Mineral Reconnaissance Programme (MRP) reports 101-110
101 Skarn-type copper mineralisation in the vicinity of Belstone Consols Mine, Okehampton, Devon
K E Beer, G S Kimbell and M J Bennett (1989)
Significant concentrations of metalliferous sulphides occur in the Lower Carboniferous Meldon Chert Formation, mainly in calc-silicate rocks. Although examined only within a limited area around the former Belstone Consols mine, similar strata occur as a narrow belt with a strike length of some 22 km between Sourton Tors and Drewsteignton. Stream sediment samples from streams crossing this belt are dominated by minerals derived from the Dartmoor granite to the south and provide little evidence of the location, nature or richness of any sulphide ores. Soil geochemical surveys, however, do indicate clearly the presence and general composition of near-surface mineralisation, even when sited on steep valley slopes or over rather narrow ore beds. Surface geophysical surveys immediately west of Belstone Consols mine detected and traced horizons of contrasting resistivity and chargeability and provide a new insight into the geological structure. Most of the geophysical markers do not relate directly to potentially economic mineralisation, although higher chargeability values were observed over the principal mineralised zones revealed by subsequent drilling. Magnetic surveys indicate that pyrrhotite is no more than a minor constituent of the mineralisation in the vicinity of the mine. Drilling proved the presence of significant Cu and As mineralisation, with little Zn. Co is not important as an accessory metal, but high values of Bi are quite common. Sn is well developed in most calc-silicate lithologies but is present mainly in the garnets. Metal values locally exceed 3%. The worked ore beds were not identified with certainty, but it seems that a previously unknown mineralised horizon can be recognised higher in the Meldon Chert Formation.
102 Geophysical and geochemical investigations of the manganese deposits of Rhiw, western Llyn, North Wales
M J Brown and A D Evans (1989)
Detailed magnetic surveys were carried out south-west from the old Benallt Mn mine as far as the old Nant mine, and northwards from Benallt towards Sarn. Rocks in the area are of Arenig and Llanvirn age (Lower Ordovician) and consist of mudstones, siltstones and sandstones with interbedded basic lavas and sills. The Mn deposits occur in a structurally complex setting in Arenig sediments, between a basic sill and a dolerite or basalt lava. That part of the Mn mineralisation which is of ore grade is unique within the British Isles because of its strong magnetisation, caused by the presence of the Fe-Mn oxide jacobsite. Soil samples from across-strike traverses proved of limited value for exploration purposes, mainly because of the variable depth of drift cover. Ti and V were useful in areas of limited drift cover as an aid to mapping the sub-crop of the basic igneous rocks. The more extensive magnetic anomalies mark the sub-crop of a basic sill (the Footwall Sill), which occurs below the sediments that host the manganese mineralisation. In addition, several very localised magnetic anomalies were identified, three of which were investigated by drilling. Two of them were found to be due to stratabound ironstones of very limited lateral extent and of particularly high magnetic susceptibility. These ironstones contain up to ~70% Fe2O3 and show a marked depletion in Mn compared with the enclosing sediments and basic igneous rocks. It is thought that other anomalies may be due to discrete bodies of Mn ore.
103 Exploration for volcanogenic mineralisation in Devonian rocks north of Wadebridge, Cornwall
R C Leake and others (1989)
Reconnaissance overburden sampling across the main outcrops of Middle Devonian volcanic rocks clearly showed the position of the contacts between volcanic and sedimentary rocks, either as sharp increases in elements like Ti or in the value of principal component 1 derived from a principal component analysis of the geochemical data. Follow-up overburden sampling delineated several types of anomaly, some of which were investigated with ground geophysical surveys. Finally eight diamond drill holes were collared to test the source of five overburden anomalies. Forty horizons of basic igneous rock were intersected in the drill holes, some clearly volcanic and others clearly intrusive, varying in inclined thickness from a few cm to over 50m. Four compositional groups of basic igneous rock were recognised on the basis of relative concentrations of the immobile elements Ti, Y, Zr and Nb. Two varieties of quartz vein were found as loose blocks during the overburden sampling, one containing boulangerite + galena and the other with arsenopyrite + pyrite. A significant amount of Au (up to 1.0 ppm) is associated with the arsenopyrite-bearing veins. No veins corresponding exactly to these two varieties were intersected in the drill holes, though quartz veins and veinlets with either manganoan siderite or ankerite are common. Associated with some of these veins and with chloritic veins are pyrite, arsenopyrite, chalcopyrite, sphalerite and galena in varying proportions and minor amounts of tetrahedrite, some of which is richly argentiferous. A second variety of mineralisation, consisting of minor amounts of bournonite, jamesonite and stibnite, is closely associated with intrusive greenstone bodies and their immediate aureoles. Stibnite and secondary products of its alteration in association with siderite is a third type of mineralisation.
