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Mineral Reconnaissance Programme (MRP) reports 71-80

71 A regional geochemical soil investigation of the Carboniferous Limestone areas south of Kendal (south Cumbria and north Lancashire) [5.48 Mb]

J H Bateson and C C Johnson (1984)

The soils of four areas were sampled and analysed for Cu, Pb, Zn, Ba and Mn. The few significant soil–geochemical anomalies can be related either to contamination or to association with minor iron mineralisation or the reefal limestones that occur in the south of the area. The latter are the most significant in view of the similarities with areas along the Craven Fault to the east and the association of economic mineralisation with Carboniferous reefs in Ireland.

72 A geochemical drainage survey of the Preseli Hills, south–west Dyfed, Wales [4.72 Mb]

D G Cameron and others (1984)

The main sources of variation in the geochemical drainage data were found to be bedrock lithology, mineralisation, contamination and hydromorphic processes. Strong geochemical signatures are shown by dolerite intrusions and by acid volcanics of the Ordovician Fishguard Volcanic Group. Dark mudstone of the Didymograptus murchisoni Beds and Sealyham Volcanic Series also show characteristic geochemical features. Over most of the area abundant monazite nodules give rise to high background levels of rare–earth elements and uranium in the panned concentrates, particularly over Llandeilo–Ashgill sedimentary rocks. Anomalies detected in three areas may be due to hitherto unrevealed mineralisation: (1) Cu, Pb, Zn and Ba near Llanfyrnach, which may represent an extension of the known Pb–Ag mineralisation of the area; (2) Ba and base metals near Crosswell–Crymmych, associated with the Fishguard Volcanic Group and overlying pyritiferous dark mudstones of the D. murchisoni Beds, with potential for massive sulphide deposits; and (3) minor native gold and chalcopyrite occurrences near Minas Dinas and Pentre Ifan.

73 Platinum–group element mineralisation in the Unst ophiolite, Shetland [12.2 Mb]

A G Gunn, R C Leake and M T Styles (1985)

The ophiolitic basic and ultrabasic rocks of Unst comprise a sequence of harzburgite, dunite, clinopyroxene–rich cumulates and gabbro. Concentrations of chromite are found in the harzburgite and dunite and, to a small extent, in the pyroxene cumulate rocks. Five alteration or hydrothermal events have been recognised in the ultrabasic rocks. Exploration for platinum–group element (PGE) mineralisation was conducted by drainage, overburden and rock sampling and up to 20 other elements were determined in the samples. Low–amplitude Ir anomalies are present in drainage samples from three discrete areas in the harzburgite, but the maximum level of 210 ppb Ir is derived from a prominent north–south zone of faulting and hydrothermal activity. This discordant zone, which extends for at least 7 km, is also marked by enrichments in Fe, Co, Ni, Cu and As. The highest Cr levels are associated with an area in the north of the harzburgite with no previous history of chromite working but where many locally derived pieces of chromite float have been discovered. Systematic collection of panned heavy–mineral concentrates from overburden samples in the Cliff area outlined a zone of coincident Pd, Pt and Rh enrichment near to, but separate from, the chromite workings known to be enriched in PGE. The distribution of Ru was entirely different, with scattered, low–amplitude anomalous zones and a maximum anomaly 300m from the chromite–rich zone. Low–amplitude Pd and Pt anomalies were detected at other locations within the dunite unit, especially in a traverse at Helliers Water across the trace of the prominent north–south fault zone adjacent to the outcrop of the cumulate unit.

