Search

The Cadia district of New South Wales contains four alkalic porphyry Au-Cu deposits (Cadia East, Ridgeway, Cadia Hill, and Cadia Quarry) and two Cu-Au-Fe skarn prospects (Big Cadia and Little Cadia), with a total of ~50 Moz Au and ~9.5 Mt Cu (reserves, resources, and past production). The ore deposits are hosted by volcaniclastic rocks of the Weemalla Formation and Forest Reefs Volcanics, which were deposited in a submarine basin on the flanks of the Macquarie Arc during the Middle to Late Ordovician. Alkalic magmatism occurred during the Benambran orogeny in the Late Ordovician to early Silurian, resulting in the emplacement of monzonite intrusive complexes and the formation of porphyry Au-Cu mineralization. Ridgeway formed synchronous with the first compressive peak of deformation and is characterized by an intrusion-centered quartz-magnetite-bornite-chalcopyrite-Au vein stockwork associated with calc-potassic alteration localized around the apex of the pencil-like Ridgeway intrusive complex. The volcanic-hosted giant Cadia East deposit and the intrusion-hosted Cadia Hill and Cadia Quarry deposits formed during a period of relaxation after the first compressive peak of the Benambran orogeny and are characterized by sheeted quartz-sulfide-carbonate vein arrays associated with subtle potassic, calc-potassic, and propylitic alteration halos.

The Cadia district, located in the eastern Lachlan fold belt of New South Wales, Australia, comprises four gold-copper porphyry deposits (Ridgeway, Cadia Quarry, Cadia Hill, and Cadia East) and two related ironcopper- gold skarn deposits (Big Cadia and Little Cadia). These deposits formed along a northwest-striking, arctransverse structural corridor within the intraoceanic Macquarie arc. This arc is composed of a belt of Early Ordovician to Early Silurian mafic to intermediate volcanic, volcaniclastic, and intrusive rocks of calc-alkalic to alkalic composition. The Cadia porphyry deposits are temporally and genetically associated with composite intrusive complexes of alkalic monzodiorite to quartz monzonite. U-Pb dating of igneous minerals from the intrusions and Re-Os dating of hydrothermal molybdenite from the deposits has revealed the presence of two temporally discrete events of magmatism and related porphyrystyle mineralization in the Cadia district. The monzonitic intrusive complex related to mineralization at Ridgeway and a quartz monzonite porphyry stock that lies immediately southwest of the Cadia Quarry deposit are early Late Ordovician (456-454 Ma). In contrast, the quartz monzonite porphyry stock that hosts the Cadia Quarry and Cadia Hill orebodies and an intermineral quartz monzonite porphyry dike at Cadia East are Early Silurian (~438 Ma). Re-Os molybdenite ages determined for quartz-sulfide veins within the early Late Ordovician quartz monzonite porphyry confirm multiple episodes of mineralization associated with the Cadia Quarry deposit, suggesting a complex history between 460 and 450 Ma. At Cadia Hill, Cadia Quarry, and Cadia East, a widespread event of porphyry gold-copper mineralization is recorded at about 443 to 441 Ma, based on three Re-Os molybdenite ages from sheeted quartz sulfide veins. Similar ages for magmatic zircon from the host intrusions support a link between dated intrusions and mineralization. These new ages are in general agreement with the ages of other porphyry gold-copper and related epithermal deposits of the Eastern Lachlan fold belt and help to constrain the relationship between the Cadia region and the evolution of the Lachlan fold belt. It is notable that the high-grade Ridgeway deposit is up to 18 m.y. older than any alkalic porphyry deposit yet discovered in the Macquarie arc, a feature that may be important for continued exploration success in this region.

Ridgeway is a high-grade gold-copper porphyry deposit (54 Mt at 2.5 g/t Au and 0.77% Cu), related to an alkalic intrusive complex of monzonitic composition. The deposit occurs within the Cadia district of New South Wales, Australia, which consists of a cluster of four Late Ordovician gold-copper porphyry deposits and two iron-copper-gold skarn deposits with a combined gold resource in excess of 574 metric tonnes (t). The deposits lie on a 7-km-long, northwest-oriented corridor of alteration and mineralization, transverse to the axis of the postulated volcanic arc. Alteration and mineralization at Ridgeway are zoned around a vertically attenuated intrusive complex of monzodioritic to quartz monzonitic composition. Distinct styles of veining and alteration are related to different intrusive phases of the monzonite complex, with the intensity of alteration and grade of mineralization decreasing from early- to late-mineral intrusions. Early-mineral intrusions are associated with intense actinolite- magnetite-biotite (calc-potassic) alteration and up to four stages of high-grade quartz-magnetite-sulfide veining. Bornite is the most abundant sulfide formed during early-stage alteration and correlates well with gold. Moderate- to weak-intensity orthoclase-biotite plus or minus magnetite (potassic) alteration accompanies the inter- and late-mineral intrusions, this alteration being associated with chalcopyrite- and pyrite-rich quartz-orthoclase veins. Propylitic and sodic (albite-pyrite) alteration assemblages are peripheral to, and locally overprint, the potassic alteration. Phyllic alteration is restricted to the margins of late-stage faults. The fluid inclusion assemblage comprises one and two salt-bearing brine inclusions, in addition to aqueous liquid-vapor inclusions of low to moderate salinity. No low-density, vapor-rich inclusions are present, indicating that the fluids from which the quartz veins precipitated did not enter the liquid-vapor field of the H2O-NaCl system. The brine inclusions undergo final homogenization to liquid via halite dissolution. This phenomenon, in addition to the absence of low-density vapor inclusions, suggests that the mineralizing fluids at Ridgeway were nonboiling hypersaline brines that exsolved directly from the crystallizing magma. The presence of mineralized aplitic vein dikes and comb quartz layering are interpreted to indicate that the early and transitional stages of mineralization at Ridgeway formed at the transition between magmatic and hydrothermal conditions.

Ed Malone. 2011. The Australasian Institute of Mining and Metallurgy, Spectrum no. 19, Victoria, Australia. 252 p. ISBN 978-1-9215522-38-3. $70 AUS (also $50 AUS in CD-ROM format). The Cadia Valley Mines: A Mining Success Story is a comprehensive account of the discovery, evaluation, development, and operation of a series of gold-copper orebodies that today comprise Newcrest Mining Limited’s Cadia Valley operations. This is a globally significant cluster of porphyry-style ore deposits hosting at least 44 Moz of gold and 8 Mt of copper, and certainly worthy of the attention to detail provided by the book’s author, the economic geologist Ed Malone. The book is laid out in 25 chapters that cover, in chronological order, a preamble on generic aspects of the Cadia district (geography, geology and mineralization, mining methods, ore reserve estimation methodologies), the pre-Newcrest exploration period, and a detailed account of the Newcrest exploration and discovery phase. This is followed by accounts, from both a technical as well as …