Exploration for porphyry-style copper mineralisation near Llandeloy

, - - Roads (selected)
- Streams
1-1 Traverse Lines , *
Figure 1 The Middle Mill and Llandeloy prospects showiw the traverse lines followed during the
reconnaissance surveys
island arc volcanic episode whereas the succeeding,
Llanvirn to Caradoc, volcanic rocks in Wales relate to
bimodal, dominantly tholeiitic, volcanism in a back arc
extensional setting (Kokelaar, 1979; Kokelaar and others,
1984). Because it is known that disseminated copper
mineralisation is associated with the end Cambrian
subduction event, any area in which there is some
evidence for the existence of volcanic rocks and
subvolcanic intrusions derived from it is worth
investigating.
The Treffgarne Andesites have not been recorded
around Middle Mill and Llandeloy, but Williams (1933)
noted that the lowest component of the Llandeloy Ashes,
which are early Arenig in age, is richly pyritic locally.
The traverse lines, set mainly to cover the intrusions
within the Cambrian and Precambrian, therefore were
extended near Llandeloy to cover the earliest of the
Ordovician volcanic rocks.
Previous work
The part of the area around Llandeloy was mapped by T.
G. Williams (1933) and later all the area was remapped
by A. C. Wilcox and H. H. Thomas, the latter responsible
for the Solfach valley. Their maps were not published,
but were lodged with the Geological Survey in 1952. The
linework on them has been used in conjunction with that
on the published work of T. G. Williams (1933) to draw
Fig. 2. However any map showing solid geology in this
area must be largely speculative. A comparison of the
Geological Survey map (Sheet 40) of 1857 with those of
Cox and others (1930), Williams (1933) and manuscript
maps of Cox and Thomas shows several different ways of
interpreting-essentially the same data.
Twentieth century written accounts of the area are
limited to Williams (1933) for the part north of
Llandeloy, Cox and others (1930) for a general account
of the area between Ramsey Island and Newgale, and
Stead and Williams (1971) who have described excursions
in the area. Rushton (1974) has reviewed all the
literature on the stratigraphy.
The drift of the area has never been properly
examined. No attempt has been made here to reinterpret
the work of Williams (1933) and Cox and Thomas except
around Treffynnon where the boreholes yielded
previously unavailable information. Some minor
adjustments have been made to boundaries around
intrusions that have been re-examined.
Very little geochemical work has taken place in the
area. U-Pb zircon age determinations have been carried
out on the St David's granophyre and the Johnston
diorite, both just outside the area (Patchett and Jocelyn,
1979), and Thorpe (1970, 1972) has discussed chemical
analyses of the Johnston intrusions. The only
geochemical drainage survey data available is that
included in the Wolfson geochemical atlas of England and
Wales (Imperial College, 1978). No outstanding features
are shown in this area except for moderately high levels
of iron and manganese. The Soil Survey of Great Britain
has published a general account of the soils across the
whole of west and central Pernbrokeshire (Rudeforth and
Bradley, 1972). More pertinent to this work Bradley,
Rudeforth and W ilkins (1978) published the results of
analyses for 12 elements on alternate soil profiles
collected at 1 km intervals across the same area. They