Exploration for porphyry-style copper mineralisation near Llandeloy

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Figure 42 Contour map of apparent resistivity at n=2 in the area around the boreholes Figure 43 VLF horizontal intensity in the area of the boreholes with contours at 206 intervals 44
Magnetic surveys Numerous magnetic anomalies were recorded with maximum amplitudes up to 500 nT on the ground and 200 nT from the detailed airborne survey (Fig. 7). Measurements on borehole cores show that susceptibilities of the magnetic rocks lie generally around 0.6 x 10-3 SI units. The magnetic data available for the Llandeloy area provide a picture of considerable complexity, but several useful conclusions can still be drawn. Strong short-wavelength anomalies due to near surface magnetic rocks are superimposed on a broad high indicating an underlying magnetic body of some depth extent. The pat tern of aeromagne tic anomalies in the wider area and their interpretation has been discussed in the regional geophysics section. In the Llandeloy area two groups of short wavelength anomalies occur separated by the line of the Tref fynnon Fault (Figs. 44 and 45). To the north-west of the fault lies a roughly triangular group of anomalies with a 250 nT high along the northern edge. There is no geological indication of the source of these anomalies which are not accompanied by high chargeabilities but do coincide in part with weak copper-insoil anomalies to the north of Ty Llwyd. To the east of the Treffynnon Fault a roughly linear magnetic high up to 300 nT on the ground runs NE-SW along the line of the major chargeability and geochemical anomalies. The anomaly is not highly peaked, and on one traverse (3700E) becomes noticeably broader. Drilling showed that this effect was due to the lacustrine deposits. Several smaller anomalies occur north of the main high, but to the south the magnetic field falls away quite rapidly. To the west the anomalies of this group end abruptly alonjg a line sub-parallel to the Treffynnon Fault but 200 to 300 metres to its east. A second fault along this line is implied, which could also explain differences in the degree of alteration between boreholes 7 and 8, and the 2 to 5 group. Double or multiple faults with this orientation occur elsewhere in the St David's area, notably south west of the city. Table 16 shows the results of the susceptibility measurements and profiles are plotted on the graphic logs (Figs. 20-28). High suscep tibillties occur almost exclusively in altered intrusive rocks, but not all members of the intrusion complex are magnetic, and petrographically similar rocks from different boreholes have widely different susceptibilities. Magnetic susceptibility cannot, therefore, be used to distinguish different types of igneous rocks in the complex, different petrographic types having overlaming ranges. This is partly at least because of two secondary factors superimposed on the primary magnetic susceptibility variation. Firstly, there is a general relationship between susceptibility and degree of alteration, the most highly altered rocks of the area intersected by boreholes two to five showing average susceptibility eight times that of the other cores. Secondly, rocks from the weathered zone give lower results. Except for some thin magnetic horizons, low results were also obtained from the shales, sandstones and volcanic rocks. The lacustrine sediments were mostly weakly magnetis but contained some material of moderate susceptibility. If this magnetism arises from detrital magnetite a local source for the sediments is implied, as there is no other magnetic rock within a considerable distance. There is good evidence, reviewed by Parker (1983) that magnetic susceptibility is distributed log-normally through a rock. Good descriptors of this distribution are the mean and standard deviation of the log-transformed data. Figure 46 shows these quantities calculated for the various lithologies at Llandeloy as well as the frequency histogram of susceptibility on a logarithmic axis. The histogram, constructed from all the data, is clearly poly modal and was split into component populations using cumulative frequency curve analysis (Sinclair, Figure 44 Aeromagnetic anomaly map of the area around the boreholes with contours at 4 nT intervals 45
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 50 and 51: than average values quoted for sand
Page 52 and 53: Introduction The main geophysical m
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 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
Page 101 and 102: RMS RESID - 6.53 FLU. PT ELEV ORS n
Page 103 and 104: MRGNET I C: TIJTERF'HFTAT 1014 . .
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ANU.YSB OF BOLPROLB CORE& TRACE ELE
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Fig.7 Aeromagnetic anomaly map of t
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Fig.27 BOREHOLE 7 E Q C Q E C 0 Q E
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Exploration for porphyry-style copper mineralisation near Llandeloy

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