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CHAPTER – III

EXPLORATION FOR DIAMOND BEARING ROCKS (PRIMARY SOURCE)

Diamond occupies a unique place among the precious stones by virtue of

hardness, high refractive index and resistance to mechanical and chemical

changes. Exploration strategy is fixed before going for exploration.

III.1 Diamond-Host rocks: Diamonds are found in different environments

and in a variety of rock types, ranging from Archaean to Quaternary.

These include igneous rocks like kimberlites, lamproites & orangites

(Mitchell, 1986) and sedimentary rocks.

III.1.1: Igneous rocks: Kimberlites, olivine lamproites and orangites which

are referred as „primary source rocks‟, contain significant

concentrations of diamonds. Besides, diamonds are also found in

traces of ultramafic lamprophyres (particularly alonites), alkali basalts,

ophiolites, komatites, peridotite and eclogite xenoliths, metamorphic

schists or metakimberlites and meteorites.

Kimberlites (Ks), Lamproites (Ls) and Orangites, ranging in age from

„Meso Proterozoic to Quaternary, are reported from several parts of the

world (Janse A.J.A. and Sheahan P.A.,1995). Extensive Kimberlite

activity appears to have taken place in Proterozoic (1100 – 1250 Ma),

Ordovician – Devonian (440-500 Ma), Jurassic (145-160 Ma),

Cretaceous (115-135 Ma), 80-100 Ma and 65-80 Ma) and Eocene (50-

55 Ma) times. It appears that certain parts of the craton have been

subjected to repeated kimberlite magmatism. The Siberian Platform,

the Kaapavaal Craton and the West African Craton each has five

distinct periods of intrusions.

The oldest kimberlites are reported from Venezuela (1700 Ma) (Nixon

et.al., 1992) and Kuruman, South Africa (1600 Ma) where as the oldest

lamproites are from India (~ 1350 Ma). The youngest Kimberlites /

Lamproites (Quaternary) are in Tanzania, Antarctica, etc.. The oldest

diamondiferous pipes are reported from Guaniamo (Venezuala),

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Premier (South Africa), Argyle (Australia), Wajrakarur and Majhgawan

(India), etc. and are Proterozoic in age.

The kimberlite and lamproite rocks from Southern and Central India

Provinces are Neoproterozoic age (~1000-1100 Ma) (Anil Kumar et.al.,

2001) whereas the lamproites from Cuddapah Basin in Southern India

are emplaced around ~1350 Ma (Chalapathi Rao N.V., 2007) and

those from the Damodar Valley in eastern India during the Crateceous

period (~1100 Ma) when large-scale breakup of the continents took

place.

III.1.2 Sedimentary Rocks: Alluvial and beach deposits (Recent placers)

and Conglomerates (Paleo-placers) constitute „Secondary Source

rocks‟ for diamond. Diamondiferous river gravels are Quaternary age.

Though diamonds are found in many rock types, kimberlites, olivine

lamproites and orangites along with the recent placers have significant

concentrations of diamond to form economically viable deposits.

III - 2: General Features of Kimberlites (Ks) and Lamproites (Ls):

III.2.1 Morphology of pipe rocks:

The characteristic features of kimberlites and lamproites diatremes are

their general shape and distinct depth zones. Size, shape and

complexityof the diatreme depend on many factors – the rock types

encounters, fracturing in the country rocks, involvement of ground

water, supply of source magma and number of different phases

reaching the surface. The classical model developed by Dawson

(1967) and Hawthrone (1975) for a carrot shaped kimberlite pipe

consists of root, diatreme and crater zone/facies. Lamproite pipes have

sherbat glass shape in which crater facies are generally intruded by

magmatic facies (Scott Smith, 1989).

Kimberlites and lamproites are inferred to be the products of

continental intra-plate alkaline magmatism forming small bodies (pipes)

relative to other volcanic rocks. These occur as vertical tube like

bodies (pipes) and dykes occupying small surface areas, ranging in

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size from a few hectares(ha) to more than one hundred hectares. A

few clusters may form a field and each field may contain 1 to 40/50

more bodies. A province is a large area containing one or more field.

III.2.2 Mineralogical Characteristics:

Kimberlites and lamproites contain a variety of major and minor

minerals that crystallize directly from kimberlite and lamproite magmas

alongwith mantle derived xenocrysts (Kirkley et.al., 1992). Olivine

occurs as xenocrysts, phenocrysts and as groundmass constituent.

