Heavy mineral beach placers in southeastern Australia; their nature and genesis

Abstract

Southeastern Australia has been one of the world's most productive heavy mineral sand provinces. Concentrations of detrital rutile, zircon, ilmenite, and minor monazite occur as beach placers and in dunes in high-stand barriers of Quaternary age along the present coast. Although individual mineral grains are of terrigeneous origin, the heavy mineral deposits themselves are not linked to specific source rocks in the hinterland. Rather, they arise under the action of waves during the building of coastal sand barriers. A number of fractionating mechanisms that concentrate heavy minerals operate during marine transgressions and under highstand conditions to produce three different types of beach placers. Type A deposits occur along erosional discontinuities in leaky embayments or between barriers of different age. Here, repeated storm reworking has produced beach face concentrations on drift-aligned sectors of coast by a process termed littoral bypassing fractionation. Type B heavy mineral deposits are found in back-barrier washover facies (and sometimes as condensed sections) at the rear of swash-aligned barriers in coastal embayments. These were produced by a process of transgressive barrier fractionation that reworked heavy minerals from shelves undergoing erosion during marine transgressions. Type C heavy mineral deposits occur in prograded barriers and represent episodic influxes of remobilized heavy mineral-rich sand that originated elsewhere through the two fractionation mechanisms mentioned above. Necessary environmental conditions for these fractionating mechanisms to operate include: low rates of clastic sediment supply and long periods of weathering and abrasion to create a mature heavy mineral suite, an energetic swell wave climate driving large sand fluxes onshore and alongshore, and changing sea levels (especially marine transgressions) that have the effect of moving heavy minerals from the shelf onto the present coast. Because of the last-mentioned effect, the mineral prospectivity for the southeastern Australian continental shelf is not high.