Contents
of October, 1999 - Vol. XX No.4
TECTONIC SETTINGS AND DIAMOND PRODUCTIVITY
OF KIMBERLITE-CLAN ROCKS
Stephen E. Haggerty
A GEOPHYSICAL PROGNOSTICATION OF
PRIMARY DIAMOND DEPOSITS IN PENINSULAR INDIA
C. Ramachandran, M. Kesavamani and R. M. C. Prasad
GEOPHYSICAL EXPLORATION IN SEARCH
OF KIMBERLITES: AN APPRAISAL FROM PARTS OF ANDHRA PRADESH
K. P. R. Vittal Rao
SEARCH FOR KIMBERLITES IN THE EASTERN
BLOCK OF THE DHARWAR CRATON: A CONCEPTUAL MODEL
S. S. Nayak, K. R. P. Rao, S. A. D. Kudari, S. Ravi, S. K. Kulshrestha and
K. S. Bhaskar Rao
N. Balakrishna Rao
OCCURRENCE PATTERN OF KIMBERLITE
PIPES BASED ON AEROMAGNETIC IMAGES
SAMPLING TECHNIQUES FOR LOCATING
PRIMARY SOURCES OF DIAMOND: PATHFINDER METHODS FOR RECOVERY OF DIAMONDS, INDICATOR
MINERALS AND GEOCHEMICAL SIGNATURES
Maureen T. Muggeridge
THE EXPLORATION CONTEXT FOR DIAMONDS
C. M. H. Jennings and N. K. Smithson
TECTONIC SETTINGS AND DIAMOND PRODUCTIVITY OF KIMBERLITE-CLAN ROCKS
Stephen E. Haggerty
Department of Geosciences, University of
Massachusetts, Amherst, Mass. 01003, USA.
Abstract
The geological and tectonic settings of
kimberlite-clan rocks (KCRs), defined as alkali Ultramafic intrusives that are
volatile and light rare-earth-element enriched, and are capable of transporting
mantle xenoliths and diamonds from depth in excess of 660 km, are reviewed from
the West African Craton, the Congo Craton, the Kaapvaal Craton, the Kimberley
Craton (NW Australia), the Sino-Korean Craton, the Siberian Platform, and for
cratons in Brazil, the USA, Canada, and India. Diamond deposits are in Archean
and Proterozoic cratons that have Lithospheric roots (keels) to depths of at
least 200 km, and the optimum conditions for diamond formation and diamond
preservation are low heat flows in tectonically stable cratons. Delamination of
diamondiferous keels and thermochemical interaction appear to be major
influences in controlling the diamond productivity of KCRs. There is a
consistent pattern of crustal lineaments, and lineament controls on KCR
diatremes by long lived reactivated faults and mega-shear systems in all of the
diamond provinces reviewed. This pattern, as well as the clustering of 10-100
pipes in all cratons, implies that successful access of explosive volcanism in
the crust, without the combustion of diamond, requires long term
preconditioning of the crust but rapid access of KCRs through conduits and
along deeply penetrating faults that may reach the upper mantle.
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A GEOPHYSICAL PROGNOSTICATION
OF PRIMARY DIAMOND DEPOSITS IN PENINSULAR INDIA
C. Ramachandran, M. Kesavamani and R. M. C. Prasad
Geophysics Division, Geological Survey of
India, Bandlaguda Complex, Hyderabad - 500 068, India.
Abstract
We propose an integrated strategy for
the geophysical prognostication of kimberlites and lamproites, which continue
to be the important hosts for diamond deposits, in Peninsular India. The
proposed strategy comprises: 1) regional and local structural controls for
kimberlites and lamproites of well studied diamond deposits from different
parts of the world, 2) recognition of the typical geophysical map patterns or
shapes of anomalies that may reflect the known kimberlites and lamproites of
Peninsular India, and 3) re-interpretation of some of the gravity and magnetic
maps of Peninsular India and selection of favourable areas for diamond
deposits.
Low heat flow regions, deep-seated
basement fractures / fault corridors and linear grabens are important
large-scale controls for diamond deposits. Dome or ring shaped features, fault
systems arranged in radial patterns, minor dome shaped features within the main
dome, splay faults of major fault corridors, dilation jogs created by strike
slip movements or fault / fractures, internal shears and cross cutting
structures in major structural lineaments, contiguous fault zones and
intersection of contiguous faults, form the regional and local controls of the
exploration model. The heat flow and regional gravity maps of India, and the
available gravity and magnetic maps of Wajrakarur, Chigicherla, Maddur, Raipur
and Panna areas are re-interpreted to recognize some of the above controls for
diamond deposits. The geophysical signature of kimberlites and lamproites can
be highly varied. 12 types of contour patterns are recognized from the gravity
and magnetic maps of Peninsular India, that may reflect the kimberlite fields /
clusters and individual pipes, and on this basis, several areas with potential
for diamond deposits are delineated.