Besides olivine, diopside and phlogopite occur as major minerals and

apatite. Perovskite and ilmenite as minor minerals suggesting similar

chemical characteristics of their respective magmas. Lamproites

exhibit an extremely wide range in mineralogy. Besides, the Ks and Ls

contain xenocryst minerals derived from the upper mantle viz., olivine,

garnet, clino and ortho pyroxenes and chromite and rarely diamond

(Table III-1).

III.2.3 Chemical composition:

Both the rock types are silica poor but rich in MgO, FeO, K2O and

volatiles (CO2, H2O and/or F) relative to basaltic rocks. Lamproites are

per-alkaline and typically ultrapotassic (containing 6% to 8% K2O

compared to 0.3% to 2% K2O in Group I and up to 5% K2O in Group II

kimberlites. Kimberlites have high K/Na and low Mg/Fe ratios, which

distinguishes them from most ultramafic rocks. Kimberlite is more

encriched in CO2 (average 8.6%) where as lamproite has <1% CO2 but

enriched in F.

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Table III-1 Mineralogy of Kimberlites and Lamproites (Kirkley et al., 1992)

Minerals that crystallize directly from Kimberlite and Lamproite magmas

Mineral Kimberlite Lamproite

Major

Olivine x x

Diopside x x

Phlogopite x x

Calcite x

Serpentine x

Monticellite x

Leucite x

Amphibole x

Enstatite x

Sanidine x

Minor

Apatite x x

Perovskite x x

Ilmenite x x

Spinel x x

Priderite x

Nepheline x

Waedite x

Xenocryst minerals derived from the upper mantle

Olivine x x

Garnet x x

Clinopyroxene x x

Orthopyroxene x x

Chromite x x

Diamond x x

X: Present

III.3.0 EXPLORATION TECHNIQUES:

For discovering kimberlites / lamproites, the age old techniques like

traditional mapping, chip sampling are less effective in view of their

relatively small size and restricted distribution. The different techniques

available for targeting of kimberlites and lamproites are many which

vary from place to place depending on the facilities / resources

available. Modern exploration programmes involve a combination of

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Remote Sensing, Aerial photography, Airborne geophysical survey,

Geological, Geochemical survey, Stream sediment sampling / heavy

mineral studies and Ground geophysical survey; of which the last two

techniques are found to be quite successful.

III.3.1 Remote Sensing Techniques:

Different remote sensing techniques used in diamond exploration rely

on reflection / radiance of earth materials and their analysis through

satellite imagery, aerial photography and multi-spectral scanning. Both

imagery and aerial photo studies are quite useful in diamond

exploration but cannot provide direct evidence of kimberlite / lamproite

location (Atkinson 1989 and Coopersmith 1993).

Lineaments, their intersections and contacts are favorable sites for

kimberlites and lamproites. Remote sensing studies are useful for

mapping of the area on regional scale, to provide first hand information

like the structure of various tectonic belts, lineaments and fracture

patterns. Remote sensing studies also helps in establishing the

corridors pertaining to kimberlite / lamproite emplacement.

The platforms of LANDSAT, SPOT, INSAT, etc.. have a fixed

resolution of 15 to 30 m and atleast 10 to 20 pixels are required to see

the signature of a feature. So, pipes of more than 20 ha sizes are likely

to be detected. For example, Tokpal kimberlite body in Bastar district,

Chhattisgarh is prominently seen on the INSAT imagery.

Based on the satellite data, False Colour Composites (FCCs) could be

generated on 1:50,000 and 1; 25000 scales. With this the direction of

the major / minor lineaments could be observed. Based on the field

visit for the ground truthing of Remote Sensing data, the geo-

environment involving major lineaments, minor lineaments, dykes and

various lithologies can be interpreted. Thematic maps can be

generated based on the degree of incidence on the ground of each

parameter. Favorable locations can be demarcated as polygons on

different FCC‟s. Subsequently thematic maps could be generated for

further search of kimberlites/lamproites using GIS softwares.

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Aerial photography is more useful in obtaining information on regional

structure, geomorphology and delineating alluvials or

kimberlites/lamproites. Many pipes and dykes have been located in

southern and central Africa, based on vegetation features, circular

depressions / mounds or tonal diffrences visible on aerial photographs

(Mannard, 1968).

