An Upper Jurassic age for the Alston block (North England) fluorite deposits from single-crystal Sm–Nd geochronology

The age of mineralisation in the North Pennine orefield is controversial, with age estimates ranging from Permian to Paleocene times. Here we present Nd-isotope data from a single large crystal of fluorite from the Boltsburn vein in Weardale that define an errochron with an apparent age of 155±21 Ma, indicating formation in the Upper Jurassic. Nd-isotope growth curves based on these data and published fluorite data from several other Alston block mineral veins intersect growth curves for the basement rocks (Skiddaw Group mudstones and Weardale granite) at the same age, confirming that the date is robust. Our proposed date coincides with a major phase of extension in the North Sea and with the time at which global sea level reached its highest point in the Phanerozoic, possibly submerging the Alston block. Upper Jurassic extension may have reactivated older fractures of Variscan origin on the Alston block and allowed seawater to penetrate deep into the basement rocks to feed the hydrothermal circulation responsible for the mineral deposits. Mineralisation followed a long period of tectonic inactivity when global sea level was generally low, and these two factors combined to inhibit hydrothermal circulation through the basement rocks. The temperature of the Weardale granite would have increased by radioactive decay during this period of inactivity.