KIMBERLITES
Kimberley (South Africa)
Lecture contents:
1.
2.
3.
4.
5.
6.
7.
8.
9.
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
Kimberlites as natural mantle samplers
Kimberlite prospection
Kimberlite mining
Market of diamonds
References.
What are kimberlites – why are they important:
1.
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.
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)
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
1.
1.
Chemical and mineralogical composition:
1
2
3
Lamp.
MORB
SiO2
35.2
31.1
33.21
52.7
51.45
TiO2
2.32
2.03
1.97
2.4
2.50
Al2O3
4.4
4.9
4.45
10.8
14.36
Fe2O3
-
-
6.78
FeO
9.8
10.5
3.43
5.1
5.50
MgO
27.9
23.9
22.78
8.4
11.06
CaO
7.6
10.6
9.36
6.7
11.49
Na2O
0.32
0.31
0.19
1.3
2.13
K2O
0.98
2.1
0.79
10.4
0.56
CO2
3.3
7.1
4.58
1.0
-
H2O
7.4
5.9
8.04
2.6
-
tot
100.30
99.24
99.23
100.00*
99.49
1, 2, 3, Lamp.: Mitchell, 1986, lamproite: Leucite Hills, USA
MORB: Cmiral et al., 1998
*recalculated to 100% volatile free
1.
Crystals:
Mg-Olivine *
Mg2SiO4
Mg-Ilmenite
(Mg,Fe)TiO3
Ti-bearing Pyrope
Mg3(Al,Ti,Cr)2[SiO4]3
Diopside
CaMgSi2O6
Phlogopite*
K2Mg6[Si6Al2O20](OH)4
Enstatite
MgSiO3
Cromite
(Fe,Mg)(Cr,Al,Fe3+,Ti)2O4
* Often replaced by serpentine and calcite
1.
Fine matrix:
Olivine
Mg2SiO4
Monticellite
Ca(Mg,Fe)SiO4
Phlogopite
K2Mg6[Si6Al2O20](OH)4
Perovskite
(Ca,Na,Fe2+,Ce,Sr)(Ti,Nb)O3
Spinel
MgAl2O4
Apatite
Ca5(PO4)3(OH,F,Cl)
Serpentine
Mg3Si2O5(OH)4
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)
2.
2.
When were kimberlites formed:
Period
Age (My)
Locality
Eocene
50-55
Namibia, Tanzania
Upper Cretaceous
65-80
Southern Cape (South Africa)
Middle Cretaceous
80-100
Kimberley (South Africa)
Lower Cretaceous
115-135
Angola, Siberia
Upper Jurassic
145-160
Siberia
Devonian
340-360
Colorado, Siberia
Ordovician
440-450
Siberia
Upper Proterozoic
810
Australia NW
Middle Proterozoic
1.100-1.250
Premier (South Africa)
Lower Proterozoic
1600
Kuruman (South Africa)
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’
Structural model of a kimberlite: Facies: Mitchell, 1986
a) Crater facies
b) Diatreme or pipe
c) Hypabyssal facies
3.
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)
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*
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
Theoretical sketch of a kimberlite
Crater facies 
Diatreme 
Hypabyssal facies
3.
3.
Realistic sketch of a kimberlite
KCF: Crater facies
K1, K2: Diatreme facies
Petrology of kimberlites:
4.
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.
4.
Melting of mantle 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
Melting of mantle
peridotite:
4.
Source rock:
Dol-Phl-bearing peridotite
P > 30 Kbar: alkaline picrites
P ~ 55 Kbar: kimberlites (low
degrees of melting)
What conditions are necessary to yield a kimberlite?
1) 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 mantleplumes highly enriched in fluids
and incompatible elements
(e.g. potassium)
4.
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) ;
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 ]
4.
Classification of 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 macrodiamonds 
5.
Geodynamical aspects:
Thermal character of Eons
Eon
Age (Gy)
Archean
4.5 – 2.7/2.5
Proterozoic
2.7/2.5 – 0.57 (0.54)
Phanerozoic
0.57/0.54 – present
Archean: microplate tectonic
Proterozoic: intraplate tectonic
Phanerozoic: plate tectonic
5.
