24

CHAPTER – I

INTRODUCTION

“Kimberlites constitutes a rare, highly alkaline volatile rich rock type that

has in many ways attracted more attention than its relative volume might

suggest that it deserves. This is largely because it serves as a carrier of

diamonds and garnet peridotites mantle xenoliths to the Earth‟s surface. Further

more, its probable derivation from depths greater than any other igneous rock

type and the extreme magma compositions that it reflects in terms of low SiO2

contents and high levels of incompatible trace element enrichment , make an

understanding of kimberlite petrogenesis important ------ Le Roex et al ( 2003).

Kimberlites along with the lamproites constitute, a fascinating group of

mantle derived rocks and are products of intraplate alkaline magmatism. These

ultramafic (and ultrabasic) rocks marked by mineralogical diversity and

geochemical variability, assume significance because of their rarity in space and

time. Though volumetrically insignificant and occur as small bodies in the

continental cratonic interiors (with exceptions to lamproites of Argyle occur in

mobile belts) they continue to garner utmost attention owing to their pre-eminent

position as the primary source rocks for diamond on the surface of the earth.

The present work relates to the identified kimberlite pipes from Kalyandurg

and Timmasamudram areas in Anantapur district, Andhra Pradesh and

assumes significance as it forms a part of the historically well known diamond

belt in Southern India.

I.1 Definition of Kimberlite:

The term kimberlite was first used by Lewis (1887) to describe the host

rock of diamond at the type locality, Kimberley in South Africa.

Owing to the great diversity in terms of their textural, mineralogical,

petrographic and geochemical characteristics, diverse definitions and

classifications for Kimberlites were proposed, resulting in a great dispute on

every front. Every term proposed for the kimberlites was greeted with hostility

and contested with equal verve, and issue could not be resolved satisfactorily

until proposed and subsequently modified by Mitchell (1979; 1986). Mitchel

25

attempted at introducing the concept of a kimberlite clan encompassing the

whole spectrum of the kimberlitic rocks including kimberlite-hosted macrocryst /

megacryst suites.

In simple terms, Kimberlites , constitute a hybrid group of rocks that

encompass a group of volatile rich (dominantly CO2) potassic, ultrabasic rocks

and that displays a pronounced inequigranular texture, resulting from the

presence of macrocrysts (and/or megacrysts) that are set in a fine grained

matrix.

It took nearly a century for gaining clarity as to what exactly should be the

definition of kimberlite - when Mitchel (1986) attemped to cover the whole

spectrum or related rocks. In general, the megacryst / macrocryst assemblage of

kimberlite is constituted by

Magnesian ilmenite, forsteritic olivine, Cr-poor titanian pyrope, Cr-poor

clinopyroxene, phlogopite, enstatite and Ti-poor chromite, that are rounded

and anhedral in outline.

Olivine constitutes the dominant member of the macrocrystic assemblage.

The mineral assemblage of ground mass or matrix include : second

generation euehdral olivine and /or phlogopite, along with perovskite, spinel,

magnesian aluminous chromite, titanian chromite, all the members of

magnesian ulvo spinel- ulvo spinel - magnetite series), diopside ( both Al

and Ti poor), monticellite, apatite, calcite and primary late stage Fe rich

serpentine (Mitchel, 1986).

Often some kimberlites contain late stage eastonite phlogopites.

Nickeliferous sulphides and rutile constitue the accessory minerals

Replacement of early formed olivine, phlogopite, monticellite and apatite by

serpentine and calcite is very common.

Some kimberlites contain late-stage poikilitic eastonitic phlogopites,

nickeliferous sulphides and rutile are common accessory minerals.

The replacement of early-formed olivine, phlogopite, monticellite and apatite

by deuteric serpentine and calcite is common.

Evolved members of the clan may be devoid of, or poor in, macrocrystic, and

composed essentially of calcite, serpentine and magnetite, together with

minor phlogopite, apatite and perovskite.”

26

The first discovery of kimberlites has its roots in the bustle related to

prospecting for diamonds along the courses of Vaal, Orange and Riat rivers of

south Africa and the earliest discovery of source rocks for diamonds ( now

being called as kimberlite ) dates back to 1869 from the mud being excavated

from a small quarry for construction of farm house. These quarries, located in

water filled depressions (popularly known in South Africa as ‘Pans’). These finds

lead to the “Diamond Rush” to these farms and were torchbearers/ forerunners

for the subsequent development of four diamond mines and the Kimberley town

in the region.

The high prevalence of diamonds occurring in these “Pans” lead to deem

that diamonds as of alluvial origin (river transported). However, by 1872, this

earlier held notion soon got dispelled as these diamonds were not of alluvial

origin and the rocks carrying diamonds were identified in the close vicinity

exhibiting cylindrical pipe-like structures.

The prospectors at that time used terms “yellow ground” (highly altered

and decomposed rock) and “blue ground” that graded into more compact and

hard rock. The blue ground corresponds to what is presently known as the

Diatreme facies kimberlite.

Though the kimberlites are marked by economic significance, a proper

description of kimberlite came to light in 1886, until Henry Carvill Lewis

described these rocks are porphyritic and mica-bearing peridotite and

recognized them as volcanic breccia , and again in 1888, it was Lewis who

coined the term Kimberlite for the first time for these diamond hosting rocks

after Kimberley town following the type locality categorization rule.

Kimberlites continued to attract the attention both the diamond explorers and

the academicians alike for two reasons:

1) they are the primary source rocks for diamond on the earth‟s surface

2) They are the carriers of a host of upper mantle rocks, they act as

windows to the earth‟s upper mantle.

3) With their petrographic diversity, perplexing mineralogy and geochemistry,

kimberlites have captured the imagination of academicians who view the

kimberlites as exciting rock types that provide glimpses of mantle

processes.

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I.2 Definition of Lamproite:

The term lamproite was introduced by Niggli (1923) for leucite bearing rocks from

Spain and Wyoming which had unusual “Niggli” parameter. Subsequently Troger

(1935) referred to Lamproites as potassium magnesian-rich lamprophyric rocks.

Later Wade and Prider (1940) used the term (as defined by Troger) to embrace

rock types in the West Kimberley area of Western Australia.

“The lamproite clan are a group of ultra potassic mafic rocks characterised by

the presence of one more of the following primary phenocrystal and /or ground

mass constituents with widely varying modal amounts: titanium, alumina – poor

phlogopite, Leucite (typically sodium – poor but may be replaced by analcime),

titanium tetra-ferri phlogopite, titanian potassic richterrite, forsteritic olivine,

diopside and sanidine.” (Modified from Mitchell 1985, Scot Smith and Skinner

1984 a).

Priderite, apatite, wadeite, spinel, ilmenite, shcherbakovite, armalcolite,

perovskite, jeppeite etc constitute the minor/accessory phases in these rocks.

Glass may be an important constituent of rapidly chilled lamproites. Other

minerals such as carbonate, chlorite and zeolite may be secondary (B.H.Scot

Smith, 1987).

Lamproite generally occur towards craton margins (eg. Bergman in press).

Volatiles in lamproites are dominated by H2O.

I.3 Classification of Kimberlites:

Numerous classification models have been developed for kimberlites and the

large textural and mineralogical variations seen in these rocks do not help to

make the task easier.

1 Classification based on the textural - genetic variations : This model

proposed by Clement and Skinner, (1979) relying on extural features identifies

three genetic facies of kimberlite rocks.

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A. Crater Facies: Crater facies are represented by pyroclastic (formed as a

result of eruptive forces) and epiclastic rocks (fluvial alteration of pyroclastic

material) and are distinguished by sedimentary (layer) deposition. This

classification is generally accepted and most widely adopted. However, it is

important to note here that there are genetic implications in this model. In the

Crater facies, kimberlites are characterised by pyroclastic and epiclastic deposits

and tuff rings forming oval or elliptical bowl shaped basin-like structures with their

margins sloping inwards at angles of 250 -700 (Hawthorne,1975) (Fig. I-1a & I-

1b). The crater facies may extend up to 150 to 300 m below the surface.

