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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‰_{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 The low temperature end member has an isotopic composition that is typical  of oxygen isotope compositions lies within meteoric groundwaters being depleted in ^{18}O with respect to ocean water. We suggest that  the range expected for of minimum temperatures (30-50$^{\circ}$C) is characteristic of the depth of burial at the time of mineralization and that the near surface hydrogeology of the platform carbonates was dominated by  meteoric groundwaters recharge. With an elevated geothermal gradient of between 40 to 50$^{\circ}$C.km^{-1} this suggests that mineralization occurred at a depth of 0.8 to 1km. Such an estimate is in good agreement with previous studies (references).  The high temperature end member has a temperature greater than 100$^{\circ}$C  and a $\delta$^{18}O value of +5‰_{VSMOW} that is typical of  sedimentary basin and oil field brines (Sheppard, 1986). It is most likely sourced from depth within the Visean-Namurian shales of the Bowland-Hodder unit in the Edale Gulf. Likely minimum source depths lie between 2 to 3 km. An interesting comparison can be made between the derived  formation waters \citep{Sheppard_1981}. water isotope composition of +5‰_{VSMOW} and that expected for a fluid in isotopic equilibrium at 100$^{\circ}C with kaolinite.  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. For 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 \citep{Sheppard:1986ta}.