"Dear John, Peter and Trevor:

You have really fired my curiosity, especially John.  In spite of having
spent quite a bit of my youth in N. Wales, I know very little about Welsh
geology
(having left the UK at 18 and spent very little time there since), but
John's
lists of mineral assemblages/elemental compositions certainly does have some
weird ones.  The Snowdon Caldera, for example, which I have usually seen
referred to as a porphyry copper type of district, with its >Cu-Zn-Fe-Pb-As
would certainly seem to be a mixed up porphyry.  No gold, no moly?  As and
Cu-Zn-Pb at the same level?  I was only a kid, but I don't remember any
tourmaline-bearing breccia pipes around the Snowdon mines, either."



John - the Snowdon Caldera is not really a porphyry-related area.
Mineralisation is entirely vein-type. It is genetically-linked to the
Snowdon Caldera volcanism - see ref below. Tourmaline breccias not known,
but one breccia-vein carries magnetite with minor scheelite + cassitterite
(academic levels only!).

REFs:

Colman, T.B, and Appleby, A. K, 1991. Volcanogenic quartz-magnetite-
hematite veins, Snowdon, North Wales. Mineralogical Magazine, 55, 257-262.

Reedman, A.J., Colman, T.B., Campbell, S.D.G., and Howells, M.F. 1985.
Volcanogenic mineralization related to the Snowdon Volcanic Group
(Ordovician), Gwynedd, North Wales. Journal of the Geological Society,
London, 142, 875-888.




"The Dolgellau assemblage sounds almost epithermal, but again, I don't
remember
from childhood visits the huge argillic alteration zone that "should" then
be
associated with it."




John it's in fact a classic mesothermal gold-lode province. There is some
epithermal stuff related to Tremadocian volcanism, but the gold-lodes are
typical ribbon-rock veins.

REFS:

Mason, J.S., Fitches, W.R and Bevins, R.E. (1999) Evidence for a pre-
tectonic origin for the auriferous vein-type mineralisation in the
Dolgellau Gold-belt, North Wales. Transactions of the Institution of Mining
and Metallurgy (Section B, Applied earth science), 108, B45-B52.

Shepherd, T.J. and Bottrell, S.H. 1993. Dolgellau Gold-belt, Harlech
district, North Wales. In: Pattrick, R.A.D. and Polya, D.A. (eds):
Mineralisation in the British Isles, 187-205. Chapman and Hall.



"And then

>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."

Copper-nickel sounds like an association that might be found around or in
ultrabasic intrusives, but what is Sb doing in that group?

This one really caught my eye as I spent years trying to understand the
sedimentary copper deposits of Zambia, and one clue was the presence in the
basement of ?ultrabasic ? pipes with rich Cu-Co-Ni mineralization.  They
appeared to perhaps be associated with the master faults controlling the
half-grabens in which the sedimentary orebodies formed. Since these were
found
by aerial survey in the 1950s, however, and were sparsely drilled and never
really mapped, it's hard to say what they, in fact, really were.  I have
assumed
that they were associated with the mantle/lower crustal heating and
extension
that led to the half-grabens, which I assume are in fact the remains of a
complex rift zone (cf. the Great Basin of north America).  Impossible to
know
now, I suppose, unless Harry Clemmey at Leeds had it all figured out before
life
got difficult there.

So, this Llanfair Clydogau area grabbed my attention:  is there any chance
that
it is late enough to be associated with the doming and extension that led
to the
erosion of 2 km of chalk, etc., off Wales and to the formation of the Irish
Sea?

Has anyone looked at any or all of the "late" (i.e. post-orogenic) deposits
to
see how they do relate to Mesozoic events?

Are there any good references to look up?"



John, the Central Wales stuff does open up interesting questions about the
geochemical properties of the basement underlying the thick Ordovician-
Silurian turbidite pile which hosts the veins. Will have to win the lottery
& drill a deep hole or two!

REFS:

Mason, J.S. 1997. Regional polyphase and polymetallic vein mineralisation
in the Caledonides of the Central Wales Orefield. Transactions of the
Institution of Mining and Metallurgy (Section B: Applied Earth Science),
106, B135-B144.

Fletcher, C.J.N., Swainbank, I.G. and Coleman, T.B., 1993: Metallogenic
Evolution in Wales: Constraints from lead isotope modelling. Journal of the
Geological Society, London, 150, 77-82.

Mason, J.S. 1998. Tucekite, a mineral new to Britain, and other rare ore
minerals from the Central Wales Orefield. UK Journal of Mines and Minerals,
19, 30-36.


Cheers - John