Natural Environment Research Council Institute of Geological Sciences
Mineral Reconnaissance Programme Report No.1
.1 The concealed granite roof In• south-west Cornwall
K. E. Beer, BSc, A. J. Burley, BSc, PhD and J. M. Tombs, BSc
I I
A report prepared for the Department of Industry (Reference SW 903)
London: 1975 The Institute of Geological Sciences was '(ormed by the incorporation of the Geological Survey of Great Britain and the Museum of Practical Geology with Overseas Geological Surveys and is a constituent body of the Natural Environment Research Council. t NERC copyright 1975
Ie is recommended that reference· to this report be made in the following form: BEER. K. E.. BURLEY. A. J. and TOMBS. J. M. 1975. The concealed granite . roof in south-west Cornwall. Miner. Reconnaissance Programme Rep. Inst. Geol. Sci.• No.1, 16 pp [UnpubJ.]
IGS 1462 25 10/75 rr:rneo in England by the Instit.ute of Geological Sciences This report relates to work carried out by the Institute of Geological 'Sciences on behalf of thE: Department of Industry. The information contained herein must not be published without reference to the Director, Institute of Geological Sciences.
S. H. U. Bowie Programme Manager !nstitute of Geological Sciences 64~ 78 Gray's Inn Road London we 1x SNG CONTENTS Page
BACKGROUND "I AIMS GEOPHYSICAL MEASUREMENTS 1 DRILLING 3 PRELIMINARY RESULTS 3 CONCLUSIONS 3 REFERENCES 4
APPENDIX A: GENERALISED GEOLOGICAL LOGS OF IGS BOREHOLES 5 1. Parbola Borenole 5 2. Bosworgy Borehole 13
APPENDIX B: GEOPHYSICAL BOREHOLE LOGS In pocket . 1. Par bola Borehole 2. Fraddam Borehole
ACCOMPANYING MAPS: SOUTH-WEST ENGLAND, COMPLETE BOUGUER ANOMALIES In pocket 1. St Austell area: SW 9S 2. St Austell area: SW 96 3. St Austell area: SX 06 4. St Agnes area: SW 64, 74, 75 5. Hayle-Leedstown area: SW 53, 63
ILLUSTRA TION
Fig. 1. Location map showing granite outcrops, SW England 2
iii Summary
This report ou,lines the results of grayity sun'eys and drilling to locate cusps, possibl:-' s:"nr,lierous, in the concealed granite roof north and west of ,he C"rr:n.eneii:s granite uutcrop. The results are plotted on the two majJS of the compiece Bouguer .-\nomaly (maps -l and 5 at 1: 10 560 scale) 3ccompan:-'ing this :'e,pot't. Three oth~r maps (numbered 1 to 3) are also presen:ec ShOI\:ll::'; ,be- results of similar sun'eys in the area to the north of the St ..-\:ustell g:"Z:.nite: these are not described in this text, but a full inter pre:,,:ion of all of :,fie gt'avity results is in preparation.
Ti,e SU1'':E:yS "rotmd the Carnrnencllis granite showed that shallower- depth prolo:1:;3tio::5 oi outcropping granites can be defined but that smaller or r:!ore (!ee;:>ly :J"..:ried rises in the roof are not eaSily recognisable. The :;'l:ol~:al.'" :~:"ps :;u;_,;est that the gt'anite roof exhibits less relief -than- expected It'OI~: other' geologicai indica"ions alld that relationships between mineralisation, pOl'pi:,':r:,' intrusion and granite eminences are considerably more complex than pos tula led ;:>reviou51y.
T:Je 80s\\-Or0- jorehole penetrated granite a: 173.51 m at the intersection of :he south-I\-e::;t'sZ:.rds extension of the Carn Brea - Carn .-\rthen - Pendarves ~ra:1ite ridge lI"it!: the northern prolongation of the Tregonning - Godolf,lhin l'idge. T!ll' uppe-:,!::ost 20 11\ of granite at Bosll"orgy is greisenised with 1011" gradl" cassiterite, ch ... lcopyritc and pyrite disseminations.
:\t Pat':)ola, dl'illing on a postulated granite cusp reached 665. 51 m without penetrating the 5teeply dipping southern flank ot' a granite ridge. C<:!.ssiterite Ira5 ,'ound in some nurro\\" quartz-c:hlorite veiJ;s, the richest carrying about 1 per cent O\"et' 0,25 m. ..\.t 379-382 rn there is a ·::omplex copper-Iead-zinc lode \':ith a thin s'2.nniferous quartz-chlorite margin,
Generalised ::;eological logs of the t\\'o boreholes form :\ppendix A. Appendix H provides details of geophysical logs (G,ilmrna R2.~', Xeutron, ElectriC, Caliper, DensilY, Tempe 1'2. LUre, Diffe rential Tempera lure) measu toed by British Plaster Board Limited,
iv The concealed granite roof in south-west Cornwall
K. E. Beer, BSe, A. J. Burley, BSe, PhD and J. M. Tombs, BSe
The following report is renroduced from AIMS t;'le Transactions of the Institute of Mining and Two economic aims were intrinsic to the i\letallurgyl by kind permission of the Editor. i~vestigation - the location of sh~,llow -depth cuspate granite bodies likely to carry stanni This, the second brief report upon mineral ferous vein swarms (as at Cligga, Hemerdon ami reconnaissance investigations sponsored by the St Michael's i\lount) and the'-correlation of eh'an Department of Industry (Dunham, 1973), out and lode frequency with concealed granite ridges. lines the results of gravity surveys and drilling Selection of the survey 'area was dictated by four instigated to locate and examine cuspate eleva major factors: (1) irregularity of roof surface, tions in the concealed granite roof to the north indicated by cusps and ridges cropping out at and west of the Carnmenellis Granite outcrop. Carn 1\larth, Carn Brea, Tregonning and St A total area of 330 km2 was surveyed, 1100 Michael's l\lount; (2) abundance of porphyry gravimeter readings being taken at an average dykes and productive veins within the area; (3) density of 0.9 stations per km2 - this being intensity of mineralisation and concentration of increased some tenfold in areas of indicated emanative centres (Hosking, 1964, 1966) around thin slate cover. Two boreholes were drilled and between granite outcrops; and (4) interpretative to examine postulated granite cusps and control afforded by underground and commercial abbreviated geological logs of these have been drilling information. published (IGS, 1973). Gravity data maps and full borehole logs were placed on open file in In the structurally complex and lithologically the offices of the Institute in London, Exeter, variable setting of Cornwall detailed gravimetric Leeds and Edinburgh on the date of publication surveys were considered to offer the speediest of this note. and least ambiguous method of defining the large r irregularities of the granite surface. Supporting A detailed account of the results obtained regional geochemical studies were not attempted will be published in due cou rse. because of widespread surface contamination from former mining activities. I3AGKGROUND Hosking (1969) recently restated the belief GEOPHYSICAL :\IEASUREMENTS that most of the economically important tin, Gravity observations were made initially at deposits of Cornwall occured as vein swarms Ordnance Survey benchmarks and spot heights within, or crose to, the apices of granitic at a density of approximately one station per CliSpS or in ,rrnjor vein structures flanking km2 by use of a vehicle-borne gravity meter, granite ridges. If this generalisation is true, In two areas selected for detailed ,,;urvey graviT.Y it follows that the location of concealed granitic and levelling traverses were made on foot along cusps and ridges could provide valuable guidance roads and tracks and, occasionally, fields to in exploration for hypothermal ore bodies. give the required density of cover. Worden and La Coste and Romberg gravity meters were used. The form of the' batholith roof is imperfectly known. Widely spaced gravimetric observations After correction for gravity meter drift and (Bott, Day and l\lasson Smith, 1958) indicated tides, observed gravity values were derived by the large-scale relief and small-amplitude the use of. the existing IGS gravity base net and a ridges have been demonstrated in South Crofty value of 981. 265 gal at PendUlum House, Cambridge. mine (l\lacAlister, 1903) and in recent drilling Combined eleva;.ion corrections to sea-level and nort.h of Redruth. Circumstantial geological a partial terrain correction were made with a and minerological evidence prompted Hosking density of 2.67 g cm- 3, and the preliminary (l!.lG!:)) to postulate the positions of subsurface Bouguer anomaly was obtained by subtracting gl'anite ridges; but, hitherto, there has been normal gravity according to the 1930 lnte rnai.icr,;;.l no attempt to verify or modify the model. Gravity Formula (recently compiled data now
ITrans. lnst. l\Iin. Metall. B., Vol. 84, pp. 1925. .so 60 70 80
.so .so SCAlf o S 6. 7 8 9 10 K~
TRURO +
.leh.dy RfDRUIH + Crofty Mine ...... t~r;~::. 'J"8,e-q:"" + GWENNAP
Po'COIC S ..
