Having read the other replies, I Should point out that in our glorious
Temperate Maritime climate the development of major supergene silver
deposits is a bit limited compared to areas like the Australian Interior
(e. Broken Hill etc).
OK, there are some general supergene deposits that formed during Tertiary
deep weathering, but north of a line from about Pembroke-Essex many of
these were subsequently eroded away mechanically by glacial action.
Survivors of glacial erosion occur in Central Wales (eg. Llechweddhelyg,
Bwlchglas, Geufron), North Wales (eg. Great Orme, Parys Mountain), the
Lakes (eg. Roughton Gill, Driggeth) and the Leadhills orefields (eg.
Susanna, Bay mines). Down in SW England the deep supergene zones are/were
still present.
Few of the examples above were particularly noteworthy for silver
enhancement. Silver is in fact a surprisingly mobile element in our wet
climate, and it is worth noting that in the silver-rich veins of Central
Wales (eg. Darren), a multitude of obscure Cu-Pb-Zn-Sb-As-Ni secondary
minerals have been found but not one silver-bearing specie. The point may
be reinforced by noting that at Alva, where many samples of native silver
have been collected, the only supergene indications are black sulphide
tarnishes on the surface of the silver dendrites. Conclusion must be that
in virtually all cases it is the nature of the primary mineralisation that
controls historical Ag grade distribution. Cheers - John
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