1. Crystals: * Often replaced by serpentine and calcite

  9. 1. Fine matrix:

  10. Part 2

  11. 2. Where are kimberlites: 10 1. Navajo-Hopi 2. Brasile 3. West Africa 4. Angola 5. Tanzania 6. Namibia 7. South Africa 8. Yacutia, 9. Australia, 10. NWT - Canada (da A. Gregnanin – personal communication)

  12. 2. When were kimberlites formed: October 1869: first diamonds found at Bultfontein and Dorsfontein farms (“Pans”, South Africa) July 1870: more gems found at Koffiefontein and Jagersfontein. Opening of mines and birth of ‘Kimberly’

  13. Part 3

  14. 3. Structural model of a kimberlite: Facies: Mitchell, 1986 a) Crater facies b) Diatreme or pipe c) Hypabyssal facies

  15. 3. Crater facies • Laves (Igwisi Hill, Tanzania) • Pyroclasts(highly erodible; Tanzania, Botswana) • Volcanoclasts (massive process; Tanzania, Botswana) • Sedimentary volcanogenic deposit (terrigenuos fraction + volcanic fraction, inside natural lows; Yakutia, Russia)

  16. 3. Diatreme facies Diatreme: conic shapes thinning downwards, composed of angular and rounded clasts (maybe xenoliths) with or without matrix • Variable morphology • Lithics, minerals, matrix • Origin not clear: • a) Hydrovolcanism • b) Volatile condensation and rapid quenching • - pelletal lapilli and autoliths*

  17. 3. Hypabyssal facies • Located at diatreme roots: dikes and sills • medium-coarse grain size, homogeneous or with evidences of • segregation processes (gas condensation, immiscibility) • presence of globular segregation features (up to 10 cm) • giving the aspect of a conglomerate

  18. 3. Theoretical sketch of a kimberlite Crater facies  Diatreme Hypabyssalfacies

  19. 3. Realistic sketch of a kimberlite KCF:Crater facies K1, K2:Diatreme facies

  20. Part 4

  21. 4. Petrology of kimberlites: Kimberlitic melt: generated by low degrees of fusion of a phlogopite bearing carbonated-peridotite, at the right T, P and volatile species fugacity (essentially H2O & CO2). Peridotite mineralogy: Olivine (>60%), Clynopyroxene, Orthopyroxene and one aluminum phase (for growing depths plagioclase, spinel or garnet) Peridotite chemistry: MgO (~ 40%), SiO2 (~ 45%), Al2O3, CaO, FeO all of them < 10 wt%, Na2O e K2O almost negligibile.

  22. 4. Meltingofmantle peridotite: Peridotite ‘solidus’ intersects the ’geotherm’ . 4 main types of solidus : • 1) “Anhydrous”: • 2) “Water-bearing” (> 0,3-0,5 wt% H2O); • 3) “CO2-bearing” (> 5 wt% CO2); • 4) “Fluid-absent amphibole-bearing” (water • vapour present only during first melting) ~45 km

  23. 4. Meltingofmantle peridotite: Source rock: Dol-Phl-bearing peridotite P > 30 Kbar: alkaline picrites P ~ 55 Kbar: kimberlites (low degrees of melting) shield

  24. 4. What conditions are necessary to yield a kimberlite? • Type of ‘solidus’: H2O & CO2 present but in small amounts • 2) Phlogopite-bearing peridotite (Potassium enrichment) “METASOMATIC MANTLE” • The generation of this type of source-rock is thought to be bond with upwelling of mantle-plumes highly enriched in fluids and incompatible elements • (e.g. potassium)

  25. 4. 3) Low degrees of partial melting at very high P, ≥ 40 kbar (below continental shields); This set of conditions could be realized only below a craton or a ‘mobile belt’ So there are 2 types of kimberlites: - On-craton: located in the middle regions of cratonic areas - Off-craton: placed at the boundaries between cratons, on mobile belt terrains (withouth diamond) ;

  26. 4. • From these sketches it is evident that the solidus of interest intersects the cratonic geotherm (cold geotherm) where diamond is the stable carbon polymorph, whereas the stable phase would be graphite when below a mobile belt (hot geotherm). [ Kirkley, M., in ‘The Nature of Diamonds’, 1998 ]

  27. 4. Classificationof kimberlites: Kimberlites might be divided in three different types based on the geodynamic context of their emplacement. Evidence is found in the type/s of xenoliths occurring in kimberlites. K1: no diamond K2: eclogitic diamond K3: peridotite-type macro-diamonds 

  28. Part 5

  29. 5. Geodynamical aspects: Thermal character of Eons Archean: microplate tectonic Proterozoic: intraplate tectonic Phanerozoic: plate tectonic

  30. 5. Thermal character of Eons Komatiites Brittle regime and oceanization Diapirism and underplating (ipersolidus) Ductile flow (subsolidus) Intermediate depth crust average temperature variation with geologic time (Wynne & Edward, 1976) Achaean heat-flux probably 4-5 times larger than present. Vigorous mantle convection

  31. 5. Tectonic setting of kimberlite-bearing areas: The following models have been proposed to describe possible emplacement settings for kimberlite magmas. 1. Lithospheric faults: Lithosphere is crossed by a limited number of deep faults extending towards the upper mantle kimberlites would rise through these fractures, which represents permanent upwelling channels 2. Extension of transform faults: Their localization would be determined by pre-existing continental fractures as leftover of transform faults on continents kimberlites would rise through these fractures;

  32. 5. Tectonic setting of kimberlite-bearing areas: 3. Hot-spot magmatism: Hot-spot below continental lithosphere with following thinning and rifting kimberlites are emplaced before rifting; 4. Subduction related magmatism: For mature subduction, melting of oceanic material with production of peridotite and eclogite restites. These leftovers would then melt at larger depths to generate kimberlites evidences of kimberlites located parallel to fold belts.

