87 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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87
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),
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