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Mineral Reconnaissance Programme (MRP) reports 111–120

111 Gold and platinum group elements in drainage between the River Erme and Plymouth Sound, South Devon [8.77 Mb]

R C Leake, D G Cameron, D J Bland and M T Styles (1990)

Significant amounts of gold have been found in drainage sediment at several localities in the area of south Devon bounded on the east by the estuary and the lower part of the River Erme and on the west by Plymouth Sound. Gold is particularly abundant at a site about 2km to the southwest of Holbeton, where many of the grains are dendritic or otherwise extremely irregular in shape. The dendritic gold shows internal zonation in Pd abundance very similar to that seen in material in carbonate veins in limestone at Hope's Nose, Torquay. Other grains show incomplete rims rich in Pd (up to 13%) or are close to pure gold in composition. Grains with silver–rich rims are much less common than in drainage and overburden samples from east of the River Erme. Associated with the dendritic gold from southwest of Holbeton are dendritic, elongate knobbly and subrounded grains rich in either Pt or Pd or both, forming around 5% of the total. Though some grains are either pure potarite (PdHg) or Au–bearing potarite without significant internal compositional variation, the majority, including all those containing a significant amount of Pt, show very complex compositional zonation. Six PGE–bearing compositional types have been distinguished. The cores of grains are generally Au–rich: either Au with only small contents of other metals, or Au with significant Pd and Hg. Rims are generally Pt–rich, often with a significant Au or Cu content which may be recognisable as a distinct thin marginal zone. Other compositions include Pt with a significant Hg content, and a combination of roughly equal amounts of Pd, Hg, Au and Pt. Some grains show clear concentric zonation and there may be very large compositional differences between zones only a few microns thick. Cinnabar occurs in small amounts in several drainage samples from the area but is particularly abundant (up to 2000 ppm Hg) in samples derived from an area underlain by Middle Devonian rocks, chiefly mafic volcanics, in the northeast of the area. The distribution of Mg, Ca, Ti, Mn, Fe, S, Ni, Cu, Zn, As, Rb, Zr, Sn, Sb, Ba, W, Pb and Bi in drainage samples is also discussed. Au concentrations show greatest positive correlation with Mn and Fe but a correlation with Sn is also significant.

112 Geophysical and geochemical investigations on Anglesey, North Wales [7.86 Mb]

D C Cooper, I F Smith, M J C Nutt and J D Cornwell (1990)

This report describes a number of surveys carried out on Anglesey and not covered by previous reports in the series. A gravity survey of the island identified two large amplitude lows: one is associated with volcanic rocks and granite cropping out southeast of the Menai Strait Fault; the other is centred off the north–west coast and is possibly caused by a concealed granite. If of Caledonian age, such a granite would have influenced the distribution of base metal mineralisation on the island. Positive anomalies are associated with metabasic rocks in the south–east of the island whilst Carboniferous sedimentary rocks give rise to gravity lows between Malltraeth and Dulas. Geophysical orientation studies of the Ordovician volcanogenic massive sulphide Cu–Pb–Zn–Ag mineralisation at Parys Mountain showed that this style of mineralisation generates strong chargeability anomalies but only weak EM anomalies, prone to interference from artificial sources. VLF(EM) proved useful for detecting steeply dipping conductors, and magnetic anomalies are produced by some basic rocks. A gravity survey detected Bouguer anomalies which two seismic refraction lines showed may be caused by concealed acid volcanic rocks. IP traversing indicated that no substantial mineralisation was associated with the Bouguer anomalies. Ground geophysical surveys confirmed airborne EM and magnetic anomalies at Bodewryd, Rhosbeirio, Treferwydd and Tyntywyn. At Rhosbeirio and Tyntywyn the cause of the EM ground anomalies remains uncertain whilst at Bodewryd and Treferwydd basic dykes are the probable source of magnetic and EM anomalies. Soil sampling was carried out around Cerrigceinwen, City Dulas, Llanbadrig, Llandyfrydog and Lligwy to investigate promising indications of mineralisation arising from earlier regional surveys. In addition, geochemical groundwater surveys were carried out around Cerrigceinwen and Llanbadrig, geophysical traversing at Llanbadrig and City Dulas, and rock sampling at Llandyfrydog. Anomalous results related to mineralisation, possibly of similar style to that found at Parys Mountain or Carmel Head, were recorded at Llanbadrig. Geochemical and geophysical anomalies probably caused by hitherto undiscovered mineralisation were also found at City Dulas. At Llandyfrydog large base metal anomalies in soils were ascribed to metal–rich water, derived from the Parys Mountain mine, flooding across and percolating into superficial deposits. Some smaller anomalies are probably derived from weak base metal vein mineralisation. In the Cerrigceinwen area stream sediment and groundwater survey data suggest that mineralisation might be associated with spilitic rocks within the Mona Complex and the basal Carboniferous succession, but limited soil sampling across these lithologies only located a few isolated base–metal anomalies. The single soil traverse sampled across the basal Carboniferous at Lligwy produced similar results.

