Slowhands,
as I understand it a miner of coal is traditionally termed a "collier", whereas a miner of iron ore is a "miner". In your considerable experience, do you know if these meanings are reflected in the occupation terms used in the PRs ? Certainly I've seen the "coal miner", "coal hewer", or "collier" used wrt my Lydbrook ancestors who were indeed coal miners, but off hand I don't have experience of researching other "miners" in the PRs.

Collier and Hewer - Coal mining
Miner - generic term used for coal or iron