Potential for stratiform massive sulphide mineralisation in south-west England

The aim of the work described in this report was to determine areas favourable for the occurrence of massive stratiform base-metal deposits in Devon and East Cornwall. Deposits of this type are major sources of zinc, lead and copper worldwide and they provide large targets, economically more attractive than the vein mineralisation which was the mainstay of metal mining in south-west England for many centuries. Assessment of the geology of the central part of the study region, 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. In addition 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. Furthermore the Middle Devonian volcanic rocks of south Devon contain minor sulphide mineralisation comparable with sedimentary exhalative (SEDEX) mineral deposits associated with bimodal volcanic rocks in intracontinental settings. A large amount of exploration activity has been undertaken in the target region by commercial companies and by BGS and is reviewed in this report. Analysis of existing geological, geochemical, borehole and mineral occurrence data indicates that stratiform sulphide mineralisation occurs at more than 60 sites. A compilation of the mineral workings and trials in the region is provided in the report and has been used to identify the geological formations of particular interest. On this basis, and by analogy with the settings of major deposits elsewhere in Europe, the most prospective geological units are: Lower Carboniferous strata containing black shales in the central region and around the northern margins of the Bodmin Moor and Dartmoor granites; Devonian volcanosedimentary formations in south Devon; and Middle–Upper Devonian slates on Exmoor. Summary geological logs for two boreholes north of Bodmin Moor and on Exmoor are provided in the report. Previous drilling of a strong annular magnetic anomaly close to the northern margin of the Bodmin Moor and Dartmoor granites identified magnetic pyrrhotite as the likely cause of the magnetic anomalies. The pyrrhotitebearing rocks generally exhibit a very low magnetic susceptibility but have been shown to have a variable, locally strong Natural Remanent Magnetisation (NRM) acquired at the time of granite intrusion. Remagnetisation of primary stratiform pyrite contained in prospective Fammenian-Tournaisian-Visean formations carrying mafic volcanic rocks and black shales implies that the magnetic anomaly can be interpreted to represent primary syngenetic sulphide deposits. In this study new interpretations of the regional magnetic data, including calculation of the analytic signal and 2D and 3D depth solutions, have identified the source depths and positions for many of these magnetic anomalies. In many cases these sources are relatively shallow (<500 m) and have not been tested by drilling. These parameters have been included as positive evidence in the assessment of the mineral potential of the area. Extensive, but incomplete drainage geochemical surveys have been carried out across most of the prospective geological units. About 3000 stream sediment and panned concentrate samples provide evidential data for prospectivity analysis. The report provides a summary of these data. Stream-sediment data for Cu, Pb and Zn, filtered for proximity to known mineral workings, has been used in the analysis, together with anomalous panned-concentrate data for Sb, Ba, Mn, Ag and As. The potential for the occurrence of stratiform sulphide deposits in the region has been assessed by knowledgebased prospectivity analysis using a binary weights of evidence model with criteria and weights derived empirically from established models for this style of mineralisation. New targets have been identified using selected geological, geophysical and geochemical data in conjunction with the distribution of known stratiform mineralisation. These targets occur primarily within the stratigraphical intervals identified as favourable on geological grounds or in regions where prospective formations are presumed to occur beneath shallow cover sequences. The degree of confidence which can be placed in the prospectivity maps depends on the accuracy of the deposit models utilised, in terms of their applicability to the target area, and the availability of adequate reliable data of sufficient quality. The extent to which the mineral deposit model is represented by the available data is especially important. The quality of the analysis can be improved as more data become available or as the reliability of the deposit models is improved. Nevertheless several unexplored targets, located outside areas with designated planning restrictions, warrant further investigation. The recommended exploration approach will depend on the specific target deposit type and on the local geology. For targets in the central area, where IPB type deposits are to be expected, the exploration practice should include highresolution gravity surveys. These should be followed by electrical surveys over positive gravity anomalies to define drilling targets.