Mineral Reconnaissance Programme Report
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-- Natural Environment Research Council r _ Institute of Geological Sciences Mineral Reconnaissance Programme Report .. __ A report prepared for the Department of Industry .’ - I This report has been generated from a scanned image of the document with any blank pages removed at the scanning stage. Please be aware that the pagination and scales of diagrams or maps in the resulting report may not appear as in the original No. 2 Geochemical and geophysical investigations around Garras Mine near Truro, Cornwall Natural Environment Research Council Institute of Geological Sciences I Mineral Reconnaissance Programme Report No. 2 I Geochemical and geophysical I investigations around Garras Mine 1 near Truro, Cornwall I R. C. Jones and J. M. C. Tombs, BSc 1 with analytical contributions by B. A. R. Tait, BSc and Miss L. Hearn A report prepared for the Department of Industry I_- I London: 1975 The institute of Geological Sciences was formed 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 It is recommended that reference to this report be made in the following form. JONES, R. C. and TOMBS, J. M. C. 1975. Geochemical and geophysical investigations around Garras Mine near Truro, Cornwall. Miner. Reconnaissance Programme Rep. Inst. Geol. Sci., No. 2, 14 pp. [ Unpubl.] IGS 1475 25 1 O/75 Printed in England by the Institute of Geological Sciences CONTENTS Page Summary INTRODUC TlON Location and Topography Geology Former Mining GEOCHEMICAL INVESTIGATIONS Introduction Sample Preparation and Analysis Analytical Results Interpretation of Results GEOPHYSICAL INVESTIGATIONS Introduction Geophysical Instrumentation and Methods Results and their Interpretation CONCLUSIONS 13 ACKNOWLEDGEMENTS ‘, 4 REFERENCES 14 ILLUSTRATIONS Fig. 1. hIap &owing location of Garras area, Cornwall Fig. 2. Localities mentioned in the text Fig. 3. Structural map of fiIic(-* (Joinwall and West Cornwall Fig. 4. Geophysical traverse lines near Garras &Mine, Cornwall Fig. 4a. Geophysical traverse lines near Garras Mine, Cornwall Fig. 5. Garras: Gcochemical soil anomalies Fig. 6. Cu, Pb, %n and Ni concentrations in soil sample traverses in the vicinity of the Garras Mine 8 Fig. ‘7. Cu, P!), %n and Ni concentrations in soil sample traverses in the l:iciniry oi the Garras Mine 9 Fig. 8. LIppar-ent resis-dvlty and chargeability along geophysical traverse line GD 11 Fig. 9. ,Ipparcnt resistivity and chargeability along geophysical traverse line GA 12 iii Geochemical and geophysical investigations around Garras Mine near Truro, Cornwall R. C. Jones and J. M. C. Tombs, BSc Summary of past generations of mineral prospectors. In the valley of the River Allen north of Location and Topography Truro there are three old mine workings and four trials arranged in a sub-linear manner, of The area under investigation extends from which Garras is the principal. In the interven- the northern boundary of the City of Truro to ing unworked ground applied geochemical ahd Newlyn Downs, which is a distance of 5 miles. geophysical investigations have been carried out. The area is devoted to agriculture and forestry, Ten soil traverses were arranged to locate and and is cut by the River Allen and its tributaries investigate the possibility of continuity of which form a dendritic pattern draining to the mineralisation. The results of these investi- south. The lo\ver part of the valley is deeply gations did not indicate such continuity, though incised and the valley sides arc well wooded. they did show that the lode at Garras Mine The ancient road pattern follows the interfluvial persists for a short distance beyond the former ridges, and the A30 Redruth to Bodmin road workings. Several low order geochemical runs along the watershed between the River anomalies were located but they seem to be of Allen and the Gannel river system draining to a restricted size and of little importance. Four the north beyond the area. There are no geophysical anomalies were found, two of which communities of any size in the area, though the are close to Garras Mine, and two are thought villages of Zelah and Shortlanesend are nearby. not to be due to mineralisation. Within the area there are three hamlets, Idless, St Allen, and Gwarnick. The land rises to 450 In view of these disappointing results, ft above sea level at Xeu-lyn IX:\\lis, but in the these investigations have been brought to a remainder of the area the summons of the 1sidge.s close. lie at about 350 ft. Geology l IKTRODUCTLON The area under investigation lies \vhollj In 197 3 an investigation into the economic \ifithin the outcrop of the Gramscatho Beds potential of the Garras urea ix:as undertaken as (mainly of nliddle Devonian age) which al-t‘ part of the Department of Industry’s Mineral here represented by interbedded sandstont’s, Reconnaissance Programme. The features of grits, siltstones and slates. To the nortll tllis the area which attracted attention were the iithology is underlain by the more calcareous I distance over which mineraliscd structures succession of the Meadfoot Beds and a short occurred; the restr.