Mineral Reconnaissance Programme (MRP) reports 31–40
31 Geophysical investigations in the Closehouse-Lunedale area
J D Cornwell and A J Wadge (1980)
The area around the Lunedale Fault and the Closehouse baryte mine, at the northern edge of a deep Carboniferous sedimentary basin, was investigated by an airborne magnetic, EM and radiometric survey. The Whin dolerite gives rise to pronounced magnetic features, some of which indicate previously unknown intrusions. The Closehouse baryte deposit is related to the distribution and alteration of the Whin dykes, and ground investigation of the aeromagnetic anomalies is recommended. Ground follow-up of EM anomalies was incomplete, but the four areas that were studied show no indications of mineralisation.
32 Investigations at Polyphant, near Launceston, Cornwall
M J Bennett, K Turton and K E Rollin (1980)
Geological examination, shallow boreholes and geochemical soil sampling over the Polyphant ultrabasic igneous mass indicate a marked variation in composition, with at least two types of peridotite and two of gabbro. The distribution of Ni, Co and Cr in overlying soil gives an impression of macro-layering within the peridotite, but the concentration levels of these metals, and of Cu, are normal or low for peridotite and gabbro and offer little prospect of layered base-metal orebodies being found. No significant mineralisation was indicated by the investigations, but high IP values at the southern margin of the complex may be due to minor sulphide concentrations.
33 Mineral investigations at Carrock Fell, Cumbria. Part 1-Geophysical survey
D J Patrick (1980)
A geophysical survey was conducted in the vicinity of the Carrock tungsten mine to establish an optimum geophysical exploration procedure for the location of the style of mineralisation known at Carrock. The VLF-EM method proved to be the best tool for this environment. It recorded only weak or indistinct anomalies over much of the known mineralisation, but a weak anomaly coincident with the Emerson vein was traced northwards for 1km. Several similar linear features were recorded in the area on trends favourable for mineralisation, and two, at Poddy Gill in the east and Arm o'Grain in the west, are coincident for part of their strike with exposed mineralisation. These three anomalies appear to warrant investigation by drilling. Resistivity measurements indicated that most fault structures have coincident low-resistivity zones; a detailed traverse across the Emerson vein showed a minor high-resistivity peak within the low zone. Induced polarisation, magnetic and self-potential anomalies were recorded only within the gabbro on the extrapolated positions of the Smith and Wilson lodes. It is concluded that these anomalies are caused by discontinuous near-surface lenses of pyrrhotite (which have little VLF response).
34 Results of a gravity survey of the south-west margin of Dartmoor, Devon
J M C Tombs (1980)
A gravity survey (station density 4-6 per km2) of the southwestern margin of Dartmoor, including the Hemerdon stockwork, was interpreted with the aid of computer techniques to indicate the depth to buried granite. The results show that the Hemerdon Ball granite is an isolated block that does not extend to depth and that no vertically continuous shallow granite occurs at any distance from the known outcrop. Various computer-graphics presentations of the data are given.
35 Geophysical investigation of chromite-bearing ultrabasic rocks in the Baltasound-Hagdale area, Unst, Shetland Islands
C E Johnson, C G Smith and N J Fortey (1980)
Economic deposits of chromite in Unst were worked intermittently until exhaustion of the known near-surface deposits in 1945. Since it is likely that further comparable deposits exist at shallow depth, detailed geophysical surveys employing gravity, magnetic and electrical methods were carried out over 1 km2 of the area of known mineralisation to test the feasibility of detecting and delineating them. Seven of 16 small positive gravity anomalies were tested by shallow boreholes, but only two were attributed to chromite concentrations and the efficacy of geophysical techniques in the detection of chromite concentrations is unproven.
36 An appraisal of the VLF ground resistivity technique as an aid to mineral exploration
R D Ogilvy (1980)
A study of the VLF ground resistivity method confirmed that it is well suited to the mapping of broad mineralised zones, flat-lying conductors of limited lateral extent, or abrupt changes in conductivity associated with geological contacts. In resistive terrains the method offers distinct operational advantages over galvanic resistivity methods. The principal disadvantages of the technique relate to interpretational ambiguities associated with the complex behaviour of surface impedance at VLF and the fact that the operator has no effective control over the depth of investigation.
