Mason: THE GENESIS OF THE CENTRAL WALES OREFIELD
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taken place in Central Wales since Bronze Age times (for copper), with
a recently-discovered smelting-site near Talybont providing evidence
that the Romans worked lead in the area. Later mining was for lead and
silver (Elizabethan and Civil War times)
and zinc became important from the mid-19th century onwards. Modest
amounts of marcasite and barium minerals have been produced locally and
one mine has been looked at as a potential calcite resource. Mining
ceased in the
early 20th century, but much research has since been undertaken, with a
host of unusual ore minerals being identified from the mid-1980s
Orefield is a polyphase mineralised terrain,
in which the mineralisation is epigenetic vein-type and at least twelve
of fracturing and mineralisation, represented by successive
crosscutting mineral assemblages, have been identified. The
mineralisation is entirely hosted by Upper Ordovician to Lower Silurian
clastic marine sediments.
The veins are apparently all post-Acadian in age, with a range of Pb-Pb
suggesting emplacement from Devonian through to Permian times.
mineralisation is very different from the pre-tectonic
metallogenic province known as the Dolgellau Gold-belt, only a short
distance to the north.
The Gold-belt, however, is a mesothermal metamorphic gold-quartz vein
province hosted by a highly
altered suite of Cambrian sediments and intrusives (including some with
porphyry-type Cu mineralisation): the paragenetic and
alteration patterns are completely different and the veins themselves
have, via Acadian deformation, been strongly folded, boudinaged and
Back to Central Wales: in mineralogical terms, the veins are dominated
by quartz with two major
generations of carbonate, the early one being ferroan dolomite and the
later one calcite. Baryte and witherite are late-stage components in
the eastern part of the orefield. Fluorite is absent so far as we know.
Sulphides are locally abundant and may form large masses or ribs.
Galena and sphalerite are broadly speaking the most abundant, with
lesser chalcopyrite and tetrahedrite and locally important marcasite. A
wide range of accessory ore minerals, particularly cobalt
and nickel-bearing species, occur in certain assemblages.
The paragenetic determinations that allowed the sequence of emplacement
of the assemblages to be
determined (as set out in the table below) were based on observations
of cross-cutting veins and rebrecciation. A frequent feature in Central
Wales is to find one assemblage occurring in a vein cutting
another, or to find one assemblage rebrecciated and cemented by
another. In such cases it is straightforward to say that one came
first, the other later. Moreover regional patterns, in which such
observations repeat themselves time and again, tend to build up as more
sites are studied.
a view of part of Cwmystwyth
mine, one of the larger mines of Central Wales, where extraction of
ores has intermittently occurred since the Bronze Age. Several of the
twelve Central Wales mineral assemblages may be identified here.
The twelve successive mineral assemblages I deduced in my MPhil
research & later
publications may conveniently be divided into Early (A1) and late (A2).
The original terms ("early-complex" and "late-simple") I now utterly
loathe and would discourage their use!
suggested ages are
based on a lead isotope study using paragenetically selected galena
samples collected in a joint piece of work with geochemists at the
British Geological Survey in
the late 1980s. The model isotopic ages were certainly consistent with
the "paragenetic relative ages" of the assemblages. The paragenetically
last Central Wales assemblage - the A2-f marcasite
assemblage, has not been studied isotopically - no galena! If the Pb-Pb
dates are correct - they date material up to the A2-C assemblage, which
appears to be of Permian age - then it appears that
events continued, perhaps into post-Permian times.
Interestingly some of the later Central Wales assemblages with
consistent Permian Pb-Pb dates are texturally very similar to the MVT
assemblages of the UK Pennine Orefields, and evidence
exposed at Ogmore in South Wales demonstrates conclusively that the
responsible for the MVT deposits of South Wales continued at least into
Central Wales features, in
addition to the more typical Pb-Zn-Cu ores, a number of unusual
metalliferous minerals, and particularly several species containing
nickel and cobalt. This is a sample, magnified x1.5, of pinkish grey
metallic siegenite (Ni,Co)3S4
associated with brassy yellow chalcopyrite in quartz-cemented breccia.
The table below gives details of the primary paragenesis. The
assemblages are listed in order of deposition as are the minerals in
each assemblage. Species underlined are major economic components,
those in italic only occur in trace amounts; the rest are minor phases
occurring at more than two localities.
These commonly feature a high mineral species diversity and often have
a fine grainsize, so that
ore petrology is absolutely vital for their full paragenetic study. The
most diverse stage, the A1-C
assemblage, is very likely to yield more ore mineral species with
further sampling and petrology.
A1 assemblages typically occur as breccia-cements, in all cases
featuring tough, compact milky quartz
radially grown about rock clasts. Vugs carry elongated prismatic quartz
crystals. Wall-rock alteration is not obvious in hand specimen. Those
from which galenas have been analysed have yielded early
Devonian Pb-Pb isotopic ages, and it is generally thought that they
developed during post-Acadian relaxation.
