alluvion vs placer

alluvion and placer both are nouns.

Word	Noun	Adjective	Verb	Adverb
alluvion	Yes	No	No	No
placer	Yes	No	No	No

As nouns, placer is a hyponym of alluvion; that is, placer is a word with a more specific, narrower meaning than alluvion:

alluvion: clay or silt or gravel carried by rushing streams and deposited where the stream slows down
placer: an alluvial deposit that contains particles of some valuable mineral

Other hyponyms of alluvion include delta.

alluvion (noun)	placer (noun)
clay or silt or gravel carried by rushing streams and deposited where the stream slows down	an alluvial deposit that contains particles of some valuable mineral
the rising of a body of water and its overflowing onto normally dry land
gradual formation of new land, by recession of the sea or deposit of sediment