Preservation of the Cadia Valley porphyry Au–Cu district, NSW, Australia: Silurian basin formation and subsequent inversion

Abstract

Preservation of pre-Cenozoic porphyry Cu deposits requires geological events that bury the deposits before significant exhumation. Burial may be due to basin development over the top of the porphyry system or to burial beneath thrust sheets. The porphyry Au–Cu deposits in the Cadia district of central New South Wales underwent both events, and thus this metal-rich part of the crust survived the overprinted tectonic events that shaped eastern Australia. The Cadia porphyry Au–Cu deposits are associated with the Upper Ordovician to lower Silurian Cadia Intrusive Complex emplaced into the Upper Ordovician to lower Silurian Forest Reefs Volcanics and Middle Ordovician Weemalla Formation during the final phase of Macquarie Arc magmatism. Published stratigraphic evidence suggests west-northwest-striking faults influenced stratigraphic facies and thicknesses in the Forest Reefs Volcanics. Generally north- to east-northeast-striking faults that developed early in the district’s history are likely a product of exhumation of the porphyry Au–Cu deposits and their host rocks during and after the Benambran Orogeny. Alteration halos of several of the porphyry Au–Cu and skarns were exhumed at this time. Later during the Silurian (Wenlock to early Ludlow), the partially exhumed porphyry systems were buried beneath sediments, correlated with the Waugoola and Mumbil groups, in a rift–sag basinal sequence deposited initially in local fault-bounded basins that must be connected regionally to formation of the Cowra and Hill End troughs to the west and east, respectively. This basin geometry was controlled by the reactivation of the extant basement faults. Shortening of the Silurian sedimentary succession during younger Paleozoic orogenies was accommodated in thin-skinned fold and thrust faults forming a complex network of bedding-parallel detachments, ramps and anticlines that are linked to steeply dipping reverse and oblique-reverse slip faults in the underlying Forest Reefs Volcanics and the Cadia Intrusive Complex. KEY POINTS The world-class porphyry Au–Ci deposits were initially exhumed for a relatively brief interval during the early Silurian as a result of the Benambran Orogeny. The porphyry Au–Cu deposits were subsequently buried beneath Silurian sediments of the Waugoola Group accumulating in rifted basins formed during the Wenlock and Ludlow epochs of the Silurian, with sedimentary deposition continuing into the Pridoli (latest Silurian) in the lower part of the Mumbil Group. The Silurian basin was inverted with thin-skinned fabrics in the Silurian sedimentary rocks and thick-skin fabric localised along pre-existing faults, burying the deposits.