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Geological Society, London, Special Publications; 1987; v. 29; p. 325-342;
DOI: 10.1144/GSL.SP.1987.029.01.25
© 1987 Geological Society of London

Part III Descriptive

Aspects of veining in the Welsh Lower Palaeozoic Basin

Bill Fitches

Department of Geology, Llandinam Building, The University College of Wales, Aberystwyth, Dyfed SY23 3DB

In the Welsh Lower Palaeozoic marginal basin, veins of diverse origin are widely distributed in space and time. The veins are categorized as pre-, syn- and post-tectonic with respect to folding and cleavage development; these sub-divisions denoting different types of veining processes. Among the pre-tectonic veins are those described as regional veins which were emplaced diachronously by hydraulic jacking and related processes due to overpressured pore fluids affecting newly lithified rocks while sedimentation continued above. Other pre-tectonic veins are those around Snowdon which were due to processes closely associated with the development of an Ordovician resurgent caldera. Syn-tectonic veins are uncommon. They include the auriferous lodes of Ogofau in south-central Wales. Post-tectonic veins are the most widespread type and are exemplified mainly by those of the Harlech and Plynlimon Domes. These late veins are attributed largely to regional extension and dewatering of the Lower Palaeozoic pile caused by the early Variscan extensional regime.