Exploration for porphyry-style copper mineralisation near Llandeloy

intrusive rocks are affected, as are the volcanic and
sedimentary rocks they intrude (see Figs. 20 to 29).
Discrete veinlets of intrusive breccia occur in
porphyritic microtonalite in borehole 1 (7.28 m) and a
5 cm vein is present within porphyritic microdiorite
(22.74 m) in borehole 5. In the latter example the breccia
is composed of angular fragments no more than 8 mm
long of porphyritic microdiorite in a matrix
predominantly of epidote, but with some small angular
fragments of feldspar. A 4 cm thick vein intersects
volcanic rocks in borehole 6 (22.45 m).
Intrusive breccia is most common within the
succession of Solva Group sedimentary rocks
encountered in borehole 8. At several levels lengths of
core as much as four metres long display intense crackle
breccias, through which are threaded either several veins
of breccia containing rotated and in some parts
subrounded fragments, or a single breccia vein in the
middle. No truely exotic fragments were observed in
these breccias, but the fragments were not always the
same as the immediately adjacent wall rock. In one vein
(E 53837) fragments of mudstone with aligned sericite
were rotated after the formation of the schistosity.
Elsewhere, clearly defined breccia veins penetrated
unbroken rock. In some the fragments are set in quartz
cement, or less commonly, epidote. Usually, however,
the matrix consists of a small portion of finely
comminuted rock and sandsize quartz with abundant
chlorite and, in some rocks, associated sulphides. The
chlorite fills most voids and penetrates fractures in
fragments.
Alteration
All the rocks encountered show evidence of alteration
though it is irregularly developed. Veining, sulphide
mineralisation and brecciation are all spatially related to
the alteration and together appear to reflect the effects
of a hydrothermal system comparable to that associated
with classic porphyry copper deposits. Four principal
types of alteration are present, two of which are
essentially potassic and two propylitic. The potassic
alteration is represented by a high temperature form in
which K-feldspar and amphibole are stable, and a
presumed lower temperature form in which biotite is the
main alteration mineral and K-feldspar minor or absent.
A primary propylitic alteration is recognisable locally,
but the most widespread alteration is late-stage
propylitic. This has been imposed on all other alterations
and local epidotisation is related to it. Widespread, but
localised alteration to carbonate is the last event.
K-feldspar alteration Despite the widespread late-stage
propyli tic alteration relict mineral assemblages of
earlier, high temperature events are preserved and in
boreholes 4 and 5 there are minerals and textures
characteristic of the high temperature potassic
alteration. The characteristic minerals are K-feldspar,
pale green amphibole and biotite. In both boreholes there
are zones of recrystallisation consisting of an aggregate
of sutured equant grains in veins forming a network
through the rock. Veins may cross large feldspar
phenocrysts. The recrystallised veins contain euhedral
crystals of potash feldspar, the amount of which varies
from minimal (E 53250) to moderately abundant (E
53252). Potash feldspar also is present, derived from
recrystallized plagioclase, in the walls adjacent to
mineralized fractures. Some of the potash feldspar is
introduced in veins, for example, in microdiorite in
borehole 4 (E 53253). The slightly microperthitic potash
feldspar is associated in recrystallized areas with pale
green amphibole and pyrite and in veins with pale green
amphibole, epidote, sphene and pyrite, in places with
late calcite. Pale green amphibole replaces the
magmatic hornblende and itself is chloritised or replaced
by secondary brown mica. Fine-grained pale green mica
is present in some rocks in both of these boreholes and
this also is chloritised.
In borehole 4, near the top, no K-feldspar was
identified in the rock, but primary brown-green
hornblende is replaced by pale green amphibole which
also forms veins traversing the rock. In all of these rocks
the plagiocw is albitised and altered mainly to sericite
but both may be a result of the latestage
propylitisation. Magmatic amphibole and biotite has
been replaced mostly by secondary biotite which is now
c Nor i t ised.
The original extent of this type of alteration is
difficult to determine but veinlets containing K-feldspar
occur in boreholes 1 and 2 where the rocks are
dominated by low temperature, presumably re tragrade,
mineral assemblages.
Biotite alteration Alteration to biotite is evident in all
boreholes except 3A and 6, though little of the biotite is
preserved in borehole 4. The effect of retrograde
propylitic alteration, which generally masks this
alteration, are least visible in borehole 5. In the tonalite
from this borehole fresh brownish-green biotite replaces
both primary hornblende and biotite. Relict crystals of
pale green amphibole, similar to the variety which
characterises the potassic zone, remains in parts of the
rock and there are tiny exsolution blebs or replacement
patches of K-feldspar in plagioclase. Original zoning is
visible through the alteration in these rocks and in
another lightly altered rock from a thin sill of
porphyritic microtonalite (E 53856) containing biotite
pseudomorphs after amphibole in borehole 8; suggesting
that the original plagioclase is stable in the biotite zone.
Strictly speaking this alteration is potassic, but it is a
slightly lower temperature form. In most rocks the
partly chloritised biotite is dusty brown and besides
replacing original ferromagnesian minerals it forms
clusters of small crystals, stringers, network veinlets and
disseminations through the body of the rock.
The characteristic assemblage of minerals replacing
the argillaceous matrix in the quartz wacke and
siltstones encountered in borehole 8 contains greenish-
brown or brownish-green biotite and sericite, in places
with disseminated fine pyrite and a little chlorite. The
micas are fine-grained and not strongly foliated. Veinlets
of biotite and chlorite with pyrite are present, in places
forming a network. The sills within the succession
contain biotite pseudomorphs after amphibole or contain
chlorit ised secondary biotite and it is considered that
both the sills and sedimentary rocks have been subjected
to biotite alteration.
Primary propylitic alteration Common characteristics
of the rocks showing this type of alteration are an
alteration assemblage of clinozoisite, epidote, chlorite,
sericite and tremolite; the common, but not necessarily
total preservation of primary brownish-green hornblende,
and the presence of the original, though mildly altered,
zoned plagioclase.
The least altered rock in all the boreholes is quartz
diorite in borehole 3A. Zoned plagioclase is patchily
altered to clinozoisite and minor sericite. Poikilitic
brownish-green hornblende is fresh, though clumps of
small crystals of brown biotite show selective
chloritisation. This alteration may be late stage deuteric
or even metamorphic, but it differs only by degree from
that in other boreholes (for example in borehole 7) which
is probably hydrothermal. Quartz-m icrodiorite in
borehole 7 contains saussuri t ised zoned plagioclase,
chloritised biotite and pseudomorphs of fibrous
amphibole, chlorite, yellow epidote and quartz after
amphibole. Rosettes of fibrous amphibole occur
throughout the rock (E 53296). There is some
disseminated pyrite. Veins of fibrous amphibole, minor
quartz and chlorite with pyrite and some chalcopyrite
are also present.