Abstract: Two metamorphic alteration styles have been observed in basic lavas and volcaniclastic rocks of the Builth Volcanic Series of Ordovician age exposed at Llanelwedd Quarry, in the Builth lnlier, Wales. The first alteration style is characterized by the development of a mineral assemblage comprising albite+chlorite+ sphene+ pumpellyite ± prehnite ± calcite ± white mica, although original textures are largely preserved. This alteration pattern corresponds with that developed in volcanic and volcaniclastic rocks elsewhere in Wales, and which is attributed to the effects of burial. The second alteration style is characterized by the development of metadomains dominated by pumpellyite or, more rarely, prehnite. In the metadomains no original textures are seen in hand specimen. This alteration pattern has, so far, not been observed elsewhere in Wales, and it is here suggested that it might result from a submarine hydrothermal circulation system.