A reinterpretation of the mineralisation processes involved in the formation of the Tomnadashan sulphide deposit, Loch Tay, Scotland, UK

The Tomnadashan sulfide 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 mineralization 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 contextualized these ages within a paragenetic interpretation. Our results show that the age of the molybdenite is c. 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 mineralization event at Loch Tay. Our new data and literature review suggest that although Tomnadashan is a magmatic-related ore deposit, the porphyry that crops out is unlikely to have provided the mineralizing fluids associated with the mineralization. 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 mineralization is coeval with gold mineralization at Cavanacaw in Northern Ireland, giving rise to the possibility of a previously unrecognized mid-Silurian magmatic–hydrothermal episode of gold and base metal mineralization throughout the Grampian Terrane.