Thermomechanical erosion of ore-hosting embayments beneath komatiite lava channels: Textural evidence from Kambalda, Western Australia

Abstract

Archean komatiite-hosted nickel deposits at Kambalda, Western Australia, are located in embayments within channelized lava flows. The origin of these embayments is controversial. The currently favoured model invokes pre-existing topographic features filled in by the komatiite and sulfide melt and subsequently modified by thermomechanical erosion. New evidence from sulfide deposits on the eastern limb of the Kambalda Dome includes well preserved magmatic textural relationships of the sulfides with the surrounding rocks suggesting the entire embayment was formed by thermomechanical erosion. These textures are particularly well preserved within the Moran deposit of the Long-Victor Complex which lacks the younger intense structural deformation that other Kambalda deposits have undergone. The vestiges of an older, broad concave embayment are represented by sediment-sulfide textures present on the flanks up to 150m away from the younger steep-sided embayment. Sediment-sulfide contacts are marked here by a globular silicate-sulfide intergrowth, interpreted as emulsion, and cumulate-like mono-sulfide solid-solution (MSS) grains surrounded by interstitial silicate melt. Textures preserved from the formation of a steep-sided embayment are frozen into the basalt-sulfide contact and include an undulating contact with sulfide-filled microfractures and a ferrichromite layer, basalt-sulfide breccia and interpreted crystallized silicate-sulfide liquid emulsions and basalt plumes rising into the sulfides. Textures from the last step of embayment formation are found in the pinchouts, i.e. where the massive sulfide ore is bounded top and bottom by basalt. They include the same textures that were formed during the excavation of the steep-sided embayment but they also feature interpreted basalt-sulfide emulsions on the upper contact, cm-scale silicate-sulfide-ferrichromite layering on the upper contact, and a vesicular basalt “scum layer” floating on the sulfides beneath the solid older basalt.All these textures represent frozen examples of different stages during the excavation and formation of the embayment, and reflect physical processes responsible for thermomechanical erosion. The initial disposition and orientation of ore-hosting channels was controlled by paleo-topography, probably defined by syn-volcanic faults, but the textures we describe show that within the channel the geometry of the entire embayment was the product of erosion. The sulfide melt itself played a major role in the process owing to its high density, high heat content and very low viscosity, but the hydrated nature of the underlying altered basalt was also critical. Fluids were released from the basalt due to the high temperature in the 1–2cm immediately beneath the sulfide melt with instant flash boiling creating a network of microfractures. Pre-existing fracture networks and breccia in the basalt allowed the sulfide melt to locally infiltrate the substrate to a greater depth increasing the hydraulic head at the tip of the fracture, thus accelerating the erosion process. Once a critical sulfide melt thickness is reached, partly aided by a growing pile of matrix (net-textured) sulfides, the pressure difference on the embayment floor compared to the basalt at the bottom edges of the embayment becomes high enough to allow the liquid sulfides to melt sideways to form a pinchout.