Mineral sands occurrences in the Murray Basin, southeastern Australia

The Murray Basin in southeastern Australia is proving to be a major mineral sand province that eventually will replace Australia's east and west coast s in production of rutile, zircon, and ilmenite. Concentrations of relative ly coarse-grained heavy mineral occur as beach placers in the Pliocene Loxt on-Parilla sands in the upper part of the Murray Basin sequence. These form ed as 400-km-long barrier complexes in the "Murravian Gulf" under the actio n of long-period ocean swell waves. We think the main source of barrier san d, at least initially, was From erosion of Miocene sands on the bed of the Murravian Gulf progradation was a response to sea level fluctuations linked to Milankovitch climatic cycles in the Pliocene. In most areas, the result ing 400-km-wide barrier strand plain is now overlain by fluvial, aeolian, a nd lacustrine deposits. Typically, the heavy mineral deposits are ilmenite rich, with 30 to 40 perc ent rutile and zircon. They occur as single, or as multiple, stacked strand line deposits, are often more than 10 m thick, have mineral grades that exc eed 20 percent in places, are several hundred meters wide and 10 to 25 km l ong; some contain up to several million tonnes of heavy mineral. The rutile and zircon are comparable in grain size and quality to minerals traditiona lly mined Australia. Many of the deposits are associated with topographic r idges-the Neckarboo and Iona Ridges are the best known-that appear to be fa ult-bounded blocks. Deposits of major commercial significance found so far contain a total of over 12 million tonnes (Mt) of rutile, zircon, and ilmen ite. The total, coarse-grained mineral sand resources in the Murray Basin a re conservatively estimated to be over 50 Mt. The distribution of mineral sands in the Murray Basin seems to be associate d with two aspects of the region's geology and geomorphology: (1) a zone bo rdering the central part of the basin where the Pliocene barriers were deri ved from underlying Miocene sands that probably already contained some mine ral concentrations and (2) growth faulting with deposits preferentially occ urring on upfaulted blocks especially in the zone defined by (1) above. We speculate that localized uplift during the formation of the Loxton-Parilla barriers was sufficient to modify coastal processes on uplifted blocks so a s to increase the rate of alongshore sediment bypass compared to nearby are as. This phenomenon has been simulated in computer modeling. Where the barr ier sands were already enriched in heavy minerals, winnowing by storm waves formed. beach placers on the uplifted fault-blocks. Based on criteria (1) and (2), the prospective areas of the Murray Basin account for 80 percent o f the beach placers found to date.