Abstract Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a c. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic of bulk rock compositions and is associated with strongly depleted Th–U–Nb–Ta contents at high Cs–Rb–Ba–K concentrations. The resultant high large ion lithophile element/high field strength element ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones. The Bushveld-intersecting orangeite dykes have strongly enriched Sr–Nd–Hf isotopic compositions (initial 87 Sr/ 86 Sr = 0.70701–0.70741; ε Nd = −10.6 to −5.8; ε Hf = −14.4 to −2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti–Nb–Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2–1.1 Ga Namaqua–Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma. Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H 2 O–CO 2 –F in the petrogeneses of these potentially diamondiferous alkaline rocks.

Abstract Our pilot study reveals potential Li isotope fingerprints recorded in the Mesoproterozoic ( c. 1.4–1.1 Ga) kimberlites, lamproites and lamprophyres from the Eastern Dharwar Craton and Paleocene (62 Ma) orangeite from the Bastar Craton in India. The new data are interpreted in the context of available Li isotope composition of lamproitic to lamprophyric rocks occurring in Variscan (Bohemian Massif) and Alpine–Himalayan (SW Tibet) orogenic belts formed in response to Gondwana–Pangea amalgamation and break-up. As a result of the development of supercontinents, kimberlites from the Eastern Dharwar Craton and ‘orangeite’ from the Bastar Craton show clear presence of a component with a heavy Li isotope signature (δ 7 Li up to 9.7‰) similar to ancient altered oceanic crust, whereas the Eastern Dharwar Craton lamproites (2.3–6.3‰) and lamprophyres (3.3–6.7‰) show Li isotope signatures indicative of a dominant contribution from heterogeneous lithospheric mantle. Variscan lamprophyric to lamproitic rocks and post-collisional mantle-derived (ultra)potassic volcanic rocks from SW Tibet, i.e. rocks from the orogenic belts outside the cratonic areas, are characterized by a clear Li isotope shift towards an isotopically lighter component (δ 7 Li as low as –9.5‰) comparable with the involvement of evolved continental crust and high-pressure metamorphic rocks in their orogenic mantle source. Such components with isotopically light Li are strikingly missing in the source of cratonic kimberlites, lamproites and lamprophyres.

Abstract The Bunder diamond project comprises a cluster of seven known diamondiferous volcanic pipes and dikes known as the Saptarshi field. The largest of these is Atri, which comprises two adjacent coalesced volcanic pipes, Atri North and Atri South. This paper reports data that have been compiled into a new three-dimensional geologic model and, together with new geochemical and geochronological information, provides further insight on the internal geology, emplacement history, classification, and age of the Atri pipes. The range of texturally diverse geologic units within the Atri pipes suggests a complex emplacement history, with variations in eruption energy and source magmas identified. The steep-sided pipes were infilled by multiple phases of primary pyroclastic material as well as variably coherent material now locally preserved along the pipe margins. Atri North postdates Atri South and displays a marked change in both the locus and style of volcanic activity. Comparison between the Saptarshi intrusions and the Majhgawan and Hinota diamondiferous pipes (the only other known diamondiferous deposits on the craton) reveals similarities in the marginal cratonic setting, petrogenesis, and age of emplacement. The classification of the Atri pipes within the traditional kimberlite-orangeite-lamproite scheme is not possible due to conflicting discrimination evidence. The magmatic mineral assemblage of the Atri pipes (olivine, phlogopite, apatite, spinel, rutile/anatase, and ilmenite) is not diagnostic. The expanded dataset of phlogopite mineral chemistry has both lamproite and orangeite affinities, while the Sr and Nd systematics of the pyroclasts ( 87 Sr/ 86 Sr 0.7038–0.7048, ε Nd +1.6 to –1.8) are more consistent with archetypal kimberlites. Many of these characteristics are similar to those of the nearby Majhgawan and Hinota pipes. Consequently, these pipes are best classified as members of the alternative metasomatized lithospheric mantle magma group. Rb-Sr dating of phlogopite indicates a pipe emplacement age of 1079 (± 6) Ma, similar to published phlogopite ages from Majhgawan (1067–1084 Ma, recalculated).

Abstract Diamonds are lithologically widely distributed, and are found in unconsolidated and consolidated sediments (placers and paleoplacers), various igneous rock types of deep-seated origin (kimberlite, orangeite, lamproite, alnoite, aillikite, picritic monchiquite, alkali basalt), high pressure mantle xenoliths, high pressure metamorphic rocks, and also meteorites and their impact structures. Of these, only diamond-bearing kimberlite, orangeite, and lamproite, plus associated placers and paleoplacers, are economically viable. Prior to 1960, more than 80% of all diamonds were derived from secondary deposits; by 1990, this figure was less than 25% (Levinson et al., 1992). Diamond is the only mineral commodity extracted from kimberlite- or lamproite-hosted deposits. Diamonds are subdivided into industrial, near-gem, and gem quality stones. However, they are also described as being either ‘cuttable’ or ‘industrial’ (Levinson et al., 1992). Based on 1992 world production figures, approximately 50% by weight of a total production of 105 Me (where Me = million metric carats; c = metric carat = 0.2 g) was industrial grade, the remainder being cuttable. Industrial grade stones are used for a variety of purposes, but compete with synthetically produced industrial diamonds (estimated 1993 production 450-500 Me; G.T. Austin, pers. comm., 1994). Only primary diamond deposits are discussed here. These have been subdivided into two groups on the basis of their host rocks, which are either kimberlites or lamproites. In addition to their host rock differences, these deposits also differ in morphology, mineralogy, and other respects. These differences between the two types are discussed in the summary accounts that follow.

Paleoproterozoic to Cenozoic lamprophyres, lamproites and related rock types (e.g., orangeites, kimberlites) are volatile-rich mafic magmatic rocks with a unique potential for the investigation of processes affecting mantle reservoirs. They originated from primary mantle-derived melts that intruded both cratons and off-craton regions, which were parts of former supercontinents – Columbia, Rodinia and Gondwana–Pangea. Well-known for hosting economic minerals and elements such as diamonds, base metals, gold and platinum-group elements, they are also significant for our understanding of deep-mantle processes, such as mantle metasomatism and mantle plume–lithosphere interactions, as well as large-scale geodynamic processes, such as subduction-related tectonics, and supercontinent amalgamation and break-up. This book aims to provide a timely overview of the state-of-the-art and recent advances as achieved by various research groups around the world. Mineralogical, geochemical, geochronological and isotope analyses are used to decipher the complex petrogenesis and metallogenesis of these extraordinary rocks, and unravel a complete history of tectonic events related to individual supercontinent cycles.