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Mineral Reconnaissance Programme (MRP) reports 41–50

41 Metalliferous mineralisation near Lutton, Ivybridge, Devon [3.72 Mb]

K E Beer and others (1981)

Localised U and base–metal mineralisation in Devonian slates and volcanics was traced by radiometric and geochemical soil surveys. The mineralisation is confined to two narrow structures in a fault zone trending north–west–south–east, and at surface it has a strike length of no more than 200m. Percussive drilling down to the shallow water–table indicated persistence of the secondary metalliferous minerals, but cored drilling failed to intersect any recognisable well–mineralised structure. It remains uncertain whether a small ore shoot exists below the surface anomalies; if so, it must be presumed to pitch south–eastwards. Only oxidised, and possibly enriched, mineralisation was sampled; this yielded a little cassiterite, sphalerite, pyrite, pyrrhotite and covellite, abundant hydrated iron and manganese oxides with adsorbed U, Pb, Bi, Zn, Cu and As, and flakes of secondary U and Ag minerals. Radiometry confirmed gross U disequilibrium.

42 Mineral exploration in the area around Culvennan Fell, Kirkcowan, south–western Scotland [3.45 Mb]

M E Parker, D C Cooper, P J Bide and P M Allen (1981)

Reconnaissance geochemical and geophysical surveys were concentrated on the west of the Culvennan diorites, where numerous dykes, mainly of intermediate composition, and three small bodies of intrusion breccia intrude folded greywacke, quartz wacke, silty mudstone, siltstone and calcareous mudstone of the Silurian Gala Group. A zone of high chargeability was defined, within which there are areas of low resistivity and narrow magnetic anomalies. The cause of these anomalies is most likely to be strata–bound concentrations of sulphides within the sedimentary succession and the dykes, and there is no evidence to show that the high chargeability is associated with porphyry–style mineralisation. The results of the geochemical survey substantiate this, though minor secondary concentrations of metals and weak, local Cu–As–Fe–Pb mineralisation were indicated.

43 Disseminated copper–molybdenum mineralisation near Ballachulish, Highland Region [4.91 Mb]

H W Haslam and G S Kimbell (1981)

Chalcopyrite–pyrite–molybdenite mineralisation, with minor scheelite, occurs in disseminated, veinlet and fracture–filling forms in microadamellite and surrounding adamellite in the Ballachulish igneous complex. The mineralisation is best developed in and around the eastern part of the microadamellite over an area of about 250 m x 450 m, where it was observed over a vertical interval of 250m from the highest exposure to the base of a borehole. An IP survey showed that chargeability values are slightly higher in this area. In 3m lengths of core the maximum Cu content was 260 ppm and the maxiumum Mo content 500 ppm, but the average tenor over the mineralised area is not more than 50–100 ppm Cu and 10–30 ppm Mo. Selected mineralised outcrop samples gave values of up to 2400 ppm Cu, 9200 ppm Mo, 2400 ppm W, 0.31 ppm Au and 8 ppm Ag. It is thought that the ore minerals were introduced by a hydrothermal system, which, compared with those of classic porphyry models, was small in extent and weak in intensity. Sericitic alteration is generally associated with the mineralisation, but no potassic alteration is evident and the standard zonation of porphyry copper deposits is absent. There is very little K or Rb metasomatism, the best defined chemical change being a loss of Sr in altered rocks. Anomalously low Rb and high K/Rb values in the unaltered microadamellite are attributed to the separation of a Rb–rich aqueous fluid from the microadamellite before or at the time of consolidation of the rock.

44 Reconnaissance geochemical maps of parts of south Devon and Cornwall [883 Kb]

R C Jones (1981)

These geochemical maps cover, in four sheets at a scale of 1:50 000, the area bounded by National Grid lines 170 and 290E and 040 and 100N. The data presented are for Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, Sn, Ba, Pb and U in stream sediments and Ti, Mn, Fe, Ni, Cu, Zn, Sn, Sb, Ba and Ce in panned concentrates.

45 Mineral investigations near Bodmin, Cornwall. Part 2–New uranium, tin and copper occurrences in the Tremayne area of St Columb Major [3.77 Mb]

B C Tandy and others (1981)

Investigations of radiometric anomalies in slates and calc–silicate rocks of Devonian age 2km northeast of St Columb Major indicate that at the main (Tremayne) occurrence a body containing 600—1000 t of U at a grade of 0.2% U may exist within 60m of the surface. A Sn stockwork, similar to those of the nearby Mulberry and Prosper mines, may be indicated, but Sn values are erratic. Radiometric reconnaissance and gridding outlined three main groups of surface anomalies greater than 15 ┬ÁR/h in an area 1.2km x 0.7km. They probably represent separate but related mineralised zones located where northerly–orientated structures intersect the easterly–trending boundary between calc–flintas and slates. Two inclined boreholes were drilled to examine the largest radioactive structure. Meta–autunite and meta–torbernite occur at true depths of 34–38m in the first borehole and at 18–29m and 34–36m in the second. Analytical and gamma log data indicate broad zones of moderate–grade U, which reach a grade of at least 0.2% U over 3m in the second borehole. Cu, Zn, Co and Sn values are also high, and further drilling is recommended.

