placer vs alluvium

placer and alluvium both are nouns.

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

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

placer: an alluvial deposit that contains particles of some valuable mineral
alluvium: 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, alluvion.

placer (noun)	alluvium (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