The commonly used diamond exploration rechniques for primary

sources is provided in Table III-2.

Table III-2: Commonly Used Techniques in Diamond Exploration:

Method / Technique Remarks

Aerial Photography It is a basic tool for studying structural environment of primary rocks. Gravel beds can be identified.

Satellite Imagery Good for structural analysis. Many kimberlites and lamproites are too small to be observed directly.

Airborne multi-spectral scanning

Requires rigorous preliminary field reflectance studies over known intrusions under specified climatic conditions to obtain a significant spectral signature.

Geochemical surveys

Elements like Cr, Ni, Sr, Co, Ba, may be used if the background values are appropriate to identify the source.

Ground and airborne magnetic methods

Applicable in areas of low magnetic background, Airborne surveys are successfully used but topogra[hic irregularities, may give false anomalies.

Resistivity methods Used locally for delineating pipe rocks and also in locating gravel horizons.

Electrical/ Electromagnetic

Techniques depend on weathering characteristics of the pipe rocks. These are generally used for local search/ delineation of bodies. Airborne EM techniques are also quite useful.

Gravity The methods, used in conjunction with magnetics, depend on the density contrast between the pipes and the country rocks.

Heavy Mineral studies

To identify indicator minerals associated with Ks and Ls are trace them to the source rocks.

III.3.2 Airborne Magnetic Survey:

Aeromagnetic survey constitutes important tools for identification of

potential areas for kimberlite / lamproite search. It is primarily based on

the principle that the magnetic response of kimberlites (measured in

„nT) may be recorded as high or low as compared to surrounding

rocks, and the contrasts thus obtained, in addition to delineation of

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deep-seated lineaments are useful in the search of kimberlites. The

magnetic responses in the Ks and Ls are due to the content of

Magnetite (Fe2O3), Ilmenite (FeTiO3) and Ulvospinel (Fe2TiO4) and

their relationship with each other in solid solution. Fresh kimberlites

have 5 to 10% iron oxides. Unweathered kimberlites/lamproites have

strong magnetic susceptibilities than that of the weathered ones.

Magnetic susceptibility for kimberlites ranges from 10 to 10,000 x 10-5

and for lamproites, 180 x1800 .The magnetic responses are

complicated by weathering and multiple or zoned intrusives.

The search parameters for using the aeromagnetic survey, particularly

flight line spacing and ground clearance, depend upon the expected

size of the kimberlite pipe / lamproite bodies. As most of the pipes are

of the order of 150-250 m in diameter, the suggested line spacing is

<300 m with a terrain clearance of <80 m (60 m in flat terrain).

As aforementioned, confined nature of geophysical anomalies are

better picked up by low flight height and close space of flight lines,

heliborne magnetic survey is preferred which suits for low flying. In

India, this needs necessary clearance from DGCA.

III.3.3 Stream Sediment Sampling / Indicator Mineral Techniques:

One of the earliest used techniques in diamond prospecting i.e.,

Indicator Mineral sampling techniques form a major component in any

exploration programme framed for targeting diamondiferous kimberlite

and lamproite deposits. These are classic but indirect methods used

for regional and detailed sampling of prospective areas for kimberlites

and lamproites Since the discovery of first kimberlite in 1870‟s in South

Africa, indicator mineral techniques have been widely used and

responsible for many discoveries all over the world (Muggeridge,

1995). For example, three kimberlites in Kalyandurg cluster of

Wajrakarur Kimberlite Field have been discovered (Sravan Kumar et

al., 2004, Abhijeet Mukherjee et al., 2007).

The kimberlites and lamproites contain a suite of heavy minerals such

as garnet, chromite, ilmenite and chrome diopside. These minerals,

which accompany diamonds are more common than the diamond itself

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and commonly used as pathfinders / tracers for locating kimberlites

and lamproites. Besides, enstatite, olivine, phlogopite and zircon are

also used as indicators. Diamond itself is used as a pathfinder in many

cases. Most of these resistant minerals are accumulated in drainage or

alluvial sediments and also in loams and tills, on release from

weathered kimberlites and lamproites. Relative durability and high

specific make them more significant for using in sampling programmes.

The minerals in the order of decreasing resistance are zircon,

chromite, ilmenite, garnet, chrome-diopside and olivine whose specific

gravities range from 3.1 t o 4.7. These minerals with unique physical

and chemical characters are

(a) Resistant to weathering,

(b) Dispersed in to surface environment

(c) Transported over considerable distance and

(d) Diagnostic of their source.