104 Stratabound barium and base-metal mineralisation in Middle Dalradian metasediments near Braemar, Scotland
M J Gallagher and others (1989)
Stratabound mineralisation comprising baryte, Ba silicates, sphalerite, galena and other sulphides has been discovered in the upper part of the Ben Eagach Schist Formation 11-13km south-south-east of Braemar. This Formation forms part of the Middle Dalradian (late Precambrian to Cambrian) sequence of metasedimentary and meta-igneous rocks. The new mineralisation occurs at the same stratigraphical position as the Aberfeldy and Loch Lyon deposits, 45km and 90km south-west respectively along the regional strike in the Grampian Highlands. Up to 10% Zn + Pb is present in Coire Loch Kander in quartzite which also contains hyalophane and armenite, the rare hydrated Ba-Ca aluminosilicate. Bedded quartz-baryte rock, some 5m thick, is exposed in the headwaters of Allt an Loch, 1-2km south of Loch Kander. A Ba anomaly in the overburden extends over 1.6km along the strike of the bed. The mineralisation was found as a result of integrated geochemical-geophysical-geological investigations north-eastwards along the presumed strike of the Ben Eagach Schist from the Glenshee district. Overburden sampling and geophysical (VLF-EM, magnetics, IP and SP in part) measurements were conducted along 40km of across-strike lines running for 11km from Glen Brighty in the south, across the mountains of Glas Maol and Cairn of Claise to the Allt an Loch district and Coire Loch Kander. Host-rocks are graphitic schists and quartzites, regionally metamorphosed to amphibolite grade, lying at or within a few tens of metres of the top of the Ben Eagach Schist against a thick, sill-like amphibolite body incorporated into the Ben Lawers Schist Formation. Sharp variation in the thickness of the Graphitic Schist Member from 0m to 300m may be partly attributable to folding but along-strike facies variation is probably of greater significance. Younger igneous rocks include a stock-like diorite which has contact metamorphosed both the bedded sulphide mineralisation and a thin baryte-galena vein unaffected by the regional metamorphism. The geochemical and geophysical information provides an excellent guide to the bedrock geology which is very poorly exposed except in Coire Loch Kander. The Zn-Pb sulphide enrichment in quartzite is accompanied by pyrite, actinolite and diopside as well as by armenite, hyalophane and traces of baryte, while the massive fine-grained quartz component of this rock is rich in fluid inclusions. The precursor assemblage may have been a hydrothermally altered sediment or a chemical exhalite. The sulphidic quartzite is interdigitated with 15 m of highly pyritic graphitic quartz schist, regarded as a distal exhalative iron-sulphur concentration in carbonaceous mud, and banded calc-silicate schist. In contrast, the bedded, quartz-baryte rock occurs in a gossanous clastic quartzitic sequence containing very little graphitic schist 1km to the south along strike.
105 Investigations at Lambriggan Mine, near St Agnes, Cornwall
K E Beer and K E Rollin (1989)
Ground geophysical and soil geochemical surveys were employed in an attempt to define the extensions of Pb-Zn mineralised veins, in Devonian slates, formerly exploited in the Lambriggan mine. The geochemical surveys did not indicate any extensions to known mineralisation nor any new mineral veins. Of the geophysical methods used, only the IP method offered any promise and even that yielded a somewhat speculative interpretaion. It seems unlikely that a significant body of Pb or Zn ore awaits discovery in the immediate area of the Lambriggan mine.
106 Marine deposits of chromite and olivine, Inner Hebrides of Scotland
M J Gallagher, I R Basham and others (1989)
A reconnaissance survey was carried out of near-shore marine deposits considered to have been derived from Tertiary ultrabasic rocks in southwest Skye and southern Rhum, rocks known to be enriched in chrome spinel and forsteritic olivine. Dive sampling close to the rocky coastlines supplemented grab sampling at surveyed locations in water depths of 50m or less. In the bay off Harris, southern Rhum, a heavy mineral sand deposit 3km2 in area occurs within 2km of the coast in waters of 20-25m average depth. A 1 km2 deposit is present up to 1km off Dibidil, also in southern Rhum, in an average water depth of about 20m. Using a wet density of 2.2 g.cm-3 some 9 million tonnes of sand are calculated to be present in the topmost 1 m of the deltas. Shell calcite forming about 20% was removed prior to chemical analysis. The analytical results indicate that the surficial 1 m of sand contains some 70 000 tonnes of chrome spinel averaging 32% Cr2O3 at a grade of nearly 1%. Also present are 1.5 - 2 million tonnes of olivine averaging 47% MgO at 25% grade. Accompanying minerals are ilmenite and vanadiferous magnetite, and traces of platinum-group elements have been detected. The minerals occur in sand-size fractions (125-500 µm), from which concentrates of 86% chromite and 78% olivine at recoveries of 60% and 50% respectively have been achieved in the laboratory. Most of the 63 seabed samples from Loch Scavaig and the Soay Sound, southwest Skye, are grey glacial sandy clays averaging only 0.5% Cr2O3 and 2% Mg after carbonate dissolution. Heavy mineral sands derived from the Cuillins igneous centre may nevertheless underly the glacial deposits. Geophysical surveys and profile sampling are required to determine the thickness and grade of the heavy mineral sands.