Thermal character of Eons
Komatiites
Diapirism and
underplating
(ipersolidus)
Ductile flow
(subsolidus)
Brittle regime
and
oceanization
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
Tectonic setting of kimberlite-bearing areas:
5.
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;
Tectonic setting of kimberlite-bearing areas:
5.
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.
Tectonic setting of kimberlite-bearing areas:
5.
5. Nucleus model: V.M. Moralev e M.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.
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 emplacement (Kopylova et al., 1998). The
“stratigraphy” of the paleo-mantle of the
craton was also prepared.
6.
Kimberlites as natural mantle samplers:
6.
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
Kimberlites ore prospection:
In different stages of the ore prospection (strategic or tactic) the
following methodologies are employed:
1) 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).
2) Airborne and land electromagnets: anomalies due to the
presence of conductive material at the surface of kimberlite
terrains.
3) Airborne and land gravimetry: crater and diatreme shows
negative gravitational anomalies;
4) Radiometric and spectrometric methods: negative and/or
positive anomalies due to presence of U, Th and K within respect
to the background terrain.
7.
7.
Kimberlites ore prospection:
5)
Remote sensing;
6)
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.
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.
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
7.
8.
Kimberlites ore mining:
Several approaches depending on topography and mining stage:
- Open pit: up to a certain (safe) depth
Yacutian Open Pit, Russia
Finsch Diamond Mine, South Africa
8.
Kimberlites ore mining:
- Sublevel Caving (used in Kimberley)
 Open pit exhaust
 Sub-levels net
 Actual Mining
Market of diamonds:
Two main uses of diamond crystals:
1) 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.
2) 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.
9.
Diamond Anvil Cells: used in experimental petrology
Record pressure 4.6 Mbar !
Our lab: HDAC very important !!!
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);
9.
More diamonds:
9.
Diamonds resources: in millions of carats
Country
Australia
Production (106 ct)
40.8
Zaire
Botswana
Russia
20.0
16.8
12.5
South Africa
South America
Angola
Namibia
9.1
2.0
1.9
1.3
Ghana
0.8
Canada (expected)
World total
ca. 3
107.9
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
Market of diamonds: all values in million carats
800
700
600
500
400
300
200
100
0
-100
698
644
334
297
9.
Resources
and in
Risorse
e riserve
carati
al 311999
Dicembre
reserves,
1999
Movements,
2000 nel
Movimenti,
in carati,
2000
239
Adjustments, in
2000
Aggiustamenti,
carati,
nel 2000
142
98
-1
-8
-29 -8
-46
Presumed
Risorse
resources
Presunte
Indicated
Risorse
resources
indicate
Probable
Riserve
resources
probabili
Resources
and in
Risorse
e riserve
carati
al 312000
Dicembre
reserves,
2000
Resources: the actual yearly production of ore (diamond, metal, etc.)
Reserves: estimated amount of resources in already operating mines
Movement: it reflects the removal or exploitation of reserves or resources
Adjustment: it reflects increment or decrement of resources or reserves
References:
Books:
Kimberlites, Roger H. Mitchell, Plenum Press, 1986 – 442 p.
Petrology of Lamproites, R. H. Mitchell & S.C. Bergmann, Plenum Press, 1991 – 451 p.
The Nature of Diamonds, various authors, Cambridge University Press, 1998, 278 p.
Articles:
Kopylova, M.G., Russell, J.K., Cookenboo, H., Upper-mantle stratigraphy of the Slave
Craton, Canada: insights into a new kimberlite province, Geology, vol. 26, 1998, p.
315-318.
Mitchell, R.H., Aspects of the petrology of kimberlites and lamproites: some
definitions and distinctions, in Proceedings of the Fourth International Kimberlite
Conference ,1986, vol.1, p. 7-45.
Mitchell, R.H., Kimberliets and Lamproites: Primary Sources of Diamonds, in Ore
Deposit Models, 1993, vol. 2, Geoscience Canada Reprint Series 6, p. 13-28.
Moralev, M.V., Glukhovsky, M.Z., Diamond-bearing kimberlite fields of the Siberian
Craton and the Early Precambrian geodynamics, Ore Geology Reviews, vol. 17, 2000,
p. 141-153.
Big Hole, Kimberley
http://www.zolfo78.com/lectures
More info available from me:
Office 2056 – Phys. Scie. Bldg.
or gsolferi@stfx.ca