Lead isotope ratios of 36 galenas from the Northern Pennines

B. Scaife, B.A. Barreiro1, J.G. McDonnell2 and A.M. Pollard2

1: NIGL, c/o BGS, Keyworth, Nottingham, NG12 5GG, UK
2: Dept. of Archaeological Sciences, University of Bradford, BD7 1DP, UK

Abstract

The lead isotope ratios obtained from 36 galenas collected from 8 sites in the Northern Pennines, England indicate that Lead Isotope Analysis is unlikely to prove to be of more than limited value in the provenacing of archaeological artefacts in this area.

Introduction

There are no proven Bronze Age lead mines in the Northern Pennines despite evidence of the use of lead throughout the period (Craddock, 1994). Any tentative associations of lead mining activity with pre-Roman periods are outside the Northern Pennine region. It is reasonable, however, to make the assumption that there was Roman and Early Mediaeval extraction of lead from the Northern Pennine orefield. The existence of Roman pigs of lead with inscriptions indicating a Brigantian origin and the possible existence of contemporary furnaces at Dacre in Nidderdale (Shepherd, 1993) make it almost certain that the lead ore came from this region. Indeed as the Romans used lead for a wide range of purposes in Britain, including water distribution, roofing, and coffins (Shepherd, 1993), it would seem odd if they ignored one of the largest orefields in the country. The abundant surface outcrops of the veins could have lead them to the region and these could easily be exploited by hushing (building up a head of water above the outcrop using a dam and then flooding the outcrop to wash away the overburden). There is, however, no undisputed evidence of Roman or prehistoric lead mining in the Northern Pennines and that for the Early Mediaeval period is little better. Both general texts (e.g. Cranstone, 1992; Shepherd, 1993) and specialist conference reports (e.g. Barker & White, 1992; Pickin, 1992) bemoan the lack of hard evidence. Obviously, Mediaeval boles must have been relatively close to the origin of the ores but they also had to be situated with regard to the supply of fuel and wind. Similarly, the Roman furnaces Dacre in all likelihood processed ore from the Greenhow area five miles away but could have also served other ore sources; possibly even placer deposits now exhausted.

There would seem to be some value to any tool that would potentially give us more information about the location of early mining in this region. If we could identify parts of the region that were more likely candidates for ancient sites then we make our task of finding them simpler. The rationale behind this study was to determine if there are any distinctive patterns in the lead isotope patterns of galenas collected from the Northern Pennines. It would be useful to discover if there are any distinctive sub-regions based on the isotope ratios or if particular veins or mines are likely to exhibit 'compact' or unusual signatures that could exclude them from consideration when looking at artefacts of known isotopic composition but unknown origin.

The exploitable mineralisation of the Northern Pennines is in the form of either nearly-vertical veins formed along fissures in the surrounding rock or horizontal metasomatic flats. The metasomatic flats form when mineralising fluids from a vein gain access to a flat bed of limestone susceptible to replacement. A single oreshoot may form a network of veins and flats but the ground between these will be not be mineralised to any appreciable extent, certainly not to a commercial level. Veins tend to form the majority of workable deposits (Dunham, 1990). The ores were deposited by circulating hydrothermal fluids from at least two sources and have a complex mineralogy; buried heat sources and the directions of fluid flow tend to affect the distribution of isotope patterns in lead and sulfur. The orefield is split into distinctive geological areas or blocks: the Askrigg and Alston blocks. The Askrigg block runs from Craven to as far north as Stainmore Pass, between Bowes in Durham and Brough in Cumbria. Although having many features in common the underlying rocks through which the mineralising fluids would have passed are different for each block (Dunham & Wilson, 1985).

Thirty-six analyses were made to estimate the isotopic variability of the orefield on as many scales as possible and thus to gain some information on the possible value of lead isotope analysis for the provenancing of archaeological objects in this area.

Sampling strategy

In order to be totally sure of the provenance of reference material, samples should be taken directly from veins in situ. Given that many old mine workings are now inaccessible this was not possible in all cases. Because a wide coverage of the area was desirable, both geological blocks of the region were sampled. It was also important to determine variation on a smaller scale, and hence several samples were taken from each mine visited. Two veins in particular (Coldstones Sun and Waterhole) had a significant number of samples (seven and eight) taken to investigate the level of variation at this very small scale.

Within the Askrigg block samples were taken from Gillfield Mine in Nidderdale, from two spoilheaps on Grassington Moor and from Bunting (Bunton in some sources) Level, Gunnerside, Swaledale. At Gillfield Mine seven samples were taken from Coldstones Sun Vein, over a distance of approximately 20m, and seven samples from Waterhole Vein, over a distance of approximately 75m. On Grassington Moor samples were taken from spoilheaps by two shafts (Sarah and Cottingham). Descriptions of workings in Gill (1993) and Dunham and Wilson (1985) suggest that these shafts were good candidates as they probably served individual veins (Cavendish Vein and Middle Vein respectively). Five samples were taken from the spoil heap and dressing area around Sarah Shaft; three samples were taken from the dressing area by Cottingham Shaft. The Old Rake Vein was sampled at Bunting Level, on Gunnerside Gill, Swaledale.