Y'!-.~=I • HQ~D!on
.so 70 80
Fig. 1. Location map showing granite outcrops, SW England
2 obtainable from IGS incorporate a full terrain 'PRELIMINARY RESULTS correction and the 1967 IGF). The Bouguer A refined model of the subsurface granite anomaly is dominated by the negative trough form awaits detailed computer processing of following the Cornubian granite chain, but where all available geophysical and geological data, the granite is covered by kUlas, the anomaly but semi-quantitative analysis of the Bouguer becomes relatively positive because the killas anomaly and residual Bouguer anomaly maps has a higher average density than the underlying indicates a pattern of relief differing somewhat granite. Before the Bouguer anomaly can be from published postulations. used to determine killas thickness, the gravitational effect of the killas must be An extension of the Carn Brea - Carn Arthen separated from that of the granite. To achieve - Pendarves granite ridge can be traced south this the Bouguer anomaly over areas of exposed westwards to meet a northern prolongation of granite is used to deduce the cross section of the Tregonning-Godolphin ridge. The Bosworgy the batholith in th~se areas. The effect of a borehole was sited in the vicinity of this inter fictitious batholith,. which reaches the surface section. The distribu,tion and abundance of everywhere and whose cross -section varies tin-copper veins and porphyry dykes in the gradually between the known cross-sections Gwinear area may be related to this concealed at the areas of exposed granite, is then granite ridge. There is no clear definition of subtracted from the observed Bouguer anomaly. a ridge linking the Godolphin and St Michael's This yields a residual Bouguer anomaly that is Mount granites. Surprisingly, there is no roughly proportional to killas thickness. suggestion of a shallow cuspate granite near Modelling studies by use of three -dimensional the emanative centre at Wheal Hampton computer techniques to obtain the depth to (Hosking, 1964, 1966). A limited north grani'te are continuing; after consideration of westerly extension of the Carn Marth mass is borehole and surface density measurements evident, but near-surface connexion with the a density' contrast of 0.1 g cm - 3 between killas Cligga - St Agnes ridge is uncertain. The and granite has been chosen as representing a latter trends roughly parallel to the coast, where reasonable average for the area. Borehole sharp terrain effects confuse the gravity results, derived depths to granite have not been used and it remains uncertain whether the ridge may directly as control but only to provide a check continue westwards offshore (Hosking, 1969). for gross inconsistency. This is because undulations of the granite roof of short wave North of the Carn Brea - Bosworgy ridge length compared with their depth would be un the granite surface is presumed to dip gently detectable by.. gravity survey even' if their north~ards, and any irregularities on its amplitudes were moderately high; since gravity surface are too deep or too small to be survey gives average rather than exact local recognisable. Certainly, there is no indication depths, a single borehole would not necessarily of granite close enough to surface to account for provide a reliable control. the tin mineralisation and spotted slates recorded on the coast north of Tehidy (Hosking, 1949). DRILLING Two unexplained gravity' highs' are apparent Two boreholes were drilled for the dual near St Erth and southeast of Hayle; both lie purpose of supplying interpretative control and immediately north of wide greenstone belts and examining indicated granitic cusps. At may relate to locally thick developments of Bosworgy (NGR SW 5806 3367) the hole entered basic rocks. Formerly important copper lodes batholithic granite at a depth of 173. 51 m, and lie close to the north of both anomalies. at Parbola (SW 6157 3633) the borehole was drilled close to the steeply dipping southern . Postulated east-west granite ridges beneath flank of a granite ridge without penetrating the the highly mi~erali sed areas of Gwennap (Rayment, igneous body before termination at 665.51 m. Davis and Wilson, 1971) and Wheal Vor do not The uppermost 20 m of granite at Bosworgy are give rise to recognisable. gravity anomalies. In greisenized and carry low-grade disseminations both areas the slate cover is known to be deep of cassiterite, chalcopyrite and pyrite. In the (in excess of 300 m), however, and this may Parbola cores cassiterite occurs in some mask relatively small-amplitude features. narrow quartz-chlori~e veins (the richest about Perhaps more Significant is the fact that both 1 per cent over 0.25 m), chalcopyrite is districts are located adjac:ent to granite cupola disseminated in greenstone over 1.3 m at 316 m walls where steep gravity gfadients suggest the depth and accompanies tourmaline in a 2 m lode possibility of major faulting. cutting elvan at 290 m, and at 379-382 m there is a complex copper-lead zinc lode with one CONCLUSIONS 10 cm selvage of stanniferous quartz-chlorite This investigation has shown that regional veins tuff. gravity surveys with closely.spaced stations can
3 j J outline, with reasonable certainty, the ----1969. The nature of the pr~mary tin ore2 ! shallower-depth prolongations of outcropping of the south-west of England. Pp. 1157- granites, but the interaction of unresolved 1243 in A second technical conference on complex structure and unsuspected major tin, Vol. 3. (Bangkok: ITC). lithological variations (eg the thick arenaceous INSTITUTE OF GEOLOGICAL SCIENCES 1974. succession of the Parbola borehole) prevents the IGS Boreholes 1973. Rep. Inst. Geol. I'(~cognition or definition of sr.-.aller or more Sci. Rep. No. 74/7 (London: HMSO). deeply buried rises in the granite roof. In the !I.'lacALISTER, D. A. 1903. A cross-section and present state of the science it seems doubtful some notes on the tin and copper whether this could be achieved without a large deposits of Camborne, with .special number of deep control boreholes. reference to the limits of productive ore ground. Trans. R. Geol. Soc. Visual examination of the gravity anomaly Cornwall, Vol. 9, pp. 773-795. maps suggests that the granite roof exhibits RAYMENT, B. D., DAVIS, G. R. and WILSON, less relief than was hitherto expected from J. D. 1971. Controls to mineralisation other geological indications, and it is clear at Wheal Jane, Cornwall. Trans. lnstn. that relationships between mineralisation, Min. Metall. Vol. 80, pp. B223-237. porphyry intrusion and granitic eminences are considerably more complex than were depicted by (Hosking, 1969). Nonetheless, hypothermal mineralisation in various structural patterns is associated spatially and presumably, genetically with all the known granite cusps and ridges, and the location of concealed major apical bodies is likely to define areas of future mineral pntential. In the present investigation the occurrence of stanniferous greisenised granite in the Bosworgy cusp demonstrates the validity of this concept.