The concentration and type of indicators in a dispersion train will

depend on the abundance and types present in source rocks. The

mineral content and grain size vary considerably within and between

the pipe rocks. Kimberlites may contain 0.1% or more of certain

indicators and not all are present in all kimberlites. The detection of

olivines and chrome diopsides in sampling trails usually indicates

proximity to source but in warmer and humid tropical climates, these

are readily destroyed within a few kilometers from source. More

resistant minerals such as chromite, survive long transport and are

found in the drainage sediments for several tens of kilometers away

from source rocks. The absence of particular indicators in a dispersal

train does not indicate that pipes are absent in source regions.

The characteristics of different indicator minerals and or exploration

techniques have been reviewd by Dawson (1980); Nixon (1980);

Gurney (1984); Mitchell (1986); Atkinson (1989); Smith et al (1991);

Gurney & Moore (1993); Cooper Smith (1993); Muggeridge (1995);

Fipke et al (1995) and others. The indicator minerals, which are

commonly used in tropical climates like that of India, are magnesian

garnet, chromeferous chromite, magnesian ilmenite, chrome-diopside

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and zircon. The different indicator minerals and their general

characteristic properties are given in Table III-3.

Table III – 3: General Characteristics of the Indicator Minerals

Mineral Composition Colour H S.G.

Garnet Mg3Al2(SiO4)3 Orange to red, purple to pale lilac, mauve, rarely green.

6.5 – 7.5

3.5 – 4.3

Picro- Ilmenite

FeTiO3 with chromium Black to grey-brown 5 - 6 4.5 – 5.0

Chromite FeCr2O4 Brown to black 5.5 4.5 – 4.8

Chrome-diopside

CaMg(Si2O6) Bright green to emerald green

5 - 6 3.2 – 3.6

Zircon ZrSiO4 Colourless, yellow, honey coloured, pink, reddish-brown and grey

7.5 4.7

Diamond Carbon Colourless, shades of yellow, brown, often fancy coloured.

10 3.52

Olivine (Mg,Fe)2 SiO4 Shades of green, pale-yellow to pale-brown.

9-7 3.2 – 4.3

Phlogopite KMg3(AlSi3) O10

(OH,F)2 Bronze, reddish brown 2.5 -

3 2.78-2.85

(H: Hardness; S.G. : Specific Gravity)

III.3.4 Sampling Programme:

The different sampling techniques followed, sample density and size,

sample collection, sample trap sites, field and laboratory processing of

samples are discussed in detail by Gregory and White (1989); Davison

(1993); Muggeridge (1989, 1995) (Fig. III-1) and Towie and Seet

(1995).

Exploration programmes aimed at detecting indicator or pathfinder

minerals are (i) stream or drainage sampling, (ii) loam sampling, (iii)

glacial till sampling, and (iv) anthill or burrow mound sampling

(Muggeridge,1995). The first two techniques are commonly used.

Stream bed sampling is effective in areas of high relief or heavily

dissected terrain. Loam sampling is applicable to low relief areas with

poor drainage.

Important clues to actual proximity to or away from source rocks are

(a) freshness of the indicator minerals, (b) morphological features, and

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(c) abrasion and transportation characters (Mc Candless, 1990;

Afanasev et. al., 1984; Mosig, 1980).

Fig. III-1 : Different Sample Trapsites (Muggeridge,1995)

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III.3.5 Orientation Surveys:

Regional programmes depend on the composition of the country rocks,

geomorphological history, mineralogy of the pipe rocks, size of the

samples, nature of drainage, trap sites, laboratory methods of

concentration and skills of mineral pickers. Properly executed and

interpreted orientation (reconnaissance) surveys are vital in successful

implementation of heavy mineral sampling programmes.

Reconnaissance surveys are wide spaced and cover large areas. For

reconnaissance survey, samples rangefrom 25 to 200 Kg to cover 4 to

20 Sq.Kms area (Gregory and White, 1989). The programme reveal

different aspects for consideration, such as (a) the type of mineral to

be sampled, (b) suitable grain size, (c) likely distance of transportation,

(d) size of sample; (e) number of samples to be collected from a given

area, and (f) nature of heavy minerals being contributed by the country

rocks (Cooper smith, 1993).

