Chapter 2 diamond exploration principles and practices

Chapter – 2 Diamond Exploration- Principles & Practices
47
2.1 Introduction
2.1.1 Diamond Exploration Philosophy
‘The diamond content of a kimberlite magma arriving at the earth’s surface is
defined by ascent processes. Once a pulse of magma arrives close to the earth’s
surface, the diamond distribution within the resulting consolidated kimberlite
depends on the final emplacement processes. Multiple pulses of kimberlite
typically form a single body and each pulse has a different diamond content and
emplacement history. Thus, the diamond distribution within single kimberlite
bodies can be complex. The understanding of the geology and emplacement
history of kimberlite bodies plays a critical role from early exploration through
to mineral deposit evaluation, resource determination, deposit economics,
mining and resource reconciliation’ -------- Howard Coopersmith et al., (2006)
Majority of the investigations pertaining to kimberlites, culminate at assessing the
diamond potentiality, and therefore it implied that the exploration of kimberlites is equated
mostly with diamond exploration. The philosophy of diamond exploration lies in discovering a
mineable kimberlite pipe which involves an integrated approach with the aid of geological,
geophysical and geochemical methods. Locating these volumetrically small intrusions is a tough
task for the geologist and it demands a religious systematic approach of exploration. It is
imperative that a comprehensive systematic approach is adopted by kimberlite explorationists,
right from selecting the area, prospecting on regional scale followed by that on prospect scale and
target testing by drilling. First of all, locating a new kimberlite pipe is important, immaterial
whether it is diamondiferous in the initial exploration stage.
The Clifford‟s rule (1966) states that diamondiferous kimberlites are exclusively found in
regions forming tectonically stable Archaean cratons that have mantle roots which is evident from
viable economic deposits worldwide (Gurney et al., 2005). The distribution of majority of notably
diamondiferous „on-craton‟ kimberlite occurrences in Africa, Russia, Canada, North America and
India is an attestation to this fact. The exception to Clifford‟s rule is the Argyle lamproite of
Australia, which is „off-craton‟ and occurs in a Proterozoic belt. Seemingly diverse and
genetically related, yet interesting rock types like, kimberlites, lamproites, ultramafic
lamprophyres and carbonatites have been increasingly under scrutiny (Gaspar and Wyllie, 1984;
Haggerty, 1989; Haggerty and Fung, 2006; Tappe et al. (2008, 2013) and reported occurrences
and association of any of these rock types is likely to increase the exploration impetus towards
successful discovery.
Archaean cratons possess relatively cool lithospheric roots with downward deflection of
isotherms and a corresponding upward deflection of the diamond stability field. This region of
high temperature (~1200o
C) with relatively low pressure provides a „window‟ in which diamonds

48
can form and be preserved. Kimberlite magmas are generated at or below these depths and may
sample the lithospheric roots. Thus these magmas en route the diamonds to the earth‟s surface.
Thus the philosophy of diamond exploration lies in appropriate area selection for
exploration considering several factors like suitability of the area to possess mantle deep fracture
systems for kimberlite emplacement, geological setting, presence of already discovered
kimberlite and/or related rocks and choosing the right exploration methodology. A clear
understanding of different types of diamond deposits, regional and local geology, and different
exploration techniques will lead to exploration success towards discovery of new kimberlite
pipes, out of which some or one could be diamondiferous. It is seen that the philosophy of
exploration is understood well in the present day and the same was put into practice to evaluate
the diamondiferous kimberlites, with decisive conclusions on relevant parameters. The assurance
of the findings is necessarily high as global estimates while being at reasonable level of
confidence even as local estimates.
2.1.2 Types of Diamond Deposits
World diamond occurrences were first found in placer deposits which in due course of
time helped in unveiling of many primary kimberlitic source rocks. Diamonds occur in different
geological environments and in a variety of rock types like kimberlites, lamproites and
orangeites, ranging in age from Archaean to Quaternary (Mitchell, 1986) and sedimentary rocks
like diamondiferous conglomerates and sandstones and also in metamorphic rocks of ultra-high
pressure. Owing to this fact, diamond deposits can be classified as primary (kimberlites,
lamproites and related rock types and their metamorphic equivalents) and secondary (alluvial and
marine).
Primary Deposits: Kimberlites, olivine lamproites and orangeites which are referred to as
„primary source rocks‟, contain significant concentrations of diamonds. Besides these, diamonds
are also found in traces of ultramafic lamprophyres (mainlyalonites), alkali basalts, ophiolites,
komatiites, peridotites and eclogite xenoliths, metamorphic schists or metakimberlites, meta-
lamprophyricbreccias and meteorites.
i. Igneous Rocks: Diamondiferous kimberlites, lamproites and orangeites are recorded through
almost entire geological time, ranging in age from Mesoproterozoic to Quaternary distributed
across the globe (Janse and Sheahan, 1995). From the study of literature, it is evident that profuse
kimberlitic magmatism occurred during 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

49
Eocene (50-55 Ma) times. The oldest kimberlites are reported from Venezuela (1700 Ma)
(Nixonet.al., 1992) and Kuruman, South Africa (1600 Ma) (Simon, 2005) whereas the oldest
lamproites are from India (~1350 Ma) (Chalapathi Ra., 2007). The youngest Kimberlites/
Lamproites (Quaternary) are in Tanzania, Antarctica, etc. The oldest diamondiferous pipes are
reported from Guaniamo (Venezuala), Premier (South Africa), Argyle (Australia), Wajrakarur
and Majhgawan (India), etc. and are Proterozoic in age. In general, the kimberlite and lamproite
rocks from southern Indian province are of Neproterozoic age ranging from ~100-100 Ma (Anil
Kumar et al., 2001). The kimberlite rocks from southern and central India provinces are of
Neoproterozoic age (~1000-1100 Ma) (Anil Kumar et.al., 2001) whereas the kimberlite from
Timmasamudram (TK-1) recorded a Late Cretaceous (90Ma) age (Chalapathi Rao et al., 2015)
ii. Metamorphic Rocks: Records of diamonds exist even from metamorphic rocks.
Microdiamonds (<1mm) have been reported from the metamorphic rocks viz., gneisses and
marbles of crustal affinity in the Kokchetav Massif of northern Kazakhstan (Shatsky et al., 1995).
Diamonds were recorded from garnet-muscovite quartzo-feldspathic gneisses at Erzebirge of
Germany (Chopin, 2003 and Dobrzhinetskaya et al. 2006b, 2007). Diamonds are also reported
from metamorphosed poorly sorted polymictic breccia (Lefebvre et al. 2005) and conglomerate
(Ryder et al. 2008) suites of Wawa region of Ontario, Canada.
Secondary Deposits: Secondary diamond deposits are formed from sedimentary process. The
diamonds present in exposed kimberlite/lamproite bodies, along with other valuable minerals get
dissociated by the surficial processes like weathering, transportation and deposition. The
dissociated diamonds might occur in consolidated rocks or as placer deposits in unconsolidated
alluviums in present times. Hence the secondary diamond deposits are sub-divided into two types,
viz., consolidated and unconsolidated deposits.
Gravel and conglomerate beds in many parts of the world, have been looked upon as
store-houses of diamonds. The famous south African mine at Saxendrift is a best example for
diamondiferous terrace gravels (Fig. 2.1). Other famous examples include Witwatersrand
conglomerates in South Africa, late Proterozoic Karro conglomerates of Somabula forest,
Rhodesia, Precambrian Carains and Diamantina conglomerate of Brazil etc. The conglomerates
of Upper Preoterozoicage reported at Banaganapalli of Cuddapah Supergroup, Andhra Pradesh,
Bagain conglomerates of Vindhyan Supergroup, Madhya Pradesh comprise diamondiferous
horizons.