46 Gold mineralisation at the southern margin of the Loch Doon granitoid complex, south–west Scotland [4.20 Mb]

R C Leake, H A Auld, P Stone and C E Johnson (1981)

Following the identification of native gold and arsenopyrite in the area, geochemical soil sampling of the southern margin and aureole of the Loch Doon plutonic complex found several As anomalies with levels in excess of 1000 ppm within the pluton and its aureole. Seven shallow boreholes were drilled to test their source. Geophysical surveys by magnetic, Slingram EM, VLF and IP methods were carried out, but none showed anomalies that correlated with zones of high As in soil.

The Loch Doon plutonic complex in this area is intrusive into a sequence of graded turbidites of probable Caradocian age. Swarms of concordant minor intrusive rocks of quartz monzonite and granodiorite, which predate the pluton, have been encountered in its aureole. Major differences in chemistry exist between these minor intrusions and the composition of the margin of the plutonic complex. Pervasive metasomatism has affected the sedimentary rocks throughout the area. Two phases of Au–bearing, As–rich mineralisation have been recognised. The earlier comprises disseminations of pyrrhotite, arsenopyrite and pyrite in the margins of monzonitic minor intrusions and disseminations of arsenopyrite in the adjacent metasediments. This mineralisation occurs in zones of thickness up to at least 18 m, within which As levels reach 3000 ppm and Au levels 0.16 ppm in samples of c. 1 m of core. It is probable that the majority of soil As anomalies originate from this type of mineralisation. Superimposed upon this is a series of discordant quartz veins and stringers, which trend roughly south, cutting all rock types, and which may be richly mineralised with arsenopyrite and some pyrite and may also contain minute grains of native gold. Individual veins range up to 30 cm in thickness, but thicker stockwork zones also exist. Arsenic levels in 200–300 g samples of veined material exceed 3.5% and Au assays up to 8.8 ppm have been obtained. A separate, minor phase of sphalerite and galena mineralisation also occurs within the area, usually in association with carbonate veinlets.

47 An airborne geophysical survey of the Whin Sill between Haltwhistle and Scots' Gap, south Northumberland [7.49 Mb]

A D Evans and J D Cornwell (1981)

The survey, conducted with magnetic, electromagnetic (VLF–EM) and radiometric equipment, covered the outcrop of the Whin Sill (a dolerite sill in Lower Carboniferous limestones and associated detrital sediments), its downdip extension, and the Haydon Bridge mining district. Maps are presented of the magnetic and VLF results. The aeromagnetic map shows a clear correlation between the distribution of anomalies and the mapped outcrops of the sill, and in drift–covered areas allows more accurate delineation of the subcrop of the sill. The magnetic data also indicate that the outcrop pattern consists of a series of linear segments, and it is suggested that the form of the sill was subject to control by the pre–existing joint or fault system during intrusion, as well as to extensive modification by later faulting. In the Settlingstones mine area the magnetic anomalies show a clear spatial relationship to the known veins and have been used to guide the search for vein extensions; elsewhere, comparable anomalies suggest new sites to be considered for detailed exploration.

48 Mineral investigations near Bodmin, Cornwall. Part 3—The Mulberry and Wheal Prosper area [4.17 Mb]

M J Bennett and others (1981)

The results of geochemical soil sampling and geophysical surveys (gravity, VLF–EM and IP) in an area of Lower Devonian sediments north of the St Austell granite, between the former opencast tin workings of Mulberry and Wheal Prosper, suggest that the most promising ground for future mineral exploration lies to the south of the latter workings. Geochemical and geophysical evidence points to the presence of a previously unrecognised mineralised zone sub–parallel to the Prosper vein sheets and some 200m south of them. The presence of Sn, Cu, Zn and a little W is indicated. Traced westwards, the Prosper mineralisation becomes more tenuous and, as seen in core from shallow boreholes, uneconomic.

49 Seismic and gravity surveys over the concealed granite ridge at Bosworgy, Cornwall [2.35 Mb]

K E Rollin, C F O'Brien and J M C Tombs (1982)

An attempt to define the form of the concealed granite ridge by seismic surveys carried out at a station separation of 20m was unsuccessful, and it was concluded that if the seismic method were to be effective a more intensive coverage would be needed, possibly with a 2m station separation. The detailed gravity survey was useful in providing an approximate shape for the granite ridge and an order of magnitude to the actual depths.

50 Geochemical drainage survey of central Argyll, Scotland [10.0 Mb]

J S Coats, B C Tandy and U McL Michie (1982)

A reconnaissance geochemical drainage survey of 720 km2 of Dalradian outcrop in central Argyll is described, and geochemical maps are presented to show the distribution of Cu, Pb, Zn, Ni, U and Mo in stream sediments and of Cu, Pb, Zn, Ni, Ba, Sb, Sn, Fe, Ce, Ca, Mn and Ti in panned concentrates. The survey identified base–metal anomalies in the Pyrite Zone, Ardrishaig Phyllites, Loch Tay Limestone and the Green Beds. The distribution of metal content within these formations has been modified by faulting and igneous intrusion. Resampling and the investigation of anomalous stream courses defined parts of Glen Fyne, the Garabal Hill–Glen Fyne igneous complex, the southerly outcrop of the Pyrite Zone and the Loch Tay Limestone as zones of base metal mineralisation in which further investigation is recommended.