RE-OS ARSENOPYRITE GEOCHRONOLOGY OF QUARTZ VEINING AND AU MINERALIZATION, MEGUMA TERRANE, NOVA SCOTIA, CANADA

The absolute timing of quartz veining and related Au mineralization in the turbidite dominated Meguma Terrane, Canada, is presently constrained by mica Ar-Ar age data. These dates from bedding-parallel, bedding-discordant, or saddle-reef quartz veins suggests that all vein types (and associated Au) were formed late in the deformational history of the Meguma Terrane at ~ 370 - 380 Ma, and that all deposits are of similar age regardless of location. To further constrain the origin of these auriferous quartz veins, we have determined Re-Os ages for arsenopyrite (asp), an abundant vein mineral having a cogenetic relationship with gold. Meguma asp contains only ~ 1 - 10 ppb Re but shows very low common Os (0 - 9 ppt ¹⁹²Os) and Os is thus highly radiogenic and amenable for Re-Os dating. Re-Os ages were calculated by either an isochron approach using multiple samples from one vein, or as single-mineral model ages if the abundance of common Os was zero. Arsenopyrite from a bedding-parallel quartz vein at The Ovens in the SW Meguma Terrane yields an isochron age of 406 +/- 5 Ma (2s). Bedding-discordant quartz veins here contain asp having zero common Os and a model age of 407 +/- 2 Ma (2s). In the NE Meguma Terrane, asp from saddle-reefs at Dufferin yields an isochron age of 381 +/- 3 Ma (2s). One asp sample here also has zero common Os and gives a model age of 381 +/- 2 Ma (2s). The Re-Os asp dates show that at The Ovens, bedding-parallel and bedding-discordant veins are coeval. However, the ~ 407 Ma Re-Os age is significantly older than Ar-Ar mica from this locality (376 Ma). Thus, the mica either formed at a younger time relative to the veins, or potentially it is coeval with veining and has been thermally reset. At present there are no Ar-Ar ages for vein mica from Dufferin. At both Dufferin and The Ovens, structural analysis indicates veining occurred late in the deformational history, but the Re-Os asp dates indicate either a diachroneity of deformation across the Meguma Terrane, or the occurrence of a separate, younger mineralizing event (i.e., fold reactivation event) for saddle-reef ore at Dufferin. In addition, The Ovens asp data provide a minimum estimate of timing for Acadian deformation of 406 Ma. These results confirm that the Re-Os asp geochronometer is a useful tool for evaluating the timing of asp-bearing Au deposits.