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A mineral reconnaissance survey of the Llandrindod Wells/Builth Wells Ordovician inlier, Powys

By T.R. Marshall, R.C. Leake and K.E. Rollin


A reconnaissance drainage geochemical survey was carried\ud out at a density of about 2 samples per km2 of the\ud Llanclrindod Wells/ Builth Wells Ordovician inlier in\ud Powys. The rocks of the area comprise a sequence of\ud mudstones and shales ranging in age from the Llan\.irn to\ud the Caradoc containing volcanic horizons and a lrolcanic\ud complex of acid/intermediate composition overlain in the\ud south by a further volcanic sequence of more basic composition.\ud An area of about 65 km2 was sampled and from\ud each site a wet screened minus BSI 100 mesh (< 150\ud micrometres) fraction and panned heavy mineral concentratc\ud samples were obtained. Sie\rcd sediment and concentratc\ud samples were analysed for B, V, Cr, Mn, Fe,\ud Co, Ni, Cu, Zn, As, Zr, MO, Ag, Sn, Ba and Pb and Ca,\ud Ti, Mn, Fe. Ni, Cu, Zn, Sr, Zr, MO, Sn, Sb,Ba, Ce, Pb\ud and U respecti\.ely. Several Pb and Zn anomalies were\ud apparent many of which appeared to be associated with\ud the outcrop of the main volcanic unit of the area and 4 of\ud these were confirmed by further grab stream sediment\ud and bank sampling. Drainage sampling was supplemented\ud by 27 reconnaissance soil tra\rerscs mostl)\ud across the main outcrops of the \.olcanic- rocks which tencled\ud to form the most ele\rated ground. The soil samples\ud were sie\.ed to give a minus 85 BSI mesh (< 180\ud micrometres) sample for analysis for Cu, Pb, Zn, Ni and\ud either Mn and Fe or Ag and Co. Samples collected from\ud an art‘s of‘old lcaci workings in the west of the inlier failed\ud to pro\ridc c\icience of further vein mineralisation beyond\ud the limits of the workings. Elswhere the biggest concentration\ud of‘ Pb and Zn anomalies and the greatest\ud amplitude of anonlaly (2000 ppm Pb) wcrc located over\ud the northern outcrop of the the main unit of tuffaceous\ud volcanic rocks, particularly in the \.icinity of the farm Pen\ud Rhiw Frank.\ud A clctailccl geological, geochemical and geophysical\ud survey of the Pen Rhiw Frank area was carried out using\ud photogeological interpretation, further soil sampling and\ud a combination of IP/resistivity, VLF, SP and magnetic\ud surveys. Geological interpretation indicated that the area\ud was divided into fault blocks and the geophysical work\ud re\realecl a zone‘ of high resistivity with roughly coincident\ud low amplitude chargeability maxima and a VLF crosso\.\ud t‘r o\c‘r part of the zone. There was also a general\ud association of this zone with lead in soil anomalies and the\ud presence of significant amounts of weathered pyrite in the\ud limited outcrops. The lead in soil anomalies when contoured\ud in the light of the photogeological interpretation\ud can by sc‘cbn as broadly following the local strike of the\ud rocks. Four holes were drilled to in\restiage the source ot\ud the> geochemical anomalies and to investiage any association\ud bt~tn~ecn this and the obser\red geophysical\ud anonlalies. Thy holes wcrc sited to test the down dip extension\ud of‘ apparently stratabound surface soil anomalies.\ud l’uff~accous rocks of andcsitic composition with a horizon\ud of clacitic. tuffs and minor lrcsicular andcsitic 1aL.a wt‘rc intcrsccted\ud in the holes. Mineralisation occurs in the for-m of\ud secondary lc~ad minerals within a poorly rcco\rcred soft\ud clay-rich section 5.8 m thick near the top of ant‘ of the\ud holes. 1~~1 lc\.els up to 0.52 %I o\‘t‘r 3.4 m were found in\ud the zone which is located at the interface between a dacitic\ud tuff unit chcn~icnlly distinct from the rest of the \.olcanic\ud scqucrlcXX and an andesitic la\,a probably of linlitcd c\;-\ud tent .‘I’hc (h-ill holes suggest that thcl dip of the strata is\ud around 5 dcgrccs rather than the 30 degrees d~cluc~d from\ud th<> t,xisting gc,ological map of the arca and so the holes\ud wcr(‘ not sitccl at locations which wc’rc’ likcsly to iI1tc‘rsc‘c.t\ud any stratabound mineralisation at depth below the zone of\ud surface Lveathcring. A further consequence of the flat dip\ud of strata is that by accidents of topography extensive overburden\ud anomalies could be produced from relatively\ud minor statabound base-metal enrichments on the hill\ud tops

Topics: Earth Sciences
Publisher: British Geological Survey
Year: 1987
OAI identifier:

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