KIMBERLITES - PowerPoint Presentation

Slide1

KIMBERLITES

Kimberley (South Africa

)Slide2

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.Slide3

Part 1Slide4

1.

Kimberlites are a very rare type of magma with extremely deep origin (>150 km

). Their key characters are high K, Mg and fluids (CO

2

) 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.

What are kimberlites – why are they important:Slide5

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)Slide6

1.

Chemical and mineralogical composition:

-

Ultramafic

magmatic rocks (SiO

2

25-35 wt%, MgO up to 38 wt%) - K2O/Na2O = 2 K

2

O up to 2 wt% (highly alkaline)

- CO

2

+ H

2

O

 8-15 wt% (high fluid content) - Inequigranular texture - textural components:

Macro- & Megacrystals Fine matrix Peridotite and Eclogite xenoliths Diamonds Slide7

1.

Chemical and mineralogical composition:

1

2

3

Lamp.

MORB

SiO

2

35.2

31.1

33.21

52.7

51.45

TiO

2

2.32

2.03

1.97

2.4

2.50

Al2O34.44.94.4510.814.36Fe2O3--6.78FeO9.810.53.435.15.50MgO27.923.922.788.411.06CaO7.610.69.366.711.49Na2O0.320.310.191.32.13K2O0.982.10.7910.40.56CO23.37.14.581.0-H2O7.45.98.042.6-tot100.3099.2499.23100.00*99.49

1, 2, 3, Lamp.: Mitchell, 1986,

lamproite

:

Leucite

Hills, USA

MORB:

Cmiral

et al., 1998 *recalculated to 100% volatile freeSlide8

1.

Crystals:

Mg

-

Olivine

*

Mg

2

SiO

4

Mg

-

I

lmenite

(

Mg,Fe)TiO

3

Ti

-bearing

Pyrope

Mg

3(Al,Ti,Cr)2[SiO4]3DiopsideCaMgSi2O6Phlogopite*K2Mg6[Si6Al2O20](OH)4EnstatiteMgSiO3Cromite(Fe,Mg)(Cr,Al,Fe3+,Ti)2O4* Often replaced by serpentine and calciteSlide9

Fine matrix:

Olivine

Mg

2

SiO

4

Monticellite

Ca(Mg,Fe)SiO

4

Phlogopite

K

2

Mg

6

[Si

6

Al

2

O

20

](OH)

4Perovskite(Ca,Na,Fe2+,Ce,Sr)(Ti,Nb)O3SpinelMgAl2O4ApatiteCa5(PO4)3(OH,F,Cl)SerpentineMg3Si2O5(OH)41.Slide10

Part 2Slide11

Where are kimberlites:

2.

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

)

10Slide12

When were kimberlites formed:

2.

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 Cretaceous115-135Angola, SiberiaUpper Jurassic145-160SiberiaDevonian340-360Colorado, SiberiaOrdovician440-450SiberiaUpper Proterozoic810Australia NWMiddle Proterozoic 1.100-1.250Premier (South Africa)Lower Proterozoic1600Kuruman (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’Slide13

Part 3Slide14

Structural model of a kimberlite: Facies:

Mitchell, 19863.

a) Crater facies

b

) Diatreme or pipe

c

) Hypabyssal faciesSlide15

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.Slide16

3.

Diatreme facies

D

iatreme

: 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*Slide17

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 conglomerateSlide18

3.

Theoretical sketch of

a kimberlite

Crater

facies



Diatreme



Hypabyssal

facies

Slide19

3.

Realistic sketch of a kimberlite

KCF:

C

rater

facies

K1, K2:

Diatreme faciesSlide20

Part 4Slide21

4.

Petrology of kimberlites:

Peridotite mineralogy:

Olivine (>60%),

Clynopyroxene

, Orthopyroxene and one aluminum phase (

for growing depths

p

lagioclase

,

spinel

or

garnet)

Peridotite chemistry:

MgO

(

~

40%), SiO

2

(

~

45%), Al2O3, CaO, FeO all of them < 10 wt%, Na2O e K2O almost negligibile. 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).Slide22

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% H

2

O);

3) “CO

2

-bearing” (> 5 wt% CO

2

);

4) “Fluid-absent amphibole-bearing” (water

vapour present only during first melting)

~ 45 kmSlide23

4.

