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Abstract:

Rhenium-osmium geochronology using arsenopyrite was undertaken for three gold deposits in the Meguma terrane, Nova Scotia, Canada, in order to better constrain their age of formation and to assess the utility of arsenopyrite for dating similar deposits globally. Analyses of arsenopyrite from bedding-concordant veins from The Ovens locality, southwest Meguma terrane, yield a precise Re-Os isochron age of 409 {+/-} 5 Ma. Arsenopyrite analyses from a bedding-discordant vein at this locality indicate an identical Re-Os age of 407 {+/-} 4 Ma. Saddle-reef veins from the Dufferin deposit, northeast Meguma terrane, contain arsenopyrite with a precise Re-Os isochron age of 380 {+/-} 3 Ma. The lack of common Os in some arsenopyrite samples from both The Ovens and Dufferin permit calculation of single mineral model ages for each deposit, which are identical to those determined using the isochron method. Initial Os compositions for the two vein types at The Ovens suggest a predominately crustal source of Os in the mineralizing fluids, whereas a less radiogenic initial Os composition for arsenopyrite from Dufferin does not as clearly define a crustal metal source. At a third locality, the Touquoy deposit, the Re-Os systematics of arsenopyrite associated with disseminated gold mineralization do not define a precise formation age, possibly as a result of mixing of Re and Os derived from the mineralizing fluid and the shale host rock. The Re-Os ages of arsenopyrite indicate that there were at least two distinct periods of gold deposition in the Meguma terrane coinciding with widespread tectonothermal events: regional deformation and metamorphism associated with Acadian orogenesis, and widespread generation of meta- and peraluminous granites and high-grade metamorphism within the basement rocks under the Meguma terrane. The ca. 407 Ma age for The Ovens arsenopyrite provides the best estimate for the timing of regional Acadian deformation in the Meguma terrane and is slightly older than previous estimates based solely on 40Ar/39Ar dating.