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^sub 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^sub 2^O^sub 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.PUBLICATION ABSTRACT
New SIMS U-Pb (zircon) data for intrusive rocks of the Macquarie Arc and adjacent granitic batholiths of the Lachlan Orogen (southeastern Australia) provide insight into the magmatic and tectonic ...evolution of the paleo-Pacific Gondwana margin in the early Paleozoic. These data are augmented by Re-Os dates on molybdenite from four Cu-Au mineralised porphyry systems to place minimum age constraints on igneous crystallization. The simplicity of the zircon age distributions, and absence of older inheritance, stands in contrast to previous geochronological studies. The earliest magmatism within the Macquarie Arc is registered by a ca. 503 Ma gabbro from the Monza igneous complex, whereas a monzodiorite from the same drillhole records the youngest (ca. 432 Ma). Igneous activity in the Macquarie Arc thus overlapped deformation and magmatism in the craton-proximal Delamerian Orogen to the west, and the emplacement of the Lachlan granitic batholiths at 435–430 Ma; the thermal pulse associated with the latter may have triggered the formation of richly mineralised Silurian porphyries in the Macquarie Arc. The juvenile Hf isotope signature of the Monza Gabbro, together with the lack of zircon inheritance and the radiogenic Hf-Nd isotope systematics of Ordovician Macquarie Arc rocks, is consistent with early development of the arc, or a precursor magmatic belt, in an oceanic setting remote from continental influences, and with the arc being built on primitive Cambrian mafic crust. Outboard arc magmatism in the Cambrian may have initiated in response to convergent Delamerian orogenesis adjacent the Gondwana margin. Overlapping radiogenic isotope-time trends are consistent with the evolution of the Macquarie Arc and the Gondwana continental margin being linked from the Cambrian to the Silurian. These data provide further evidence for the growth of continental crust along the southeastern Australian segment of this margin being related to the dynamics of an extensional accretionary orogenic system.
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•Zircon U-Pb ages from the Macquarie arc intrusives range from 503 Ma to 432 Ma.•Molybdenite Re-Os dates overlap or slightly postdate zircon ages.•Porphyry Cu-Au mineralisation overlapped with an S-type granite ‘flare up’.•Macquarie arc magmatism initiated in an intra-oceanic setting.•The Ordovician Macquarie arc was built on juvenile Cambrian mafic arc crust.
In colonially breeding marine predators, individual movements and colonial segregation are influenced by seascape characteristics. Tidewater glacier fronts are important features of the Arctic ...seascape and are often described as foraging hotspots. Albeit their documented importance for wildlife, little is known about their structuring effect on Arctic predator movements and space use. In this study, we tested the hypothesis that tidewater glacier fronts can influence marine bird foraging patterns and drive spatial segregation among adjacent colonies. We analysed movements of black-legged kittiwakes (Rissa tridactyla) in a glacial fjord by tracking breeding individuals from five colonies. Although breeding kittiwakes were observed to travel up to ca. 280 km from the colony, individuals were more likely to use glacier fronts located closer to their colony and rarely used glacier fronts located farther away than 18 km. Such variation in the use of glacier fronts created fine-scale spatial segregation among the four closest (ca. 7 km distance on average) kittiwake colonies. Overall, our results support the hypothesis that spatially predictable foraging patches like glacier fronts can have strong structuring effects on predator movements and can modulate the magnitude of intercolonial spatial segregation in central-place foragers.
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