Heavy mineral beach placers in southeastern Australia: their nature and genesis. Economic Geology 94

Southeastern Australia has been one of the world's most productive heavy mineral sand provinces. Concen-trations of detrital rutfie, 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 hemselves 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 con-ditions 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 transgres-sions. Type C heavy mineral deposits occur in prograded barriers and represent episodic influxes of remobi-lized heavy mineral-rich sand that originated elsewhere through the two fractionation mechanisms entioned above. Necessary environmental conditions for these fractionating mechanisms to operate include: low rates of elas-tic sediment supply and long periods of weathering and abrasion tocreate a mature heavy mineral suite, an en-ergetic 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. Be-cause of the last-mentioned ffect, the mineral prospectivity for the southeastern Australian continental shelf is not high