ALBERT

Description: The Caledonian orogenic belt of northern Britain hosts some significant quartz vein-hosted gold deposits. However, as in orogenic belts worldwide, the relationship between gold mineralization and regional tectonics, magmatism, and metamorphism is a matter of debate. This is primarily due to the absence of precise temporal constraints for the mineralization. Here we report high-precision 40 Ar/ 39 Ar and Re-Os ages for the largest known gold deposit at Curraghinalt (2.7 Moz) in Northern Ireland and use these ages to constrain the regional geologic setting of the gold mineralization and establish a genetic model. The gold resource is contained in a suite of quartz sulfide veins hosted by Neoproterozoic (Dalradian) metasediments, which have been thrust over an Ordovician island arc (Tyrone Igneous Complex). Previous studies recognized two generations of gold sulfide mineralization and we have identified a third in microshears that cut the veins. In the absence of precise geochronological data, mineralization ages from Ordovician to Carboniferous have been proposed. We have dated muscovite ( 40 Ar/ 39 Ar) in quartz vein-hosted clasts of Dalradian wall rock to 459.3 ± 3.4 Ma (all 40 Ar/ 39 Ar and Re-Os ages herein are reported at the 2 confidence level including all sources of uncertainty), an age that we interpret as representing the regional cooling path and which provides a maximum age constraint for all gold mineralization. This is consistent with the quartz veins postdating the end of main-stage deformation in the Grampian event of the Caledonian orogeny (ca. 465 Ma). Molybdenite (Re-Os) and sericite ( 40 Ar/ 39 Ar) from the newly identified gold-bearing microshears (third generation of gold mineralization) yield indistinguishable Re-Os models and 40 Ar/ 39 Ar ages, with a combined age of 455.8 ± 3.0 Ma. The radioisotope ages and field evidence temporally constrain gold mineralization at Curraghinalt to the lower Late Ordovician. Data show that the gold mineralization was emplaced during the Grampian event of the Caledonian orogeny. The ca. 10 Ma maximum possible mineralization interval (462.7–452.8 Ma) for all three episodes of gold emplacement is postpeak metamorphism and main deformation, coinciding with a period of rapid uplift and extensional tectonics following orogenic collapse. While previous studies have suggested the involvement of magmatic fluids in the deposition of the primary gold resource, the absence of magmatism throughout most of the mineralization interval and the nature of the geologic setting suggest that crustal orogenic fluids should also be considered. Overall Curraghinalt displays most of the characteristics of orogenic gold deposits but also some important differences, which may be explained by the geologic setting. The timing of mineralization at Curraghinalt broadly coincides with the shift from compressional to extensional tectonics. The extensional regime, rapid uplift, and a crustal profile comprising metasediments overlying a still hot island arc were ideal for creating large and long-lasting hydrothermal systems deriving heat, metals, and some of the fluids from the underlying arc.