Structural controls of superposed komatiite-hosted Ni and orogenic Au mineralisation at Beta-Hunt, Western Australia

Abstract

The Beta-Hunt Au-Ni deposit hosts coinciding orogenic Au and komatiite-hosted Ni-Cu-(PGE) mineralisation. The close spatial relationship between the two diachronous mineralisation styles suggests that long-lived local structural architecture played a role in the superposition of the two deposit types. Deposit-scale field geological and structural analysis was conducted to define the deformation paragenesis and structural framework associated with Ni-Cu-(PGE) and Au mineralisation. Structural reconstruction suggests that komatiite-hosted Ni-Cu-(PGE) sulfides were preferentially deposited within a pre- or syn-magmatic SW-dipping growth fault array during early extension (D BH1 ), associated with a ~ 2700 Ma regional SW-NE directed rifting event. During subsequent ~ 2690–2630 Ma rift inversion (D BH2 – D BH3 ), the early growth faults were reactivated by fluid-mediated deformation and were associated with brittle-ductile shearing and Au mineralisation. Early D BH2a horizontal shortening and ductile reactivation of D BH1 growth faults resulted in the formation of shear zones hosting low-grade Au mineralisation. This was followed by a rotation of the local stress field to vertical shortening during D BH2b , synchronous with mean stress reduction and increased fluid pressure associated with an influx of hydrothermal fluids. The associated brittle reactivation of the shear zones caused the development of a D BH2b extension and hydraulic breccia vein network, which promoted advective open fracture fluid flow and the formation of high-grade to locally ultra-high-grade Au mineralisation. Structural inheritance and reactivation during subsequent deformation events were, therefore, key to the superposition of komatiite-hosted Ni-Cu-(PGE) and orogenic Au mineralisation at Beta-Hunt.

Bibliographic Information