Structural controls on the formation of the Cadia East porphyry Au-Cu deposit, NSW, Australia

Cadia East is hosted by the Forest Reefs Volcanics, a subaqueous volcano-sedimentary succession that was deposited in an east-trending sedimentary basin. Two stratigraphic marker beds in the upper part of the stratigraphy are systematically offset by north- and south-dipping normal faults that bound a local sub-basin. Stratigraphic thickening of these marker beds into the hanging wall of the sub-basin indicate that these faults were active at the time of sedimentation. Reactivation of the sub-basin bounding faults in the late Ordovician allowed the intrusion of andesitic dykes and sills that represent the final stage in the deposition of the Forest Reefs Volcanics. These faults were reactivated again in the earliest Silurian and facilitated the emplacement of a series of steeply dipping narrow monzonite and quartz-monzonite dykes oriented sub-parallel to the east-trending volcano-sedimentary sub-basin. Hydrothermal fluids expelled from these dykes fractured the host rocks at depth (1500 to 500 m below present surface) forming a sheeted array of quartz-sulphide-calcite veins that define a high-grade ore zone. These veins trend east- to southeast and dip steeply north and south. A 100 to 200m thick zone of disseminated low-grade Au-Cu mineralisation occurs above this sheeted vein set, restricted to volcanic breccias and bedded units. The two contrasting mineralisation styles define an elongated mineralised zone 2.5 km in length, ~0.8 km wide and extending vertically for over 1.5 km. This ore zone geometry is atypical for porphyry deposits and reflects the influence of the stratigraphic and structural architecture of the Cadia East sub-basin on mineralisation.