The metallic minerals that occur as alluvial deposits in flood plains or valleys are known as _______.

The correct answer is placer deposits.

Key Points

• Placer deposits, or "placers," are collections of valuable minerals that have been naturally concentrated in overburden, stream sediments, or beach materials.
• The minerals are liberated from solid rock by mechanical and chemical weathering.
• Then they are typically carried to their final resting place by water or wind action.
• Large minerals or minerals with a higher density are deposited when the water's velocity slows.
• Heavy minerals, such as gold, diamond, Ti, Cr, Sn, and magnetite, will be concentrated in locations where water current velocity is low because they will be deposited at a faster rate than low density minerals, such as quartz.
• Placer deposits have a specific gravity above 2.58.

Additional Information

• Polymetallic nodules:
  • Manganese nodules, also known as polymetallic nodules, are mineral concretions on the ocean floor that are made of a central core and concentric layers of iron and manganese hydroxides.
  • They contain four essential battery metals: cobalt, nickel, copper and manganese, in a single ore. 
• Vein deposits:
  • Veins are mineral deposits that develop when fresh mineral material fills a pre-existing fissure or crack inside a host rock. Aqueous solutions are often circulated to perform mineral deposition.