Exploration for porphyry-style copper mineralisation near Llandeloy

the Brunel Beds or the Tetragraptus Shales. The intrusive
episode can be dated, therefore, as being pre-Arenig and
later than middle Cambrian.
The form of both the individual intrusions and
complexes is difficult to resolve. Porphyritic
microdiorite intersected in borehole 8 shows markedly
discordant relationships with the Solva Group and in
other boreholes the porphyritic microdiorite appears to
form veins intersecting other intrusive rocks. Porphyritic
quartz-microdiorite and microtonalite in borehole 8,
however, form concordant sheets. There is evidence of
repeated injection of sheet-like bodies of the same rock;
in borehole 7, for example, four sheets of quartz-
microdiorite were intersected showing intrusive contacts
against each other. It is not uncommon to find cognate
xenoliths in quartz-microdiorite and porphyritic
microtonalite. Where both the top and bottom of a sheet
were located none exceeded 16.5 m in thickness. The
intrusive complexes are unlikely to be stocklike. It is
most probable that they are concordant or semi-
concordant laccolithic bodies, the results of repeated
injection of thin sheets of magma, with, perhaps the
porphyritic microdiorite forming a late dyke phase.
Relative orders of intrusion among some of the rock
types can be established by the examination of contacts
and identification of xenoliths in some boreholes.
Extrapolation between them, however, is not easy and no
order of intrusion applicable to all boreholes can be
devised for all the rock types. This particular problem is
compounded by the considerable amount of variation
within some rock types and among transitional types:
thus, though distinct rock names are given it is
conceivable that differently named rocks could belong to
the same intrusive phase and vice versa.
Petrography of the intrusive rocks
The seven distinct rock types, each of which is described
below, can be grouped into three broader divisions which
may be more meaningful when considering magma
genesis. These are:
a) Porphyritic microdiorite
b) Microdiorite
Quartz-microdiorite
Porphyritic quartz-microdiorite
Quartz diorite
c) Tonalite
Porphyritic microtonalite
The porphyritic microdiorite, though displaying some
textural variation is outstanding in being quartz-free or
containing a very low content of it.
The second group, with the exception of the
porphyritic quartz-microdiorite, which is of restricted
occurrence, is texturally homogeneous. The principal
variations are in the quartz content and grain size. The
quartz, however, is unevenly distributed even within a
single slide and the microdiorite and quartz-microdiorite
may comprise a single series.
The third group is consistently quartz-rich and
texturally distinct. All the microtonalites are strongly
porphyritic and the tonalites, though differently named,
may be regarded as densely porphyritic microtonalites.
Among the rocks sampled at surface no equivalent was
found of the porphyritic microdiorite or of the
microdiorite to quartz-microdiorite series. The
Hollybush quartz diorite compares with the quartz
diorite in borehole 3A. The porphyritic quartz-
microdiorites found at surface are similar to and as
uncommon as among the borehole rocks. Most of the
surface rocks are porphyritic microtonalite and, though
no textural equivalents of the tonalites were found, they
may be compared directly with the tonalite-porphyritic
microtonalite group in the boreholes.
Microdiorite This rock (E 53249-53) is sparsely
porphyritic with phenocrysts of plagioclase and
amphibole up to 5 mm long. The bulk of the rock is
composed of stubby plagioclase crystals with a median
size of 0.5 mm patchily altered, mostly at the crystal
centres, to sericite, epidote and minor chlorite. Small
amounts of (?)exsolved K-feldspar occurs within the
plagioclase. Relict zoning shows through the alteration.
Hornblende, when fresh is pale green and forms poikilitic
crystals, but mostly it is replaced by chlorite and
calcite. Quartz is interstitial, locally recrystallised and
comprises no more then 4% of the rock, though it is
irregularly distributed. Accessory minerals include
magnetite and sphene.
Three sheets of this rock occur in borehole 4 (Fig. 24).
In all cases, there is a fine-grained porphyritic margin,
containing euhedral amphibole and plagioclase
phenocrysts, which passes into coarse-grained only
sparsely porphyritic rock. In the upper sheet this rock
shows little variation through over 14 m of core. In the
middle sheet, only 0.58 m thick, the porphyritic variety
reappears in the middle; but in the lower sheet, 4.44 m
thick, there is complete mixing of the two types with
irregular transitions between them. Patches of medium-
grained uniform feldspar and amphibole rock merge into
rock containing feldspar crystals from 0.25-2.5 mm long
dispersed in irregular concentrations through fine-
grained feldspar, hornblende and quartz.
Porphyritic microdiorite In borehole 4 a sheet of
porphyritic microdiorite beneath the lower microdiorite
sheet passes inperceptibly into it and, like it, it displays
some textural heterogeneity. Plagioclase phenocrysts (E
53254) up to 2 mm long, showing sutured margins, are
altered to sericite and epidote. Some phenocrysts of
greenish brown, poikilitic, euhedral hornblende are
slightly smaller. The groundmass comprises plagioclase,
hornblende, minor quartz with abundant chlorite and
epidote, but is partly recrystallised.
In boreholes 1 and 5 porphyritic microdiorite forms
irregular, discordant veins and minor intrusions no more
than 1.48 m thick. They display sharp, chilled contacts
with flames and lobes penetrating the wall rock. The
rock (E 53169, 53244-5, 53248) is texturally quite
different from the type in borehole 4. It is stongly
porphyritic with euhedral and subhedral amphibole
phenocrysts commonly up to 5 mm long, rarely up to
1 cm lpng and abundant somewhat smaller plagioclase
phenocrysts. The pseudomorphs after amphibole are
composed of chlorite and ?magnetite; in places
containing plagioclase inclusions. The feldspar
phenocrysts are altered to sericite and albite, but display
relict twinning and zoning. Typically, the groundmass
consists of feldspar laths about 0.05 mm long with
variable amounts of chlorite and opaque dust. The
texture, however, is obscured by alteration. One rock
only has a fine-grained recrystallised matrix. Quartz is
very rare. Accessory minerals include apatite, sphene
and hematite.
The two-metre thick discordant intrusion intersected
in borehole 8 (E 53846) differs from the others in being
fine-grained and having closely packed phenocrysts. The
rock is chlorite rich and it is veined by slightly more
acid, uniformly fine-grained feldspar-quartz rock.
Quartz-microdiorite Two large complexes of this rock
were intersected; one in borehole 2, the other in
borehole 7. The rock bears a strong resemblance
texturally to the non-porphyritic microdiorite in
borehole 4, but the quartz content is higher, ranging
from 6.5 to over 10%. Quartz is irregularly distributed
through this rock giving concentrations as high as 12.5%
in parts, but the overall content is below 10%.
Four units of this rock were drilled in borehole 2. They
ranged from 0.91 to 8.32 m thick. Each of the top three
units has a richly xenolithic basal zone, the inclusions all
being of quartz-microdiorite. The rock in borehole 2