Remnant lenses of komatiitic dykes in Kambalda (Western Australia): Occurrences, textural variations, emplacement model, and implications for other komatiite provinces

Abstract Remnants of komatiitic dykes have been intersected for the first time at Kambalda (Western Australia), a classical locality for komatiite-hosted Ni deposits. The dykes consist of metamorphosed lenses, aligned on three planar and parallel surfaces. They are composed of brecciated komatiitic rocks, differentiated lenses with spinifex- and hopper-textured olivine and clinopyroxene cumulates (all now composed of variable amounts of tremolite, chlorite and biotite), and chloritite. Igneous textures (spinifex and cumulate, skeletal chromite, amygdales) are well preserved and their combination with a low carbon content (carbonation is common in more altered komatiites) implies an isochemical metamorphic alteration. The composition of the komatiite dyke near the contact is similar to the Kambalda Komatiite (but lower in MgO) and changes to a clinopyroxene composition in the dyke centre. This is interpreted to be a clinopyroxene cumulate crystallized from a komatiitic basalt. Furthermore, trace elements in whole rock samples from the centre of the dyke record contamination by felsic crust. This means one drill hole intersecting the dyke has a record of contaminated to uncontaminated mantle melts within a few centimetres. The rare occurrence of komatiite dykes compared to sills in all komatiite occurrences worldwide is probably a consequence of three factors – the komatiite melt erupting above its liquidus, the melt's very low viscosity and its laminar flow. In areas of obstacles (e.g., change in lithology), local eddies can cause a local temperature decrease which leads to crystallization of the liquidus phase. A decrease in temperature might be accelerated by locally induced convection cells of pore fluid in the host rock. In the laminar flowing part of the dykes, however, the overall liquidus of the melt is only reached when the eruption ceases. Due to decreasing melt pressure, the fracture closes to adjust to the changing pressure, squeezing the melt out before the liquidus is reached. Such dykes leave no trace of their former existence. These areas of changes in flow direction therefore form chains of komatiite lenses.