The Ridgeway Gold-Copper Deposit: A High-Grade Alkalic Porphyry Deposit in the Lachlan Fold Belt, New South Wales, Australia

Abstract

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 ± 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 H 2 O-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.

Platinum-group minerals in the Skouries Cu-Au (Pd, Pt, Te) porphyry deposit

“…The mineralisation and alteration at Skouries is broadly similar to that in other alkali Cu-Au porphyry deposits such as the Ridgeway deposit, New South Wales (Wilson et al, 2003), and the main stage PGM-bearing veins are similar to those in Elatsite, another PGE-enriched Cu-Au porphyry deposit. The observation of PGE-bearing minerals within the main stage of mineralisation agrees with previous work by Tarkian et al (1991) and by Economou- Eliopoulos and Eliopoulos (2000) but extends the range of recorded minerals.…”

Geochronology and isotope geochemistry of the Baogutu porphyry copper deposit in the West Junggar region, Xinjiang, China

“…3). The orebody dips northeast at about 45-55°suggesting that significant tilting may have occurred after ore formation, given that porphyry deposits typically have sub-vertically oriented ore shells (e.g., Hedenquist et al, 1998;Wilson et al, 2003).…”

Ordovician to earliest Silurian history of the Macquarie Arc, Lachlan Orogen, New South Wales

“…Late Ordovician volcaniclastics and lavas of the Forest Reefs Volcanics in the vicinity of Cadia mine conformably overlie the Weemalla Formation (Holliday et al 2002;Wilson et al 2003). Conodonts from calcareous strata towards the exposed base of the Weemalla Formation on the Belubula River suggest an early Darriwilian (Da2) age (Zhen & Percival 2004b), and at nearby Cadiangullong Creek, south of Cadia, siltstone slightly higher in the formation contains a diverse graptolite assemblage of Da3 age, listed (but not illustrated or described) by Smith (1966).…”