REFERENCES
BOTT, M. H. P., DAY, A. A., and !\IASSONS!\UTHD. J. 1958. The geological interpretation of gravity and magnetic surveys in Devon and Cornwall. Philos. Trans. R. Soc. A, Vol. 251, pp. 101 -1 91. D1XES, H. G. 1956. The metalliferous minir.f, region of south-west England. Mem. Geo!. Sun·. G. B. (London: Hi\ISO).
Du?\HA1\I, K. C. 19.3. Report of the Director. Pp. 3-4 in Annual Report for 1972. (London: Institute of Geological SCie>nces.) 241 pp. HOSKING, K. F. G. 1949. Fissure systems and mineralisation in Cornwall. Trans. R. Geo!. Soc. Cornwall, Vol. 18, pp. 9-49. 1966. Permo-Carboniferous and later primary mineralisation of Cornwall . and south-west Devon. Pp. 201-245 in Present views of some aspects of the geology of Cornwall and Devon. Commemorative volume of the Royal Geological Society of Cornwall for 1964.
4 APPENDIX A: GENERALISED GEOLOGICAL LOGS OF IGS BOREHOLES
1. Parbola Borehole
Location Parbola Farm. Gwinear. Hayle. Cornwall Collar situated at G R SW 6157 3633. 640 m at 241 0 (T) from Cathebedron Cross. I" Geoi. Sheet NS 351/8.
Driller Drilling and Prospecting International Ltd
Date February - April 1973
Surveys Vertical to 149 m. below which the hole inclines at 850 towards 1470 T. Collar height approx. 80.8 m above O. D.
Sizes 0- 9.14m H size 9.14 - 149.35 m HQ size 149.35 - 665.51 m NQ size
Classification Description Thickness Depth m m Overburden Soil and clayey subsoil 0.91 0.91
Head Compacted clay with fragments of slate and vein quartz 3.71 4.65
l'vlylor Series Weathered slates. fragmental recovery. 6.63 11.28 Dark grey. smooth to lustrous slates. somewhat iron stained at the top. Locally very hematitic and lower down there are pyrrhotite flakes developed on cleavage surfaces. Major cleavage dips at 10-30°. Rare quartz veins and intercleavage bands. and some clay filled jOints. Well displayed tight folding disposed horizontally 23.31 34. 5.9
Quartz Vein Massive white quartz stained by iron oxides. No ore minerals other than pyrite and hematite. Dips at 400 1.01 35.60
Mylor Series Soft. dark grey. vaguely spotted slates with pyrrhotite on cleavage which dips at 200 • Tight. near horizontal folds. Sporadic development of low angle quartz veining 14.83 50.54
Granite Vein Dips at 300 • almost parallel to cleavage in host slates. Partly fine grained aplitic composition and partly very coarse grained and kaolinised. The basal 50 mm is brecciated slate cemented by ? aplitic material and may be an explosion breccia 0.25 50.68
5 Classification Description Thickness Depth m m i\[ylor Se ries Darkish grey. soft. smooth slates· 1.32 52.00
Intrusi .. "" Sreccia Granitic and aplitic material cementing partly rounded brecciated slate fragments 0.33 52.33
~·I:"lor Series Dark grey soft slates with a little pyrite 0.26 52.59 intrusive Breccia As body above. of which it may be a thin offshoot 0.10 52.69
.\iybr Series Darkish grey smooth slates with major cleavage dipping at 200 . Some low angle quartz veining. Slates vaguely spotted. sometimes knotted 13.76 66.45
Quartz \-ein Broken massive white quartz with some inclusions of baked slate. A little chlorite. Dips at 600 0.88 67.33 i\Iylor Series Grey slates. variably hematised. Well spoited and slightly indurated. Cleavage dips at 20 - 30° . Some tight folding with axial plane parallel to main cleavage. Clusters of low angle quartz veins 63.18 130.51
Quanz \"ein Broken recovery of massive white vein quartz with patchy chlorite and chlori tised slate. Dips at 350 . Joints coated by chlorite with abundant sphalerite. some chalcopyrite. pyrite and a little arsenopyrite 2.43 132.94
:\[ylor S!a:es Dar!, grey to near black. smooth. tough slates cleaved at 150 alternating with zones of striped siliceous slates banded at 20-250 dip. Some tight. near horizontal folding 10.42 142.75
Flat lying band of massi ve white quartz with a little chlorite 0.43 143.18
~"Iy!vr Sl".:es Grey slates_inter.bedded with pale grey silty sandstone layers. Intensely sheared and folded. Bedding variable from 100 to 30 0 dip; cleavage less steep apparently . 3.00 146.18
(~uar.L \" "in Dips at 55°. Cellular in uppermost 40 mm and thereafter massive. Some chlorite. little pyrite 0.86 147.04
.\Iylor Ser:'es Grey to silvery grey. micaceous silty slates. finely banded with banding usually steeper dipping than the cleavage inverted? Little chlorite and traces of pyrite. Some zones of low angle quartz veining" Vaguely spotted or knotted.
