A Review of the Kimberlitic Rocks of Western Australia

Abstract

Abstract As the result of a planned exploration programme for diamonds spanning 10 years, CRA Exploration Pty. Limited and the Ashton Joint Venture have discovered four diamond bearing kimberlitic provinces in Western Australia. Three of these provinces are marginal to the Kimberley craton in the north of the State (Fig. 2) and one lies in the Carnarvon Basin, adjacent to the Yilgarn Block, 1300 km to the southwest. The distribution of the kimberlitic rocks suggests that emplacement was controlled by major fractures associated both with early Proterozoic mobile zones and with later Phanerozoic rifting and continental break-up. Radiometric dating suggests intrusion of kimberlitic rocks took place 160 m.y. ago in the Carnarvon Basin and 25 m.y. ago in the West Kimberley, but other occurrences in the Kimberley region could be older. The bodies range in size from dyke-like features less than one metre wide to volcanic crater deposits with surface areas up to 128 hectares. These craters are champagne-glass shaped in cross-section, the narrow stem corresponding to the pipe feeder. The craters are infilled with volcanoclastic and epiclastic sediments. A late stage phase in many of the West Kimberley bodies was the emplacement of massive, igneous-textured, magmatic lamproite, rising to the surface and intruding into the crater. The exploration discoveries were facilitated by early recognition that in petrography, mineralogy and chemistry the kimberlitic rocks in Western Australia vary from classical kimberlites resembling those of Kimberley, South Africa, to the soda-rich rocks of Wandagee in the Carnarvon Basin, to the olivine lamproites (terminology of Jaques et al, this volume) of the Kimberley region of Western Australia which are composed essentially of phenocrysts of olivine ± clinopyroxene ± phlogopite ± potassic richterite ± glassy groundmass. Mantle nodules recovered range from dunite to lherzolite; graphic-textured intergrowths of picroilmenite and silica (after diopside?) occur at the Skerring pipe. Heavy mineral concentrates from the kimberlitic rocks yield pyrope, picroilmenite, chrome-diopside, chromite and zircon, the former two minerals being more abundant in the classical types of kimberlite. Recognition of the diamond-bearing potential of the olivine lamproites strongly influenced exploration techniques with use being made of such minerals as chromite, andradite and zircon as indicator minerals during heavy mineral gravel sampling. In regions where host rocks display low magnetic responses it was found that the kimberlitic bodies produced recognisable magnetic anomalies from detailed aeromagnetic surveys and this technique has been an important aid to exploration. To date only rare diamond has been found in the classical kimberlites of Western Australia, and in the more sodic varieties at Wandagee. Western Australia's rich diamond deposits have been found in the highly potassic olivine lamproites and many of the associated olivine-poor leucite lamproites contain rare diamond. Feasibility studies of the Argyle AK1 lamproite are based on a 2.9 million tonnes per annum operation to come into production in 1985, producing some 25 million carats per year, while limited commercial production from associated alluvial deposits commenced in January 1983. Lamproites thus constitute a new terrestrial primary source for diamond, hitherto thought solely restricted to kimberlite.