Hydrothermal fluid sources in the development of shallow potassic-sodic alteration at the Cadia East alkalic porphyry Au-Cu deposit, NSW

The Cadia East deposit occupies a mineralized zone 2 km in strike length, 600m in width and >1500m in vertical extent. Resource estimates indicate that >28 million ounces of gold and >5.5 million tonnes of copper may be contained in the deposit. Alkalic monzonite and monzodiorite dikes intrude a 2000m thick sequence of submarine basalt and basaltic-andesite volcanic and volcaniclastic rocks that are the principal host to mineralisation. Sheeted quartz-sulphide veins are spatially and temporally associated with these dikes, defining a high grade resource at depth. High-level, broadly stratabound, disseminated Cu–Au mineralisation is overprinted by a pervasive quartz–orthoclase–albite– muscovite–calcite alteration assemblage (upper potassicsodic) which may be >200m thick and extends laterally over the majority of the deposit. Stable isotope (O, D and S) data suggests that magmatic hydrothermal fluids were implicit in the development of both the upper potassicsodic alteration zone, and the deeper vein related alteration-mineralisation. There is limited evidence for the incursion of external waters into the hydrothermal system. The contrasting mineralisation and alteration styles imply that fracture permeability controlled the movement of mineralising fluids from source intrusions at depth whereas the development of the broadly stratabound mineralisation and subsequent alteration in the upper parts of the deposit was strongly influenced by host rock permeability.