Lead isotope abundance studies on mineral occurrences in the British Isles and their geological significance

Abstract

Lead isotope abundances are reported for ninety-eight galena specimens from Great Britain and Ireland. The analyses were made with a solid-source mass spectrometer. Comparison analyses show excellent agreement with results from other laboratories using solid-source techniques, but differences of 1 to 5% may occur for individual abundances when comparison is made with laboratories utilizing the lead tetramethyl vapour technique. The model chosen for calculation of ages from the isotopic composition is that of Holmes and Houtermans, using the published values of Patterson for the isotopic comparison of primeval lead in iron meteorites and modern lead in ocean sediments. This model permits calculation of the parameters uranium-238/lead-204 and thorium-232/uranium-238 in the source of the ores, which may exhibit small regional differences. The Holmes-Houtermans model ages of three suites of galenas from south-west England, northern England and southern Norway give excellent agreement with published values of the absolute ages of genetically associated igneous rocks. Other models used for interpreting lead isotope abundances do not generally give such satisfactory agreement. The significance of the isotope data from Great Britain and Ireland is discussed regionally in terms of the age of mineralization as well as the possible correlation and origin of different deposits. Of the ninety-eight leads investigated, eighty-six are assumed to be normal and to obey the conditions of the Holmes-Houtermans model. The remaining twelve are B -type leads, as defined by Houtermans, i.e. the model ages are demonstrably older than the true age of mineralization. The main criteria for recognizing normal leads are, first, the close regional grouping of isotope abundances and, secondly, that the model age does not exceed the age of the enclosing sedimentary rocks on the basis of the most recently published geological time-scales of the fossiliferous strata. Detailed consideration of normal leads suggests the existence of six periods of mineralization in the British Isles, ranging in age from Lower Palaeozoic to Upper Mesozoic. The two most important and clearly defined groups are associated with the Caledonian and Hercynian orogenies, respectively. There is, as yet, no isotopic evidence for Tertiary mineralization in the British Isles. A discussion of the causes of normal lead isotope abundances indicates that the latter could be the result of large-scale crustal homogenization processes in continental geosynclinal-orogenic belts. However, there is not enough critical evidence to identify definitely the source of normal lead ores with either crust or mantle. B -type leads probably arise by comparatively localized remobilization and regeneration of lead from metamorphic basement complexes with high lead/uranium ratios, or low radiogenic lead content. The source of such leads frequently appears to be somewhat heterogeneous and ore solutions may not have the opportunity for extensive mixing before the site of deposition is reached. However, B -type leads in some cases give an approximation to the true age of the basement rocks from which they are derived. Processes of this type probably account for the 5-type leads in the north-west and central highlands of Scotland and in County Galway, western Ireland, where the occurrences are situated in metamorphic basement rocks. No cases have been definitely recognized within the British Isles in which lead has a negative, or anomalously young, model age ( J -type leads). The results presented in this paper do not support the view of Russell and co-workers that most vein-type deposits which have traversed sedimentary rocks exhibit J -type anomalies—a consequence of their suggestion that B -type leads, as defined by Houtermans, should be regarded as normal leads.

Human teeth as historical biomonitors of environmental and dietary lead: some lessons from isotopic studies of 19th and 20th century archival material

“…The latter, which is itself in agreement with other proxy values for atmospheric lead in 19th century Scotland, for example from dated ombrotrophic peat bogs (Farmer et al 1997), represents the average 206 Pb/ 207 Pb ratio of lead emitted to the atmosphere primarily via the smelting of indigenous lead ores and the combustion of coal (Farmer et al 1999 (Moorbath 1962;Sugden et al 1993), water in contact with domestic lead water pipes (Sugden et al 1993;Farmer et al 1994), and Scottish coal (Farmer et al 1999 Table 2) collected in 1993, the influence of lead of much lower 206 Pb/ 207 Pb ratio (e.g. the 1.04 of Australian lead, as used in combination with other sources in the leaded petrol additives introduced from the 1930s) is apparent.…”

“…The best straight line through the points is defined by 208 Pb/ 206 Pb = 2.983-0.735Â( 206 Pb/ 207 Pb), with r = )0.560. Also plotted are mean values of the corresponding ratios for: Australian lead ore (Chow et al 1975); Australian-lead-influenced leaded additives in petrol on sale in Scotland from 1989 to 1998 resulting in car-exhaust emissions of lead to the atmosphere (Farmer et al 2000); indigenous lead ore from Leadhills/Wanlockhead in Scotland (Moorbath, 1962;Sugden et al 1993); drinking water with domestic lead water pipes (Sugden et al 1993;Farmer et al 1994); and Scottish coal (Farmer et al 1999) the combustion of which resulted in emissions of lead to the atmosphere.…”

Isotopic age determinations on clay minerals from lavas and tuffs of the Derbyshire orefield

“…The lack of mineralization in the nearest Jurassic of the Midlands would suggest emplacement prior to Lower Liassic times. Additional confirmation is given by Moorbath (1962) who reports isotopic ages of Midland galenas as 180 + 40 m.y. (i.e.…”

“…An age of 220 m.y. from Millclose Mine was obtained from one sample only, and can only be substantiated by comparison with the galena ages obtained by Moorbath (1962) from Millclose, Riber, Moletrap and Wheel's Rake Mine whose mean age was 220 ±30 m.y. The youngest ages obtained which are considered reliable are around 190 m.y.…”

“…They advanced the hypothesis that a succession of distinct metasomatizing events related to hydrothermal activity, can be recognized by the numerous discrepant ages obtained from altered basic rocks. Prior to these findings, Moorbath (1962) had obtained from lead-isotope studies, a mean modal age for galenas of 220 ± 30 m.y., and postulated a Mesozoic age (Upper Triassic) for mineralization. Primary mineralization was indicated by Fitch, Miller & Williams (1967) as commencing around 290 and was associated with the Variscan (Hercynian) orogeny, with subsequent hydrothermal events at 280-265, 225, 170 and 130 m.y.…”