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Mineral Reconnaissance Programme (MRP) reports 21–30

21 A geochemical drainage survey of the Fleet granitic complex and its environs (8.38 Mb)

R C Leake, M J Brown, T K Smith and A R Date (1978)

Data from multi–element analysis of stream sediments and heavy mineral concentrates are presented in geochemical maps. (The elements are the same as in Report No. 19, except that Sn is not considered in sediments.) Broad–scale patterns exhibited by some elements make possible the division of the sedimentary rocks into eight distinct geochemical units, each characterised by different element distibution patterns. The Fleet and Loch Doon plutons are sub–divided on the same basis. Follow–up investigations of drainage anomalies led to the discovery of both structure–controlled and disseminated base–metal mineralisation in the Penkiln drainage basin within the southern aureole of the Loch Doon granite. The distribution of Cu, Pb and Zn, to the south and southwest of the Fleet granite suggests a zonation of vein mineralisation, with Cu prominent adjacent to the granite contact and Pb and Zn having a wider dispersion away from the granite. Other anomalies delineate a mineralised lineament following the regional strike of the Lower Palaeozoic sediments, southeast of the Fleet granite.

22 Geochemical and geophysical investigations north–west of Llanrwst, North Wales (3.88 Mb)

D C Cooper and K E Rollin (1978)

Anomalous Zn values had been found in previous regional–scale stream sediment surveys over Ordovician volcanic and sedimentary rocks northwest of the former Lanrwst mining field. To find the source of these anomalies stream sediment, panned concentrate and stream water samples were collected from 28 sites in the Afon Dulyn and Llyn Eigiau areas and analysed for Cu, Pb and Zn in all three sample types, Fe, Mn and Ba in sediments and concentrates and Ce, Ca, Ni, Ti and Sn in concentrates only. Statistical treatment of the results led to the conclusion that most of the high Zn in stream sediment was related to hydrous oxide precipitates. It is apparent that a source of Zn exists in the general area, perhaps on the wide interfluves, but the nature of this source is not known. Ground magnetic traverses indicate that an airborne anomaly over Foel Fras is a complex anomaly caused by the Foel Fras Volcanic Complex and Conwy Rhyolite Formation, and that an anomaly near Drosgl was related to a diorite body.

23 Disseminated sulphide mineralisation at Garbh Achadh, Argyllshire, Scotland (6.19 Mb)

R A Ellis, G R Marsden and N J Fortey (1978)

A small stock of biotite–feldspar porphyry, 0.25 km2 in area, outcrops in a sequence of Dalradian schists and quartzites with interbedded epidiorites. Disseminated sulphides occur within the porphyry and the hornfelsed epidiorite but do not normally exceed 3% of the rock by volume. Assays of both rock types showed a maximum level of 0.24% Cu. Hydrothermal alteration is prominent within the porphyry, with the widespread development of sericite and kaolinite. The results of geological, petrographic, geochemical and geophysical studies demonstrate the presence of several features characteristic of porphyry–style mineralisation, but the small surface area and low grade of the deposit, combined with a lack of encouraging geophysical responses at depth, suggest that there is little chance of discovering an economic orebody. Several small strata–bound lenses of massive sulphide within the metasediments were recorded, but were not investigated in detail.

24 Geophysical investigations along parts of the Dent and Augill Faults (2.73 Mb)

J D Cornwell, D J Patrick and J M Hudson (1978)

Airborne electromagnetic (AEM), radiometric and magnetic surveys were conducted over selected parts of the Dent and Augill Faults, followed, at the most promising anomalies, by ground EM surveys employing VLF, Turam and Slingram methods and by detailed geological mapping. A gravity survey was undertaken to provide Bouguer anomaly data typical of the margins of the Lower Carboniferous blocks in the northern Pennines. Five AEM anomalies were attributed to conductive shale or mudstone horizons in the Carboniferous sequence. The VLF and Turam methods produced anomalies at Kitchen Gill and Birkett Common, which correlate with faults and mudstone outcrops. Anomalies at Long Rigg and Dowgill are thought to be due to conductive boulder clay. The regional gravity survey indicated that the main faults are characterised by weak Bouguer anomaly highs. Other anomalies probably reflect variations in the basement. Such changes may be generally useful in defining the boundaries of uplifted blocks of basement rocks.

