Analogue modelling of a Tabberabberan fold-thrust belt in the Cobar region and implications for the origin of Cobar-type mineral deposits

Abstract

The Cobar Supergroup is a package of Siluro-Devonian sedimentary, volcanic and volcaniclastic rocks in central New South Wales that underwent significant deformation during the Middle Devonian Tabberabberan Orogeny. The Cobar Supergroup, particularly within the Cobar Basin, constitutes one of the most mineral-rich rock packages in New South Wales, being endowed with Au, Cu, Pb, Zn and Ag, mainly in what are commonly referred to as Cobar-type deposits. Inconsistencies in the traditional stratigraphic interpretation of the area have led to a re-evaluation of its structural history, in particular the role of the Rookery Fault, which bounds the eastern side of the basin. Evidence suggests that that structure may dip to the east, rather than the west, as is traditionally assumed, leading to the possibility that Tabberabberan deformation in the area took the form of a west-verging fold-thrust belt. Analogue modelling is employed to examine the consequences of that scenario. Such a mechanism explains the known distribution of stratigraphic units of the Cobar Supergroup and its basement, as well as observed structures, in particular a high strain zone immediately west of the Rookery Fault. The modelling indicates that deeper parts of the crust were thrust up along the Rookery Fault, which, along with substantial burial, led to perturbation of the geotherm and heating of the footwall. This accounts for the higher metamorphic grade immediately west of the Rookery Fault. Furthermore, heating and overpressuring of basement rocks in the footwall led to mobilisation of metal-bearing fluids and transport of them along the high strain zone and subsequent deposition of those metals within favourable structural traps, with variable interaction with basinal fluids, to form Cobar-type deposits. A fold-thrust mechanism with ongoing erosion of the thrust wedge can also explain the distribution of Middle to Upper Devonian rock sequences in the region. KEY POINTS A west-verging fold-thrust model is proposed to explain Tabberabberan deformation of the Cobar Supergroup and its basement. Analogue modelling is employed to demonstrate the fold-thrust model. A fold-thrust model explains the distribution of Ordovician to Upper Devonian stratigraphic units in the Cobar region, as well as associated structures, metamorphic grade distribution and the origin of syn-deformational Au, Cu, Pb, Zn, Ag mineral deposits.