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Minerals Report - International Seabed Authority

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In most placer deposits, economically valuable minerals have been<br />

mechanically concentrated in rivers along beaches, fan-aprons and river<br />

deltas as a result of their higher density (>3.2 gcm -3 ) compared to the bulk<br />

of detrital minerals, which consist mostly of quartz and feldspar (with a<br />

density of 2.7 gcm -3 ). These placer minerals, sometimes called heavy<br />

minerals, are derived by weathering from continental rocks of mostly<br />

volcanic, plutonic, or metamorphic origin and have a broad compositional<br />

range. Therefore, the majority of heavy mineral marine placer deposits are<br />

intrinsically linked close to their geological sources and to near-shore or<br />

shallow continental shelf environments.<br />

The majority of placer deposits are found along many present<br />

shorelines and are predominantly a result of beach migration during<br />

periods of marine transgression. Although few shelf deposits large enough<br />

INTERNATIONAL SEABED AUTHORITY 685

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