6 1 Classification Description r:-" ::£:ress Depth ::L m
rarely .conspicuously so. Rare thin sandstone horizons below 157.53 m. Occasionally hornfelsed. Much ~ : . ., shearing in parts '-----.. 233.86
!\lylor Series? Dark grey to brownish grey somewhat hornfelsed striped sediments consisting of alternating slates and silty sandstones. Locally the slates are preponderant and 1 the sandstones become more discrete and thicker. Scattered quartz veins. Sediments dip at 20-40°, cleavage 1 generally is less .. steep. Generally spotted, locally intensely so 260.63
l Elvan Upper contact sharp and irregular, I dipping about 300 • Slate above slightly brecciated. Fine grained throughout with 1 slightly finer chilled margin. Very finely I. porphyritic and rich in tourmaline and dark chloritic spots. Abundant tourmaline veining below 282.17 m. Locally kaolinised 'J.~-_ .. _.~- 288.90
Lode Upper part of shattered siliceous elvan cut by abundant tourmaline veins. Central zone essentially of quartz with tourmaline veining recut by massive chlorite carrying abundant chalcopyrite. Basal section of tough siliceous elvan with tourmaline veins and some later chlorite/ chalcopyrite veining. Apparently sub -ve rtical 290.93
::::lvan Tough, fine grained equigranular elvan similar to that above vein. Locally kaolinised or hematitic 297.92
Explosion Breccia Ang!.Ilar to rounded fragments of slaty rocks in a clastic granitic matrix. Contads poorly cored 299.77
Elvan Fine grained equigranular elvan, rich in tourmaline and rather siliceous 305.19
Explosion Breccia Wedge shaped body similar to that above and with one fragment of granite 305.64
Elvan Very fine grained grey siliceous elvan with glassy groundmass. Very finely porphyritic. No chilled margin. Lower contact dips at 100 308.13
Portscatho Se ries? Finely banded pale grey sandstones interbedded with darker grey micaceous slates. Tight flat folds cut by later shearing. Some irregular quartz veining 311.18
7 Classification Description Thickness Depth m m
Elvan Pale grey, tough, fresh, finely porphy ritic elvan with narrow chilled margins. Dips at about 250 0.76 311.94
Ponscatho Series Pale grey sandstones interbedded with darker grey micaceous hornfelsed slates. Flat, tight folding 0.33 312.27
Elvan Coarsely porphyritic, pale, slightly greenish grey elvan with narrow fine grained chilled margins. Dip 50 - 600 • Very fresh 3.20 315.47
Greenstone Dark greyish to brownish green, tough, compact, fairly fine grained igneous rock. Patchy development of quartz chalcopyrite -sphalerite near the top. Chloritic near base 5.19 320.66
Elvan Upper contact is irregular, stepped, at about 400 dip. Coarsely porphyritic generally pale grey rock with narrow chilled margins. Locally altered by kaolinisation or chloritisation; some streaky ferruginous discolouration. Locally intense tourmaline veining. Basal contact dips at 250 8.86 329.52
Greenstone Tough, hard, compact, fairly fine grained dark greyish blue to greyish green igneous rock. Chloritic adjacent to elvan. Rare chalcopyrite in actinoli tic veining 3.33 332.85
Elvan Dips at 20-300 . Medium grey coloured porphyritic rock with much finely clustered chlorit(~ and some tourmaline. Some chlorite veins carry traces of chalcopyrite 4.67 337.52.
Greenstone Dark green, brown and bluish, hard, fairly fine grained, compact igneous rock. Some chloritic veining. Some sediment ·at base. Dip 100 2.10 . 339.62
Pon::;catho Series Thin grey sandstone separated by very thin layers of brown biotitic hornfelsed slates. Banding dips at 10-20°. Rare thin quartz veins 3.46 343.08
GreenstOne Dark green and brown, tough, compact, fine grained rock. Some epidotic and sideritic veining. Dip 100 0.30 343.38
8 Classification' Description Thickness Depth m m
Portscatho Series Predominantly light grey, fine sandstones with thin greenish grey chloritic slates and brown biotitic slates. Bandirig dips 0 at 0 _20 • Tightly folded but disturbed by later shearing. Rare quartz veining. Quartz veinlet with cassiterite at 349.27 m and rare quartz -chlorite -sphalerite veining below this 12.55 355.93
Greenstone Fine grained, tough, dark greyish green to bluish rock. Massive and poorly jOinted. Both contacts ill-defined 3.q7 359.00
Portscatho .Series Interbedded thin grey sandstone and bluish to greenish slates, becoming more arenaceous in depth. Bedding varies from, 00 to 250 dip. Flat lying tight folding. Clusters of near horizontal quartz veins 8.49 367.49
Essentially bluish grey smooth, spotted slates with thin sandstone bands. Intensely sheared in parts and frequently veined by low-angle quartz 5.71 373.20
Interbedded sandstones and slates, intensely sheared and stretched. Bedding at 200 dip. Thin quartz banding. Chloritised below 378.10 m 5.69 378.89
Lode Banded quartz/ chlorite and white quartz vein complex with the basal 0.5 m of brecciated slate in a cement of siderite and quartz. Throughout there is a· mixture of sulphides, mainly sphalerite. galena, chalcosite, ·chalcopyrite, pyrite and arsenopyrite. Neither contact is clear but banding suggests dip of about 700 3.02 381.91
Portscatho Series? Pale grey silty slates, highly sheared. Foliation at 250 dip. Spotted at base 1.99 383.90
Greenstone Bluish to brownish green, hard. tough. fine grained, compact igneous rock. Near horizontal 1.32 385.22
Portscatho Series? Pale grey silty slates with thin sandstones, dipping at 20-250 0.53 385.75
Greenstone Tough, compact, hard. fine grained dark green, brown or blue igneous rock, somewhat biotitic 0.76 386.51
Portscatho Series Olive green silty slates with thin sandstone horizons. Cleavage dips at 200 , bedding variable. Tight folding evident 8.97 395.48
9 Classification Description Thickness Depth m m
Greenstone? Two bands of tough. fine grained highly biotitic rock. apparently altered green stone; separated by pale grey sandstones and ·dark grey slates 0.68 396.16
Portscatho Series Pale grey sandstones with thin interbedded darker grey slates. Banding dips at 300 or less and is often tightly folded 14.38 410.54
