Mineral Reconnaissance Programme (MRP) reports 141-146
141 Assessment of the potential for gold mineralisation in the Southern Uplands of Scotland using multiple geological, geophysical and geochemical datasets
R C Leake, K E Rollin and M H Shaw (1996)
Digital geological, geophysical and geochemical datasets for the Southern Uplands of Scotland have been combined and evaluated using computer-based visualisation methods to enable selection of areas potentially favourable for the occurrence of gold mineralisation. Datasets utilised include residual polar aeromagnetic anomalies, lineations derived from images of aeromagnetic and gravity data, significant deviations from the regional strike of the Lower Palaeozoic rocks, the distribution of As, Sb, Bi, Cu, Pb, Ag and Ni in -150 microns stream sediment samples and the locations of known gold occurrences.
Particular attention was paid to: (1) areas where widespread strike deviations were possible indications of anomalous relative tensional stress fields which could favour mineralisation, (2) intersections and foci of geophysical lineations and (3) small groups of As and Sb anomalies which could indicate centres of minor intrusive igneous activity. Three primary target areas and 19 smaller secondary target areas were identified containing variable combinations of favourable geological, geophysical and geochemical features. Reconnaissance drainage and rock samples were collected from the three primary target areas and, on a smaller scale, from eight of the secondary target areas. In one of the primary target areas, south-east of Moffat, widespread gold was found in drainage and 2.23 ppm Au was recorded in a sample of brecciated and altered red greywacke siltstone. Gold was also found in drainage in the other primary target areas to the south of Peebles and west of Castle Douglas, though amounts were small in the latter area. Significant amounts of alluvial gold were found in three of the secondary target areas and lesser amounts in two others. The results of the test sampling validate the methodology and are sufficiently promising in several areas to merit follow-up investigations to define and assess the source of alluvial and bedrock gold.
142 Industrial mineral potential of andalusite and garnet in the Scottish Highlands
C J Mitchell and D J Harrison (1996)
A desk review of andalusite and garnet occurrences in the Scottish Highlands, was followed by a programme of field sampling, laboratory characterisation and mineral processing to identify their potential for use as industrial raw materials. Bulk samples were collected from selected occurrences of andalusite schists in the Dalradian metasedimentary rocks of north-east Scotland, and from garnetiferous Lewisian, Moine and Dalradian rocks and beach sands at sites throughout the Highlands.
Following petrographical analysis, five andalusite samples and nine garnet samples were chosen for laboratory mineral processing trials whose aim was to produce concentrates of the target minerals with high grades and recoveries. The most promising samples were then subject to pilot-scale mineral processing trials.
The andalusite concentrates produced by both laboratory and pilot-scale mineral processing do not match the chemical specifications required for industrial grade andalusite. The Al2O3 and Fe2O3 contents, which are particularly critical, are respectively too low and too high.
The garnet samples, however, yielded concentrates with high garnet contents and recoveries. Concentrates gave densities of 4 g/cm3, indicating their suitability as industrial-grade garnet products. The more promising samples were collected from garnet-mica schists of Dalradian age at locations from near Huntly in north-east Scotland and near Loch Fyne in the south-west Highlands. Further investigations are recommended to prove the scale of resources and to carry out more detailed mineral processing and quality assessments.
143 Gold mineralisation in the Dalradian rocks of Knapdale-Kintyre, south-west Highlands, Scotland
A G Gunn and others (1996)
Following reappraisal of regional geochemical and geophysical data for the northern part of the Kintyre Peninsula, a geochemical survey was conducted to search for the presence of gold in the Dalradian rocks of the area.
Drainage sampling identified gold enrichment in several areas underlain by Argyll Subgroup Dalradian metasedimentary rocks. Associated lithogeochemical sampling confirmed the presence of local elevated gold concentrations in bedrock. The highest levels, up to 22 ppm Au, were found in base metal-bearing quartz veins at the disused Stronchullin lead mine in the north-east of the project area. Similar veins carrying high levels of gold have been found elsewhere in the Stronchullin valley, in the Inverneil area in the north, and in the south of the project area near the shore of West Loch Tarbert. Gold enrichment also occurs in association with discordant copper veins in the Meall Mor area.
Analysis of geophysical and remotely-sensed lineations indicates the importance of structural control on the distribution of gold in the area. The drainage geochemical data identify several additional prospective zones possibly associated with east-west lineations.
In the light of recent discoveries of potentially economic gold mineralisation elsewhere in the Dalradian belt in Scotland and Northern Ireland, further investigations in the south Knapdale area of the Kintyre Peninsula are recommended. Priority should be given to the Stronchullin area and the other known base metal vein occurrences.
144 The potential for gold mineralisation in the British Permian and Triassic red beds and their contacts with underlying rocks
R C Leake and others (1997)
A review of the potential for gold and related mineralisation in Permian and Triassic red beds and their contacts with other sequences has been carried out for the whole of Britain, using the mineralisation model of gold transport within oxidising saline solutions and deposition at interfaces with more reduced rocks, and in fault zones carrying more reduced fluids. Drainage and rock sampling was carried out in several areas considered prospective to search for direct evidence of gold mineralisation.
In Scotland, gold, compositionally similar to that found in the Crediton Trough in Devon, was found to be widespread and locally abundant in alluvial sediment from the Mauchline Basin of red beds and alkali basalts of Permian age. Two samples of a sandstone raft within the alkali basalt were found to be impregnated with native copper and two grains of gold were extracted from an outcrop of hydrothermally altered basalt. Streams draining the nearby Thornhill Basin were also auriferous.
Gold was also found in stream alluvium at sites in north-west England, Charnwood Forest, near Redditch, the southern Malvern Hills, adjacent to the Mendip Hills and north of Tiverton in south-west England.
