Deposits of mineral sands are formed along ancient coastlines where the heavier minerals have been concentrated by wave and wind action. Most mineral sand deposits are found in unconsolidated fossil shorelines several hundred metres to tens of kilometres – or occasionally hundreds of kilometres – inland from the present coastline.
The common observation that heavy mineral concentrations are found on beaches, particularly after storms, provides a link with mineral sand deposits and marine erosion.
The components of mineral sands all have high specific gravity and tend to lag during storms when lighter components, like quartz and feldspar, are carried offshore or along shore by strong littoral drift.
Repeated storm erosion and reworking over centuries or millennia may progressively enrich a mineral sands deposit. Over geologically longer periods, subsidence of coastal sediments, changing sea levels caused by ice ages or isostatic adjustment of continental margins may cause the shorelines to migrate inland (marine transgression), reworking earlier accumulations into larger deposits, or to migrate seaward (marine regression) leaving reworked deposits preserved inland.
Most mineral sands deposits mined today were formed during the Holocene and Pleistocene periods (over the past 1.8 million years) but may date back into the Mid Miocene period (12 million years ago). Over this period, changes in the sea level of between minus 300 metres and plus 350 metres from current sea level, caused principally by ice ages, resulted in repeated reworking of sediments deposited by rivers in coastal shorelines.
The size and grade of mineral sands deposits vary considerably. Along the shoreline, deposits are typically 100 or 200 metres wide, 5 to 20 metres thick and 2 to 20 kilometres long. Heavy mineral grades vary from several per cent to 90 per cent. Dunal deposits close to the shore tend to be larger, more irregular and lower grade.
The composition of mineral sands deposits reflects the type of rocks from which the sands containing the heavy minerals are derived. For example, granitic and gneissic source rocks principally provide ilmenite and zircon, while metamorphic rocks provide ilmenite and rutile.
During erosion cycles, the river courses change providing different sediment loads and heavy mineral suites to the ocean. Repeated reworking of a particular deposit may also cause mineral zoning. Later weathering may enhance the value of a mineral sand deposit by leaching iron out of the ilmenite. This may increase the titanium dioxide content of ilmenite from 55 per cent to a maximum of about 90 per cent.
Geologists select areas for exploration by researching an area's geology, topography, soil types and geological history. Areas are drilled with a widely spaced grid to identify any occurrence and concentration of minerals. If the results are promising, samples are taken from a more closely spaced grid.
Identifying mineral for new mine sites involves a systematic process of surveys, sampling analyses and evaluation. The key elements of the exploration program are:
- a review of the geological features of potential mineral provinces to identify the most likely deposits;
- drilling to collect samples from areas taken up by the company as exploration tenements;
- laboratory analyses of the mineral content of the samples; and
- an exhaustive process of computer modelling evaluation to establish the profile and viability of each deposit.
When a favourable deposit has been identified, the main exploration method is drilling. Usually small, 4WD-mounted Reverse Circulation (RC) drilling rigs are used. The RC drilling method – where air or water is forced down an annular tube and cuttings are returned up the central tube – produces a clean uncontaminated sample at the surface. These are bagged at 1 or 1.5 metre intervals and, if heavy mineral is present, sent to a laboratory for analysis. Samples are wet sieved and the amount of heavy minerals, clay and sand determined.
During drilling, attention is paid to recording the presence of ground water and rock as these can substantially reduce the profitability of a potentially economic deposit. After each hole is drilled, it is filled in or plugged using cuttings to prevent injury to livestock or native animals. If drill samples contain significant heavy minerals, further analysis determines the proportions of valuable minerals and their suitability for commercial use.