A revised stratigraphic model for the 2.7 Ga Agnew-Wiluna greenstone belt, Yilgarn Craton, Western Australia

The lower stratigraphy of Agnew-Wiluna greenstone belt is composed of two main elements; a mafic/komatiite domain and a felsic/komatiite/basalt domain. Previous stratigraphic models show the mafic domain overlying the felsic domain. Komatiites in the latter host the vast majority of the nickel sulphide endowment of the belt (>20 significant deposits) whereas those in the mafic domain contain 3 and 4 relatively small deposits. Recently published geochemical data from well-preserved mafic domain rocks exposed in the Agnew area opens up the possibility to match these units with mafic rocks within the more structurally disrupted felsic domain. Analytical data from basalts at the Cliffs and Mount Keith Ni deposits and from the Wiluna Au mine sequence show that these can be matched to the basalt sequence stratigraphically below the Agnew Komatiite at Agnew and also show that basalts previously thought to occupy different stratigraphic positions (Centenary Bore and MacFarlanes Basalts) are laterally equivalent but structurally displaced. The revised stratigraphic model together with available age dates show that komatiites in both domains, Mount Keith and Cliffs/Agnew Komatiites, are laterally equivalent and part of the 2705 Ma Kalgoorlie-Kurnalpi komatiite LIP. This greatly enhances the Ni prospectivity of komatiites within the mafic domain which, previously being thought younger, were historically considered less prospective. The footwall to the komatiite is composed of basalt (Never Can Tell Basalt, in the mafic domain) and felsic sequences (Mount Keith Dacite in the felsic domain) that are laterally separated but occupy the same stratigraphic position and together with the komatiite correlate with the Kambalda Sequence in the south of the Kalgoorlie Terrane. The oldest crystallisation ages from the Mount Keith Dacite are 2719–2725 Ma but whether these rocks belong to the Kalgoorlie or Youanmi Terrane is currently unknown. The Kalgoorlie-aged sequence has an unconformable contact with underlying Youanmi-aged sequence (the latter including dates of 2724–2729, 2734, 2749 Ma) composed of basalt, komatiitic basalt, komatiite and minor felsic volcanic (in decreasing stratigraphic order; felsic volcanics, Songvang Basalt, Hickies Bore Basalt, Donegal Komatiite, Butchers Well Basalt). The Youanmi sequence is exposed throughout the AWB, is present in the Leonora area to the immediate south and extends eastward to other areas within the northern part of the Kalgoorlie-Kurnalpi Terranes.