The petrography of the mineralised dykes of W. Scotland

Abstract

The epigenetic lead-zinc-barium-strontium mineralisation of the Loch Sunart area of Argyllshire has taken place along channels provided by dykes and faults. Dolerite and lamprophyre dykes of Permo-Carboniferous age constitute the most important structural controls. These were intruded along lines of displacement some of which were initiated before and some after the intrusion of the Strontian Caledonian Granitic Complex. The re-opening of certain faults after dyke intrusion and prior to mineralisation was connected with movements of probable Permian age. The dykes are strongly altered at mineral vein contacts Mafic minerals have been replaced by chlorites, serpentines and carbonates, plagioclases severely albitised, magnetite-ilmenite grains converted to leucoxene and rock textures disintegrated. In rare cases, potash feldspar and iron-magnesian carbonates have been formed. During mineralisation, iron, magnesium and sodium have been lost to the mineralising fluids and carbon-dioxide, potassium, sulphur and sometimes calcium, bariurn and water gained by the dyke-rocks. Comparative data for mineralised dykes from Lanarkshire, Linlithgowshire and Shropshire have been collected. The important Blackcraig Vein dyke (Kircudbrightshire) is not significantly mineralised. In the geological history of the Loch Sunart area, numerous lamprophyre sheets, hornblendic schist bands and amphibolites were formed within the Moine Basement Complex before the emplacement of the strontian Granite. Small numbers of minette, spessartite vogesite, kersantite and. felsite dykes are of post-Granite, (probably Devonian) age. Later abundant W.N.W. trending dykes, often mineralised, are considered to be connected with Carboniferous and Permian volcancity. Among them are broad quartz-dolerite intrusives which have thermally altered the adjacent country-rocks to a marked degree. Narrow dykes of normal dolerite, olivine- dolerite and hornblende-bearing olivine-dolerite display a complete petrographical gradation into camptonite through camptonitic dolerite. They are, however, of an earlier age than members of the W. Highland camptonite-monchiquite swarm. The differentiation of camptonite from olivine-basalt magma is suggested. The classification of the lamprophyres is also discussed, A detailed study of harmotome from the Strontian Mines has been made. North-south trending dykes of quartz-dolerlte, normal dolerite and olivine-dolerite cut through the mineral veins. They are believed to be connected with the W. Scottish Tertiary Volcanic Centres. Magnetometer investigations have aided the mapping of the Loch Sunart Intrusives and palaeomagnetic measurements the determination of their age-relationships.