Fig I-1a: Crater Facies Kimberlite (From Mitchell 1986)

Fig I-1b: From Mitchell (1986)

B. Diatreme Facies: The diatreme facies in kimberlite is characterised by a

carrot shaped body with near circular or elliptical outline on the surface and

steeply dipping (800-850) walls. This facies sometimes may exceed 2 km in

depth.

29

The diatreme facies are characterised by fragmental nature and the presence of

angular to rounded country rock fragments (ranging from a few centimetres to

sub-microscopic size) imparts a distinct identity. This facies is constituted by

autoliths (rounded fragments of earlier generations of kimberlite), pelletal lapilli,

(large rounded to elliptical lapilli sized clasts represented by a large anhedral

olivine or phlogopite in the form of a nucleus, that is enclosed in a optically

unresolvable microphenocrystal matrix) nucleated autoliths, fragmented mantle

xenoliths that are represented by discrete and fractured grains of olivine garnet,

clinopyroxene and ilmenite set in a product of magnetic crystallisation consisting

of microphenocrysts and groundmass. The matrix, supporting the pelletal lapilli,

autolithic and xenolithic clasts, consists essentially of diopside and serpentine

(when fresh) and less commonly of phlogopite (Michell, 1986).

The pelletal lapilli are the large rounded to elliptical lapilli sized clasts of

kimberlite, with a large anhedral olivine or phlogopite or rarely country rock

fragments occurring as a nucleus at the centre enclosed in a microphenocrystal

kimberlite where the microphenocrysts are concentrically oriented around the

nucleus and the matrix is optically unresolvable and appears as amorphous dark

material (Mitchell 1986). The outstanding feature of the matrix is that all the

crystals are extremely fine grained or cyrptocrystalline and that the normal

igneous granular or poikilitic texture is absent. The matrix may be uniform or may

show discrete segregations of serpentine and diopside.

C. Hypabyssal or Root Zone Facies: The hypabyssal facies kimberlites are

rocks formed by the crystallisation of volatile rich kimberlite magma.

Macroscopically they are massive rocks in which the macrocrystal olivine and

other macrocrysts (ilmenite, phlogopite, garnet) are commonly visible. They show

the igneous textures and effects of magmatic differentiation.

Some of the characteristic textural features of this facies include:

1. Absence of pyroclastic fragments and textures,

2. Presence of late stage poikilitic growth of phlogopite,

3. Segregation textures involving segregation of calcite and serpentine.

4. Globular segregation and

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5. Flow banding marked by the preferred orientation of microphenecrysts.

Multiple intrusions (pulses) of kimberlite magma within the same body are quire

common in the hypabyssal facies rocks, which also commonly contain country

rock xenoliths. They are believed to crystallise from hot and volatile –rich

kimberlite magmas.

II Classification based on wide variations in mineralogy, petrography and

isotopic characteristics:

This classification was initially proposed by Wagner (1914) identifies two groups

namely micaceous and nonmicaceous kimberlites.

Subsequently Smith (1983) based on difference in their isotopic composition

classified the kimberlites in to two groups viz., Group-I and Group – II kimberlites,

which correspond to the non-micaceous and micaceous kimberlites. Later the

detailed study of the petrography, content of mantle xenoliths and xenocrysts,

whole rock geochemistry, isotopic characters and distribution carried out by

Smith et al (1985) and Skinner (1989) established the fact that the Group –II

kimberlites are distinctly different from Group -I kimberlites.

The Group-I kimberlites (also termed as archetypal kimberlites, Mitchell,1995)

contain, apart from xenoliths, a macrocryst assemblage consisting of anhedral

olivine (foresteritic), phlogopite, Mg-rich ilmenite, Cr-rich spinel, Mg-rich garnet,

clinopyroxene (chrome diopside) and orthopyroxene (enstatite) set in a matrix

consisting of euhedral micro-phenocrysts of olivine, phlogopite together with

calcite, serpentine, clinopyroxene, monticellite, apatite, spinel, perovskite and

ilmenite. The macrocrysts may be xenocrysts (disaggregated constituents mantle

rocks like lherzolite, harzburgite or eclogite) and / or parts of megacrysts/cognate

xenoliths (disaggregated cumulates). They are distinctly encriched in CaO, H2O

and CO2 and poorer K2O and SiO2 in comparision to the other group.

The Group –II kimberlites (ca. Micaceous kimberlites of Wagner 1914),

although texturally similar to Group-I kimberlites, are characterised by the

presence of abundant phlogopite as macrocrysts, phenocrysts and in the

groundmass (showing composition range from phlogopite to tetraferriphlogopite),

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rounded olivine megacrysts and highly magnesian olivine (Fo 91-93 mol%)

phenocrysts, groundmass diopside, spinels of magnesiochromite to

titanomagnetite composition, Sr- and REE rich perovskite Sr-rich apatite, Mn-rich

ilmenite, monticellite and magnesian ulvospinel and Ba-rich phlogopite are

conspicuously absent. Group-II or micaceous (phlogopite) kimberlites are richer

in K2O and isotopically similar to potassic volcanic rocks in continental settings.

While the Group-I kimberlites are found through out the world, the Group-II

kimberlites are found in found only in South Africa (Skinner, 1989).

In view of the petrological, geochemical characteristics of the Group-II kimberlites

are distinctly different from the Group-I. Mitchell (1994, 1995) proposed that the

Group-I kimberlites alone considered as kimberlites. He also revived the term

“Orangites” of Wagner (1928) to designate the Group-II kimberlites of Smith

(1983) and Smith et al (1985) and also suggested that the term Group-II

kimberlites be scrapped. The I.U.G.S. Sub-commission on the systematic of

Igneous Rocks, however, has not sanctioned the term “Orangite” and still retains

the two-fold classification of kimberlites into Group-I and Group-II (Woolley et al,

1996) and also endorsed the mineralogical – genetic classification of kimberlites

and lamproites (Wolley et al 1996).

Morphology and Facies Concept:

Kimberlites are the small volume igneous rocks of limited aerial extent. Based on

the studies of numerous diamondiferous deposits, 3 distinct units based on their

morphology and petrology. These units are:

1) Crater Facies Kimberlite

2) Diatreme Facies Kimberlite

3) Hypabyssal Facies Kimberlite

The Picture (Fig.I-2) shows the Model of idealized kimberlite magmatic system

illustrating the relationships between the Crater, diatreme and hypabyssal facies

rocks (After Mitchell 1986).

1) Crater Facies Kimberlites:

The surface morphology of un-weathered kimberlite is characterised by a crater,

up to 2 kilometers in diameter, whose floor may be several hundred meters

32

below ground level. The crater is generally deepest in the middle. Around the

crater is a tuff ring which is relatively small, generally less than 30 meters, when

compared to the diameter of the crater. Two main categories of rocks are found

in crater facies kimberlites; pyroclastic, those deposited by eruptive forces; and

epiclastic, which are rocks reworked by water.

A. Pyroclastic Rocks: These rocks are found preserved in tuff rings around the

crater and within the crater. Tuff rings have small height; crater diameter

ratios and are preserved in very few kimberlites. Igwissi Hills in Tanzania and

Kasami in Mali are the pipes with well preserved tuff rings (Dawson, 1995).

Heights range from 1-4 meters on one pipe, and 15-50 meters in one

kimberlite field. Deposits are commonly bedded, vesicular and carbonatised.

Tuff deposits preserved within the crater are also rare; however, the Igwissi

Hill pipes in Tanzania have been examined and revealed three distinct units.

From top to bottom, they are:

1. Well-stratified tuffs –layers defined by lapilli and ash size particles. Graded

bedding and depositional features appear absent. Believed to be products

of air fall and possibly settling through water.

2. Poorly stratified coarse pyroclastics – recognised by deposits of complex

folding and slumping.

3. Basal breccias.

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NOTE:

DIKES & SILLS

EMPLACED AT

HIGHER LEVELS

THAN THE

ORIGIN OF

DIATREMES

CONCORDANT

TABULAR

PRE-DIATREME

DIKES & SILLS

DISCORDANT

TABULAR

CLASS 1 PIPE

Fig. I-2: Model of an idealized kimberlite system, illustrating the hypabyssal

dyke-sill complexleading to a diatreme and tuff ring explosive crater. From

Mitchell (1986) Kimberlites: Mineralogy, Geochemistry and Petrology. Plenum

Newyork, Winter (2001). An introduction to igneous and Metamorphic Petrology,

Prentice Hall.

