Isotopic evidence of the depositional environment of Late Proterozoic stratiform barite mineralisation, Aberfeldy, Scotland

Abstract

Sulfur, carbon and oxygen stable isotope analyses of sulfides, barite, quartz, magnetite and carbonate and initial ⁸⁷Sr/⁸⁶Sr ratios (I_Sr) of barite have been undertaken on representative samples from barite horizons mainly in the Foss sector of the Late Proterozoic, Middle Dalradian stratiform mineralisation, near Aberfeldy in the central Scottish Highlands. The main objective of the study was to use new isotope studies to test a SEDEX depositional model which involved mixing of reduced hydrothermal solution with contemporaneous anoxic and oxygenic seawater.

The assemblages studied were: massive pyrrhotite+pyrite +sphalerite+galena (scarce, and probably formed sub-seafloor only); barite +pyrite (common); and barite +magnetite (relatively uncommon). The sulfur isotope results (gd¹⁸SCDT) for the Foss samples, mainly from the open pit area, are: pyrrhotite and associated sulfides ( + 21 to + 24‰); pyrite (+ 27‰) with barite (+40‰); and barite (+35 to +38‰) with magnetite. Oxygen isotope analyses (δ¹⁸0_VSMOW) are: about + 14‰ for barite with pyrite and about + 16‰ for barite with magnetite. The variation in I_Sr is small; I_Sr being about 0.715 for barite with pyrite and about 0.714 for barite with magnetite. δ¹⁸O of magnetite is around +7.5‰.

The observed assemblages and the analytical results are considered to be reasonably consistent with the mixing model but with oxygenic seawater playing a very minor and localised role. The massive sulfide has inherited a δ³⁴S value of about +22‰ from hydrothermal sulfide. The end-member isotopic compositions of the sulfate resulting from mixing are likely to have been: sulfate from hydrothermal sulfide oxidised using oxygen dissolved in seawater, $\text{δ}{}^{34}\text{S}\text{=+22‰}$ and $\text{δ}{}^{18}\text{O}\text{= +25±5‰}$ and seawater sulfate, $\text{δ}{}^{34}\text{S}\text{= +40‰}$ and $\text{δ}{}^{18}\text{O}\text{= +14‰}$. A plot of δ³⁴S against δ¹⁸O for barite sulfate gives a line with a correlation coefficient of 0.766 (n=10), which is significant above the 99% level although most points cluster close to the seawater end of the expected variation. The I_Sr values of the barite suggest that the dominant source for this Sr was from the hydrothermal solution. There is no evidence for a significant contribution to sulfide by bacterial reduction of sulfide and this lack of a geochemical barrier of bacteriogenic sulfide could account for the dispersal of hydrothermal Pb and Zn and thus the apparent lack of economic base metal concentration.