50
Figure 2.1 Exposure of diamondiferous terrace gravels, Saxendrift Diamond Mine.
(Source: www.rockwelldiamonds.com)
The majority of large placer or unconsolidated diamond deposits are in South America,
Africa or Asia. Different terminologies are being used to describe the placer deposits, based on
the distance of placer deposit from the source (Fig. 2.2). Alluvial diamonds are most common
among the secondary deposits.
Figure 2.2 Different types of diamond placer deposits
Diamondiferous in-situ rubble and clay occurring on kimberlite pipe rock in Tanzania is a
typical example of eluvial deposit. Diamondiferous placer of Namibian desert is a classic
example of aeolian deposit. Glacial moraines of Michigan and Wisconsin, USA are examples of
glacial placers.

51
2.1.3 Exploration Guides and Tools
Geological: Known occurrences of kimberlite/lamproite/lamprophyre and related rock types will
accelerate the possibility of finding a new pipe. The fenitisation of country rocks which is
commonly observed at the kimberlite- country rock contact, kimberlite rock float or kimberlitic
calcrete float act as important field guides to locate a new pipe. Float rocks of kimberlite or
kimberlitic calcrete can be the most useful guides to locate a new pipe. It is observed that
intersection of lineament with domal morphology is found to be potential zones for kimberlite
exploration (Nayak et al., 1988, Ramadass et al 2006 and Phani, 2015a). Indicator minerals are
the most important tools to provide a likelihood of presence of a pipe. Indicator mineral chemistry
provides clues to assess the diamond potentiality at the reconnaissance stage. Presence of
kimberlite/lamproite indicator minerals like pyropes, Cr-diopside, Mg- ilmenite, chrmoite,
phlogopite, richterite, zircon etc. act as field guides in locating the kimberlite/lamproite pipes
(Coopersmith, 1993 and Morris et al., 1994).
Geochemical: Since kimberlites commonly have high Ti, Cr, Ni, Mg, Ba and Nb content in
overlying residual soils or regolith, enrichment of these elements may act as guide to unravel the
the presence of a pipe. However a careful assessment is warranted to ascertain the presence of
kimberlite using these pathfinder elements. Residual regolith over Aries kimberlites, Western
Australia showed that elements viz., Cr, Ni, Co, Cu, Nb, Zr, Ti, P ans REE are significantly
enriched in the mottled and duricrust zones (Singh and Cornelius, 2006). At Lattavaram
kimberlite cluster, elements like Ti, Cr, Nb, Ni, La and Zr are well noticed to be enriched in the
regolith (Phani and Srinivas, 2016).Yet, a careful scrutiny is warranted to ascertain the presence
of kimberlite using these pathfinder elements. Thus geochemistry acts as an effective tool in
kimberlite exploration.
Geophysical: Although geophysical techniques cannot offer vital clues on diamond content, they
can flash the distinct anomalies which deserve ground check. Various geophysical techniques
serve as vital tools in search of kimberlites. By virtue of their enrichment in magnetic content,
kimberlites exhibit either low or high „bulls-eye‟ anomalies. Deeply weathered kimberlites give
negative responses whereas fresh kimberlite gives a positive gravity anomaly. In general,
magnetic surveys are proved to be essential primary tool in locating the source areas of indicator
mineral trains. Resistivity survey is also a useful tool in exploring crater as well as diatreme
facies kimberlite pipes. Gravity and seismic refraction surveys are instrumental in determining
the depth of the kimberlite pipe (Power and Hildes, 2007).

52
2.1.4 Importance of Kimberlite Geology in Diamond Exploration
The ultimate aim of kimberlite exploration is to find economically viable kimberlite pipes
containing diamonds. After an in-situ outcrop of kimberlite/ lamproite is discovered, the next step
is to conduct the large scale geological mapping aided with pitting/trenching. The lithological
variations are demarcated based on the field expressions like outcrops, float rocks, calcrete,
yellow ground distribution etc. An initial assessment of diamond potentiality is made using bulk
sampling and caustic fusion method.
The diamond potentiality of a kimberlite magma rising towards earth‟s surface is
governed by ascent processes. The distribution of diamonds depends on the emplacement process.
Even though the pipe appears as a single intrusion, it might have been emplaced through multiple
magmatic phases and pulses during its formation. In general a single kimberlite eruption
experiences multiple pulses and each pulse will have different diamond content and emplacement
history (Coopersmith et al., 2006).
Kimberlites typically occur in fields comprising up to 100 individual intrusions which
often group in clusters. Each field displays enormous diversity with respect to the petrology,
mineralogy, mantle xenolith and diamond content of individual kimberlites. Economically
diamondiferous and barren kimberlites can occur in close proximity. Controls on the differences
in diamond content between kimberlites are not completely understood. They may be due to: (a)
depths of origin of the kimberlite magmas (above or below the diamond stability field); (b)
differences in the diamond content of the mantle sampled by the kimberlitic magma; (c) degree of
resorption of diamonds during transport; (d) flow differentiation, (e) batch mixing or, (f) some
combination of these factors. Therefore it is envisaged that diamond distribution in a single pipe
may vary drastically and a thorough understanding all the geological variables is warranted. Such
internal geological aspects of the kimberlitic rock are best studied with the aid of chip and
outcrop mapping and/or drillhole logging data (Coopersmith, 2006). Moreover, diamond
distributions reflect the mode of emplacement and nature of each kimberlite phase in a deposit
and even the contamination by crustal material may exhibit wide variations. Thus understanding
of the geology, emplacement, and crustal contamination of kimberlite body serves as a valuable
input, right from early exploration until mineral deposit evaluation, resource calculation,
feasibility studies, mining and resource reconciliation (Coopersmith et al., 2006).
The 3D models of pipe intrusion are constructed in conjunction with geological,
petrographic interpretations, diamond incidence etc. which will aid in delineating the pipe
configuration. The geological models are outcome of integration of data pertaining to geophysical
parameters, density, volume, clay mineral content, geological units, their geometry, diamond

53
distribution, grade and continuity or variation of grade, geotechnical and metallurgical
characteristics. Clement (1982), Hawthorne (1975) and Field and Scott Smith (1999) have
proposed individual geological models of kimberlite pipes, according to which each
kimberlite/lamproite pipe may have its own emplacement history and geological affiliations.
Hence understanding complete geology of a kimberlite pipe and building an appropriate
geological resource model forms a crucial step in proceeding to the next stages of exploration.
2.1.5 Diamond Exploration Potential of India
Diamond itself and many famous historical diamonds (Fig.2.3) were introduced to the world by
India, wherein production was largely by alluvial mining that was spread over seven States (i.e.
Andhra Pradesh, Chattisgarh, Jharkhand, Karnataka, Maharashtra, Madhyapradesh and
Telangana). is the country which introduced diamond to the world. Diamond mining has been
known at Panna, Hirapur and Hinota of Madhya Bharat (Madhya Pradesh) since 18th century in
the dynasty of Bundela Kingdom. Since ancient times, in the coastal tracts and river beds of
Andhra Pradesh also, alluvial diamonds were derived from a variety of rock types and geological
environments ranging from Early Proterozoic conglomerates to Tertiary palaeochannels, terrace
deposits and recent colluvium (Fig. 2.4).

54
Figure 2.3 World famous diamonds from Krishna River alluvium, Guntur district, Andhra
Pradesh, India (Source: www. andhra-rayalaseema-diamondmines.com)
Figure 2.4 An ancient map showing alluvial diamond mining localities of Golconda Emprie,
along the banks of Krishna River Raichur (Karnataka) and Guntur (Andhra Pradesh).
(Source: www.andhra-rayalaseema-diamondmines.com).

