Cobar-Bourke Geological Mapping Project
Cobar-Bourke Mapping Project
Project status
The Cobar-Bourke Mapping Project forms part of the New Frontiers initiative and the Geological Survey of NSW 10 year strategic mapping plan. The project aims to update our knowledge of the geology on map sheets adjacent to the economically mineral rich Cobar 1:100 000 map sheet in north central New South Wales. These areas have previously only been mapped at 1:250 000 scale.
Mapping of the Sussex and Byrock 1:100 000 map sheet areas are now available as provisional geological maps. An explanatory note for both areas is in press.
Geology
The oldest recognised rock package in the region is the Girilambone Group. The recent mapping has allowed subdivision of the Girilambone Group into two major packages - an Early Ordovician turbidite package with minor, thin (approximately 1 m) chert units and mafic volcanics, including the Mount Dijou Volcanics; and a late Middle to early Late Ordovician turbidite package with more significant and thicker (to 40 m) chert units and basaltic volcanics. Whole rock chemical analyses indicate that the basaltic volcanic rocks of both packages were erupted within an intraplate setting, probably as oceanic islands. Identifications of conodonts found within chert intercalated with pillow basalts of the Mount Dijou Volcanics indicate that they are of Early Ordovician age. Deformation of the Girilambone Group occurred in the Late Ordovician to early Silurian Benambran Orogeny. During that deformation the rocks were heated mostly to greenschist facies but in the northern part of the Byrock sheet area they reached lower amphibolite facies. Numerous granites intrude the Girilambone Group and recent zircon U-Pb dating of some of those intrusions within the Byrock map sheet area indicates both Early Silurian and Early Devonian emplacement ages.
100 km northeast of Cobar
The Siluro-Devonian Cobar Supergroup unconformably overlies the Girilambone Group. Within the Cobar Basin the Cobar Supergroup is divided into the Nurri Group and the overlying Amphitheatre Group. The Nurri Group comprises basal conglomerates fining upwards into sandstones and siltstones and locally contain volcanic rocks. The Amphitheatre Group comprises finely bedded, deep-water turbidites. To the east of the Cobar Basin, Cobar Supergroup rocks are assigned to the Kopyje Group, which is equivalent to the Nurri Group. It contains a similar assemblage of rock types with local limestone units. Mapping of the Sussex and Byrock 1:100 000 map sheet areas has shown that the basal conglomerate of the Kopyje Group interfingers with quartzite which locally contains worm burrows, indicating a shallow marine, possibly a beach, depositional environment. The recent mapping also indicates that the Cobar Supergroup rocks in the Sussex and Byrock areas occur as sheared synclinal keels and are interpreted as remnants of a once laterally extensive sheet.
Deformation of the Cobar Supergroup and underlying Girilambone Group occurred in the late Early Devonian with inversion of the Cobar Basin. The Cobar style Au-Cu-Pb-Zn-Ag mineralisation, which occurs along the eastern edge of the basin and on the Kopyje Shelf (eg Mount Boppy mine), are considered to have been emplaced at that time. This mineralisation has been localised within high strain zones. Deposition of the dominantly terrestrial Mulga Downs Group continued from the late Early Devonian to the Late Devonian.
The recognition of strained synclinal slivers of Cobar Supergroup rocks within the Sussex and Byrock map sheet areas indicates that this area is prospective for Cobar-style mineralisation.
For more information on the mapping project please contact the Geological Mapping Team.
(Last updated on 20/05/2009)