0361-0128/01/3239/435-17 $6.00 435 Introduction ANALYSIS of fluid inclusions in irradiated samples by noble gas mass spectrometry allows the simultaneous determination of both the naturally occurring noble gas isotopes ( 40 Ar, 36 Ar, 84 Kr, 129 Xe) and those formed by neutron activation of the halogens ( 38 Ar Cl , 80 Kr Br , 128 Xe I ). This technique is an extension of Ar-Ar methodology and can provide definitive information on fluid origins and the acquisition of salinity (Kelley et al., 1986; Bohlke and Irwin, 1992a, b; Turner and Bannon, 1992). In addition, complementary He isotope analysis of fluid in- clusions provides a conclusive means of assessing the involve- ment of mantle and crustally derived volatiles, as these have different 3 He/ 4 He ratios (e.g., Simmons et al., 1987; O’Nions and Ballentine, 1993; Kendrick et al., 2001a). 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 M. A. KENDRICK, †, * R. BURGESS, R. A. D. PATTRICK, AND G. TURNER Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, 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 re- leased inclusion fluids that were analyzed for noble gas isotopes ( 40 Ar, 36 Ar, 84 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 analy- ses were obtained from unirradiated fluorite. The 3 He/ 4 He ratio of the Pennine ore fluids (≤0.1 Ra, where Ra = atmospheric 3 He/ 4 He ratio of 1.4 × 10 –6 ) 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 mod- els. The low 40 Ar/ 36 Ar ratios with most <400 are compatible with surface recharge. The range in Br/Cl mol ra- tios is similar to that of other Mississippi Valley-type districts. Values of 2.6 to 4.0 × 10 –3 indicate that fluids from Hucklow Edge acquired salinity by the evaporation of seawater and were evaporated beyond the point of halite saturation (26 wt % NaCl equiv). In contrast, fluids from the Dirtlow rake replacement body contain >50 to 60 percent halite dissolution water (Br/Cl mol ratio = 0.57–0.92 × 10 –3 ). 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 <10 wt percent NaCl equiv and an air-saturated water 36 Ar concentration, compatible with dilution of these late fluids by 60 to 90 percent meteoric water. Elsewhere at Hucklow Edge high 36 Ar concentrations and 84 Kr/ 36 Ar greater than air-saturated water indicate that adsorbed noble gases have been acquired from fine-grained sedimentary material. The concentration of 40 Ar excess ( 40 Ar not attributable to either in situ radiogenic decay of 40 K or an atmospheric origin) of up to 9 × 10 –4 cm 3 cm –3 H 2 O is higher than can be explained by fluid interaction with only the K-poor host limestone, suggesting significant water-rock interaction with the overlying Namurian Shale. This is supported by high I/Cl mol ratios of 15 to 120 × 10 –6 , favoring fluid interaction with organic-rich sediments. The concentration of 4 He in fluids included within Dirtlow rake fluorite is variable, lying in the range of 1 to 50 × 10 –4 cm 3 cm –3 H 2 O. These concentrations have been used to infer a premineralization fluid residence time of >40 m.y., a time scale compati- ble with models in which fluids were derived from small local basins such as the Widmerpool and Edale Gulfs. † Corresponding author: e-mail, mark.kendrick@ngu.no *Current Address: Geological Survey of Norway, Leiv Erikssons Vei 39, 7491 Trondheim, Norway. Economic Geology BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS VOL. 97 May–June 2002 NO.3 Vol. 97, 2002, pp.435–451