Natural Environment Research Council Institute of Geological Sciences v- Mineral Reconnaissance Programme Report A report prepared for the Department of Industry 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 The information contained herein must not be published without reference to the Director, Institute of Geological Sciences.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 Institute of Geological Sciences 64-78 Gray’s Inn Road London WClX 8NG No. 11 A study of the space form of the Cornubian granite batholith and its application to detailed gravity surveys in Cornwall a Natural Environment Research Councii Institute of Geological Sciences a 1 Mineral Reconnaissance Programme Report No. 11 a 1 a A study of the space form of the Cornubian granite batholith and its application to detailed gravity surveys in Cornwall 1 a J.M.C. Tombs, BSc I a I a a I A report prepared for the Department of Industry London: 1977 The lnstitute 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. 1 c NERC Copyright 1977 1 It is recommended that reference to this report be made in the following form: TOMBS, J.M.C. 1977. A study of the space form of the Cornubian granite batholith and its application to detailed gravity surveys in Cornwall. Miner. Reconnaissance I Programme Rep. inst. Geol. Sci., No. 11, 16 pp. t Photocopied in England by the Institute of Geological Sciences ii CONTENTS Page SummaT INTRODUCTION THE GRAVITY SURVEYS Outline of the procedure for obtaining depth to granite 1 Gravity measurements 2 Density estimates 2 Borehole information 2 Other controls on the modelling 2 Construction of the batholith model 3 The detailed survq areas 3 INTERPRETATION OF THE MODELS 3 The batholith model 3 The problem of indeterminacy 3 Description of the model 3 Discussion 4 The detailed survey areas 4 a. Hayle-Leedstown area 4 b. St Agnes area 4 c. St Austell area 4 CONCLUSIONS 5 REFERENCES 5 ILLUSTRATIONS Fig, 1 Location map 6 Fig. 2(a) The batholith model - polygon approximation 7 Fig. 2(b) The batholith model - section along line A-A’ 8 Fig. 2(c) The batholith model - pseudo-perspective view 9 Fig. 3(a) Hayle-Leedstown area - regional field in mGa1 10 Fig. 3(b) Hayle-Leedstown area - residual field in mGa1 10 Fig. 3(c) Hayle-Leedstown area - depth of granite below sea level 11 Fig. 3(d) Hayle-Leedstown area - section along line A-A’ .ll Fig. 4(a) St Agnes area - regional field in mGa1 12 Fig. 4(b) St Agnes area - residual field in mGa1 12 Fig. 4(c) St Agnes area - depth of granite below sea level 12 . Fig. 4(d) St Agnes area - section along line A -A ’ 12 Fig. S(a) St Austell area - regional field in mGa1 13 Fig. 5(b) St Austell area - residual field in mGa1 14 Fig. 5(c) St Austell area - depth of granite below sea level 15 Fig. 5(d) St Austell area - section along line A-A’ 16 Fig. 6 Lands End sheet: Bouguer Gravity Anomaly Map In pocket . 111 A study of the space form of the Cornubian granite batholith and its application to detailed gravity surveys in Cornwall J.M.C. Tombs Summarl THE GRAVITY SURVEYS A three-dimensional computer model of the Cornubian Outline of the procedure for obtaining depth to granite granite batholith, defined by polygonal contours, was created so that its calculated gravity field matched the It is fir% assumed that the Bouguer anomaly variations observed Bouguer Anomaly field, both onshore and off- are due entirely to the granite having a lower density shore. The model was used to define a background than the rocks surrounding it. (= “regional”) field in three areas where detailed gravity Let one of the areas within which the thickness of surveys had been undertaken in the search for shallow killas was to be determined be called A. The technique granite. From the residual field maps of de@h to granite used was to extract from the observed Bouguer anomaly were produced by an iterative technique. Geological field a contribution, GRES, which was proportional to interpretations of the batholith model and of the depth the attraction of the killas within A. (GRES is defined maps are included. more rigorously below). If GRES were known precisely, then the thickness of killas could in principle be uniquely INTRODUCTION determined everywhere within A. In practice an approximation was obtained to the values of GRES at This report, prepared as part of the Department of points on a square grid within A, and the killas thickness Industry’s Mineral Reconnaissance Programme, is an was then derived therefrom, at the same points, by an expansion of the geophysical section of a previous report automatic omputer technique. (Beer, Burley and Tombs, 1975) in the same series. GRES is the difference between the observed Bouguer Bouguer