Very high levels of all PGE occur in rock samples from chromitite, chromite–rich dunite and dunite in the Cliff area. The proportions of the PGE—with strong relative enrichment in Pd and Pt—are similar to those in deposits in major layered basic/ultrabasic complexes and completely different from the Ru–Ir–Os–dominant assemblage typical of ophiolitic rocks. There is no correlation with Cr, and some samples of chromitite from the Cliff area contain only background levels of PGE. High to moderate levels of PGE with the same proportions of elements as the Cliff samples also occur in samples of chromitite and serpentinised dunite from the dunite unit and in samples of pyroxenite from the cumulate unit. In contrast, PGE–rich samples of chromitite from the harzburgite unit near Harold's Grave have proportions of PGE that closely resemble the pattern found in typical ophiolites. In samples from the Cliff area the platinum–group minerals sperrylite, stibiopalladinite, hollingworthite, laurite and, possibly, irarsite have been identified, mostly as grains less than 10 µm in size. A hydrothermal origin for the PGE mineralisation is proposed, probably related to the second phase of serpentinisation. Pre–existing concentrations of chromite may have acted as a precipitation barrier, causing rich platinum–group mineral deposition in the alteration haloes around chromite grains. The high levels of PGE and the evidence of widespread occurrence of the Cliff–type PGE enrichment are favourable indications of economic mineralisation. The PGE enrichments found in the cumulate complex are of potential interest as they may originally have been of magmatic origin. Targets of a larger tonnage may, therefore, be present in this unit by comparison with the likely size of structurally controlled mineralisation elsewhere in the complex.

74 A reconnaissance geochemical drainage survey of the Harlech Dome, North Wales [10.6 Mb]

D C Cooper and others (1985)

The results of a geochemical drainage survey show that the Harlech Dome area is metalliferous, containing large anomalies for a wide range of metals. Strong regional patterns are caused by bedrock lithology, hydromorphic processes, mineralisation and contamination. Geochemical signatures characteristic of the following metalliferous concentrations were identified: (1) disseminated Cu ‘porphyry–style’ mineralisation; (2) ‘gold–belt’ vein–style mineralisation in Cambrian rocks; (3) mineralisation in Ordovician volcanic and sedimentary rocks; (4) bedded Mn deposits in the Cambrian; (5) Mn vein–style mineralisation in Ordovician volcanic rocks; (6) granite–related mineralisation; and (7) dark mudstones. Eleven areas or styles of mineralisation were identified where it was considered that further work might lead to the recognition of deposits of economic or supply significance. These targets include base–metal anomalies in Ordovician volcanic rocks where there is some potential for volcanogenic stratiform mineralisation; Cu and Au anomalies in Cambrian rocks indicating the presence of further gold–belt, vein–style mineralisation; As anomalies over Ordovician acid volcanic rocks whose Au potential merits investigation; Mn and Ba anomalies related to Mn–Ba vein mineralisation in Arenig volcanic rocks; and metalliferous concentrations in dark mudstones marginal to the Rhobell volcanic centre.

75 Geophysical surveys in part of the Halkyn–Minera mining district, north–east Wales [2.58 Mb]

J D Cornwell and G S Kimbell (1985)

Induced polarisation surveys were carried out over Carboniferous limestones and sandstones at four localities in the Halkyn–Minera area. The first covered the Llandegla Moor area, but failed to produce any evidence for a north–westward continuation of the rich Minera lode system. Gravity data subsequently demonstrated a Bouguer anomaly low over the western margin of the Cefn–y–Fedw Sandstone Group, including the Llandegla Moor area, suggesting that these rocks thicken rapidly eastwards, perhaps along a concealed northerly trending fault. Trial IP surveys were conducted at three sites in an attempt to locate this fault and test its mineral potential, but with negative results. The Bouguer low is interpreted as probably due to the presence of high–porosity and rapidly thickening sandstones in the Cefn–y–Fedw Sandstone Group. For these reasons the area where the low was discovered is not strongly recommended for any future exploration for extensions of the mineral veins.

76 Disseminated molybdenum mineralisation in the Etive plutonic complex in the western Highlands of Scotland [11.4 Mb]

H W Haslam and D G Cameron (1985)

A drainage geochemical survey covering most of the Etive plutonic complex, of Caledonian age, found the highest concentrations of Mo in stream sediment (40–120 ppm) to be in streams draining an area about 5km in diameter within the Central Starav granite. Within this area molybdenite occurs sporadically, mainly in quartz veinlets. Although selected samples of mineralised rock have been shown to contain up to 0.9% Mo, the incidence of sulphide mineralisation is too sparse for a meaningful estimate of tenor to be given. Molybdenite is usually accompanied by pyrite. Chalcopyrite and scheelite are also widespread, though less common. Mild hydrothermal alteration accompanies the mineralisation, but there is no pervasive or zoned alteration, nor is there any K or Rb metasomatism. It is suggested that the ore minerals were deposited from hydrothermal fluids, which, in the absence of any structural or physico–chemical constraints, circulated freely throughout a large volume of rock, with the consequence that the ore minerals are widely dispersed.

