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Disseminated sulphide mineralisation at Garbh Achadh, Argyllshire, Scotland

By R.A. Ellis, G.R. Marsden and N.J. Fortey


A brief investigation of low-grade copper mineralisation associated\ud with a small, talc-alkaline porphyry intrusion of Caledonian age is\ud described.\ud Geological mapping has delineated a small stock of biotite-feldspar\ud 2\ud porphyry, 0.25 km in area, intruded into a sequence of Dalradian schists\ud and quartzites with inter-bedded epidiorites. Disseminated sulphides occur\ud within the porphyry and the hornfelsed epidiorite but do not normally\ud , exceed 3% of the rock by volume. Assays of both rock types obtained maximum\ud levels of 0.24% Cu. Hydrothermal alteration is prominent within the\ud porphyry, with the widespread development of sericite and kaolinite.\ud Subsequent faulting apparently exerted some control on the present limits\ud of alteration and mineralisation. Several small strata-bound lenses of\ud massive sulphide within the metasediments were recorded but were not\ud investigated in detail.\ud Geochemical rock-sampling delineated a strong arcuate copper anomaly\ud over the northern epidiorite/porphyry contact and a more subdued anomaly\ud along the southern faulted contact. Molybdenum shows a similar distribution\ud but is more closely confined to the porphyry. Overburden sampling\ud demonstrated that little or no metal dispersion occurred within the overlying\ud till, and stream sediment sampling of the catchment area showed that\ud concentrations of copper decrease to background levels within 1 km of the\ud intrusion. 1\ud Induced polarisation (IP) surveys produced a clearly defined grouping\ud of chargeability anomalies around the edges of the porphyry stock but these\ud showed no increase in magnitude at depth. Above background chargeability\ud values recorded over the centre of the intrusion increase markedly towards\ud the margins , possibly reflecting a pyritic halo. Most resistivity 'lows' coincide with fault zones and the results of a total intensity magnetometer\ud 1\ud survey showed that, while most magnetic anomalies occur over epidiorites,\ud I\ud significant anomalies are also produced in the vicinity of fault zones.\ud The results of the geological, petrographic, geochemical and\ud I\ud geophysical studies demonstrate the presence of several features\ud : characteristic of 'porphyry copper style' mineralisation but the small\ud 1\ud surface area and low grade of the deposit, combined with a lack of\ud encouraging geophysical responses at depth, suggest that there is little\ud II chance of discovering an economic orebody by exploratory drilling

Publisher: Institute of Geological Sciences
Year: 1978
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