B. Epiclastic Rocks – These sediments represent fluvial reworking of

pyroclastic material from the tuff ring in the Crater Lake formed on top of the

diatreme. They are complex and resemble a series of overlapping alluvial

fans mixed in with lacustrine deposits. They coarsen with distance from the

wall rock and become better sorted towards the centre. Fossils may be found

in these sediments. Some epiclastic deposits have been replaced with

chalcedony – evidence for late stage volcanic hot-spring activity.

C. Lavas – no concrete evidence for this.

Few kimberlites exist with well preserved crater facies. It is difficult to develop

a model with any certainty that all kimberlites will confirm to the observed

features above. Crater facies kimberlite is difficult to distinguish from diatreme

facies kimberlite. The most distinguishing feature is visible bedding.

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2) Diatreme Facies Kimberlite:

Kimberlite diatremes are 1-2 kms deep, generally carrot-shaped bodies which

are circular to elliptical at surface and taper with depth. The dip contact with

the host rocks is usually 800-850. The zone is characterised by fragmented

volcanoclastic kimberlitic material and xenoliths plucked from various levels in

the earth‟s crust during the kimberlites journey to the surface.

Some textural features of Diatreme Facies Kimberlites:

i) Country rock fragments – angular

ii) Cognate fragments (juvenile) – rounded to angular

iii) Country rock xenoliths found 100 meters below depositional unit – it is

clear that sinking occurs in the pipe.

iv) Pelletal lapilli – appear to have formed by the rapid crystallisation of a

volatile poor magma containing phenocrysts. They are characterised by

a crystal nucleus surrounded by micro-phenocrysts which align

themselves tangentially to the central crystal.

v) Nucleated autoliths – similar to pelletal lapilli but lacking microphenocryst

orientation. Kernel grain usually country rock. Magmatic nucleation about

a nucleating center.

vi) Matrix composed almost entirely of fine grained diopside, serpentine and

phlogopite.

vii) Hardly and calcite found in matrix (wheras lots of calcite is found in

hypabyssal matrix). This suggests that the magma has already

degassed.

viii) Crystallisation in diatreme occurs at low temparatures based on the lack

of thermal effects seen in intruded limestones.

ix) Contact metasomatic/metamorphic effects with the country rock are few.

x) Up warping and fractures associated with the intrusive body are absent.

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Fig. I-3: Photograph of diatreme facies kimberlite core (Mitchell (1986)

These fragments with halos of crystallised kimberlite magma are characteristic of diatreme facies rocks (Fig.I-3).

3) Hypabyssal Facies Kimberlite :

These rocks are formed by the crystallisation of hot, volatile –rich kimberlite

magma. Generally, they lack fragmentation features and appear igneous (Fig.

I-3a).

Fig. I-3a. Photograph of hypabyssal facies kimberlite from Anantapur area.

Some Textural features:

i. Calcite-serpentine segregations in matrix.

ii. Globular segregations of kimberlite in a carbonate –rich matrix.

iii. Rock fragmentation has been metamorphosed or exhibit concentric

zoning.

Inequigranular texture creates a pseudoporphyritic texture.

The general criteria for recognising kimberlite, lamproite and lamprophyre based

on their tectonic setting, texure,mineralogy and geochemical characteristics are

given at Table I.1 (Madhavan V.,2001).

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Table I.1: General criteria for recognizing kimberlite, lamproite and lamprophyre( Madhavan, 2001 )

Kimberlite Lamproite Lamprophyre

Form Pipe, sill, dyke; diatremes tend to be carrot shaped

Pipe, sill, dyke, cinder cone; olivine lamproite diatremes are funnel or sherbet-glass shaped

Dyke, sill, plug, stock, sheet, diatremes or sub-volcanic vent.

Tectonic setting

Associated with intra plate magmatism (confined to craton interiors)

Associated with intra plate magmatism (confined to craton margins)

Associated with intraplate, divergent, convergent and passive-margin magmatism.

Texture Inequigranular with two generations of olivine and phlogopite (macrocrysts and micro-phenocrysts or groundmass). These rocks cannot be identified mainly on petrographic basis.

Inequigranular with two generations olivine and phlogopite. Like kimberlite, lamproite also cannot be identified on petrographic investigations alone.

Inequigranular with porphyritic-panidiomorphic with euhedral phenocrysts of mafic minerals. Felsic minerals are strictly confined to groundmass

Globular structure

Absent Absent Present (though not always)

Glass Absent Present (occasionally) Present (occasionally)

Minerology Phlogopite

TiO2 < 6%

TiO2 < 6% atleast

Biotite-Phlogopite. TiO2 < 6%

Atypical minerals

Picro-ilmenite, Cr-spinel, pyrope, pyroxene, calcite, monticellite and perovskite

Sanidine, leucite, diopside, foresteritic olivine, perovskite, apatite

Sanidine, orthoclase, plagioclase, nepheline, analciltene, pyroxene (diopside, salite, augite, Na-pyroxene).

Other Minerals Feldspar, feldspathoid, andradite, schorlomite

Primary plagioclase, melilite and/or monticellite, nepheline, primary calcite, melanite

Geochemical characteristics

Undersaturated ultrabasic rocks with low Al2O3 (<5%) and Na2O/K2O<0.5 Group-II kimberlites have many geochemical similarities with lamproite than with Group-I kimberlites. Kimberlites in general contaIn high Cr, Ni and LREE.

High K2O, TiO2, P2O5 and low Cao, FeO. Highest K2O/Na2O>3 (ultrapotassic), K2O/Al2O3 > 1 (perpotassic) and Na2O+K2O/Al2O3~1 (agpaitic-peralkaline), Highly enriched in Rb, Sr, Zr, Ba and REE.

Highly variable because of wide compositional variations. The order of LREE enrichment and slope of chondrite normalized pattern is less in lamprophyre than in lamproite.

(Source: V.Madhavan /Journal of Asian Earth Sciences 19 (2001) 321-332)

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I.4 Kimberlites of world and India: a glance ii

For the first time, kimberlites were identified in their type locality in the Kimberley

region of South Africa by Lewis in 1887 and later in 1914, Wagner provided a

comprehensive petrographic description of kimberlites, in his monograph entitled

„ the diamond fields of South Africa” and around 1932 Williams gave an updated

imformation of the petrography of kimberlites. It can be stated that these earliest

reports are most invaluable and were the forerunners for the subsequent research

work carried out on kimberlites in the world. The identification of new kimberlite

occurrences in several areas has catapulted the significance of kimberlites around

1950‟s and resulted in a flood of information in the literature with reference to

their geology, petrography and geochemistry. Several aspects of kimberlites were

being looked into by pioneering workers who gave prominence to the distribution

of kimberlites of economic importance (Clifford 1966, Janse 1985), the structural

and tectonic controls of their emplacement (Arsenyev 1962, Marsh 1973, Bailey

1964, 1974, Pretorious 1973, Sharp 1974, Crough et al 1980, Haggerty 1982,

Taylor 1984, Helmstaedt and Gurney 1984) and their mineralogy, geochemistry

and Petrology (Mitchell 1986). For the first time an account of the different facies

(Crater, diatreme and hypabyssal or root zone) of kimberlite magmatism has been

provided by Hawthorne (1975). The publication of Nixon ( 1973 ) entitled “

Lesotho Kimberlites”, a compilation of the papers, the proceeding volumes of the

successive international kimberlite conferences starting from the first one held in

1973 in Cape Town, South Africa to the recent one held in India ( 2012) have

contributed a lot to the modern understanding toeh kimberlite geology.

The research work carried out on the kimberlites during 1970 – 1985

mainly focused on petrographic, mineralogical and geochemical characterisation

of kimberlites and their xenoliths (Mitchell 1979, Skinner & Clement 1979,

Clement et al 1984). Mitchell (1986) provided a detailed compilation of these

studies and Dawson (1980) and Nixon (1987) have given a detailed account of

the mantle xenoliths in kimberlites. During the 1980‟s and 1990‟s major thrust of

the research work on kimberlites was on petrogenetic aspects including the

nature of source, depth of magma, generation, contamination and ascent of the

magma based on mineralogical and petrochemical (including REE and Isotopic)

38

characterization of the kimberlites and related rocks . Most important conclusion

of these studies are:

1. Kimberlites are formed from high temperature magmas, which are products

of very low volume partial melting of metasomatised peridotitic mantle.

