page_title: Heavy mineral sands ore deposits
section_title: Trap
hierarchical_section_title: Heavy mineral sands ore deposits / Trap
caption_attribution_description: English: Magnetite, hematite and gold. Black sand concentrates from panning placer gravels.
context_page_description: Heavy mineral sands are a class of ore deposit which is an important source of zirconium, titanium, thorium, tungsten, rare-earth elements, the industrial minerals diamond, sapphire, garnet, and occasionally precious metals or gemstones.
  Heavy mineral sands are placer deposits formed most usually in beach environments by concentration due to the specific gravity of the mineral grains. It is equally likely that some concentrations of heavy minerals exist within streambeds, but most are of a low grade and are relatively small.
context_section_description: The heavy minerals within the source sediments attain an economic concentration by accumulation within low-energy environments in 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 and become stranded because they are denser than the quartz grains they accompany. 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 forming in modern times suggest that the greatest enrichment tended to occur in storm events energetic enough to remove most of the beach's sediment load—a process favoring the lighter minerals. The resultant 'clinker' sands left behind were mined during low tide following major storm events, suggesting that most beach placer deposits are formed during such cycles.
  Fossilised dune systems often 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 that is drowned by the subsidence of a coastline may be preserved, often for millions of years, until it either is 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, where low-energy zones trap sediments carried along by the longshore drift. Also, sand bars developed at the mouths of rivers that feed the placer deposits are rich trap sites where the winnowing action of the waves is most efficient, because heavy minerals too heavy to be moved will deposit at an isthmus in preference to drifting farther down the beach.
caption: Black sand concentrates
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