A seabed radiometric survey of Haig Fras, S. Celtic Sea, U.K.

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At Haig Fras an Hercynian intrusion, probably a separate body from the S.W. England batholith, forms a seabed ridge made up of 3 main outcrops of well-jointed granite. The granite intrudes Devonian/Carboniferous metasediments in an inlier bounded by Cretaceous and younger strata. Relatively low and high radioactivity areas of granite result mainly from variations in Th content. This is ascribed to the breakdown of monazite by hydrothermal alteration. The alteration may also have mobilised U since U concentrations, interpreted as vein mineralisation, occur in slates adjacent to low activity granite. The high activity granite is depleted in U compared to fresh S.W. England granite but this probably results from surface weathering. Enhanced U concentrations near the base of the chalk are believed to reflect a uraniferous phosphatic horizon. The fine to medium grained granites from Haig Fras differ from the coarse grained megacrystic varieties prevalent in S.W. England, but this may be a sampling anomaly. Foliated granite, prominent in early dredge samples, appears to comprise only a subordinate marginal facies. Granitic permeation gneisses may represent a sliver of crystalline basement emplaced along a faulted margin of the granite, where basic dykes appear also to have been introduced.

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