Exploration for porphyry-style copper mineralisation near Llandeloy

than average values quoted for sandstone and slates
(Turekian and Wedepohl, 1961). The high results are
probably caused by analytical interference from rare
earth elements, which are much higher in the
sedimentary roclts than the igneous rocks.
It is uncertain whether the relatively weak mineral-
isation and small changes in LIL elements and their
ratios in borehole 8 is a function of alteration zonation
or host rock control. Evidence from the borehole is
inconclusive. Some of the intrusions intersected apqear
to show a different type of alteration and enrichment in
Cu compared with the adjacent sedimentary rocks, for
example the porphyritic quartz microdiorite cut bet~veen
45.34 and 46.44 m. Also, ratio plots of the analytical
results for borehole 8 intrusives show more scatter than
those of the host rocks, although none of the values
match the extreme outlying results reported from
boreholes 2, 4 and 5. The sedimentary roclis in borehole 8
are not of a type which might be considered particularly
resistant to alteration and, in most porphyry systems,
host rock lithology has little effect on the intensity of
alteration: a comparable example is Coed y Brenin
which is emplaced into a very similar sequence of host
rocks. It is concluded, therefore, that the sedimentary
rocks in borehole 8 were most probably peripheral to the
hydrothermal system during the main (propylitic-
potassic) phase of hydrothermal activity despite the
presence of biotite alteration. The presence of intrusion
breccias, a feature of the peripheral areas of porphyry
deposits, supports this conclusion.
Regional alteration Surface as well as borehole samples
show signs of alteration which has affected the igneous
rocks across the whole area. The alkali metals show
great variation in the Llandeloy-Middle Mill area as is
Spilite
A
/
/
.O /
Rhyodacite
Trachyte
Rhyolite
+r X . , /xx
,
Alkali Basalt, . . +
illustrated using the plot devised by Hughes (1973). On
this plot the Llandeloy rocks cut right across the field
defined by Hughes as containing most common fresh
igneous rocks (Fig. 39). It suggests that many of the
rocks collected frorn available exposures have suffered
soda metasomatism and some borehole samples potash
metasomatism. Great local variation is expressed by the
sanples from a small part of the intrusion exposed in
Middle Mill quarry. These rocks show quartzsericite-
chlorite-epidote alteration and it is not clear whether
the chemical variation here is partly or entirely due to a
regional event or hydrothermal alteration associated
with the intrusion.
Loss of Na2O and gain of K20 is one of the
characteristics of the potassic alteration associated with
the mineralisation (see above), so on Fig. 39 the borehole
rocks define a clear trend extending from the field
occupied by soda metasornatised roclts, across the
'igneous spectrum' of Hughes into the potassic field. The
rocks with the highest K2O contents and containing K-
feldspar alteration plot in the salne sector as mineralised
intrusives from Coed y Brenin (Allen and others, 1976)
though the Llandeloy rocks are richer in total alkalis. It
is likely that these roclcs have suffered both potash and
soda metasomatism. Na20 does not show a strong
positive correlation with any other element in surface or
borehole samples, only a weak association with Si02 and
Sr, and it is concluded that Na metasornatis~n was not
accompanied by any other element determined.
There is no evidence from the borehole data to equate
soda metasomatism with late stage propylitic alteration,
although data is insufficient to be certain. On Nugi;esl
diagram (Fig. 39) late stage propylitic alteration shows
conflicting trends. One interpretation of the sketchy
data indicates that late stage propylitic alteration
superimposed on early propylitic alteration increases
/
/
/ / K Rock showlng I(-feldspar alteration
Surface Borehole
0 Porphyritic rnicrotonalite
%
Tonalite m
Quartz microdlorite (BH 2) +
Quartz rnicrodiorlte(8H 7) X
0 Quartz diorite 4
Porphyrltlc rnicrodiorite T
/ Microdior~te A
Mnst Cnnd-v-hmnin rocks
Porphyritic rnicrotonalite from
Middle Mill quariy
Figure 39 Plot of (r(?O/K20 + Na2O) x 100 v (K20 + Na20: showing the fields occupied by common igneous
rocks and mineralised Coed y Brenin rocits
. .
. .