Kimberlites - petrology

1. Kimberlites

2. 1. Introduction
2. Mineralogy
3. Morphology
4. Terminology
5. Petrology
6. Classification
7. Origin
8. Kimberlite emplacement models
9. Economic importance
10. Indian occurence
11. Conclusion
12. Reference

3.  Kimberlite is a volatile-rich, potassic, ultrabasic, igneous
rock which occurs as small volcanic pipes, dykes, and sills.
 It has a porphyritic texture .
 Inequigranular texture consisting of large crystals or rock
fragments (xenoliths) enclosed in a much finer grained matrix.

4.  This named after the town of Kimberly in South
Africa.
 It was found for the first time in Kimberly
provinance.

5.  It is mainly consist’s of olivine, phlogophite, ilmenite,
garnet, pyroxenes and groundmass.
 Olivine- alteres to serpentine.
 Phlogophite- replaced by chlorite, vermiculate, magnetite,
Ilmenite.
 Garnet- rich in pyrope. It commonly shows kelyphitic
rims.
 Groundmass- fine grained, consisting of serpentine &
magnetite, upto 50% primary calcite.
 Pyroxenes- enstatite may show rims or internal lamellae of
clinopyroxene, garnet.

6.  Kimberlites occur as carrot-shaped, vertical intrusions termed ‘pipes’. This
classic carrot shape is formed due to a complex intrusive process of
kimberlitic magma which inherits a large proportion of both CO2 and H2O.
 The morphology of kimberlite pipes and the classical carrot shape, is the
result of explosive diatreme volcanism from very deep mantle-derived
source.

8.  Historically, kimberlites have been subdivided into two
distinct varieties termed 'basaltic' and
'micaceous‘(Wagner, 1914). It was re-named by Smith
(1983) as Group I and Group II based on the isotopic
affinities of these rocks .Mitchell (1995) , showed that
Group II kimberlites actually show closer affinities to
lamproites than Group I kimberlites.

9.  Group- I kimberlites are of CO2-rich ultramafic
potassic igneous rocks dominated by a primary
mineral assemblage of forsteritic olivine, ilmenite,
Cr-pyrope, phlogopite, enststite. Group-I
kimberlites exhibit a distinctive inequigranular
texture.

10.  Group II kimberlites are ultrapotassic rocks rich in
volatiles. The distinctive characteristic of phlogopite
macrocrysts and microphenocrysts, together with
groundmass micas that vary in composition from
phlogopite to “tetraferriphlogopite” (Fe-rich phlogopite).

11.  Kimberlite classification is based on the
recognition of differing rock facies. These differing
facies are associated with a particular style of
magmatic activity, namely
Crater facies
Diatreme facies
Hypabyssal facies

13. • The surface morphology of an unweathered
kimberlite is characterized by a crater, upto 2 Km
in diameter.
• The crater is generally deepest in the middle.
• Around the crater is a tuff ring which is relatively
small generally less than 30 meter, when
compared to crater diameter.
• Two main categories of rocks are found in crater
facies kimberlites, namely Pyroclastic and
Epiclastic.

14. Crater facies kimberlite

15.  Kimberlite diatremes are 1-2 Km deep, generally carrot-
shaped bodies which are circular to elliptical at surface and
taper with depth .

16. Diatreme facies kimberlite

17.  These rocks are formed by the crystallization of hot,
volatile rich kimberlite magma. Generally, they lack
fragmentation features and appear igneous.

18. Hypabyssal facies kimberlite

19.  Kimberlites appear to originate at depths of 100 to 200 km
in the mantle asthenosphere.
 At a depth of 200 km the pressure is 60,000 times greater
than the surface and the temperature is about 1500°c.
 Kimberlite magmas are rich in carbondioxide and water
which brings the magma quickly and violently to the surface.
 Kimberlites occurs in the earth’s crust in vertical structures
known as kimberlite pipes.

20. Mitchell (1986) consider several theories and presents
a more comprehensive critique of each emplacement theory.
Here we will examine three theories :
1. Explosive volcanism theory
2. Magmatic (fluidization) theory
3. Hydrovolcanic theory

21.  Kimberlites are the most important source of primary
diamonds.
 Many kimberlite pipes also produce rich alluvial
diamond placer deposits.
 Only about 1 in 200 kimberlite pipes contain gem
quality diamonds
 Kimberlites are very significant rock group both
academically and economically.

22.  Kimberlite are found in Anantapur, Prakasam and
Mahboob nagar district. The well known Wajrakarur
diamond field in Anantapur district and area about 250 sq
km, out of 12 pipes identified, pipes 2 and 5 are considered as
micaceous kimberlite and the rest are typical kimberlite, these
kimberlites are diamondiferrous. The kimberlite diatreme of
late protrozoic age(840 to 1020 my) are intrusive into the
archean granites and magmatite gneiss.
 Maddur- Mahboob nagar sector, covers an area of about
400 sq km. In kimberlite bodies are reported around Maddur
and some bodies around Kotakonda.

23. Occurrence of diamond

26.  Kimberlite magmas are rich in carbondioxide and water
which brings the magma quickly and violently to the
mantle.
 Kimberlite is a gas rich potassic ultramafic igneous rock.
 Australia is currently the world’s largest producer of
diamonds are low quality and used for industrial purposes.
 The crater facies kimberlite is recognized by
sedimentary features.
 The diatreme facies are recognized by pelletal lapilli .
 The hypabyssal facies is commonly recognized by the
presence of abundant calcite.

27. Jhon M. Guilbert & Charles F. Park, Jr.(1985), Geology
of Ore Deposits, W. H. Freeman & company/New York
,Pp: 346-352
Loren A Raymond, Petrology, (1995) ,Wm.c.Brown
publisher, volume 1, Pp:83-118.
P.K.Raman and V.N.Murthy, (1997), Geology of Andhra
pradesh, Geological society of India, pp:114 to 121.
www.wikipedia.com/kimberlite.htm
www.ias.ac.in/currsci/apr25/articles13.htm