Greenstone Dips at 20 _25 0 and is tough. fine grained green to brown compact rock. Traces of sulphides 3.79 414.33
Pprtscatho Series Alternating pale grey sandstones and thin. micaceous and hornfelsed darker grey slates. Bedding usually dips at 250 or less and is frequently folded or sheared. The sandstone;slate ratio varies from t:1 to 9;1
A probable altered greenstone - now a .dark brown.quartz-biotite-garnet rock. near horizontal extends from 434.17 to 434.32 m 31.22 445.55
Grey and silvery micaceous slates. intensely folded 0.51 446.06
Quartz Vein Massive milky white quartz with a little chlorite. Dip 65-700 1.04 447.10
Portscatho Series Dark grey and brownish grey spotted slates. soft and altered. Contorted and abundant zones of quartz veining. Small greisen veinlet at 448.73 m 2.57 449.67
Pale· grey sandstones with thin inter calations of sheared hornfelsed micaceous slates. Banding dips at 20;.30° often highly folded. Occasional quartz veining and a 0.15 m greisen veinlet at 450.66 m 13.32 462.99
Dark grey to near black spotted micaceous slates WIth scattered paler sandstone bands. Tightly folded with cleavage at 15-20° dip .. Bedding dips at 300 9.93 472.92
Pale grey sandstones with interbedded darker grey micaceous hornfelsed slates. Sandstones are quartzitic. Tightly folded. Rare greisen veinlets 4.40 .477.32
Dark grey slightly hornfelsed micaceous slates with thin sandstone layers. dipping at 200 1.72 479.04
10 Classification Description Thickness Depth m m
Pale grey sandstones with up to 25% of grey and brown intercalated slate. Sub-horizontal folding. Few quartz veins 2.16 481.20
Dark brownish grey slates and occasional paler sandstones dipping at 200 • Tight folding 1. 73 482.93
Pale grey, rather quartzitic sandstones. Some horizons up to 0.66 m. Thin inter bedded micaceous slates, dark grey to brownish grey. Rare quartz veining. Slate up to 20% of whole 17.73 500:66
Grey, silty, spotted micaceous and hornfelsed slates. Still preserve slaty cleavage dipping at 10 0 1.02 ·501.68
Hornfelsed sandstones and slates, the former pale grey and quartzitic, the latter darker grey or brown. About 60% of lithology is sandstone. Gentle dip. Thin greisen at 511.48 m . 11.91 513.59
Greisen Vein Greiseny aplitic vein dipping at 75-800 0.58 514.17
Portscatho Series Mainly pale grey quartzitic sandstones with lustrous silvery or dark grey. micaceous and hornfelsed slates inter bedded. Variable sandstone: slate ratio but at least 50% sandstone. Some tnin greisen veinlets and locally abundant. quartz veins 13.24 527.41
Portscatho Series? Dark grey, spotted or hornfelsed silty slates with streaky, phacoidal texture. Foliation dips at 10-20° and major cleavage at 0-10°. Well folded. Zones rich in quartz veins. Rare sandstones 29.54 556.95
Portscatho Series Predominantiy grey quartzitic sand stones with dark grey, greenish or brownish hornfelsed micaceous slates. Highly disturbed strata with many flat folds. Rare greisen veinlets 12.04 568.99
Quartz Vein Massive milky white quartz dipping at 20°, not quite parallel to bedding in the sediments. Cut by steep greisen veinlet 1.42 570.41
Portscatho Series Pale grey sandstones, with darker lustrous slates interbedded. Sand stones comprise at least 70% of sediments 7.67 578.08
11 Classification Des~ription Thickness Depth m m Interbedded slates and sandstones. Slates are micaceous and occasionally chiastolitic. Abundant quartz veining. Down to 580.00 m slate predominates, thereafter sandstones represent at least 60% of the . lithology 5.94 584.02
Aplite-pegmatite Vein of aplitic granite with irregular Vein patches of quartz and an upper margin of pegmatite. Whole is chloritised. Dips at 50 _55 0 0.38 584.40
Portscatho Series Pale grey sandstones with. interleaves of darker grey hornfelsed slate. Dip 200 2.70 587.10
Greenstone Dips at 150, almost parallel to bedding in slates. Tough, compact, fairly fine grained greenish grey igneous rock. Cut by quartz and pyrite veinlets with traces of chalcopyrite 7.29 594.39
Portscatho Series Banded sandstories and brown biotitic hornfelsed slate 0.40 594.79
Aplite ope gmatite Dips at about 400 • Upper part is pegmatitic Vein and lower is aplitic 0.46 595.25
Portscatho Series Very tough, foliated, adinolised slate 0.08 595.33
Greenstone Fine grained, . tough green and brown rock. Extremely dense. Near horizontal 0.68 596.01
Portscatho Series Intensely biotitic, hornfelsed slates interbedded with pale biotitic sandstones 0.18 596.19
Greenstone Horizontal body. Dark green, very tough, compact, fine grained rock 0.99 597.18
Ponscatho Series Pale grey sandstones interbedded with brown biotitic hornfelsed slates 0.41 597.59
Greenstone Compact, tough, greenish fine grained rock, in part biotitised. Apparently horizontal though contacts difficult to define 0.38 597.97
Portscatho Series Succession predominantly of pale grey, tough, quartzitic turbidite sandstones with thinner greenish, brownish or dark grey hornfelsed micaceous slates. Much folding and shearing of original bedding which dips at 200 •. Rather rare quartz veining. Two thin greisen veins 47.70 645.67
Quartz Vein Massive quartz, milky white, dipping at 700. A little chlorite but no ores 0.91 646.58
12 Clas sification Description Thickness Depth m m
Portscatho Series Mainly dark grey quartzitic hornfelsed sandstones with thin bands of green, dark grey and brown hornfelsed micaceous slates. Intensely sheared, consisting in parts of superimposed fold hinges with intervening limbs stretched out. Rare thin greisen veinlets 18.93 . 665.51
End of borehole 665.51 metres
2. Bosworgy Borehole
Location Bosworgy Farm, Townshend, Fraddam, Cornwall Collar situated at G R SW 5806 3367, 1420 m at 30 31 ~ (T) fro~ Townshend crossroads. I" Geol. Sheet NS 351/8
Driller Drilling and Prospecting International Ltd
Date January - February 1973
Surveys Vertical, maximum deviation 20. Collar height approx. 83.8 m above O. D.
