Structural controls on late Cambrian mineralisation in the Stavely Arc

The middle-late Cambrian Stavely Arc forms the eastern boundary of the Delamerian Orogen in Victoria, Australia. A pulse of mineralised magmatism at ca. 500 Ma coincided with the collision of VanDieland and related deformation. Porphyry intrusion during this event has previously been inferred, from tectonic reconstructions, to coincide with a change from D1a transpression to D1b transtension, with intrusions having exploited transtensional structures. The outcropping area of the Stavely Arc is a small proportion of the total terrane, and informative outcrops and data are sparse, providing limited supporting evidence to date. This work presents new insights into the structural geology of the Stavely Arc during porphyry emplacement. A bottom-up approach is informed by structural observations and exploration activity from the Thursday's Gossan-Cayley Lode Project of Stavely Minerals Limited (ASX:SVY). This data-orientated approach utilises measured structures and assay results from exploration drilling, as opposed to a top-down approach interpreted from the regional geodynamics. An evaluation of mineralised versus non-mineralised structures was used as a proxy for active versus inactive structures, where active structures represent pathways for migration and deposition of mineralising fluids exsolved from an intrusive porphyry system at depth. Faults and shear zones variably host elevated Cu-Au-AsAgNiCo dependant on orientation during the porphyry emplacement event; veins do not appear to host substantial metal anomalism. By comparing the orientations of active versus inactive structures, an overall NW-SE directed axis for maximum compressive stress is interpreted at the time of mineralisation. The results of this work imply that porphyry intrusion and mineralisation occurred within a compressive D1a stress regime, during VanDieland collision, and prior to D1b extensional tectonism. The collision event and associated disruption to subduction zone processes are therefore implicated in triggering porphyry emplacement. These findings will be important for understanding the prospectivity of the Stavely Arc and targeting future exploration activity.