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.