The Murray Basin (MB) is an intracratonic sedimentary basin of Cainozoic age (66Ma – present) that extends across 300,000km2 of New South Wales, Victoria, and South Australia. The upper sequences of the MB, principally the Loxton-Parilla Sands (LPS), are known to contain economic accumulations of heavy mineral sands (HMS).
The Murray Basin and major heavy minerals (HM) deposits – note the Avonbank deposit is just north of WIM 150
Valuable heavy minerals (VHM), which include rutile, zircon, ilmenite, and Leucoxene, typically have a higher specific gravity than other minerals. Due to the 'heavy' nature of the VHM, they can be mechanically sorted through marine processes in a beach or offshore environment, to form placer or offshore heavy mineral (HM) deposits. The accumulation of HM is illustrated in the below picture of a present day beach (the black material is the HM).
HM that has accumulated on the foreshore of a Sydney Beach
The concentration of HM is due to the due to the higher specific gravity of the HM, compared to barren sands. The depositional environment and structural geology of the underlying basement can also play an important role in forming a HM deposit. For example, headlands can act as a trap like mechanism for HM and over time HM strandlines can form. In the Murray Basin HM deposits typically form arc-like strandlines along the shore face and also 'sheet-like' offshore deposits.
In the present day form of the Murray Basin these deposits represent ancient beach systems that are often hundreds of kilometres from the present day coast line and typically lie below various sedimentary cover sequences.
The size and grade of strandline deposits varies greatly – on average a strandline deposit would be 100-150m wide and approximately 2-20km long. HM grades may vary from approximately 2-90% HM with the strandline itself.
Offshore deposits typically have different mineralogy to strandline deposits, the VHM is finer grained and usually zircon-rich. The geometry of offshore deposits differs from strandlines, commonly being several kilometres wide.
Fine grained 10-12%, zircon-rich HM in a panning dish from Avonbank