Mineral systems of the Murray Basin , New South Wales

The Murray Basin in southeastern Australia is an intracratonic sedimentary basin of Cainozoic age that extends across 300 000 square kilometres of New South Wales, Victoria and South Australia. The upper sequences, principally the Loxton–Parilla Sands, Calivil Formation and Shepparton Formation, contain economic accumulations of heavy mineral sands; bentonite; kaolin; and gypsum. Mineral sands are the most significant commodities in the Murray Basin because of their large resources of high-quality coarse-grained rutile, zircon and ilmenite. Quartz sand amenable to construction applications occurs in many places, primarily as aeolian dune deposits, alluvial sequences associated with modern drainage courses, and ancient stream channels along the eastern margin of the basin. Mineral deposits of the Murray Basin formed in a large geological province in which critical interactions of depositional, eustatic, climatic and tectonic processes influenced their development. High-quality kaolin deposits along the eastern margin of the basin are believed to have been derived from weathering of nearby basement rocks. The kaolin accumulated in lacustrine or floodplain settings. Thick deposits of heavy mineral sands that accumulated in early Pliocene sequences in the northern Murray Basin are believed to represent coastal barrier stacking that occurred during multiple sea level fluctuations of at least 40 m. Bentonite deposits scattered across much of the southern and central Murray Basin were probably derived from volcanic ash emitted from inferred late Pliocene eruptive centres, now concealed by younger sedimentary units. Cainozoic sequences of the Murray Basin are disrupted in many places as extensive faulted and uplifted basement ridges that mainly trend northeast to southwest. Tectonic activity played a major role in the development of heavy mineral sands concentrations by the formation of uplifted fault blocks upon which beach placers preferentially formed. Structural depressions formed in response to subtle downwarping provided favourable sites for volcanic ash accumulations subsequently altered to bentonite. During the last 0.5 Ma, large quantities of marine salts from the Southern Ocean have been deposited over much of southern Australia. Those salts leached into shallow aquifers resulting in their typically saline groundwater from which salt is extracted commercially at several locations. Favourable structural or stratigraphic pathways in many places allowed groundwater to migrate to the surface to form hypersaline lakes that dried out leaving numerous deposits of gypsum.