Tuesday 30 December 2014

Understanding the Tokapal Kimberlite.

Kimberlites are uncommon geological formations formed by the extrusion of mantle material from deep within the Earth. Despite the relatively small number of kimberlites known, they have been the subject of extensive studies for many years, as they are the source of the deepest known rocks found on the surface of the planet, frequently contain xenoliths of other material that they have accumulated during their passage, and because they are the best known source of diamonds for the mineral extraction industry. There are generally considered to be three forms of kimberlites: Group I Kimberlites, the best known form, associated with Archaean cratons (ancient pieces of the continental lithosphere, emplaced before 2500 million years ago) across the globe. Group II Kimberlites (or orangeites), which have a different mineralogical and chemical composition, and which were thought to be restricted to the Kaapvaal Craton of Southern Africa, until recent discoveries in Russia and India. Group III Kimberlites (or transitional kimberlites) were also thought to be restricted to Southern Africa, but are now known from West Africa, South America, Russia, Australia and India.

Recent discoveries of diamond-bearing Group II Kimberlites in the Bastar Craton of central India have provoked considerable interest in Indian Kimberlites, both for their commercial potential as sources of diamonds, and due to their apparent relationship to the Deccan Traps Flood Basals, emplaced at the end of the Cretaceous.

In a paper published in the journal Geoscience Frontiers on 19 December 2013, Chalapathi Rao of the Department of Geology at Banaras Hindu University, Bernd Lehmann of Mineral Resources at the Technical University ofClausthal, B.K. Panwar and Alok Kumar, also of the Department of Geology at Banaras Hindu University and Datta Mainkar of the Directorate of Geology and Mining in Raipur discuss the results of petrographical and geochemical study of samples from the Tokapal Kimberlite in the Bastar Craton.

(A) Generalised geological map of India showing the location of the Tokapal kimberlite in the Bastar craton, central India. The occurrence of other lamproites and kimberlites in various cratons is also shown. CBF = Chitradurga fault; CITZ = Central Indian tectonic zone; CUB = Cuddapah basin; CB = Chhattisgarh basin; VB =Vindhyan basin; SMB = Singhbhum mobile belt; CIS = Central Indian shear zone; W =Wajrakarur kimberlite field; R = Raichur kimberlite field; Np = Narayanpet kimberlite field; T =Tokapal kimberlite; M =Mainpur kimberlite field; Dv =Damodar valley; Mg = Majhgawan; S = Saptarshi; K = Khadka; N = Nuapada; G =Garledinne; Vk = Vattikod; Kr = Krishna; Rd =Ramadugu. (B) Geological setting of the Tokapal kimberlite system. Rao et al. (2014).

The Tokapal Kimberliteis a roughly saucer-shaped sheet of volcanic rock 2.5 km wide in the Western Indrāvati Basin. It is one of the world’s largest crater kimberlites, covering over 5.5 km2) and being more than 50 m thick across much of its extent, exceeding 90 m in places.

Tokapal tuff exposed in a pit. Rao et al. (2014).

The samples show little contamination by crustal material, but have apparently been somewhat metamorphicly altered since they were emplaced, showing low-temperature serpentinisation and carbonate alteration. This means that of the three major mineral types in the rock (olivines, carbonates and spinels) only the spinels are preserved in their original form. Garnierite is present in the samples, apparently formed from the serpentinisation of olivine, but diamonds are absent.

Back scattered electron (BSE) image of the Tokapal pipe showing olivine macrocrysts (Ol), titanite (Ti) and spinel (Sp). Rao et al. (2014).

Despite this metamorphic influence, the proportions of major elements is thought to have been preserved, enabling inferences to be made from the bulk element composition. The Lanthanum-Ytterbium ratio of the samples are comparable to those seen in the Wajrakarur and Narayanpet kimberlite fields in the south of India, implying a common origin. Other element ratios show strong similarities to those seen in Group II Kimberlites from Southern Africa and Central India, and Group III Kimberlites from Southern India and Southern Africa.

The Tokapal, Wajrakarur and Narayanpet kimberlites, the Majhgawan diatreme and the lamproites of the Cuddapah basin show similar depleted-mantle Neodymium ages (date obtained by comparing the ratio of Samarium147 to its decay product Neodymium143), implying a potential common origin, with the dates obtained suggesting a possible link with the breakup of the ancient supercontinent of Columbia, which formed between 2.0 and 1.7 billion years ago and lasted till about 1.3 billion years ago.

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