Sizes 0-2.90 m H size 2.90 - 149.18 m HQ'size 149.18 - 214.15m NQ size
Classification Description Thickness Depth m m
'Overburden No recovery 1.37 1.37
Clay with abundant rock fragments 0.92 2.29
Mylor Series Slates, ferruginous, in matrix of clayey decomposition products 0.61 2.90
Weathered grey and brown slates, rather ferruginous. Horizontal cleavage .. Well broken .8.28 11.18
Partly hematitic greenish grey slates with alternating spotted and unspotted layers. Major cleavage varies from 100 to 350 . Several low angle quartz veins 12.19 23.37
Well spotted and chiastolitic grey slates with some distinctly hornfelsed layers. Rarely ferruginous. Some quartz veining 8.00 31.37
Vein Zone White massive quartz at top and bottom separated by argillised broken slate. Little grey copper ore. Dip 60-700 0.81 32.18
13 Claet3ification Description Thickness Depth m m
Mylor Series Light to medium grey, . somewhat horn felsed, micaceous spotted slates. Locally altered. Major cleavage dips 0-200. Some quartz veining 6.15 38.33
Vein Zone Broken white vein quartz with slate inclusions. Patches of chlorite 0.99 39.32
Mylor Series Brown, purple and light grey slates with alternations of spotted and unspotted layers. Some quartz veining 6.40 45.72
Vein Zone White quartz vein in w.all rock of siliceous, chloritised and hematised slates. Dips at 650 • No ore minerals 1'.83 47.55
Mylor Series Variably altered, argillised, intensely spotted slates. usually grey in colour but with occasional thin, unspotted purplish bands dipping at 200 , almost parallel to cleavage. Some quartz veining 4.67 52.22
Similar lithology but not argillised except around some jOints. Some greenish, chlorite slate bands occur 8.54 60.76
Vein Zone White vein quartz with hematitic walls. A little chlorite but no ores 0.40 61.16
Mylor Series Medium to greenish grey micaceous spotted slates cleaved at 10-200 • Little quartz. Become highly argillised below 60.90 m 9.53 70.69
Vein Zone Massive white vein quartz. with bands of soft hematite. Dips at 400 0.76 71.45
Mylor Series Purplish and greenish grey slates, universally spotted and micaceous, often argillised. Cleavage dips at . 0-200 . Some quartz veins and rare greisen veins. Locally hornfelsed to the stage of losing thermal spots 42.14 113.59
Vein Zone Wide zone comprising chloritised and hematitic slates and massive white vein quartz. No ore minerals 4.32 117.91
Greisen Greisen vein dipping at 750 • Traces of arsenopyrite but no cassiterite 0.99 118.90
Mylor Series Alternations of grey and purplish brown hornfelsed and spotted micaceous slates, usually somewhat argillised. Some low angle quartz veins. Cleavage at 100 • Rare greisen veinlets with traces of pyrite 19.10 138.00
14 Classification Description Thickness Depth m rri
Greisen Intensely hematised greisen veinlet with patches of chlorite and quartz. A little pyrite. Dip BOO 0.40 13B.40
Mylor Series Grey and purplish brown slates, spotted or hornfelsed with cleavage dipping at 100 . Some folding apparent. Cut by 5.31 143.71 small greisen veinlets
Aplite Sugary quartz and feldspar with abundant white mica and scattered tourmaline. Traces of arsenopyrite. Dip about BOO 1.30 145.01
Mylor Series Argillised hornfelsed and spotted slates in alternating greenish and grey purplish brown layers. "Cleavage at 100 dip, banding about 200 • Some quartz veins 7.03 152.04
Granite Vein Rotted granitic vein, medium grained. Dips at 600 0.97 153.01
Mylor Series Alternating greenish grey and brownish purple slates," heavily spotted to distinctly hornfelsed. Frequent quartz veining 4.BO 157.Bl
Granite Vein Upper contact dips at 650 • Uppermost 0.6 m is white quartz. Granite is medium to fine grained and equigranular. Partly kaolinised and partly ferruginous at the top. Some pegmatitic feldspars around 159.4 m, 160.2 m, and 160.9 m. Lower contact dips at 75 -BO° 10.B4 168.65
Mylor Series Hornfelsed greenish grey micaceous slates with occasional purplish bands. Remnant cleavage dips at 100 , steepening near the granite contact. No flinty hornfels developed. Several quartz veins 2.11 170.76
Granite Contact almost horizontal. Upper 0.06 m very micaceous then coarsely pegmatitic for 1 m. In this zone there is develop ment of coarse biotite and a little arsenopyrite and cassiterite. The main granite is medium grained and" equi granular with scattered specks of chalcopyrite and free cassiterite. Has a somewhat greiseny appearance, becoming fresher in depth. Chalcopyrite pe"rsists to about IB7 m and cassiterite a little further. Below 192 m tourmaline is common as an accessory and feldspars become larger and more conspicuous, tending towards a ~inely megacrystic stage 43.39 214.15
End of borehole 214.15 metres
15 I SLoH..:::IfY)
