Exploration for porphyry-style copper mineralisation near Llandeloy

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APPENDIX 5 COMPUTER MODELLING OF AEROMAGNETiC AND RESISTIVITY DATA Aeromagne tic anomaly Three profiles from the aeromagnetic map (Fig. 7) were subjected to computer interpretation. The IGS Applied Geophysics Unit Program GAM2D2 was used (Lee, 1979). Profiles were taken from the contour map and sampled at 250 m intervals. The Llandeloy magnetic anomaly lies within the influence of two larger anomalies to the south: that due to the rocks in the core of the Hayscastle anticline, and that which extends along the south coast of Wales. The mutual interference of these anomalies made it impossible to define the regional field accurately. A value of 2nT km-1 was used though there are indications for a gradient of up to 5nT kmwl negative northward. At Llandeloy, the width and gentle flanks of the anomaly indicate a source extending to some depth. The base of the model was therefore set at 3 km, and the shape of the top surface and the susceptibility were adjusted to give the best possible fit of calculated to observed anomalies, while permitting the body to be exposed at the surface where appropriate. The susceptibility which gave the best fit within these limitations was 7.5 x 10-3 cgs units, which is close to the average value for acid to intermediate igneous rocks often given in the literature. The shape provided for the interpreted sources are all generally similar. A steep southern margin dips southwards at 60-70° from the surface just south of the maximum anomaly, The northern margin, corresponding at the surface with the steepest gradient, has dips as little as 20" and is therefore somewhat concave on two of the three profiles. The individual profiles and their interpretations are presented below (Figs. A5.1 to A5.5). For profile two, an iterative interpretation method was also used, which adjusts the length of prisms (extending to infinity at right angles to the paper) above a base, which was again assumed to be 3 km. The two bodies from the automatic iterative process and the manual polygon process correspond closely to one another. Thickness of conductive overburden An attempt was made to model the resistivity vertical profile at two points using data from the pseudosections. The dipole-dipole array is not well suited to this use however, because it is very sensitive to lateral variations, and because the relatively large dipole length makes for poor resolution of the shallower layers. The two sites chosen were 3700E/500N where borehole 3 intersected over 20 m of lacustrine sediments, and 4100E/675NY where the pseudosection indicated freedom from lateral effects. It was not possible to produce a realistic model for the former site, suggesting that lateral variations are predominant here. The model for the more easterly site is shown on Fig. A5.6. Even with this relatively thin overburden, the maximum measured resistivity will be only half the true resistivity of the bedrock,
C* -- FL Cl PT C'I FV 11B5 ONUM CAI-C. CIW1.I HFSI rl O 000 (J. 250 0. SO0 0. 750 1. 000 1. 250 1 500 1. 750 2. 00Cl 7. 250 2 500 2 750 3. 000 9. 2fi0 3. 500 3. 750 4. 000 4. 2PmO 4 500 1. 750 5. 000 5. 250 5 500 5. 750 6.. 000 A. 250 6. 500 0. 19 I 91 4 17 '7. 14 11. 00 16. 03 22. 6s 31. 37 43. 15 59. a0 85. 82 13'9. B.5 224. 81 182 16 153. 17 51. 0.3 ZC. 12 14 5.3 7. 71 3. 42 0. 38 -2. -5.57 -8. 69 -11. 8EI -15. 05 -18. 12 ClEF'7HY (C:M +VE DOWN) ANTI EL EVS (EM +VE LIP ) --3 3. 500 2. BCIO 2. 100 I. 400 0. 760 NCIMBEH8 HEFFR 70 PC4 YCiONS E L2 El EVATIONS + - POI Y EXTENT15 RFYNlJli FIELD Fig. A5.1 1.1-ANISEl.liY MACiNFTI C: ANFJEIAI .Y I ( COARSE ) FOK EACH PC11 YCiOlJ : [IENC'; :;. W-NSITY CONTRAST IN Ci/f':C AIF AZI1.llJTH CIF PKOFI1.F UI tiEI::TICIN CJ. Fr T PtAu NCH'TH UIPF = 1NC:L INATI CIN OF EARTHS F IELTJ ( +VE rlL3WIJ) AZM = AZltN17H OF M(WF-7 ISATION VECl'nR W. R T PRCtF'ILE I1 I F'H .: I NC:l . I NKr I (SN OF W T I SAT 1 TJN VEr:'TOR ( +VE IinWIJ ) AMAG = I I.S'TEN5 I TY OF MAONET I SAT I CIN ( NANOl ESL A ) DEP'rHS IN KH ( +VE rlOWN), FI-EVS IN MFTRFS (+OE UP). ANGLES IN DEGS -------- I IlFlJS = 0. 000 AZF c: 4. 0 DIPF = '70 0 Azl.1 = 4 0 UIPM =. '70. 0 AMfiO :. 31 0Cl0 X 2 2. 500 0. 600 1 &tCI 8. 0CN.J 7. 500 3. 0130 ' 7. t
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Natural Environment Research Counci
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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 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 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 101 and 102: RMS RESID - 6.53 FLU. PT ELEV ORS n
Page 103 and 104: MRGNET I C: TIJTERF'HFTAT 1014 . .
Page 105 and 106: ANU.YSB OF BOLPROLB CORE& TRACE ELE
Page 107 and 108: . Sample BH , Depthv ,(m); -, - . ~
Page 109: . , I , . . . . .. . , . , '," . ',
Page 112 and 113: Fig.7 Aeromagnetic anomaly map of t
Page 115 and 116: \ Fig.13. Compilation map of resist
Page 117 and 118: Q Q E C E CCt C E Ct C Q Ct CtQ C Q
Page 119 and 120: !2 CO N A 4 ul o ~n o ul 0 DEPTH IN
Page 121 and 122: Fig.25 BOREHOLE 5 C C C C Ct C E K
Page 123 and 124: Fig.27 BOREHOLE 7 E Q C Q E C 0 Q E
Page 125: 81 82 83 84 Fig.30. : Location of c
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Exploration for porphyry-style copper mineralisation near Llandeloy

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