Exploration for porphyry-style copper mineralisation near Llandeloy

sedimentary rocks, which, though unfossiliferous,
Williams (1933) attributed to the Tetragraptus Shales of
Arenig age.
Tertiary deposits Apart from sedimentary infill in
solution cavities in limestone south of Milford Haven no
deposits of Tertiary age have previously been found in
southwestern Dyfed, However, in seven of the boreholes
a succession of lacustrine sediments 2.44-20.0 m thick
was encountered, which Allen (1981) has argued is most
likely to be Tertiary in age. The sediments consist of
deeply weathered thinly interbedded feldspar sand and
clay, with some thick beds of clay, silt and sandy gravel.
The deposits were probably laid down in the marginal
parts of a lake in a landscape of deeply rotted rock now
substantially reduced by erosion. The extent of the
thickest part of the deposit was defined by resistivity
measurements (Fig. 191, but it is likely that remnants of
an originally extensive cover of lake sediments could be
found in many parts of this district.
Drift deposits The approximate distribution of drift
deposits was determined during the soil survey (Fig. 19).
Details of the deposits were obtained from diillhole
samples.
The most extensive deposit is a variously coloured silty
or sandy pebbly clay. The pebbles (or cobbles) are mostly
subangular or subrounded and their composition tends to
reflect the underlying bedrock: thus large pebbles of
black or dark grey mudstone are abundant only in the
north of the area and occur no more than two hundred
metres beyond the faulted margin of the Tetragraptus
Shales. This lithology, however, is an important
constituent of the sand-size fraction in drift deposits
over much of this area,
0 100200JO[)
In four of the boreholes (3A, 3B, 6 and 7) structureless
clay with pebbles (0.12-1.77 m thick) was found to
overlie pebbly silty clay (0.21-0.94 m thick) with thin
beds, laminae and lenses of clay, silt, coarse sandy clay
and gravel. It is not known whether the two units
together represent an undermelt drift sequence or
whether they are a complex of water-lain deposits and
geiifluc ted till, laid down under periglacial conditions.
Whichever is the case the material in the deposits is
locally derived and ice-related,
Locally filling hollows in this drift deposit and
occupying the bottom of most of the valleys is grey,
sticky clay, in places containing rounded pebbles. The
clay overlies alluvial gravels in one locality and itself is
covered in places by alluvium. The origin of the clay is
unknown, but it may be late-glacial, alluvial or
lacustrine,
Intrusive rocks
One major intrusive complex, penetrated by five of the
boreholes, and several smaller ones which may be
connected to it at depth, are present in this area. Using
textural and compositional criteria seven rock types may
be named, among which are equivalents of all the
intrusive rocks exposed at surface. They include
microdiorite, porphyritic microdiorite (005% quartz),
quar tz-m icrodiorite, porphyritic quartz-m icrodiori te,
quartz diorite (5-10% quartz), tonalite and porphyritic
microtonalite (>lo% quartz), Intrusive contacts were
observed against rocks of the Sdiva Group and the
Treffynnon Group. Xenoliths found in the intrusive rocks
include basic rock types, diorite and sedimentary and
volcanic rocks from these groups. There is no evidence
to show that any intrusions were emplaced into either
4 Solid outcrop
- Drift bound.ry
Approxinuto positton of --- transition between d~ffermt
- - Rwd
LOC~UO~ md number
a3 of borehole
Figure 19 Distribution of outcrop and superficial deposits in the area around the boreholes
2 1
St~cky gfoy cly wearhmd wng. w
b m : wnh r0und.d pObbll8 Ud
Onmp-bfmn undy cly wkh vanour
Gny-yellow adty cloy wth 70rdomm
Pmbable area underlam by tho thtckeat