The Tomnadashan sulphide deposit, which is located on the southern margin of Loch Tay (Scotland, UK), was mined for copper during the 19th century. The genetic processes at Tomnadashan remain poorly understood, and the mineralisation has never been dated. To gain an improved understanding of this mineral system, we have dated the molybdenite at Tomnadashan using the Re-Os chronometer. Furthermore, we have contextualised these ages within a paragenetic interpretation.

Our results show that the molybdenite is ca. 423-419 Ma, and it occurs early in the paragenesis (the second stage out of six). Based on the paragenesis of molybdenite, this age is likely to reflect the initial Caledonian mineralisation event at Loch Tay. Our new data and literature review suggest that whilst Tomnadashan is a magmatic-related ore deposit, the outcropping porphyry is unlikely to have provided the mineralising fluids associated with the mineralisation. A concealed intrusion or granitic dykes within the porphyry may be the source of the magmatic-hydrothermal fluids. The age data indicate that the Tomnadashan mineralisation is coeval with gold mineralisation at Cavanacaw in Northern Ireland, giving rise to the possibility of a previously unrecognised mid-Silurian magmatic-hydrothermal episode of gold and base metal mineralisation throughout the Grampian Terrane.

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