placer vs alluvial sediment

placer and alluvial sediment both are nouns.

Word	Noun	Adjective	Verb	Adverb
placer	Yes	No	No	No
alluvial sediment	Yes	No	No	No

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

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

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

placer (noun)	alluvial sediment (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