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To help us understand these processes we have used clumped isotope thermometry to determine the temperature at which hydrothermal vein calcite precipitated in dilation voids of a Variscan strike-slip fault in the southern Pennines of the United Kingdom. The veins are associated with Pb-Zn and fluorite mineralisation and the fault was extensively worked as an economic deposit. Located at the margin of a lower Carboniferous platform on which shelf carbonates were deposited, the fault separates the platform from a deep water basin infilled with deep water facies limestones and shales of lower to upper Carboniferous age.   Clumped isotope geothermometry relies on the fact that the heavy isotopes ^{13}C and ^{18}O are ordered in the carbonate lattice with the degree of ordering being an inverse function of temperature. This is a result of the greater stability of the ^{13}C-^{18}O bond compared to bonds involving either no, or a single isotopic substitution. As temperature increases the isotopes tend towards a more random or stochastic distribution. Measurement of the degree of ordering allows us to estimate the temperature at which the distribution of isotopes in the calcite structure are locked in. This is analogous to the concept of a closure temperature for radiogenic isotopes or cation ordering in minerals. In this study this temperature is taken as the precipitation temperature of the calcite. A key advantage of the method is that the temperature estimate is based on the distribution of carbon and oxygen isotopes within a single phase and not on the partitioning of oxygen isotopes between calcite and it’s parent fluid as in the conventional oxygen isotope geothermometer. Thus combining the clumped isotope temperature (T($\Delta$_{47})) with the bulk oxygen isotope composition of the carbonate we can use published fractionation factor calibrations to calculate the isotopic composition of the parent fluid. Using these techniques we find:  (i) the calcite precipitated at temperatures between 40° and 100°C.  (ii) the parent fluids range in isotopic composition from -4‰VSMOW -4‰_{VSMOW}  to +5‰VSMOW +5‰_{VSMOW}  and represent mixtures of a cool, meteoric water and a more evolved formation water. (ii) the temperature at which the calcite precipitated is a conservative tracer for the fluid mixing. This implies that heat is rapidly advected as the hydrothermal fluid flow is focussed along the fault plane.  (iii) The calcites exhibit zoned development characterised by cyclic and pulsed evolution of precipitation temperatures and fluid compositions as a result of varying mixing ratios of the two fluid end-members.  (iv) The fluids have evolved under low fluid:rock ratios.