55
The most commonly followed diamond exploration practices in India until 1980s
involved conventional methods such as surface geological mapping, geophysics and
drilling. However, with the advent of latest technologies, supported with slightly
increased investment and availability of in-house facilities in late 1990s, the public sector
organisations have acquired state-of-the-art technologies and started using advanced
techniques like remote sensing, indicator mineral chemistry, high-resolution geophysical
and geochemical surveys for exploration in search of kimberlites. The strategic plan led
to discovery of many new pipes in India and attracted investments by international
companies, with the advent of economic liberalisation. In late 1990s, explorations by
various multinational companies with huge investments, several new discoveries were
made. In the period 2001-20015 Rio Tinto Exploration has discovered about 26
diamondiferous kimberlites in the states of Andhra Pradesh and Karnataka. The Bunder
Project in Chattarpur district, Madhya Pradesh discovered by Rio Tinto, contains a cluster
of eight kimberlite pipes intruding to Vindhyan sandstones. Even the De Beers company
also had come out with equal number of discoveries but diamond potential is unknown
(IRL, 2009). A new occurrence of olivine lamprophyre at Bayyaram, Khammam district,
Telangana, by the GSI, opens up a new vista for kimberlite exploration in that area
(Meshram et al., 2015). A recent lamproite dyke discovery at Chintalapalle, Nalgonda
district, Telangana also stands out as an example of prospectivity of the EDC for
kimberlite exploration. (Alok Kumar et al., 2016). Several such new discoveries made by
private sector companies remain unpublished and reams of data have not seen the light of
the day, owing to their proprietary reasons due to confidentiality pertaining to commerce.
More than 100 kimberlite/lamproite pipes have been discovered in India so far,
distributed in the form of fields/clusters (Fig. 2.5). In some provinces the high level
erosion, for instance an estimated 3 km at the 19 hectare Pipe-1 of Wajrakarur, helps to
explain the widespread distribution of diamonds and the historical focus on alluvial
diamonds. The discovery of crater facies kimberlite at Tokapal, Chattisgarh covering an
area of 550 hectares illustrates the magnitude of some of kimberlitic volcanic events and
also that in certain regions the pipes have possibly been subjected to significantly less
erosion.

56
Much of the Indian subcontinent consists of stable, ancient cratonic blocks that are
geologically and structurally prospective for kimberlite emplacement, diamond formation
and its preservation. The distribution of alluvial diamonds, their quantity and quality is
obvious evidence of congenial mantle conditions for the formation and preservation of
diamonds within the primary kimberlitic rock in the Dharwar, Bastar, Singhbhum and
Bundelkhand Cratons. Many alluvial diamond occurrences have been reported with
unexplained source provenance. India forms a favorable ground for diamond exploration
and salient encouraging criteria in target selection are:
 Precambrian granitoid basement that has been stable for more than 1500 Ma.
 Thick (35-50 km) continental crust to provide a low geothermal gradient.
 Deep mantle lithospheric root (>150 km) to access the diamond stability field.
 Deep-seated, mantle-tapping faults as conduits for kimberlite.
 Crustal uplift and basement doming.
 Associated alkaline and flood basalt volcanism.
 Absence of mantle metasomatism provoking carbonization and resorption of diamond
and
 Many historical old workings and new diamond/kimberlite occurrences
With such splendorous history, geological prospectivity, amplified by the widely spread
alluvial diamond locations and with a vast area of craton, India‟s diamond potentiality can be
brought to its past glory, with a sustained exploration and continued foreign investment in future
to unveil many more diamond bearing kimberlites, some of which may likely prove economic.
2.2 Diamond Exploration Principles
Diamond exploration is generally conducted in a sequential manner (Atkinson, 1989,
Coopersmith, 1993, McKinlay et al., 1997) commencing with area selection. There are three
broad stages in diamond exploration viz., regional prospecting, detailed exploration or prospect-
scale exploration and deposit evaluation, which are described hereunder.
2.2.1 Regional Prospecting
In the initial phase, prospecting on a regional scale starts with studying regional geology
including lithological set-up, structural attitude, geomorphology, degree of dissection, known
kimberlite occurrences and/or diamonds and so on. Regional sampling is usually started with
indicator mineral sampling in the first, second and occasionally in 3rd order streams. The samples

57
are collected, so as to represent the whole catchment, from best trap-sites available. The quality of
trap-site is also important to obtain good quantity of quality indicator minerals that can help
interpreting the provenance. The samples are sieved for -1mm size and subjected to heavy
mineral concentration. The non-magnetic mineral concentrate is further classified under the
microscope into appropriate mineral species. The selected mineral grains are geochemically
studied for major oxides, minor and trace elements using Electron Probe Micro Analysis (EPMA)
to decipher the origin, kimberlitic affinity and diamond inclusion studies with the aid of graphical
analysis. At this stage, regional geophysical data is spatially used in conjunction with indicator
mineral data to assign priority of the catchment. Depending on the quality and quantity of heavy
minerals, thus the catchments are prioritized.

58
Figure 2.5 Locations of kimberlites and lamproites in India Geological map (Modified after
GSI, 1993). WKF- Wajrakarur Kimberlite Field, NKF-Narayanpet Kimberlite Field, TKF-
Tokapal Kimberlite Field, RLF- Ramadugu Lamproite Field, NKF- Narayanpet Kimberlite
Field, NLF- Nalgonda Lamproite Field, BLF- Bhuj Lamproite Field, KLF- Krishna
Lamproite Field. WDC- Western Dharwar Craton, EDC- Eastern Dharwar Craton, SGT-
Southern Granulite Terrain

59
2.2.2 Prospect-scale Exploration
Once priority catchments are identified, the next step is to conduct ground surveys to
locate the causative body for liberation of indicator minerals. Based on the quality and quantity of
indicator minerals, the promising catchments are usually designated as prospects. During the
course of ground surveys, it may be possible that a kimberlite outcrop may be straight away
found. Sometimes surficial expression in the form of calcrete and other associated oxidized
products of kimberlite like yellow/blue ground may be encountered during the ground surveys. If
no surficial expression is found, the catchment may be taken up for in-situ soil geochemical
surveys and close spacing ground magnetic traverse to identify anomalous areas. After locating
the anomalies, pitting, trenching is carried out or auger holes are placed targeting the causative
body. At this stage, priority anomaly areas, drilling is carried out to ascertain the pipe continuity
and overall configuration. This is followed by preliminary bulk sampling to evaluate the diamond
grade of the kimberlite pipe.
2.2.3 Deposit Evaluation
The economic evaluation of diamondiferous kimberlites/lamproites is generally carried
out in four stages. Expenditure tends to increase by an order of magnitude at each successive
stage. At the end of each stage, the sample results should be judgmentally appraised before
deciding to continue to the next phase. In the first stage, even before individual kimberlite bodies
have been discovered, the indicator mineral geochemistry will give a first rough idea of the
diamond potential. The relative abundance of harzburgitic pyropes (subcalcic chrome-rich) is
often directly correlated with the diamond grade. In the second stage, when the kimberlite body
has been discovered, a relatively small sample of a few hundred kilograms will be enough to
recover sufficient microdiamonds to allow an extrapolation of the size distribution towards the
commercial-sized diamonds and an approximate grade estimation. If positive, the third stage
should be a limited bulk sampling programme (around an order of 200 tonnes) to determine the
commercial-sized diamond grade, expressed as carats per hundred tonne (cpht). The final stage
aims at obtaining a parcel of the order of 1,000 carats to estimate the average commercial value of
the diamonds (Lus Rombouts, 2002).
2.2.4 Diamond Exploration Flow Chart
The diamond exploration contains two broad stages viz., regional reconnaissance and
detailed prospect scale exploration and the third stage involves evaluation (Fig.2.6). If 100

60
kimberlites are discovered, 10 among them may be diamondiferous and one or none can be
economically viable for mining.
Figure 2.6 Flow chart showing different stages of diamond/kimberlite exploration.
Area Selection
Remote Sensing
Field Geological
Mapping
Petrography
Airborne Geophysics &
Stream Sediment
Sampling (KIMs)
Catchment
Prioritization
High Resolution Ground Surveys
(Ground Geophysics & Geological
Mapping)
Soil/Regolith/Loam
(Orientation
Sampling)Geochemical
Analysis
Anomaly Identification
Detailed Geochemistry
Pitting/Trenching
Drill Sample Analysis
KIMBERLITE*
RECONNAISSANCE DETAILED EXPLORATION
Drill Target Selection &
Drilling
EVALUATION