anomaly data relating to detailed surveys, which anomaly field and the field which would be observed if were presented in the former report, together with a the killas within A were replaced by granite. If a separate compilation of regional Bouguer anomaly data, computer model (Ml) could be created which was an have been processed by computer to give contour maps of exact representation of the (anomalous) density distribution depth to the shallowly-concealed roof of the Cornubian everywhere (this is obviously impracticable), its computed granite batholith. As a necessary step in the data gravitational attraction (Gl) would be identical to the processing new information on the possible space form of observed Bouguer anomaly (GOBS) at all points. A second the granite body has been obtained. It is hoped that the model M2, differing from Ml only in that granite reaches results of this investigation will provide not only significant surface everywhere within A, would have an attraction G2 new evidence in the search for a direct relationship between differing from Gl, and hence from GOBS, by GRES. granite plutons and mineralisation, but also a geophysical In the present work a model Mla had been set up to interpretation technique of general applicability in similar approximate Ml. To avoid excessive computing it was geological settings. necessary to limit both the complexity of the model and A mDre detailed knowledge of the relationship between the amount of Bouguer anomaly data used to control its high-temperature mineralisation and the boundary between shape. Mla was designed to reproduce the anomaly at the batholithic granite and metamorphosed slate cover (known intersections of a S km square grid covering the whole in south-west England as killas) is of importance in the region of influence of the batholith; it therefore represents continued search for economic mineral deposits of the tin- a very smoothed version of the actual granite surface tungsten-copper type. The object of the geophysical beneath the killas. From Mla a model M2a, approximating surveys was to produce a detailed picture of depth to M2, was obtained directly. The difference between the granite in three areas (Fig. 1) where the killas cover was attractions G2 and G2a should be small within A excepe believed to be shallow, The principal method used was near its periphery. G2a was computed and subtracted from gravity: previous work in south-west England (Bott, Day GOBS over a grid within A of much closer spacing than and Masson Smith, 1958) and elsewhere had clearly was used to derive Mla. Hence an approximation to demonstrated the negative gravity. anomalies of granite GRES was obtained which made full use of detailed plutons. It was hoped that refinement of both the quality Bouguer anomaly data within A while also allowing for the of the raw gravity data and interpretation techniques would broader background effect of the batholith as a whole. permit an accurate quantitative interpretation. It is evident that to carry out this work the following In the Bosworgy area seismic reflection techniques have data were required: also been used: the results will be presented as a separate (a) detailed gravity measurements over the areas of report. interest, together with less detailed cover over a large surrounding area; boreholej, Non-granite densities range from 2.49 (Culm (b) estimates of the densities of granite and killas so that sandstone) to 2.99 (Lizard hornblende schist). Intrinsic to their relative gravity effects could be determined; the measurements are the following possible sources of (4 borehole information, where available, to provide error: control on the interpretation. (4 Samples of slaty killas taken from de$h may open out The collection and use of the above data are discussed slightly along their cleavage planes leading to a low in the following sections. measured density. This effect can usually be detected at the time of measurement and is not thought to be Gravity measurements a significant source of error (McCann, 1973). w Weathering of samples taken from the surface would In 1973 IGS completed its onshore regional gravity survey lead to the low measured densities but, again, can in south-west England, stations having been established at normally be recognised. bench marks and spot heights at a density of 70-80 per (4 Any small sample may have a density untypical of 100 km2, using vehicle-borne Worden or LaCoste and its formation as a whole. Romberg gravity meters. The regional survey was (4 There are obvious dangers in extrapolating near-surface supplemented in 1972 and 1973 by detailed gravity and granite density measurements down to great deph.