77 Follow–up mineral reconnaissance investigations in the Northumberland Trough [4.25 Mb]

J H Bateson, C C Johnson and A D Evans (1985)

A regional mineral reconnaissance of the Carboniferous sediments of the Northumberland Trough (Report No. 62), including detailed airborne geophysical coverage, enabled several areas to be defined for further investigation, largely on the basis of magnetic data. At Todridge Fell, Wheathill and Brown Moor, soil geochemical data indicate that faulting in the Whin Sill is accompanied by mineralisation. The amount and type of such mineralisation are, however, not determinable—except at Brown Moor, where a short drilling programme indicated alteration of the Whin Sill accompanied by Mn, Ba and Pb mineralisation. At Ewesley a linear zone of Ba enrichment in soils was identified, with associated Pb and Zn. A weak linear magnetic feature does not coincide with the surface geochemical ‘highs’. The source of the geochemical anomalies can only be determined by drilling; mineralisation associated with a fault structure or a strata–bound concentration is possible. The work in these four areas provides further evidence of the value of magnetic methods in identifying fault structures affecting the Whin Sill, with which alteration and mineralisation may be associated.

78 Exploration for porphyry–style copper mineralisation near Llandeloy, southwest Dyfed [9.71 Mb]

P M Allen, D C Cooper and others (1985)

Geological, geochemical and geophysical surveys followed by drilling in the area around Llandeloy have located disseminated Cu mineralisation of porphyry type associated with intermediate intrusive rocks masked by thick overburden. Geological examination combined with a stream–sediment survey revealed the presence of weak polymetallic sulphide mineralisation associated with the margin of a tonalitic intrusion at Middle Mill. Six traverse lines, totalling 10.5km in length, were surveyed by IP, VLF–EM and magnetic methods. Soil samples were collected at 25m intervals along the same lines and analysed for Cu, Pb and Zn. Few anomalies were located, and it was concluded that no substantial body of disseminated Cu mineralisation is present at or near the surface in this area. The mineralisation found in Middle Mill quarry is thought to be minor epigenetic mineralisation associated with the intrusion. At Llandeloy traverses totalling 37 line km in length were surveyed by IP, VLF–EM, magnetic and radiometric methods. Soil samples were collected at 50m intervals along all traverses and analysed for Cu, Pb and Zn. Gravity data were also collected from some traverses and sites. Several strong soil Cu and geophysical anomalies were identified. Nine boreholes were drilled to investigate their cause. Disseminated Cu mineralisation was intersected, occurring principally within a concordant or semi–concordant sheeted complex of dioritic and tonalitic rocks, believed to be latest Cambrian or early Arenig in age, whose composition is consistent with emplacement within a volcanic–arc setting. The intrusions and their host rocks have suffered a two–phase, pervasive, hydrothermal alteration which is inseparable from the sulphide mineralisation and which was recorded in boreholes over an area of 1 km2. The alteration shows features common to porphyry Cu systems, consisting of an early, patchy and irregularly developed, porphyritic and potassic alteration overprinted by widespread and locally intense, late propylitic alteration. Cu–Fe–S mineralisation accompanied the alteration. Cu levels are generally modest, the best intersection being 0.1 wt% over 3.4 m. Cu, and particularly the Cu/S ratio, are generally highest in the most altered rocks, but locally high levels of Cu may be found in weakly altered rocks. Mo enrichment is weak and erratic, and high levels of Cu and Mo show only a weak correlation. There are localised, very weak enrichments of As, Pb and Zn. It is suggested that the present erosion level cuts a deep section through a Cu porphyry deposit, thus explaining the imperfectly developed zonation, low Cu content and abundant magnetite. The richest material has probably been removed by erosion, and some of it may be represented in the overlying lacustrine sediments, which contain abundant magnetite, clay, feldspar and up to 640 ppm Cu.

