Paragenesis of the Munster Basin Upper Devonian polymetallic veins, SW Ireland

The Late Paleozoic Munster Basin of SW Ireland is predominantly composed of the non-marine siliciclastic-dominated fine-grained alluvial sediments of the Upper Old Red Sandstone magnafacies. Copper mineralisation in this sedimentary basin is important, either as sediment-hosted stratiform or locally abundant polymetallic vein-hosted copper. In the polymetallic extensional veins, the ore phases include chalcopyrite, tetrahedrite-tennantite, galena, and molybdenite, with gangue minerals commonly quartz, carbonates, chlorite, barite, and Fe-oxides. Recent Re-Os geochronology on molybdenite proved the latter veins opened ca. 367-366 Ma, during Upper Devonian basinal extension, and were deformed before ca. 316-312 Ma by the Variscan orogeny. However, the role of these two major geodynamic events on copper mineralisation was never studied in detail, such that the vein-hosted copper mineralisation and remobilisation processes are still poorly understood. A collection of mineralised vein samples from the western Munster Basin are characterised using reflected light microscopy, Raman spectrometry, and LA-ICPMS trace element analysis to better define the mineralised vein paragenesis. We have identified a pre-mineralisation chlorite veinlet generation. This generation appears to have been reopened by the quartz-rich polymetallic veins in a syntaxial manner, such that the chlorite rims the polymetallic veins. Both vein types show evidence of Variscan deformation (i.e., buckling, displacement). These new observations are critical as 1) the presence of chlorite may allow for precise geothermometry on the veins and 2) the veins appear to have used the same conduits, which may indicate important physicochemical variations (e.g., T, P, pH, fO2, etc.) and/or a pivotal switch in the fluid source(s).