Mineral sands are derived from the weathering of certain kinds of rocks. These rocks were most likely igneous (ie. crystallised from magma, such as granite or basalt), or metamorphic (ie. formed from even older rocks by the action of extreme pressure and heat, such as eclogite or schist). As well as rutile, ilmenite and zircon, these older rocks also contained other, more abundant minerals, such as quartz, feldspar and garnet.
In south-western Western Australia these rocks are Archean in age, meaning that they formed more than 2500 million years (Ma) ago. They were uplifted to the surface much more recently by fault movement which produced the Darling Escarpment, and lead to development of the higher country a few kilometres to the east of the south-west coast of Western Australia.
Uplift lead to weathering and erosion of the Archean rocks. Their constituent minerals were washed by rainwater into streams, and rivers, flowing in a westerly or south westerly direction towards the sea. The valuable minerals grains of rutile, ilmenite and zircon tended to accumulate in river channels or along coastal shorelines because they are denser than the more abundant minerals quartz and feldspar. The action of breaking waves further concentrates the heavy minerals. As waves break and wash up onto the shore they carry sand up the beach. When the wave recedes it takes some of the lighter minerals in the sand (eg. quartz and feldspar) with it, leaving denser minerals such as rutile, ilmenite and zircon behind. Repeated wave action over hundreds or thousands of years has resulted in large accumulations of sand very rich in valuable heavy minerals.
The picture shows a mineral sand deposit being mined near Capel. The bright red colour is due to iron oxides in the mineral sand deposit.
In south western Australia, mineral sands deposits occur in strand lines. Strand lines are fossil beaches, which record the position of the shore-line of this part of Australia at various times in the geological past (the position of the shore line is affected by the sea-level.) The past shoreline may have been up to 30 km away from its present position! The heavy minerals occur in high-grade deposits at the base of the strand lines. Strand lines may be 100 to 400m across, but individual deposits can be up to 5 km long. They are buried to depths up to 30m by lower grade sand dunes, which consist of sands washed or blown over the heavy minerals deposits. The strand lines being mined in the Capel area formed at various times over the past 1.8 million years.
Schematic cross section showing the geology of a mineral sands deposit.
