LAUSR

Abstract The Southern Uplands-Down-Longford Terrane (SUDLT) within the British Caledonides hosts several economically significant gold occurrences, including a 45 km-long gold trend in central Ireland that includes the proven deposit at Clontibret. However, the region is relatively underexplored for gold and the well-constrained geology and tectonic history of the terrane provide a firm geological framework for investigating the roles of tectonic, magmatic and metamorphic processes in gold mineralisation during the transition from subduction of oceanic lithosphere to soft (thin-skinned) continental collision. The SUDLT is an Ordovician-Silurian fore-arc accretionary complex that evolved into a foreland basin fold-and-thrust-belt during continental collision in earliest Devonian times. Comparable terranes in Phanerozoic orogenic belts globally host significant orogenic gold deposits. However, the SUDLT exhibits a very low metamorphic grade and mineralisation exhibits similarities with orogenic, intrusion related and post-subduction porphyry gold systems. Gold in the SUDLT predominantly occurs as a lattice constituent of arsenopyrite and pyrite within quartz-carbonate veins and disseminated within phyllic and propylitic potassic alteration haloes around quartz-carbonate veins and some minor intrusions. Fluid inclusion data indicate that the gold was deposited from a low salinity mesothermal (∼330 °C) carbonic fluid of mixed magmatic-metamorphic origin consistent with Caledonian orogenic conditions. Gold mineralisation occurred at shallow crustal depths of less than ∼10 km and exhibits a broad spatial association with major NE-SW-trending Caledonoid shear-zones (D1). Economically significant gold mineralisation in the SUDLT is most commonly hosted by transverse ∼NW-SE- and ∼N-S-trending fractures (D3) constrained to between 418 and 410 Ma that cut anchizone-epizone facies volcaniclastic turbiditic metasedimentary host rocks. The mineralised D3 structures cut, and are cut by, broadly coeval polyphase diorite and granodiorite intrusions. Gold mineralisation is associated with the early, dioritic I-type metaluminous oxidised phase of Trans Suture Suite magmatism that straddles the Iapetus Suture but predates the later emplacement of S-type granitic magma. The host structures were subsequently reactivated during Late Palaeozoic, Mesozoic and Cenozoic times and host younger base metal deposits (Pb-Zn, Cu, Sn, Sb). Mineralisation occurred in Early Devonian times (∼418 and ∼410 Ma), following arrival of the Avalonian continental margin at the subduction trench at ∼420 Ma, coeval with the onset of regional transtension, post-subduction lithospheric mantle delamination and K-lamprophyric and mafic to intermediate calc-alkaline magmatism. Gold in the SUDLT, therefore, provides a case study for relationships between post-subduction porphyry and orogenic gold mineralisation in soft continental collision zones. The transient geodynamic setting between subduction and transtension, common during soft continental collision, provided the critical elements of the mineralising system: a metasomatically fertilised mantle source; transient geodynamics; favourable lithospheric architecture for the rapid transfer of mass and energy from subcrustal to upper crustal levels; thin-skinned deformation and relatively low exhumation favours the preservation of mineral deposits in the upper crust.