Placer Formation in a Holocene Barrier System, Southwestern Australia

Abstract

The location of coastal titanium-zirconium placers within coastal barrier sequences provides significant problems for traditional exploration model development, because of the geomorphic (as opposed to geologic) nature of the processes responsible for placer formation. This paper describes an economic placer at Minninup, southwestern Australia, and develops a model of formation from a geomorphic viewpoint.

The Minninup placer deposits lie on the contemporary coast about 160 km south of Perth, Western Australia. The sediments form part of a Late Holocene transgressive barrier sequence, with placer development occurring in transgressive dune, elevated Mid Holocene beach and nearshore units, as well as contemporary environments. Sea-level history is a significant factor in placer development and preservation.

Formation of the Minninup placers is the result of processes operating at different temporal and spatial scales. Heavy mineral segregation due to differential entrainment in the nearshore and swash zones, differential transport, shear sorting, and longshore and offshore sediment movement all contribute to concentration and preservation. Seasonal variation in wave energy and direction, coastal morphology, and barrier evolution are also crucial. A model, based on geomorphological concepts, showing how these factors interact to produce economic placers, reveals the value of alternative approaches to exploration model development.