Base metal mineralisation associated with Ordovician shales in south-west Scotland

Abstract

Several narrow, lenticular belts of black cherty mudstone and siltstone (the Moffat Shales), alternating with thick greywacke sequences, strike north-east to south-west across Galloway with uniformly steep dip. In the Penkiln Burn area, 13 km NNE of Newton Stewart, one such belt is hornfelsed and considerably broadened near the southwest margin of the Loch Doon granitic pluton. Base metal anomalies in drainage and overburden are spatially associated with the broadened section of the shale belt, which is host to weakly disseminated and epigenetic Pb-Zn-Cu mineralisation. Within the Moffat Shale sequence highly siliceous mudstone and siltstone are interbedded with chert, greywacke and possibly thin tuffaceous horizons. The broadening is structurally controlled, caused by the interference of early structures with a major reciined fold plunging to the south-east. Several phases of faulting and minor intrusion have been recognised, and the abundance of dykes is an unusual geological feature of the area. Lead is particularly enriched in drainage samples, reaching approximately 1% in pan concentrates collected close to a mineralised gossan-like zone. The main leadbearing mineral identified in the anomalous concentrates, and the in situ gossan material, is the secondary lead phosphate plumbogummite. Overburden sampling proved anomalous metal values extending for 2.3 km along strike and 500 m across strike. Lead again shows the greatest enrichment, with values ranging up to about 0.5 % , in soil close to the gossan. Zinc and copper give a weaker response in both overburden and drainage, but drilling showed that zinc, in the form of disseminated sphalerite, has a greater incidence at depth than was suggested by the surface anomalies. Three varieties of mineralisation have been recognised. The earliest consists of fine disseminations, chiefly of sphalerite and pyrite, in the hornfelsed sediments. It is characterised by zinc levels between 500 and 1000 ppm over several metres of drill core; lead levels rarely exceed 300 ppm. The second phase of mineralisation occurs in thin quartz veinlets, which in this case contain accessory sphalerite, galena and pyrite. Where the veining is intense, lead concentrations reach 7000 ppm and those of zinc 1500 ppm, but these values persist over only a few tens of cm of core. Finally, a low-temperature mineral assemblage in which plumbogummite is dominant is associated with the altered margins of dykes and gossanlike zones occupying a north-south fault system. Lead levels in the dyke margins range up to 1.5 % in zones generally less than 50 cm thick, but 4.5 %I Pb has been recorded in one specimen from the exposed gossan. Fine stratiform pyrite iaminae in mudstone interbedded with chert containing disseminated pyrite and sphalerite suggests that at leas: some of the early mineralisation is synsedimentary. Later mineralisation phases are, however, structurally controlled and the origin of the majority of the base metal mineralisation remains problematical. The unusual abundance of minor intrusions in the mineralised zone is strong circumstantial evidence for an igneous source. Full details of the soil geochemical surveys and the geophysical surveys are available for inspection at the Keyworth office of BGS