2. Existence of kimberlites in different facies viz; crater, diatreme and

hypabyssal.

3. Kimberlite bodies consist of multiple pulses of intrusions.

4. Diamonds in kimberlites are xenocrysts resulting from the disintegration of

their hosts (i.e., peridotite and eclogite) in the mantle (Meyer 1985).

5. The mantle xenoliths suites as denoted by the coexisting mineral phases

indicate the depth of generation of kimberlite magma.

6. Kimberlite / lamproite emplacements denote remarkably synchronous

events in geological time span with global events clustered around ~ 1Ga

(India, Africa, Brazil, Australia, Siberia, and Geenland); ~450 to 500 Ma (

Archangel in Baltic region China, Canada, South Africa and Zimbabwe);

between ~370 and 410 Ma (Siberia and USA); between ~200 Ma

(Botswana, Canada Swaziland and Tanzania); ~80 to 120 Ma (South

Central and West Africa, Brazil, Canada, India Siberia and USA) and ~50

Ma (Canada and Tanzania), with minor intrusions in the Ellendale field of

NW Australia at ~22 Ma.

India is known since ages for its most beautiful, large sized diamonds and the

presence of primary diamond source rocks came to light only during 1930 with the

identification of Majhgawan pipe as kimberlite (Sinor, 1930). Infact diamond

production at Majhgawan can be traced back to 1829. The Majhgawan kimberlite

was later termed as Lamproite (Scotsmith, 1989). The discovery of tuffaceous

rocks at Wajrakarur in Andhra Pradesh as kimberlites ( Rao and Phadtare, 1966)

can be viewed as major contribution in stepping up the search for kimberlites in

this part of the Indian subcontinent and the consisting efforts by the Geological

Survey of India has lead to the discovery of ~ 70 kimberlite bodies in the

Dharwar craton, five kimberlite bodies in the Bastar cratons and two kimberlite

bodies in the southern part of the Bundelkhand craton in the recent years. In

39

addition, at the eastern margin of the Dharwar craton around 60 lamproites bodies

have been reported.

Among the the Indian Kimberlites those of the Dharwar craton and the Majgawan

pipe have been studied in greater detail as compared to the recently discovered

kimberltes of Mainpur and Tokapal fields in the Raipur district in Chhattisgarh.

Akella et al (1979), Middlemost and Paul (1984), Reddy (1987) and Scott-Smith

(1989) have made the initial studies on the petrological and mineralogical

characters of the kimberlites of Wajrakarur area in South India and brought out

their similarity with the kimberlites of South Africa and Ekutia. Though Reddy

(1987) opined that two of the Wajrakarur pipes (P-2 and P-5) might be lamproites

later studies by (Scott Smith 1989) confirmed that they area kimberlites. The

petrology and geochemistry of Maddur – Narayanpet kimberlites were discussed

by Nayak et al(1988), Chalapathi Rao and Madhavan (1996), Rao et al (1998),

Nayak et al (1999, 2001) dealt with the geological and tectonic setting of the

kimberlites of Dharwar craton. Based on detailed petrological, mineralogical and

petrochemical characters Chalapathi Rao et al (2004) have suggested that the

Dharwar Kimberlites (including those of Wajrakarur and Maddur-Narayanpet area)

are comparible with teh Group-I kimberlites of South Africa. The Majgawan pipe

was initially identified as micaceous kimberlite (Mathur and Singh 1971). However

Scott Smith (1989) based on petrological and mineralogical characters felt that

Majgawan and Hinota pipes are olivine lamproites. Chalapathi Rao et al (2005)

and Paul et al (2006) felt that the Majhgawan pipe has petrographic, mineralogical

and geochemical characters overlapping those of kimberlites and lamproites and

as such does not fit into the existing definitions of kimberlite, lamproite or

orangeite. Paul et al (1975a, 1975b), Scott Smith (1989) and Paul et al (2006)

discussed the geochemical characters of Indian kimberlites. Ganguly and

Bhattacharya (1987) and Nehru and Reddy (1989) attempted the P-T estimates

for the Wajrakarur kimberlites based on the studies on the mantle xenoliths.

Reddy (2000) and Rao et al (2001) also gave a detailed account of the

petrography of the mantle xenoliths from teh kimberlites of Dharwar craton. Paul

et al (1975a) Anil Kumar et al (1993, 2001), Chalapathi Rao et al (1996, 1999,

2005) gave the emplacement of ages of the Indian Kimberlites.

40

I.5 Distribution of Kimberlites in the world:

Kimberlites find distributed in all the continents of the world. On the

basis of the distribution patterns of the kimberlites across the world, Clifford

(1966), observed that the economically viable kimberlites occur primarily on Pre-

Cambrian Cratons – particularly those of Archaean age. This observation later on

came to be known as “Cliffords Rule “.

Janse (1992) modified this “ Clifford‟s Rule” and invoked the terms like

Archons, Protons and Tectons for cratons, that correspond to the old and stable

part of the continental crust that has survived the merging and splitting of

continents and super continents for at least 500 million years, are part of the

continental interiors and extending to a depth of 200km .

Janse has thus classified the cratons into three categories that are „period

specific‟ and mentioned below;

1. Archons: consisting of rocks from the Archaean era, older than 2,500

Million years.

2. Protons: consisting of rocks from the early to middle Proterozoic era with

ages between 1600 million years and 2500 million years.

3. Tectons: consisting of rocks from the late Proterozoic era, with ages

between 1600 million years and 800 million years.

Janse opined that all the economically viable kimberlites are confined to Archons.

Lamproites on the other hand, occur in Ancient Mobile belts (Protons) that have

accreted the cratons. The Archons that are bestowed with great diamond

potential , find a global distribution in 12 regions that are scattered in all the

seven continents. (Fig.I-4). Episodes of kimberlite and lamproite magmatism

worldover are shown in Table I - 2.

South Africa : Being the most prominent Archons, the African Continent

remains unique and has three largest cratonic blocks, that namely the South

African, the Central African and the West African blocks . It is pertinent to

mention that kimberlites discovered from 16 African countries a part of these

three Cratonic blocks.

41

Fig. I-4: Worldwide distribution of kimberlites and lamproites (modified after Dawson,

1980).

42

Table : I-2: Kimberlites and Lamproites across the continents of the world across the

Geological Time Scale

Time Age

(Ma)

Kimberlites Lamproites

Quaternary Igwisi, Tanzania Gaussberg, Leucite hills

(SW Uganda)

Miocene 5 - 25 SE Apain, Algeria,

Tunisia, Elledale*

(W.Australia), Utah

(Uganda)

Oligocene 26 - 28 Smoky Butte, Montana

Eocene 45 - 53 Tanzania

Eocene –

Paleocene

50 - 64 Namibia, Tanzania, Canada Yellow Water Butte,

Montana

U.

Cretaceous

65 - 79 SW Cape, Namibia, Mbuji-Mayi,

Zaire

M.

Cretaceous

80 - 114 Kimberley; Kundelungu, Zaire;

Lesotho; Orapa; Brazil; Sask,

Canada; Huangjiacum, China;

Sierra Leone –Guinea – Mali.

Hills Pond, Kansas;

Prairie Creek*, Arkansas;

Coromandel*, Brazil.

L.

Cretaceous

115-144 S.Africa GpII; Angolia, Kuoika,

Siberia; Liberia; India GpII

(Damodhar valley)

Murun, Irkutia

U. Jurassic 145-174 Newyork; Swatruggens,

S.Africa; Pri-Lina, L.Olenek,

Yakutia; SE Lesotho;

E.Griqualand; SE Australia.

