QUANTITATIVE HEAVY-MINERAL ANALYSIS OF BEACH PLACER DEPOSITS IN SOUTHEASTERN AUSTRALIA USING THE AUTOGEOSEM

Heavy mineral placer deposits have been extensively mined along both the east coast and west coast of mainland Australia. Recent exploration has also included both the Murray Basin and Eucla Basin, situated in inland southeastern Australia. This has resulted in the discovery of numerous heavy mineral deposits, such as the Bondi Main (Murray Basin) and Jacinth (Eucla Basin) deposits. Such deposits are characterised by ilmenite, rutile and zircon typically concentrated as beach placers.

Thirty-two heavy-mineral separates, each comprising approximately 2000 grains, were extracted from the Loxton-Parilla sands, which is the host of the heavy minerals at the Bondi Main deposit. These were analyzed using the Automatic Geological Scanning Electron Microscope (AutoGeoSEM). This analysis was undertaken to study the influence of depositional environment on the distribution of heavy-minerals throughout a regressive Pliocene coastal sequence. The heavy-mineral suite was found to be dominated by stable to ultrastable minerals such as zircon, rutile, and tourmaline, along with ilmenite, various intermediate titanates, and anatase. Sillimanite, andalusite, topaz, chrome spinel/chromite, monazite, and muscovite also occur. These heavy-minerals are heterogeneously distributed throughout the Loxton-Parilla sands. The main process controlling the heterogeneous distribution of these heavy-minerals throughout the Loxton-Parilla sands appears to have been the hydraulic conditions, which are intimately linked with the environment of deposition. Consequently, the dense heavy-minerals (density > ~ 3.5 g/cm3) are concentrated in relatively high-energy environments such as the breaker zone and swash zone. The less dense minerals (density < ~ 3.5 g/cm3) are concentrated in the shoaling zone and shelf environments. Post-depositional weathering has altered the heavy-mineral assemblages in all units. Accordingly, some primary minerals, such as ilmenite, appear to have altered to secondary minerals, such as pseudorutile and anatase.