79 Volcanogenic and exhalative mineralisation within Devonian rocks of the South Hams district of Devon [7.95 Mb]

R C Leake and others (1985)

Soil samples collected across the outcrop of the Devonian volcanic rocks between the River Yealm and Totnes in the South Hams district display several geochemical anomalies, the most extensive and highest–amplitude of which comprise: Ba with smaller amounts of other elements in the Burraton area; Ba and other elements in the Higher Ludbrook area and further northeast; Sb in the Ladywell area; As in the extreme west of the area; Cu in association with a diabase body near Weeke; and Zn and Pb around Willing Cross. In the Burraton and Higher Ludbrook areas resistivity/IP, VLF–EM, VLF–R and some detailed gravity surveys were conducted. Around Burraton resistivity anomalies were generally coincident with soil Ba anomalies, but there was no coincident gravity anomaly. In the Higher Ludbrook area a massive carbonate horizon found by drilling is responsible for a zone of high apparent resistivity and a residual Bouguer anomaly high; IP anomalies indicate that disseminated pyrite–rich mineralisation may be extensive, although the results of EM and resistivity surveys suggest that the massive pyrite intersected in one of the boreholes is of limited lateral extent. Geophysical surveys were also carried out near Ba anomalies around Whetcombe Cross and near Fursdon in an area of diffuse geochemical anomalies. A small–amplitude IP anomaly in the Fursdon area indicates a possible zone of disseminated, pyrite–rich mineralisation. In the Higher Ludbrook area the drilling proved a sequence of massive ankeritic–carbonate–quartz rock about 25m thick, underlain by massive pyrite up to 7 m thick resting on highly altered tuffaceous volcanic rocks; the sequence is interpreted to be of exhalative origin. Associated with the carbonate rock are high Zn, minor baryte and veinlets containing pyrite, tetrahedrite and chalcopyrite. The carbonate rock also contains some inclusions of highly altered schistose tuff with more than 5 wt% Ba. In this rock, and also in similar volcanic rocks beneath the pyrite, Ba appears to be accommodated chiefly in muscovite. The massive pyrite is lensoid in shape, with very minor chalcopyrite. Pyrite in layers up to 0.25m thick and as rich disseminations also occurs in the upper part of the volcanic rocks beneath the pyrite rock. The tuffaceous volcanics are highly altered basic rocks enriched in potassium. They contain minor amounts of discordant tetrahedrite, chalcopyrite and Co–Ni mineralisation. In the Burraton area argillaceous sedimentary rocks were the dominant rock type in the drill–holes, a 10–m volcanic horizon also being present. Quartzite and baryte occur within a 2–m zone, which may be similar to the massive, layered baryte float seen in the vicinity. The Ladywell borehole intersected an inverted sequence of volcanic rocks similar to those from the Higher Ludbrook area. No significant Sb mineralisation was intersected, and the source of the surface anomalies remains uncertain. The extensive exhalative mineralisation and disseminated sulphide in altered volcanic rocks in South Hams suggest the activity of large, convective hydrothermal cells. The geochemistry suggests that enrichments of Au may also occur in the region.

80 Mineral investigations in the Ben Nevis and Ballachulish areas of the Scottish Highlands [1.24 Mb]

H W Haslam (1986)

A drainage geochemical survey of the Ben Nevis area revealed above–background levels of Mo that are associated with the outer margins of the Porphyritic Outer granite and probably related to small–scale vein–type mineralisation, such as that exposed in the Allt Daim. Disseminated pyrite, sometimes with pyrrhotite and/or chalcopyrite, is quite common in the dioritic, appinitic, trondhjemitic and ultrabasic rocks near the Ballachulish igneous complex. Cu values are low, and the sulphide disseminations are probably primary.