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 K 2 O 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 P 2 O 5 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.

Magmatic characteristics and geochronology of Ordovician igneous rocks from the Cadia – Neville region, New South Wales: implications for tectonic evolution

“…Together with the Nullawonga Latite Member, the majority of these units represent the plumbing system associated with a major volcanic centre, some of which may have been removed by erosion and thus have no counterparts among the sampled lavas. The monzodioritic to monzonitic suites are strongly porphyritic, show pronounced shoshonitic affinities (Wyborn 1992) and are also commonly associated with magmatic -hydrothermal Cu -Au mineralisation such as at Cadia (Holliday et al 2002). The Burnt Yards Basalt, Gooleys Basalt, Sundew Basalt and the hornblende basaltic andesites are considered here to represent part of the same intrusive system, though Wyborn and Henderson (1996) interpreted the basaltic lavas of the Burnt Yards Basalt as recording the commencement of a second phase of volcanism in the Forest Reefs region.…”

“…Hydrothermal alteration is locally intense (Holliday et al 2002), and only carefully selected, least-altered samples were used for this study. Detailed petrographic descriptions are provided in Squire (2001), and here we provide only a brief description of the keys units.…”

“…The shoshonitic compositions of these Ordovician volcanic and intrusive rocks have been used to support tectonic models that involve magmatism either unrelated to subduction (Wyborn 1992) or accompanied by coeval subduction Blevin 2002;Holliday et al 2002). Understanding the significance of the shoshonitic magmatism is, therefore, critical for interpretation of the Ordovician tectonic setting.…”

The petrographic and compositional character of variably K-enriched magmatic suites associated with Ordovician porphyry Cu-Au mineralisation in the Lachlan Fold Belt, Australia

“…Cadia (Holliday et al 2001, this volume) represents the largest Cu-Au deposit group within the Ordovician with pre-mine resources of 470 t Au and 2.0 Mt Cu, followed by the Endeavour deposits at Goonumbla (36.9 t Au and 0.65 Mt Cu combined resources for the E22, E26, E27 deposits; Heithersay et al 1990). The Copper Hill deposit is much smaller and subject to ongoing exploration efforts.…”

Gold-copper mineralization in alkaline rocks

“…For instance, the world-class epithermal gold deposits Cripple Creek, USA (Kelley and Ludington 2001, this volume), Emperor, Fiji (Setterfield et al 1991), and Ladolam and Porgera, Papua New Guinea Ronacher et al 2001, both this volume) are all hosted by potassic calc-alkaline or alkaline rocks. Similarly, there are equivalent host plutons associated with the porphyry gold-copper deposits at Bajo de la Alumbrera, Argentina (Mu¨ller and Groves 2000), Bingham, USA (Maughan et al 2001, this volume), Cadia, Australia (Holliday et al 2001, this volume), Grasberg, Indonesia (Pollard and Taylor 2001, this volume), Skouries, Greece (Kroll et al 2001, this volume) and Ok Tedi, Papua New Guinea (Rush and Seegers 1990).…”