Abstract

The Kambalda nickel sulphide deposits are located in the 2700 Ma Eastern Goldfields Greenstone Belt, Yilgarn Craton, Western Australia. The main lithologies are basalts and komatiites. Nickel sulphide ore occurs within the komatiite at the contact with the underlying basalt. This contact is structurally complex, the result of several phases of compressive deformation that post-date ore formation. Ore is confined to trough structures at the basalt-komatiite contact, these may be tens to hundreds of metres wide, several kilometres in length and tens of metres deep. Troughs are subparallel to each other and to the regional structural trend, ore bodies have a characteristic ribbon-like geometry. Various models have been proposed to explain trough formation but none account for the variety of features seen in the area. A new model is presented explaining the Kambalda structures by analogy with present-day graben structures associated with Icelandic-style fissure swarms. It is proposed that the initial geometries of the Kambalda trough structures, prior to compressive deformation, were similar to the present-day structures seen in Iceland. Komatiitic lavas subsequently exploit this graben topography, resulting in distinctive orebody geometries. Thus, it is suggested that the Kambalda troughs reflect an early phase of extension in the region.