107 Mineral investigations near Bodmin, Cornwall. Part 6—The Belowda area
K E Beer, B R Mountford and R C Jones (1989)
A series of soil sampling traverses were made across the Belowda Beacon granite and across slates of the Devonian Meadfoot Beds, for a distance of 1.7km east of the granite outcrop, to test the extensions of known and formerly worked mineralisation. In two areas gridded auger sampling was undertaken. All samples were analysed for Sn and its common associates, Cu, Zn, U, As and B. Trenching was used to investigate the geochemistry of the area around Brynn Tye mine and Wheal Tregoss: the bedrock was examined for Sn using a hand-held portable radioisotope fluorescence analyser, and significant mineralisation was channel sampled for laboratory assay. Anomalous Sn values in one of the two gridded areas indicate extensions to two veins which may have been previously worked. Trenching at Brynn Tye mine demonstrated that Sn mineralisation is associated most commonly with quartz-tourmaline veining and alteration of the slates. Two wide zones of low-grade ore were defined by the sampling, one of which was recognised in two trenches, thus defining a limited strike length, but two inclined diamond drillholes failed to find depth continuation to the veining or the cassiterite distribution.
108 Geochemical investigations around Trewalder, near Camelford, Cornwall
K E Beer and R C Jones (1989)
Stream sediment geochemistry over Devonian slates to the north of St Teath revealed two Zn anomalies, suggestive of an apparently unexplored extension to the Pb lode of Trewalder mine. More detailed stream sampling confirmed these findings, with a significant length of the River Allen reporting anomalous Zn levels in sediments and panned concentrates. There is no correlation with either Pb or Ag. Some 150 soil samples were collected for analysis from six lines across the strike of the Trewalder Lode and its possible extensions to the north. Log-probability plots failed to show any significant correlation between the distribution populations and the mapped lithology and mineralisation. Traverse profiles for Pb, Zn and Ba, on the other hand, can be interpreted as showing both particulate and hydromorphic anomalies, suggesting that there may be more than one mesothermal lode in the Trewalder area, extending north of that hamlet for some distance, and that to the north of the River Allen, near Helland, the lode may change in character to become essentially baryte. It appears from the low tenor of the anomalies that the prospects for significant economic ore concentrations must be extremely small. Some previously unexplored Pb-Zn-Ba mineralisation seems to be indicated on the slopes to the west of the River Allen.
109 Copper and molybdenum distribution at Shap, Cumbria
K E Beer and G S Kimbell (1989)
Examination of rock outcrops in and around the Shap granite and percussion drilling behind the Pink Granite Quarry confirmed that Cu and Mo are present over a wide area, though in amounts which everywhere are subeconomic. IP surveys did not provide any evidence of anomalously high concentrations of sulphide mineralisation, although a VLF-EM anomaly indicates a possible northward extension of a fracture zone which is mineralised where exposed in the quarry face. A model suggesting a more deeply buried porphyry-type deposit is not wholly disproved, but the evidence obtained from drillholes and geophysical surveys is not encouraging.
110 Mineral investigations near Bodmin, Cornwall. Part 7—New uranium occurrences at Quoit and Higher Trenoweth
T K Ball, B C Tandy and K Turton (1990)
Radiometric anomalies over Devonian slates near Quoit and Higher Trenoweth were investigated to determine their subsurface continuity and their potential for associated concentrations of elements such as Co and Ni. Surface geochemical investigations and drilling indicate that the individual surface anomalies at Quoit are too isolated to offer significant targets for further exploration. The low levels of Cu, Pb and Zn suggest that there is little likelihood of this structure being of any significance for mineralisation. Although the anomaly at Higher Trenoweth is traceable for a strike length of at least 300m it still presents too small a structure to merit further attention. Some high surface Sn and B values are indicated at the southern end of the structure, where kaolinised elvan is known to occur, but although the values reflect the mineralisation they are considered too low to warrant further investigation.
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