$1million is being spent on drilling exploratory boreholes, which could lead to the creation of a new lead and zinc mine in the North Penines.
If the project goes ahead, up to 500 jobs could be created at the mine just outside the village of Nenthead, near Alston, just a few miles from the border between Cumbria, County Durham and Northumbria.
One 350 metre borehole has already been drilled by the mineral exploration company Minco Plc.
Two further boreholes are planned, which could rise to eight, depending on what the initial ones reveal.
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The 12.5 hour walkout by 63 GMB members got under way at 6am this morning and will continue until 6.30pm this evening.
37-year-old Graeme Haddon pleaded guilty to the offence which happened in his home village of Lamancha near West Linton at the weekend.