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).

Igneous Rock Associations 26. Lamproites, Exotic Potassic Alkaline Rocks: A Review of their Nomenclature, Characterization and Origins

“…The rocks are similar to the Majhgawan lamproites and contain many vesicular pyroclasts. Mica compositions follow the lamproite trend of compositional evolution from phlogopite to tetraferriphlogopite with Ti-and Aldepletion coupled with Fe 3+ enrichment (Das et al 2018).…”

“…Models presented by Mathur and Singh (1971) and Rao (2007) have suggested the vent consists of concentric bands of volcaniclastic rocks all of which taper to a common focal point at unknown depth. Given the complexities of other lamproite vents, in particular the Atri Pipes (see below; Das et al 2018), revealed by recent studies, such a model is highly improbable and should be re-evaluated. The vent fill consists of diverse heterolithic olivine lamproite breccias distinguished on the basis of colour, grain size, content of xenoliths, and abundance of mica (Fig.…”

“…Bunder Diamond Project (Das et al 2018) are a cluster of seven lamproite vents and dykes located 80 km west-southwest of Majhgawan. The lamproites were emplaced in Vindhyan Kaimur Group supracrustal sedimentary rocks contemporaneously with the 1080-1067 Ma Majhgawan-Hinota vents.…”

“…Five distinct lamproite pyroclastic or volcaniclastic units are present in the South pipe and three in the North pipe. These lamproites, described in detail by Masun et al (2009) and Das et al (2018), consist of diverse melt-bearing olivine phlogopite pyroclasts with country rock clasts and rarer autolithic clasts. Individual units (Fig.…”

“…Unfortunately, regardless of the mineralogical and petrological evidence, Das et al (2018) have incorrectly classified the Atri Pipes as "transitional kimberlite-orangeite-lamproite" or "majhgawanite". As noted above this nomenclature is petrogenetically impossible and should be discontinued.…”