VOLCANOLOGY OF THE ARCHEAN LUNNON BASALT AND ITS RELEVANCE TO NICKEL SULFIDE-BEARING TROUGH STRUCTURES AT KAMBALDA, WESTERN-AUSTRALIA

The Lunnon Basalt is an Archaean, subaqueous, tholeiitic metabasalt su ccession, with a minimum inferred thickness of 1750 m. It forms the ol dest exposed stratigraphic unit at Kambaida in the nickel sulfide-rich Norseman-Wiluna Greenstone Belt, Western Australia, and is dominated by variable proportions of massive basalt, pillow basalt, and basalt b reccia. These facies form intimately interlayered massive, pillow and complex lava flow units, with an average thickness of about 20 m. The stratigraphy of the basalt can be subdivided into a `lower' MgO-rich m ember and an 'upper' less MgO-rich member, these being separated by a sedimentary horizon. Of the possible palaeovolcanic and tectonic setti ngs (layer 2 of oceanic crust, a large shield volcano, or a tectonical ly-topographically ponded sea-floor lava field), the Lunnon Basalt app ears to be the remains of a ponded (?rift) lava succession, The presen ce of inherited zircon xenocrysts indicates that the subjacent crust a t the time of formation was sialic, Archaean crust. Komatiite-associat ed nickel sulfide is normally localised in or over depressions in the upper surface of the Lunnon Basalt. The origin of these depressions or `troughs' is uncertain. The four most-favoured models are: synvolcani c faulting: original volcanic topography; thermal erosion: and structu ral, Most depressions have faulted margins but there is very little fi eld evidence to support synvolcanic faulting. Furthermore, physical ev idence for thermal erosion is non-existent in the exposures seen. Pala eoflow indicators from the pillow basalts and basalt breccias indicate flow approximately parallel to the major depressions, suggesting a ca se for volcanic topography, Since significant relief occurs on the sur faces of all modern lavas, if is therefore clear that the surface of t he Lunnon Basalt must have had depressions which may have controlled t he behaviour of the succeeding komatiites, and therefore perhaps the l ocation of the nickel sulfide mineralisation, However, the depressions confining the nickel sulfide mineralisation at Kambaida are extremely complex and variable in nature, and their present form is most likely a result of several processes.