25 Mineral investigations near Bodmin, Cornwall. Part 1—Airborne and ground geophysical surveys (21.5 Mb)

J M C Tombs (1978)

Airborne magnetic and electromagnetic surveys and ground follow–up showed that airborne geophysical methods are unsuited to this area. No significant new mineralised areas were located.

26 Stratabound barium–zinc mineralisation in Dalradian schist near Aberfeldy, Scotland: Preliminary report (3.93 Mb)

J S Coats, C G Smith and others (1978)

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 4.3m. 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.

27 Airborne geophysical survey of part of Anglesey, North Wales (1.46 Mb)

I F Smith (1979)

An airborne magnetic, electromagnetic and radiometric survey was carried out over most of the northwestern half of Anglesey. The EM and radiometric results show little of interest, but the magnetic results, presented as an aeromagnetic map, show prominent anomalies associated with the Carmel Head Thrust, with Tertiary dykes and with various hornfels bodies. Possible extensions of known ultrabasic bodies and areas where other such potentially mineralised bodies may be concealed are indicated, in particular near Llandyfrydog and near the coast south of Valley.

28 A mineral reconnaissance survey of the Abington–Biggar–Moffat area, south–central Scotland (5.87 Mb)

J Dawson, J D Floyd and P R Phillip (1979)

Panned heavy mineral concentrates from an area underlain mostly by Lower Palaeozoic sedimentary rocks were examined for heavy detrital minerals and analysed for Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, Mo, Sn, Sb, Ba, Ce and Pb. Numerous new occurrences of Pb, Zn, Cu and Ba minerals were found, and nine areas are recommended for further investigation. Minor amounts of baryte and traces of cupriferous pyrite were identified in the basal breccia of the New Red Sandstone deposits in Annandale. The Hg mineral, cinnabar, was identified for the first time in Scotland. Chromiferous spinel is a major constituent in the majority of panned samples. Corundum is widely dispersed in trace amounts. Historical references to a wide distribution of particle gold were confirmed and many new occurrences found. Some placer concentration of gold and chromiferous spinel is likely in the alluvium of the valleys of the River Tweed and the Megget Water. Six greywacke formations were mapped in the project area, each distinguished by a characteristic lithology and heavy mineral content.

29 Mineral exploration in the Harlech Dome, North Wales (23.8 Mb)

P M Allen, D C Cooper and I F Smith (1979)

This report presents the results of an airborne geophysical (electromagnetic, magnetic and radiometric) survey of the eastern part of the Harlech Dome and the consequent geological, geochemical and geophysical investigations carried out on the ground and in the laboratory. The area is underlain predominantly by Lower Palaeozoic sedimentary and volcanic rocks. The airborne magnetic and EM results show a complex pattern of anomalies, but little variation is apparent from the aeroradiometric data. Ground studies of aeromagnetic anomalies demonstrated that they are related either to magnetite present in some sandstones and igneous rocks, or, more importantly, to pyrrhotite enrichments in several lithologies. Mineralogical studies showed that, whereas some enrichments in siltstones and mudstones are probably syngenetic in origin, others are possibly related to epigenetic concentrations produced during vein mineralisation. A study of the EM anomalies showed that many of them are related to comparatively low concentrations (<3%) of carbonaceous material or non–commercial quantities of sulphides in the dark mudstones of the Dolgellau Member and Clogau Formation. An interpretation of the radiometric data confirmed that the only indications of uraniferous enrichment occur in the black mudstones of the upper Cambrian Dolgellau Member. A rock geochemistry study indicated that statistical treatment of rock analysis could be used to distinguish intrusions associated with prophyry–style mineralisation. It also showed that the porphyry–style mineralisation at Coed–y–Brenin is probably cogenetic with the end Tremadoc magmatism that gave rise to both the volcanic pile on Rhobell Fawr and intermediate intrusions in the Cambrian, and therefore is quite distinct from the end–Silurian quartz–sulphide vein mineralisation. Intrusion breccias, which are also linked to the Rhobell Fawr volcanism, were recognised in the area for the first time. One of them contains the worked–out Glasdir Cu deposit. Fluid inclusion studies showed differences between quartz veins associated with the vein and disseminated styles of mineralisation, which may be useful in exploration.