This desk study and reconnaissance exploration has demonstrated that gold is associated with the Permian red beds of Britain, especially where alkali basalts are present within the sequence, and to a lesser extent with Triassic rocks. The findings support the red bed mineralisation model, developed from previous MRP work in Devon.
Further exploration is recommended in several areas, particularly the Mauchline Basin.
145 Exploration for stratabound mineralisation in the Argyll Group (Dalradian) of north-east Scotland
B C Chacksfield, M H Shaw, J S Coats, C G Smith and D Stephenson (1997)
The Mineral Reconnaissance Programme (MRP) identified several horizons of stratabound mineralisation within the Argyll Group during investigations of the Dalradian Supergroup in Scotland between 1973 and 1987. These included the multi-million tonne Aberfeldy baryte deposits. Most of the discoveries were made in the central and south-western Scottish Highlands, where the Dalradian succession is well recognised. The Dalradian of north-east Scotland, because of its poorer exposure and greater complexity, did not receive the same level of attention.
The project reported here was set up to assess the potential of the Argyll Group in north-east Scotland for stratabound mineralisation, based on the geological models developed in the central Highlands. An initial desk study involved the digitisation (where necessary), integration and review of the following datasets: (i) geological mapping, including the results of new BGS mapping, (ii) BGS airborne aeromagnetic data, (iii) BGS geophysical ground survey results, (iv) BGS Geochemical Baseline Survey of the Environment drainage data, (v) existing MRP data, (vi) BGS traverse-based soil sampling results, and (vii) datasets collected by Exploration Ventures Ltd, principally detailed airborne and ground geophysical information.
Assessment of these datasets identified three broad areas which contained geochemical, geophysical and geological features that merited examination to determine their sources and indicate the potential for economic stratabound mineralisation. These areas were Ballater-Strathdon, Upper Deveron and Huntly-Portsoy. Geochemical and geophysical ground surveys were carried out in parts of all three of these areas and followed-up locally by pitting and trenching to clarify the sources of individual anomalies.
The studies indicated that the areas with the most potential for stratabound mineralisation occur to the east of the Portsoy Lineament and/or in rocks which cannot be easily correlated with Argyll Group rocks elsewhere. These rocks tend to be lithologically different from the typical Argyll Group sequence and are generally more fractured, sheared, brecciated and altered.
The most promising area for the discovery of stratabound mineralisation is considered to be in the Upper Deveron area at Wellheads. Here, high levels of lead and zinc in overburden are probably enhanced by hydromorphic processes, but the source of the lead has been traced to quartzites and the zinc to adjacent pelites within the Corinacy Pelite Member of the Blackwater Formation. The predominantly lead-zinc mineralisation is similar to that at Glenshee and Dericambus. Detailed VLF EM and IP surveys are recommended to assist in defining trenching and drilling targets. Lead and coincident IP anomalies at Succoth-Gouls also merit follow-up investigation.
In the Ballater-Strathdon area a large EM and magnetic anomaly on Creagan Riabhach, although apparently low in base-metal content, merits further investigation on the grounds that: (1) a significant amount of sulphide mineralisation is present at shallow depth, (2) there is good evidence for stratabound mineralisation, and (3) the rocks are thought to be similar to the Ben Lawers Pyrite, where copper mineralisation occurs sporadically.
In the Upper Donside part of the Ballater-Strathdon area several lead-zinc anomalies have been identified from drainage sampling, but very little mineral exploration work has been carried out over the c. 10 km strike length of the Argyll Group metasedimentary rocks. The Glenbuchat Graphitic Schist Formation has a volcanic component here, providing the potential for hydrothermal mineralisation. In the Glen Avon part of the area lead and zinc drainage anomalies have been identified and their potential is enhanced by proximity to the Lecht mineralisation.
146 Mineral exploration in Lewisian supracrustal and basic rocks of the Scottish Highlands and Islands
J S Coats and others (1997)
Supracrustal rocks in the Lewisian of north-west Scotland have several of the characteristics of 'greenstone belts' in basement gneiss terrain, which host many of the world's gold deposits. Often these deposits are closely associated with banded iron formations (BIFs) and iron-rich exhalites are present in the Loch Maree Group of mixed volcano-sedimentary origin. The exhalites include massive and microbanded iron oxide facies, together with various banded silicate and sulphide facies rocks, which form well-defined and laterally extensive suites.
A sub-economic, Besshi-style Cu-Zn-Au deposit was discovered by Consolidated Goldfields in the Loch Maree Group supracrustals near Gairloch, and an extension to this mineralisation has been found 11 km to the south-east at Flowerdale Forest. Sulphide-bearing and banded iron formations have been located with significant gold values up to 4 g/t. A significant VLF-EM and magnetic anomaly is found in the area and further exploration should be concentrated along strike under a thin cover of Torridonian sedimentary rocks. A separate outcrop of the supracrustals, north-east of Loch Maree, contains two BIF horizons and, also, chlorite schists similar to those forming the footwall to the Gairloch deposit. The BIF horizons were traced over 6 km by measuring their magnetic susceptibilities and contain up to 0.8 g/t Au. The tectono-stratigraphy of the Loch Maree and Flowerdale areas has been clarified and can now be compared to the better known Gairloch succession.
Reconnaissance lithogeochemical sampling of other supracrustal successions in Coll and Tiree, North and South Uist, and Benbecula was less promising. Investigations in other areas of the Lewisian, east of the Caledonian front at Glenelg and Scardroy have been described in MRP Report 140 and MRP Open File Report 12. The Central Mainland belt of the Lewisian contains a number of dismembered relicts of mafic and ultramafic parentage and limited sampling of these layered complexes for PGEs and Cu-Ni was inconclusive.
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