61
2.3 Reconnaissance Exploration Practices
2.3.1 Area Selection
The most likely settings for kimberlite emplacement are Cratons; the oldest and most
deeply rooted rock formations that make up the stable cores of the continents (Clifford, 1966).
These cratons have undergone relatively little structural change over hundreds of millions up to 4
billion years and are generally the thickest parts of the Earth‟s lithosphere (crust + upper
mantle).There exists a relation between Archaean basement and occurrence of diamondiferous
kimberlites can be explained theoretically by considering the structure of cratons and the
pressure- temperature relationship between graphite and diamond. In general, areas for diamond
exploration can be selected on the basis of following criteria (De Beers, 2001).
 Global, regional and local tectonics: location and presence of preserved Archaean
cartons.
 Lithospheric structure and composition: Lithospheric thickness extending into the
diamond stability field.
 Diamond formation and preservation: Occurrence of any processes like rifting that can
destroy diamonds.
 Known host rock petrogenesis and emplacement: Various rock types in the given area
that can host diamonds.
 Country rock and source rock geochronology: Possibility or evidence of kimberlite
intrusions in the sedimentary cover.
 Local tectonic controls: Any local structures that can control the kimberlite emplacement.
Kimberlite pipes in Butuobiya, Yakutia province, Russia, are closely associated with
domal structures (Kamintsky, 1995). Similarly in WKF also, Lattavaram and Anumpalli
kimberlite pipes occur on the peripheries of Marutla and Katrimala domes whereas those of
Kalyanadurgam and Timmasamudram are localized within a structural corridor trending NE-SW
(Phani, 2015). Hence it is important to select areas that are tectonically congenial for kimberlite
emplacement with reference to pre-existing discoveries.
2.3.2 Remote Sensing& GIS
A variety of remote sensing analyses using high resolution satellite, air photos and drone
imagery through different types of sensors like infrared (IR), near infrared (NIR), multi-spectral
scanning (MSS) etc., have been used by several workers to map the kimberlite bodies. All these
interpretations are based on the analysis of various spectral signatures responded by different rock

62
types in a terrain. However in case of kimberlite mapping, remote sensing techniques cannot
provide direct evidences of presence of a pipe (Atkinson, 1989 and Coopersmith 1993). But
remote sensing approach can be used as part of an integrated exploration strategy.
The circular nature of majority of kimberlite pipes and their cross-cutting relationship
with local stratigraphy makes their outcrops relatively easy to recognize on air photography and
high resolution satellite imagery. The distinctive mineralogical composition of pipes can also be
detected using spectral measurements in the shortwave red infrared wavelengths for minerals
such as phlogopite and serpentine. However Kruse and Boardman (2000) reported great difficulty
in interpreting Hymap airborne scanner imagery over kimberlites in Wyoming due to the intense
weathering and very limited outcrop of the distinctive minerals.
Mapping of lineaments by using satellite imagery is a common technique in the initial
phase of kimberlite exploration. With the aid of visible and near infrared data, remote sensing
techniques can identify lineaments which may reflect crustal weakness zones along which
kimberlite pipes are emplaced (Tsyganov et al., 1988). Owing to the occurrence of kimberlite
pipes at the intersections of lineaments, assessing the trend of lineaments is often used in regional
reconnaissance surveys. Majority of the kimberlites of WKF are localized at the peripheries of
large domal structures. In case of WKF, kimberlite pipes in the Lattavaram, Anumpalli,
Chigicherla clusters occur in the proximity of Marutla, Katrimala and Cherlopalli domes and also
at the lineament intersection trending NNE-SSW, NW-SE, NE-SW whereas the pipes at
Kalyanadurgam and Timmasamudram are emplaced along the lineament intersection corridor
trending mostly NE-SW, EW, NNE-SSW (Phani, 2015). Even the investigations of Arindam et
al., (2013) strengthen the concept of lineament control on kimberlite localization, exemplifying
Narayanpet Kimberlite Field (Arindam et al., 2013).
The Alberta Geological Survey has successfully applied portable infrared mineral
analyser (PIMA) spectral imaging with the aid of Short Wave Infrared (SWIR) to detect
kimberlites in Alberta. In their investigations, it was found that minerals found in kimberlitic
terrains may be easily distinguishable from surrounding Phanerozoic shales and sandstones in the
mid-infrared spectral range. Certain alteration minerals like clay minerals, serpentine, smectite,
saponite, chlorite etc., which are derived from kimberlites can be easily detected at certain
absorption wavelengths (Table. 2.1).

63
Table 2.1. Major absorption wavelengths of kimberlitic alteration minerals (Hauff, 2001)
A distinct variation in the tone & texture of soil over the kimberlite when
compared to the soil of country rock may be visible in the satellite imagery. At
Lattavaram, the Pipe-4 and 9 exhibit a conspicuous tonal difference in the distribution of
soil (Fig.2.7A & B).
Figure 2.7.An example of composite image of Landsat TM and IRS IC LISS III image of
Lattavaram kimberlites. A. Pipes- 3& 9. B. Pipe-3, 4, 8 & 9 of Lattavaram. Note the tonal
variation at Pipes (green outlines). Pipe-3 &4 are manifested by abundant kimberlitic
calcrete cover
Morphometric analysis of the area with the aid of remote sensing data and GIS
will give considerable amount of information in preliminary assessment of the terrain and
assist in planning the stream sediment sampling programme appropriately (Phani, 2014).
Mineral/Compound Molecule Wavelength Position
Illite, Kaolinite, Smectite, Talc, Pyrophyllite OH and H2O ~1.4µm
Smectite, Saponite, Kaolinite, Illite, Micas, Serpentine Al-OH ~2.2µm
Chlorite, Fe- Illite Fe(OH) ~2.2-2.6µm
Amphiboles, Chlorite, Micas, Talc Mg(OH) ~2.3µm
Carbonates, Calcite CO3
-2
2.29-2.35µm
Phosphate P-O-H ~2.38µm
A B

64
2.3.3 Geological Mapping & Petrography
Geological mapping is usually the first exploration method undertaken on the ground.
The field geological mapping work may include prospecting for kimberlite float (loose pieces of
rock at surface), locating zones of fenitisation and other guides along with initial sampling. If a
sufficiently favourable target exists, more detailed ground geophysical and geochemical surveys
over the anomalies are undertaken.
Once kimberlite pipe gets emplaced in the earth, it leaves a surficial expression similar to
a volcanic eruption. After emplacement, the pipe is subjected to weathering and erosion leading
to removal of rock over millions of years. In due course of time, the removed debris is deposited
in downstream portions leaving a modern eroded surface, exposing deeper parts of the diatreme,
thus leaving a modern landscape where kimberlite is further subjected to surficial weathering
processes (Fig. 2.8). Such remnant outcrops act as useful guides in geological mapping.
Figure 2.8 Erosion of a kimberlite pipe producing modern landscape, thus leaving surface
outcrops of the pipe (Phil, 2014)
The information gathered in the initial stages is sufficient to prepare a geological map the
exploration area by recording the rock types and structures. Systematic geological mapping
provides a comprehensive understanding of structural control and the field studies around the
newly discovered pipe are authoritative and have been proven to be of high value in search of
kimberlite pipes. Geological mapping in general reveals structural control that can lead to the
discovery of other intrusives as kimberlite pipes occur in the form of clusters. Even though