Ground follow–up work showed indications of mineralisation in many of the areas examined. Among these, the possibility of vein and strata–bound Pb and Zn mineralisation was found at Hengwrt Uchaf and Benglog. At Bryn Coch, Tyddyn Gwladys and Hafod Fraith possible associated but separate bodies or extensions to the proved porphyry–type Cu deposit at Coed–y–Brenin were identified, as was, also, modest vein mineralisation. In three areas—Mynydd Foel Uchaf, Hafod–y–fedw and Y–Gors—dispersed epigenetic sulphide mineralisation in bedrock was found, mainly pyrrhotite with sub–economic base–metal sulphides. Similar metalliferous concentrations were tentatively identified in a number of other areas, which included Garth Gell, where pilot studies were carried out on coincident EM and magnetic anomalies. At Mynydd Bach, Craiglaseithin and Dol Haidd there are indications of either new veins or extensions to known veins containing Cu, Pb and Zn and, at Craiglaseithin and Dol Haidd, feeble disseminated Cu mineralisation also; here, too, the metal concentrations are considered sub–economic. Ffridd Dol–y–moch and Waun Hir are both drift–covered areas that may conceal mineralisation, but exploration is hampered by contamination problems. At Nannau, slight Cu enrichment was found in volcanic rocks which are believed to be cogenetic with the Coed–y–Brenin porphyry Cu deposit. Recommendations are made for further work at Glasdir, in the Coed–y–Brenin area, at Hengwrt Uchaf and Benglog, at Ffridd Dol–y–moch and Nannau. Furher investigations are also recommended on the nature and extent of the sulphide concentrations in the Clogau Formation.

30 Porphyry style copper mineralisation at Black Stockarton Moor, south–west Scotland (10.4 Mb)

M J Brown, R C Leake, M E Parker and N J Fortey (1979)

A Caledonian multiphase subvolcanic complex intruding Lower Palaeozoic turbidites to the west of the Criffel granodioritic plutonic complex has been mapped. An induced polarisation survey delineated an arcuate anomaly about 6km long, and a geochemical soil survey showed that there is a zone with anomalous levels of Cu (from 140 to 5500 ppm) in the southern part of the area and that it is essentially parallel to the IP anomaly but partially displaced to the east. Three deep and nine shallow drill–holes confirmed the widespread presence of both veinlet and disseminated pyrite and Cu mineralisation of the porphyry type. Regular zonation can be observed in the style and intensity of mineralisation and hydrothermal alteration. An outer, propylitic alteration zone occurs to the west, passing eastwards into a sericitic zone. Pyrite is most conspicuous within rocks of the outer sericite zone, the outcrop of which coincides roughly with the axis of the IP anomaly. Further east, pyrite decreases but chalcopyrite and bornite with some chalcocite become relatively conspicuous, and Cu levels are among the highest attained (in the 400 to 1100 ppm range). Mn, Zn, As and Pb are enriched in the outer propylitic zone, Ba in the sericitic zone and Cu in the inner sericitic zone, whereas As, Sb and Au are concentrated with Cu and Mo in isolated brecciated sections.