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KIMBERLITE

Introduction

Kimberlite is defined as an mantle derived, potassic ultramafic intrusive igneous rock which

may carry diamond xenocrystals. It was originally described as a mica peridotite which contains

both mantle derived cognate and xenolithic inclusions. Kimberlite was named after Lord Kimberley(Colonial ecretary in outh !frica" by #.C. Lewis in $%&'.

Mineralogy & Petrography

Essential mInerals:

livine, pyroxenes, and mica constitute the essential minerals. livine dominates the others

in abundance. )he olivine composition may range from forsterite to monticellite. *yroxenes may

either be orthorhombic+ enstatite to ferrosilite or monoclinic+ chrome diopside in composition. ica

may be biotite to phlogophite in composition. )he essential minerals typically occur as anhedral

phenocrystals and often altered by metasomatic processes+ olivine to serpentine or talc,carboniti-ation magnesite or calcite.

Accesssory minerals:

!nhedral phenocrystal picroilmenite, chrome spinel, g garnet, perovskite, ilmenite, garnet,

and apatite constitute the common matrix minerals. /iamond occurs as a xenocrystal mineral.

Petrography

)he dominant texture is ine0uigranular anhedral prophyritic texture. 1enoliths of dunite,

peridotite, pyroxenite, eclogite, and crustal fragments have been observed in kimberlites. Chromite,

23)i spinels, g34e spinels may occur in essential 0uantities.

Mode of ccurence

Kimberlite generally occurs as a carrot shaped vertical intrusion termed as pipe or diatreme

and as tabular dykes, rarely as a sill. )he kimberlite diatreme consists of a tapered basal root -one

in the subsurface which gradually widens towards the surface and forms a crater like body on the

surface. )heir vertical extent is variable. )he surface crater is composed of epiclastics and

pyroclastic fragments. )he contact aureoles are poor or absent.

Association and Tectonic !ettings

Kimberlites mainly occur in *rotero-oic cratons with thick overlying continental crust (5 6.7

8a in age". Cratoni-ed regions which act as a focus for kimberlite diatremes are termed as

kimberlite provinces which is composed of several smaller kimberlite fields. Kimberlites within a

field are genetically related to one another with similar or differing intrusive ages. )he diameter of a

typical kimberlite field ranges from 79 to :9 km. )he number of kimberlite bodies varies from field

to field. ! number of kimberlite pipes which occur close to each other forms a cluster in a field.

Kimberlites are generally associated with continental intraplate magmatism associated with

ma;or lineaments or fault -ones which dissect the craton.

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"lassification of Kim#erlites

Kimberlites have been classsified based on mineralogy, mineral facies, petrography, and

outcrop appearance.

!$inner & "lement%s !cheme '()*+

)his scheme used accessory or matrix mineralogy to classify the kimberlite as+ diopside

kimberlite, serpentine kimberlite, and others.

,agner%s !cheme '('*+=agner classified kimberlites into two types based primarily on mineralogy and texture+

>!!L)IC KI>?

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