Determination and distribution of gold and the platinum group elements in the Devonian and Carboniferous rocks of south-west England
A method has been developed for the determination of gold and the platinum group elements at the low concentrations found in most geological materials, utilising the advantageous properties of Inductively Coupled Plasma-Mass Spectrometry (lCP¬MS) analysis. The method has been applied to assess the distribution of the precious metals in south-west England. Results from this study are combined with previously published data on the distribution of precious metal enrichments and new fluid inclusion data, to determine the origin of these enrichments. The methodology developed uses a sodium peroxide fusion of a small (1.0 g) sample of powdered geological material, with separation by tellurium co-precipitation and determination by ICP-MS. Isotope dilution calibration is used where possibe to overcome losses through the procedure, but data for monoisotopic elements (Au, Rh) are obtained by external calibration. Detection limits for the method are excellent, being < 0.5 ng g-l for Au, Pt, Pd and the minor PGEs: Ru, Rh and Ir. Good agreement with reference data for certified reference materials WGB-l, TDB-l and UMT-l was obtained. Pd data were reliable throughout the method testing. Data for Pt and Au improved through the method development phase. The minor PGEs gave reliable data for high concentration reference materials. Field sample data were only reliable for Pd, Pt and Au; minor PGE concentrations were too low to allow quantitation. Low concentration Ir results showed a systematic error.Small quantities of gold have previously been found in south-west England. New data confirm the presence of gold in mineral materials from Hope Nose and LoddisweIl. Most of the area shows low precious metal concentrations, probably due to the lack of potential source rocks. Anomalous precious metal concentrations are found associated with deep-seated cross-course structures. These may have allowed the ingress of precious metal-bearing fluids from deeper crustal levels. A spatial association with greenstone basic volcanics is also seen, but the greenstones analysed do not show anomalously high precious metal concentrations. Palladium shows a greater dispersion than either Au or Pt; small amounts being detected in most areas sampled.