Most mineral sands deposits are found in unconsolidated fossil shorelines several hundreds of metres to tens of kilometres and occasionally hundreds of kilometres inland from the present coastline.
In shallow marine sediments, the heavy minerals tend to lag or concentrate during storms when lighter components, such as quartz, are carried offshore or along shore by strong littoral drift. Heavy mineral accumulation occurs during periods of fair weather beach building and it is this concentration that provides the basis for the thicker strandlines formed during major storm events.
Repeated storm erosion and reworking over centuries or millennia may progressively enrich a mineral sand deposit. This can be observed within individual deposits being mined today and can result in enrichment through winnowing out of lighter, trash or gangue heavy mineral within a deposit.
- Deposit preservation occurs over geologically longer periods through subsidence of coastal sediments, changing sea levels caused by ice ages or isostatic adjustment of continental margins. This may cause shorelines to migrate inland (marine transgression), potentially resulting in reworking older HM accumulations into larger deposits. Alternatively migration seaward (marine regression) occurs, leaving reworked deposits preserved inland.
Most mineral sand deposits being mined today in Australia were formed during the Holocene and Pleistocene periods (that is over the past 1.8 million years) but some date back into the Eocene period or 58 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, interglacials and associated eustatic adjustments, resulted in repeated reworking of sediments deposited by rivers in coastal shorelines and existing HM laden coastal sediments.