At 15:00 13/04/2008, John Mason wrote:
>Many Central Wales veins are multiphase. First active in Devonian times,
>there were phases of reactivation in the Carboniferous and Permian. Later
>mineralisation along existing and new fractures deposited galena that is
>pretty much inclusion-free and hence 5-10 times less argentiferous than the
>earlier stuff. Thus, some mines were getting 20-30oz Ag per ton whilst
>others just managed 3 to 7. I wonder if a similar pattern exists in the
>Combe Martin mining district?
John,
The Combe Martin deposits are probably syngenetic in origin. Richard
Scrivener, late district geologist with the BGS, is of the opinion
that they were laid down in the Devonian period on a contemporary
seabed, subsequently overlain with sediments and subject to folding
before being remobilised by groundwater movements in later
north-south cross-courses. The tetrahedrites are relatively rare at
Combe Martin - Scrivener failed to find much on the dumps - compared
with the later mineralisation at Bere Ferrers in the Tamar Valley.
Peter
______________________________________________
Dr Peter Claughton,
Blaenpant Morfil, nr. Rosebush, Clynderwen, Pembrokeshire, Wales SA66 7RE.
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