Geochemistry, mineralogy and petrology of a new find of ultramafic lamprophyres from Bulljah Pool, Nabberu Basin, Yilgarn Craton, Western Australia
Abstract
Four variously pipe or sill-like, Carboniferous (≈ 305 Ma) bodies have been located near the NE edge of the Archaean Yilgarn craton. The rocks comprise Basbnd Ti-bearing tetraferriphlogopite-tetraferriannite, low Alsbnd Ti-diopside, calcite, perovskite and groundmass titanomagnetite-chromite (up to 41.3% Cr2O3), with minor apatite, Mgsbnd Mn ilmenite, rare-earth phosphate, Ksbnd Ba-feldspar (up to 17% BaO), baryte and an unidentified Basbnd Zr silicate. The last three reflect very high whole-rock Ba (up to 5,652 ppm). Aegirine-rich pyroxenes occur in fenitic alteration assemblages. Together with high {Si}/{Al} and low {Mg}/{Ca} whole-rock geochemistry, these features are diagnostic of ultramafic lamprophyres (damkjernites and aillikites), although the rocks also show some affinities with classical kimberlites. Mineral concentrates from loam samples yield an array of minerals of mantle origin, including garnets (Dawson and Stephens' groups G1, 3, 5, 9 and 10), chromian diopsides (up to 6.2% Cr2O3), magnesiochromites (up to 20% MgO, 70% Cr2O3) and four compositional groups of ilmenites (low-Mn picroilmenites, Mn-rich, Mg-poor and two moderate Mnsbnd Mg compositions). Actual spinel-lherzolite nodules are common in one body and the presence of spinel-and/or garnet-lherzolites can be inferred in the others from the concentrates. The Bulljah bodies are therefore of deep mantle origin, as confirmed by the recovery of a single microdiamond. They thus extend the field of potentially (if not necessarily economically) diamondiferous rocks beyond kimberlites and lamproites. When added to other recent lamprophyre finds, the Bulljah discoveries suggest that the Yilgarn craton could, like many other ancient cratons, be ringed and/or dotted by a diverse array of alkaline and lamprophyric rocks of varying ages which remain to be discovered.
- Publication:
-
Lithos
- Pub Date:
- August 1990
- DOI:
- 10.1016/0024-4937(89)90048-0
- Bibcode:
- 1990Litho..24..275H