Minor early Cu: this is rather
localised in occurrence.
Minerals: quartz + chalcopyrite + ferroan dolomite
Castell mine has more of this than most, and it is easier to spot there.
Early sphalerite assemblage: this
was locally important.
Minerals: quartz + pyrite + sphalerite
disease) + ferroan dolomite +
and Erglodd mines:
at the latter, it is rebrecciated and cemented by the A1-C assemblage,
as it is at most mines. Castell reveals the "cleanest" material and was
Polymetallic assemblage: this
widely developed assemblage has a great
mineralogical diversity. Economically by far the most important of the
A1 assemblages, it supported the silver-mining industry of the
centuries, due to its locally significant argentiferous tetrahedrite
Minerals: quartz + pyrite + siegenite + cobalt
+ millerite + chalcopyrite
+ tucekite + ullmannite + gersdorffite
+ bournonite + boulangerite
Eaglebrook, Brynyrafr mines: most occurrences are on the west side of
the Plynlimon "dome" with relatively few to the east.
Minor late veining, often cutting
sheared sulphides e.g. "steel ore" galena. Of no economic importance.
Minerals: chalcopyrite + galena + "honey-blende" sphalerite
occurs at most
mines where the A1-C assemblages are well-developed.
Ferroan dolomite influx - copious
quantities of iron-bearing carbonate which cement rebrecciated A1-C and
quartz + ferroan dolomite
Eaglebrook, Hafan mines
Late cavity-filling: widespread in
minor to trace amounts - forms fracture-veins cutting ferroan dolomite
quartz + siegenite + cobalt pentlandite + millerite + chalcopyrite +
Eaglebrook and Henfwlch
Both breccia-cements and crustiform open-space fillings; the quartz is
always glassy-white to clear and tends to be vuggy with squat,
pyramidal crystals. Sulphides are coarse-grained and often
inclusion-poor or inclusion-free. Wall-rock alteration
is noticeable in A2-b, A2-d and A2-f assemblages where silicification
has developed to a modest extent. Pb-Pb dating of some stages has given
an early Carboniferous to Permian spread of isotopic
ages: it is reasonable to suggest that these assemblages were deposited
mainly during pre- and post-Variscan extension during
Major Pb-Zn assemblage: this was
of major economic importance for large-tonnage orebodies with abundant
galena and sphalerite. Silver values in lead concentrates were an order
of magnitude below those of the A1-C assemblage.
quartz + sphalerite
chalcopyrite + galena
Coarse-grained (to 10mm)
ullmannite-bearing Pb-Cu assemblage. This was worked on a small scale
locally where combined with the subsequent A2-C assemblage. It forms
breccia-cements and composite veins in which the quartz and sulphides
reveal a layered appearance. Ullmannite (not recognised by the miners)
often overgrows idiomorphic galena epitaxially. Wall-rock very close to
deposits of this assemblage is silicified in places.
quartz + chalcopyrite
ullmannite + galena
Calcite-dominated assemblage: this
tends to manifest itself as a crustiform-banded open fissure-fill as
opposed to a breccia-cement, although it is sometimes found cementing
shattered A2-b veinstone. Galena is coarse-grained, often occurring as
large octahedral crystals.
quartz + galena
+ chalcopyrite +
and Nantiago mines
Coarsely crystalline quartz: large
(to 15cm), always well growth-zoned quartz crystals with dendritic iron
sulphide and chalcopyrite inclusions. The original vugs that occurred
in this assemblage - especially at Nantymwyn mine, must have been quite
a sight! At Cefngwyn, the quartz overgrows A2-C calcite.
quartz + chalcopyrite + pyrite
Barium minerals assemblage:
breccia-cements and open fissure-fills, sometimes as masses occurring
in uncemented fault-gouge, localised but large
quantities of minerals where well-developed in the east of the Orefield.
sphalerite + galena + calcite + baryte
Bryntail, Penyclun, Gorn
mines and an isolated trial to the W of Esgairhir.
Iron sulphides assemblage:
this is widespread and locally occurs in major quantities causing
environmental problems. At such localities it was worked for sulphuric
acid production: at others it was an annoying contaminant, netveining
earlier deposits of galena and sphalerite, which caused ore-dressing
and smelting issues. This stage occurs as breccia cements, as cements
to fault-gouge and as open-space crustiform fissure-fills.
Silicification of wall-rock may be strong.
quartz + sphalerite + pyrite + marcasite
Brynyrafr and Cwmrheidol
images in the menu above cover some of the more interesting ore
minerals from Central Wales. For the uninitiated, apart from the
breccias page, these images are taken through a high-power microscope
which views the samples in reflected light. Each sample is a block of
opaque ore minerals that has been painstakingly polished to an optical
mirror-finish using diamond pastes down to 1 micron in size.