Understanding metallogeny around Loch Tay, Scotland: insights from isotope geochemistry and petrographic studies

The southern margin of Loch Tay, which encompasses a 100 km2 portion of the Scottish Grampians, hosts several polymetallic veins that consistently trend 120-140 ° and were initiated contemporaneously during the Caledonian Orogeny. At Lead Trial and Tomnadashan, the veins crosscut granitoids, whereas several other veins throughout the region are hosted in metasediments. To characterise some aspects of the mineral system (the ages of mineralisation and magmatism, and potential sources of mineralising fluids), an integrated approach involving geochronology (Re-Os, U-Pb), isotope geochemistry (δ34S, Pb) and petrography was implemented.

Petrographic characterisation has enabled the construction of novel parageneses for Lead Trial, Tomnadashan, and the Glen Almond Vein. The paragenesis of Tomnadashan demonstrated that molybdenite occurs in the second paragenetic stage out of six, indicating that the Re-Os dates (c. 425-417 Ma) pertain to the initial mineralisation processes at this locality. CA-ID-TIMS U-Pb dates of zircons from four granitoids throughout the Loch Tay region yielded ages of c. 420-418 Ma. These datasets demonstrate that mineralisation and magmatism around Loch Tay are coeval. This interpretation is supported by the δ34S values and Pb isotope ratios recorded from the mineralisation at Lead Trial and Tomnadashan, which are typical of magmatic-hydrothermal processes. However, the δ34S and Pb datasets also indicate a crustal contribution to the mineralising fluids associated with the metasediment-hosted veins.

The results have been used to interpret a metallogenic model for Loch Tay; mineralisation around Lead Trial and Tomnadashan involved a fluid that was derived from I-type granites. However, the genesis of the metasediment-hosted veins was linked to a fluid that developed as a result of mixing between magmatic and crustal sources of sulphur (e.g. the Dalradian Supergroup). The isotope datasets are indicative of at least two mineralisation events around Loch Tay, although the age of the metasediment-hosted veins remains unknown.