Nick,
Wilson, in 'The Lead Zinc, Copper & Nickel Ores of Scotland' (1921), states "A lead mine is said to have been worked for argentiferous galena near Croggan, but all signs of it are now obliterated. In all probability it is the one mentioned be Sir A Murray of Stanhope in ' The Interests of Great Britain Considered', published in 1740".
Landless, in his Gazeteer of Scottish mines (1993), writes "No sign of this mine now remains".
Probably best to give up the search & retire to the nearest tea shop to drown your disappointment in a pot of tea & a large sticky bun!