Exploration for porphyry-style copper mineralisation near Llandeloy

SUMMARY
Geological, geochemical and geophysical surveys
followed by drilling in the area around Llandeloy, south-
west Dyfed, have located disseminated copper
mineralisa tion of porphyry type associated with
intermediate intrusive rocks masked by thick
overburden.
Intermediate intrusive rocks in the area were selected
for investigation as potential hosts for disseminated
copper mineralisation on the basis of the known geology
and tectonic setting. An initial assessment of the area
involved revising the geological maps, analysing rocks
from surface exposures, studying available geophysical
data and carrying out a stream sediment survey in the
catchment of the River Solfach. This work revealed the
presence of weak poly metallic sulphide m ineralisation
associated with the margin of a tonalitic intrusion at
Middle Mill. In view of the very poor exposure, more
detailed geochemical and geophysical surveys were
carried out across the two areas underlain by intrusive
rocks of dioritic or tonalitic composition.
At Middle Mill six traverse lines, spaced 300 m apart
and totalling 10.5 km in length, were surveyed by IP,
VLF-EM and magnetic methods. Soil samples,
subsequently analysed for Cu, Pb and Zn were collected
at 25 m intervals. Few anomalies were located. Most of
those found could be ascribed to artificial sources and it
was concluded that no substantial body of disseminated
copper mineralisation was present at or near the surface
in the area. The mineralisation found in Middle Mill
quarry is thought to be minor, epigenetic mineralisation,
associated with the intrusion.
At Llandeloy 13 traverse lines spaced 600 m apart and
covering an area of 12 km2 were surveyed by IP, VLF-
EM, magnetic and radiometric methods. Soil samples
were collected along these lines at 50 m intervals and
analysed for Cu, Pb and Zn. In about 4 km2 around
Treffynnon additional lines were sampled and measured
to close the spacing to 200 m. Gravity data were also
collected from some traverses and sites to supplement
the Hational Gravity Survey. Several strong copper-in-
soil and geophysical anomalies were identified. Nine
boreholes were drilled to investigate the causes,
Disseminated copper mineralisation was intersected in
the boreholes. It occurs principally within a concordant
or semiconcordant sheeted complex of dioritic and
tonalitic rocks, believed to be uppermost Cambrian or
low Arenig in age, whose composition is consistent with
e mplacement within a volcanic arc setting. The
intrusions and their host rocks have suffered a two
phase, pervasive, hydrothermal alteration which is
inseparable from the sulphide mineralisation and
recorded in boreholes over an area of 1 km2. The
alteration shows features common to porphyry copper
systems, consisting of an early patchy and irregularly
developed propylitic and potassic alteration, overprinted
by a widespread and locally intense late propylitic
alteration. The potassic alteration is only well preserved
locally and is divisible into K-feldspar and biotite types.
When intense, the potassic alteration is characterised by
substantial changes to the bulk chemistry of the rocks
involving increases in K, K/Na, K/Rb, Rb/Sr, Cu/S and,
erratically Ba and losses of Na, Sr and Ca. In the most
altered rocks so called 'immobile1 elements such as Y a~d
Nb are redistributed. The late propylitic alteration
affected rocks in all boreholes except one and gave rise
to the dominant alteration assemblage of sericite,
chlorite, epidote, albite, pyrite and magnetite.
Introduction of Fe and S appears to have accompanied
this event but any other bulk chemical changes are
confused by host rock variation. Retrograde effects on
potassic alteration, such as the lowering of Rb/Sr, K/Na
and Cu/S are probable but not clearly defined.
Mineralisation, involving the introduction of Cu, Fe
and S, accompanied the first phase and ?Cu, Fe and S the
second phase of alteration. Cu levels are generally
modest, the best intersection being 0.1% over 3.4 m in
borehole 2. Cu and particularly the Cu/S ratio are
generally highest in the most altered (potassic) rocks but
locally high levels of Cu may be found in weakly altered
rocks, There is only weak and erratic enrichment in Mo,
and high levels of Cu and Mo show only a weak
correlation. There are localised very weak enrichments
of AS, Pb and Zn. Au was not determined.
It is suggested that the present erosion level cuts a
deep section through a copper porphyry deposit, this
explaining the imperfectly developed zonation, low Cu
content and abundant magnetite. The part of the system
most likely to have contained ore grade material has,
therefore, been eroded away and some of the material is
found in the overlying lacustrine sediments which
contain abundant magnetite, clay, feldspar and up to
640 ppm Cu. The style of mineralisation, chemistry of
the rocks and geological setting all suggest that
mineralisation took place in conditions consistent with
an island arc setting. The detailed geology of the area is,
however, imperfectly understood because of the
extremely poor exposure, and the possibility exists that,
because of downfaulting and tilting, parts of the deposits
may be preserved and concealed to the north and east of
the area drilled.
The case history shows that in such areas of low relief
and thick overburden drainage sampling can be an
ineffective mineral exploration technique. Surface rock
sampling also failed to indicate the presence of the
deposit, partly because of poor exposure and partly
because of the patchy, multi-phase alteration pattern.
The locally thick sequence of interbedded sands and
clays containing copper and magnetite overlying the
deposit confused to varying degrees all the geochemical
and geophysical survey results except lithogeochemistry.
If the sands and clays had not contained anomalous
copper the deposit would probably not have been located.
Borehole results suggest that Cu, S and the Cu/S ratio
provide the best li thogeoche m ical targets.
INTRODUCTION
Two adjoining prospects, one centred on Middle Mill near
Solva, the other a broad belt of country north of
Llandeloy (Fig. I), both in the catchment of the River
Solfach, were covered by reconnaissance geochemical
and geophysical surveys. The results of the work around
Middle Mill were not encouraging and the area was
abandoned. In the Llandeloy prospect, however, the
geochemical and geophysical results were of sufficient
interest to justify further investigation. Detailed
geophysical surveys were made and soil samples
collected for chemical analysis along 200 metre spaced
traverse lines across a target area around Treffynnon.
Anomalies were identified and nine boreholes were
drilled to investigate their cause.
The surface of both areas is a peneplain incised deeply
by the lower reaches of the River Solfach. All the land is
farmed and outcrop of any kind is rare. Many of the
quarries recorded by early workers have now been filled
in and grassed over.
In both prospects the target was disseminated copper
mineralisation. Inter mediate intrusions within the
Cambrian and Precambrian of this region are similar to
those in the eastern part of the Harlech dome - a
comparison made by Cox and others (1930). While their
age is not known for certain, the possibility exists that
the intrusions were co-magmatic with the andesites at
'I'reffgarne (Thomas and Cox, 1924) which are either late
Tremadoc or early Arenig in age. In the Harlech dome
the intermediate intrusions and the associated Coed y
Brenin porphyry copper mineralisation (Rice and Sharp,
1976) are believed to be derivatives of the late
Treinadoc magmatism that gave rise to the Rhobell
Volcanic Group. Furthermore, it has been suggested that
the andesitic Rhobell Fawr volcanism formed part of an