Exploration for porphyry-style copper mineralisation near Llandeloy
Exploration for porphyry-style copper mineralisation near Llandeloy 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 . . . .
potash con tents whilst in K-feldspar altered rocks a If the results of the elements and ratios which most reduction in potash and increase in soda occurs. Comparison of surface and borehole rock results From the mineral exploration viewpoint the differences between the surface and borehole results are of great importance, because surface rock sampling gave no clear indication of the disseminated mineralisa tion intersected by the boreholes. Precise comparisons between the borehole and surface rock data are hampered by the lack of dioritic rocks in the surface samples, which in itself is significant. The closest chemical comparison that can be obtained is shown in Table 15. Alteration in the borehole samples is dominantly of weak potassic (biotite) type, overprinted with variable intensity by late stage propylitic alteration. In the surface samples alteration is very variable but frequently of quite intense propylitic type with quartz-chlorite-epidote-sericite assemblages developed. The two groups have virtually the same mean silica content and similar levels of most elements concentrated in basic rocks. Cu is the only element to show a great contrast in the two groups, with the lowest borehole result only slightly less than the highest level recorded in the surface rock group. The borehole samples may also contain higher levels of Pb, Zn, As, Ba and Rb and lower CaO, Na2O and Zr. As the medians indicate, some of these differences are greatly emphasised by the presence of one or two very high results and the differences may not be significant, but with the exception of Zr, they are changes which might be predicted when moving toward the centre of a porphyry system and changing from propylitic to potassic alteration zones (e.g. Chaffee, 1976). The higher Zr content of the surface samples may be related to weathering. clearly show variation in the vicinity of the mineralisation (for example Cu, S, Cu/S, Na/K, Rb/Sr) are plotted out on a regional basis for the surface rocks no clear pattern emerges, and most variables show as great a variation within one outcrop as across the whole area. Only tenuous differences, such as the absence of very high soda in rocks collected in the vicinity of the mineralisation, can be discerned and it is concluded that no regional pattern useful for exploration purposes can be discovered from the surface rock sample results, even with the use of hindsight. Samples taken closest to the boreholes give no chemical clue to the nearby mineralisation. This is surprising considering the extent of the mineralisation and can only be attributed to chance. The quartz diorite sample taken from Hoilybush quarry (Table 2, no. 2) was collected within 200 rn of borehole 5 (tonalite at bedrock surface with 300 ppm Cu in drill mud) and contains the highest copper (46 ppm) and Fe (9.05%) content of any surface sample. It also contains low Zr and Na20, but these are features entirely consistent with the more basic composition of this lithology compared with other surface rocks. Only a slightly elevated Mo content suggests that it is close to disseminated mineralisation. It is therefore concluded that inadequate sampling caused by a lack of surface exposure, together with the masking effects of regional alteration and the complicated pattern of alteration associated with the disseminated mineralisation, are the reasons which have prevented the detection of extensive disseminated mineralisation by whole rock analysis of samples from available exposures in this area. It shows that in such an area this type of approach, used in isolation, could be a most misleading prospecting tool. Table 15 Comparison of surface and borehole analyses of igneous rocks M~O CaO Na2O K20 p205 Surface Rocks1 Maximum Minimum Median Mean Major elements quoted as percent of oxide, trace elements as ppm. Borehole samples2 Mean Median Minimum Maximum 1 Twenty porphyritic m icrotonali tes from outcrops in the Llandeloy-Middle Mill area. 2 Sixteen porphyritic microtonalites and tonalites from boreholes 1, 2 and 5.
