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\subsection{Fluid isotope compositions, sources and mixing processes}  In Figure 6(d) we have plotted T($\Delta$_{47}) versus the isotopic composition of the fluid (fluid $\delta$^{18}O) that is in equilibrium with the vein calcite. This composition is determined using the Kim and O'Neill (1998) calibration of the calcite-water fractionation factor as a function of temperature. The data defines a two component mixing line with a hot and cool end member. The hot end member has a temperature >100$^{\circ}$C and a $\delta$^{18}O value of approximately +5 +5‰_{VSMOW} whilst the cool end member has a temperature of 30-50$^{\circ}$C and a $\delta$^{18}O value of -2 to -4‰_{VSMOW}. This range of oxygen isotope compositions lies within the range expected for meteoric groundwaters and sedimentary formation waters \citep{Sheppard_1981}.   We are reassured that the derived fluid $\delta$^{18}O values are consistent with our understanding of the isotopic compositions of naturally occurring fluids and suggests that the clumped isotope temperatures are robust. Foe example were the calcite temperatures to have been as high as the maximum reported fluid inclusion homogenization temperatures of 240$^{\circ}$C then the derived fluid $\delta$^{18}O values would need to have been in excess of 18 - 20‰_{VSMOW}. Such values are in excess of any reported data for modern geothermal and formation waters.