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
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
OCCURRENCE PATTERN OF KIMBERLITE
PIPES BASED ON AEROMAGNETIC IMAGES
K. K. Sharma, V. M. Padmakumari and K. Mallick
THE EXPLORATION CONTEXT FOR
C. M. H. Jennings and N. K. Smithson
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.
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.
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.
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.
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.
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.
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.
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.