placer vs alluvion

placer and alluvion both are nouns.

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

As nouns, alluvion is a hypernym of placer; that is, alluvion is a word with a broader meaning than placer:

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

Other hypernyms of placer include alluvial deposit, alluvial sediment, alluvium.

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