Tectonic environment and mineral prospectivity of Rockley-Gulgong volcanic belt, Oberon region, New South Wales, Australia

The Oberon region is dominated by two major lithological successions: The quartzrich turbidite succession is represented by the Adaminaby Group comprising mainly medium- to thick-bedded sandstone, siltstone, shale, and thinly bedded chert with a minimum thickness of 750 m. The Adaminaby Group was deposited on the eastern Gondwana margin in a distal submarine fan. The other succession faulted against the Adaminaby Group is the Rockley-Gulgong Volcanic Belt of the Ordovician Macquarie Arc, which is represented by the Budhang Chert, the Triangle Formation, the Rockley Volcanics, the Fish River Breccia, and the Swatchfield Monzonite. The Budhang Chert is characterized by moderately to highly deformed dark thinbedded chert interbedded with siliceous mudstone ranging between Early to early Late Ordovician (Bendigonian-Gisbornian). The Middle to Late Ordovician Triangle Formation conformably overlies the Budhang Chert and comprises thin- to medium-bedded mafic volcaniclastic fine-grained sandstone and minor conglomerate with common greenschist facies. The Triangle Formation is overlain by Late Ordovician Rockley Volcanics. The Rockley Volcanics are composed of pyroxene-phyric mafic to ultramafic breccia, lava, and volcaniclastic conglomerate and sandstone. The Triangle Formation and the Rockley Volcanics are unconformably overlain by the Fish River Breccia. This is a new unit proposed in this study to describe pyroxene-plagioclase-rich mafic to intermediate volcaniclastic pebbly siltstone breccia occurring near the Fish River 5 km to the east of Oberon. This youngest unit contains minor quartz-rich sandstone clasts which indicate mixing between the Adaminaby Group and the Rockley-Gulgong Volcanic Belt. Comparisons of the overall stratigraphy of the Macquarie Arc rocks in Oberon with the central Molong Volcanic Belt shows that the rocks around Oberon tend to be finergrained and more distal to the volcanic centers that were active in the Ordovicia. Whole-rock geochemistry of the volcaniclastic and volcanic rocks was characterized as shoshonitic to high-K calc-alkalic with the Triangle Formation possibly correlated with Phase 2 magmatism of the Macquarie Arc and the Rockley Volcanics correlated to Phase 4 magmatism. The Fish River Breccia are correlated with the coarse conglomerates at the base of the Waugoola Group. Detrital zircon U-Pb age determination from several quartz-rich sandstones in the Oberon and Black Springs region indicate maximum depositional ages in the Early to Late Ordovician. However, a slightly different provenance in the source of zircons was detected between the Oberon and Black Springs quartz-rich sandstones with a much larger 500 Ma peak in the sandstones from Oberon. The Fish River Breccia contains detrital zircons that are uncommon in volcanic/volcaniclastic rocks of the Macquarie Arc with old continental-derived zircons recorded both in mineral separates and in situ in polished rock mounts suggesting that this unit is post-collisional deposit formed during the Silurian to Early Devonian. U-Pb zircon age determinations and whole-rock geochemistry of intrusive igneous rocks in the Oberon region shows that the Swatchfield Monzonite is a Late Ordovician intrusion that may be made up of several intrusive bodies. The Greenslope Porphyry and Racecourse Porphyry were determined as Early to Middle Silurian and are unrelated to the intrusive suites of the Macquarie Arc. The whole-rock geochemistry, U-Pb age dating, and stratigraphy demonstrate that the Ordovician volcanic rocks of the Oberon region formed in a distal zone relative to the main volcanic centers and are less likely to host Cu-Au mineralization.