Facies architecture of the Lewis Ponds carbonate and volcanic‐hosted massive sulfide deposits, central western New South Wales

Abstract

The Lewis Ponds carbonate and volcanic‐hosted Zn–Pb–Cu–Ag–Au–rich massive sulfide deposits are located near the western margin of the Hill End Trough in the eastern Lachlan Fold Belt of New South Wales. Two stratabound massive sulfide zones, Main and Toms' occur in a tightly folded Upper Silurian succession of marine felsic volcanic and sedimentary rocks. Siltstone, mudstone and quartz crystal‐rich sandstone overlie thick, tabular beds of poorly sorted mixed‐provenance breccia, limestone‐clast breccia and pebbly granular sandstone. The sedimentary rocks unconformably overlie a succession of strongly foliated, quartz‐plagioclase‐phyric dacite. Hydrothermal alteration and deformation have obscured primary textures and contact relationships in the footwall volcanic succession. However, three lithofacies associations were distinguished using phenocryst abundance, overall texture and immobile element compositions. Thick (up to 100 m) conformable or fault‐bound units of poorly sorted limestone‐clast breccia, megabreccia and massive limestone occur throughout the Lewis Ponds host sequence. Clasts vary in size from small pebbles to 10 m–long boulders and 90 m–thick lenses. Where primary textures are preserved, the bioclastic limestone contains crinoid fragments, brachiopods, bivalves, solitary corals, volcanic quartz and feldspar crystals and rare Silurian conodonts. Although originally deposited in shallower water, the allochthonous limestone occurs in a host sequence of predominantly siltstone and mudstone. The mixed‐provenance breccia, limestone‐clast breccia, pebbly granular sandstone and quartz crystal‐rich sandstone were deposited on the flanks of a high‐level intrusive dacite centre in a moderately deep, below wave‐base slope environment. Volcanic and sedimentary components originated from multiple source areas at the basin margin and in the adjacent hinterland. The Lewis Ponds host sequence records a transition from coarse‐grained polymictic volcaniclastic breccia and sandstone, upwards into mudstone, siltstone and minor volcanogenic sandstone, indicating deposition in a deepening, more restricted environment as volcanism waned and the basin progressively subsided.