GEOLOGY OF THE ALKALINE GIFFORD CREEK COMPLEX, GASCOYNE COMPLEX, WESTERN-AUSTRALIA

The Mesoproterozoic Gifford Creek Complex is a recently recognised sui te of high-level alkaline intrusions located in the Gascoyne Complex o f Western Australia. Two alkaline magmatic episodes are recognised: a swarm of ultrabasic intrusions accompanied by an extensive zone of met asomatism located along the Lyons River and emplaced prior to the depo sition of the Bangemall Group sediments at 1679 +/- 6 Ma; and a younge r phase of high-level dykes and sills of carbonatitic affinity that in trude Bangemall Group sedimentary rocks. Ultrabasic intrusions along t he Lyons River occur in two forms: as a swarm of metasomatised sills e xhibiting preserved primary igneous textures, and as intensely deforme d and remobilised lenses. Interpretation of the pseudomorphed igneous textures in the Lyons River sills suggest that the original rock was c omposed of olivine macrocrysts or phenocrysts, with a groundmass compr ising mica, perovskite, titanomagnetite and carbonate. Textures, such as centimetre-scale layering and gravity settling of phenocrysts are s imilar to those described from the Benfontein carbonate-rich kimberlit e sills in South Africa. The carbonatitic suite consists of ferrocarbo natite-magnetite-rare-earth element enriched veins (Yangibana 'ironsto nes'), ne-phlogopite-apatite-magnetite-pyrochlore-bearing sills (Spide r Hill sills), irregularly shaped, weathered intrusions of unknown ori gin within basal layers of Bangemall Group sedimentary rocks (Bald Hil l intrusions) and fluidisation breccias (not commonly observed due to susceptibility to weathering). The nature of the primary igneous rocks is not certain due to intense metasomatism and/or extreme weathering. However carbonatitic affinity is indicated by the ferrocarbonatite dy kes, and the presence of pyrochlore in the Spider Hill sills and gorce ixite in the Bald Hill intrusions. It is suggested that the Lyons Rive r ultrabasic sills were emplaced due to reactivation of a deep-seated structural zone during a tectonothermal event related to the initial r ifting of the Bangemall Basin. The later carbonatitic intrusions cut t he Bangemall sedimentary rocks and may reflect continued, or renewed, extension within the basin.