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

Abstract

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