The role of ground melting in the genesis of komatiite-associated nickel sulfide deposits at Kambalda and Widgiemooltha, Western Australia

[Truncated] Komatiite-associated nickel sulfide deposits are the world's premier source of sulfide nickel from Archaean greenstone belts. These deposits comprise two main associations in Western Australia: i) low-grade disseminated nickel sulfide mineralization in thick komatiitic-dunite bodies, and ii) high-grade disseminated/matrix/massive nickel sulfides at the base of multiple-flow komatiite sequences. Kambalda is the type-example of the latter association, and these deposits, together with similar deposits at Widgiemooltha, are best suited for study of the genesis of their nickel sulfide ores.
The timing of sulfur saturation which resulted in the segregation of sulfide-oxide rich melts, that subsequently settled and accumulated at the base of the mineralized komatiite flows forming the nickel sulfide ores, is one of the more controversial aspects of ore genesis at Kambalda. Further controversy arises from the difficulty indiscriminating between mantle and sedimentary sources of sulfur in the sulfide ores.
Previous studies at Kambalda have emphasized the volcanic features of these deposits. The mineralized komatiite sequence is overlain and underlain by pillow basalts, and sulfidic and carbonaceous sedimentary rocks occur at regular intervals in the volcanic succession, particularly in the lowermost komatiite member, indicating a deep marine volcanic setting. The komatiites display characteristic flow-top breccias, polygonal jointing, and a wide variety of spinifex, crescumulate, and equant cumulate textures, consistent with their emplacement as lava flows. The nickel sulfide mineralization is restricted to the lowermost komatiite flows, which are the thickest and most magnesian komatiites of the entire komatiite sequence. In addition, nickel ores are confined to discrete volcanic environments which are characterized by thick, highly magnesian, cumulate-dominant flows, partially confined to highly elongate depressions in the underlying basalt sequence. The laterally-contiguous, barren, flanking-ore environments comprise several thinner, less-magnesian komatiite flows overlying relatively planar basalt substrates. Ore environments are n o w generally accepted as lava channels in which there was substantial turbulent flow of highly magnesian komatiite partly localised in primary topographic depressions (lava-channel fades). The relationships between ore environments and the barren flanking-ore environments are complex and indicate time stratigraphic discontinuities in the lava pile. The flanking-ore flows probably accumulated in lava levee areas (sheet-flow fades) by the repeated spillage of komatiite from the channel during periodic lava emplacement.