Mineralogical and geochemical controls on the formation of the Woods Point Dike Swarm, Victoria, Australia: evidence from the Morning Star Dike and implications for sourcing of Au within orogenic gold systems

The Woods Point dike swarm of eastern Victoria, Australia, has produced over 133 tonnes (t) of Au. Here, as in a number of other orogenic Au provinces, a close spatial relationship is observed between orogenic Au mineralization and intrusive rocks; this study focuses on the well-endowed Morning Star dike, a gabbro to gabbro-diorite dike that has produced similar to 28 t of hydrothermal quartz-carbonate-pyrite vein-hosted Au and a further 24 t of placer gold derived from primary rocks in the area immediately around Morning Star. The fractionated magma that formed the dikes within the Woods Point dike swarm was sourced from a staging magma chamber at a depth of 16 to 23 km; here, mantle-derived magmas were driven to S saturation by assimilation of crustal material, leading to the formation of Cu-Ni-platinum group element (PGE)-Au-bearing sulfides, most of which segregated and settled toward the bottom of the staging chamber. Emplacement of the dike initiated with tapping of the staging chamber and release of the magmas within the chamber, thrilling the Woods Point dike swarm; flow differentiation and the formation of dike bulges enabled the segregation of sulfide-bearing mafic units and intermediate sulfide-barren units within these dikes.