113 Mineral investigations at Tredaule, near Launceston, Cornwall [2.34 Mb]

R C Jones and K E Beer (1990)

Five sediment samples from the two small streams east of Tredaule yielded panned concentrates with anomalous contents of Sn and W, suggestive of local mineralisation. A single soil sampling traverse was sited parallel to the main stream and in the analyses of 34 soils from this line a small group of coincident Sn and W anomalies were reported, as well as a marked pair of Ag anomalies farther south. In an endeavour to determine the source of these anomalies a gridded pattern of soil samples was collected. A total of 379 samples were analysed for a range of ore metals and associated elements. For some elements the results were combined with those from the adjacent traverse line prior to statistical treatment. From these results it is possible to recognise several soils anomalous in Sn, usually with associated elevated levels of W, and a different set anomalous in Ag. The latter are sometimes associated with anomalous levels of Cu, but there is a separate grouping of Cu anomalies which may have a closer relationship either to the Sn anomalies or to the volcanic rocks over which they are located. It remains an open question as to whether the anomalies have been fully defined in this restricted geochemical programme or whether they continue to the east of the Tredaule stream. The correlation between W and Sn, and the location of their anomalies relative to those of Cu and to the mapped geology, suggests the presence of an east–west hypothermal vein.

114 The Mineral Reconnaissance Programme 1990 [37.0 Mb]

H W Haslam, D G Cameron and A D Evans (Compilers)

Summaries of MRP reports 1–113 adapted from compilations first published in the Transactions of the Institution of Mining and Metallurgy. The text of this document has been used in the preparation of this electronic copy (Author's Note).

115 Platinum–group elements in ultramafic rocks of the Upper Deveron Valley, near Huntly, Aberdeenshire [6.74 Mb]

A G Gunn and others (1990)

The results of reconnaissance drainage surveys over an area of mafic and ultramafic Caledonian intrusives showed a good correlation with the known geology. Following the identification of a grain of sperrylite (PtAs2) during panning at a site within a poorly–exposed serpentinite near Bridgend, detailed overburden and rock sampling, guided by a ground magnetic survey, defined a zone of Pt enrichment in basal overburden close to the margin of the serpentinite. The reconnaissance survey was supported by drilling at Red Burn in the Succoth–Brown Hill intrusion. Elevated PGE contents (up to about 270 ppb Pt + Pd, with many values in excess of 100 ppb) were recorded in ultramafic lithologies. The highest levels were often accompanied by an increase in Pd relative to Pt, and sometimes by elevated Au values. Automated searching on the microprobe revealed several grains of Au, together with Pt–Cu and Pd–Sb minerals, in association with base–metal sulphides in sheared and altered host rocks. A hydrothermal origin for the mineralisation is proposed. Drilling was conducted to investigate a zone in a similar setting near Kelman Hill. One borehole intersected a zone of serpentinite enriched in platinum–group elements (PGE), up to 280 ppb Pt + Pd. The host rock has elevated Cr levels with sporadic enrichment in As. A new technique of automated searching for rare phases using the electron microprobe was applied to this drill core, resulting in the detection of several complex PGE–bearing grains intergrown with nickel arsenide.

116 Gold in the Ochil Hills, Scotland [7.89 Mb]

J S Coats and others (1991)

Reconnaissance drainage sampling in the late 1970s identified gold in heavy–mineral concentrates in the Ochil Hills. Subsequent detailed sampling showed that alluvial gold is present over a large area of the central Ochils and eastwards to the Firth of Tay. The most anomalous catchment, Borland Glen, was the focus for further integrated geological, geochemical and geophysical studies, over Lower Devonian andesitic lavas and pyroclastics intruded by a diorite body and porphyry dykes. A large IP anomaly between Borland Glen and Coul Burn was interpreted as a steep–sided zone of disseminated pyrite with associated hydrothermal alteration. Overburden sampling proved Au and Hg anomalies. Seven boreholes, drilled to a maximum depth of 102m, revealed intense hydrothermal alteration and brecciation affecting the lavas and pyroclastics in the central, IP–anomalous zone, accompanied by pyritisation with associated minor base–metal sulphides. Gold values in the drill core reach a maximum of 505 ppb Au and it is concluded that the bedrock source of the alluvial gold has not been proved.

However, the intense hydrothermal alteration in the setting of an evolved calc–alkaline volcanic complex is indicative of a large epithermal system, and a more fertile source may yet be discovered in the area. Other Au sources are indicated in the central Ochils and further detailed investigations are thought to be warranted.

