Exploration for porphyry-style copper mineralisation near Llandeloy

contains sparse subhedral plagioclase phenocrysts (E
53235-411, reaching 2 mm long, and 3 mm long poikilitic
hornblende crystals. Most of the rock is composed of
stubby plagioclase crystals roughly 0.5 mm long, which
are strongly altered though they often retain a clear rim
of albite. Brownish-green hornblende forms anhedral
poikilitic crystals and is usually partly replaced by
tremolite or chlorite, calcite and magnetite. Quartz is
interstitial to feldspar.
Four units of this rock were also intersected in
borehole 7; the top two separated by a fault whereas the
lower two show intrusive relationships. A generation of
large pseudomorphs after amphibole, in places with
remnants of green hornblende, is common to all the
specimens (E 53295-9, E 53300-1) and there are some
phenocrysts of plagioclase in the 2 mm size range. Most
of the rock contains closely packed subhedral to anhedral
feldspar crystals 0.25-1.00 mm long between which is a
generation of small crystals not exceeding 0.15 mm with
quartz, pale brown mica, epidote and chlorite.
Chloritised biotite is present in some specimens. All the
feldspars are altered, though relict zoning is visible.
Magnetite is present in this rock as a primary magmatic
phase, partly altered to hematite, and also as a
secondary mineral. Ilmenite, partly leucoxenized, is the
main accessory mineral.
A rock similar to this (E 53170-1) forms a sheet 3.50 m
thick in borehole 1 within the thick composite body of
porphyritic microtonalite.
Porphyritic quartz-microdiorite A sill of this rock just
over a metre thick intrudes sedimentary rocks in
borehole 8. The rock (E 53848) contains plentiful
subhedral and euhedral phenocrysts, up to 2 mm long, of
zoned plagioclase which is usually altered to sericite.
Relict fresh crystals are oligioclase, but are possibly
albitised. Pseudomorphs of chlorite and magnetite, some
with pyrite, are probably after amphibole. There are a
few recrystallized quartz phenocrysts no more than
0.25 mm in size. The groundmass, median grain size
about 0.5 mm, consists of subhedral altered feldspar,
with chlorite and some quartz. There is abundant pyrite
in this rock.
guartz diorite In borehole 4 this rock intrudes the
overlying microdiorite complex and shows an irregular
sharp contact with it. No other rock was in contact with
it in borehole 3A. The rock (E 53242, 53255-6) is
composed of euhedral and subhedral, normally zoned,
plagioclase crystals up to 2.5 mm long and plentiful
brownish-green hornblende, which varies from euhedral
to poikilitic anhedral. In places the hornblende may form
5 mm long phenocrysts. The plagioclase is usually lightly
altered whilst some hornblende is completely altered.
Chlorite and epidote pseudomorphs are suspected to be
after biotite. There is a little interstitial microperthite
in the samples from borehole 4. Quartz, which is also
interstitial, comprises about 10% of the rock and is
unevenly distributed.
Tonalite Textually, this rock (53246-7, 53257) is
difficult to classify. It consists mainly of subhedral to
euhedral crystals of plagioclase ranging in size from 0.25
to 3 mm, but with a median size of about 1 mm. The
feldspar crystals, which in places are closely packed and
elsewhere form an open framework, are set in a matrix
of equant grains of quartz, rarely more than 0.15 mm in
size, and small plagioclase crystals. The plagioclase,
though strongly altered in some rocks, shows excellent
relict zoning. Many crystals have clear albite rims.
There are large pseudomorphs of chlorite and epidote, or
chlorite with pale brown biotite and tremolite, after
amphibole and some chloritised biotite. Where unaltered
the hornblende is brownish-green. The quartz content
ranges from a little over 10% to 25%. Rutile occurs as
an accessory mineral.
Porphyritic microtonalite This rock is present in
boreholes 1, 2, 6 and 8. It is always porphyritic, locally
glomeroporphyritic, containing abundant euhedral or
subhedral phenocrysts of plagioclase, 0.4-3 mm long, less
abundant pseudomorphs after biotite and amphibole and
small phenocrysts of quartz in a groundmass of equant
grains of quartz, subhedral or euhedral feldspar and
chlorite with a grain-size range 0.02 to 0.2 mm (E 53165-
8, 53172-5, 53233-4, 53294, 53857, 53860-1). The
feldspar phenocrysts are altered mostly to sericite and
albite, some with chlorite, or to saussurite. Relict zoning
and twinning is commonly visible. Fresh crystals in
borehole 6 display cores of labradorite, and in a sill in
borehole 8 they show oscillatory zoning. Exsolved or
replacement K-feldspar is present in samples from
borehole 1. Euhedral pseudomorphs of chlorite, biotite
and magnetite after amphibole, in some slides enclosing
plagioclase, fall in the same size range as feldspar
phenocrysts. Most rocks contain chlorite and magnetite
pseudomorphs after biotite. Quartz phenocrysts are not
common. They rarely exceed 0.5 mm in size. They are
euhedral or irregularly embayed, clear and locally
slightly strained. Ilmenite is a common accessory
mineral. The total quartz content ranges up to 25%,
most of which is in the groundmass.
Weathering
Pervasive weathering was observed in boreholes 1 to 7
down to depths ranging from 8 to over 23 m and below
these depths down to over 40 m in zones adjacent to
joints. In borehole 8 weathering only along joints was
observed immediately at rockhead.
Most intersections through the uppermost glacial
deposits revealed oxidised clay throughout, though in
places lengths of residual grey clay were evident. The _
underlying Tertiary lacustrine sediments are uniformly
deeply weathered. Feldspar crystals in the feldspar sands
are almost completely kaolinised. At all depths there are
ferruginous or manganese-lined fractures. At certain
levels pebbles in gravel beds have been completely
pseudomorphed by manganese minerals.
The weathered zone beneath rockhead varies from 4 to
>37 m in thickness. The complete profile consists of an
upper zone of deeply weathered rock, a middle zone in
which deeply weathered rock alternates with less
weathered or even fresh rock, and a lower zone in which
weathering is confined to rock immediately adjacent to
joints. This full profile was recorded even in the borehole
with the thinnest weathered zone, but it is not always
complete. Intrusive rocks are preferentially weathered
and in borehole 8 weathering in the sedimentary rocks,
which extended only to 18 m depth was along joints only.
The volcanic rocks in boreholes 1, 6 and 7 all weather
less readily than the intrusive rocks, which in the upper
part of borehole 1 are reduced to orange-brown crumbly
clay with white kaolinite pseudomorphs after feldspar
phenocrysts adjacent to much less intensely weathered
volcanic rock.
Common to the weathered zones in all rocks are
ferruginous or manganese-filled joints and fractures,
manganese-lined cavities and hematite-altered pyrite.
Brecciation, alteration and mineralisation
Brecciation
Rocks in all the boreholes are brecciated and fractc-ed.
Zones of closely spaced, planar fractures or irregular
intersecting fractures are commonplace. There are,
however, many examples of network hair fracturing and
crackle breccia also in zones, and in places these are
associated with clearly defined veins of intrusive
breccia. Brecciation is not confined to any one rock
though it is confined to zones within them: all the
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