Exploration for porphyry-style copper mineralisation near Llandeloy

concluded that except for Zn the results showed similar
spatial patterns to those obtained from the analysis of
stream sediment reported in the Wolfson atlas (Imperial
College, 1978), though careful inspection of the soil
results indicate copper enrichment in soil over this area
not evident on the published stream sediment maps.
REGIONAL STUDIES
The work and results described here are common to both
the Middle Mill and Llandeloy areas. Most was
undertaken during initial studies of the area.
Stratigraphy
The area is underlain by sedimentary and volcanic rocks
of Precambrian, Cambrian and Ordovician we. The rocks
are folded, probably as a result of Caledonian
deformation, but this area lies on the edge of the
Hercynian front and it is possible that early structures
have been modified by this event. Most rocks examined
contain chlorite and sericite, and at Solva the
manganif erous sedimentary rocks contain spessar tine,
There is not a strongly developed cleavage. The
metamorphism is low grade, possibly low greenschist
facies. In several places spotting was observed in
mudstone not adjacent to exposed intrusions which
suggests the presence of a widespread thermal event
separate from the regional metamorphism.
The stratigraphic succession is shown in Table 1. No
attempt has been made to modernise the terminology
except where following Rushton (1974). The brief
descriptioils that follow &e taken mainly from Williams
(1933).
Table 1 Stratigraphic subdivisions
- -- --- -- -
8 TetagraptusShales
7 Brunel Beds including Arenig Ordovician
Llandeloy Ashes
6 Lingula Flags Merioneth
Series
5 Menevian Group St. David's
4 Solva Group Series Cambrian
3 Caerfai Group Comley
Series
2 Treffynnon Group
Treglemais Group Pebidian Precambrian
1. Treglemais Group: variously coloured
conglomeratic tuffs with interbedded banded
porcellani tes and halleflintas, and less com mon
coarse crystal tuff and rhyolitic tuff. According to
Williams (1933) pyrite and, in less abundance,
chalcopyrite, galena and arsenopyrite occur in all
the rock types.
2. Treffynnon Group: well-bedded, fine-grained, green
to brown rhyolitic tuff thinly interbedded with
quartzite.
3. Caerfai Group: thinly bedded, green or purple,
arqillaceous, quar tz-f eldspar sandstones with minor
horizons of purplish mudstone.
4. Solva Group: green, coarse, pebbly sandstone,
overlain in turn by feldspathic sandstone, purple
and green thinly interbedded mudstone and
sandstone, and green and grey flaggy sandstone and
mudstone. In Solva harbour there are thin beds,
laminae and disconnected nodules of spessartine
rock.
5. Menevian Group: dark and light grey laminated
mudstone with silty and sandy lenses and, in piaces,
ashy beds. Thickly bedded dark grey sandstone
occurs at the top in places.
6. Lingula Flags: thinly interbedded pale grey,
siliceous, coarse siltstone and greenish-grey
m icaceous muds tone.
7 Brunel Beds: dark blue to grey sandy slate and
mudstone with interbedded ashy, pebbly sandstone.
The Llandeloy Ashes comprise lithic and crystal
tuff of intermediate composition, tuffite,
sandstone and mudstone.
8. Tetragraptus Shales: blue to blue-black cleaved
mudstone with thin, pale ashy beds.
Classification of Williams (1933)
The many intrusions in this area were divided by
Williams (1933) into two groups of different ages. The
Dimetian group intruded the Precambrian volcanic rocks
and was believed to be Precambrian in age; whereas a
younger group intruded Cambrian rocks. The
nomenclature he adopted for these rock types reflected
this division into age groupings: hornblende porphyry is
found only among the Dimetian rocks intruding the
Treffynnon Group; hornblende-diarite porphyrite intrudes
only the Cambrian; quartz porphyry intrudes the
Treglemais Group.
Hornblende porphyry Williams (1933) commented on the
diversity in the texture, in the proportions of hornblende
and feldspar, and in the alteration -in these intrusions. He
recognised potash feldspar in the rocks and from his
descriptions they appear to be granodioritic in
composition. The largest of the intrusions, exposed in a
quarry near Hollybush, is bordered by diorite but
separated from it by a thin wall of tuff. Intrusions of this
rock are cut by pegmatite veins and thin basic dykes.
Williams (op. cit.) observed chilled contacts against tuff
at the margin of the Ty-llwyd intrusion and found intense
pyritisation locally. He compared these rocks with
similar bodies believed to be Dimetian in age within the
Precambrian in neighbouring areas.
Hornblende-diori te porphyrite Williams (op. cit.)
recorded several laccoliths and sills of this rock type
within the Cambrian strata southwest of ~reffynnon.
They show a diversity in character similar to the
hornblende porphyries intruding the Treffynnon Group
and, except for the absence of K-feldspar in these rocks,
they resemble them petrographically. Intrusions of this
type were reported to be numerous by Cox and others
(1930) around Solva.
Quartz porphyry Williams (op. cit.) recorded that the
only intrusion within the Treglemais Group is quartz
porphyry near Llanhowel. The relative proportions of
quartz, plagioclase and potash feldspar phenocrysts vary
considerably as does the overall grain size. The rock is
always heavily altered, chiefly to sericite and chlorite.
Williams (op. cit.) classified this rock among the
Dime tian intrusives,
Reclassification of the igneous rocks
Despite the poor exposure all the major intrusions and
some minor bodies in the survey area were re-examined;
samples from some were analysed for major and trace
elements. Considerable textural variation was observed
even among compositionally similar rock types, and the
rock types themselves were gradational into each other.