The quarries used to be the main employer in the area
A quarry at Blaenau Ffestiniog, in north Wales, is being closed at the end of April because it is too dangerous.
Welsh Slate said it regretted the move but there had been subsidence into old mine chambers at Ffestiniog (Oakeley) Quarry in the past month.
The company added safety experts had concluded there was an unacceptable level of risk to site personnel.
Welsh Slate Ltd employs 55 people at Ffestiniog. Around half will be offered a move to Penrhyn Quarry in Bethesda.
"We deeply regret the mothballing of Ffestiniog Quarry," said Welsh Slate managing director Alan Smith.
Mr Smith said he wanted to thank the employees at Ffestiniog for their hard work in the past three weeks.
"However, the issues involved are beyond the control of the employees or the company," he added.
In a statement, the company said activities at the quarry had been suspended when subsidence was discovered.
"Following investigations with independent safety experts it was concluded there was an unacceptable level of risk to site personnel.
"We have therefore decided to mothball activities in the quarry, and roofing production will cease at the end of April 2010," it said.
The Health and Safety Executive have also been made aware of the situation.
The company added that around half the 55 employees would be offered jobs at Penrhyn Quarry in Bethesda where production levels would be increased.
No other operations or orders would be affected, it said.
Customers have the option of buying slate from the company's other quarries, Cwt-y-Bugail, Pen yr Orsedd and Penrhyn.
Options for re-commencing activities in Ffestiniog Quarry will be reviewed in the near future, the company added.