mining vs placer mining

mining and placer mining both are nouns.

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

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

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

Other hyponyms of mining include opencast mining, strip mining, fracking, salt-working, salt working.

mining (noun)	placer mining (noun)
the act of extracting ores or coal etc from the earth	mining valuable minerals from a placer by washing or dredging
laying explosive mines in concealed places to destroy enemy personnel and equipment