placer mining vs excavation

placer mining and excavation both are nouns.

Word	Noun	Adjective	Verb	Adverb
placer mining	Yes	No	No	No
excavation	Yes	No	No	No

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

placer mining: mining valuable minerals from a placer by washing or dredging
excavation: the act of extracting ores or coal etc from the earth

Other hypernyms of placer mining include mining.

placer mining (noun)	excavation (noun)
mining valuable minerals from a placer by washing or dredging	the act of extracting ores or coal etc from the earth
	the act of digging
	a hole in the ground made by excavating
	the site of an archeological exploration