- Page 1: Natural Environment Research Counci
- Page 5: Report No. 78 @ Crown copyright 198
- Page 9 and 10: SUMMARY REGIONAL STUDIES Intrusive
- Page 11 and 12: SUMMARY Geological, geochemical and
- Page 13 and 14: concluded that except for Zn the re
- Page 15 and 16: The only rock containing evidence o
- Page 17 and 18: Table 3 Analyses of miscellaneous r
- Page 19: - 28 St. Brides Bay -
- Page 22 and 23: Hercynian orogenies (M. 3. Arthur,
- Page 24 and 25: Figure 14 Location of soil anomalie
- Page 26 and 27: producing a very limited dispersion
- Page 28 and 29: -1 00- Contours of VLF horizontal i
- Page 30 and 31: area and periglacial and glacial de
- Page 32 and 33: the Brunel Beds or the Tetragraptus
- Page 34 and 35: contains sparse subhedral plagiocla
- Page 36 and 37: There is some variation in the alte
- Page 38 and 39: The cause of other isolated weak Cu
- Page 40 and 41: j Table 9 Summary of analyses of in
- Page 42 and 43: increase the Zr content. The divisi
- Page 44 and 45: variation between pu8M microdioritn
- Page 46 and 47: T/I~~/~~IPYS openjoi~~tcd limestone
- Page 48 and 49: Table 13 Summary statistics for sed
- Page 52 and 53: Introduction The main geophysical m
- Page 54 and 55: Figure 42 Contour map of apparent r
- Page 56 and 57: Figure 45 Contour map of tow magnet
- Page 58 and 59: Pigum? 47 Cumulative f reguency plo
- Page 60 and 61: it to this late propylitic alterati
- Page 62 and 63: HOLWSTER, V.F. 1974. An appraisal o
- Page 64 and 65: APPENDIX 1 ABBREWATEI) BOREHOLB LOG
- Page 66 and 67: BOREHOLE 2 Registration Number STvI
- Page 68 and 69: BOREHOLE 3B Registration Number SM
- Page 70 and 71: BOREHOLE 5 Registration Number SM 8
- Page 72 and 73: BOREHOLE 6 Registration Number SM 8
- Page 74 and 75: BOREHOLE 8 Registration Number SM 8
- Page 76 and 77: SANDSTONE: as above CLAY GOUGE: SAN
- Page 78 and 79: --- APPARENT ESISTIVITY pa IN OHM M
- Page 80 and 81: APPARENT ESISTMTY H Bbs ys 06 q6 CH
- Page 82 and 83: z ~ l ~ ~ ~ l ~ l . ~ l ~ , ~ l l A
- Page 84 and 85: z ~ 1 ~ 1 ~ 1 ~ 1 ~ 1 APPARENT FEJS
- Page 86 and 87: z ~ l $ l l l l l ~ l l ~ l l l l ~
- Page 88 and 89: MAGNETIC FELD IN NAFTESLA Y /-*-.-.
- Page 90 and 91: !IN@ a 8 g e s s I! s F N : , S ? I
- Page 92 and 93: ~ N & E e 8 g B 8 8 7 a F N . S ~ 9
- Page 94 and 95: PHYSICAL PROPERTY MEASUREMENTS ON B
- Page 96 and 97: Borehole 8 (diameters of core 9-72
- Page 98 and 99: APPENDIX 5 COMPUTER MODELLING OF AE
than average values quoted <strong>for</strong> sandstone and slates<br />
(Turekian and Wedepohl, 1961). The high results are<br />
probably caused by analytical interference from rare<br />
earth elements, which are much higher in the<br />
sedimentary roclts than the igneous rocks.<br />
It is uncertain whether the relatively weak mineral-<br />
isation and small changes in LIL elements and their<br />
ratios in borehole 8 is a function of alteration zonation<br />
or host rock control. Evidence from the borehole is<br />
inconclusive. Some of the intrusions intersected apqear<br />
to show a different type of alteration and enrichment in<br />
Cu compared with the adjacent sedimentary rocks, <strong>for</strong><br />
example the porphyritic quartz microdiorite cut bet~veen<br />
45.34 and 46.44 m. Also, ratio plots of the analytical<br />
results <strong>for</strong> borehole 8 intrusives show more scatter than<br />
those of the host rocks, although none of the values<br />
match the extreme outlying results reported from<br />
boreholes 2, 4 and 5. The sedimentary roclis in borehole 8<br />
are not of a type which might be considered particularly<br />
resistant to alteration and, in most <strong>porphyry</strong> systems,<br />
host rock lithology has little effect on the intensity of<br />
