Influence of volcano-sedimentary facies architecture on strain partitioning during the evolution of an orogenic-gold lode system, Stawell, western Victoria

Abstract

The localised distribution of gold in a series of ore shoots adjacent to the western flank of the Cambrian Magdala Basalt is controlled by formation and partial reactivation of faults and shear zones that closely follow the geometry of the basalt. The recognition of the controls on the geometry of the basalt and overlying metasedimentary units is therefore vital for exploration in the region. Detailed structural mapping around one of the steep, west-dipping basalt sheets that comprise the Magdala Basalt, as well as the overlying metasedimentary units, has been used to compare the recorded structural history in the basalt relative to that in the metasediments to determine how strain has partitioned during the evolution of the deposit, and if the geometry is a product of primary volcanic processes or early ductile deformation. The results of this work show that much of the deformation history at the Magdala mine is recorded within the metasediments, while the basalt itself appears only affected by the late brittle deformation. The lack of evidence for early ductile deformation in the basalt and the partitioning of strain into the Albion Formation early in the deformation process suggests that the variable geometry of the basalt is a product of primary volcanic processes, rather than ductile deformation. The irregular geometry of basalt flows along the southwestern flank of the Magdala Basalt may be related to the dominance of distal volcanic facies, primarily pillow basalt, the variability of which influences localised deformation and gold mineralisation during an episode of low displacement east – west compression.