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 The generation of the Earth’s continental crust modified the composition of the mantle and provided a stable, buoyant reservoir capable of capturing mantle material and ultimately preserving ore deposits. Within the continental crust, lithospheric architecture and associated cratonic margins are a first-order control on camp-scale mineralization. Here we show that the evolving crustal architecture of the Archaean Yilgarn Craton, Western Australia, played a key role in controlling the localization of camp-scale gold, iron and nickel mineralized systems. The age and source characteristics of Archaean lithosphere are heterogeneous in both space and time and are recorded by the varying Nd isotopic signature of crustal rocks. Spatial and temporal variations in isotopic character document the evolution of an intra-cratonic architecture through time, and in doing so map transient lithospheric discontinuities where gold, nickel and iron mineral systems were concentrated. Komatiite-hosted nickel deposits cluster into camps localized within young, juvenile crust at the isotopic margin with older lithosphere; orogenic gold systems are typically localized along major structures within juvenile crust; and banded iron formation (BIF)-hosted iron deposits are localized at the edge of, and within, older lithospheric blocks. Furthermore, this work shows that crustal evolution plays an important role in the development and localization of favourable sources of nickel, gold and iron by controlling the occurrence of thick BIFs, ultramafic lavas and fertile (juvenile) crust, respectively. Fundamentally, this study demonstrates that the lithospheric architecture of a craton can be effectively imaged by isotopic techniques and used to identify regions prospective for camp-scale mineralization.

Abstract We report new, highly precise, U–Pb and Ar/Ar ages for seven Cretaceous rhyolites, tuffs and granites from across Zealandia spanning a 30 Ma period from arc magmatism to continental break-up. Combined with previously published data, these reveal a strong episodicity in Cretaceous silicic magmatism outside the Median Batholith. 112 Ma tuffs are known only from the Eastern Province in association with a Cretaceous normal fault system. Both 101 and 97 Ma groups of rhyolites and tuffs occur across the entire width and half the length of Zealandia from near the palaeotrench to the continental interior, indicating widespread and effectively instantaneous extension. We attribute an increase in A-type character with time (112–101–97–88–82 Ma) to the progressive thinning of the Zealandia continental crust whereby, with time, there is less opportunity for crustal contamination. Extension directions associated with 101, 97 and 82 Ma magmatism and associated core complex exhumation across Zealandia are all oriented c. 30° oblique to the margin. These observations suggest Zealandia rifting was controlled by either >83 Ma capture of Zealandia by the Pacific Plate and/or <83 Ma Zealandia–West Antarctica spreading, rather than by laterally migrating triple junctions, slab windows or plume heads.

Abstract An Nd-Sr isotope database, including c. 200 new analyses, is presented for Palaeozoic and Mesozoic metasedimentary successions extending through southeastern Australia, New Zealand, West Antarctica and the Antarctic Peninsula to southern South America. Combining with U-Pb detrital zircon age data, this enables characterization of New Zealand terranes, especially within the Eastern Province, where there is a progression from westernmost terranes of both volcanic/volcaniclastic and accretionary origin with primitive sediment sources, to easternmost terranes with mature continental sediment inputs. In southern South America, West Antarctica and the Antarctica Peninsula, similar accretionary complexes have Nd model ages principally reflecting mixing of sedimentary material from multiple sources, both mature and juvenile. A mature Gondwana continental provenance dominates in sedimentary basins inboard of the active margin, especially in the Palaeozoic (Western Province, New Zealand, interior West Antarctica and NW Argentina), and contributes significantly to pre-Mesozoic sedimentary rocks of Patagonia east of the Andes. Along the Gondwanaland margin, Nd systematics for younger (late Palaeozoic to early Mesozoic) accretionary complex metasediments reflect younger source inputs, notably in the Scotia metamorphic complex. Many of the accretionary complex deposits must involve significant crustal reworking. There is no apparent South American equivalent of the primitive provenance of the westernmost accretionary terranes of New Zealand.