65
remote sensing studies can provide favourable zones of kimberlite emplacement, they cannot
provide direct indication of presence of kimberlite. Hence, geological mapping is mandatory to
find a variety of small volume mantle-derived intrusives like kimberlites. Many kimberlites
possess a thin layer of highly oxidized and weathered capping predominantly composed of
calcrete or caliche that occurs as „yellow ground‟ or „blue ground‟. This thin layer acts as a
yellowish-brown „gossan‟ on the pipe rock exposure. In addition, emplacement of kimberlite
causes metasomatic changes at the contact of kimberlite and the country rock leading to alteration
of minerals in the host rock. The alteration may enrich the iron and alkalies and finally produce a
rock of different composition. This process is called fenitization and the resultant rock type is
called as a fenite. The occurrence of fenitised granites is evidenced at many locations in the
WKF. These features are used as field exploration guides in search of kimberlites, especially in
Archaean terrains of India like Dharwar, Bastar or Bundelkhand provinces. In Lattavaram and
Anumpalli areas, the country rock granitoids in the contact zone of kimberlite, exhibit enrichment
of Fe giving rise to deep red alkali feldspar, indicating effect of fenitisation (Fig. 2.9).
Figure 2.9 Deep red tone of alkali feldspar at the contact zone of kimberlite-
granite at Muligiripalli
From the initial stage of geological mapping and, while carrying out the field traverses
coupled with collection of samples of suspicious rock types, petrographic studies will help the
explorationist to reach the goal of identifying an unusual rock type which may be proven as
kimberlite. In addition, up to a few kilograms of rock material from outcrops are collected using
hand-held tools like picks, chisels etc. Rock chip samples are commonly collected during
geological mapping programs. During the geological mapping, float rocks of suspicion are also
collected and analysed for their geochemistry. As a follow-up, after the discovery of a new

66
kimberlite pipe, it is vital to map the different facies of the kimberlite/lamproite pipe which can
throw light on the detailed geology of the pipe and its diamond potentiality.
2.3.4 Airborne Geophysics
Airborne magnetic surveys are usually carried out to select discrete anomalies (targets)
and to obtain a picture of the structural set-up of investigated area. Though not always possible, it
is useful to have an idea of the expected target size since the line spacing of the survey affects the
ability to detect a target of a given size. Probability studies may be undertaken to assess the
likelihood of finding a target of a particular size, based on a given survey line separation and if
the distance between lines is too large it could mean missing an important target. Choosing a line
separation distance that is smaller than required will generate higher quality data but may be
unnecessary for finding suitable kimberlite anomalies and costly to the exploration budget. The
Euler solutions for the airborne magnetic anomalies provide useful information of the depth
estimate of the magnetic sources. The Euler deconvolution map of southern India shows that the
Wajrakarur area has deepest sources ranging from 11141 to 16240 meters (Rajaraman et al.,
2003). This helps enormously in area selection for kimberlite exploration (Fig.2.10).
Figure 2.10 Aeromagnetic image showing depth of magnetic source
Euler’s deconvolution (Rajaraman et al., 2003)

67
The location of airborne surveys is usually dictated by positive mineral chemistry
results obtained from previous soil sampling programs. The airborne data is then used in
combination with the soil chemistry results to select and prioritize targets for follow-up ground
work. The airborne data provides important information for the structural and geological
interpretation of the area is. Airborne datasets also contain digital terrain information useful for
correlating with selected anomalies. For example, a magnetic anomaly coinciding with a
topographic depression would be a good kimberlite target if the kimberlite rock is expected to be
softer than the host rock.
Geophysical methods are particularly useful in diamond exploration since kimberlite
pipes, generally have properties that are different from the surrounding country rocks. In addition,
the data provide most of the guidance on where to position drillholes. On a lease hold area
scale airborne surveys will be used to cover large swaths of land. On a local or prospect scale,
ground surveys are used to site and orient a drill hole. These surveys are collectively used to
assess the target‟s size and to model the anomalies at depth. The main methods used in diamond
exploration are listed below (Table 2.2). Each method targets a specific physical property by
measuring a parameter.
Table 2.2 Different geophysical methods used in kimberlite exploration
Method Physical Property Parameter
Magnetic Magnetic susceptibility and
remanence
Spatial variations in the strength of the
Earth‟s magnetic field
Gravity Density Spatial variations in the strength of the
gravitational field of the Earth
Electromagnetic Electrical conductivity and
Inductance
Response to Electromagnetic Radiation
Radar Dielectric constant Travel times of reflected radar pulses
Seismic Density and elastic moduli, used
to determine the travel speed of
seismic waves
Travel times of reflected/refracted
seismic waves
Traditionally magnetics have been proved to be the best and most economical survey
method to use in kimberlite exploration. Magnetic data are collected continuously by a
magnetometer located on a fixed wing or helicopter platform and the data are used to select
anomalies. Those anomalies are then resurveyed from the ground at smaller line spacing. Other
survey methods that work well, depending on the environment are gravity and electromagnetics.
In the aeromagnetic map of southern India the Wajrakarur area holds high magnetic susceptibility

68
(80-197 nT) owing to the high magnetic and ferromagnesian content of lithological units
(Fig.2.11, Rajaraman et al., 2003).
Electromagnetic surveys work well where the exploration team expects either a more
conductive „calcrete cap‟, the weathered material usually forming the top few meters of a
kimberlite, or the kimberlite is expected to be more resistive than the background rock at depth.
Electromagnetic sensors are often placed on the same airborne platform as a magnetometer to
reduce costs significantly while significantly increasing the ability to detect anomalies.
Figure 2.11 Aeromagnetic map of southern India, showing high magnetic anomaly at
Wajrakarur Kimberlite Field (Rajaraman et al., 2003)
The gravity method also works well since kimberlite typically has a specific
gravity that is lower than older Archean basement rock and this produces a gravity low
response. However, in sedimentary basins kimberlite may have a gravity high response if
the background rock has lower density. However, gravity surveys can be time consuming
and expensive because the measurements require more time and equipment. Gravity
methods may be extremely good at finding kimberlites given the right conditions but they

69
are extremely sensitive to any type of perturbation. When choosing to carry out a gravity
survey at the exploration level, the geophysicist generally expects to find kimberlites that
have no discernible magnetic or electromagnetic response. Again, the expected contrast
should be the driver behind the choice in survey method. Often, airborne survey
platforms will combine instruments for various methods to reduce costs, so that
kimberlitic signatures flashout. Interestingly, both positive and negative responses of
gravity and magnetic signatures have been observed in the Indian kimberlites irrespective
of the geological terrain (Guptasarma et al., 1989).
Seismic and Radar methods have proven useful when more detailed structural
information about a known kimberlite is needed. These exploration methods are not
generally used to discover an unknown kimberlite because they tend to be more
expensive compared to the other methods mentioned.
Gradient magnetic surveys are also common. Magnetic gradients are measured
using at least two sensors, separated by a distance. The difference in measurements
between the sensors is used with the separation distance to produce a gradient value at a
location between the two sensors. These methods can contribute to a significant reduction
in survey costs by allowing a larger line separation or increasing the capacity to detect
targets from one survey line.
Lastly, gradiometer gravity surveys represent some of the most advanced
technologies in geophysical surveying. Full tensor gravity gradiometers measure the rate
of change of gravitational acceleration along three perpendicular directions at the same
time (for example: up-down, east-west, and north-south). The detection ability of these
systems is greatly affected by the survey sensor altitude and speed. Ultimately, the
explorer must understand the detection capability of the system(s) used with respect to
the target sought.
Airborne radiometric surveys interpreted in conjunction with lithology provide
useful information on the K, Th and U anomalies. This type of interpretation has been
proved invaluable for not only regolith mapping but also to understand the geomorphic
processes (Wilford et al., 1997). Depending on the content of K, Th and U in the rock and
its weathered material, the signatures are different. Ramadas et al (2015) attempted an
integrated study of aeromagnetic, radiometric and lithological data on Narayanpet

70
Kimberlite Field resulted into identifying 21 favorable locations for occurrence of
kimberlites (Fig. 2. 12).
2.3.5 Stream Sediment Surveys
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. Though this is an indirect method, indicator
mineral survey defined a classic/traditional approach used for regional and detailed sampling of
prospective areas for kimberlites and lamproites. Eversince the discovery of first kimberlite in
1870s in South Africa, indicator mineral techniques have been extensively employed successfully
for many discoveries all over the world (Muggeridge, 1995). Closer to home, in India, three
kimberlites in Kalyanadurgam cluster of Wajrakarur Kimberlite Field have been discovered using
the same methodology (Sravan Kumar et al., 2004, Mukherjee et al., 2007).
Figure 2.12 An example of Narayanpet Kimberlite Field showing geophysical
prognostication for Kimberlite locations with the aid of radiometric data
interpretation (Ramadas et al., 2015).
The kimberlites and lamproites comprise 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 and commonly used as pathfinders / tracers for

