0361-0128/05/3542/1229-14 $6.00 1229 Introduction ALTHOUGH the ability to accurately and precisely determine the absolute age of gold mineralization is necessary for devel- oping reliable ore deposit models, this continues to be a diffi- cult problem in many types of gold mineralizing environ- ments (e.g., Foster et al., 1998; Hofstra and Cline, 2000). The 40 Ar/ 39 Ar method of dating vein-associated hydrothermal minerals, commonly micas, is frequently used for constrain- ing the age of gold mineralization (e.g., Goldfarb et al., 1991; Haeussler et al., 1995; Foster et al., 1998; Pettke et al., 1999), but uncertain mineral paragenesis and the potential of ther- mal resetting and prolonged cooling histories are inherent is- sues, thus raising concerns about determined ages. U-Pb geochronology of hydrothermal zircon (Yeats et al., 1996), titanite (Lin and Corfu, 2002), rutile (Clarke et al., 1990), and more recently monazite and xenotime (Vielre- icher et al., 2003) also have been explored as a means of dat- ing orogenic gold deposits. Some of these approaches have shown good potential, but some specific limitations hamper their routine use. These include the paucity of required min- eral phases in many gold deposits, uncertain paragenetic re- lationship to gold mineralization, uncertainty of initial Pb compositions, excess 206 Pb in some monazites, and/or the po- tential of isotopic resetting of monazite and (possibly) xeno- time at temperatures >400ºC due to dissolution and recrys- tallization (Teufel and Heinrich, 1997). The Re-Os radioisotope system, due to the chalcophile na- ture of both elements, provides a direct method of dating sul- fide and cogenetic ore mineralization (e.g., Stein et al., 2000, 2001; Arne et al., 2001; Kirk et al., 2002; Selby et al., 2002). This method has recently been used to successfully deter- mine the age of several deposit types. Specifically, Re-Os geochronology of molybdenite (Selby et al., 2002) and pyrite (Stein et al., 2000) have been used to elucidate the timing of gold mineralizing events, and Re-Os geochronology of gold itself has been explored (Kirk et al., 2002). Re-Os geochronology of arsenopyrite offers a unique op- portunity to precisely constrain the timing of gold mineraliz- ing events, as this mineral is commonly present in vein-hosted deposits and typically has a discernable cogenetic relationship with gold in most major gold deposit types (e.g., Bierlein and Crowe, 2000; Hagemann and Cassidy, 2000). The potential of Re-Os chronometry of arsenopyrite was first recognized by Frei et al. (1998), who incorporated analyses of arsenopyrite from the Kimberly mine, Zimbabwe, into a multimineral Re- Os isochron diagram to obtain an imprecise age broadly Rhenium-Osmium Geochronology of Arsenopyrite in Meguma Group Gold Deposits, Meguma Terrane, Nova Scotia, Canada: Evidence for Multiple Gold-Mineralizing Events RYAN M. MORELLI, † ROBERT A. CREASER, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 DAVID SELBY, Department of Earth Sciences, University of Durham, Durham, United Kingdom DH1 3LE DANIEL J. KONTAK, AND RICHARD J. HORNE Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, Nova Scotia, Canada B3J 3M8 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 ar- senopyrite 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. Arsenopy- rite analyses from a bedding-discordant vein at this locality indicate an identical Re-Os age of 407 ± 4 Ma. Sad- dle-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 de- termined using the isochron method. Initial Os compositions for the two vein types at The Ovens suggest a pre- dominately 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 metamor- phism 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 40 Ar/ 39 Ar dating. † Corresponding author: e-mail, rmorelli@ualberta.ca ©2005 Society of Economic Geologists, Inc. Economic Geology, v. 100, pp. 1229–1242