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II -\--- l I I i=1 '\ ",L"-___ / 7~ - ~ ...... ,,' . - --- ,:'>.,.-~ ',------I----! 14\4 \ --I r ! ~~, -19.4 ~\ \\ i, I / \ -lB.4 ~-18.2 -19.6 \ \ / ~ \ \ \ \ \ \ "-- -18.0 \ \ -178 -176;It \ \. \ \ / L17.4 , \ \ , \ \ \ \ \ : -\ I I \ \ \ '. \ \ -17.8 L-- \ \ \ I I \ \ \ • -18.0! -19.2 \ \ \ ,{r---/ -184-1(l . -182 i \ \ I \ \ \ \ I I -17.8 -140 -150 I -16.0 -170 i \ I -18.0 c18\~.ic18nl 2_ \\~f190 057 ~-- ._____ .--,-----.. -.-.. . I -176 -18.4 -18.0 -174 I I _..L --~- . --" ---~ --~-- 056 I I .... __ .__ ... ____J . 191 192 193 194 195 196 197 19B 199 200 SOUTH WEST ENGLA NO COMPLETE BOUGUER ANOMALIES ST. AUSTELL AREA: S.W. 95 (SHEET 1 OF 3) Dens'ty 2 70 Gravity daturn' I GS N 71 INTERNATIONAL GRAVITY FORMULA 1967 • Scale: 1 : 10560 CONTOUR INTERVAL 0.2 MILL/GAL PREPARED UNDER CONTRACT TO THE DEPARTMENT OF TRADE AND INDUSTRY lJC /Mr2j-:}S .' .~: '" ""- " . / i, /;~:,,=,,~'::~~~"~'~::::~"':"O'~~~'~:~:'~ __ C:cC~~'~:~'_''~'______~ ____ ~-,~",;, ______~ ______,-______-,______-'______-,______~ ______,_------o:----"-'--'c"c' ______-'~ ______c_------______c·,'.'", ______"'"--',.'"'""'-______-'______~ ______;\:_-----"'---~-:-: ______,_------:,:_------"'] , I 055, I'. /'104 .1I --l , I' / . , _122 -14 ~ ______I l- , -14.8 -106 II , 1 /I /" -12 4 j50" -12 6 I / I -91, / -10 8 / I I 1 / -10 6 -12.8 -14 2 _____--=-:.-:15:;" 4::,." '" " -15 2: -15,:.0 i 055 :~_ -~95 ' --~.------,------lJ. -f -14.4 ~ ______t=:=,-, ---j 8 1D , , - ,O -10 B -15.0 -15,2 1// -llQ1l2 14 ! -130 -146 ..------,- ; "--- ~ -154 _____ - 1-11'?Jl 8112.0 -13 2 ~ , I' ///-148 ------~----- , / , ,/ / , ~' / /--,/. I /// / / -13.4 / ------I 1/ -1135 -- " ,-- / -14.0 .c/<~///~~- ~/- ,~ 1/ / -;:;:;/ ~ /// , / ------' // '/ ( i " // '\ //;/ / 1// / /./" -15d -15,6 ~ /// // '--1 I ",,/~ // -110 ////,. .... ~~~~~ 054 /' /' , y"------j+-- >~-~--,--I ~---'----- ~···-t/- --=~ /~.~ ~-, / / /r'-- '-11 2 . I-\\"~- ~ -11,4 ~~ .. ~ ~ -11,6 ------1 6,4 \ ~,i -11.8---- \ 'I --- -16.6--- -12.0 // ,---- / ------16,8 ~ I - 170 i -~ / . ~ -r '------172 \1 / .~ /'~ , 174 1-12.2 J.------__ ------r /---/ /,/ I i ~~~d/ ...------17.6 1-124 1/ , ~y , ------17.8 -,,~-- 1-12.6 / -18.0 I / J ------/ / -l 1-12.8 ,// ---,_____-- ~8.2 , I ,-130 I I '. ,/ - --r - ... ----,~-~----. // 7/ 7/ // // II II 7/ II ! V ~~._/ -----,--/------~-=---- ,'----, 053t:1ii --+------"'''~'-=--. ---,------,------+------·--I------+I-~ - -==------// . / /. .. - /1 ------1------'-" I /' ? / /' I '-.~ -13.4 -~"- -18,4 I / - 18.0 -18.6 \ --/ \ ///// I .1 /~// /-~" _/ / -18.8 I /\ \\ /-/// , -'- //I~ : / 1_ / -19.0: \ 135 /1 ... / -17.6 I ( ---~ -19,2 I . - 18,4 \ ! -178 !-13.8 -18.6 '"'I..--~ ( -19.0-188 i-KO , n " -18.0 \. -18 4 -14. 2 UJ/: // / - 18.:2 ) G'156 -178 \ G4 -14 4 -184 , -14.6 ---~ , i I J __ • 1-';'" .--'-- ~ 052 !--'--- '14~~5,O_J5.2 --- ,_-~----m-~'-. __ 7/ _____ ' --- ,--,(-j-j -.. ------.------J --- --/'" -"" - ,--, -.-. - i- r-,j - -- -1,,4 -156 -15~16D / --- y-- i --- // ~ ,/ I -\ )80 1-18L -162 • /) / / ---.-- . ( ~ // / I, /,--~ II ~ ! /. / / N\ '. '. ~ / I I I .. i ~ /~ ,/ '\ ( '\ / (/\ ( (( I I , ) ) 1 • o )- -17 6 I / / \ \. -17.0-168 -168-17.0-172 -174 -176~-17.8 " 1/. ~ /,.-:,~ ) I / \ r .:.------.,/ .) I / I 051 ~--- ' ------J--·~I/------I . / _-1 --'j ('I -16.6 --~~ i ---r--c-;~1--T------,- ~- - ,-- )----'-- -\;---- C'-- 1/ .--rr--- ___-186 I . -16! \ ' __ -...... __ ~ ! I 1 /" ~-- ~: -178 -186 -18.4 -182 -184-186 1 ~ -188' I ~ -18.0 ~. \ -18 t. ~ .1 / \ \ -18.8 ----- \ -19.0 ~ l __ ,,---- lej8 \ /_l-l_~ 1 \ ' r -18\\-1~\2 \ ( i " • -164 -~ " " \ -1 g, 2 -\. _/ \ , ~\--~\ /' ------~ ) -19 4 ! -19 6 /-,----- . J '., ------II i , I I /" I , ;.// ~ 'I, i/\/ / . /'1 , ------\ / i ------, .------'.~~- ./ / I j-~ --- // ~--..--~ .... / ------/' , L~L" ./~-=. _~ ..... ____ , L ~ ,------ /' \\\ / 050 L L~ __ 1 1 ...J 191 192 193 194 195 196 197 198 199 200 SOUTH WEST ENGLAND COMPLETE BOUGUER A~JOMALI ES ST. AUSTELL AR EA S. W 96 (S HEET 2 0 F 3 I Densi ty' 2· 70 GrQvity dQtum I G S N 71 l~nERNATIONAL GRAVITY FORMULA 1957 SCQle \ 1: 10560 CONTOUR INTERVAL 0.2 MILUGAL PREPARED UNDER CONTRACT TO TH E DEPARTMENT OF TRADE AND INDUSTRY \;JC! N KI =tS <. --,------~----~~-,--.-~--~~-, .. ~----.-.-. -j 066, I -14.4 )44 ;; 5 l -''-. -14 2 ~"~·i' / ~~~ -14.0 I --- , I II , / I ---, . -14.2 - -14 t... '" \ _146· 1 / ''''~, , \ -""-- -1/,5 ______. " -> '", ~ I / , \ I / ---- ~. '- , ---- ~ ------1 4_ 8 ;, // ~ '-'-" -14 5 / -15.0 '- \ \ ! ~ -14~ '. ! ____-15.2- '" / '-15 4 I -14.8 ~ '---l ~/ '~ -15.0 ~ ----- , ------,-\- "------.- ·t·- --'------.~ 1,5 ..0.---, 065E--. -15.2 ______...... -...:: ~. ''0" '" --...... ,. --+- , //~~ \ -15.2 -154-_ (J____ -- 15.6 - ____ --- .------:: \ . ~ -1S.2 - 15 8 ______~~ ) . '-" "- -15 4 --- .~ ", , -16.0 ------.\ -15.8 ~~ '- ...... '~--- I --16.2 ------16.0 -'- ---- I I -15.4 . /. -158 -162 -15.6 - - ~ , I -155~ ------. . - 16 4 ______) - ~ ~ u" I ____ .~~ t / ______-158 ----=:::: ~ --- 1------~-166 -'---- ...- ,16.8 ------"- - I' j - .-.--.-.~ / ~ -~-- -16.0 -----=~ ". -1'1.0, ------,1 /1 ____ I ---, ~ . I _ - __ , ----~ ...... / , --1- __ ._ -----____ .. __ , I , ~. , ------, ._.__ L ______~ -f- -I . , 064 ;---.. ____ .:-,-'1.0 __---_ '" ',,--' \ ~---"---'-- -.) / :=:==l=:::::--~-~~~",,\-----l- .:.::c----.., '<,,,, ~ ~ ~~~------.....~ .