Exploration for porphyry-style copper mineralisation near Llandeloy, southwest Dyfed

Geological, geochemical and geophysical surveys followed by drilling in the area around Llandeloy, southwest Dyfed, have located disseminated copper mineralisa tion of porphyry type associated with intermediate intrusive rocks masked by thick overburden. Intermediate intrusive rocks in the area were selected for investigation as potential hosts for disseminated copper mineralisation on the basis of the known geology and tectonic setting. An initial assessment of the area involved revising the geological maps, analysing rocks from surface exposures, studying available geophysical data and carrying out a stream sediment survey in the catchment of the River Solfach. This work revealed the presence of weak poly metallic sulphide m ineralisation associated with the margin of a tonalitic intrusion at Middle Mill. In view of the very poor exposure, more detailed geochemical and geophysical surveys were carried out across the two areas underlain by intrusive rocks of dioritic or tonalitic composition. At Middle Mill six traverse lines, spaced 300 m apart and totalling 10.5 km in length, were surveyed by IP, VLF-EM and magnetic methods. Soil samples, subsequently analysed for Cu, Pb and Zn were collected at 25 m intervals. Few anomalies were located. Most of those found could be ascribed to artificial sources and it was concluded that no substantial body of disseminated copper mineralisation was present at or near the surface in the area. The mineralisation found in Middle Mill quarry is thought to be minor, epigenetic mineralisation, associated with the intrusion. At Llandeloy 13 traverse lines spaced 600 m apart and covering an area of 12 km2 were surveyed by IP, VLFEM, magnetic and radiometric methods. Soil samples were collected along these lines at 50 m intervals and analysed for Cu, Pb and Zn. In about 4 km2 around Treffynnon additional lines were sampled and measured to close the spacing to 200 m. Gravity data were also collected from some traverses and sites to supplement the Hational Gravity Survey. Several strong copper-insoil and geophysical anomalies were identified. Nine boreholes were drilled to investigate the causes, Disseminated copper mineralisation was intersected in the boreholes. It occurs principally within a concordant or semiconcordant sheeted complex of dioritic and tonalitic rocks, believed to be uppermost Cambrian or low Arenig in age, whose composition is consistent with e mplacement within a volcanic arc setting. The intrusions and their host rocks have suffered a two phase, pervasive, hydrothermal alteration which is inseparable from the sulphide mineralisation and recorded in boreholes over an area of 1 km2. The alteration shows features common to porphyry copper systems, consisting of an early patchy and irregularly developed propylitic and potassic alteration, overprinted by a widespread and locally intense late propylitic alteration. The potassic alteration is only well preserved locally and is divisible into K-feldspar and biotite types. When intense, the potassic alteration is characterised by substantial changes to the bulk chemistry of the rocks involving increases in K, K/Na, K/Rb, Rb/Sr, Cu/S and, erratically Ba and losses of Na, Sr and Ca. In the most altered rocks so called 'immobile1 elements such as Y a ~ d Nb are redistributed. The late propylitic alteration affected rocks in all boreholes except one and gave rise to the dominant alteration assemblage of sericite, chlorite, epidote, albite, pyrite and magnetite. Introduction of Fe and S appears to have accompanied this event but any other bulk chemical changes are confused by host rock variation. Retrograde effects on potassic alteration, such as the lowering of Rb/Sr, K/Na and Cu/S are probable but not clearly defined. Mineralisation, involving the introduction of Cu, Fe and S, accompanied the first phase and ?Cu, Fe and S the second phase of alteration. Cu levels are generally modest, the best intersection being 0.1% over 3.4 m in borehole 2. Cu and particularly the Cu/S ratio are generally highest in the most altered (potassic) rocks but locally high levels of Cu may be found in weakly altered rocks, There is only weak and erratic enrichment in Mo, and high levels of Cu and Mo show only a weak correlation. There are localised very weak enrichments of AS, Pb and Zn. Au was not determined. It is suggested that the present erosion level cuts a deep section through a copper porphyry deposit, this explaining the imperfectly developed zonation, low Cu content and abundant magnetite. The part of the system most likely to have contained ore grade material has, therefore, been eroded away and some of the material is found in the overlying lacustrine sediments which contain abundant magnetite, clay, feldspar and up to 640 ppm Cu. The style of mineralisation, chemistry of the rocks and geological setting all suggest that mineralisation took place in conditions consistent with an island arc setting. The detailed geology of the area is, however, imperfectly understood because of the extremely poor exposure, and the possibility exists that, because of downfaulting and tilting, parts of the deposits may be preserved and concealed to the north and east of the area drilled. The case history shows that in such areas of low relief and thick overburden drainage sampling can be an ineffective mineral exploration technique. Surface rock sampling also failed to indicate the presence of the deposit, partly because of poor exposure and partly because of the patchy, multi-phase alteration pattern. The locally thick sequence of interbedded sands and clays containing copper and magnetite overlying the deposit confused to varying degrees all the geochemical and geophysical survey results except lithogeochemistry. If the sands and clays had not contained anomalous copper the deposit would probably not have been located. Borehole results suggest that Cu, S and the Cu/S ratio provide the best li thogeoche m ical targets.