  33. 5. Tectonic setting of kimberlite-bearing areas: 5. Nucleus model: V.M. MoraleveM.Z.Glukhovsky, 2000 This model try to explain the genesis of crustal diamonds, by assuming the existence of hyper-pressure zones in the Archean (diamonds age: 4-2 Ga). Complex model. Kimberlite upwelling spots are located in the central portions of these hyper-pressures areas.

  34. Part 6

  35. 6. Kimberlites as natural mantle samplers: Kimberlite magmas carry xenoliths of peridotite and/or eclogite rocks original of the deepest upper-mantle (maybe even lower-mantle) otherwise not accessible to human investigation. An interesting case study is the reconstruction of the paleo-geotherm of the Slave Craton in Canada. Analyses of eclogite and peridotite xenoliths of the Jericho kimberlite (172 Ma) and high-T-P experiments, aimed to reproduce the phase assemblages of those xenoliths, lead to the reconstruction of the P-T curve below the Slave Craton at the time of Jericho Kimberlite empla-cement (Kopylova et al., 1998). The “stratigraphy” of the paleo-mantle of the craton was also prepared.

  36. 6. Kimberlites as natural mantle samplers: Moreover diamonds often contain inclusion-minerals which are also a source of precious information to reconstruct the composition and the conditions of the mantle below continents at the time of their emplacement. - Baddleyte inclusions in diamond (ZrO2): Compositional analyses on these inclusions from diamonds of the Mbuji Mayi kimberlite (Congo) indicate provenience from the lower mantle. - Mg-wustite (Mg,Fe)O inclusions: Often found in macro-diamonds, they also require a lower-mantle origin

  37. Part 7

  38. 7. Kimberlites ore prospection: • In different stages of the ore prospection (strategic or tactic) the following methodologies are employed: • Airborne and land magnetometry: magnetic anomalies due to the presence of ferromagnetic material (ilmenite) are mapped. Those anomalies could be positive (e.g. South Africa), or negative (e.g. Australia). • Airborne and land electromagnets: anomalies due to the presence of conductive material at the surface of kimberlite terrains. • Airborne and land gravimetry: crater and diatreme shows negative gravitational anomalies; • Radiometric and spectrometric methods: negative and/or positive anomalies due to presence of U, Th and K within respect to the background terrain.

  39. 7. Kimberlites ore prospection: • Remote sensing; • Chemical analyses of pathfinder or index minerals: Garnet, Ilmenite, • Clinopyroxene, olivine, zircon • Blue and yellow ground: field observation! Ternary diagram where the peculiar composition of ilmenites from diamond bearing kimberlites is highlighted (Finger, 1972) Ni vs Cr plot of fertile-kimberlite-ilmentes: Ni/Cr ratio spans between 2 and 15.

  40. 7. Kimberlites ore prospection: INDEX MINERALS: Garnet: Unusually low in Ca, high Mg, Cr; referred to as G10. Ilmenite: Mg-bearing ilmenite. Diopside: Emerald-green, high-Cr-diopside These minerals are usually found in re-deposited volcanoclastic sediments in rivers, downstream within respect of the actual location of the diamondiferous pipe. A case study is the reconstruction of the “migration” path of microdiamonds and index minerals found in glacial (till) sediments in Northwest Territories, leading to the discovery of the Ekati kimberlite.

  41. 7. Kimberlites ore prospection: New type of prospection: Analyses of natural and induced resorption or/and etching features on natural diamond crystal to deduce fertility of kimberlite pipes. Operated with SEM and in a close future also with AFM (Atomic Force Microscopy). Prof. Yana Fedortchouck (Dalhousie University) is the world leader for these types of investigation. More info from me and at: http://earthsciences.dal.ca/people/fedortchouk/fedortchouk_y.html

  42. Part 8

  43. 8. Kimberlites ore mining: Several approaches depending on topography and mining stage: -Open pit: up to a certain (safe) depth Yacutian Open Pit, Russia FinschDiamond Mine, South Africa

  44. 8. Kimberlites ore mining: -SublevelCaving(used in Kimberley) Open pit exhaust • Sub-levels net • Actual Mining

  45. Part 9

  46. Market of diamonds: 9. Two main uses of diamond crystals: • Industry: High quality abrasive, cut-tools, drilling tools, digging (oil perforation), laser optic element, diamond windows, supercomputers, electronic devices, atom smasher (LHC, Geneva), flat panel displays. • Jewellery: most priced gem. There exists exceptionally large stones. Gem size is expressed in carats (1 carat = 0.2 grams). The “Cullinan” diamond was a 3’016 carats. From it many gems were cut, among those the Cullinan I, 516 ct.

  47. 9. Diamond Anvil Cells: used in experimental petrology Record pressure 4.6 Mbar ! Our lab: HDAC very important !!!

  48. 9. The Diamond: Crystallographic and mineralogical characteristics: - Color: All, mainly colorless; - Crystallographic system: Cubic; - Formula: C; - Hardness: 10 (Mohs), 8’000 (Knoop) - Density: 3,52; - Opacity: Transparent; - Fluorescence: yellow, green, pink, often also blue (useful to distinguish from synthetic crystals);

  49. 9. More diamonds:

  50. Diamonds resources:in millions of carats 9. Data from: Metals & Minerals Annual (1996) More recent data: Canada: Ekati ~ 4 Mct/y Diavik ~ 8 Mct/y Jericho ~ 0.4 Mct/y Total Canada: ~ 12.4 Mct/y

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