Hydrothermal Fluid Origins in a Fluorite-Rich Mississippi Valley-Type District: Combined Noble Gas (He, Ar, Kr) and Halogen (Cl, Br; I) Analysis

摘要

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 fur 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 (~(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 analyses were obtained from unirradiated fluorite. The ~3He/~4He ratio of the Pennine ore fluids (<=0.1 Ra, where Ra = atmospheric ~3He/~4He ratio of 1.4 X 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 models. The low ~(40)Ar/~(36)Ar ratios with most <400 are compatible with surface recharge. The range in Br/Cl mol ratios is similar to that of other Mississippi Valley-type districts. Values of 2.6 to 4.0 X 10~(-3) indicate that fluids from Hucklow Edge acquired salinity by the evaporation of seawater and were evaporated beyond the point of halite saturation (26w percent 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 X 10~(-3)). The salinity of fluids included in the main stage of the Hucklow Edge mineralization (avg of 22 w percent 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 X 10~(-4) cm~3cm~(-3)H_2O 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 X 10~(-6), favoring fluid interaction with organic-rich sediments. The concentration of ~4He in fluids included within Dirtlow rake fluorite is variable, lying in the range of 1 to 50 X 10~(-4) cm~3cm~(-3)H_2O. These concentrations have been used to infer a premineralization fluid residence time 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.