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GEOPHYSICAL EXPLORATION IN SEARCH OF KIMBERLITES: AN APPRAISAL
FROM PARTS OF ANDHRA PRADESH
K. P. R. Vittal Rao
Geological Survey of India, Southern Region,
Hyderabad.
Abstract
Kimberlites are the primary source
rocks for diamonds that originate at mantle depths, of over 100 km. They are
confined to uplifted block of continental platforms and are related to large
scale structural features such as deep fractures, disjunctive zones, lineaments
etc. The application of geophysical techniques in kimberlite exploration helps
in identification of the blocks with a favourable structural setting for the
emplacement of kimberlites and also for conducting semi-detailed / detailed surveys
for targeting kimberlite itself.
Regional gravity and magnetic maps
generated at 1: 50,000 scale with a coverage of one station per sq. km are
utilized to trace the underlying structural deformations which are related to
the emplacement of kimberlites. Gravity and magnetic lineaments, second order
trends, flextures, terminations and their intersection points are indications
of the density and magnetic susceptibility changes in the rocks during several
tectonic processes. Therefore these form a favourable structural situation for
the emplacement of kimberlites.
Blocks, wherein the above deformational
features are inferred from a study of regional gravity and magnetic data, are
selected for semi-detailed surveys. The features of the semi-detailed maps are examined,
correlating with the geological information. Prominent lineaments, either
'lows' or 'highs', isolated anomalies etc., are carefully sorted out and
anomalous features are further tested by resistivity profiling. Exploratory
pits are opened at anomalous locations to expose the bed rock. After a visual
examination of the exposed rock, petrological and chemical analyses are
carried-out to confirm the kimberlite affinity.
Geophysical exploration in Wajrakarur
kimberlite field (WKF) in Anantapur district of Andhra Pradesh was carried out
for nearly 20 field seasons while geophysical work in the recently discovered
Narayanpet Kimberlite field (NKF) in Mahbubnagar district of Andhra Pradesh and
Gulbarga district of Karnataka was only for 3 field seasons.
After such a relentless search, 3
kimberlite pipes in Wajrakarur and Lattavaram Tanda areas of Anantapur district
and one in Kamusanpalli block of Mahbubnagar district were located. The review
is restricted to the above 4 case studies, where the 'kimberlite fields' are
concealed under soil cover and one case study from Chandaraki block of
Maddur-Narayanpet field where the geophysical efforts are not successful in
locating the kimberlite inspite of the corroborative geophysical indications.
Pipe-7 from Venkatampalli area and CG-1, CG-2 from Chigicherla area are also
included to demonstrate the effectiveness of E. M. method in the former case
and the close association of basic dykes in the latter.
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SEARCH FOR KIMBERLITES
IN THE EASTERN BLOCK OF THE DHARWAR CRATON: A CONCEPTUAL MODEL
S. S. Nayak, K. R. P. Rao, S. A. D. Kudari, S. Ravi, S. K.
Kulshrestha and K. S. Bhaskar Rao
Operations: Andhra Pradesh, Geological Survey
of India, Hyderabad - 500 068.
Abstract
Dharwar Craton, hosting the
historically world famous South Indian Diamond Province, is endowed with both
primary (kimberlites and lamproites) and secondary (conglomerates and gravels)
sources of diamond. The primary source rocks are extremely rare, small volume
igneous bodies of mantle origin and are preferred to secondary sources for
exploration world over.
Because of their small areal extent and
rarity of occurrence a successful programme of search for primary sources in
vast continental regions has to concept oriented. Emplacement of kimberlites in
the eastern block of the Dharwar Craton is controlled mainly by the major
ENE-WSW to E-W trending basement faults which were reactivated around 1100Ma
period evidences of which are recorded in the sediments of the Cuddapah Basin.
It is proposed that emplacement of kimberlites in the Archean
granite-greenstone terrain of the eastern block of the Dharwar Craton is caused
by the reactivation of the major ENE-WSW to E-W trending basement faults during
the culmination of the Eastern Ghats Orogeny. Intersections of these faults
with the NW-SE trending major faults and closure parts of the structural domes
formed favourable loci for emplacement of kimberlites.
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AN APPRAISAL OF THE STRUCTURAL
SET-UP OF WAJRAKARUR AND LATTAVARAM AREAS USING POTENTIAL FIELD AND RADIOMETRIC
DATA
N. Balakrishna Rao
Geological Survey of India, Southern Region,
Hyderabad.
