The Willyama Supergroup in the Broken Hill and Euriowie Blocks, New South Wales

Abstract

The Willyama Supergroup consists of highly deformed metasedimentary schists and gneisses with abundant quartzofeldspathic gneisses, lesser basic gneisses and minor ‘lode’ rocks. Prograde metamorphism ranged from andalusite through sillimanite to granulite grade. Radiometric dating in the Broken Hill Block indicates separation of the source of the quartzo-feldspathic gneisses from the mantle at 2300-2100 Ma. A Rb-Sr model source age of 1820 ± 60 Ma has been interpreted as the maximum age of deposition. Deformation (D 1 and/or D 2) was coincident with high-grade metamorphism at 1660 ± 10 Ma. Minor Mundi Mundi-type granitoids and some pegmatites were intruded at 1490 ± 20 Ma, minor pyroxenites at 561 ± 7 Ma, dolerite dykes soon after, and zoned pegmatites at ∼ 500 Ma, in association with the Delamerian Orogeny which reactivated retrograde schist zones at 520 ± 40 Ma. Subsequent history mostly comprises faulting, weathering, uplift and erosion. The Willyama Supergroup has an estimated total thickness of 7–9 km, with neither top nor basement exposed. No unconformities have been recognized in the sequence. The Supergroup was deposited in a deepening environment, passing upward from sandy facies to more shaly and fine sandy facies, and from interpreted bimodal acid/basic volcanics, to non-volcanic facies. The metasediments have been interpreted as shelf sediments overlain by deeper water turbidites and minor contourites. Recently they have been reinterpreted as shallow marine, with the quartzo-feldspathic gneisses interpreted as fluvio-deltaic arkoses. In this paper the sequence to the top of Thackaringa Group is interpreted as mainly fluvio-deltaic and lacustrine. The Broken Hill Group and Sundown Group are interpreted as shallow marine. The Paragon Group may have comprised shelf muds overlain by delta-front and lacustrine sediments, in turn overlain by deeper-water fine-grained turbidites. The Broken Hill orebody, near the top of Broken Hill Group, consists of a series of stacked lenses. Much recent work has concluded that the deposit is volcanic exhalative in origin, probably formed from seawater convected through the sequence, and driven by heat from high-level magma chambers. However, some features of the deposit are more appropriate to a sedimentary exhalative origin, and models have been proposed involving ‘seismic pumping’ of connate water, with ore deposition on or below the seafloor. Devolatilization of the mantle has been proposed as a source of ore constituents. Deposition may have occurred in a narrow NE-SW trending seafloor depression. The Willyama Supergroup was probably deposited in a rift zone with thin crust. The basic gneisses are interpreted as Fe-rich tholeiites similar to basalts from some oceanic spreading ridges. The intense folding, post-folding leucogranitic intrusions and extensive granulite/amphibolite facies metamorphism could have resulted from a collision following subduction. However, there is no direct evidence of oceanic crust or subduction, and the low-pressure metamorphic path does not suit a collision model. The rocks were probably deposited in a continental rift, with some similarities to those interpreted for Proterozoic fold belts in northern Australia.