STRUCTURAL EVOLUTION AND GOLD MINERALIZATION IN THE SCOTCHMANS FAULT ZONE, MAGDALA GOLD MINE, STAWELL, WESTERN VICTORIA, AUSTRALIA

The mid-greenschist facies turbiditic and subaqueous volcaniclastic me tasediments at Stawell, Western Victoria, are overprinted by two shear zone systems that host gold mineralization in the Magdala mine. The y ounger Scotchmans fault zone overprints the older Central lode system and both systems have a reverse sense of movement. There is a systemat ic reorientation of the Central lode system structures and foliation i nto parallelism with structures of the Scotchmans fault zone. The boun daries of the Scotchmans fault zone are defined by discrete master fau lts along which are emplaced gold-bearing laminated quartz veins. The hanging-wall and footwall master faults are linked by an irregular arr ay of subsidiary faults which form duplex structures containing slicke nlines, 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 q uartz veins to greater than or equal to 10 g/t. Microfabrics in the la minated quartz veins indicate vein accretion from either wall or from the center and quartz fibers attesting to vein growth occurring as a s eries of incremental events rather than being a single event. The gold -bearing fluids overprint earlier laminations but are coeval with late r vein accretion associated with sulfides and ankerite. The mineral as semblage of pyrrhotite-pyrite-chalcopyrite-galena is deformed together with the host graphite mica schists and hosts no gold at all. This as semblage is overprinted by the shear zone-forming events. Subsidiary f aults exhibit fault jogs in which quartz, pyrite, arsenopyrite, and mi nor chalcopyrite have precipitated. The assemblage of pyrite-arsenopyr ite-chalcopyrite, which is always associated with the gangue minerals of quartz, ankerite, and minor calcite, is present in the laminated qu artz veins and hosts the gold mineralization. The gold occurs typicall y as inclusions or in fractures within pyrite and arsenopyrite. Retrog rade minerals are present, associated with the formation of pressure s olution structures such as stylolites and quartz fibers overprinting t he gold-bearing ore assemblages. Tetrahedrite and enargite replace pyr ite and arsenopyrite, and some new pyrite has recrystallized from olde r grains. In the retrograde assemblages no gold is enclosed within ind ividual minerals but it occurs on grain boundaries. This low-temperatu re mineral overprint is due to diffusive mass transfer that has remobi lized the gold that was locked in pyrite and arsenopyrite as inclusion s. Extensive dissolution of pyrite and arsenopyrite has resulted in go ld redeposition into fractures, cracks, grain boundaries, and stylolit es.