Exploration for porphyry-style copper mineralisation near Llandeloy

elated to the presence of mafic silicate minerals,
magnetite and pyrite in the rocks. In the epigenetic
sulphide group Cu shows the greatest contrast to
background: high levels are related to the presence of
chalcopyrite (samples 21, 35, 37, 38 and 391, bornite
(sample 371, covellite and malachite (sample 38).
Paradoxically sphalerite was noted in sample 2 1 which
whilst no discrete Zn minerals were observed in the . /
sections of other samples containing much higher Zn , /&@'
levels. These discrepancies are attributed to the poor ,* -$ssH
.r
.-.!
//--//
shows only the weakest indications of Zn enrichment //
H ~
sample represented by a thin section when the rocks are
/ ///
I
grossly inhomogenous,
There is no obvious relationship between elements
concentrated in sulphides and those which may be
related to alteration. Ba, Rb, Sr and Ca yield high levels
in individual samples but, as in the intrusive rocks, there
is no regular pattern which can related to observed
alteration.
It is concluded that both weak syngenetic and
epigenetic mineralisation may be present in the
sedimentary rocks at Middle Mill, with the high levels of
Cu, Pb and Zn in some of the rocks concentrated by the
latter process. The epigenetic mineralisation may be
connected with the intrusive episode but the lack of
disseminated sulphide and any enrichment of chalcophile
elements in the intrusive rocks suggest that the
mineralisation exposed in the quarry is not of porphyry
type.
Soil sampling survey
Four hundred and thirty one soil samples of about 200 g y
were collected at 25 m intervals along the traverse lines
from two or more holes driven as deeply as possible using
a 120 cm hand auger (Fig. 14). In addition, as an w
orientation exercise, 23 samples were collected at the
same interval and by the same method from around the
margin of Middle Mill quarry. Samples consisted mainly
of unconsolidated material derived from glacial or
periglacial deposits and weathered bedrock from below 9
the organic-rich topsoil. A few samples collected in the
floor of the Solfach valley contained recent alluvium. All
samples were dried, sieved and the -85 mesh BSS
(0.15 mm) fraction analysed for copper, lead and zinc by
atomic absorption spectrophotometry following
Figure 15 Cumulative frequency plots for Cu, Pb and
Zn in soil samples from Middle Mill and Llandeloy
dissolution in hot concentrated nitric acid for one hour.
Analytical results are summarised in Table 6.
Threshold levels were determined from cumulative
frequency plots (Fig. 15) following the methods of
Lepeltier (1969) and Sinclair (1976). Copper shows an
approximately lognormal distribution whilst lead has a
binormal or truncated sigmoidal form. Minor deviations
in both plots are attributed to stepping in the analytical
data. Zinc shows a more complex form suggesting the
presence of a background normal population and a higher
lognormally distributed group. Threshold for all three
elements was set at the 97.5 percentile level, equivalent
to the mean plus twice the standard deviation for a
perfect lognormal distribution. For lead and zinc the
9 7.5 percentile level coincided approximately with the
departure of the background population from a straight
line.
The threshold levels (Cu 38 ppm, Pb 53 ppm, Zn
81 ppm) were applied to the results of the 23 samples
collected around the margin of the Middle Mill quarry.
None of the results was anomalous. All the results fall in
the central part of the sample populations for the whole
area: Cu 10-25 ppm, median 15 ppm; Pb 20-40 ppm,
median 30 pprn; zinc 50-70 ppm, median 60 ppm. The
lack of anomalous results in samples collected from as
closely as possible to bedrock mineralisation was
unexpected. The lack of response cannot be accounted
for by thick drift deposits (they appear to be generally