Exploration for volcanogenic mineralisation in south-west Wales

South-west Wales has extensive and voluminous extrusive and intrusive volcanic rocks of Grdovician age interbedded with black shales and other sedimentary rocks, a sequence which has proved to contain economic base-metal deposits in other areas of the British Isles and elsewhere. No significant mineral deposits are known in the area apart from a small lead mine at Llanfymach which was abandoned in the last century. The Mineral Reconnaissance Programme has completed several projects in the area in the past 20 years and this report is a compilation of data collected during the programme but not previously published, together with data from additional new investigations in the Treffgame, Crosswell and Llangynog areas. Regional interpretations of the geophysics (gravity and aeromagnetics), lithogeochemistry and petrogenesis are presented in the report. These include the physical properties of many of the rock types and a study of the rare-earth-element (REE) contents of examples of the volcanic rocks to determine their origin and their potential for volcanogenic massive sulphide (VMS) and preciousmetal mineralisation. More detailed investigations were carried out in the Treffgame area, following on from earlier MRP work which included the drilling of three cored boreholes. The published summary report (Brown et al., 1987) contained little data as these were in a separate unpublished dataset which was later released on open file. The current report publishes some of the results of the earlier work, including details of the drilling. No sulphides other than pyrite were found during the earlier survey but up to 8.6% Ba (as baryte) was found in rhyolite lavas and tuf& in Borehole 1 on Dudwell Mountain. This prompted further investigations, described in this report, which included additional soil geochemistry coupled with geophysical techniques not used in the earlier survey. The aim was to investigate the possibility that volcanogenic mineralisation at depths of up to 100 m may occur between the widely spaced lines of the initial survey. Geophysical methods included gravity, electromagnetic (EM) and self-potential (SP). Although no signi&ant base-metal mineralisation was located, massive baryte and disseminated pyrite were found in a metre-wide zone of hydrothermal alteration in Rock Farm Quarry at the western end of the Roth Rhyolite Group. Work in the Crosswell area showed anomalous levels of Ba in soil (exceeding 2000 ppm). There, coincident strike-parallel EM conductors are associated with black shales overlying volcanic rocks of the Fishguard Volcanic Group. A regional lithogeochemical survey demonstrated that the Sealyham Volcanic Group contains acid volcanic rocks with similar intensity of alteration to that observed in the Treffgarne area; it may also be prospective for volcanogenic mineralisation. Further work is recommended in a number of districts including the ground from Plumstone Mountain to Rock Farm Quarry in the Treffgame area, the Crosswell locality and parts of the Sealyham Volcanic Group.