The geology of the Kambalda nickel field, Western Australia

Abstract

The Kambalda nickel field contains the major concentration of volcanic peridotitc-associated nickel ores in the Archean of Australia. It occupies an area of 400 km 2 underlain by two sequences of ultramafic, mafic, and felsic volcanics, and sedimentary rocks, which can be broadly correlated with previously erected regional stratigraphic sections. Most ores occur in the lower (Kambalda) sequence at the base of the Kambalda ultramafic rocks; the ultramafic rocks of the upper (Bluebush) sequence contain only minor mineralization. Four phases of deformation occurred over an interval of more than 300 m.y., resulting in a dominant north-northwest structural trend. Faulting was active during original volcanism and continued intermittently until after mafic-felsic dike intrusion. The ultramafic rocks have been serpentinized, and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism.The Kambalda ultramafic rocks are of variable thickness and in places exceed 1,000 m. The lower member of the ultramafic rocks, occupying approximately one-third of the sequence, comprises thicker high magnesium flows (i.e., generally > 36% volatile-free MgO) commonly separated by sulfidic quartz-albite sediments. Thinner, lower magnesium flow units characterize much of the upper member. The ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few, grouped, ribbonlike original orebodies. Eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt-ultramafic contact; these troughs are thought to be original volcanic depressions which focused later deformation. The remainder is hanging-wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow. Hanging-wall orebodies occasionally grade into stratigraphically equivalent interflow sediments.The ores contain varying proportions of massive, matrix, and disseminated sulfides. The hypogene mineral assemblage is pyrrhotite-pentlandite-pyrite-(chalcopyrite), but millerite is locally significant. The massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position. Metamorphism has caused recrystallization of the ores and some redistribution of elements. There is commonly marked variation of mineralogy and S/Ni ratios between contact and overlying hanging-wall ores, and within and between original ore surfaces within shoots, but the relative contributions of primary composition and metamorphism to this variation are uncertain.In ore environments, the footwall basalt-ultramafic contact is more deformed than in non-ore environments, the basal ultramafic flows are thicker and richer in magnesium, the trend toward lower magnesium flows upward through the ultramafic formation is less well developed, and there is better textural development and compositional differentiation within flows. Non-ore environments usually contain interflow sediments, whereas there is generally a sediment-free prism of ultramafic rocks above ore environments.Most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor. Volcanic exhalations probably contributed to minor ore in sulfidic sediments.