71
locating kimberlites and lamproites. Besides enstatite, olivine, phlogopite and zircon are
also used as indicators. Under some rare situations, diamond itself can act as an indicator.
Most of these resistant minerals are accumulated in drainage or alluvial sediments and
also in loams and tills, upon release from weathered kimberlites and lamproites. Relative
durability and high specific gravity make them more significant for using in sampling
programmes. It is worth mentioning that the pipe-7 of Venkatampalli in Wajrakarur
Kimberlite Field was the first pipe discovered with the aid of indicator mineral survey
(Guptasarma et al., 1986).
The minerals in the order of decreasing resistance are zircon, chromite, ilmenite,
garnet, chrome-diopside and olivine whose specific gravities range from 3.1 to 4.7. These
minerals with unique physical and chemical characters are
(i) Resistant to weathering,
(ii) Dispersed in to surface environment
(iii) Transported over considerable distance and
(iv) 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 kilometres from source. More resistant
minerals such as chromite, survive long transport and are found in the drainage sediments for
several tens of kilometres 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 reviewed 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 and zircon. The
different indicator minerals and their general characteristic properties are given in Table 2.2 &
Fig. 2.13.

72
Table 2.2 Diagnostic properties of kimberlitic indicator minerals
Figure 2.13 Kimberlite indicator minerals showing colours and surface textures. A. Cr-
pyrope, B- Cr- diopside C. Cr-spinel, D. Mg-ilmenite, E. Mg-olivine and F. pyrope-
almandine garnet. Source: McClenaghan and Kjarsgaard (2007).

73
Stream Sediment Sampling Technique: This is the most common method used in diamond
exploration, especially in the early stages of exploration well before the considerably expensive
geophysical methods are employed. This method, which was first used in 1902 for the “Premier
Mine” in South Africa, consists of looking in secondary environments (soil, streams, rivers, etc.)
for minerals characteristically associated with diamond-bearing kimberlites and retracing them
back to their source (Bari, 2001).
The different sampling techniques deployed, sample density and size, sample collection
methods, indicator mineral trap sites, field and laboratory processing of samples were discussed
in detail by Gregory and White (1989); Davison(1993); Muggeridge (1989, 1995) (Fig. 2.7) and
Towie and Seet (1995).
Exploration programmes aimed at detecting indicator or pathfinder minerals are (i)
stream sediment 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 (c)
abrasion and transportation characters (McCandless, 1990;Afanasev et. al., 1984; Mosig, 1980).
Sediments are collected from suitable trap sites where there can be possibility of accumulation of
heavy minerals. Different types of trap sites are illustratively described by Muggeridge (1995)
(Fig. 2.14).
Approximately two kilogram samples of sediment are collected within drainage. Three
samples are usually taken at the junction of two creeks: one downstream of the junction and two
upstream of the junction (in each of the merging drainage lines). Samples are typically extracted
using hand tools and may be sieved during collection.

74
Figure 2.14 Different types of heavy mineral trap sites (Redrawn after Muggeridge, 1995)
Typically, samples are wet screened after weighing. If geochemical analysis of the
sample matrix is also required, all or part of the sample may be dried and screened to remove the
–0.18 mm material to ensure recovery of the clay sized and finer material which can be critical as
the host of mobile elements and a carrier of geochemical signals. Sample sizes can vary
dramatically from a few hundred grams to 50-100 kg. Often samples may have been pre-screened
in the field to remove +2 mm material to reduce transportation costs. An attrition mill may be
used for sample disaggregation, particularly for clay rich samples that may be indurated to some
degree. After screening, the sample goes directly to heavy liquid separation.

75
The selection of screen is made on the basis of type of sediment. Fluvial or alluvial
sediments ain tropical countries are generally fine-grained. Hence in general 1-2 mm size screens
are used. Many multinational companies use -1 mm fraction for indicator mineral collection.
Standard screen sizes for till samples are 0.85 mm and 0.25 mm.
The screened residual fraction is dried before being fed to a Wilfley table fitted with a
slurry pump to re-circulate the tails. The table set up parameters can be adjusted by an
experienced operator to ensure that the complete heavy mineral train is collected. If quality
control (QC) tests indicate inadequate heavy mineral recovery, the table tails will be processed
through a Permroll magnet to recover any remaining magnetic minerals that might normally
report to the heavy mineral concentrate. In the indicator mineral processing lab, specific minerals
are separated using an array of magnetic and electrostatic instruments.
The heavy mineral portion may then be acid washed prior to drying and processing in
methylene iodide diluted to a specific gravity (SG) of 3.1. Tetrabromoethane may be used if a
lower specific gravity is preferred. After removal of ferromagnetic minerals with a hand magnet,
the concentrate is divided into two fractions by screening at 0.5 mm.
If the concentrate is large, 30 g will be riffled out to speed processing. The remainder is
processed if the indicator mineral content is low. A Frantz magnetic separator divides the sample
into paramagnetic and non-magnetic mineral fractions for the picking lab.
Using high quality stereo microscopes with a variable 10 to 50 times zoom lens the grains
are picked up. Questionable grains are checked by a scanning electron microscope (SEM)
equipped with an energy dispersive system (EDS) or Electron Probe Micro Analyser (EPMA)
(Fig.2.15).
Figure 2.15 Kimberlitie indicator mineral processing flow chart. (Source: SGS Mineral Services)

76
In addition to the utility of stream sediments for kimberlitic indicator mineral study, the
finer clay fractions are also being used to understand the geochemical signature of a terrain. In the
Indian context, National Geochemical Mapping (NGCM) of GSI is one such project which
generates enormous amounts of data and corresponding spatial dispersion patterns of about 63
elements. The Lattavaram catchment area conspicuously showed higher Nb, Ni, Ti contents in the
stream sediments in the proximal zone of kimberlite pipes (Phani, 2014).
2.3.6 Anomaly Identification/Catchment Prioritization:
After the chemistry of indicator minerals is obtained, several graphical plots are
constructed to study the geochemical signature of the mineral grains. The stream sediment
samples which possess maximum populations of kimberlitic indicator minerals and whose
chemistry indicates that they are kimberlitic in nature are prioritized for further ground surveys
(Fig. 2. 16). For example, a good number of G10 garnets will be prioritized than a catchment with
few ilmenite grains which fall in non-kimberlitic fields.
Fig. 2.16 Conceptual diagram showing prioritization of drainage catchments based on
stream sediment indicator mineral population and chemistry
2.4 Detailed Exploration practices
After the identification of positive catchments, next step would be carrying out ground
truth surveys, to locate the actual causative body, i.e., kimberlite pipe. In this process, a
combination of geological, geophysical and geochemical techniques is deployed, which is
discussed in the following sections.

