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Mineral Reconnaissance Programme (MRP) reports 61–70

61 Mineral reconnaissance at the Highland Boundary with special reference to the Loch Lomond and Aberfoyle areas [3.29 Mb]

W G Henderson, N J Fortey, C E Johnson and A Grout (1983)

The chromite geochemistry and hornblende–schist mineralogy of the serpentinite–spilite–black–shale–chert assemblage tend to confirm its ophiolitic character. Relict textures indicate that the serpentinites were derived from peridotitic precursors, but one unaltered ultrabasic sample comprises mainly chromian diopside. Magnetic and VLF traverses across the Highland Boundary Fault near Helensburgh identified several anomalous zones. One may be due to a concealed serpentinite sheet. The most mineralised serpentinite body showed Cr values in the range 1000–3035 ppm. Such concentrations, though not economically significant, may indicate that larger, unaltered serpentinites elsewhere at the Highland Boundary merit investigation.

62 Mineral reconnaissance in the Northumberland Trough [12.7 Mb]

J H Bateson, C C Johnson and A D Evans (1983)

A reconnaissance geochemical drainage survey across Lower Carboniferous sediments identified not only the known mining areas but also a number of other areas with anomalously high metal values. Ba in stream sediments and panned concentrates is a reliable indicator of mineralisation and identifies the Whinnetley–Settlingstones–Fallowfield area. High Ba values in concentrates were obtained from an area to the south of Rothbury (Ewesley Farm); subsequent soil sampling also revealed high levels of Ba, and it is considered that unexposed Ba mineralisation exists in the area, probably associated with a fracture cutting dolerite of the Whin Sill. A number of linear magnetic anomalies were identified from the results of a detailed low–level airborne geophysical survey over the part of the basin underlain by the Whin Sill at shallow depth. Several of these can be equated with known fault structures or their probable extensions, some of which have carried significant mineralisation. Selected anomalies were further examined by geochemical (soil sampling) and ground geophysical techniques, but the soil samples contained generally low values of ore elements. Four boreholes were drilled on a magnetic anomaly that indicates an eastward extension of the Sun Vein near Newbrough, to test the fault structure affecting the Whin Sill as interpreted from the magnetic data. Considerable variations in the texture and alteration of the quartz dolerite sill were evident from the cores. Base–metal mineralisation was identified associated with this alteration and in some of the carbonate sediments. Chemical analyses of samples from the Whin Sill quantify the changes in composition effected by the hydrothermal alteration. Magnetic susceptibility values determined on the Whin Sill core show great variability, consistent with the variation in the degree of alteration.

63 Exploration for volcanogenic sulphide mineralisation at Benglog, North Wales [4.45 Mb]

D C Cooper and others (1983)

The Ordovician Aran Volcanic Group southeast of the Harlech Dome was explored for volcanogenic sulphide mineralisation, and work is described for one of three areas that were selected for detailed investigation. The volcanic rocks of the Benglog area are acid and basic in composition; the acid rocks are mostly ash–flow tuffs derived from outside the area, whereas the basic rocks have a local derivation. They are all interbedded with dark grey or black silty mudstone and were probably erupted in a submarine environment. Contemporaneous dolerite sills were intruded into the wet sediment. This environment was suitable for the formation of volcanogenic exhalative sulphide deposits, and indications of a metallogenic horizon were found at the top of the Y Fron Formation in the form of abundant pyrite, minor pyrrhotite and minor base–metal enrichment. Soil samples were collected for Cu, Pb and Zn determination, and geophysical surveys were conducted along 11 east–west traverse lines 300m apart across the succession. Indications were found of minor vein mineralisation at dolerite intrusion margins and locally along faults. Very high chargeability and low resistivity anomalies over mudstones do not coincide spatially with geochemical anomalies in soil, which in many cases may be transported. Geochemical drainage data, in conjunction with rock analyses, show strong Ba enrichment in mudstones, which could be volcanogenic in origin and related to two separate eruptive episodes.

