Dolomitic breccia veins as evidence for extension and fluid flow in the Dalradian of Argyll

Abstract

Veins of red dolomite occur extensively in the Dalradian rocks of Argyll, Scotland and adjacent areas. The veins represent brittle extensional deformation, preferentially reactivating Caledonian quartz veins. The dolomite is associated with reddening of the adjacent Dalradian country rock, which it partially replaced. Dolomite was also precipitated in overlying Old Red Sandstone, and probably dates to late Carboniferous–early Permian. Fluid inclusion studies show that the veining involved moderate- temperature (75 to 115 °C) fluids. Stable isotope data suggest that these fluids were basinal brines. Traces of chalcopyrite, paragenetically late in the veins, may reflect the mineralization which occurs more widely in the Dalradian rocks of Argyll. The red colour of the dolomite is due to abundant haematite crystallites that grew in the dolomite crystal fabric. Palaeomagnetic analysis yields a consistent late Permian–early Triassic age for the haematite growth in the dolomite veins and the reddened Dalradian country rocks. This age represents the time of haematite precipitation from iron-rich dolomite that may have been related to deep oxidizing weathering. Gold anomalies associated with reddened basement rock must be of this age or younger.