High-resolution gravity surveys at Egloskerry and Combe Martin, SW England

This report describes the results of two detailed high-resolution (HR) gravity surveys at Egloskerry, Cornwall and Combe Martin, Devon. These areas were selected on the basis of existing exploration data and prospectivity appraisal as potential sites for massive sulphide deposits.
Assessment of the geology of the central part of SW England, between Bodmin Moor and Dartmoor, reveals many similarities with the Iberian Pyrite Belt (IPB) in southern Spain and Portugal, where numerous sediment-hosted massive sulphide (SHMS) deposits occur. Furthermore the geological environment of the Exmoor district is similar to the setting of the major polymetallic sulphide deposit at Rammelsberg in the Rhenish Massif in Germany. A review of existing geochemical, borehole and mineral occurrence data (Rollin et al., 2001) indicated that apparent stratabound sulphide mineralisation occurs at more than 60 sites in SW England. At Egloskerry, near Launceston, extensive exploration in the early 1980’s had identified stratabound galena within Lower Carboniferous shales and mudstones of the Liddaton and Crackington formations. At Combe Martin, coastal exposure of the Middle Devonian Lester Slate and Sandstone Formation indicates mineralisation of galena with quartz and carbonate showing evidence of stratigraphic control. This mineralisation is typical of the ore worked in historical times in the Combe Martin silver-lead mines.
At Egloskerry, a detailed HR gravity survey was conducted over an area of about 20 km2 within the region 225–230 kmE, 083–087 kmN. Previous commercial exploration including drilling and geochemical soil and deep overburden sampling had been supported by ground geophysics using induced polarisation (IP), self-potential (SP), Very Low Frequency (VLF) and magnetic surveys. No previous gravity survey had been carried out. A total of 181 new gravity stations were observed at station positions located by GPS. This compares with only 14 regional gravity stations in this block.
At Combe Martin, North Devon, extensive historic mining for lead-silver in part of the Lester Slate Formation is apparent at several localities inland and minor stratabound galena (sphalerite) quartz-carbonate veining can be observed at Lester Point (Scrivener and Bennett, 1980). There are however no known ground geophysical surveys in this district and the main surveys of the Mineral Reconnaissance Programme (MRP) work in this region was focused towards regional drainage geochemistry and ground magnetic surveys to explore the linear magnetic anomalies across Exmoor. At Combe Martin a total of 167 new gravity stations were observed at station positions located by GPS within a survey block of about 40 km2 defined by 257–265 kmE, 143–148 kmN. This compares with 25 regional gravity stations observed previously in this area.
For both areas the new gravity data were collected using LaCoste and Romberg gravity meters with elevation and position determined using a Leica 530 GPS in Real-Time-Kinematic (RTK) mode. Gravity data were reduced to Ordnance Datum at a density of 2.70 Mgm-3, and corrected for terrain effects out to 48 km (Hammer Zone X) using a high resolution (50 m) and a low resolution (1 km) DTM. In both regions observed gravity was referred to the National Gravity Reference Net 1973 (NGRN73) and showed no significant difference from the observed regional gravity data. Terrain corrections at duplicate stations also showed no significant or systematic errors with the regional data. The data for each of the surveys was gridded at an interval of 50 m. A regional field based on the existing survey was generated and subtracted from the new grids to provide residual anomaly maps for both regions.
At Egloskerry, a positive residual gravity anomaly of up to about 1 mGal shows a strong correlation with the digital 1:50 K geology and also with detailed geochemical data. Anomalous concentrations of zinc and lead in soil also show good correlation with the gravity residual anomaly.
At Combe Martin, a small positive residual anomaly shows a general correlation with the outcrop of the Lester Slate Formation to the east of the main mining district and might warrant further exploration with a ground contact electrical method such as self potential SP or induced polarisation IP.
Future exploration for stratiform base metal deposits will depend on the particular style of mineralisation of the target. For targets in the central region where IPB type deposits are to be expected the exploration practice might include further high-resolution gravity and subsequent investigation of minor positive gravity anomalies with an electrical survey to define drilling targets. In other regions, such as Exmoor, where the prospective targets are sedimentary exhalative (Sedex) deposits, there has been relatively little previous ground geophysical survey there is potential for targeted exploration of several horizons, particularly over existing residual gravity anomaly features.