Geology of the alkaline Gifford Creek Complex, Gascoyne Complex, Western Australia

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Abstract
The Mesoproterozoic Gifford Creek Complex is a recently recognised suite of high‐level alkaline intrusions located in the Gascoyne Complex of Western Australia. Two alkaline magmatic episodes are recognised: a swarm of ultrabasic intrusions accompanied by an extensive zone of metasomatism located along the Lyons River and emplaced prior to the deposition of the Bangemall Group sediments at 1679 ± 6 Ma; and a younger phase of high‐level dykes and sills of carbonatitic affinity that intrude Bangemall Group sedimentary rocks. Ultrabasic intrusions along the Lyons River occur in two forms: as a swarm of metasomatised sills exhibiting preserved primary igneous textures, and as intensely deformed and remobilised lenses. Interpretation of the pseudomorphed igneous textures in the Lyons River sills suggest that the original rock was composed of olivine macrocrysts or phenocrysts, with a groundmass comprising mica, perovskite, titanomagnetite and carbonate. Textures, such as centimetre‐scale layering and gravity settling of phenocrysts are similar to those described from the Benfontein carbonate‐rich kimberlite sills in South Africa. The carbonatitic suite consists of ferrocarbonatite‐magnetite‐rare‐earth element enriched veins (Yangibana ‘ironstones'), aegirine‐phlogopite‐apatite‐magnetite‐pyrochlore‐bearing sills (Spider Hill sills), irregularly shaped, weathered intrusions of unknown origin within basal layers of Bangemall Group sedimentary rocks (Bald Hill 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 dykes, and the presence of pyrochlore in the Spider Hill sills and gorceixite in the Bald Hill intrusions. It is suggested that the Lyons River ultrabasic sills were emplaced due to reactivation of a deep‐seated structural zone during a tectonothermal event related to the initial rifting of the Bangemall Basin. The later carbonatitic intrusions cut the Bangemall sedimentary rocks and may reflect continued, or renewed, extension within the basin.