77
2.4.1 Ground Geophysical Surveys
Once the anomalous areas/catchments are demarcated, close spacing ground geophysical
surveys such as magnetic, electromagnetic and gravity methods are carried out to locate the
source of indicator minerals and other surface geochemical signatures if any. The gravity,
magnetic and resistivity highs are observed over fresh unweathered kimberlites whereas low
signatures of these parameters are observed over weathered kimberlites in India (Bose, 1980).
Usually the line spacing in ground geophysical surveys in the initial stage would be 200-250
meters. In case of additional geological information and occurrence of kimberlitic float in the
area, the spacing may be reduced as close as 25 meters. The discovery of pipe-8 at Lattavaram
Tanda (Chayanulu and Singh, 1992) which is concealed under 1-2 meter thick calcrete, stands as
a best example of utility of electrical resistivity survey.
An example of resistivity and ground magnetics of Lattavaram pipe-3 and 9 is shown Fig.
2.8. The pipe-3 shows weak surficial conductor, in resistivity property, coinciding and off-setting
to the SW of the kimberlite boundary. The pipe-9 being very small in areal extent shows no
prominent response. The magnetic image shows low RTP (reduced to pole) values within a
background of Peninsular Gneissic Complex granitoids. A linear trend of RTP magnetic low is
observed connecting both pipe-3 and 9 (Fig. 2.17).
Figure 2.17 Geophysical survey images of Pipes-3 and 9 Lattavaram. A. 3 kHz Resistivity
image (Data courtesy Surya Naik, Lattavaram) and B. Ground magnetic image at line
spacing 150 meters (Data Source: AMSE Aeromag Contour map)
100m
BA
Pipe 3
Pipe 9
100m
Pipe 3
Pipe 9

78
2.4.2 Regolith Geochemical Surveys and Pitting
Regolith is residual soil that originates in-situ from the parent rock. As it represents the
mineralogical and geochemical characteristics of its parent rock, study of regolith geochemistry
helps in understanding the underlying parent rock. Regolith is often represented by weathered
rock turned into calcrete/ silcrete/ ferricrete/ dolocrete. At James Bay Lowlands, Attawapiskat
area of northern Canada, kimberlites are concealed under 2-4 meter thick peat cover. The peat
compositions are found to be influenced by the underlying rocks and show drastic variations
where kimberlite is underneath, even though there are thick layers of Quaternary glacial tillites in
between (Hattori and Hamilton, 2008). The colour of regolith is governed by the degree of
oxidation. In case of kimberlites in arid to semi-arid countries, the regolith is pale yellow or pale
green in the initial stages of oxidation and as oxidation improves the colour grades to brown or
orange. Kimberlitic regolith contains relict textures of kimberlite and preservance of altered
pseudomorphs of olivine and/or serpentine along with other kimberlitic indicator minerals. By
virtue of presence of kimberlitic minerals, kimberlitic regoliths are enriched in elements like Cr,
Ni, Nb, La and Zr which may give clues of kimberlite presence (Phani and Srinivas, 2016).
However caution should be exercised as these elements are also enriched in other alkaline rocks.
The granitic regoliths contain semi-altered or fresh feldspar and cryptocrystalline silica.
Distinction between kimberlitic and granitic regolith is shown in Fig. 2.18.
Figure 2.18 A. Kimberlitic calcrete regolith (Kalyanadurgam) (Source: Ravi et al., 2009) B.
Granitic regolith with calcrete (Muligiripalli)

79
Hand operated tools such as shovels, picks and hand augers are used to collect soil and
subsoil samples. In general practice, samples are collected on a regular grid pattern and involve
collection of small (approximately one kilogram) samples of soil. In addition to hand driven
augers, power augers may also be used. The soil samples collected typically are of residual to
obtain the in-situ geochemical signature, as sampling the transported soil material will result in
erroneous results. In case of shallow deposits concealed under top soil/ calcrete, it may be
necessary to test the anomaly by pitting.
Mineral deposits have been discovered directly as a result of their association with
calcrete for centuries. According to McGillis (1967), the use of calcrete as a geochemical guide to
metal deposits may have begun in Russia during the late 1950s. However, its potential may have
been recognised earlier by Cuyler (1930) who suggested Ca-rich waters, rising under hydrostatic
pressure, deposit calcrete in faulted areas. Calcretes also have the capacity to indurate and trap
components of the soil matrix, fresh rock fragments and/or discrete minerals. That‟s the reason
why calcretes often contain relict parent rock fragments. In case of kimberlites, calcretes contain
not only the well-preserved indicator minerals but also pipe rock fragments of varying sizes. Bulk
geochemistry and petrographic characteristics of inclusions can readily indicate the kimberlite
bedrock. When compared to calcretes of surrounding rocks, those derived from kimberlites show
a distinct enrichment in incompatible elements like Cr, Zr, Nb, La, Co etc. Thus calcretes may
serve as a potential sampling medium in kimberlite exploration.
2.4.3 Anomaly Identification and Drill Target Selection
After a thorough analysis based on geological, geochemical and geophysical data, targets
for drilling are selected. At this stage, higher density sampling (infill sampling) may be further
necessary to define the mineral train. Since the vast majority of kimberlites have a detectable
magnetic response ground geophysical surveys will usually be placed at the head of a KIM train.
If the ground geophysical data is interpreted as the likely source of a kimberlite-like target, it will
be recommended for drill testing. Prioritizing anomalies can be a balancing act between deciding
on good targets that are small and large targets that do not have much geochemical or geophysical
data support.
Once a general area is outlined based on the results from a regional sampling schemes, it
is possible to fine-tune the exploration with additional sampling programs and airborne
geophysical surveys. Positive stream sediment samples generally indicate a nearby source, which
helps the exploration team to prioritize areas based on the abundance of KIMs. Experienced
geologists and geochemists will generally be able to interpret the geochemical data as pertaining

80
to a diamondiferous source or not. Priority areas identified during early exploration are targeted
for high density sampling programs and higher resolution geophysical surveys. Exploration team
members will work together to combine their interpretations of geology, surficial sampling, and
geophysics to decide on a list of targets that suggest the source of a mineral train leading back to
the kimberlite. Since most of the kimberlites possess discrete magnetic signature, geophysical
surveys are positioned at the head of KIM train. Since the vast majority of kimberlites have a
detectable magnetic response ground geophysical surveys will usually be placed at the head of a
KIM train.
A conceptual illustration is shown in Figure 2.19, describing the methodology of target
selection. The line spacing is a crucial parameter that defines the types of anomalies that can be
detected.
Figure 2.19. Conceptual scheme showing horizontal survey lines with different types of
targets. Targets A and C are anomalies. Target B is very common and should not be
classified as an anomaly
Based on the surface geological characteristics, surface geochemical studies, petrography,
quality and quantity of indicator minerals and geophysical inferences anomalous zones are
targeted for drilling. Prioritizing anomalies can be a balancing act between deciding on good
targets that are small and large targets that do not have much geochemical or geophysical data
support (Fig. 2.20). If no targets are recognized in the data, the sampling process may be extended
to confirm the mineral train. Furthermore, alternate geophysical methods (e.g. electromagnetic
or gravity surveys) may be used to help the interpretation. In some cases, if the overburden layer
is not very thick, it may be possible to find outcropping kimberlite (i.e. kimberlite rock exposed at

81
the surface) at the site of a geophysical anomaly. Geophysical inverse modeling of anomalies is
common in order to estimate the depth of targets prior to drilling.
Figure 2.20 An example of various geophysical products to choose one anomaly. Magnetic
data is shown in comparison with electromagnetic section (Source: www.geosoft.com)
2.4.4 Drilling & Deposit delineation
The two main methods for drill testing kimberlite anomalies are core-drilling and reverse-
circulation (RC) drilling. Core drilling is based on a diamond coated drill bit that bores into the
rock to provide sections of solid rock (Fig. 2.21). This type of drilling is more expensive than RC
drilling but makes geological interpretation easier. RC drilling is based on a perforating
mechanism, using a piston-like action known as a “hammer” that drives a tungsten-steel drill bit.
Drill cuttings or chips are brought to surface in RC drilling. RC drilling is generally based
on a more mobile platform. This makes RC drilling useful for testing a large number of shallower
targets economically and quickly; especially when the number of targets to be tested is many.
However, core drilling is usually the norm for delineating the extents of kimberlites. An angled
drill-hole is usually planned across the anomaly in order to test it and obtain an initial sense of its
geometry using “pierce points”. The core drill holes are generally range from 30 to 250-m deep,
while RC holes are shallower.

82
Figure 2.21 Kimberlite core, HQ diameter from WKF (Courtesy: Pravir Mukherjee)
The areal extent of the kimberlite pipe is mapped using the geophysical signatures and
then in conjunction with the drillhole logs using appropriate software (e.g. Geosoft, ESRI etc.)
and 3D pipe models are constructed (Fig. 2.22). This stage thus is termed as „deposit delineation‟
or „order of magnitude study‟. During this stage, large diameter drilling may also be carried out to
assess the different lithological facies of the kimberlite pipe discovered. Then the diamond
incidence is evaluated in each kimberlite facies carefully followed by a pre-feasibility analysis.
Figure 2.22. A. Examples of stages in deposit delineation from BK-11 kimberlite pipe.
Integration of ground magnetic data and delineation drillholes. B. BK-11 pipe 3D model
showing different kimberlite horizons (Source: Firestone, 2015).

