CAVERS - Poor people, often old women, who searched and reworked the hillocks for discarded ore.
The word first occurs about 1538.
The number of cavers increased dramatically after about 1570-1580, after the introduction of the sieve and vat and ore hearth smelting furnace.
Glossary of Derbyshire Lead Mining Terms" by J.H.Rieuwerts (and twitchy on m-e)