Exploration for porphyry-style copper mineralisation near Llandeloy

The only rock containing evidence of secondary
minerals of a higher pressure-temperature stability
range than the minerals in these assemblages is the
quartz diorite from Hollybush (RPR 2). In this rock there
is abundant chlorite after biotite, much of it forming a
rough network through the rock. It also appears that
these chlorite pseudomorphs occur in chlorite
pseudomorphs after amphibole. It is possible, therefore,
that the biotite was a secondary mineral in part
replacing amphibole and both minerals have undergone
retrograde alteration.
There is no regional pattern to the relative distribution
of the two alteration types. Both may occur in close
proximity, e.g. both are present in Middle Mill quarry,
also in neighbouring intrusions at Solva and parts of the
same intrusion south of Tre-nichol. Highly altered rocks
occur in all parts of the area and there is no obvious
pattern to their distribution. The most intensely
sericitised rocks occur around Solva harbour where
hydrothermal breccias are particularly common and at
Dinas Fawr, south of St. Elvis, where mineral veins have
been worked.
Mineralisa t ion
The quartz-sulphide veins near St. Elvis, said by Hall
(1971) to have been worked for lead and copper in the
sixteenth century, and the sulphide minerals found in
Middle Mill quarry (see below) are the only occurrences
of base metal mineralisation known in this area. The
alteration styles resembling the propyli t ic, phyllic and,
at Hollybush, possibly potassic (biotite) types around
porphyry copper systems, give a hint that hydrothermal
activity was associated with igneous intrusion in the
area. Outcrop is too poor, however, for any firm
conclusions to be made other than that the area around
Hollybush appears to contain signs of high temperature
alteration assemblages consistent with porphyry copper
mineralisation.
Geochemistry
All available exposures of intrusive rocks around
Llandeloy together with selected exposures in the
general area were sampled. In addition, because of the
absence of any exposure over three of the mapped
intrusives around Llandeloy, some samples of float were
collected (Fig. 2). Samples weighing about 2 kg were
crushed, and a split ground in a 'temal mill with lelvacitel
binder prior to pelletising and analysis for a range of
elements by XRFS. Major elements were determined on
intrusive rock samples by B-probe. The results and
detection limits are given in Tables 2, 3 and 4.
The majority of sedimentary rock samples are
mineralised siltstones and sandstones of the Solva Group
from Middle Mill quarry. These are discussed in more
detail in the section describing that area. Two samples
of laminated siltstone collected from quarries in the
Brunel Beds have composi tibns very similar to Beaufon t
Group siltstones and world averages for shales compiled
by Beeson (1980) except for a very low Ca content and
less pronounced lower levels of Sr, Zn and Ni (Table 3).
There is no indication of any concentration of siderophile
or chalcophile elements despite the presence of pyrite
both disseminated and on joints in one sample.
Comparisons of tuffaceous rocks are more difficult
because of their extremely variable composition, but no
exceptionally high levels of any of the elements
determined are present in the three samples from the
Treglemais Group. The relatively high Ee level in the
sample from a fault zone may be caused by pyritisation,
but if so, no accompanying enrichment in other
chalcophile elements is evident.
Representatives of all four groups of intrusive rocks
distinguished petrographically were analysed. Two of the
groups, however, are only represented by a single sample
and a third group by two samples whilst the remaining
twenty samples are all of porphyritic microtonalite
(Table 2). This imbalance and the small total number of
samples precluded rigorous statistical treatment of the
r esulis.
As a guide to the most likely differences in chemistry
between samples of float and bedrock samples, samples
of fresh and weathered porphyritic microtonalite were
collected from one quarry. The results (Table 2, nos. 1
and 26) suggest that weathered samples may be
relatively enriched in SiO2, A1203, Nb, Y, Cu and As and
depleted severely in CaO, but a high degree of
uncertainty must exist in a single sample comparison of
this type.
Although the quartz feldspar porphyry group is only
represented by two fl~at samples it is clear that
chemically it forms a distinct group. The samples may be
separated solely on their high Si02 content, though K20,
Rb, Pb and Nb are also high whilst total iron, MgO, TiO2,
Mn, Ni, Zn and Y are lower than in the other intrusive
rocks (Table 2). A highly altered rock sample collected
from the faulted margin of the quartz feldspar porphyry
intrusive (Table 2, no. 25) has a composition more similar .
to the porphyritic microtonalites and is included in that
group. However it is uncertain whether the rock
represents highly altered quartz porphyry, or whether
there is a slice or small body of microtonalite in the
fault zone at this point.
The compositional range within the porphyritic
microtonalite group is produced by a combination of
primary geochemical variation, patchy and variable
alteration and weathering. For example, the most acid
rock in the group (Table 2, no. 42) which also contains
the lowest Ti02 and MgO is quartz veined, whilst the
second lowest level of CaO and highest A1203 and Y are
recorded in a deeply weathered sample (no. 26). The
highest K20, Na2O and Rb and lowest K20 and Ba
results are all in samples taken from a small part of a
variably altered intrusion exposed in Middle Mill quarry.
Perhaps the sample with the most extreme composition
is that of a pyritised microtonalite from an intrusion
exposed by Solva harbour (Table 2, No. 8): it contains the
lowest Na20, A1203 and Rb and highest As, Cu, CaO and
K/Rb in the group. Another feature which is taken as
indicative of the variable alteration is the absence of a
clear pattern of inter-element correlations. The only
two closely correlated groups of elements are a 'maficl
group, probably concentrated in amphiboles and chlorite,
characterised by Ti-Fe-Ni-Mg-Zn-Y, and a 'potash groupf
of K-Rb-Ba-? Pb. Several elements show M, close ties
whilst others, such as Mo and Pb, generate positive
correlations which may be spurious, because the bulk of
the results are close to or below the analytical detection
limit and appear as near constant values in the data
matrix.
The sample representative of the quartz diorite group
can be readily distinguished from the porphyritic
microtonalites by its low SiO2 content and high Ti02,
total iron, MgO, Cu and Sr levels (Table 2, no. 2). The
porphyritic quartz microdiorite sample, which came
from a small intrusion separate from the main body in
Middle Mill quarry has a composition which falls between
the quartz diorite and porphyritic m icrotonali tes (Table
2, no. 15). Further samples of these two lithologies were
obtained from boreholes at Lhndeloy and their
chemistry is discussed in that section.
The overall composition of the intermediate rocks
collected at surface suggests a calc-alkaline parentage
and comparison with analyses from elsewhere indicate
that low Ti02 and Nb contents are particular features of
these rocks.
There are no clear indications of nearby disseminated
copper mineralisation in the chemistry of these rocks.
No Cu result is above the expected background level for
these lithologies whilst Pb and Zn show Iithophile rather
than chalcophile affinities in their inter-element
relationships. Only one sample, collected near a fault