Metamorphism in the Cobar Basin: current state of understanding and implications for mineralisation

A metamorphic map of the Siluro-Devonian Cobar Basin highlights a partial disconnect between penetrative deformation, metamorphism and mineralisation in the region. The sedimentary sequences of the Cobar Basin preserve a subtle increase in burial-related metamorphic grade from diagenetic shelf sequences (west) to anchizone basin sequences (east), transitioning back into late diagenetic to low anchizone shelf and marginal basin sequences in the south and east. Volcanogenic sequences preserve rift basin metamorphic grade ranging from zeolite facies to epizone greenschist facies, with local biotite-zone greenschist facies conditions around syn-volcanic intrusions. Inversion of the Cobar Basin, along with reactivation of basement and/or basin-forming faults, folding and localised penetrative cleavage development, occurred from 405 to 380 Ma. The development of slaty cleavage(s) is in part spatially related to metamorphic grade, with high strain rates along faults and shear zones characterised by ductile fabrics associated with epizone ‘hydrous’ regional metamorphism. Mineralisation in the main Cobar mining field is associated with these high strain zones. Importantly, hydrothermal metamorphic highs are also associated with mineral deposits in areas of comparatively lower strain (e.g. Nymagee copper mine, Hera gold mine). These deposits are proximal to the same, or similar, long-lived fault systems as the deposits located within high strain zones. Hydrothermal metamorphic grade around the lower strain deposits reaches biotite-zone greenschist to lowest amphibolite facies (or albite–epidote to pyroxene hornfels facies; T >450°C) conditions, clearly predates the development of epizone penetrative deformation fabrics, and is intimately associated with mineralisation. Localised high-temperature biotite-rich metasomatism, biotite–plagioclase–magnetite veining, garnet– quartz–sulfide veining and areas of garnet–quartz–tremolite–sulfide ± actinolite–zoisite–scheelite–phlogopite–K-feldspar– anorthite–talc–chlorite alteration characterise these zones. Formation of these early-formed, high temperature associations commands a proximal magmatic heat source or hot magmatic fluid. Magmatism in the region occurred c.420–415 Ma, ~ 10 Ma prior to the first recorded deformation-related hydrothermal alteration. It is likely the burial field gradient of the basin was locally perturbed through igneous activity prior to, and possibly up to, the onset of basin inversion. High temperature hydrothermal alteration and mineralisation accompanied this igneous activity. The established, perturbed thermal regime promoted further regional-scale epizone hydrothermal metamorphism during basin inversion, leading to further mineralisation and probable metal remobilisation to from Cobar-type mineralisation in zones of high strain.