Swatruggens, S.Africa;

(Wandagee, W.Australia)

L. Jurassic 175-204 Pennsylvania; Jwaneng; E.

Grikualand; SE Australia

Triassic 205-239 Kimberlites, W.Geenland Kapamba*, Zambia

Permian 240-290 Kentucky; Cross, Canada

Pennsylvan

ian

305 (Bulljah Pool,

W.Australia)

Devonian 340-409 Alakit-Daldyn, Male

Botuobinsk, Yakutia; Colorado-

Wyoming Somerset Island,

43

Canada.

Silurian-

Ordivician

410-499 Muna, Yajutia; Shandong and

Liaoning, China; Arkhangeisk,

Russian Paltform

Mt. Bayliss Antarctica.

Cambrian 500-600 Zimbabwe; Venetia, S.Africa;

W.Greenland

Upper

Proterozoic

800-

1250

Nort Western Australia; India;

Premier, A.Africa; Bubiki,

Tanzania.

Holsteinsberg, Western

Greenland; Majhgawan*,

India; Argyle*, Western

Australia; (Ngualla,

Tanzania)

Middle

Proterozoic

1300-

1500

Gabon; Liberia Bobi*, Ivory Coast;

Chelima, India.

Lower

Proterozoic

1600-

1800

Kuruman, S.Africa; Venezula Disko Bugt: W.Greenland

Lower

Proterozoic

Ca.2000 (Burkina Faso)

The word kimberlite is derived from the town of Kimberley in South Africa. The

African continent still remains the largest supplier of diamonds. The South African

block comprises three major components namely the Kaapvaal craton, the

Zimbabwe craton and the Limpopo mobile belt. The Kaapvaal and Zimbabwe

cratons are welded together by the Limpopo mobile belt to form the Kalhari

archon, which in turn is bordered by the Proterozoic mobile belts, such as the

Orange River belt, the Namaqualand Gneissic Complex, and the granite gneiss

complex of Namibia. The important pipes in south African block include

Bultfontein, Dutoitspan, Jagersfontein, Koffiefontein, De Beers, Kimberley,

Premier, Finsch, Venitia, Letseng La Tera, Kao in Lesotho, Dokolwayo in

Swaziland, Orapa and Jwaneng. The Central African block , which forms another

important kimberlite province, extends from Angola in the south to Cameroon in

the north and Tanzania in the east. Economically viable kimberlites are restricted

to Archaean parts of the Central African craton. The important kimberlites pipes

include Mbuji Maye, Kundulung in Zaire, Catoca in Angola, Mwadui in Tanzania.

The West African block constitutes another important Kimberlite province, which

includes Koidu limberlites in Sierra Leone and Soquinex in Guinea.

44

Russia is endowed with 20 kimberlite field and consists of more than 1000

kimberlite pipes and dykes representing another important kimberlite province.

The kimberlites are emplaced in to the Siberian craton, which comprises of two

Archons viz; the Anabar and the Aldan. The major diamond producing pipes in

this craton are Mir, Internatsionalnaya, 23rd Congress, Udachnaya, Aikhal,

Sytykanskaya, Zarnitsa and Yubileinaya.

Canada, arrival on the kimberlite scenario has been a little late. Though relatively

a latecomer on to the kimberlite map of the world since 1956, has turned out to

the most outstanding province in the world. This country has the largest extent of

cratonic blocks (or) Archons with intervening mobile belts (or) Protons. The most

important Canadian field are within the Slave craton of the Northwest Territories

(NWT) and the Superior craton, which came into prominence by the discovery of

diamondiferous kimberlite from Ponit Lake in the Lac De Gras area of Slave

craton in 1991. After these discoveries more than 600 kimberlites have been

discovered, some of which are of high economic potential. A newcomer (since

1998) as a supplier of gem quality rough diamonds to the world market, Canada is

presently ranking the third most important diamond producer (by value) in the

world.

Australia, with its large cratonic blocks like the Yilgarn and Pilbara

Archons(western Australia) and the Kimberley Archon (northwestern Australia),

contains kimberlites and lamproites. The first three kimberlite pipes were reported,

some of them with good diamond potential. However, the only diamond producing

mine of Australia is the Argyle olivine lamproite mine, which is a major global

contributor of diamonds. This lamproite is located in the Halls Creek Mobile Belt

bordering the eastern margin of the Kimberley Archon, northwestern Australia.

The other areas of kimberlite occurrence in the world include China with two

working mines, the USA with more than 25 kimberlite intrusions, and the

Arkhangelsk province, Ukraine, Belorus, Finland and Sweden with both

diamondiferous and barren kimberlites.

45

I.6 Indian Scenario:

As early as 300 BC , the grandeur of diamonds was introduced to the world by

India. Till the end of the 19th century India enjoyed the monopolous position by

being the exclusive supplier of diamonds to the entire world. Many well-known

diamonds like the Great Moghul (787 ct), the Koh-i-Noor (186 ct), the Pitt/Regent

(41ct), the Nizam (440 ct), the Hope (67 ct), the Orloff, the Darya-i-noor etc.. were

recognized and have been obtained from the mines along the courses of the

Krishna River, ( referred to as the diamond river by Ptolemy) in the state of

Andhra Pradesh, in southern Peninsular India. Since ages India is known its

wealth of most beautiful, famous and large sized diamonds, the existence of the

primary diamond source rocks were identified only in 1930 with the identification

of the Majhgawan pipe (where diamond production started a century back ) as

kimberlite (Sinor 1930) which was later termed as lamproite (Scot Smith 1989).

Subsequently a tuffaceous rock found near Wajrakarur in Andhra Pradesh,

(diamond mining was known since centuries), was recognised as kimberlite (Rao

and Phadtare 1966). These exciting works were the torch bearers that

provided an impetus to the geologists of the country to carry out the exploration

for diamonds in India. It is only with the ardous and sustained efforts by the

personnel of Geological Survey of India and other Government Agencies to locate

kimberlites and lamproites have have resulted in the identification of more than

sixty kimberlite bodies in the Dharwar Craton, five kimberlite bodies in the Bastar

Craton and three kimberlite bodies in the Southern part of the Bundelkhand craton

in recent years. In addition , around sixty lamproite bodies were also reported

from the region close to the eastern margin of the Dharwar Craton (Mitchell and

Bergman, 1991, Reddy et al 2003). The vast Archaean cratonic areas of

Peninsular India are ideal geological milieu for the emplacement of kimberlites. Of

the five ancient Cratons of India viz; Dharwar, Bastar, Singbhum, Bundelkhand

and Aravali, kimberlite emplacements are recorded from three (the Dharwar, the

Bastar and the Bundelkhand ) cratons only. (fig. I-5 & Fig.I- 5.1). It is only at the

Panna diamond belt (Madhya Pradesh) in Bundelkhand craton and the

Wajrakarur area (Andhra Pradesh) in Dharwar craton where diamond – mining

activity was known since a few centuries.

46

The Bundelkhand cratons has the distinction of hosting two kimberlite pipes –

Majhgawan and Hinota – in the Panna district of Madhya Pradesh. The

Majhgawan kimberlite has the distinction of being the sole working mine in India,

accounting for nearly 99% of the Indian diamond production. Sixty years prior to

the discovery of the kimberlites in south Africa, the Majhgawan pipe was

discovered in 1827 in the Bundhelkhand craton. The true identity of the

Majhgawan and the Hinota pipes as kimberlites was established only in the

1970s (Mathur and Singh 1971).

The Bastar craton hosts two kimberlite fields in the state of the Chattisgarh viz;

1. The Mainpur Kimberlite field (4 pipes) in the SE part of the Raipur district, rh

(Newlay, S.K. and Pashine, J.1993) and 2. The Tokapal kimberlite field, Bastar

district. In addition to these two pitpes, diamondiferous kimberlite diatremes are

reported from parts of Orissa adjoining the Mainpur Kimberlite Field and located

close to the contact of the Bastar craton with the Eastern Ghat mobile belt.

Apart from these identified bodies, a few alkaline intrusions occurring in the

Barakar formations in the Damodar Valley Coal fields of Eastern India that form a

part of the Singhbhum Craton. These rocks intrude into the fault bounded

Gondwana rift basin underlain by rocks of the Chotanagpur granite gneiss,

migmatite, granulite complex, These exotic rocks emplaced into Jharia, Raniganj

and Bokaro Coalfields were inititally classified as Lamprophyres/ Mica –

peridotites ( Ghosn, 1949) and later as calc alkaline lamprophyres ( minnettes)

and lamproites ( Middlemost et al., 1988 , Paul, 1991; Rocks et al.,1992 ).

