Archaean period was notable for intense magmatism, which brought about development of large metallogenic provinces and diamond genesis. Continental crustal formation during early Archaean was negligible and whatever formed was recycled back into the convecting mantle. The few that survived either retained or built up thick Archaean mantle keels (between 200 and 400 km) which served as repositories (Figure 1) for the early formed diamonds1,5–7. Some of these Archaean crusts or cratons have remained unaffected for billions of years by orogenic activities like subduction. Typical of these Archaean cratons are greenstone belts (mainly rocks of ultrabasic and basic magmatism) and granitoids (products of younger magmatism). Peridotites, eclogites, komatiites, kimberlites, lamproites, basalts, andesites, and dacites are the usual intrusive rock groups in these greenstone belts and graded volcanoclastic sediments and greywacke are the sedimentary sequences noted here. Considerable thickness of such geological terrains lie scattered in a number of countries like Western Australia, Canada, South Africa, Norway and Greenland, while in India, the Dharwar craton and the Singhbhum craton are the two classic occurrences.

Most of the world’s diamonds come from kimberlites, intrusive into theArchaean cratons. Lamproites (volcanic lamprophyres), another class of intrusive rocks, also carry diamonds and they have been more associated with Proterozoic mobile belts adjacent to some of these cratons8 (Figure 1). About 500 diamondiferous kimberlites are known all over the world and 15 among them are reported to be active mines with hopes of new ones coming up in Canada, Eastern Europe and Russia8. Indian kimberlites and lamproites, worked for diamonds are Proterozoic volcanic intrusions between 846 and 1446 m. y. ago8–10. Notable occurrences are in Andhra Pradesh (Wajra Karur, Lattavaram), Madhya Pradesh (Majhgawan and Hinota near Panna), Uttar Pradesh (Jungel valley), and Orissa (Sambalpur). At Majhgawan is the only mine working lamproites for diamonds, while the others in Andhra Pradesh and Orissa are alluvial deposits of Krishna and Mahanadi rivers. Microdiamonds occur in lamproite dykes at Chelima, Andhra Pradesh. Bulk of world’s diamond production comes from Africa, Australia, Russia and the recently discovered new deposit from the Canadian Arctic region. China, Borneo, Brazil and India produce smaller amounts. Now a new type of diamond occurrence has been reported11 from komatiites, which like kimberlites are early ultrabasic lavas generated by partial melting of mantle. This reported occurrence of diamonds adds a new dimension to diamond prospecting in Archaean terrains where komatiites are predominant.

During the late 1960s, geologists working in one of the ancient cratons in South Africa – the Barberton Mountain Land greenstone belt came across certain interesting ultrabasic intrusives within the 3.49 b.y. old Komati Formations of the Onverwacht Group. These showed unusual chemistry, pillowed structure and a distinct spinifex texture (radiating bladed olivine crystals) that develops during quench crystallization of highly magnesian high temperature lava. They had high MgO, very high MgO/FeO ratio, low alkalies and TiO2, Al2O3/CaO ratio less than unity, a chemical composition quite unlike that of average basalt and peridotite. These rare chemical and mineralogical features warranted classifying them as a separate rock group – ‘komatiites’12, named after the Komati river formations where they were first noticed. These were further classified as basaltic komatiites (with MgO 10–20%) and peridotitic komatiites (with MgO >20%). Discoveries of komatiites were soon reported from Western Australia (Pilbara craton)13,14, India (Kulamara, Singhbhum Thrust Belt and the Kolar Schist Belt)15,16, Canada (Abitibi Belt)17–19, Finland (Satasvaara greenstones)20, Norway (Karasjok belt)21, Columbia (Gorgonia Island)22 and French Guiana (Inini greenstone belt)11. Petrological, geochemical and isotopic investigations have indicated that komatiites are products of high degree melting (typically 50–100%) of mantle peridotite and those with high ages around 3 b.y. are found to be alumina depleted (e.g. Barberton, Pilbara) while those with younger ages, around 2.7 b.y., are alumina undepleted (e.g. Abitibi, Kolar Schist Belt)23. There exists a view regarding a decline in komatiite abundance (and the average MgO contents) and increase in kimberlite occurrence with time; however, examination of some 40 greenstone belts worldwide does not support this view; the study, however, has indicated peaking of these rocks in late Archaean greenstone belts6.

The stability of diamonds plus their remote age (most of the diamonds show ages >2.5 b.y.) make them ideal time capsules to study early mantle geochemistry and mineral genesis. In fact, few minerals have locked up within them so much information about early mantle, its mineralogy, chemistry and about volatiles as diamonds have and this has led to a lot of experimental work and lengthy debates about diamond genesis and distribution. It is interesting to note that most of the diamonds in the world were formed around 2.8–3 b.y. and were emplaced by the volcanic pipes in rocks of age group >1 b.y. which strongly suggests that conditions specific to diamond genesis as well their subsequent emplacement must have prevailed globally at these times in the earth’s interior7. This inference emerges from extensive studies made not only on mantle xenoliths caught up in kimberlites and associated lamproitic rocks but also from mineral inclusions such as sulphides, garnet, olivine, pyrite, chromite in the diamonds24, trace element patterns (K, Na, Ba, Sr, REE, Ti, Zr, Nb and P)7 and radiogenic isotopes25,26. Based on similarity of inclusions to olivine-bearing peridotites or eclogites, diamonds are grouped as P-type (peridotitic) or E-type (eclogitic). While the former shows d 13C values within a limited range, in agreement with that of lithospheric mantle, the E-type shows variable d 13C which has given rise to speculation about its derivation. Recent studies27 on the latter type diamonds attributed the different d 13C ranges in them to different evolutionary trends taken by the carbonatitic melts from which diamonds crystallize. The nature of the mineral inclusions, Fe or Ni rich sulphides, is also found to be helpful in deducing the principal host rocks, peridotite or eclogite1 as well as the depths from which diamonds were sampled, whether from the transition zone or deeper.

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