Question

What are placer deposits
A. Metals that occur as sandy deposits in sands of the sea floor.
B. Metals that occur as sandy deposits in sands of land floor.
C. Minerals that occur as alluvial deposits in sands of valley floor.
D. None of the above.

Answer 1

Hint: Placer deposits are natural concentrations of heavy minerals which are caused by the effect of gravity on moving particles.

Complete step by step answer:
The accumulation by gravity separation, for valuable minerals due to gravity, are known as placer deposits. The heavy and stable minerals are broken out from its matrix by the weathering processes, they slowly wash down and reach downslope into water streams which quickly wash away the lighter flux material into the stream. Thus, the heavy minerals become concentrated in stream, beach, and residual gravels and constitute as ore deposits. Minerals that form placer deposits have higher density, and are chemically resistant to weathering. They are durable and stable. Examples of some of the minerals found in the placer deposits are gold, platinum, cassiterite, magnetite, chromite, ilmenite, rutile, native copper, zircon, monazite, and various gemstones.
Since, these deposits accumulate on hill sides etc, they are dense and do not weather easily. The minerals are denser than quartz, the material of sand. Also, the spanish word for placer is alluvial.
Therefore, the correct answer is option C. minerals that occur as alluvial deposits in sands of valley floors. These deposits generally contain minerals which do not corrode by water.

Note: An important source of gold is placer mining and it is the main technique which was used in the early years of many gold rushes. Types of placer deposits include alluvium, eluvium, beach placers, and paleo placers.