Aspects of veining in the Welsh Lower Palaeozoic Basin

Abstract

Summary 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.

Constraints on the age and genesis of the Llanrwst and Llanfair–Talhaiarn orefields, North Wales from K–Ar and Rb–Sr studies

“…The Llanrwst orefield K-Ar ages cluster in the middle Devonian to Tournasian (386-359 Ma) with two samples indicating younger, Visean and Westphalian (336 and 307 Ma) ages. These younger ages related to the later M 3 veins probably reflecting later remobilization and/or fresh fluid injection associated with extensional tectonism in north Wales (Fitches, 1987). The M,, M 2 and M 3 veins all have very similar mineralogies and styles of emplacement suggesting common genetic factors even though the M 3 veins formed approximately 50 Ma later than the M, and M 2 veins.…”

“…Models proposed for the formation of these deposits include circulating meteoric waters for Leadhills (Samson & Banks, 1988), the hydrostatic migration of fluids from the developing Carboniferous basins for the Vale of Newlands, Greenside, Plynlimon and West Shropshire orefields (Pattrick & Russell, 1989) and metamorphic fluid expulsion during uplift is suggested for the Dolgellau Gold Belt (Bottrell et al 1990) and the Plynlimon orefield (Phillips, 1972(Phillips, , 1983. A model of post-tectonic Lower Palaeozoic dewatering associated with Variscan crustal extension is proposed by Fitches (1987). By constraining the timing of mineralization in the Llanrwst and Llanfair-Talhaiarn orefields the importance of these various genetic models for these orefields can be evaluated.…”

Lineaments in enhanced Landsat images from a portion of west Wales

“…The formation of the joints probably occurred during the relaxation phase at the end of the Caledonian deformation (Price 1962). The formation of the mineralized faults must post-date this event and, according to Fitches (1987), are probably due to early Variscan extension.…”

The genesis of the Llanrwst and Llanfair veinfields, North Wales: evidence from fluid inclusions and stable isotopes

“…Fitches (1987) suggests that some of the mineral occurrences in the Welsh Basin (Plynlimon and West Shropshire) formed in fractures initiated during strain relaxation after regional deformation, with a common east-northeast to east-southeast trend, in the early Carboniferous and related to Visean tectonism. The M 2 veins at Llanrwst have a east-northeast trend and the later M 3 veins a east-southeast trend.…”

“…Vein genesis models include circulating meteoric waters for Leadhills (Samson & Banks, 1988), hydrostatic migration of fluids from developing Carboniferous basins for the Vale of Newlands, Greenside, Plynlimon and West Shropshire (Pattrick & Russell, 1989;Pattrick & Bowell, 1991) and metamorphic fluid expulsion for the Dolgellau Gold Belt (Bottrell et al 1990) and Plynlimon (Phillips, 1972(Phillips, , 1983. A model of post-tectonic Lower Palaeozoic dewatering associated with Variscan crustal extension is proposed by Fitches (1987). These studies indicate that formation of Pb-Zn-Cu mineralized vein systems was widespread in the Lower Palaeozoic of Britain and that mineralization took place at different times over a period spanning the Devonian to .…”