Quantitative Heavy-Mineral Analysis of a Pliocene Beach Placer Deposit in Southeastern Australia Using the AutoGeoSEM

Abstract Thirty-two heavy-mineral separates, each comprising approximately 2000 grains, 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 majority of these heavy minerals were separated from the Loxton-Parilla sands, the host of the Bondi Main heavy-mineral beach placer deposit in the Murray Basin of southeastern Australia. The remaining heavy minerals were separated from a number of other Neogene units encountered at the deposit. The heavy-mineral suite is 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 both the Loxton-Parilla sands and the remaining Neogene units. 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 heavy minerals (density < ~ 3.5 g/cm3) are concentrated in the shoaling zone and shelf environments. The erratic distribution of the various heavy minerals in some of the other Neogene units is also apparently due to varying depositional conditions. Postdepositional 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.