Compositions of garnet and spinel from the Aries diamondiferous kimberlite pipe, central Kimberley Block, Western Australia — implications for exploration

Abstract

Aries pipe is a weathered, micaceous kimberlite (≈820 Ma) consisting of four lobes: South, Central, North and North Extension. It is the largest (≈18 ha) and most diamondiferous of the few kimberlites currently known in Australia. The pipe contains abundant country-rock (basalt, quartzite) xenoliths, but the only mantle-type inclusions so far recovered are individual grains of Cr-spinel, rare chromian pyrope-garnet and diamond. Aries differs from other kimberlites of similar age in the Kimberley Block by the absence of megacrystic minerals. Chromian pyrope-garnet has compositions which indicate garnet lherzolite and garnet wehrlite in the upper mantle, but harzburgitic “G10” garnets considered the paramount indicator of diamondiferous kimberlite in southern Africa have not been recovered at Aries. Spinel has been the most important indicator mineral in the discovery of Aries. It can be distinguished, morphologically and compositionally, from spinel in local basaltic country-rocks. The spinels in the pipe have a range of internal features which can be allocated into six types. Some morphological varieties show apparent subgrain development, with 120° triple point junctions, indicating post crystallisation deformation and recrystallisation, whereas others have textures suggesting rapid crystallization from a chromium-saturated melt. Compositional variations suggest that the six morphological types can be combined into two broader classes. Class 1 spinel compositions are consistent with derivation from garnet peridotite in the upper mantle and include some which are similar to diamond-associated spinels in other deposits world-wide. As such, the most Cr 2O 3-rich Class 1 spinels have been used during exploration to indicate the potential of the Aries pipe to host diamond. Class 2 spinels are more Fe-rich than Class 1 spinels and the compositions are unlike any described to date in the common rock types from the upper mantle, but the internal textures suggest that these are xenolithic in origin. Both Class 1 and Class 2 spinels may be associated with Ti-rich Cr-spinel which is compositionally similar to titaniferous Cr-spinel from the groundmass. Class 1 spinels are typically cross-cut and rimmed by Ti-rich Cr-spinel and may have originated from either metasomatised, Cr-spinel rich dunite or metasomatised harzburgite. Class 2 spinels are only rimmed by Ti-rich Cr-spinel. The Ti-rich variety which either cross-cuts or rims the xenolithic spinels probably developed during metasomatic/magmatic events in the mantle, and is perhaps related to kimberlite formation.