The Bunder Diamond Project, India: Geology, Geochemistry, and Age of the Saptarshi Lamproite Pipes

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 (87Sr/86Sr 0.7038–0.7048, εNd +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).