alluvium vs placer

alluvium and placer both are nouns.

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

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

alluvium: 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 alluvium include delta.

alluvium (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