Research overview presentation.
The bedrock sources of widespread alluvial gold in modern stream sediments in the Southern Uplands-Longford Down Terrane (SULDT) in Scotland and Ireland are enigmatic . The geotectonic evolution of the terrane is well constrained and provides an excellent framework for investigating the processes and controls of mineralisation. Data from 11 known bedrock gold anomalies indicate that gold is dominantly refractory with rare native grains <10 µm and is geospatially associated with D1 Caledenoid shear-zones, and commonly hosted by D3 transverse structures of probable Early Devonian age [2, 3]. Fluid inclusion data indicate that gold was deposited from a low salinity mesothermal (~330°C) carbonic fluid of probable mixed magmatic-metamorphic origin consistent with Caledonian orogenic conditions [4, 5]. Auriferous veins exhibit bleached sericite-chlorite alteration haloes and disseminated arsenopyrite. Mineralisation is spatially and temporally associated with Late Caledonian minor intrusions at several localities . The same relations are seen at the Black Stockarton Moor subvolcanic complex, interpreted as a porphyry Cu deposit . Caledonian gold in the SULDT therefore appears compatible with both orogenic and intrusion-related gold (IRG) deposit types and postsubduction porphyry Cu-Au and related epithermal systems [6-8]. A contribution from magmatic fluid has been indicated for numerous orogenic gold deposits globally e.g. the Birimian of West Africa, the Lachlan Belt, Australia or Val D’Or and Timmins, Canada . With careful consideration of the complex polyphase history of deformation and fluid flow the well-constrained geology of the SULDT provides opportunities to investigate the relationships between various global models of gold mineralisation.
Throughout the SULDT gold occurrences are geospatially associated with Pb-Zn quartz-carbonate veins of probable Carboniferous age that formed from a high salinity, low temperature aqueous fluid [10, 11, 12, 13]. Regionally, gold was remobilised by low temperature brines during post-orogenic reactivation of Caledonian structures [9, 10,14]. Gold was remobilised by high salinity oxidising fluids, probably during Permo-Triassic times in relation to development of tectonically active red-bed basins [8, 15]. Understanding the influence of secondary mobilisation processes on regional gold endowment requires further study.
29 Nov 2016
Scottish Universities Environmental Research Centre, United Kingdom