The Gays River Zn-Pb deposit of southern Nova Scotia, Canada, represents an example of Mississippi Valley-type mineralization (reserves ca. 2.4 million metric tons 8.6% Zn and 6.3% Pb). The deposit is hosted by Visean-age, dolomitized carbonate rocks (bank and interbank facies) that form part of a series of carbonate banks (i.e., the Gays River Formation). The banks developed on paleotopographic highs underlain by lower Paleozoic metaturbidites of the Meguma Group, but locally there is an intervening basal breccia unit which contains fragments (centimeter to meter scale) of Meguma Group lithologies within a mineralized (Pb > or = Zn) dolostone matrix. Petrographic examination of fragments in the breccia unit indicate traces of hydrothermal alteration which is considered to have been coincident with mineralization at Gays River. Maximum temperatures associated with the alteration ranged from ca. 300 degrees (fluid inclusions) to ca. 350 degrees C (chlorite geothermometry). Five 40 Ar/ 39 Ar step-heating experiments of metasedimentary clasts in the basal breccia give nearly identical age spectra profiles, with low-temperature gas fractions indicating apparent ages of ca. 300 Ma followed by plateau ages of ca. 380 to 400 Ma for high-temperature gas fractions. Whereas the latter ages are consistent with previously determined whole-rock 40 Ar/ 39 Ar ages for regional deformation and metamorphism of the Meguma Group, the ca. 300 Ma age is considered to reflect thermal overprinting and is consistent with recent palcomagnetic determinations suggestive of a mid-Mississippian to late Pennsylvanian age for mineralization. Model calculations of the age spectra, assuming degassing of micaceous phases and volume diffusion, suggest that the most plausible interpretation is a reheating event of ca. 250 degrees to 300 degrees C of ca. 1- to 6-m.y. duration. The anomalously elevated temperatures determined from this study, in conjunction with the presence in Maritime Canada of Late Devonian-Carboniferous mafic-felsic magmatism and the widespread resetting of radiometric systems in the Meguma terrane at ca. 300 Ma, suggest that the Gays River mineralization may be related to the same broad-scale tectonic processes that formed the Martimes basin and involved regional-scale structures and probably magma generation.