117 Exploration for vanadiferous magnetite and ilmenite in the Lizard complex, Cornwall [4.30 Mb]

R C Leake, M T Styles and K E Rollin (1992)

Drilling near magnetic anomalies over the Trelan gabbro proved extensive oxide–rich gabbro with 10–15% combined ilmenite and magnetite. Ilmenite is the predominant oxide and only rarely is magnetite more than a minor constituent. The V content of the magnetite is commonly 2–3%, reaching up to 5% V2O3. The oxides occur in clusters and vein–like growths, suggesting that a filter–pressing mechanism may have caused local coalescence of oxide–rich liquid prior to crystallisation. No economic concentrations of oxide were found in the solid rock, but the friable near–surface weathered oxide–rich gabbro is a potential ilmenite resource. Part of the Crousa gravel is a channel deposit up to 9 m deep, cut through clay–rich material interpreted as glacial till, with boulders of gabbro from the Crousa gabbro on the coast to the east. At its base there is concentration of heavy minerals of exotic origin, including cassiterite and tourmaline. The till is thought to have been deposited from sea ice pushed up against the coast during an early glacial event. The results of limited power–auger sampling from the north–east of the complex suggests potential for exhalative mineralisation. Siliceous sediments with anomalously high Ba occur in association with probable mafic volcanics. A provisional revised geological map of the eastern part of the Lizard based on geochemical mapping of the overburden samples and showing the extent of the Trelan gabbro is included in the report.

118 Mineral exploration in the Cockermouth area, Cumbria. Part 1: Regional surveys [10.2 Mb]

D C Cooper and others (1991)

Revision geological mapping over Carboniferous rocks between Caldbeck and the coast at Maryport discovered many new occurrences of baryte, particularly in the Dinantian and Namurian rocks of the Tallentire–Bothel area. The mineralisation usually comprises epigenetic fracture fillings of baryte with, in some places, minor chalcopyrite or malachite. Baryte also occurs in disseminated and veinlet form in the Hensingham Grit. Lead–zinc mineralisation is less common; it occurs rarely as epigenetic fracture fillings and locally as syngenetic or diagenetic concentrations in mudstones and shales of the Coal Measures. A geochemical drainage survey confirmed the widespread occurrence of baryte and suggested that hitherto undetected mineralisation may be present around Ruthwaite (Ba), Tallentire Hill (Ba, Cu), Broughton Moor (Ba, base metals), near Binsey (polymetallic) and south of Stockdale (Zn, Pb). Gold was observed in 21 panned concentrates, mostly over Upper Carboniferous rocks in the west of the area. It probably comes from glacial deposits derived from mineralised Lower Palaeozoic rocks in southern Scotland and the north–east Lake District. Cinnabar was identified in 22 concentrates and is believed to be derived locally from the epigenetic mineralisation, as some samples of mineral veins and altered wallrocks contain appreciable Hg (up to 40 ppm). It was concluded that the baryte mineralisation may be present locally in sufficient quantities to be of economic interest. The epigenetic mineralisation is considered to be Upper Carboniferous to Lower Permian in age, the product of fluid flow through open fractures at the margin of the Solway Basin.

119 Investigations for Cu–Ni and PGE in the Hill of Barra area, near Oldmeldrum, Aberdeenshire [1.75 Mb]

A G Gunn and M H Shaw (1991)

Following promising indications from commercial surveys, limited investigations for Cu–Ni mineralisation were conducted at the eastern end of the Insch intrusion over a sheared and disrupted zone of olivine–bearing cumulates regarded as potentially favourable for PGE enrichment associated with the base–metal mineralisation. Magnetic data delineated the southern contact of the intrusion and highlighted structural discontinuities within the cumulates. Enhanced levels of Cu and Cu/Ni ratios in overburden above these structures indicated potential for hydrothermal base metal enrichment. Precious metal concentrations were generally low, but one site produced values of 88 ppm As and 12 ppb Au. A small suite of rock samples showed no significant enrichment in base or precious metals.

120 A gravity investigation of the Middleton granite, near Inverurie, Aberdeenshire [1.98 Mb]

G S Kimbell (1991)

An 8 km2 detailed gravity survey was conducted around the Middleton granite (which is associated with Mo and W mineralisation) at a station density of 30 per km2, and a surrounding area of 120km2 was surveyed at about two stations per km2. A residual Bouguer anomaly low over the granite indicates a granite subcrop centred to the north–west of its previously mapped location and a shallow granite ridge extending about 1.5km to the south–south–west. This structure enlarges the target area for further mineral exploration. Modelling indicates that the granite is a cupola projecting from the buried roof of the Bennachie component of the Eastern Highlands batholith and delineates a north–east trending feature interpreted as the concealed margin between the Bennachie granite and the more dense and/or thinner components of the batholith to the south–east.