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\subsection{Fluid isotope compositions, sources and mixing processes}  In Figure 6(d) we have plotted T($\Delta$_{47}) T($\Delta_{47}$)  versus the isotopic composition of the fluid (fluid $\delta$^{18}O) $\delta^{18}$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 $\delta^{18}$O  value of approximately +5‰_{VSMOW} +5‰$_{VSMOW}$  whilst the cool end member has a temperature of 30-45$^{\circ}$C and a $\delta$^{18}O $\delta^{18}$O  value of -2 to -4‰_{VSMOW}. -4‰$_{VSMOW}$.  The low temperature end member has an isotopic composition that is typical of meteoric groundwaters being depleted in ^{18}O $^{18}$O  with respect to ocean water. We suggest that the range 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 recharge. With an elevated geothermal gradient of between 40 to 50$^{\circ}$C.km^{-1} 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 $\delta^{18}$O  value of +5‰_{VSMOW} +5‰$_{VSMOW}$  that is typical of sedimentary basin and oil field brines (Sheppard, 1986). It is most probably 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 water isotope composition of +5‰_{VSMOW} +5‰$_{VSMOW}$  and that expected for a fluid in isotopic equilibrium at 100$^{\circ}$C with shales containing clay minerals. In Figure 7 we have replotted the data of Figure 6(d) and included the trajectory of evolution of pore waters in a shale assuming: (i) the pore waters are connate and have an initial marine isotope composition of 0‰_{VSMOW}; 0‰$_{VSMOW}$;  (ii) the system eveolves evolves  under closed system conditions with a porosity of 20\%; (iii) isotopic equilibrium is attained between the clay minerals and pore water, and; (iv) the initial $\delta$^{18}O $\delta^{18}$O  value of the kaolin in the shale is +20‰_{VSMOW}. +20‰$_{VSMOW}$.  The oxygen isotope fractionation factor for the kaolin-water system is taken from \citet{Sheppard:1996vq}.