Baryte and copper mineralisation in the Renfrewshire hills, central Scotland

Lower Carboniferous volcanics have been mapped and sampled in detail in an attempt to establish the controls, age and genesis of known mineral veins and to locate possible sites of new occurrences. A study of mining records provided additional information on the nature of worked veins and a reconnaissance stream sediment and panned concentrate survey indicated a potential for further geochemical exploration. Baryte veins are particularly concentrated in massive, open-jointed rocks of the Misty Law trachytic complex where they occupy a variety of fracture directions within the limits of a NW-SE swarm of Tertiary dolerite dykes. IS-Ar isotopic dates (Moore, 197913) on veins which are cut by the dykes indicate a Triassic age. Outside the trachytic complex, in the less massive basaltic sequence, baryte veins are confined to major ESE to ENE-trending fault zones and the margins of ENE-trending late- Carboniferous quartz-dolerite dykes, with which they are probably contemporaneous. Similar baryte vein deposits elsewhere in Scotland are also located on major NW-SE or NE-SW fracture systems. It is suggested that baryte mineralisation occurred at intervals from the late Carboniferous onwards during tensional stress regimes when increased heatflow circulated low-temperature, bariumrich brines, which combined with sulphurous groundwaters in near-surface oxidising conditions. Barium may have been leached from Devonian and Lower Carboniferous elastic sediments, or from trachytic rocks within the volcanic pile. Copper mineralisation occurs in a wide variety of environments ranging from replacement of plant debris by malachite in sandstones to veins of chalcocite, chalcopyrite and malachite on the margins of quartzdolerite dykes. Mineralised rocks include basal Carboniferous to Lower Limestone Group sediments and volcanics and late-Carboniferous dykes. Some of the copper has a direct late-stage hydrothermal association with the basaltic magmas and it is suggested that cupriferous veins were deposited by later, possibly late-Carboniferous hydrothermal fluids which leached copper from the basalt pile. Several new discoveries of isolated, wide veins of pure baryte could be economic if worked on a small scale, and follow-up geochemical work may reveal more extensive deposits. Panned concentrate sampling and analysis is the most sensitive method of detecting outcropping baryte veins near streams, backed-up by stream sediments which may be more effective in detecting finer-grained, more widely-dispersed material. Follow-up in selected upland target areas of relatively thin drift could be possible by overburden sampling and all three methods would benefit from the use of rapid field analysis for barium by portable X-ray fluorescence equipment. Suitable areas for further exploration include extensions of structures with known economic mineralisation, an area of thin drift cover with abundant baryte float, and areas with barium anomalies in drainage samples. Drainage geochemistry is successful in detecting known copper mineralisation and reveal further occurrences in regional surveys.