Mixing matters: resolving the fluid sources of metasediment-hosted auriferous veins in the Grampian Terrane of the Northern British Isles

Throughout the Grampian Terrane of Britain and Ireland, there are multiple occurrences of vein- hosted gold deposits, some of which have commercial value. However, it is unclear whether the fluid source of several auriferous veins in the Grampian Terrane is magmatic or metamorphic. The veins are structurally controlled and mineralogically simple, although the fluid inclusion data and regional geochemical signatures are typical of magmatic mineralization processes. To evaluate the genesis of these more unusual occurrences, we conducted sulfur and lead isotope studies on gold deposits around Loch Tay, which is situated in Central Scotland, and compared the results to regional data from the Grampian Terrane. Pb isotope data from galena in several Scottish gold deposits support a mantle-derived source, although the sulfur isotope profiles suggest that at some localities mantle-derived magmatic fluids have mixed with crustal fluids, obscuring the isotopic signature. The results from our study represent an important case study for other researchers of the challenges that can arise when interpreting isotopic datasets associated with mineralization processes involving fluid mixing, which has been influential in determining the isotopic signatures of gold deposits in Scotland.