Trevor,
With specific reference to Wales, here we have episodic metalliferous
mineralisation covering a broad timespan, from Lower Palaeozoic through to
Mesozoic and quite possibly Tertiary in some cases. Much of the
mineralisation is vein-type and as such will exploit previously-existing
fracture systems if the extensional stresses are favourably orientated.THat
allows it to overprint existing mineralisation and during mining the whole
lot may well end up getting stoped.
This means that, for example, in the case of Central Wales we have early
(Devonian) Pb-Cu-Ag-Sb dominated assemblages in veins that historically
yielded relatively high Ag values from desilvered galena - eg. up to
1000ppm - Darren, Goginan etc. As Bernard said, these galenas incorporated
tetrahedrite inclusions, the tetrahedrite carrying up to 18 wt% Ag from
electron microprobe analyses.
Later (Carboniferous-Mesozoic) Central Wales assemblages with usually low
Ag values (Pb-Zn) occupied both these and subsequently-created fractures so
that there is a second population of mines with very low silver values,
although as in the case of Van, the sheer tonnage avbvailable meant that
total recovered silver was still considerable. Grade: 50-150ppm Ag with no
silver carriers identified.
Thirdly in Central Wales there is an odd group of assemblages which is very
late but is marked by Pb-Cu-Ni-Sb, no silver minerals found (so far) but
very high Ag values recorded. Llanfair Clydogau mines are the classic
example and deserve further investigation although the mines themselves are
pretty much obliterated. Grade: historically >>1000ppm Ag in galena
concentrates - but Pb grade in bulk low-ish!
Esgairmwyn is an interesting case that certainly deserves more study
although as this would require underground sampling it's unlikely to happen!
Going northwards we have the Dolgellau Gold Belt where argentiferous
tetrahedrite again occurs but in much lesser amounts than in Central Wales.
Here it is associated with a complex and variable Au-Ag-Bi-Te-Sb-Pb
assemblage which has an extremely pockety distribution in Early Ordovician
mesothermal gold-quartz type lodes. Later (Mesozoic????) cross-veins
locally carry base-metal sulphides but are poor in Ag.
Futher north again we have the Mid Ordovician Snowdon Caldera Cu-Zn-Fe-Pb-
As veins which are generally silver-poor, while to the NE again we have the
big Llanwrst Orefield where probably Carboniferous-Mesozoic vein
mineralisation yielded big tonnages of base-metal sulphides but with a low
Ag grade not dissimilar to the Halkyn Orefield.
That's a quick tour through some complex and some less complex Welsh
orefields: the main point though is that it is absolutely vital to
understand the paragenetic structure of an orefield on a regional basis
prior to attempting to decipher the minstats. The mineralogy & paragenesis
are the key. Cheers - JOhn
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