III.3.6 Detailed Surveys:

Though orientation surveys will broadly guide planning of detailed

sampling programmes, it is vey difficult to define sample density. The

factors related to sample spacing, size and minimum grain size for

examination are (i) topographic variation and density, (ii) degree of

weathering and erosion of country rocks, (iii) depth and type of

overburden, (iv) drainage density, and (v) overall regional trap site

quality (Muggeridge, 1995).

Stream sediment or loam sample weight is commonly kept at 60-80 kg

to obtain around 40 kg of 1 to 2 mm size material (Muggeridge, 1995).

It is desirable to select good heavy mineral trap sites for sample

collection, which include crevices and joints cutting across the

drainage, depressions in the river channel, boulder bars, basal gravel

accumulations, under tree roots, etc. Gregory & White (1995) and

Muggeridge (1989, 1995) (Fig. III-1) classified the trap sites in to five

categories, ranging from good to poor. Samples are normally collected

using hand – shovels, pick axes, trowels, brushes and dustpans.

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III.3.7 Processing and Analysis of Samples:

Success of any sampling programme depends mostly on experienced

and specialist laboratory. Recovery of rare indicator minerals relies

entirely on accurate and meticulous processing and observation

techniques by competent and trusted staff. Extreme care must be

taken to avoid sample contamination or loss of indicator grains.

Field concentration of samples may be conducted to screen the coarse

size material down to two fractions, viz., 1.2 mm to 600 # and 600 # to

300 # approximately by using screens, portable jigs, deep conical

pans, etc.. Conventional panning requires greater skills. The field

operations depend on the type equipment used and the skill of the

operators. Commonly used laboratory techniques involve jigging,

tabling, heavy liquid separation (Bromoform / Tetra Bromo Ethane),

alkali fusion, magnetic separation and observation. The various

processes and steps involved and equipment used have been detailed

by Gregory and White (1989), Davison (1993), and Towie & Seet

(1995). Normal size ideal for microscopic observation is 1.2 mm to 0.6

mm and 0.6 to 0.3 mm.

The flow chart adopted for processing of stream sediment samples and

grid loam samples during the recent kimberlite discoveries in

Kalyandurg is as under:

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Flow Chart of Stream Sediment Sample /Loam Sample Processing

Dry Screening

Dry Screening

1.18 mm -1.18 mm to +600 # -600 # to +300# -300 #

Reject Reject

(After Observation)

Jigging 500 # screen Jigging 250 # screen

Conc. +600 # to – 1.18 mm Conc. +300 # to – 600 #

Over all Conc. +300# to – 1.18 mm

TBE Seperation

Float Sink

Reject Sink

Non-Mag Conc

Mag. Conc.

Visual Picking

Analysis of indicator minerals is normally done using either Electron

Probe Micro-Analyser or Scanning Electron Microscope with EDX

facility.

40 Kg (-4 mm) Stream Sediment Sample / 20 Kg (-4 mm) Loam Sample

40 Kg (-4 mm) Stream Sediment Sample / 20 Kg (-4 mm) Loam Sample

Diamond Cr-diopside Garnet Cr-Spinel Picro-

ilmenite

EPMA Follow-up Kimberlite

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III.3.8. Calcrete Chemistry as exploration technique :

Many KCR‟s have calcrete cappings. Calcretes over KCR‟s, granite

and granite gneiss are not very much different in their physical

appearance. Major oxide chemistry was therefore attempted to bring in

focus target area. Roy et.al (2002) also attempted studies on the

calcrete over KCR‟s, granite and granite gneiss based on their major

oxide chemistry in WKF and inferred the parent rock based on the

chemistry of calcrete. Roy et. al (2002) also indicated that the Maturity

Index (MI) of calcrete is directly proportional to its CaCO3 content.

MgO content is not necessarily higher in calcrete associates with

KCR‟s and so MgO value of calcrete cannot be related to its parent

rock. However, in KL-5 of Kalyandurg it is found that MgO is related to

CaO/SiO2 ratio (MI) and these two attributes have a high correlation

with a=0.76.

The high degree of linear correlation between MgO and the ratio

CaO/SiO2 is noticed. It is observed that in KL-5 of Kalyandurg, the M.I.

<3.0 for gneissic rocks where as the M.I. > 3 or 3.5 for KCR. Similarly

the MgO < 2 for Gneissic rocks and >2 in KCR. This technique also

has been utilised in KL-5 discovery of Kalyandurg (Abhijeet Mukherjee

et.al. 2007).