Three sites were sampled in the Alston block. At Old Carr Level, near the village of Nenthead in Cumbria, samples were taken from Carr's Vein from an area about 5m square. Middlegrove Vein, close to the Killhope Mine in Weardale, was exploited where it outcrops at the surface. Samples were taken from the large boulders that are still lying on the surface. There are many old mine workings along Great Eggleshope Beck in Teesdale, however most appeared unsafe to enter, therefore samples were taken from spoilheaps along the valley.

Table 1. Locations of sites sampled


Sample code Geological Block Area Location Grid reference
GFS Askrigg Nidderdale (Coldstones) Sun Vein, Gillfield Mine SE 115 649
GFW Askrigg Nidderdale Waterhole Vein, Gillfield Mine SE 115 649
SAR Askrigg Grassington Spoil heap, Sarah Shaft (Cavendish Vein) SE 673 036
CTH Askrigg Grassington Spoil heap, Cottingham Shaft (Middle Vein) SE 670 038
BUN Askrigg Gunnerside Old Rake Vein, Bunting (Bunton in some sources) Level NY 940 011
OCL Alston Nenthead Carr's Vein, Old Carr Mine NY 785 430
MGV Alston Weardale Middlegrove Vein, Killhope NY 821 432
GEB Alston Teesdale Spoil heaps, Great Eggleshope Beck NY 975 300

Measurement of lead isotope ratios

For each sample, one to three milligrams were dissolved first in nitric, then in hydrochloric acid. They were evaporated and taken up in five millilitres of deionised water: no further chemical treatment was undertaken. One microlitre of this solution was loaded onto a previously outgassed rhenium filament with 1M phosphoric acid and silica gel. The samples were analysed automatically on a MAT 262 mass spectrometer in static mode. Within-run precision, based on counting statistics, was better than 0.01% for each isotope ratio. Of the thirty six samples measured, nineteen filaments were run more than once, the results presented in table 3 are the means of multiple runs.

Twelve separate loadings of 150ng of the NBS 981 standard were run at the same time as the galenas. Six of the loadings were measured twice by the mass spectrometer. The uncertainties in the measurements of NBS 981 (table 2) must be taken as the minimum estimate of reproducibility of the samples. (The variation exhibited between different runs of the same sample was well within the reproducibility errors reported.) The errors are highly correlated in all ratios. Measurement of lead isotope ratios on the mass spectrometer is affected by two main sources of error: uncertainty due to mass fractionation during analysis, and error in the measurement of 204Pb due to poorer counting statistics on the least abundant lead isotope. The high correlation between 208Pb/206Pb and 207Pb/206Pb suggests that mass fractionation during analysis is the major source of variation. The observed correlation due to mass fractionation is not the same as the overall correlation between all the galena data, implying that the range of observed variation in the data set cannot be attribute wholly to analytical error and must be due to real, natural variations.

Table 2. Measurement errors based on eighteen repeats of NBS 981


Ratio % error (2σ) Correlation with 207Pb/206Pb error Slope with 207Pb/206Pb error Correlation with 206Pb/204Pb error Slope with 206Pb/204Pb error
208Pb/206Pb 0.1272 0.999 4.59
207Pb/206Pb 0.0656
206Pb/204Pb 0.1324 0.946 35.22
207Pb/204Pb 0.1958 0.994 1.34
208Pb/204Pb 0.2569 0.988 4.15

The data

The data have Cumming-Richards model Pb ages ranging between 102-247Ma, suggestive of a Permo-Triassic age. These dates tend to confirm the developing opinion for the date of this mineralisation obtained from various techniques (e.g., Shepherd et al., 1982; Dunham and Wilson, 1985; Dunham, 1990). The majority of analyses have isotopic compositions which are similar to those of galenas from many other areas such as the South Pennines, the Craven Basin and some of the more radiogenic ones from the Lake District (Jones et al., 1991; Haggerty et al., 1996). Whilst these areas have a similar geological setting and history to that of the North Pennines, deposits from Wales do not but there are ore deposits there which have isotopic compositions overlapping those of the North Pennines (Fletcher et al., 1992). Indeed, comparison of the data presented here with the British data of Rohl (1996) shows that the isotopic composition of the North Pennine galenas overlaps the isotopic space of many other regions. If the data is plotted it is possible to discern a number of smaller groups within it. These could be the result of a number of discrete mineralising episodes. They could also, however, be the result of sampling small patches of a regionally varying continuum of isotopic composition. Whichever of these scenarios is correct, it remains the case that the Northern Pennine orefield shows great variation in its isotope ratios at all scales. At the finest level studied here galenas from each of the two veins sampled at Gillfield Mine exhibit almost half the variation observed for the entire region. This fact combined with the overlap of the whole North Pennine isotope field with those of many adjacent (and some more distant) regions suggests that lead isotope analysis will be of limited value in provenance studies in this region.

