>I have been searching for data on the above but have drawn a complete
blank. I am trying to determine a comparison of the percentages of silver
per tonne of lead ore contained in the various lead ore fields in the UK. In
other words, Derbyshire, Mendip,
>S. Wales, Aberystwyth, Devon ect.
Trevor,
For a modern geological overview see Ridgeway, J. M., Silver, (Mineral
Resources Consultative Committee, Mineral Dossier 25, London, 1983). But you
are not going to get the full story from one publication. Silver yields are
extremely variable. Many of the mining fields regarded as low in silver in
the modern period did in fact produce significant amounts of silver in the
medieval - the Northern Pennines is a classic example, as is Mendip and
other parts of upland Britain.
Weathering and the action of surface waters on the upper parts of mineral
deposits is primarily responsible for the enhanced silver content of the
base ores accessible to the medieval miner. Supergene action through
leaching by acidic waters partially removed the primary minerals in
solution, leaving a concentration of silver in the unaffected portion. Below
the area of leaching was the oxidised zone, in which the base metal deposits
were enriched with carbonate minerals from the zone above. The carbonates
had a lower silver content but that of the sulphide ores, i.e.. galena,
which remained unoxidised was enhanced. Lower again, below the water
table, was the 'zone of secondary sulphide enrichment' where the base
metallic elements leached from above, but deprived of oxygen, combined with
the primary sulphides to form high grade base ores without enhancing the
silver content.
Oxidised and secondary sulphide enrichment is particularly prominent in
deposits in the South-West of England, largely unaffected by glacial action.
Peter
______________________________________________
Peter Claughton, Blaenpant Morfil, Rosebush, Clynderwen,
Pembrokeshire, Wales SA66 7RE.
Tel. 01437 532578; Fax. 01437 532921; Mobile 07831 427599
University of Exeter - School of Historical, Political and Sociological Studies
(Centre for South Western Historical Studies)
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