Porphyry gold–copper mineralisation in the Cadia district, eastern Lachlan Fold Belt, New South Wales, and its relationship to shoshonitic magmatism

Abstract.

The Cadia porphyry gold–copper district is the largest hydrothermal, intrusion-related gold deposit in eastern Australia. Discovered in 1992 by Newcrest Mining Limited, pre-mine resources in the district were in excess of 585 t Au and 2.35 Mt Cu. The Cadia district lies within shoshonitic subaqueous volcanic rocks of the Late Ordovician Molong Volcanic Belt in the eastern Lachlan Fold Belt (LFB) of New South Wales. An island arc tectonic setting is envisaged for the formation of these volcanic rocks, but the nature of the arc basement and geometry of the subduction zone(s) is debated. Mineralisation occurs in four principal porphyry deposits (Cadia Hill, Cadia Ridgeway, Cadia East/Cadia Far East and Cadia Quarry) as sheeted and stockwork quartz–sulfide veins, and locally as broadly stratabound disseminated mineralisation (Cadia East) and skarn (Big Cadia and Little Cadia). All of the porphyry deposits show a close spatial association with shoshonitic monzodiorite to quartz monzonite dykes and stocks of the Cadia Intrusive Complex (CIC). Gold–copper mineralisation is hosted by these intrusions and also by the enclosing Forest Reefs Volcanics (FRV) wall rocks. Hydrothermal alteration associated with mineralisation is potassic, which is overprinted by selectively pervasive propylitic and silica–albite assemblages. Petrological studies and major and trace element analysis of rocks of the CIC and FRV, as well as of other intrusions in the Cadia district that appear unrelated to mineralisation, have been conducted. Unaltered samples from the CIC are characterised by high K₂O contents (up to 6.5 wt%) and molecular K/Na ratios consistently >1, confirming the alkalic and shoshonitic nature of the complex. The CIC samples also fall just within the shoshonite field on Ce/Yb–Ta/Yb and Th/Yb–Ta/Yb plots, although immediately adjacent to the calc-alkaline field. The FRV are geochemically similar to the CIC, although lower in P₂O₅ and Ce (LREE). Dacitic and hornblende porphyry intrusions from within the district, but spatially unrelated to mineralisation, have a distinctive geochemical signature and are not shoshonitic in composition. The close spatial association between gold–copper mineralisation and the shoshonitic, monzonitic CIC argues strongly for a genetic link between the two. A similar association occurs at the economic Goonumbla porphyry gold–copper deposits in the eastern LFB, while sub-economic gold–copper mineralisation at Cargo and Copper Hill is associated with calc-alkaline quartz diorite and dacitic intrusions. The Cadia district is considered to be an excellent example of the association of alkaline, potassic magmas and gold–copper porphyry style mineralisation.