Melting

of

mantle

peridotite:

shield

Source rock:

Dol-Phl-bearing

peridotite

P > 30

Kbar

: alkaline

picrites

P ~ 55

Kbar

: kimberlites

(low degrees of melting)Slide24

4.

What conditions are necessary to yield a kimberlite?

Type of

‘solidus’: H

2

O & CO

2

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)Slide25

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) ; Slide26

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

]Slide27

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 macro-diamonds

Slide28

Part 5Slide29

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.

Geodynamical aspects:Slide30

Achaean heat-flux probably 4-5 times larger than present. Vigorous mantle convection

Thermal character of Eons

Diapirism

and

underplating

(

ipersolidus

)

Ductile flow (

subsolidus

)

Brittle regime and

oceanization

Intermediate depth crust average temperature variation with geologic time (Wynne & Edward, 1976)

Komatiites

5.Slide31

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

;

5.Slide32

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.Slide33

Tectonic setting of kimberlite-bearing areas:

5.

5.

Nucleus model

:

V.M.

Moralev

e M.Z.Glukhovsky, 2000This 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.Slide34

Part 6Slide35

6.

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.

Kimberlites as natural mantle samplers:Slide36

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.

- Mg-wustite (Mg,Fe)O inclusions: Often found in macro-diamonds, they also require a lower-mantle origin

- Baddleyte inclusions in diamond (ZrO

2

): Compositional analyses on these inclusions from diamonds of the Mbuji Mayi kimberlite (Congo) indicate provenience from the lower mantle.

Kimberlites as natural mantle samplers:Slide37

Part 7Slide38

7.

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.

Kimberlites ore prospection:Slide39

7.

Kimberlites ore prospection:

Chemical analyses of

pathfinder or index minerals

:

Garnet, Ilmenite,

Clinopyroxene, olivine, zircon

Remote sensing;

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.

Blue and yellow ground: field observation!Slide40

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.Slide41

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.htmlSlide42

Part 8Slide43

8.

Kimberlites ore mining:

-

Open pit: up to a certain (safe) depth

Finsch

Diamond

Mine,

South Africa

Yacutian

Open

Pit

, Russia

Several approaches depending on topography and mining stage

:Slide44

-

Sublevel Caving (

used

in

Kimberley

)

Sub-levels

net



Open pit exhaust

Actual Mining

8.

Kimberlites ore mining:Slide45

Part 9Slide46

9.

Market of diamonds:

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.Slide47

9.

Diamond Anvil Cells: used in experimental petrology

Record pressure 4.6 Mbar !

Our lab: HDAC very important !!!Slide48

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.Slide49

More

diamonds

:

9.Slide50

Diamonds resources

:

in millions of carats

Country

Production (10

6

ct)

Australia

40.8

Zaire

20.0

Botswana

16.8

Russia

12.5

South Africa

9.1

South America

2.0

Angola

1.9Namibia1.3Ghana0.8Canada (expected)ca. 3World total107.9Data from: Metals & Minerals Annual (1996)9.More recent data:Canada:Ekati ~ 4 Mct/yDiavik ~ 8 Mct/yJericho ~ 0.4 Mct/yTotal Canada:~ 12.4 Mct/ySlide51

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

9.

Market of diamonds:

all values

in million carats

Resources and reserves, 1999

Movements, 2000

Adjustments, 2000

Resources and reserves, 2000

Presumed resources

Indicated resources

Probable resourcesSlide52

ReferencesSlide53

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.Slide54

Big

Hole, Kimberley

More info available from me:

Office 2056 – Phys.

Scie

. Bldg.

or

gsolferi@stfx.ca

http

://www.zolfo78.com/

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