Recently based on the detailed geological, geochemical and petrological studies(

Kent et al., 1998) classified them as Orangeites.

The emplacement of kimberlite / lamproite clusters in different diamond provinces

of India with their age given at Table I-3 & I-4.

47

Fig.I-5: Cratonic areas of India and distribution of kimberlites and lamproites (modified

after Radhakrishna, 1989).

48

Fig. I-5.1: Schematic distribution of Cratons (WDC= Western Dharwar Craton; EDC=Eastern Dharwar Craton), mobile belts (EGMB=Eastern Ghats Mobile Belt; CIMB= Central India Mobile Belt; DMB=Delhi Mobile Belt), rifts (GR=Godavari rift; MR=Mahanadi Rift), and kimberlite-clanintrusives (#) in the India sub-continent. The gravity lineament between Mumbai and Chennai seperates diamond-bearing KCRs to the south (Wajrakarur, B), from “non-diamond bearing” rocks in the north (Narayanpet,C). CB is the Cuddapah Basin, and SGT is the Southern Granulitic Terrane. (B) Distribution of diamond – bearing intrusive in the Wajrakarur Province. (C) Distribution of “non-diamond bearing” intrusive in the Narayanpet Province (Neelakantum,2000) (Source: Haggerty et.al, 2004).

I.7 Empalcement Ages of Kimberlites and Lamproites:

Kimberlites and Lamproites emplaced throughout geological timespan from

middle Proterozoic to Quaternary. The oldest kimberlites are in Venezuela (1700

Ma) and Kuruman-South Africa (1600 Ma) while the youngest emplacement

events recorded from Tanzania, Antarctica are of Quaternary age (50 – 55 Ma).

The ages of global kimberlites emplacement events indicate that not many

49

kimberlites erupted in the later Cenozoic period. However, many lamproites

eruptions like those of Leucite Hills, Smoky Butte in North America, Gaussberg in

Antarctica, Ellandale and Fitzroy Basin inWestern Australia, Murcia – Almeria of

Spain, Sisco in France took place around this time (Mitchell and Bergman, 1991).

Table: I-3: The summary of the emplacement of Kimberlite/Lamproite clusters in different Diamond Provinces of India and their age : S.NO Diamond

Province

K/L Field Craton

(Intruded

into)

District No.of

pipes /

bodies

reported

Emplace-

ment /

Age

1 CIDP Panna K/L Field Bundelkhand

-

Aravalli

Panna 02 Upper

Proterozo

ic

2 EIDP Mainpur K/L

Field

Bastar 06 - do -

3 EIDP Tokapal K/L Field Bastar Bastar 02 – do -

4 EIDP Nawapara K/L

Field

Bastar 02 -do-

5 SIDP Wajrakarur

Kimberlite Field

SIDP Wajrakarur

Cluster

Dharwar Anantapur 6 – do -

SIDP Lattavaram

Cluster

Dharwar Anantapur 6 – do -

SIDP Chigicherla

Cluster

Dharwar Anantapur 05 - do -

SIDP Kalyandurg

Cluster

Dharwar Anantapur 06 - do -

SIDP Timmasamudram

Cluster

Dharwar Anantapur 04 - do -

6 SIDP Maddur

Narayanpet

Kimberlite Field

Dharwar Mahboob-nagar

& Gulbarga

34 - do -

7 SIDP Raichur /

Tungabhadra

Kimberlite Field

Dharwar Raichur &

Kurnool

05 - do -

8 SIDP Chelima –

Ramannapet L

Field

Nallamalai

Fold Belt

03 - do -

9 SIDP Krishna Lamproite

Field

Dharwar Khammam 25 - do -

Total 106

Bundelkhand (M.P.) – 2; Bastar (CG & Orissa) – 10; Dharwar (A.P.) – 91; Dharwar (Karnataka) – 03, Nallamalai Fold Belt – 3.

Globally synchrounous kimberlite emplacement events are clustered

around ~1Ga (Africa, Brazil, Australia, Siberia, India and Geenland; ~450 to 500

Ma (Archangel in Baltic region, China, Canada, south Africa and Zimbabwe);

50

between ~370 and 410 Ma (Siberia and USA); ~200 Ma (Botswana, Canada,

Swaziland and Tanzania); ~80 to 120 Ma ( South Central and West Africa, Brazil,

Canada, India, Siberia and USA) and ~50Ma (Canada and Tanzania), with minor

intrusions in the Ellandale field of NW Australia at ~22Ma. The oldest

diamondiferous pipes are at Guaniamo, Premier, Argyl, Wajrakarur, Majgawan,

etc. and are of Proterozoic age. Whils the Russian pipes were emplaced mostly

during paleozoic period around 500-300 Ma, the African pipes are emplaced at

peak period during Cretaceous around 135-60 MA, which was the time of of large

–scale continental breakup. Canadian and Tanzanian emplacements are of

Eoceneage around 65-45 Ma. But globally the most profuse emplacement of

Kimberlite / lamproite took place during the last 200 million years. Though most of

the Indian Kimberlites and Lamproites known so far; indicate their emplacement

during Neo-Proterozoic age, Chalapathi Rao et al (2005) showed that the

Kodumali Kimberlites of the Mainpur Kimberlite Fields is of Paleozoic age (Table

I-4).

Table I-4: Empalcement Ages of the Kimberlites and Lamproites of India

Kimberlite /

Lamproite

Field

Pipe No. Age (Ma) Systematics Reference

Wajrakarur P-1 840±33 1090

K-Ar (W.R) Rb-Sr (Phl)

Paul et al 1975 Anil Kumar et al,1993

P-2 851±33 1092±15

K-Ar (W.R) Rb-Sr (Phl)

Paul et al 1975 Anil Kumar et al,1993

P-3 966±38 K-Ar (W.R) Paul et al 1975

P-4 1023±40 K-Ar (W.R) Paul et al 1975

P-5 1145±88 Rb-Sr (Phl) Anil Kumar et al,1993

P-7 1091±10 Rb-Sr (Phl) Anil Kumar et al,1993

Narayanpet KK-1 1363±48 1085±14

K-Ar Rb-Sr (Phl)

Chalapathi Rao et al, 1996 Anil Kumar et al, 2001

NK-3 1099±12 Rb-Sr (Phl) Anil Kumar et al, 2001

Nallamalai Chelima

lamproites 1200 1350±52 1417±8.2

Rb-Sr

(W.R.Phl) K-Ar (Phl) 40

Ar/39

Ar

(W.R)

Crawford & Compston, 1973

Chalapathi Rao et al,1996 Chalapathi Rao et al,1999

Krishna

lamproite

Field

Ramannapeta 1384±18 1221±18

K-Ar Rb-Sr(Phl)

Chalapathi Rao et al,1996 Anil Kumar et al,2001

Majhgawan 974±30 1067±31

K-Ar (W.R) Rb-Sr(Phl)

Paul et al 1975 Anil Kumar et al,1993

Mainpur

Kimberlte

Field

Kodumali 478±2 40Ar/

39Ar

(W.R) Chalapathi Rao et al,2005

51

I.8 Southern Indian Shield and Kimberlite / Lamproite Fileds:

South India is known from antiquity, for its most most beautiful, famous and

large sized diamonds. The ancient diamond mining activity was concentrated

mostly along the alluvial tracts of the Krishna River in the modern State

Andhra Pradesh (Fig.I-6). The South Indian Diamond Province (SIDP) spread

over >1,00,000 sq.kms extends from Kalyandurg in Andhra Pradesh and

Raichur in Karnataka in South and West to the eastern coast of India in the

east. India, the pioneer in diamond mining and trade, produces very small

quantity of the order of 90,000 carats, annually from its only producing mine

over the Majhgawan pipe near Panna in Madhya Pradesh. The production is

not able to meet even 0.1% of its annual domestic requirement of nearly 100

million carats, which accounts 80 -90% of the world production. Realizing the

need to locate new resources to meet the country‟s demand for rough

diamonds, the Geological survey of India along with other Govt. Agencies

have started diamond exploration which resulted in the discovery of many

primary source rocks – Kimberlites and Lamproites.