~ 'I, 1----___ 17 i --- ~ , 'j ------17.4 __ (j.o / "\, ~) ~, '. ",'--- ~I ' ~ ...... ,. -16.0 ------1:---.. ~-,. -,/ ----- I -~------~-17.5 0'1.0 -172 ...- ~.~------/~ __ I ~ ~---:: I .-- ~ " 17 --- ~ -I -----=----- r;--:=I ____~ =:::= _- ~I"i. \ ~ -. -15 8 , -17.0 i ------17 8 1------~'---------~ ::=::== ~ ~ -18.2 ~ -'" -16.0 -- " ,18.4 -18.0 ______---1'--- ~ " 053 ...... ~ -. ~----~~-:::i--~--~: ------==t --" . ,---''---.. ::------v----- I ~----~--t_:,--~ -18.5 " . ~ .-158 ""-18.2 -18.8 -176 ·171, 17" ,,} . 2 -18.6 1-18 2 C}2 ·18.8 ----lB4 _ -17.B / / -18.4 I , )).//: 1 , -180/~ // -170 -17 r) ------19.0 ./-;72 .--- -19. Z ! -19.0 ~ ------~-19.2 ~ -174 // ,1-____ ------19.4 - -19.4 --- //" / 1 / -' ...---/ . I ~ ~. ------t------19 6 -18.5 / ..------~ ______J ~ ...------. .i 062 "/1>;//-188 / .-/ -19.0 ~ '--:- ~------/ / / -19.8 -t ~------v - 2 0 . 0 I ______- 19 2 / / -194/ ------~ -19.0 ~ -19.6 ------/ -20.4 ------/ -19~20.0 - - 19.B --- -20.0 - 20. 2 I 051 , I ------, 1 I ,i i I I • ! .L__ 050 ._ .. __. _____ J L J 200 201 202 203 204 205 206 207 20B 209 SOUTH WEST ENGLAND COMPLETE BOUGUER ANOMALI ES ST. AUSTELL AR EA • S X 06 (SHEET 3 OF 3) Density 2'70 Gravity datum: IGSN 71 INTERNATIONAL GRAVITf FORMULA 1967 Scale: 1 • 10560 CONTOUR INTERVAI_ 0.2 MILLIGAL PREPARED UNDER CONTRACT TO THE DEPARTMENT OF TRADE AND INDUSTRY lJC /MfC/1-:) 052, - , ._- 1 -170 -150- 148 -146 -144 / -142 -140 ! / _158-156 -160 I /4 -152 '-162 I -164 I " I' i I I I / ,/ 051 ~- . -1------~------: ------j~----I /' i / -166 , / -1>Jm /=-=-~ 'II i! / .1 I !iiiIi " / / Ii / / / / ,/ /1/ /! / ./ / ~ /,' .I //. ! ! Iii! / / "~! ,I I ___ - /%1 ~ /;,,1;- ____ I' _L~/ ! I , ! 1 /:I!/ / .,/ I ~- -18~//l:/ i 050 ------···F··· .. ------j ! , , -11fO~<: II II / / I' , -178// I. i I. / ! -176. / ' '.'/ ¢! -174/ . . //1 I / ! I -172 1/ /,' / / / / I -170 \ 1:;1 / / ' / ! , I ,,"If! I . / ! -154/ !! / / \ -162 \ ( \ \ / I \ I L. ______049 ------! ---I ------/ -~ -160 , / ! \ / • I I / I -158 / \ / \ / ; / I -154 ( -138 -;52---- I \ / ! ~ -150 I "----- _-- -140 048 f--- i-- -- I, -148 ~-- -142 -144 -146 • -148 i -148 047 ------I --~------+--~+--;' ...."L. ______...... , __ ------150 -150 -152 -154 -152 -154 -154 L------~------156 -156 ~______------+---~~ ----158 ------~---+--- __---160 -158 ------r---- 046 f------c------+------~------r_------___t---.------=-=-~~----:-....--=-=-=- ===-==-= -162 -158 -156 -154 -150 -152 -164 i -166 045f--1------I-----.--j-----j-_+_ -+--+--/---j +---+-+----( ---J-----¥::::::=::::::::--,..~------"'_----______t_ -170 I '148 044,-- /------1---+--- / -174 -180 -186 188 -168 -164 _166 -162 -154 -160 -156 -158 -186 -188 -170 -172 -174 -176 ______.______-1~~ 80-182 -184 I 043'------170 --'-"-=""'---:"17-:-1------·------,1 L72:------173 167 168 169 SOUTH WEST ENG LAND' COMPLETE BOUGUER ANOMALIES ST. AGNES AREA. SW64.74.75 (SHEET 1 OF 1 ) . Density: 2·70 Gravity datum: IGSN 71' INTERNATIONAL GRAVITY FORMULA 1967 Scale: 1: 10560 CONTOURS IN g.u. AT lNTERVALS OF 2g_u, PREPARED UNBER CONTRACT TO THE DEPARTMENT OF TRADE AND INDUSTRY ; ~ ., • -122 -'-. ;122 -168 -120 -160 -164-166 \ -158 '16)' -122_124 ~~2~1~ho -132• -152 -124 C --- / ~ ------T: i -128 / / -152 -138 I / -132 -130 -140 -134 -144 -138 / ~ -- \ \ -142 / / -128 -136 ;/' ~ ~ ) I ~ \-J/ / // / ~--- __~ _____ J ) / / I ------~ ! /" -0: -17~ ;-140 .~------/ -176\ / '''~ I / ~~------~------~ ----- / 037 - ... ----.:''''"-~""" -"'---- / I ~- /' -154 \ .~~--- I / -156 -158 / -1[.2 / - -13( I -160 -130I -128/ -126~/ ______~~ '. -16 2 ______~~- \ ' / \ -164 -lI.k _~ -166 ______/. )! ~------~ -1521 -~"- 17 / / ~ I / : -168 - 0 I ./ 1 ~ \ / / ( ~ ~13~36/? ~/ f ____ /L ___ I_____ -"--'--~-- '------~~--~--~ ...... "i"" \ \ \ \ '\ " 036~ l- .... '" '" 1\ \\\ " I -140 I -138 -138 I / -166 ------/ / // _------142 -164 I ------~ / -lL4 -162 -180! -178 ~ ---- ~ i:------'lL~14{ ______~ ~ ~ ~ ______- : -150 -160 -158 -152 -fi) -156 -154 -156 -140 I -152 \ I 0351 -138 ( _ .__ + /;15~~1' ------'cl-i~----~------\~ _132-130-/1+--1L8 - 136 -134 ;' ( -174 -128 /; i I ! -154 -13Dr32 / -134 _ / / ( 1 6 / ·"-138 i -176 / J , / -'" .)"-170 / ~ / \ / -140_\142 ~ ------~ , y / ~ '\ / , ------) / 1------\ -144 .~ I -lL6 034--~------1--+ \ , ~ --===------:: ---- -148 -17C -162 '\ \ l 1-160 ) • .\ 1 ~ ? / -- -162 \ / \166 -164 ------~ ~ (/ \ -166 -160 -150\-152 ~I \ , \ -158 /1 -156 I -154 )' \ ; If! \ -16B -156, I -158 \ \ '15z/ -156/ I -172 ~ ------150_148 -170 0331-1------~--- ~~~~ :;..=-- / ----_------'-/---FI-15-Q -152:1?4_._ \-\ -/--:/I----~ --- .. -148. ~ 1 ------~ \ \ : -168 / ( \ \ -176 };I \ \ \ \ \ '/1 I \ \ '---'---- ->---- \ \ \ ". \ \ \ \ 1-170 \ -17L. \ -172 \ -172 " "" '" -170 ~ ______-1714 ' -168 _l ----~-,- 032 .~ 154 150 156 157 159 J 158 160 161 162 163 164 SOUTH WEST ENG LAND - COMPLETE BOUGUER ANOMALIES HAYLE - LEEDSTOWN AREA SW53,63 (SHEET 1 OF 1 ) Density: 270 Gravity datum: IGSN 71 INTERNATIONAL GRAVITY FORMULA 1967 Scale: 1: 10560 CONTOURS IN g.u. AT INTt'RVALS OF 2 g.u. PREPARED UNDER CONTRACT TO THE DEPARTMENT OF TRADE AND II'lDUSTRY / r'-1r2