-icted extent of the old work- distance west occur the easternmost outcrops ings as indicated by old shafts, surface trials, , of nIyl0r slates, which also may underlie tl?c dumps etc; the persistence aL depth of Cramscatho sediments. EL cryivhere in tilt’ mineralisation in the !,;ost extensively mined area the beds dip southwards at moderate angies working (up to 70 fathoms in Garras Mine); and, where clearly recognisable, the slat- and the high silver co:ltent of the lead ore, cleavage also dips in this direction, u.-ually I: reputedly 1730 g,‘tor:nt- (L)ines, 1956). a shallower inclination. The Allen Valley was relatively undisturbed Cornwall may be dix-Ided into two structural by past mining acti\ Tity which thereby enabled units separated by a line from Perranportil to soil sampling to ‘be cndertaken with little fear Pentewan, a line the significance of ~vhich is a of spurious results 6,e ~0 contamination. It matter of some debate. Garras lies in the was considered that since the valley lies out- southern region where the major fold axes side the main establ:s!led mining areas it would generally trend north-eastwards. Potasaium- not have been subjected to the intense scrutiny 1 100 GRANITE 0 1p 20 Miles 0 10 20 30 km 200 Fig. 1. Map showing location of Garras area, Cornwall 78 79 I 80 81 82 83 84 85 86 87 55. I f+fi!c hei II I 52, .__ I /------- bI z -\ I, II - Fig. 2. Localities mentioned in the text 2 argon dating by Dodson and Rex (1910) indicates worked from 1853 to 1861. By 1860 the mine that this area was first subjected to intense fold- was 80 fathoms (146 m) deep working a north- irig in a Bretonic (end-Devonian) event; these folds south lode up to 1.8 m wide and locally contain- are highly recumbent, commonly horizontal, tight ing solid galena 0.4 m in width. Dines (1956, p. or isoclmal and invariably face northwards. They 506) quotes a total production for the period are overprinted by a later, Variscan phase of open 1855-1860 of 1960 tonnes of 54 per cent lead folding in which the axes plunge gently north-east ore, 314 750 g of silver and 11 tonnes of pyrite. and axial planes dip at about 50” to the south- Provis (1874) states that two grades of ore east. The most significant of these axes, that of were produced, one with 75 per cent of lead and the West Cornwall (or Truro) anticline passes to 527 g/tonne silver, the other with 29 per cent the south of the Garras region. lead and 721 g/tonne silver, but these figures do not agree with Hunt’s Mineral Statistics for Bouguer anomaly maps derived by Bott and 1856 and 1857 which show silver contents of ‘others (1958) suggest that batholithic granite, if 1900 and 1660 g/tonne respectively. It is present below the Garras area, lies at great depth. interesting to note that from the figures of Its closest approach to surface appears to be as a Provis the poorer grade lead concentrate is ridge connection between Fraddon and Cligga, richer in silver, suggesting the presence of a passing just north of Newlyn Downs. second silver-bearing mineral in addition to galena. At abandonment the mine was still Igneous activity in the area is represented producing well from its northern end, though to by two types of minor intrusive bodies. To the the south the lode was poorly mineralised. west of the Garras stream a narrow lamprophyre intrusion runs approximately north-south and Trading upon the success of South Garras dips westwards at about 40”. It is always highly two small trials started further east in 1855 decomposed and was formerly pitted to provide under the names of East Garras Mine and Pol- a crude agricultural top dressing, locally known whele Lead and Silver Mine, Adits were driven as ‘morrow’. Between the pits it can be traced at both sites but no production is known to have with relative ease by virtue of its radioactivity. resulted. It was claimed that here the lodes also Cooper, in an unpublished report’(i974), records coursed north-south. Other small mines were the following metal ‘contents (in ppm): U 25, Th worked for lead in the near vicinity but their 120, Cr 420, Co 40, Ni 420, Cu 120, Zn 180, location is now uncertain; West Garras lasted Pb 30, Ba 4000, Zr 1500 and up to 2.4 per only a few weeks in 1858, Wheal Powell (1855- cent P205’ In common with other lamprophyres 1858) reached at least 14 fathoms (25 m) depth, in south-west England this body seems to have and Wheal Collins (1859-1861), sunk 40 fathoms no direct bearing upon local ore deposition though (73 m), produced a little ore reputedly contain- it has a similar strike to the lead lodes.