Mineral exploration in the area of the Fore Burn igneous complex, south-western Scotland

The Fore Burn igneous complex consists mainly of quartz-microdiorite, tonalite and feldspar porphyry forming semiconcordant or concordant bodies within early Devonian volcanic and sedimentary rocks, just north of the Southern Upland Fault, some 24 km east of Girvan. There is evidence that the complex has been folded. Several small bodies of intrusion breccia occur within both the complex and the country rock and there is a zone of monolithologic breccias along a fault followed by the Fore Burn itself. The mineral composition of rocks throughout the area has been modified by alteration to sericite, carbonate and chlorite. Tourmaline is also widely distributed, but it is most abundant within the zone of breccias along the Fore Burn. The rocks in this zone are the most intensely altered in the area. They contain locally abundant disseminated sulphides and are cut by veins rich in sulphides. The rocks in this zone also contain secondary apatite. A drainage survey revealed high levels of B and As in stream sediment samples within an area of about 4 km2, and suggested the presence of Cu-As- B-AU-MO mineralisation concentrated along the Fore Burn breccia zone. Ba values are locally high, but it appears that they represent a totally separate episode of barite mineralisation. Rock geochemistry and detailed mineralogical examinations confirmed the general findings of the drainage survey. Arsenopyrite, pyrite and chalcopyrite are the main sulphide phases in the breccia zone. Smaller quantities of tennantite, tetrahedrite, cobaltite and native gold were also noted. Cu, As, MO, Au, Sb, Bi, Co, Ni, Pb and Zn were all enriched in analysed samples of mineralised rock. An examination of the so-called stable elements in the igneous rocks showed that in the highly altered rocks they had undergone remobilisation, probably under the influence of Na metasomatism. Geophysical surveys located three small areas of low resistivity, one of which is in the breccia zone, which contain concentrations of massive sulphides in veins at surface. The IP chargeability levels do not indicate any widespread, significant, near-surface disseminated sulphide mineralisation. Comparison with similar examples from western Canada suggest that the mineralisation exposed at Fore Burn may represent the upper and outer parts of a Cu-MO-AU porphyry system.