Ore deposits related to mafic igneous rocks Diamonds - GLY 361 Lecture 3

Transcription

1 Ore deposits related to mafic igneous rocks Diamonds - GLY 361 Lecture 3

2 A short history of diamonds Derived from the ancient Greek αδάμας (adámas): unbreakable Thought to have been first recognized and mined in India for at least 3,000 years but most likely 6,000 years. In 1772, Antoine Lavoisier showed that the only product of the combustion was CO 2, proving that diamond is composed of carbon. In 1869 the first diamond was found in kimberlite in Kimberley, South Africa. In 1870 South Africa is main supplier of diamonds.

3 A short history of diamonds Largest ever rough diamond: Cullinan diamond 3106 ct was found in 1905 in SA was cut into 105 pieces Largest ever faceted diamond: Golden Jubilee diamond 545 ct was found in 1985 in SA

4 World diamond production 1867 to 2012 Worldwide production including Africa 2.92 bn carats (580 tonnes) Africa s contribution 2.02 bn carats 69% 31% Share of World Rough Diamond Production (Carat Basis), 2012 Rest of the world Africa

5 Worldwide diamond deposits 90% of the world s diamonds have been extracted from placer deposits.

6 Diamond deposits

7 Classification of diamond deposits Primary deposits diamonds associated with mantle derived igneous rocks (predominantly kimberlites, less commonly other rock-types, e.g., lamprophyres). Secondary/ Placer deposits: Alluvial deposits - diamonds eroded from the primary source and deposited in sediments related to present or ancient rivers. Marine deposits diamonds introduced into the marine environment by river systems where they are redistributed by longshore ocean currents.

8 Kimberlites

9 The Big Hole, Kimberley the type locality for kimberlites

10 Kimberlite an early definition Pipes or dikes of mafic igneous rock containing mantle phases, commonly including diamond. However, this definition also included rocks such as carbonatites, peridotites, etc.

11 Kimberlite new definition Potassic ultramafic igneous rock forming small volcanic pipes, dikes and sills. Volatile-rich (H 2 O, CO 2 ) Dominated by olivine with subordinate minerals of mantle derivation Characterized by the dominance of: Monticellite (calcium olivine, Ca 2 SiO4) Phlogopite (magnesium biotite) Diopside (Ca-Mg clinopyroxene) Serpentine ((Mg, Fe) 3 Si 2 O 5 (OH) 4 ) or calcite minor amounts of apatite, magnetite, chromite, garnet, diamond, and other high-pressure, high-temperature upper mantle minerals.

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13 Kimberlite classification 1. Phlogopite kimberlite (44 67 wt.% phlogopite) 2. Serpentine-phlogopite kimberlite (25 35 wt.% serpentine, wt.% phlogopite) 3. Serpentine-phlogopite-calcite kimberlite (11 19 wt.% serpentine, wt.% calcite, 1 wt.% phlogopite) 4. Phlogopite-diopside kimberlite (22 34 wt.% phlogopite, wt.% diopside) 5. Monticellite-phlogopite kimberlite (18 31 wt.% monticellite, wt.% phlogopite) 6. Calcite kimberlite (29 50 wt.% calcite) 7. Diopside kimberlite (27 54 wt.% diopside) 8. Opaque mineral-rich calcite kimberlite (20 33 wt.% opaques, wt.% calcite)

14 Kimberlites Diamonds are very rare constituents in kimberlites ores are probably lowest grade mined anywhere. Profitable mines in RSA contain: 10 carats per 100 tons ore (1 carat, ct. = 0.2 g) = 0.02 ppm diamond in kimberlite = wt.% diamond

15 Kimberlites hybrid rocks consisting of crystals/ rock fragments derived from three discrete sources: the Earth s mantle, the megacryst or discrete nodule suite, primary phases that crystallized from the kimberlite magma. The contribution from these three sources is highly variable.

16 Kimberlites mantle xenoliths Inclusions of upper mantle rock xenoliths indicating deep derivation of these rocks: Garnet lherzolite Eclogite Harzburgite Diamond

17 Kimberlites the megacryst or discrete nodule suite Large (1-20 cm) single crystals or megacrysts of: Magnesian ilmenite Cr-poor titanian pyrope Diopside (commonly sub-calcic) Enstatite Phlogopite Zircon Different composition than the peridoditic magma: upward moving perioditic magma picks up small pockets of other magma (pegmatitic veins and apophyses surrounding the kimberlite magma at depth). therefore, the kimberlite emplaced in the crust may represent a mixed or composite magma.

18 Primary phases Subhedral to euhedral phenocrysts, micro-phenocrysts, and minerals crystallized in situ to form the kimberlite groundmass are considered to represent primary phases. These include olivine, phlogopite and chromite phenocrysts and groundmass olivine, phlogopite, Ti-bearing spinels, perovskite, ilmenite, diopside, monticellite apatite, calcite and serpentine. The abundance of these minerals is also variable and depends on the bulk composition of the magma.

19 The formation of kimberlites Carrot-shaped, vertical intrusions termed 'pipes'. Classic carrot shape is formed due to complex intrusive process of kimberlitic magma Large proportion of both CO 2 and H 2 O in the system: produces a deep explosive boiling stage causing vertical flaring (Bergman, 1987).

20 The formation of kimberlites 1. Basement 2. Magma 3. Groundwater 4. Explosion chamber 5. Fractures in the wallrock 6. Tuff ring 7. Pyroclastic flow 8. Eruption column

21 The formation of kimberlites Ukinrek maar, Alaska

22 The formation of kimberlites Eifel maars, Germany Shiprock, USA

23 crater diatreme hypabyssal modified Mitchell, 1986

24 The Root Zone irregular morphology (fracture and joint control) contact breccias stoped contact thermal metamorphism complex geology: hypabyssal kimberlite hypabyssal kimberlite breccias Segregation textures (interpreted as gas-rich, closed system) gradational transition to diatreme zone modified Mitchell, 1986

25 , South Africa

26 Dutoitspan Mine Field, unpubl.

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28 The Root Zone hypabyssal facies Hypabyssal Macrocrystic (poor high) Segregation textures Porphyritic Hypabyssal contact breccia >15 vol.% xenoliths

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30 The Diatreme Zone regular morphology steep, smooth contacts country rock unaltered simple internal geology homogenous, mixed appearance floating blocks massive, no sorting, bedding or tractional structures

31 The Diatreme facies Tuffisitic Kimberlite (TK) Pelletal TK Lithic TK Crystal TK Tuffisitic Kimberlite Breccia (TKB) >15 vol.% fragments < 4 mm

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33 The Crater Zone gradational contact with diatreme zone. extreme variation in grain sizes possible. may be several hundreds of meters thick. syn-eruptive filling or gradual post-eruptive sedimentary filling. primary pyroclastic layers (fall and surge) and resedimentation from the crater walls. often playa and lake beds in centre. complex geology and diamond distribution. sorting processes, i.e. diamond concentration or dilution.

34 Mt. Rouse, Victoria Stiefenhofer

35 Clement & Skinner, 1985