83
2.5.5 Evaluation- Diamond Potentiality
Following the discovery of a kimberlite pipe, an assessment is made to confirm whether
it is related to the associated mineral train; it is possible that there could be more than one source
with different mineral chemistries as kimberlites tend to occur in clusters or “fields”.
A large scale pitting is carried out and a bulk sample in terms of few hundreds of tons of
excavated material is sent for a microdiamond analysis (diamonds < 0.5mm in at least two
dimensions) to determine if it is diamondiferous through a process known as caustic fusion. The
caustic fusion process is used for processing small samples (0.5-3 kg) of kimberlite material. In
this process the soft kimberlite rock is dissolved in a strong caustic soda solution leaving only the
harder minerals including diamonds.
The diamonds are removed using a sieve and sorted based on size. If the kimberlite is
sufficiently diamondiferous, based on mircrodiamond analysis, a mini-bulk sample (more
drilling) will usually be done to obtain an initial estimate of the grade.
Diamond grades are measured in carats per tonne (cpt), or carats per hundred tonnes
(cpht), where one carat is equivalent to 200 milligrams of diamond, or about 0.007 ounces.
Positive results from a mini-bulk sample of several hundred tonnes will usually warrant the
commencement of a larger bulk sample, using large diameter drilling and trenching. A bulk
sample consists of several thousand tonnes and provides a better assessment of grade. The bulk
sample is processed using a procedure known as dense media separation (DMS) that separates the
heavy minerals from lighter ones (Fig. 2.23A) . The concentrate of dense minerals is then further
processed to get a diamond count (Fig.2.23.B, C & D). This information is used to calculate
tonnage potential for a possible mine. An independent market valuation of the diamonds from the
bulk sample is used together with the tonnage and grade information to build a preliminary
revenue model (i.e. Tonnage x Grade x Value). Further kimberlite pipe delineation drilling is
usually done to constrain the geometry of the pipe from the surface down to a zone between 300-
400 meters in depth.

84
Figure 2.23 Processes in the initial evaluation of kimberlite. A. Excavation pit for bulk
sampling for diamond grade evaluation B. Diamond Recovery system and C.
Recovered diamonds at Firestone Diamonds Plc Expansion Project, South Africa
(Source: Firestone Diamonds, 2015)
2.4.6 Exploration Strategy & Cost
Although exploration for kimberlites involves several steps and is very expensive,
discovery of an economically viable diamondiferous pipe rock makes the hardships transformed
into jubilant success. Any general inference on a diamond deposit depends only on representative
sampling of any parameter or its attribute being studied at a given point of time; for the average
parameter of the deposit could never be known for good being totally mined out. The mean
values of samples and the true mean for the deposit are distinctly different. The degree of
proximity estimated for the sample mean to the likely deposit mean indicates the confidence level
of the estimate. The exploration geologist has to chalk out the entire programme with optimum
cost and then plan for mining without sacrificing accuracy.
In other words, for a particular degree of assurance to an estimate, the broad quantum of
work load could be chalked out depending on the geostatistical nature of the deposit, determined
in earlier stages of exploration. Needless to mention, that standardization of the exploration
methodology and its norms, would have to be chiefly guided by the standard that can be
A B
C D

85
established over the majority of the factual information gathered during the due course of
exploration. As the majority of the known diamondiferous kimberlites of commercial interest are
of Mesoproterozoic age in India, normative derivation must primarily be based on exploration of
this type of deposits aligned with geological and tectonic milieu. Most important is to meticulous
evaluation of different phases or facies of kimberlite within a pipe body. The norms can be
established when the variables within a kimberlite body are interpolated properly.
The quantity and density of observations will always be dependent on the size of the
kimberlite deposit and diamond incidence at different petrological horizons within the pipe
rock. During the last few decades, the accuracy of reserves computation, grade and physical
nature has been eased with the advent of increased use of statistical techniques. Data computation
with high degree of accuracy depends mainly on exploration method and geology of the mineral
deposit. Exploratory obligation is best fulfilled by random analyses, grid evaluation and/or cross-
sectional proving methods depending on the natural geological layout of a deposit. It may be
interesting to integrate the philosophy of exploration and the practical approach and synthesis for
the most desired result. The aims are obvious and to achieve the goals it needs enough support to
strengthen the conclusions, so rationally arrived at.
Generally geophysical surveys are charged at a „per line-kilometer (lkm)‟ or per station
reading rate. The rates can vary from US$10/lkm to well over US$120/lkm depending on several
parameters, e.g., the type of survey, the size of the survey, the difficulty of surveying the give
terrain, the number of parameters measured, and the amount of processing required. It is not
uncommon for a junior explorer to have to budget between $300,000 to over $1 million a survey
without considering fuel costs, crew accommodations and mobilization costs to and from the
survey site. A poorly designed survey not only costs money but it leads to compounding negative
effects on the selection of targets to drill, the expected discoveries, and potentially a lost mining
investment opportunity.
Throughout the exploration and evaluation stages, there are parallel processes aimed at
generating awareness of the project and soliciting feedback from stakeholders (e.g. nearby
communities). The process of collecting data for environmental baselines is usually established
early by exploration companies and environmental studies are undertaken throughout the
exploration stages. Mining licenses and permitting follow after government review of completed
feasibility, environment, and socio-economic studies. The exploration strategy for
kimberlites/diamonds involves complex decisions and has to be planned meticulously. A six stage
strategy with an integrated approach will help in successful kimberlite exploration, as shown in
Table 2.3

86
Table 2.3 Integrated stage-wise strategy for kimberlite exploration
Diamond exploration is highly expensive and a high-risk activity. Assuming that
100 pipes are discovered, 10 pipes can be diamondiferous and one or none can be
economically workable. No single exploration technique or method is available to prove
a diamond deposit. Uncompromising investment to carry out a thorough and systematic
exploration is highly warranted. An integrated exploration methodology in conjunction
with geological, geophysical and geochemical techniques, followed by drilling of
targeted anomalies will lead to a discovery. Even though the discovered pipe is
diamondiferous, the diamond content of the pipe remains uncertain until it is critically
evaluated, assessed and declared on the basis of geological information collected right
from initial reconnaissance right up to deposit delineation.
The diagnostic properties of kimberlite pipes like smaller size, complex deposit
geometry with irregular shapes, irregular distribution of inclusions or xenoliths, irregular
distribution of diamonds with respect to facies etc., are more akin to deposits mined
adopting underground mining method. But in general, the diamond mines are mined by
open-cast method. Hence the geological milieu within the pipe and also the country rocks
is of primary concern (Jakubec, 2008). A thorough understanding of the kimberlite
emplacement process may avoid adverse effects on the economic feasibility for an
upcoming diamond mine. Therefore, it is vital to methodically understand the
emplacement processes; thereby a better deposit model can be constructed to face the
mining constraints, risks so as to achieve economic viability of the diamond mine, more
accurately.
Stage 1 Area Selection
Stage 2 Remote Sensing & Regolith Mapping
Stage 3 Airborne Geophysics & Kimberlite Indicator Mineral (KIM) Chemistry
Stage 4 Catchment Prioritization, Field Mapping, Petrography, Surface Sample
Geochemistry & Ground Geophysics
Stage 5 Anomaly Identification, Pitting
Stage 6 Drilling & Discovery

Chapter 2 diamond exploration principles and practices

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Chapter 2 diamond exploration principles and practices

Similar to Chapter 2 diamond exploration principles and practices (20)

More from James AH Campbell

More from James AH Campbell (20)

Recently uploaded

Recently uploaded (20)

Chapter 2 diamond exploration principles and practices