Ar-40/Ar-39 dating of ribbon-textured veins and wall-rock material from Meguma lode gold deposits, Nova Scotia: Implications for timing and duration of vein formation in slate-belt hosted vein gold deposits

Results of 15 Ar-40/Ar-39 age spectra for whole-rock argillite samples coll ected from within and adjacent to veins from eight Meguma gold districts in the Meguma Terrane of southern Nova Scotia are presented. The samples give excellent plateau ages (i.e., concordance of plateaux, correlation, and in tegrated ages) that range from ca. 379 to ca. 403 Ma, and there is no appar ent correlation of age with bulk rock composition (K2O, Al2O3) or sampling localities. In addition, apparent ages for samples from a district are simi lar within analytical error, except for one deposit, and there is no differ ence for samples from within ribbon-textured veins versus samples collected outside of the gold districts (i.e., in areas free of quartz veining). The results of this work compare well to previous whole-rock Ar-40/Ar-39 datin g of Meguma Group samples and we concur with previous workers that the rang e in dates (i.e., 380-410 Ma) reflects diachronous cooling of the area thro ugh the intracrystalline retention temperature for argon in mica (i.e., app rox. 300-350 degrees C). However, with respect to the vein samples, there i s a marked difference between the Ar-40/Ar-39 ages of vein-hosted whole-roc k samples and hydrothermal minerals (amphibole, muscovite, biotite) from th e same deposits previously dated, which indicates that whole-rock samples h ave retained their metamorphic ages and have not been reset by the later hy drothermal event responsible for vein formation, despite being incorporated within the high-temperature fluids (approx. 400-450 degrees C). This discr epancy in ages indicates that the wall rocks and veins were in thermal dise quilibrium, as the vein temperatures were well above that required to cause diffusion of argon out of mica phases within the whole-rock samples, and i mplies therefore that the fluids must have been derived from depth. The res ults have the following important implications for models of vein formation : (i) vein formation was rapid and is consistent with models of hydrofractu ring due to fluid overpressure, and (ii) the vein-forming fluids were deriv ed from depth and cannot have been produced by a lateral secretion processe s whereby fluids and gold are derived from the Meguma Group.