Volcanology of the Archaean Lunnon Basalt and its relevance to nickel sulfide‐bearing trough structures at Kambalda, Western Australia

Abstract

The Lunnon Basalt is an Archaean, subaqueous, tholeiitic metabasalt succession, with a minimum inferred thickness of 1750 m. It forms the oldest exposed stratigraphic unit at Kambalda in the nickel sulfide‐rich Norseman‐Wiluna Greenstone Belt, Western Australia, and is dominated by variable proportions of massive basalt, pillow basalt, and basalt breccia. These facies form intimately inter‐layered 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 member and an ‘upper’ less MgO‐rich member, these being separated by a sedimentary horizon. Of the possible palaeovolcanic and tectonic settings (layer 2 of oceanic crust, a large shield volcano, or a tectonically‐topographically ponded sea‐floor lava field), the Lunnon Basalt appears to be the remains of a ponded (?rift) lava succession. The presence of inherited zircon xenocrysts indicates that the subjacent crust at the time of formation was sialic, Archaean crust. Komatiite‐associated 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: synvolcanic faulting; original volcanic topography; thermal erosion; and structural. Most depressions have faulted margins but there is very little field evidence to support synvolcanic faulting. Furthermore, physical evidence for thermal erosion is non‐existent in the exposures seen. Palaeoflow indicators from the pillow basalts and basalt breccias indicate flow approximately parallel to the major depressions, suggesting a case for volcanic topography. Since significant relief occurs on the surfaces of all modern lavas, it is therefore clear that the surface of the Lunnon Basalt must have had depressions which may have controlled the behaviour of the succeeding komatiites, and therefore perhaps the location of the nickel sulfide mineralisation. However, the depressions confining the nickel sulfide mineralisation at Kambalda are extremely complex and variable in nature, and their present form is most likely a result of several processes.