Trap
The heavy minerals within the source sediments attain an economic concentration by accumulation within low-energy environments within streams and most usually on beaches. In beach placer deposits the lowest energy zone on the beach is the swash zone, where turbulent surf washes up on the beach face and loses energy. In this zone, heavier grains accumulate because they are denser than the quartz grains they occur with and become stranded. It is for this reason that beach placer deposits are often referred to as "strand-line deposits".
The size and position of a heavy mineral deposit is a function of the wave energy reaching the beach, the mean grainsize of the beach sediments, and the current height of the ocean.
Anecdotal reports of certain beach placers which are forming in modern times suggest that the greatest enrichment of the sands tends to occur in storm events which are energetic enough to remove most of the beaches sediment load—a process favoring the lighter minerals. The resultant 'clinker' sands which were left behind were mined during low tide following major storm events, suggesting that most beach placer deposits are formed during such cycles.
In most cases, fossilised dune systems are exploited for heavy mineral sands because they are from the ocean and because they are often remnants of previous intraglacial highstands.
Tectonic activity, which results in coastlines rising from the ocean, may also cause a beach system to become stranded above the high water mark and lock in the heavy mineral sands. Similarly, a beach system which is drowned by the subsidence of a coastline may be preserved, often for millions of years, until it is either covered by sedimentation or rises from the ocean.
Specific trap sites for heavy mineral sand placer deposits are in beaches on the leeward side of headlands, as this forms a low-energy zone which traps sediments carried along by the longshore drift. Also, sand bars developed at the mouths of rivers which feed the placer deposits are rich trap sites where the winnowing action of the waves are most efficient, because heavy minerals, if they are going to be too heavy to be moved, will deposit at an isthmus in preference to drifting too far down the beach.
Read more about this topic: Heavy Mineral Sands Ore Deposits
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