Extrusive Origin and Structural Modification of the Komatiitic Mount Keith Ultramafic Unit

Abstract

Mount Keith is a large (up to 500 m thick, and >2 km long) serpentinized and talc-carbonate-altered dunite body. The geologic features of Mount Keith are similar to many of the ultramafic bodies in the Agnew-Wiluna belt and other greenstone belts within the eastern Yilgarn craton. Mount Keith hosts a large disseminated nickel sulfide orebody that is presently exploited by open pit. There are currently two hypotheses that explain the site of formation of these dunite bodies: either kilometer-scale komatiite lava channels, or subvolcanic sills intruded into the greenstone sequence are responsible. The intrusive model is essentially based on two pieces of geologic evidence: the presence of a single 20-m-scale and single 20-cm-scale ultramafic apophysis in overlying dacitic rocks; and the presence of dacite inclusions within the contact zone of the upper part of the Mount Keith ultramafic unit. New geologic data show that the western part of the Mount Keith ultramafic unit and the overlying dacite sequence are allochthonous and that the contact zone between the two comprises multiple anastomosing layer-parallel fault slices of both lithologies at a scale of meters to tens of meters. Within some of the fault blocks, primary igneous contacts are present between the top of the Mount Keith ultramafic unit and the overlying dacite. New evidence shows that both pyroxene and minor olivine spinifex-textured rocks are present along much of the upper preserved part of the Mount Keith ultramafic unit and pseudomorphs after pyroxene grains exist on a scale of 20 to 200 μ m that formed during emplacement and survived the cooling of the 500-m-thick Mount Keith ultramafic unit. Moreover, the evidence for a large-scale ultramafic intrusive apophysis is unsubstantiated, as this apophysis is shown to be a slice within a brittle/ductile fault. The reinterpretation of the small-scale apophysis is consistent with this being an ultramafic enclave within a younger dacite flow. Finally, the dacite exposed within centimeters to meters of the western ultramafic contact commonly retains delicate igneous textures and shows no evidence of thermal effects from the Mount Keith ultramafic unit. Modeling of the heat budget assuming the dimensions of the Mount Keith ultramafic unit shows that an intrusion of this size should have produced wholesale melting of roof rocks. Our findings indicate that the Western Dacite sequence was not present above the Mount Keith ultramafic unit during its emplacement and cooling. A model is favored for the extrusive origin of the Mount Keith ultramafic unit with construction of the olivine cumulate pile at the floor of an approximately 2-km-wide, intermittently sulfide-bearing, turbulent lava pathway. At the terminal stages of eruption magma flow decreased and finally ceased, allowing the lava to drain and cause the upper crust to collapse onto the top of the crystal pile, producing in places sharp juxtaposition of chilled margin rocks with coarse-grained olivine cumulates. In most places rapid flow and turbulent convection resulted in resorption of the crust, but local formation of stagnant ponds allowed survival of an olivine spinifex-textural profile. More extensive ponding of the magma allowed in situ fractionation and formation of gabbroic and pyroxenitic rocks derived from highly fractionated komatiite magma. The Mount Keith ultramafic unit is believed to be the product of extrusive magmatism from which an olivine cumulate pile developed by upward accretion at the floor of a major lava pathway; these rocks were then extensively modified by deformation.