52

Fig.I-6: Generalised Geological Map of South India showing Kimberlite / Lamproite Fields. Source: Geol. Surv. Ind. Spl. Pub. No. 58 (2000).

53

The South Indian shield hosts nearly 100 kimberlite intrusions and more than

60 lamproite occurrences known till date. The kimberltes discovered in the

Southern Indian Shield till now (excluding those found by the Multi National

Companies as their exact locations were not known) are distributed three

distinct fields, viz., (1) the Wajrakarur Kimberlite Field (WKF) located in the

western part of Anantapur district in Andhra Pradesh, (2) the Narayanpet

Kimberlite Field (NKF) in the western part of Mahabubnagar district in Andhra

Pradesh and the adjoining Gulbarga district in Karnataka and (3) the Raichur

Kimberlite Field (RKF) in the eastern part of Raichur district in Karnataka and

the adjoining parts of Mahabubnagar district in Andhra Pradesh. Lamproites

on the other hand are, distributed along the Nallamalai Fold Belt of

CuddapahBasin in Kurnool district and in the granitic terrain adjacent to the

northeastern corner of the Cuddapah Basin, close to the eastern margin of the

craton (Fig.I-6). Among all the above kimberlites and lamproite bodies, almost

all the kimberlites of WKF are proved to be diamond-bearing where as those of

NKF are reported to be devoid of diamonds. Though not proved to be diamond

bearing, the kimberlites of RKF occurring in the vicinity of Raichur, which is a

known (apparently alluvial) diamond mining area since historic times

(Sakuntala and Krishna Brahmam 1984) are very significant from the point of

view of the alluvial diamonds along the Krishna River. None of the lamproites,

reported till date, is proved to be diamond bering except the report of a

suspected diamond grain in a thin section of one of the Chelima dykes (Sen

and Narasimha Rao, 1970).

Regional Geological Milieu:

Geologically, this historic and world-famous diamond province occurs in the

Southern Indian Shield, which is divided in to the Dharwar Craton and

Southern Granulite Terrain (SGT) based on the gross lithological assemblages

(Ramakrishnan, 1973). The Dharwar craton exposes a granite-greenstone

ensemble composed predominantly of granitoids, gneisses and greenstone

(schist) belts and late-to post-tectonic granites (Closepet Granite and its

equivalents), which were intruded by mafic dyke swarms. Sediments of Meso-

to Neo-Proterozoic intracratonic sedimentary basins overlie this dyke infested

Archaean granite-greenstone terrain. The granite-green stone terrain and the

54

sediments in the north and north western parts are covered by Cretaceous –

Tertiary lava flows of the Deccan Trap. The cratonic granite – gneiss

assemblage concealed below the lava flows perhaps extends up to the Son –

Narmada lineament in the north. The craton is bounded by the Eastern Ghat

Mobile Belt in the east, the rifted continental margin – a result of fragmentation

of the Gondwana Land – in the west, the Godavari Graben in the northeast

and Palghat – Cauvery shear system in the south.

Based on the differences in the characters of the schist belts, abundance of

younger granites, type of metamorphism and age of the terrain, the granite-

greenstone terrain of the craton is divided into Eastern and Western Blocks

separated by the Chitradurga Boundary Fault (Fig.I-6). The western block is

characterised by large schist belts with predominant platformal sediments and

the older (3300 – 3000 Ma) migmatitic gneisses whereas the eastern block is

characterised by narrow linear schist belts composed essentially of an oceanic

volcanic assemblage with rare platformal lithologies. These schist belts are

surrounded by vast expanse of younger (~2600 Ma) granitoids (Ramakrishnan

1994). A geologically striking feature of the craton is the N-S to NW-SE

trending cluster of late to post tectonic granite plutons known as the Closepet

Granite emplaced close to the Chitrdurga Boundary Fault.

The sediments of Meso-to Neo-Proterozoic intracratonic sedimentary

basins – the Cuddapah, Pakhal, Kaladgi and Bhima Basins –uncomfortably

overlie the granite –greenstone terrain of the craton. A characteristic feature of

the Cuddapah Basin situated in the eastern part of the craton is an intensely

folded Nallamalai Fold Belt in the eastern half while the western half is

practically undisturbed (Nagaraja Rao, 1987).

The granite – greenstone terrain shows effects of three phases of

deformation. While the earlier two deformations gave rise to the NNW-SSE to

NW-SE trending penetrative fabric marked by the general schistosity /

gneissosity and major faults and shears parallel to it, the third one produced

broad warps along E-W to ENE-WSW trending axes. The terrain is affected by

NNW-SSE to NW-SE trending trans crustal faults / lineaments. According to

55

the existing model of the Dharwar Craton, the western block represents, major

synclinorium, while the eastern block represents a major anticlinorium, the limb

portion of which is occupied by the Closepet Granite (Swaminathan and

Ramakrishnan, 1981). The Chitradurga Boundary Fault close to the eastern

margin of the Chitradurga Schist Belt (Fig.I-6) separates the two blocks

(Ramakrishnan, 1994).

A perusal of the tectonic evolution of the Southern Indian shield shows that

the western and eastern blocks came into being as a single coherent block by

around 2500 Ma, a period which marked the culmination of the Dharwar

Orogeny, with a wide spread emplacement of the syn-to post tectonic granites

and stabilisation of the craton (Ramakrishnan,1994). This event was followed

by crustal thinning and mafic dyke emplacement during the Middle Proterozoic

(~2000-1800 Ma) and as a consequence the Cuddapah Basin evolved through

the sinking of crustal blocks along the pre-existing lineaments (Drury et al.,

1984, Nagaraja Rao et al., 1987). Episodic development of the Eastern Ghat

Mobile Belt (Paul and Sarkar 1994) perhaps by collision of the Dharwar craton

with the Enderby Land of Antarctica culminated during the Middle Proterozoic

(Ramakrishnan et al. 1994). This continent-continent collision, apparently,

produced by the intensely folded Nallamalai Fold Belt in the eastern part of the

Cuddapah Basin and gave rise to the tectonic (trusted) eastern margin of the

Basin. The Permo – Carboniferous sediments deposited along the NW-SE

trending Pranhita-Godavari Graben at the northeastern margin of the craton

and the Crataceous-Tertiary sediments along the coast represent the younger

sedimentary basins in the southern Indian shield.

The Bouger Gravity map (10 m.gal contour interval) of India (NGRI 1975) has

clearly brought out the penetrative NNW-SSE fabric as well as deep-seated

faults cross-cutting the regional trend in the southern Indian shield. The

Bouguer gravity map (1 m gal contour interval) of the area between the

Cuddapah Basin and the Chitradurga Schist Belt comprising the Eastern

Greenstone Belts (Ramachandran et al 1999) has brought out the structural

details such as the tectonic contacts of the Greenstone Belts with the granites,

apart from the strong E-W cross structural trends. The aero-magnetic data

56

also corroborate the cross–cutting regional structural trends inferred from the

gravity data.

The surface heat flow in the Dharwar craton ranges between 31± 4.1 m

Wm-2 in the western block and 40 ± 3.4 Wm-2 in the eastern block (Gupta

1992). These values are comparable with the heat flow values of the other

cratons viz., 42± 11 m Wm-2 in the Superior province of the Canadian Shield

(Drury and Taylor 1987), 37 m Wm-2 in the Zimbabwe craton, +33 ±1.5 m Wm-

2 in the Kaapvaal craton of South Africa and 37.3±6 in the Yilgarn Craton of

Western Australia (Gupta 1993).

From the foregoing brief summary of the geological events in the southern

Indian shield and its geophysical signatures it can be seen that the Dharwar

Craton remained free of any orogenic movements since ~2500 Ma resulting in

the development of a thick cool lithosphere conforming to „Archons‟ of Janse

(1992), which is considered to be the most ideal tectonic setting for the

occurrence of diamondiferous kimberlites.

