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Exploration for volcanogenic sulphide mineralisation at Benglog, north Wales

By D.C. Cooper, D.G. Cameron, Audrey A. Jackson, P.M. Allen and M.E. Parker


Exploration for volcanogenic sulphide mineralisation\ud around Benglog is one of three investigations\ud designed to assess the metallogenic potential\ud of the Ordovician Aran Volcanic Group.\ud Detailed geological mapping in the Benglog\ud area enabled an interpretation of the volcanic\ud environment, critical to such an assessment, to be\ud made. The eruptive rocks are acid and basic in\ud composition; the acid rocks are mostly ash-flow\ud tuffs derived from outside the area, whereas the\ud basic rocks have a local derivation. They are all\ud interbedded with dark grey or black silty mudstone\ud and were probably erupted in a submarine\ud environment. Contemporaneous dolerite sills were\ud intruded into wet sediment.\ud This environment was suitable for volcanogenic\ud exhalative sulphide deposits to form and indications\ud of a metallogenic horizon were found at the top\ud of the Y Fron Formation in the form of abundant\ud pyrite, minor pyrrhotite and minor base metal\ud enrichment.\ud Soil samples, analysed for copper, lead and\ud zinc, were collected and geophysical surveys were\ud carried out along eleven east-west trending traverse\ud lines 300 m apart across the volcanic succession.\ud Indications were found of minor vein mineralisation\ud at dolerite intrusion margins and locally along\ud faults. Very high chargeability and low resistivity\ud anomalies over mudstones did not spatially\ud coincide with geochemical anomalies in soil, but\ud the secondary redistribution of metals in soils and\ud variable thickness of overburden precluded\ud confident interpretation of the source of many\ud soil anomalies. Geochemical drainage data, in\ud conjunction with rock analyses, show strong\ud barium enrichment in mudstones which could be\ud volcanogenic in origin but related to two separate\ud eruptive episodes.\ud The findings of the survey were inconclusive.\ud An environment suitable for the formation of\ud volcanogenic exhalative sulphide deposits was\ud established, but the geochemical and geophysical\ud surveys located only minor vein mineralisation and\ud tenuous indications of other styles of mineralisation.\ud Recommendations are made for further work

Topics: Earth Sciences
Publisher: Institute of Geological Sciences
Year: 1983
OAI identifier: oai:nora.nerc.ac.uk:11861

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