Exploration for porphyry-style copper mineralisation near Llandeloy

producing a very limited dispersion halo, is a possible
cause of the discrepancy, but the most likely explanation
is that the base-metal minerdisation sampled in the
quarry is very localised in extent. The high base-metal
levels are confined to thin brecciated zones and no soil
samples were collected closer than 20 m from the quarry
face because of topsoil stripping. Pyritisation, though
more widespread, would not be detected in the soil
results unless accompanied by base-metal enrichment.
The position of anomalous samples collected along the
traverse lines is shown on Fig, 14. They are very
scattered, mostly of low magnitude, and form no clear
spatial pattern, All three element distributions are
dominated by one or two high results which are
concentrated in a three site anomaly at the northen end
of traverse 450E fSM 7996 25931. The middle site of this
group contains the highest levels of Cu and Zn in the
area as well as being one of only two sites to be
anomalous for all three elements determined (Fig, 14).
The anomalies are thought to be caused by
contamination as the soil was found to contain metal and
glass fragments and the sites lie close to the eastern end
of St. DavidMrfield. The second site anomalous for all
three elements is found on line 1050E ISM 8077 25733. It
contains an outlying high level of Zn, is situated at the
base of a steep slope by a stream and is close to a spring
at Clyn-yspytty. The location suggests that this is a
seepage anomaly, but whether the high metal levels are
derived from background sources or hidden
mineralisation is uncertain. The cause of the only other
distinctly anomalous result, for Pb at SM 8114 2450 on
line 7SOE, is uncertain. Other anomalies are all weak and
close to the threshold levels; a few can be related to
contamination or seepage but the cause of most is
uncertain.
None of the anomalous results or variation in
background levels along traverse lines could be related
to the known geology, but exposure is very poor and
available geological maps so uncertain that it does not
necessarily mean that a relationship does not exist.
Unless glacial deposits are exerting a very strong
masking effect, or dispersion halos are extremely
restricted, then the soil results suggest that there is no
substantiral hidden mineralisation close to surface in this
area. The results from around the quarry suggest that
dispersion is limited and that consequently small-scale
and deeply buried mineralisation may have escaped
detection.
Induced polarisation (IP) measurements were made along
the traverse lines, using the dipole-dipole array with a
dipole length (a) of 50 m. To provide some depth
information, readings were taken at dipole separations
n = 2 to n = 6, where n x a is the separation between the
dipole centres. All lines were surveyed by proton
magnetometer, and some very low frequency
electromagnetic (VLF-EM) measurements were also
made, though the latter were so strongly affected by
man-made interference that they are not presented here.
Detailed airborne magnetic and VLF-EM data were also
available for the area.
The results are summarised on Figs, 16 and 17 and
described in detail in Appendix 3. Severe problems of
artificial noise were encountered, and most of the
anomalies can be ascribed to man-made sources.
The chargeability anomalies measured are marked on
the magnetic anomaly map from the airborne survey
(Fig. 16). The porphyry intrusions in the area seem to be
weakly magnetic and coincidence of high chargeabilities
with a magnetic anomaly may indicate a mineralised
intrusion. Three such locations occur, marked A, B and C
(Fig. 16). The 1P effects at B, and both IP and magnetic
anomalies at C, however, almost certainly have man-
made sources, namely water-mains. Anomaly A extends
westwards from the Middle Mill quarry (where
mineralisation is exposed), to the airfield boundary, and
has no visible artificial origin. The forms in
pseudosection of the IP anomaly on lines 450E and 650E
are, however, indicative of a narrow near surface source,
most likely be to a water pipe. At D, minor IP and
magnetic anomalies coincide with a small porphyry body,
but powerlines also cross the traverse at this point.
Several IP anomalies occur unaccompanied by magnetic
features, but in almost all cases are due to artificial
sources, as detailed in Appendix 3.
The resistivity results (omitting the strongest man-
made effects) are superimposed for comparison upon the
contour map of VLF horizontal intensity from the
airborne survey (Fig. 17). There is a general inverse
correlation between the two quantities, except where
strong topographic VLF effects occur along the Solfach
gorge, and where superficial low resistivity material was
observed. The broad pattern of bedrock resistivity has
NE-SW (060°E) grain, but exposure is so poor that it is
not possible to see clearly the relationship to geology. A
linear conductive feature with this orientation runs
about 200 m south of the baseline. It marks the southern
edge of several magnetic features and could be a fault.
Another possible fault, running at 035' across the
northwest corner of the area forms the southern edge of
high resistivity weakly magnetic rock, probably the
porphyry exposed in the quarry.
In summary the geophysical surveys at Middle Mill did
not suggest the presence of substantial m ineralised
intrusions at or near the surface. Small mineralised
structures may occur to the west and south of Middle
Mill quarry, but no geophysical anomaly was completely
free of man-made interference.
No indications of nearsurface disseminated copper
mineralisation were revealed either by the soil survey or
by geophysical measurements. The copper with minor
lead and zinc mineralisation in sedimentary rocks from
Middle Mill quarry is thought to represent minor
epigenetic mineralisation associated with an intrusion of
microtonalite. Some metalliferous concentrations in the
sedimentary rocks may be of syngene tic origin,
remobilised by the igneous event. The intrusive rock
itself is highly altered, probably by late-stage
hydrothermal activity, but except for pyrite, which
occurs commonly on joint surfaces and sparsely in
disseminated form, the intrusion is not mineralised in the
quarry.
Exposure in the area is very poor. The little outcrop
that is available for examination reveals a complex
tectonic histqry. It cannot be concluded from this survey
that the intrusion exposed in the quarry is not associated
with mineralisation at depth.
LLANDELOY
Introduction
Reconnaissance survey lines for soil geochemistry and
geophysical measurements were laid out to cover the
eight intrusions shown by Williams (1933) to have been
emplaced within the Cambrian and Precambrian rocks.
Thirteen N-trending lines at 600 m spacing with two
intermediate lines at 300 m spacing at the western end
of the area, covering about 12.5 km2, were surveyed
first. The results indicated an area of special interest
covering about 4 km2 around Treffynnon, in which
additional traverse lines were placed to close the
traverse spacing to 200 m (Fig. 30). Radiometric, IP and
magnetic measurements were made and soil samples
were collected along all the lines. Several geochemical