Investigations of radiometric anomalies 2 km\ud north-east of St Columb Major, Cornwall, indicate\ud that at the main (Tremayne) occurrence a body\ud containing 600 to 1000 tonnes of uranium at a\ud grade of 0.2 % U may exist within 60 m of the\ud surface. A tin stockwork, similar to the nearby\ud Mulberry and Prosper mines, may be indicated, but\ud tin values are erratic.\ud The surface radiometric anomalies (150 pR/h\ud maximum) lie in close proximity in an area of\ud altered Lower Devonian talc-silicate rocks and\ud slates, and trend north to south. The largest\ud anomaly measures 600 m by 170 m at the outer\ud (15 pR/h) contour. Radiometric reconnaissance\ud and gridding outlined three main groups of surface\ud anomalies greater than 15 ,uR/h. The individual\ud anomalies are well defined and isolated, and are\ud contained in an area 1.2 km by 0.7 km. They\ud probably represent separate but related mineralised\ud zones located where northerly oriented structures\ud intersect the easterly trending boundary between\ud talc-flintas and slates.\ud Soil and trench samples have high uranium, tin,\ud copper and zinc contents. Deep trenches showed\ud extensively altered rocks cut by generally subvertical\ud northerly trending quartz veins and\ud fractures rich in radioactive iron and manganese\ud oxides. A few flakes of meta-torbernite on joint\ud faces were exposed in one narrow fissure.\ud Two inclined boreholes were drilled to examine\ud the largest radioactive structure. Core recovery was\ud poor, and technical problems led to both boreholes\ud being stopped before reaching their intended\ud depths. Meta-autunite and meta-torbernite occur at\ud true depths of 34 to 38 m in the first borehole core\ud and 18 to 29 m and 34 to 36 m in the second.\ud Analytical and gamma log data indicate broad\ud zones of moderate grade uranium. A maximum of\ud 0.2 % U over a core length of 3 m is the conservatively\ud assessed value in the second borehole.\ud Capper, zinc, cobalt and tin values are also high,\ud and further drilling is recommended
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