Abstract

The Yakabindie area hosts significant deposits of disseminated Fe-Ni-Cu sulfide in the Six Mile Well (~2.0 Mt contained Ni) and Goliath North (~0.6 Mt contained Ni) dunite lenses. These dunite lenses form part of the same komatiitic ultramafic unit that hosts the giant Mount Keith MKD5 deposit (>2.6 Mt contained Ni) to the north, but the morphology and internal stratigraphy of the host unit differs significantly. Three-dimensional modeling of the Yakabindie region reveals that each dunite lens is located at the intersection between steeply dipping NNW- and NNE-trending synvolcanic faults, which appear to have acted as conduits from which komatiitic lava vented onto the seafloor. The igneous contacts dip more shallowly to the west and describe a W-facing wedge of olivine-cumulate rock, probably less than 1 km in plunge extent and cored by adcumulate rock. The mineralization at both Six Mile Well and Goliath North consists of multiple stacked lenses of disseminated Fe-Ni-Cu sulfide associated with the olivine adcumulate core. This core is interpreted as the pathway position that focused the flow of komatiite lava away from the vent. Rather than being linked by sheeted bodies of orthocumulate rocks, as is typically the case along strike to the north, the dunite lenses at Yakabindie appear to be capped by a strike-continuous sequence of thin komatiite flow units. There is no evidence of the fractionated sequences of pyroxenite and dolerite, which occur in pathway positions within the Mount Keith ultramafic unit to the north and which reflect late-stage ponding and in situ fractionation of komatiite lava.

Three sets of synvolcanic structures are recognized in the broader Agnew-Wiluna belt and a model of oblique extension at ca 2.7 Ga is proposed. A NNW-trending set of structures marks the overall rift orientation at ca 2.7 Ga but is probably inherited from an earlier ca 2.81 Ga phase of rifting. A NW-trending set is thought to represent ca 2.7 Ga transfer faults, but may also have much older roots. These structures produce marked sinistral offsets in both the greenstone stratigraphy and the gravity response and can be used to divide the Agnew-Wiluna belt into rift segments. The third set of early structures trends NNE, subparallel with the volcanic grain of the greenstones as inferred from the orientation of komatiite lava pathways. These are thought to have originated as extension-orthogonal normal faults at ca 2.7 Ga. At the regional scale there is a strong spatial association between the larger Fe-Ni-Cu sulfide deposits of the Agnew-Wiluna belt and the locus of postulated NW-trending transfer zones, whereas at the deposit scale, intersecting NW- and NNE-trending synvolcanic faults exert a control on the location and best development (in terms of thickness and/or grade) of Fe-Ni-Cu sulfide mineralization in some deposits. These spatial associations have major implications for exploration targeting at both regional and deposit scale.

You do not currently have access to this article.