Structural, mineralogical and geochemical constraints on the atypical komatiite-hosted Turret deposit in the Agnew–Mt. White district, Western Australia

Abstract

In the Norseman–Wiluna belt, Yilgarn Craton, the Agnew–Mt. White district is the host of many gold deposits. Located in the hinge of the regional Lawlers anticline, the Turret gold deposit is structurally controlled by the Table Hill shear zone that transects the Agnew Ultramafic unit. Geochemistry, coupled with petrographic data, allowed the delineation of the paragenetic sequence associated with gold mineralisation and include (1) a pervasive talc-carbonate alteration assemblage, (2) a pre-mineralisation stage associated with pervasive arsenopyrite + chalcopyrite + pyrrhotite + pyrite alteration, followed by (3) a late deformation event along a dilatational segment of the main Table Hill shear zone, leading to the formation of a breccia hosting a Cu–Bi–Mo–Au (± Ag ± Zn ± Te ± W) metal assemblage. The presence of Au–Ag–Cu alloys, native bismuth, chalcopyrite and other Bi–Te–S phases in the mineralisation stage suggest that gold may have been scavenged from the hydrothermal fluids by composite Bi–Te–Cu–Au–Ag–S liquids or melts. Using this mineral paragenetic sequence, together with mineralogical re-equilibration textures observed, we show that the gold deposition at Turret occurred over a temperature range approximately between c. 350 and 270 °C. This temperature range, together with the structural control and typical mesothermal alteration pattern including carbonate–chlorite alteration, shows that the Turret deposit shares common characteristics with the orogenic gold deposit class. However, the metal association of Cu, Au, Bi, and Mo, the quartz-poor, and high copper-sulphide content (up to 15 %) are characteristics that depart from the typical orogenic gold deposit mineralogy. Through comparison with similar deposits in the Yilgarn Craton and worldwide, we propose that the Turret deposit represents an example of a porphyry-derived Au–Cu–Bi–Mo deposit.