Table3. Lead isotope ratios of Northern Pennine Galenas.
(Figures are the average where more than one run was made. The data have been corrected for mass fractionation by a factor of 1.0 per mil per amu)


Sample no Number of runs 208Pb/206Pb 207Pb/206Pb 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb
GFS-1 2 2.0815 0.84520 18.488 15.626 38.482
GFS-2 1 2.0786 0.84369 18.515 15.621 38.486
GFS-3 1 2.0774 0.84172 18.568 15.629 38.574
GFS-4 1 2.0810 0.84454 18.506 15.629 38.511
GFS-5 1 2.0776 0.84227 18.563 15.635 38.566
GFS-6 2 2.0830 0.84560 18.478 15.625 38.489
GFS-7 3 2.0802 0.84323 18.562 15.652 38.613
GFW-1 2 2.0766 0.84135 18.582 15.634 38.587
GFW-2 2 2.0833 0.84605 18.474 15.630 38.486
GFW-3 1 2.0731 0.84118 18.568 15.619 38.494
GFW-4 1 2.0717 0.84128 18.555 15.610 38.440
GFW-5 2 2.0733 0.84190 18.558 15.624 38.477
GFW-7 2 2.0822 0.84531 18.488 15.628 38.495
GFW-8 2 2.0831 0.84612 18.475 15.632 38.485
SAR-1 2 2.0801 0.84579 18.436 15.593 38.349
SAR-2 1 2.0755 0.84093 18.564 15.611 38.530
SAR-3 1 2.0804 0.84418 18.508 15.624 38.504
SAR-4 1 2.0752 0.84129 18.562 15.616 38.520
SAR-5 1 2.0777 0.84355 18.491 15.598 38.418
CTH-1 1 2.0732 0.84097 18.562 15.610 38.483
CTH-2 1 2.0738 0.84099 18.559 15.608 38.487
CTH-3 1 2.0786 0.84388 18.492 15.605 38.438
BUN-1 2 2.0862 0.84847 18.425 15.633 38.439
BUN-4 2 2.0822 0.84531 18.482 15.623 38.484
OCL-2 1 2.0875 0.84994 18.379 15.621 38.366
OCL-3 2 2.0876 0.85015 18.365 15.613 38.339
OCL-4 2 2.0894 0.85036 18.391 15.639 38.426
OCL-5 2 2.0882 0.85029 18.376 15.625 38.373
MGV-1 2 2.0872 0.84808 18.444 15.642 38.497
MGV-2 2 2.0821 0.84544 18.485 15.628 38.487
MGV-3 2 2.0845 0.84575 18.502 15.648 38.567
MGV-4 2 2.0939 0.84639 18.645 15.781 39.040
GEB-1 2 2.0880 0.84958 18.389 15.623 38.397
GEB-2 1 2.0854 0.84815 18.406 15.611 38.384
GEB-3 1 2.0842 0.84739 18.426 15.614 38.404
GEB-4 1 2.0875 0.84808 18.444 15.642 38.502

Acknowledgements

We are grateful to Bob Willan for assistance with fieldwork. One of the authors (BS) was supported during this research by a studentship from NERC.

References

Barker, L. & R. White, 1992. Early smelting in Swaledale and Arkengarthdale: a further look, in eds. L. Willies & D. Cranstone, Boles and Smeltmills, pp. 15-18. Historical Metallurgy Society, Matlock Bath.

Cranstone, D., 1992. Monuments Protection Programme: The Lead Industry. Step 1 Report. English Heritage, internal report.

Dunham, K.C., 1990. Geology of the Northern Pennine Orefield. Volume 1 Tyne to Stainmore, HMSO, London.

Dunham, K.C. & A.A. Wilson, 1985. Geology of the Northern Pennine Orefield. Volume 2 Stainmore to Craven, HMSO, London.

Fletcher, C.J.N., I.G. Swainbank & T.B. Colman, 1992. Metallogenic evolution in Wales: constraints from lead isotope modelling. Journal of the Geological Society 150: 77-82.

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This is a stable document
Title: Lead isotope ratios of 36 galenas from the Northern Pennines
Authors: Brett Scaife
B.A. Barreiro
J.G. McDonnell
& A.M. Pollard
URL: http://www.brettscaife.net/lead/npennine/npennine.html
Version: 1.2
Date: 22 July 2008
Changes to previous version: Typographical errors corrected. Contact details for McDonnell updated

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