Description of the Kimberlite Fields:

Kimberlites discovered till now in Southern India are restricted to the Eastern

Block of the Dharwar Craton and are distributed in three distinct fields, viz.,

(1) The Wajrakarur Kimberlite Field (WKF)

(2) The Narayanpet Kimberlite Field (NKF)

(3) The Raichur Kimberlite Field (RKF)

The Lamproites on the other hand are distributed along the Nallamalai Fold

Belt within the Cuddapah Basin and in the granitic terrain adjacent to the

Northeastern corner of the Cuddapah Basin, close to the eastern margin of the

Craton.

1. The Wajrakarur Kimberlite Field (WKF) :

The WKF (Fig.I-7 & Fig.I- 8) measuring 120 km x 60 km encompasses the

area adjoining the western margin of the Cuddapah Basin. The kimberlite

pipes here are emplaced in to the granite – greenstone terrain forming a

57

part of the Eastern Block of the Dharwar Craton. The area exposes the

meta-volcano sedimentary sequence of the Ramagiri-Penakacherla schist

belt surrounded by gneisses and granites, which are, in turn, intruded by

the late –to post-kinematic granites (Closepet Granite and its

equivalents).Quartzo-feldspathic veins and pegmatites intruded these

granites and schist belt units. NW-SE and ENE-WSW trending dolerite,

gabbro and lamprophyre dykes constitute the younger mafic intrusive

bodies. The dominant structures controlling the emplacement of the

kimberlites in this field are the major ENE-WSW trending faults and their

intersections with NW-SE faults/fractures (Nayak et al, 2001). Some of

these ENE-WSW faults could be traced from Chitradurga Schist Belt in the

west to the East Coast through the Cuddapah Basin. A total of 42

kimberlite bodies are so far known to occur in this Field. The surface

configurations of the Kimberlite bodies in this field are near circular,

elliptical or linear bodies with dimensions ranging from 40 m x 20 m to 1200

m x 1000 m.

58

Fig. I-7 : Generalised geological map of Wajrakarur kimberlite Field (after Nayak et

al.,2001).

59

Fig.I- 8: Geological Map and location map of kimberlites of the Wajrakarur Kimberlite

Field (modified after Nayak and Kudari, 1999)

60

2. The Narayanpet Kimberlite Field (NKF):

The NKF is located about 150 km southwest of Hyderbad and 200 km

north of the WKF (Fig.I-9). it measures 60 km x 40km in extent, covering

the western parts of Mahabubnagar district in Andhra Pradeh extending

into the eastern parts of Gulbarga district in Karnataka. This field too is

located in the eastern Block of the Dharwar craton exposing the Archaean

migmatitic gneisses and granites of the Peninsular Gneissic Complex

(PGC) and the meta-volcano sedimentary sequence constituting the

northwestern extensions of Gadwal Schist Belt. In Maddur –Kotakonda

area, which forms the eastern part of this field, massive granitoids are the

dominant rock type compared to the schistose and gneissic rocks which

are the dominant lithounits in the western part of Narayanpet. Because of

this basic difference in the lithology, there is marked structural anisometry

observed in this field. The younger granites dominate the eastern part of

the field i.e., the Maddur-Kotakonda area. Here the E-W trending major

fractures and the NE-SW trending minor tensional fractures are the

controlling focal points for kimberlite emplacements. In the western part of

the NKF, dominated by gneisses and schist patches there are marked

strike slip faults trending E-W and NW-SE.

The surface extent of the kimberlites in this field is much smaller in general

(dimensions range from 5 m x 2m to 150 m x 50m) though one body (a

dyke) measures 2000 m x 50 m. All the known kimberlites of the NKF are

located either along the E-W trending faults or at their intersection with that

NNW-SSE faults or NE-SW trending resultant faults (Rao et al., 1998). All

the kimberlites of the NKF are emplaced into the migmtite gneisses and

younger granites of the PGC. A total of 49 kimberlite intrusions are

recorded in this Field.

61

Fig.I-9. Generalised Geological Map of the Narayanpet Kimberlite Field.

Source: Geol. Surv. Ind. Spl. Pub. No. 58 (2001).

3. Raichur Kimberlite Field (RKF):

The Raichur Kimberlite Field (RKF) is located about 70 km south of the

NKF and 120 km north of the WKF. This field measures about 20 km and

30 km in the E-W and N-S directions respectively. The Kimberlite intrusions

are distributed in the eastern parts of the Raichur district (Karnataka) and

the western part of the Mahabubnagar district (Andhra Pradesh). This field

too is located in the eastern Block of the Dharwar craton exposing the

granitic gneisses and granites of the Peninsular Gneissic Complex (PGC),

of which the massive granitoids are the dominant rock type with narrow

62

linear enclavial bands of schistose rocks of the Raichur Schist Belt. In this

field too the major E-W trending faults control the emplacement of the

kimberlite intrusions (Fig.I-10). A total of eight kimberlite intrusions are

reported in this field till date. The surface extent of the kimberlite bodies in

this field ranges from 26 m x 14 m to 600 m x 150 m.

Fig. I-10: Generalised geological map of Raichur Kimberlite Field.

63

C. Description of the Lamproite Fields:

The lamproites in this shield are distributed in two fields viz., 1. The

Nallamalai Lamproite Field occurring along the Nallamalai Fold Belt with in

the Meso-to Neo-Proterozoic Cuddapah Basin and 2. The Krishna

Lamproite Field occurring in the granitic terrain adjoining the northeastern

corner of the Cuddapah Basin.

1. Nallamalai Lamproite Field:

The Nallamalai Lamproite Field occurs along the Nallamalai Fold Belt,

where the rocks belonging to the Nallamalai Group represented by

Bairenkonda Formation (Quartzite, shale, basic intrusive) and Cumbhum

Formation (shale, phyllite, tuff, dolomite & limestone, quartzite) of the Meso-

to Neo-Proterozoic intracratonic Cuddapah Basin are tightly folded and

refolded giving rise to a series of dome –basin structures. The lamproite

dykes intrusive into this folded meta-sedimentary sequence include those

around Chelima and Pachcherla and around Zangamrajupalle. This nearly

N-S trending linear field extends over 80 km from Zangamrajupalle in the

south to Chelima in the north (Fig,I-6). In the Chelima –Pachcherla area

alone more than 30 small lamproite dykes are known to occur over a strike

length of 10 kms. The lamproitr dykes around Chelima are emplaced along

NW-SE trending fractures and faults and occur in an enchelon pattern close

to the southern part of a domal structure called the Iswarakuppam Dome

whereas the lone body recorded so far near Zangamrajupalle occurs close

to the closure part of another domal structure.

Appavadhanulu (1966) first described the Chelima dykes as minnets, which

were considered as rocks intermediate between carbonatites and

kimberlites by Sen and Narasimha Rao (1970). Bergman and Baker (1984)

and Bergman (1987) classified these rocks as lamproites – aterm reiterated

by Scott Smith (1989). Mitchell and Bergman (1991) termed them as olivine

phlogopite lamproites. The Zangamrajupalle body too is reported to have a

similar composition (Bhaskara Rao 1976).

64

2. Krishna Lamproite Field:

The Krishna Lamproite Field (KLF) located just outside the peripheral parts

of the northeastern corner of the Cuddapah Basin (Fig. I-6) falls north of the

Krishna River, close to the eastern margin of Dharwar Craton in Krishna

and Nalgonda districts. Occurence of lamproite in this part of the terrain,

which is in the close vicinity of the world famous alluvial diamond deposits

along the Krishna River, was first reported by Nayak in 1985 (Nayak 1991).

The area exposes the granites and gneisses of the PGC intruded by mafic

dykes trending WNW – ESE and ENE-WSW (Reddy et al 2003). The KLF is

spread over an area of about 160 sq.kms and comprises 25 lamproite

bodies. The lamproites occur as narrow (0.5 m to 5.0 m wide) dykes with

lengths varying from about 100 to 400 m, in close association with the

dolerite dykes. The lamproite bodies occur as clusters around nine

localities. Reddy et al (2003) have reported a range of petrological variants

of lamproites viz., Phlogopite – Leucite – Diopside –Richtertite – Lamproite,

Diopside-Lamproite and Olivine-Lamproite from this field.