Celtic Energy is closing the Selar site, near Glynneath, next March, blaming a 40% fall in the world price of coal.
It said it could not keep the site running along with Nant Helen and the soon-to-be expanded East Pit.
Glynneath councillor Eddie Jones called the move another blow to the area.
"It was totally unexpected and came out of the blue," he told BBC Wales.
"My heart goes out to those people who are going to lose their jobs because there's nothing around at the moment.
"This is the second big thump we've had - the Aberpergwm mine was mothballed and now this. It's really, really knocking our village."
Some of the jobs could go to Nant Helen and East Pit, but most staff are expected to take voluntary redundancy.
Will Watson, chief executive of Celtic Energy, said the firm "very much regret the impact" of the closure.
The decision to mothball a Neath Valley mine with the loss of 70 jobs came out of the blue, a local councillor has said.