Geology & Exploration in the Murray Basin

Why the Murray Basin?

The MB is an intracratonic sedimentary basin of Cainozoic age (66 Ma- present) that extends across 300 000 km2 of NSW, 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.

The Murray Basin and major HM deposits – note the Avonbank deposit just north of WIM 150

What are Heavy Minerals?

Valuable Heavy minerals (VHM) include: Rutile, Zircon, Ilmenite & Leucoxene; that 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 HM deposits. The accumulation of HM is illustrated in the below picture of a present day beach (black material is HM).

Heavy Mineral (HM) that has accumulated on the foreshore of a Sydney Beach

How do Deposits of Heavy Minerals Form?

The concentration of HM is due to the due to the higher specific gravity of the HM, compared to barren sands. The depositional environment & 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 & 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.

Size, Grade & Mineralogy of Mineral Sands Deposits

The size & 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 & 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