alteration: a comparable example is Coed y Brenin<br />
which is emplaced into a very similar sequence of host<br />
rocks. It is concluded, there<strong>for</strong>e, that the sedimentary<br />
rocks in borehole 8 were most probably peripheral to the<br />
hydrothermal system during the main (propylitic-<br />
potassic) phase of hydrothermal activity despite the<br />
presence of biotite alteration. The presence of intrusion<br />
breccias, a feature of the peripheral areas of <strong>porphyry</strong><br />
deposits, supports this conclusion.<br />
Regional alteration Surface as well as borehole samples<br />
show signs of alteration which has affected the igneous<br />
rocks across the whole area. The alkali metals show<br />
great variation in the <strong>Llandeloy</strong>-Middle Mill area as is<br />
Spilite<br />
A<br />
/<br />
/<br />
.O /<br />
Rhyodacite<br />
Trachyte<br />
Rhyolite<br />
+r X . , /xx<br />
,<br />
Alkali Basalt, . . +<br />
illustrated using the plot devised by Hughes (1973). On<br />
this plot the <strong>Llandeloy</strong> rocks cut right across the field<br />
defined by Hughes as containing most common fresh<br />
igneous rocks (Fig. 39). It suggests that many of the<br />
rocks collected frorn available exposures have suffered<br />
soda metasomatism and some borehole samples potash<br />
metasomatism. Great local variation is expressed by the<br />
sanples from a small part of the intrusion exposed in<br />
Middle Mill quarry. These rocks show quartzsericite-<br />
chlorite-epidote alteration and it is not clear whether<br />
the chemical variation here is partly or entirely due to a<br />
regional event or hydrothermal alteration associated<br />
with the intrusion.<br />
Loss of Na2O and gain of K20 is one of the<br />
characteristics of the potassic alteration associated with<br />
the <strong>mineralisation</strong> (see above), so on Fig. 39 the borehole<br />
rocks define a clear trend extending from the field<br />
occupied by soda metasornatised roclts, across the<br />
'igneous spectrum' of Hughes into the potassic field. The<br />
rocks with the highest K2O contents and containing K-<br />
feldspar alteration plot in the salne sector as mineralised<br />
intrusives from Coed y Brenin (Allen and others, 1976)<br />
though the <strong>Llandeloy</strong> rocks are richer in total alkalis. It<br />
is likely that these roclcs have suffered both potash and<br />
soda metasomatism. Na20 does not show a strong<br />
positive correlation with any other element in surface or<br />
borehole samples, only a weak association with Si02 and<br />
Sr, and it is concluded that Na metasornatis~n was not<br />
accompanied by any other element determined.<br />
There is no evidence from the borehole data to equate<br />
soda metasomatism with late stage propylitic alteration,<br />
although data is insufficient to be certain. On Nugi;esl<br />
diagram (Fig. 39) late stage propylitic alteration shows<br />
conflicting trends. One interpretation of the sketchy<br />
data indicates that late stage propylitic alteration<br />
superimposed on early propylitic alteration increases<br />
/<br />
/<br />
/ / K Rock showlng I(-feldspar alteration<br />
Surface Borehole<br />
0 Porphyritic rnicrotonalite<br />
%<br />
Tonalite m<br />
Quartz microdlorite (BH 2) +<br />
Quartz rnicrodiorlte(8H 7) X<br />
0 Quartz diorite 4<br />
Porphyrltlc rnicrodiorite T<br />
/ Microdior~te A<br />
Mnst Cnnd-v-hmnin rocks<br />
Porphyritic rnicrotonalite from<br />
Middle Mill quariy<br />
Figure 39 Plot of (r(?O/K20 + Na2O) x 100 v (K20 + Na20: showing the fields occupied by common igneous<br />
rocks and mineralised Coed y Brenin rocits<br />
. .<br />
. .