Exploration for porphyry-style copper mineralisation near Llandeloy

area and periglacial and glacial deposits form blanket
deposits masking them and the older solid rocks.
Stratigraphy
Solva Group According to Williamsf (1933)
interpretation, the intrusive rocks in this area are
emplaced entirely within the Tre ffynnon Group. Borehole
8, however, penetrated a thick succession of sedimentary
rocks which showed strong affinities to the Solva Group.
Nowhere here are similar rocks exposed, but the rock
types encountered in the borehole are dominant in float
in several places.
The succession in borehole 8 (Fig. 28) consists of
north warddipping, mainly thinly bedded, green, fine or
medium-grained quartz wacke sandstone with wisps and
laminae of mudstone in places. In the top ten metres and
at intervals below, the quartz wacke is thinly
interbedded with up to an equal proportion of silty
mudstone and siltstone. Beds of coarse-grained ar kosic
and quartzose sandstone were recorded at only three
levels.
Between 45 and 72 m depth, but mostly between 62
and 72 m, there are several thin laminae of tuffaceous
mudstone. The laminae tend to be graded, with a
concentration near the base of crystals, angular and
subangular fragments of feldspar, minor quartz and oval
(?bomb-shaped) quartzo-feldspathic lithic fragments set
in a dense, sericite matrix. These laminae are usually
closely associated with laminae of silty, sandy mudstone
and a distinctive spotted mudstone. Some of the spots,
which are mat likely to be due to thermal
metamorphism, have an amorphous ferruginous core and
a clear sericite rim; others are slightly richer in chlorite
than the host rock.
At seven levels thin be& of black magnetite sandstone
were recognised. They are usually less than 1 cm thick
and they are always associated with a heterogeneous
assemblage of sedimentary mck types in units no more
than 12 cm thick, which contrast markedly with the rock
above and below. Among the interbedded rock types are
greenish-grey fine sandstone, white qua^ tzite, purple or
bluish-grey massive sandstone an8 spotted mudstone.
There is usually some epidotisation either along beds or
fractures. These heterogeneous units ocaur elsewhere in
the core without associated magnetite sandstone.
The fine-grained chtic component in the rocks has
been recrystablised to green mica, sericite, chlorite and
microcrystalline quartz and some rocks possess a good
sericite-def ined schistosity . It is probable that these
rocks have undergone a regional metamorphism, but
there is a suggestion also (see later) of pervasive
hydrothermal alteration. The spotting noted in the
mudstones indicates that there has been a thermal
metamorphic overprint, which has possibly led to some
recrystallisation, especially of the green mica.
The sedimentary structures seem to indicate
deposition in a turbulent shallow water environment. The
sandstone beds are either massive of thinly laminated;
the coarse-grained Ms are graded. Evidence of
slumping is present in convoluted units between planar
bedded units and there is abundant soft sediment
deformation: load casts, downward sandy flames,
sandballs and microfaults are common. Flaser bedding is
characteristic of the thinly interbedded sandstone and
mudstone units and washouts characterise the base of
some sandstone beds.
The succession bears no resemblance to the Treffynnon
Group describes by Williams (1933), but shows
similarities to parts of his middle Cambrian. It compares
with the middle Cambrian Solva Group as exposed at
Solva except that no manganiferous beds are known at
Treffynnon and no magnetite-bearing beds at Solva. At
Porth y Rhaw, about 1.5 km west of Solva, Price (1963)
recorded ashy bands in the middle Menevian, which
overlies the Solva Group. In North Wales tuffaceous beds
are present in the Gamlan Formation, which correlates
with the Solva Group. Whether or not the succession
encountered in the borehole is the Solva Group, it is
considered most likely to be middle Cambrian in age,
Treffynnon Group Only two of the several outcrops of
volcanic rocks recorded bv Williams (1933) are still
ac@essible. The deeply weaihered exposure in a quarry
near Treyscaw Farm [SH 8464 28101 consists of thinly
bedded brownish yellow siltstone overlying br ecciated
massive rock. In - thin section the bedded rock is
composed of very fine-grained sericite and quartz with
quartz-rich silty laminae and lenses and is obviously of
volcanic origin. The other occurrence is a poor exposure
of weathered light grey siltstone with thin quartme
siltstone beds in an old quarry near Llanreithan [SM 8645
28651. There are no volcanic rocks in the quarry,
Williams (1933) commented that parts of this group had
been mapped as Lingula Flags by the Geological Survey,
and the rocks in this expasure certainly show similarities
with the Cambrian rocks, but the quality of the exposure
is too poor to be definite about their affinity.
Volcanic racks were encountered in borehole 1, which
is about 180 m west of this locality, and in boreholes 6
and 7 (Figs. 20, 26 and 27). in borehole 1 the volcanic
rucks are intruded by thin sills of porphyritic
microtonalite in the top 12 m. The rocks are deeply
weathered and the core is badly broken, but it appears
that a faulted stratified succession of volcanic breccias,
crystal tuff , porphyritic acid lava and thinly interbedded
muddy quartzose sandstone, tuff, lapillistone and
mudstone was penetrated. The breccia is typically
polymict with the majority of the angular fragments
composed of quartzose sandstone and laminated silty
mudstone; the remainder include highly altered
porphyritic acid volcanic rock and aggregates of quartz
and heavily altered feldspar crystals. The matrix, of
indeterminate origin, is structureless, fine-grained
quartz, feldspar, serici te and minor chlorite.
The top 15.10 m below rockhead in borehole 6 consists
of two thick units of porphyritic acid volcanic rock,
separated by a bed, 37 cm thick, of poorly sorted
recrystallised quartzite. The volcanic rock is uniformly
porphyritic with locally well-orientated serici tic
pseudomorphs after feldspar phenocrysts up to 3 mm
long and small recrystallised quartz phenocrysts, in a
matrix of equant, platy feldspar, minor quartz, sericite
and chlorite. The rock is brecciated in parts and contains
some lithic inclusions. The uppermost few centimetres of
the unit below the quartzite bed display contorted
banding. The texture of the rock is most likely to be that
of an acid lava. Rock similar to this was encountered
near the top of borehole 7, apparently intruded by
quartz-microdiorite.
The evidence gathered here suggests that the
Tref f ynnon Group consists of acid pyroclas tic and
extrusive rocks interbedded with shallow water
sedimentary rocks. The rocks are highly altered, locally
to sericite, and the quartzose sandstones have undergone
secondary silicification. The age of the rocks is not
known for certain. Williams (1933) regarded them as
being Precambrian and he surrounded their area of
outcrop with faults. The presence of probable Cambrian
in the area around borehole 8 and distribution of other
possible areas of Cambrian sedimentary rocks with
respect to the known occurrences of the Treffynnon
Group suggest that the volcanic rocks may
unconformably overlie the Cambrian rather than be
faulted against it. If this is the case the volcanic rocks,
which are intruded by the same rocks that penetrate the
Cambrian, could well be equivalent in age to the
Treffgarne andesites.
Brunel Beds Dark grey or nearly black cleaved
mudstones are exposed in ditches beneath drift deposits
in a few localities in the north of the area. The
mudstones lie within the westernmost limit of a belt of