Structural evolution and gold mineralization in the Scotchmans fault zone, Magdala gold mine, Stawell, western Victoria, Australia

Abstract

The mid-greenschist facies turbiditic and subaqueous volcaniclastic metasediments at Stawell, Western Victoria, are overprinted by two shear zone systems that host gold mineralization in the Magdala mine. The younger Scotchmans fault zone overprints the older Central lode system and both systems have a reverse sense of movement. There is a systematic reorientation of the Central lode system structures and foliation into parallelism with structures of the Scotchmans fault zone. The boundaries of the Scotchmans fault zone are defined by discrete master faults along which are emplaced gold-bearing laminated quartz veins. The hanging-wall and footwall master faults are linked by an irregular array of subsidiary faults which form duplex structures containing slickenlines, rotated shear zone foliations, and S-C fabrics. Fault breccias, developed on both master and subsidiary faults, are overprinted by a fault gouge implying multiple movements along these faults. In zones where the subsidiary faults intersect and link with the master faults, gold grades increase from an average of 4 to 5 g/t in the laminated quartz veins to greater than or equal to 10 g/t.Microfabrics in the laminated quartz veins indicate vein accretion from either wall or from the center and quartz fibers attesting to vein growth occurring as a series of incremental events rather than being a single event. The gold-bearing fluids overprint earlier laminations but are coeval with later vein accretion associated with sulfides and ankerite. The mineral assemblage of pyrrhotite-pyrite-chalcopyrite-galena is deformed together with the host graphite mica schists and hosts no gold at all. This assemblage is overprinted by the shear zone-forming events. Subsidiary faults exhibit fault jogs in which quartz, pyrite, arsenopyrite, and minor chalcopyrite have precipitated. The assemblage of pyrite-arsenopyrite-chal-copyrite, which is always associated with the gangue minerals of quartz, ankerite, and minor calcite, is present in the laminated quartz veins and hosts the gold mineralization. The gold occurs typically as inclusions or in fractures within pyrite and arsenopyrite. Retrograde minerals are present, associated with the formation of pressure solution structures such as stylo-lites and quartz fibers overprinting the gold-bearing ore assemblages. Tetrahedrite and enargite replace pyrite and arsenopyrite, and some new pyrite has recrystallized from older grains. In the retrograde assemblages no gold is enclosed within individual minerals but it occurs on grain boundaries. This low-temperature mineral overprint is due to diffusive mass transfer that has remobilized the gold that was locked in pyrite and arsenopyrite as inclusions. Extensive dissolution of pyrite and arsenopyrite has resulted in gold redeposition into fractures, cracks, grain boundaries, and stylolites.