Neoproterozoic (ca. 820–830 Ma) mafic dykes at Olympic Dam, South Australia: Links with the Gairdner Large Igneous Province

Abstract

A suite of basaltic to doleritic dykes (named the Olympic Dam dolerite) has been intersected in drill holes at the Olympic Dam iron oxide Cu–U–Au–Ag deposit in the Gawler Craton, South Australia. The dykes intrude the ca. 1600Ma Roxby Downs Granite as well as the Olympic Dam Breccia Complex, and range from several centimetres to over 100m in width, strike NW and dip subvertically. Clinopyroxene and plagioclase are the major minerals, together with accessory apatite and Ti-magnetite. Magmatic apatite crystals in medium-grained doleritic dykes have been dated by LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry), and yielded a U–Pb age of 825±18Ma. The Olympic Dam dolerite has invariably been altered. Secondary apatite and titanite crystals from post-magmatic veinlets cutting the same dykes give similar results of 821±15Ma and 817±11Ma, respectively. All these results are, within error, coincident in age with the Gairdner Dyke Swarm, the longest dyke swarm (∼1000km in length) in Australia. The Olympic Dam dolerite shows similar compositional trends and variations to the Gairdner Dyke Swarm, especially in terms of the relatively immobile High Field Strength Elements (HFSE) and Rare Earth Element (REE). We conclude that the Olympic Dam dolerite belongs to the Gairdner Dyke Swarm, expanding its known extent and compositional range. The Gairdner Dyke Swarm is a major component of the ca. 820Ma Gairdner Large Igneous Province (LIP) generated during the break-up of the supercontinent Rodinia. The ca. 1070Ma Warakurna and the ca. 820Ma Gairdner LIP in Australia are compositionally distinct, whereas the Gairdner LIP and LIP and mafic suites associated with the break-up of Rodinia in South China and North America are broadly similar. This provides geochemical support for a Gairdner LIP origin for some mafic suites, in particular the Beda Volcanics in South Australia, and for a juxtaposition of South China, South Australia and North America in the tectonic reconstruction of Rodinia at ca. 820Ma.