Jobs at a Gwynedd slate quarry are under threat after half a million tonnes of stone fell off a rock face, it has emerged.The Health and Safety Executive (HSE) is investigating last month's incident at the Penrhyn quarry in Bethesda.Parts of the quarry have closed, and around 90 of the 200 workers have had hours reduced, with others redeployed.Welsh Slate said it was looking for volunteers for redundancy but hoped to avoid compulsory job losses.A HSE spokesman said the landslip on 27 April was reported to have happened at 0530 BST, with no injuries resulting from it.