Exploration for porphyry-style copper mineralisation near Llandeloy

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Figure 45 Contour map of tow magnetic field from ground survey resuits in the area around the boreholes Table 16 Mean (R) and standard deviation b) of magnetic susceptibilities of borehole cores Rock-type or borehole no. Code N Rx 10-3 10- lop SI units Unweathered rocks of BH1 Unweathered rocks of BH2 Unweathered rocks of BH4 Unweathered rocks of BH5 Unweathered rocks of BH6 Unweathered rocks of BH7 Unweathered rucks of BH8 Lacustr ine deposits C 16 1.76 -0.2 0.73 Weathered rock W 41 0.56 -0.48 0.46 Volcanies V 6 0.43 -0.40 0.23 Quartz wackes & siltstones' S 9 8 1.49 -0.16 0.37 Tonali te T 24 5.5 0.12 0.82 Porphyritic microtonalite2: inc. borehole 1 excl. borehole 1 Quartz-m icrodiorite: borehole 2 borehole 73 QmD 14 13.23 0.68 0.78 QmD 17 1.17 -0.29 0.60 Quartz-diorite QD 5 8.1 0.52 0.81 Microdiorite mD 13 23.0 1.18 0.57 Porphyritic microdiorite mD1 12 8.01 0.12 0.71 1 excluding magnetite bands R = 0.62 2 R for borehole 1 porphyritic microtonalites is 0.21 3 separated on geochemical grounds
QnD (BH 7) 1 Log rwceptibitity (9 uniu x 1~3) 1 -0.5 0 0.5 +I +1.5 L I I 1 I f T I n Y V - t mS t 8 Y QnD (BH 2) 1 n - w " I QDI - I n Y w - I h u V - I 0.1 0.5 1.0 2 5 10 20 Figure 46 Magnetic susceptibilities of borehole samples 1976). Three co m~onent populations with geometric means of 0.22, 0.6 and 17 x 10-3 SI units were defined (Fig. 47). The largest population (mean 0.6), consisting of about 75 percent of the sample, can be regarded as the 'primary' background susceptibility of the rocks and probably contains unresolved populations related to different lithologies. The lower population is related to weathered rocks whilst the upper population can be ascribed to rocks containing magnetite generated during late stage propylitic alteration. Magnetic susceptibility may, therefore, be useful in detecting such rocks in boreholes. The magnetic complexity of the area as a whole, however, means that this magnetic signature is of little use on a wider scale because there are at least four modes of occurrence of magnetite here: secondary hydrothermal, primary magmatic, magnetite horizons within sedimentary rocks and detrital magnetite in lacustrine sediments. The identification of localised highly altered rock as the most magnetic is partly at odds with the interpretation of anomalies in the wider area discussed above. The model of an anticlinal core of magnetic Susceptibility (SI units X ~ O - ~ J Precambrian rock clearly cannot be adopted without qualification in the Llandeloy area, but the broad magnetic anomaly further west does extend unbroken to Llandeloy, and it is likely that these rocks underlie the area at no great depth. The forms of some of the ground profiles (Appendix 2) certainly suggest some local, near surface effects superimposed on a much deeper seated anomaly. An alternative possibility is that the broader anomaly at Llandeloy is due to a subvolcanic intrusion, perhaps representing the remains of a magma chamber underlying the hypothetical Treffynnon volcano. However, the magnetic source indicated by numerical modelling has sides sloping outwards and a base width of 5 km or more at a depth of about 3 km, not a likely form for a subvolcanic intrusion. The susceptibility used for the models, about 7.5 x 10-3 SI units, is much greater than that of the unaltered intrusions at Llandeloy, but more typical of the Precambrian rocks which cause the western part of this anomaly. An anticlinal core therefore provides a more likely explanation of the broad anomaly than a subvolcanic intrusion.
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 54 and 55: Figure 42 Contour map of apparent r
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 . .
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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