Cobar Explanatory Notes (1994)

Abstract

The Cobar 1:100 000 map sheet area, immediately to the north of Cobar, includes the important silver-lead-zinc Elura Mine and the copper-lead-zinc CSA Mine. Additional (subeconomic at the time of writing) deposits in the area mapped include those in near Mount Drysdale. Much of the area is underlain by rocks of the Ordovician Girilambone Group, which have been intruded by Silurian granitoids. Parts of the area are underlain by marine sedimentary rocks of the Early Devonian Cobar Supergroup - in fault contact with, or unconformably overlying, basement. Cobar Supergroup rocks accumulated in the deepwater Cobar Basin, surrounded by two shallow shelves - the Winduck (northwestern part of the area) and the Kopyje Shelf (eastern part of the area). The Cobar Basin, the site of turbiditic deposition, formed as a mixed-mode or strike-slip basin, being fault-bounded on all sides. Meridional faults on the eastern margin and within the eastern part of the basin are oblique-slip or strike-slip faults. Some west-northwest-trending faults at the northern edge and within the central part of the basin are inferred to be extensional faults. Deposition in the Cobar Basin can be divided into syn-rift facies (lower two-thirds of the Amphitheatre Group) and post-rift facies, represented by the upper Amphitheatre Group. Inversion of the Cobar Basin occurred mainly in the Early Devonian (ca 395-400 Ma). Structures developed then reflect control exerted by reactivation of the syn-depositional faults. Extensional faults in the centre of the inverted basin either became thrusts (linked to a floor thrust at depth) or controlled ramps in that floor thrust. Higher strain in parts of the area led to formation of regional cleavage, subvertical extension and variably plunging folds. The Elura Mine was interpreted as a syntectonic replacement deposit. 

Reference

Glen R.A. 1994. Geology of Cobar 1:100 000 Sheet 8035. 132 pp. Geological Survey of New South Wales, Sydney