37 Compilation of stratabound mineralisation in the Scottish Caledonides
G S Johnstone and M J Gallagher (1980)
This report forms part of the United Kingdom contribution to the International Geological Correlation Programme Project 60, 'Correlation of Caledonian strata-bound sulphides'. Details of seven deposits are presented in tabular form, accompanied by a geological map of Scotland.
38 Geophysical evidence for a concealed eastern extension of the Tanygrisiau microgranite and its possible relationship to mineralisation
J D Cornwell, D J Patrick and R J Tappin (1980)
A Bouguer anomaly low near Blaenau Ffestiniog in North Wales, in an area of Lower Palaeozoic sedimentary and igneous rocks, is interpreted as being due to a concealed eastward extension of the Tanygrisiau microgranite. The magnetite-bearing granite is also thought to be responsible for a pronounced aeromagnetic anomaly, which has a form that supports the gravity evidence for the eastward extension of the granite body but requires the extension of a magnetic body down to a depth of 15km. The mineralisation in the area consists of sulphide-bearing quartz veins. The veins occupy faults trending chiefly to the northeast, their distribution appearing to be mainly coincident with the southern flank of the concealed granite.
Summary of geological, geochemical and geophysical data for Cairngarroch Bay area (Report No. 39).
39 Copper-bearing intrusive rocks at Cairngarroch Bay, south-west Scotland
P M Allen and others (1981)
Two intrusion complexes, the Bay and the Glen, which probably represent an early phase of the Devonian magmatic episode, and a number of dykes are emplaced within a folded succession of Silurian sedimentary rocks at Cairngarroch Bay. The Bay Complex consists of microtonalite and granodiorite. The Glen Complex comprises quartz porphyry, porphyritic quartz microdiorite and quartz microdiorite. Local high-chargeability zones were identified along three geophysical traverse lines. Soil samples were collected on a 50m grid over an area of IP anomalies. In addition, water, base of stope talus and rock samples were chemically analysed. Both the intrusion complexes and some of the sedimentary rocks show locally intense hydrothermal alteration. In the Bay Complex narrow zones of bleached rock are rich in calcite, chlorite and pyrite and contain minor chalcopyrite and pyrrhotite. The Glen Complex displays network fracturing, brecciation and locally intense alteration to sericite or calcite. There is locally abundant pyrite in veins and disseminated and rare chalcopyrite. Arsenopyrite is present in wallrock adjacent to the Bay Complex. Rock geochemistry indicates a pervasive but patchy Cu-Fe-As-Mo mineralisation in all rock types, with Cu values up to 600 ppm. The mineralisation is accompanied by irregular Ba, K and Sr enrichment. The K/Rb ratios suggest that the hydrothermal liquors were not entirely late magmatic. The mineralisation and alteration have some characteristics of a porphyry system and it is conceivable that Cu enrichment might increase with depth.
40 Stratabound barium-zinc mineralisation in Dalradian schist near Aberfeldy, Scotland: Final report
J S Coats, C G Smith and others (1981)
The presence of an extensive zone of mineralisation in the area was recognised initially by a geochemical drainage survey and reconnaissance geological mapping. Subsequently, more detailed mapping and drainage sampling were supplemented by VLF-EM geophysical surveys, overburden and rock sampling, and shallow drilling.
The VLF-EM technique successfully delineated resistive rocks within the generally conductive graphitic schists and proved a valuable mapping aid in areas of poor exposure. The mineralised zone is defined by the presence of bedded baryte, sulphide concentrations, quartz-celsian rock and micaceous schists in which the muscovite is barium-rich. It varies in thickness from about 60m to 110m and extends, at least intermittently, over 7km of strike and through a vertical interval of 370m. Individual baryte bands are 2.3-15.5m thick and may extend along strike for up to 1.8km. The greatest sulphide concentration is in carbonate rock and assays 8.5% Zn and 3.6% Pb over 4.3 m. Other unusual constituents of the mineralised zone are the barium silicates, hyalophane and cymrite, and the chromian muscovite, fuchsite. It is concluded that the mineralisation is of synsedimentary origin and involved the introduction of metal-rich hydrothermal brine into an euxinic basin.
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