Controls on mineralization and alteration at the giant Cadia East porphyry Au-Cu deposit, NSW, Australia

The Cadia East alkalic porphyry Au-Cu deposit, NSW has total resources exceeding 33 million ounces of gold making it the largest porphyry deposit in eastern Australia and one of the largest globally, in terms of contained gold. Cadia East is hosted by the Forest Reefs Volcanics, a subaqueous volcano-sedimentary succession that was deposited in an east-trending sedimentary basin during the Late Ordovician. Systematic fault offsets of two stratigraphic marker horizons in the upper part of the stratigraphy define bounding faults of a local sub-basin. These sub-basin bounding faults were reactivated in the latest Ordovician allowing emplacement of andesitic dykes and sills that represent the final stage in the deposition of the Forest Reefs Volcanics. Further reactivation of the sub-basin bounding faults in the earliest Silurian facilitated the emplacement of monzonite and quartz-monzonite dykes, which dip steeply to the north- and south- and are oriented sub-parallel to the east-trending volcano-sedimentary sub-basin. Gold-and copper mineralization occurred as 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-sulfide-calcite veins that define a high-grade gold ore zone. These veins trend east- to southeast and dip steeply north and south. A 100 to 200 m thick zone of disseminated low-grade Au-Cu mineralization occurs above this sheeted vein set, restricted to volcanic breccias and bedded units. The two contrasting mineralization styles define an elongated mineralized 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 mineralization. Early hydrothermal tourmaline at Cadia East is associated with pervasive calc-potassic alteration within the high-grade ore zone. Tourmaline also occurs in late-stage breccias and veins which are temporally associated with an extensive zone (>200 m thick, 1 km wide and several kilometers in length) of pervasive feldspar-rich alteration that caps the deposit. The total range in the boron isotope composition (δ11B) of tourmaline ranges between -5.2 and +7.7‰ and the calculated boron isotope composition of fluids in equilibrium with tourmaline range between -1.1 and +10.4‰. Although high calculated δ11B values of fluids are commonly attributed to seawater or basinal brine sources, a combination of oxygen, hydrogen and strontium isotope analysis of tourmaline indicate that external fluids, including Ordovician-Silurian seawater and meteoric water, were precluded from the hydrothermal system at Cadia East. Instead, the isotopic composition of tourmaline was controlled by hydrothermal fluids that were derived from alkalic intrusions that may have boiled locally. The range in the δ11B composition of tourmaline at Cadia East is comparable to that documented in porphyry Cu and iron-oxide-copper-gold systems in the central Andes and therefore, suggests a similar magmatic-hydrothermal origin.