Exploration for porphyry-style copper mineralisation near Llandeloy

it to this late propylitic alteration event. Carbonate
veining is the last event seen to affect borehoie rocks.
Al, P, Zr arid Ti are the elements determined which
appear to have remained relatively immobik during K-
feldspar alteration. Apparent depletion of these
elements in the most altered rocks is attributed to
dilution. Ce and Th may also be immobile but results are
truncated by analytical detection limits, In the
propylitic (phyllic) alteration zones only the alkalis, the
alkaline earths and chalcoghile elements appear to have
been affected.
The only metal analysed of economic interest is
copper. It b associated with S and Fe and these three
elements produce a strong gemhemica1 signature in
mineralid rocks. The Mo content of the rocks is
generally low ((91 ppm). There are local but very weak
enrichments of As, Pb and possibly Zn, though the latter
may be a secondary concentration in weathered rocks,
All these chaEcophile elements show independent
beimviour and individual element concen tratians are not
closely related to each other or, with the possible
exception of Pb, to the alteration pattern. Gold has not
been determined but, because of the probable tectonic
setting of the deposit, it may be present in appreciable
amounts. Some chalcopyrite is disseminated but most
occurs in veins associated with the late stage propyiitic
alteration. Such veins owur both earlier and later than
pyrite-magnetite veins but the time difference 19 not
considered to be significant. There was, however, an
earlier Cu mineralisa tion associated with the propylitic-
potassic alteration, Rocks which show minimal late
propylitic alteration are Cu rich and there is a general
association of high Cu and particularly high CufS with
rocks containing evidence of remnant strong potassic
alteration. Whether the late stage propylitic alteration
was accompanied by an appreciable influx of Cu (as well
as Fe and S) or mainly redistributed Cu introduced during
the earlier event is uncerfain,
The alteration and mineralisation have many features
in common with those Hollister (1974) claimed were
characteristic of the 'diorlte typet of porphyry copper
deposit, Recent work by Tayiw and Fryer (1980) has led
to a refinement of the porphyry alteration models
provided by Lowell and Guilbert (19701, Guilbert and
Lowell (1974) and HoZlister (1974). It b now considered
probable that early potassic and propylitic alteration is
caused largely be magmatic fluids and that phyllic
alteration may be imposed on them when mixing of
magmatic fluids with barge quantities of meteoric water
takes place. At- Llandeloy the K/Rb ratios suggest either
strong partitioning of Rb into the vapour phase or a
minimal involvement of late magmatic fluids in the K-
feldspar alteration.
The mineral assemblages characteristic of the
alteration at Llandeloy are thought to be influenced by
the relatively basic original hat rock composition, but
they may also be influenced by depth. Little is known
about the bottoms of porphyry systems though Sillitoe
(1973) and Guifbert and Lowell (1974) amongst others
have predicted the probable characteristics of a root
zone. Among the predicted features which the Llandeloy
rocks share are (i) lack of phyllic (sensu latu) alteration,
(ii) low grade of mineralisation, (iii) presence of
magnetite and (iv) irregular development of alteration
zoning, Further evidence that the root zone of a
porphyry system may be exposed at Llandeloy is provided
by the overlying lacustrine sediments. These are rich in
magnetite, feldspar and Cu and were clearly derived
from the erosion of a nearby quartz-poor, magnetite and
copper-r ich mineralised intrusion complex. Structural
inferences from geophysical results, which suggest that
the mineralised area lies in the core of an anticline or
horst underlain at shallow depth by Precambrian and
bounded by faults in the nocth and southeast, also
support the deep level hypothesis.
The driiling did not define the full extent of the
mineralised area. Clearly only a small segment of a
porphyry system has been examined and the axis of the
system may have been considerably modified by post
m ineralisa tion ear th-movements, The scacce bedding
orientation data available suggest that the axis, if
perpendicular to bedding, may plunge to the southeast.
The dimsition of alteration zones in boreholes 4, 6 and
-8 may, however, be interpreted in terms of a west or
northwesterly Ut of the axis. In either case there is an
apparent westward decrease in the intensity of
alteration.
Finally, the strong northeast trending geophysical
feature near borehole 3A may be interpreted as a fault
with a downthrow to the southeast, The ground to the
southeast of this line may have relatively high IP but no
magnetic anomalies. Tetragraptus Shale is faulted
against mineralid rocks in the north and Precambrian
is faulted against them on the west. Extensions of the
porphyry system may, therefore, be concealed beneath
the shales in the north and be at depth southeast of the
northeast trending geophysical feature near borehole 3A.
CONCLUSIONS AND B&COMMENDATIONS
1. Disseminated copper mineralisation, believed to be
associated with the deep levels of a dioritic copper
porphyry system, has been discovered near
Llandeloy , southwest Dyf ed
2. Because of the deep level of intersection,
alteration zonation is imperfectly developed and
cwr mineralisation is not of economic grade, the
best borehole intersection yielding 0.1% Cu over
3.4 m.
3. Alteration and mineralisation was a two phase
event in which early propylitic-potassic alteration
and accompanying mineralisation was overprinted
by a late- stage propyritic event.
4. It is believed that the part of the copper porphyry
most likely to have contained ore-grade
mineralisation has been eroded away, and that
some of the remains are represented by immature,
rotted sand and clay deposits beneath boulder clay.
These deposits reach 20 m in thickness and contain
up to 640 ppm Cu.
5. The geology of the area is still imperfectly
understood because of the thick drift deposits, and
the possibility exists that, because of tilting and
down-faulting, parts of the copper deposit may be
concealed in areas to the north and east of the
area drilled.
6. The age of the mineralisation is uncertain, but it is
reasonable to assume that it is connected with
emplacement of intermediate intrusives, which the
tenuous evidence suggests was a late Cambrian or
early Ordovician event.
7. The style of mineralisation, the chemistry of the
associated intrusives and the known geology
suggest that mineralisation took place in thin sialic
crust, most probably in an island arc type of
environment,
8. The deposit was initially located by soil sampling
followed by shallow drilling. Drainage sampling and
rock sampling of available exposures gave little or
no indication of mineralisation in the vicinity.
Geophysical measurements (IP, magnetic, VLF-EM)
were inconclusive, partly because of the absence of
a well-developed pyrite halo and the presence of
magnetite from several sources. The thick
unconsolidated deposits confused all surface