III.3.9 Modern Exploration Techniques in Indian context:

As stated elsewhere, the use of state-of-the-art techniques in diamond

exploration programmes in the country is somewhat recent. GSI, which

has been carrying out exploration for search of kimberlites /lamproites

by using traditional techniques, made efforts to acquire modern

facilities since late 1980s. Kimberlite/ lamproite discoveries during

1960-80 came from field mapping and ground geophysics. More

discoveries came since 1990s, based on the conceptual approach,

integration of geological and geophysical data (ground and airborne)

use of remote sensing techniques, acquisition of modern laboratory

equipment such as EPMA, SEM-EDX and software for processing of

digital and field geological and geophysical data, experience /

expertise developed.

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Presently GSI, PSU‟s of Central Government, State Departments and

other MNC‟s are familiar with the different exploration techniques and

discovering more kimberlites /lamproites.

III.4.0 EXPLORATION STRATEGY:

The aim of a diamond exploration programme is to find an economic

deposit / mine and the discovery of a new deposit is major task.

Diamond prospecting in modern times began with the discovery of

kimberlite as the primary rock for diamond from the Kimberley region in

South Africa in 1869. Since then the diamond exploration has been

active in various parts of the world. In the last 140 years many

kimberlites have been reported from several countries, which are

mostly confined to the regions of continental crust underlain by the old

cratons (Clifford, 1966) and economically diamondiferous kimberlites in

„Archons‟ i.e., cratons underlain by old Archaean basement and in

„Protons‟ i.e., cratons underlain by old Proterozoic basement (Janse,

1984, 1992). For more than 100 years, Kimberlite was considered as

the only significant primary rock containing economic concentrations of

diamond. With the discovery of diamond pipe rock, known as olivine

lamproite (in many respects similar to kimberlite), at Argyle in Western

Australia in 1979, outside the cratons, prospective areas have been

widened. Lamproites occur generally either along the craton margins

or within the Proterozoic cratonised mobile belts accreted to the

cratons (Bergman, 1987). As the kimberlites/lamproites rarely occur

alone and form large provinces, there is a need to consider very large

areas for prospecting to identify a few small targets.

III.4.1: Kimberlites and Lamproites as Exploration Targets

The kimberlites and lamproites form important exploration targets

because they contribute approximately about 70% by weight and 60%

by value of the total diamond production apart from the placer

deposits, i.e., alluvial and beach sources. The advantages of

Kimberlites and Lamproites as exploration targets are (a) infrastructure

is at one place, (b) life of mine can be of several tens of years and (c)

the pipes rocks are well suited to low-cost open pit mining.

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The diamond content in pipe rocks is generally low (1 in 20 million).

The kimberlites / lamproites which contain economic diamond

concentrations are not common. Although more than 5000 to 6000

pipes and dyke rocks are known the world over, 10% of them are

diamondiferous and only 1% of them have economic concentrations of

diamonds i.e., 50 or 60 pipes turned out to be mines. Pipe rocks

generally occur in clusters / fields containing a few tens of them but

economically viable pipes are a few. The world-wide ratio of economic

and diamondiferous to barren pipes is quite different for different

tectonic and geologic settings.

A few examples are – in the Kimberley Field (South Africa) 5 out of 29

pipes are economic, in Oropa Field (Botswana) 2 out of 29are

economicand in Udachnaya Field (Russia) 1 out of 56 is economic.

The Wajrakarur Field in South India, which contains 41 pipes

(discovered till now), is yet to produce an economic pipe. Almost all the

pipes tested in this field are diamondiferous. In Central India, the

Panna Field has one marginally viable Majhgawan pipe out of

discovered kimberlite pipes so far.

The diamond content of a pipe rock depends on (i) the amount of

diamond bearing peridotite and eclogite xenoliths, (ii) the grade of the

source rocks, and (iii) the preservation of diamonds during

transportation to the surface. The diamond-bearing magmas enroute

upwards will be outside the diamond stability field for some time before

reaching the surface. If the rate of ascent of magma is not sufficiently

rapid, diamonds may be converted to graphite, or more frequently to

CO2 (Eggler, 1989), due to higher oxygen fugacity in the magma

(Haggerty,1986).