Abstract
The geophysical data comprising
gravity, magnetic vertical field (VF), aeromagnetic and potassium data have
been studied and interpreted to deduce the structures which might have played
role in the emplacement of kimberlites. The processed maps of gravity and the
synthesis of different data sets has helped in correlation and delineation of
structures. A careful look of the structural setup near the known kimberlite
pipes reveals very good similarity and it can be said that all the pipes in
Wajrakarur and Lattavaram areas are invariably located at the junction both in
regional and detailed scale maps.
A careful look of the Bouguer gravity
map of Jonnagiri area reveals that the diamond occurrence are very close to the
areas where the N-S trends take E-W swing or vice-versa. Hence, it can be
inferred that the diamond occurrences in Jonnagiri area are correlatable to
kimberlite pipes which might be in the vicinity only.
Besides the well established ' a
posteriori' approach i.e. extending the geophysical signatures corresponding to
the structures near the known kimberlite pipes to the new areas, other features
like structural breaks, proximity of circular features, and the bulge in adjacent
gravity contour pattern representing probable zones of dilation are some of the
criteria for identification of target areas. The aeromagnetic and potassium
maps very well corroborate the inferences drawn from the potential field and
the derivative maps.
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OCCURRENCE PATTERN OF KIMBERLITE PIPES BASED ON AEROMAGNETIC IMAGES
K. K. Sharma*, V. M. Padmakumari** and K. Mallick**
*Department of Applied Geology, University of
Madras, Chennai - 600 025
**National
Geophysical Research Institute, Hyderabad - 500 007.
Abstract
The location of Kimberlite pipes by
geophysical methods has assumed great importance in recent years. In an attempt
to identify patterns in the occurrence of the Kimberlite pipes, the
aeromagnetic map in the region around Wajrakarur Kimberlite pipes show an
average of 1091±15 million years.
The basic aeromagnetic data and the
residual map are presented in the image forms. It is observed that there is a
definite pattern in the occurrence of the pipes 1,2,6 and 12 with regard to the
magnetic and its residual maps. Since the Kimberlite is generally poor in iron
content and rich in magnesium, the magnetic low is the typical response of the
Kimberlite pipes. A combination of positive and negative anomalies appear in
the map due to the dipolar nature of the magnetic field.
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SAMPLING TECHNIQUES FOR LOCATING PRIMARY SOURCES OF DIAMOND: PATHFINDER
METHODS FOR RECOVERY OF DIAMONDS, INDICATOR MINERALS AND GEOCHEMICAL SIGNATURES
Maureen T. Muggeridge
Horizon Enterprises International PL, Suite 5,
286 Fitzgerald Street, Perth, Western Australia
Abstract
Tracking down a primary diamond-bearing
source entails detecting and systematically following a weak trail of subtle
clues, before a final target for testing is struck. Pathfinder minerals and
elements are important in this search as they provide direct evidence of the
presence of their host. Heavy mineral sampling to detect indicator minerals and
diamonds is a particularly useful and broadly applied technique which has led
to the discovery of many mines in various parts of the world. Critical aspects
of heavy mineral sampling methodology include sample selection, size, spacing
and processing. Special attention must be paid to the selection and evaluation
of trap sites during drainage sampling. Geochemical and geobotanical exploration
methods have a more restricted use in diamond exploration programmes.
Forethought and meticulous attention to detail are required when planning and
conducting each phase of an exploration programme involving any of the
pathfinder sampling techniques, as well as in the analysis of samples and in
progressive interpretation of results. Effectiveness of each stage of a diamond
exploration programme is directly related to the reliability of the results it
generates, which in turn are vital to the decision as to whether or not further
exploration is warranted.
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THE EXPLORATION CONTEXT FOR DIAMONDS
C. M. H. Jennings and N. K. Smithson
Southern Resources Limited, 33 Yonge Street,
Suite 1040, Toronto, Ont., M5E 159, Canada
Abstract
World natural diamond production
for1997 is estimated to be approximately 117 million carats. Of 7000 known
kimberlite / lamproite occurrences, less than 60 have been mined with only
about 15 major producers (0.21% of all pipes). Only nine mines that produce
more than 3 million carats per annum have been discovered since 1950.
Exploration strategy, world production, sizes of pipes, quality and value of
diamonds, mining, milling and economics of diamond mining are discussed. An
overview is given of area selection methods; exploration methods (indicator
sampling, geophysical techniques); mineral chemistry and petrographic
techniques; use of microdiamonds; initial and advanced evaluation of diamond
pipes and marketing of diamonds.