Metallogenesis of Cu deposits in Upper Palaeozoic sedimentary rocks, southern Ireland

Copper has been exploited in SW Ireland for thousands of years. The increased demand for copper world-wide has led to renewed interest in these occurrences and their overall implication for copper metallogenesis in Ireland. This PhD research study focuses on historically mined, mainly vein-hosted copper deposits of SW Ireland. Central to this is the identification of the dominant structural control on mineralization, which is found to be associated with the development of the Upper Palaeozoic Munster and South Munster Basins. Detailed geological mapping, including drone mapping and 3D modelling of selected mine sites in SW Ireland have led to a new interpretation of the chronological development of quartz veins hosting copper mineralization. Macro- and microstructural investigations of the West Cork peninsulas and the mining areas reveal the development of two distinct ore forming episodes. The first of these is related to metre- to kilometre-scale early extensional, basinal normal faults that pre-date the late Carboniferous Variscan Orogeny. The copper sulphide bearing, mainly E-W to ENE-WSW striking, quartz veins are directly associated with these extensional faults. This early vein system experienced subsequent late Carboniferous deformation during the Variscan Orogeny, including folding, cleavage development, stylolitisation, sinistral SW-NE strike-slip faulting, cataclastic fracturing and recrystallization. Smaller sediment hosted copper sulphide occurrences are also related to the early extensional fault systems. The second mineralization phase occurred during the early stages of late Carboniferous Variscan compression during reactivation of the early extensional, basinal fault systems. This localised reactivation precipitates quartz hosted copper lodes (e.g. Ballycummisk on Mizen Peninsula). Metre-scale syn-Variscan quartz veins in SW Ireland are predominantly formed as tension gashes (saddle reefs, en echelon veins) in semi-brittle shear zones. The copper sulphides within the quartz veins from both episodes are mainly syn-genetic chalcopyrite, tetrahedrite/tennantite, minor bornite and chalcocite. Re-Os geochronology measurements on molybdenite samples, associated with the vein hosted copper sulphides, reveal ages of 367.3 ± 5.5 to 366.4 ± 1.9 Ma (Upper Devonian, syn-basinal) for the Allihies deposit on Beara Peninsula, and from 315.5 ± 1.6 Ma to 311.8 ± 1.6 Ma (Pennsylvanian, late Carboniferous, syn-Variscan) for the Ballycummisk copper mines on the southern coast of Mizen Peninsula. Fluid inclusion studies on mineralized, pre-Variscan quartz veins reveal multiple fluid pulses with homogenisation temperatures (Th LV-L) ranging from 74 to 335 °C. Salinities vary between 3.2 and 28.5 wt% NaClequiv. Syn-Variscan quartz veins have a Th range from 101 to 270 °C with salinities between 4.8 and 19.1 wt% NaClequiv. The petro-stratigraphic architecture of the Munster and South Munster Basins suggests similarities to a Redbed-Type copper deposit, a sub-type of sediment-hosted stratiform copper ore systems (SSC). Predominantly vein-hosted copper emplacement is caused by an advanced diagenetic stage of the host rock sediments. The new structural, chronological and geochemical model of copper mineralization in SW Ireland has major implications for its genetic relationship to the base metal deposits of the Irish Midlands.

Keywords

West Cork , Allihies , Southwest Ireland , Re-Os dating , LA-ICP-MS , Munster Basin , Upper Devonian , Irish Midlands Deposits , Variscan , Copper , Mineralization , Mineralisation , Vein hosted , Ore geology , Structural geology , Photogrammetry , Drone mapping , Geochronology , Fluid inclusions , Sediment-hosted stratiform copper

Citation

Lang, J. 2022. Metallogenesis of Cu deposits in Upper Palaeozoic sedimentary rocks, southern Ireland. PhD Thesis, University College Cork.