64 A mineral reconnaissance of the Dent–Ingleton area of the Askrigg Block, northern England [3.70 Mb]

J H Bateson and C C Johnson (1983)

The western margin of the Askrigg Block of Lower Carboniferous sediments was investigated by reconnaissance geochemical drainage survey. In addition, following identification of a moss–like plant with tolerance to high levels of base metals, a soil survey was carried out on Tow Scar, 3km northwest of Ingleton. The drainage survey revealed no new major mineralisation, though minor mineral shows were located following investigation of some anomalous sites. The soil survey led to the discovery of minor mineralisation, which suggests that similar occurrences might exist adjacent to the Craven Fault system east of Ingleton.

65 Geophysical investigations in Swaledale, North Yorkshire [3.48 Mb]

A D Evans and others (1983)

An airborne geophysical survey was carried out over part of upper Swaledale and the adjacent moorland, where the Lower Carboniferous geology and the style of mineralisation are representative of the Northern Pennine Orefield. Magnetic, electromagnetic and radiometric methods were employed to assess their applicability in this environment. Eleven airborne EM anomalies were followed up with detailed ground surveys. Drilling targeted on anomalies at Oxnop Gill revealed no evidence of significant mineralisation. Further promising anomalies at Whirley Gill were not drilled and remain unexplained. Of the other airborne anomalies, some were not detectable on the ground and others were considered to reflect stratigraphical or artificial conductors. It is concluded that the airborne EM system employed is not effective in exploration for Pennine–type mineral veins.

66 Mineral reconnaissance surveys in the Craven Basin [6.43 Mb]

A J Wadge and others (1983)

A geochemical drainage survey indicated the presence of anomalously high Cu, Pb and Zn in stream sediments and panned concentrates over Lower Carboniferous limestones and mudstones in various parts of the Craven Basin. An airborne geophysical survey (magnetic, electromagnetic and radiometric) over the Craven Faults, at the northern margin of the basin, identified 25 localities that gave an anomalous EM response; ground EM surveys showed that five of these merited more detailed examination. High radiometric readings were obtained over several limestone reefs. Seismic traverses over the South Craven Fault provided information about the stratigraphy on either side of it, and regional gravity data provided information about the major structures.

Detailed geophysical, geochemical and geological investigations were carried out in 16 areas where the geological environment or the results of reconnaissance work suggested that mineralisation might be present. Sulphide mineralisation associated with limestones of reef facies was proved—notably at How Hill and Cow Ark—and evidence was found of a continuation of the mined Bycliffe vein, but on present evidence none of the areas appears to contain deposits of ore grade. Many of the minerals appear to have been emplaced by the concentration of brines in structural or stratigraphical traps in which limestones have acted as host rocks. Comparison with the important sulphide deposits in the Lower Carboniferous of Ireland suggests that the most promising area for mineralisation is near the northern boundary of the Craven Basin, possibly at depths of 300–400m.

67 Baryte and copper mineralisation in the Renfrewshire Hills, central Scotland [5.36 Mb]

D Stephenson and J S Coats (1983)

Baryte veins are concentrated in massive, open–jointed rocks of the Misty Law trachytic complex, where they occupy a variety of fracture directions within the limits of a northwest–southeast swarm of Tertiary dolerite dykes. Outside the trachytic complex—in the less massive basaltic sequence—barite veins are confined to major ESE to ENE trending fault zones and the margins of the ENE trending late Carboniferous quartz dolerite dykes, with which they are probably contemporaneous. Several new discoveries of isolated, wide veins of pure baryte could be economic if worked on a small scale, and areas are recommended in which follow–up geochemical work may reveal more extensive deposits. It is suggested that baryte mineralisation occurred at intervals from the late Carboniferous onwards during tensional stress regimes when increased heat flow circulated low–temperature, Ba–rich brines, which combined with sulphurous groundwaters in near–surface, oxidising conditions. Cu mineralisation occurs in a wide variety of environments, which range from replacement of plant debris by malachite in sandstones to veins of chalcocite, chalcopyrite and malachite on the margins of quartz dolerite dykes. Mineralised rocks include basal Carboniferous to Lower Limestone Group sediments and volcanics and late Carboniferous dykes. Some of the Cu has a direct, late–stage hydrothermal association with the basaltic magmas and it is suggested that cupriferous veins were deposited by later, possibly late–Carboniferous, hydrothermal fluids which leached Cu from the basalt pile.