The kimberlite/ lamproite rocks hosting the diamonds occur either as

vertical tube / pipe or dyke –like bodies in clusters and occupy small

surface areas. Assuming that each pipe on an average is 8 ha (which

is very much on higher side), the 5000 and odd pipes known occupy

0.00027% of the land area. These rocks are generally altered and

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degree of weathering depends on regional and local climatic

conditions. Exploration strategies employing modern techniques which

are proved successful in a particular area can not be implemented

entirely in other areas in view of different physiographic /climatic

conditions and tectonic / geological settings.

III.4.2: Stages in Exploration:

The different stages in a diamond exploration programme are given

below. The different stages indicated are generalized and may overlap

with one another.

Stage – 1: Programme design includes defining goals and targets,

literature study, preliminary geological studies.

Stage – 2 : Reconnaissance surveys for are selection based on tectonic

and structural features and geological information and

delineate target areas using remote sensing, photo geology,

airborne geophysics and stream sediment surveys including

orientation (reconnaissance) for indicator minerals.

Stage – 3: Detailed prospecting by detailed mapping, ground geophysics

and heavy mineral sampling.

Stage – 4 : Evaluation of prospect by delineation mapping, intense

heavy indicator mineral sampling, ground geophysics,

drilling, micro-diamond sampling, bulk sampling and

feasibility studies.

For area selection, understanding of the tectonic history of the area is

a must, which should be integrated with traditional search methods.

III.4.3: Area Selection Criteria:

Area selection is the most important aspect of diamond prospecting.

Prioritization of any area for prospecting is done taking into account

available tectonic, structural, geomorphological, geological and

geophysical data (Cooper Smith, 1993).

104

For the purpose, meaningful understanding of upper mantle processes

and Archean-Proterozoic tectonics is required. Information relating to

different techno-thermal / magnetic events, deep-seated fractures and

heat flow regimes is to be collected. Database of all the known

diamond occurrences and host-rocks is to be built up. There is a need

to characterize the known ultrabasic / ultramafic rocks for their

kimberlitic / lamproitic affinity.

III.4.4: Tectonic and Structural Controls:

Area selection for diamondiferous kimberlite and lamproite search can

be improved by paying attention to the geotectonic controls of diamond

formation and preservation (Helmstadet and Gurney, 1994).

a) Detection of regions with ancient mantle roots (relatively low

density and low temperature) in which diamonds may have formed.

b) Selection of areas in these regions under which diamonds may

have survived to be sampled by younger kimberlites or lamproites.

c) Establishment of regional tectonic and local structural controls for

the emplacements of potential diamond host rocks in the

appropriate areas.

The formation and emplacement of kimberlite and lamproite magmas

appear to have been associated with several tectonic features, which

were discussed in detail by Mitchell, (1986); Janse, (1992); Kaminsky,

(1995); White et al., (1995) and others. Some of the regional features

with which kimberlite and lamproite emplacements take place are:

i) Cratons and cratonised mobile belts

ii) Areas underlain by deep subcontinental lithospheric roots

iii) Shallow geothermal gradients and low heat flow areas

iv) Anticlises and synclises

v) Areas showing zonal distribution ultra-basic rocks across cratons

vi) Transform fault extensions

vii) High magmatic permeability zones etc..

The pipes have limited relationship to the rocks into which they are

emplaced whereas their geographic distribution is largely controlled by

regional structures.

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III.4.5: Exploration Programmes:

The exploration techniques used in diamond exploration programmes

are many and there is no single 100% tool or technique to detect the

pipe rocks. Hence, the need is for an integral programme comprising

remote sensing and photo geological, geological, geochemical and

geophysical techniques which will be a powerful exploration tool.

III.4.6: Evaluation of a Diamond Prospect::

The factors that are relevant to diamond evaluation are (i) total volume

of the rock / deposit, (ii) diamond content (grade in cpht), (iii) average

size of diamond (ct/stone), (iv) stone density (no. of stones/tone), and

(v) percentage of gem content. Grade estimation procedures require

effective recovery of macro-diamonds (>0.5 ct).

Extraction of diamonds from bulk samples is done using jigs, diamond

pans and heavy media separation (HMS). Final recovery of diamonds

is done by hand picking, grease tables or x-ray sorters or combination

of one or more of these methods.

Economic viability of a prospect can‟t be generalized as it depends on

several factors like location and size of the prospect, type of mining

required (open cast or underground), nature of ore (hard & soft),

availability of infrastructural facilities, type of diamonds, etc.

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