The Murray Basin sediments, deposited in a shallow Tertiary epicontinental sea in South Australia, New South Wales and Victoria, Australia, cover an area of 320 000 sq. km and contain substantial reserves and resources of heavy minerals comprising rutile, zircon and ilmenite.
The Murray Basin sequence comprises two deeper depocentres with 600 to 400 m of sediments, surrounded by platform cover of less than 200 m. Deposition is characterised by slow relative subsidence, minimal compaction and low rates of sediment supply over a flat, low lying, stable platform, susceptible to partial flooding by epicontinental seas. Sedimentation commenced in the Eocene and persisted to the Quaternary. During the Eocene the basin was covered mainly by flood plain alluvial and swamp deposits. In the late Oligocene to middle Miocene there was a marine incursion to produce lagoonal, then muddy marginal marine facies and finally deeper water limestones, but with marginal fluvial sediments.
In the late Miocene global sea level reductions caused a regression, followed by a rapid transgression at the end of the Miocene. Highstand deposition throughout the Pliocene produced the Loxton-Parilla Sands, a composite assemblage of regressive shoreface, beach, dune and back-barrier lagoonal facies that cover more than half of the basin and are the hosts to economic heavy mineral deposits. These were followed by Pliocene to Quaternary alluvial and aeolian sediments. By the mid Pliocene the seaward half of the basin was covered by a 400 km wide terrestrial sand plain with shore parallel ridges to within 100 km of the present coast.
Concentrations of relatively coarse grained heavy minerals occur as beach placers anf linear strandline deposits in the Pliocene Loxton-Parilla Sands in the upper part of the Murray Basin sequence, formed as 400 km long barrier complexes in the Murravian Gulf by the action of long period ocean swells. Concentrations of relatively coarse grained 90 to 300 µm heavy minerals representing the palaeo-strandline beach placers of the Loxton-Parilla Sands commonly reach 10 m and occassionally exceed 40 m over strike distances of 10 to 40 km, and 100s of metres wide. In low energy facies, formed offshore below the fairweather wavebase, in the south-eastern part of the arcuate belt of deposits there are relatively fine grained heavy minerals (40 to 80 µm) occurring as sheet deposits tens of kilometres long and wide, although some of these (such as at WIM 150) are probably mid Miocene in age and represent an earlier phase of concentration.
Palaeo-strandline beach placers typically contain between 1 and 50% heavy minerals, comprising mainly ilmenite with 30 to 40% rutile + zircon. Deposits of major commercial significance found to 2000 contained reserves/resources of over 12 Mt of heavy minerals comprising rutile, zircon and ilmenite. The conservative estimate of the total economic coarse grained mineral sand resource in the Murray Basin in 2000 was 50 Mt of contained rutile + zircon + ilmenite (Roy, et al., 2000).
Numerous strandline deposits have been identified, including (sources: Vic. DPI, 2008; PIRSA, 2002; Australian Zircon 2007):
- the Mindarie deposit near Loxton in South Australia (2005) - see the separate Mindarie record:
* Measured + indicated resource of 171.9 Mt @ 3.2% HM with 18.9% Zircon, 6.4% Leucoxene, 4.7% Rutile, 65.8% Ilmenite,
* Inferred resource of 5.3 Mt @ 3.3% HM with 17.0% Zircon, 5.1% Leucoxene, 3.9% Rutile, 67.2% Ilmenite,
* Proved + Probable Reserves (2005) of 59 Mt @ 4.3% HM with 21.6% Zircon, 6.1% Leucoxene, 4.7% Rutile, 65.3% Ilmenite.
- the Wemen deposit, 25 km SW of Robinvale in Victoria, with a
* Measured resource of 9.16 Mt @ 5.0% HM containing 28% Rutile, 11% Zircon, 51% Ilmenite.
- the Kulwin, Woornack and Rownack group of deposits, 30 km east of Ouyen in Victoria,
* Kulwin with an indicated resource of 24.0 Mt @ 11.5% HM containing 17% Rutile, 10% Zircon, 31% Ilmenite.
* Woornack and Rownack with indicated resources of 40.5 Mt @ 9.3% HM.
- Douglas, 60 km SW of Horsham in Victoria with 74 Mt @ 8% HM containing 5% Rutile, 9% Zircon, 43% Ilmenite.
- Ginkgo and Snapper 100 km north of Wentworth in NSW, with 184 Mt @ 3.1% HM, 12% Rutile, 10% Zircon, 41% Ilmenite.
- Jacks Tank South 180 km NE of Wentworth in NSW, with 41 Mt @ 2.6% HM, 21% Rutile, 15% Zircon, 55% Ilmenite.
- Jacks Tank North with 13 Mt @ 1.9% HM, 11% Rutile, 31% Zircon, 50% Ilmenite.
- Cylinder 130 km NE of Mildura in NSW with 29 Mt @ 3.8% HM, 18% Rutile, 13% Zircon, 49% Ilmenite.
- Birthday Gift 130 km NE of Mildura in NSW with 33 Mt @ 3% HM, 19% Rutile, 11% Zircon, 60% Ilmenite.
(Source: Porter GeoConsultancy, http://www.portergeo.com.au/, 2008)
