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Copper-bearing intrusive rocks at Cairngarroch Bay, south-west Scotland

By P.M. Allen, P.J. Bide, D.C. Cooper, M.E. Parker and H.W. Haslam

Abstract

Two intrusion complexes, the Bay and the Glen,\ud probably representing an early phase of the\ud Devonian magmatic episode, and a number of\ud dykes, are emplaced within a folded succession of\ud Silurian sedimentary rocks at Caimgarroch Bay.\ud Only the roof of the Bay Complex, which consists\ud of microtonalite and granodiorite, is exposed.\ud The Glen Complex, of uncertain form, comprises\ud quartz porphyry, porphyritic quartzmicrodiorite\ud and quartz-microdiorite. Exposure is good along\ud the shore line, but much of the area is covered in\ud thick, dmmlinised drift.\ud Local high chargeability zones were identified\ud along three geophysical traverse lines. Soil samples\ud were collected on a 50 m grid over the area of IP\ud anomalies. In addition, water, base of slope talus,\ud and rock samples were chemically analysed\ud Both the intrusion complexes and some of the\ud sedimentary rocks show locally intense hydrothermal\ud alteration. In the Bay Complex narrow\ud zones of bleached rock are rich in calcite, chlorite\ud and pyrite and contain minor chalcopyrite and\ud pyzrhotite. The Glen Complex displays network\ud fracturing, brecciation and locally intense alteration\ud to sericite or calcite. There is locally abundant\ud pyrite in veins and disseminated and rare chalcopyrite.\ud Arsenopyrite is present in wall rock adjacent\ud to the Bay Complex.\ud Rock geochemistry indicates a pervasive but\ud patchy Cu-Fe-As-MO mineralisation in all rock\ud types, with copper enrichment greatest in the Bay\ud Complex where the highest level recorded is over\ud 600 ppm Cu. The mineralisation is accompanied\ud by irregular barium, potassium and strontium\ud enrichment. The K/Rb ratios suggest that the\ud hydrothermal liquors were not entirely late\ud magmatic. The mineralisation and alteration have\ud some characteristics of a porphyry system and it\ud is conceivable that copper enrichment might\ud increase with depth

Topics: Earth Sciences
Publisher: Institute of Geological Sciences
Year: 1980
OAI identifier: oai:nora.nerc.ac.uk:11895

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