Caledonian gold mineralisation in the Southern Uplands-Longford-Down Terrane in Scotland and Ireland: a synthesis of current data

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 [1]. 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 [2]. The same relations are seen at the Black Stockarton Moor subvolcanic complex, interpreted as a porphyry Cu deposit [6]. 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 [9]. 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.