Keeling, John1, Pobjoy, Rick2, Raven, Mark3, Self, Peter4
1Geological Survey of South Australia (retired); 2Tawel Exploration, Adelaide, Australia; 3CSIRO Mineral Resources, Adelaide, Australia; 3CSIRO Land and Water, Adelaide, Australia
Reconnaissance geochemistry of Pliocene sedimentary deposits of the Murray Basin by McLennan (2016) recorded sites with anomalous levels of rare earth elements (REE). These were followed up subsequently to identify broad zones of clayey sediments, east of the Kanawinka fault zone, containing between 500 to 2800 ppm total rare earth oxides (TREO) + Y, over 1-3 m-thick depth intervals. The sediments occupy depositional environments of estuary, lagoon and interdunal swamps developed behind stranded coastal barrier sands during Late Miocene – Early Pliocene marine regression across the Murray Basin. The clayey sediments are 2-22 m thick and unconformably overlie Oligo-Miocene Murray Group shallow marine, bryozoan limestone. The REE are mostly concentrated within a few metres of the contact with underlying limestone. A broad exploration target extending from Comaum in the south to Frances in the north, bordered to the west by the Kanawinka Fault and extending beyond Apsley (Vic) to the east, outlines the Koppamurra REE project (~2000 km2) – the focus of current investigations.
Analyses of selected drill cuttings showed dominantly light REE (Ce, La and Nd) plus Y, with ~50% of REE recoverable using acid leach (pH 1) and repeated washing with NaCl solution. XRD and XRF analyses of a representative drill sample, from east of Wrattonbully, confirmed high clay content (78%) composed of co-dominant kaolin and smectite, with ~10% goethite. Electron microscopy of the clay fraction identified minor secondary REE minerals cerianite (CeO2) and Ce-monazite ((Ce,La,Nd) PO4) with trace of zircon, all with no detectable Th or U. Results are consistent with surficial mobilisation and concentration of REE, where a high proportion of REE are adsorbed on clay and iron (hydr)oxide surfaces.
The source of REE in this region of the Murray Basin is most likely mafic igneous rocks, including:
- Cambrian mafic volcanics within metamorphosed basement underlying basin sediments at >30 m depth, and in weathered outcrop on the Western Dundas Tableland, ~20 km to the east-southeast,
- Newer Volcanics (<4.5 Ma), as basalt flows capping weathered basement on the Dundas Tableland and as volcanic ash from explosive volcanic centres, in particular, early-Middle Pleistocene offshore eruptions that formed the Mt Burr archipelago, 35-44 km southwest of the Kanawinka escarpment.
Fluvial or groundwater transport of REE possibly was directed to the site of deposition by uplift along the Western Highlands-Gambier axis and defeat of drainage towards the south. REE deposition was affected by salinity change at the coast. Any windblown glassy volcanic ash deposited and concentrated in lagoonal depressions, was subsequently altered to smectite and kaolin. Lowering of the water table, in response to drier climate and retreat of the sea from Middle Pleistocene time, resulted in remobilisation of iron and REE and concentration deeper in the profile.
McLennan SM 2016. Sedimentation and geochemistry of the Loxton-Parilla Sands in the Murray Basin, southeastern Australia. PhD Thesis, University of Adelaide.
John Keeling was formerly Senior Principle Geologist and Program Coordinator of the Mineral Systems Team at the Geological Survey of South Australia. Since retirement in 2019 he has collaborated with colleagues Mark Raven and Peter Self at CSIRO to assist with progressing mineral industry exploration projects, including REE exploration.