Hydrothermal Fluid Origins in a Fluorite-Rich Mississippi Valley-Type District: Combined Noble Gas (He, Ar, Kr) and Halogen (Cl, Br, I) Analysis of Fluid Inclusions from the South Pennine Ore Field, United Kingdom

Abstract

The South Pennine (Peak district) ore field, United Kingdom, is host to several fluorite-rich Mississippi Valley-type deposits and was chosen for study as an analogue to the Illinois-Kentucky fluorspar district, United States. The study was undertaken to test the idea that the large-scale carbonate dissolution and fluorite-rich mineralogy, characteristic of both districts, may require a magmatic source of HF. The data also provide constraints for water-rock interactions and potential fluid sources relevant to the local geology of the South Pennine ore field. Samples were studied from two of the largest and most economically important deposits within the South Pennine ore field, the Hucklow Edge and Dirtlow rakes. In vacuo crushing of irradiated calcite samples released inclusion fluids that were analyzed for noble gas isotopes ( Ar, Ar, Kr) and the halogens (Cl, Br, I). This was achieved simultaneously by noble gas mass spectrometry (Ar-Ar methodology) and was combined with fluid inclusion thermometric salinity data to determine noble gas concentrations. In addition He isotope analyses were obtained from unirradiated fluorite. The He/ He ratio of the Pennine ore fluids (≤0.1 Ra, where Ra = atmospheric He/ He ratio of 1.4 × 10 ) is significantly lower than the maximum value recorded in the Illinois-Kentucky fluorspar district (0.35 Ra: Kendrick et al., 2002, this volume), indicating a crustal fluid source and supporting existing basinal brine models. The low Ar/ Ar ratios with most 50 to 60 percent halite dissolution water (Br/Cl mol ratio = 0.57-0.92 × 10 ). The salinity of fluids included in the main stage of the Hucklow Edge mineralization (avg of 22 wt % NaCl equiv) places an upper limit on the extent of meteoric recharge at 15 vol percent. A secondary vein present at Hucklow Edge has a lower salinity of 40 m.y., a time scale compatible with models in which fluids were derived from small local basins such as the Widmerpool and Edale Gulfs.