68 Polymetallic mineralisation in Carboniferous rocks at Hilderston, near Bathgate, central Scotland [6.77 Mb]

D Stephenson, N J Fortey and M J Gallagher (1983)

Five boreholes in the vicinity of the ancient Ag–Ni–Pb mine at Hilderston yielded new stratigraphic, mineralogical and geochemical information. Stratabound Zn–Pb mineralisation occurs in the lower, argillaceous part of the Petershill Limestone, which was deposited in an anaerobic lagoon on the edge of a volcanic landmass during the Lower Carboniferous (Lower Limestone Group, Viséan stage). The best intersection shows 8m of mineralised limestone, with underlying carbonaceous mudstone (1m) and tuffaceous seat rock (2m), which has an average concentration of 0.14% Pb and 0.66% Zn and maximum values of 0.6% Pb and 3.1% Zn in the carbonaceous mudstone. Late Carboniferous hydrothermal veins occur within the Petershill Limestone and in immediately overlying clastic sediments, where they are cut by east–west faults and quartz dolerite dykes. At Hilderston mine two assemblages are recognised in the vein: Ba–Fe–Ni–Co–Ag–As on a dyke margin adjacent to the clastic sediments and Fe–Pb–Zn–S at lower levels adjacent to the limestone. Zones of alteration in the dolerite dykes carry hydrocarbons and weak Ba–Fe–Cu–F mineralisation. No potentially valuable vein deposits were discovered.

69 Base metal mineralisation associated with Ordovician shales in south–west Scotland [5.94 Mb]

P Stone and others (1984)

Base metal anomalies in drainage and overburden are associated with a belt of hornfelsed black cherty mudstone and siltstone (the Moffat Shales) in the Penkiln Burn area, 13km north–north–east of Newton Stewart, which is host to weakly disseminated and epigenetic Pb–Zn–Cu mineralisation. Three varieties of mineralisation have been recognised, the earliest of which is probably syngenetic, whereas the latter two are structurally controlled. The first phase consists of fine disseminations, chiefly of sphalerite and pyrite, in mudstone and chert or stratiform pyrite laminae in mudstone. It is characterised by Zn levels averaging 500–1000 ppm over several metres of drill core; Pb levels rarely exceed 300 ppm. The second phase of the mineralisation occurs in thin quartz veinlets, which contain accessory sphalerite, galena and pyrite. Where the veining is intense Pb reaches 7000 ppm and Zn 1500 ppm, but these values persist over only a few tens of centimetres of core. Finally, a low–temperature mineral assemblage in which plumbogummite is dominant is associated with the altered margins of dykes and gossan–like zones occupying a north–south fault system. Pb levels in the dyke margins range up to 1.5% in zones that are generally less than 0.5 m thick, but 4.5% Pb has been recorded in one specimen from the exposed gossan.

70 Regional geochemical and geophysical surveys in the Berwyn Dome and adjacent areas, North Wales [12.9 Mb]

D C Cooper, K E Rollin and J D Cornwell (1984)

A gravity survey across the Berwyn Dome confirmed a broad, regional Bouguer anomaly low in the central part of the dome with smaller, irregular highs and lows, some of which may reflect small igneous bodies. The Bryneglwys Fault coincides with a 4.5 mGal anomaly, but, southwards, the two features diverge, suggesting that the density interface is related either to a splay fault or to the eastern margin of the Lower Palaeozoic Montgomery trough. The Bouguer anomalies probably reflect such factors as variation in the Precambrian basement and changes in the lithology and thickness of Lower Palaeozoic sedimentary rocks. There is no evidence for a large granitic body in Lower Palaeozoic rocks underlying the mineralisation at Llangynog. A drainage geochemical survey provided evidence of hydrous oxide precipitation processes; contamination from human activities; base–metal and baryte mineralisation; the presence of monazite nodules; hitherto unrecorded Au mineralisation; and lithological variations. The latter were related principally to shale–sandstone variation, but geochemical signatures attributable to basic intrusions, phosphatic rocks, coal measures, sandstones, limestones and volcanics were also discerned. A few geochemical anomalies deserve further investigation, notably those associated with (1) Au mineralisation in the northwest of the area; (2) baryte, perhaps accompanied by base–metal mineralisation associated with Caradocian volcanics and phosphatic rocks; (3) mineralisation associated with Llandeilian limestones and volcanic rocks north of Llanrhaeadr; and (4) Cu mineralisation associated with intrusives near the eastern margin of the dome.