AR-40-AR-39 DATING OF FLUID MIGRATION IN A MISSISSIPPI VALLEY-TYPE DEPOSIT - THE GAYS RIVER ZN-PB DEPOSIT, NOVA-SCOTIA, CANADA

The Gays River Zn-Pb deposit of southern Nova Scotia, Canada, represen ts 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) t hat form part of a series of carbonate banks (i.e., the Gays River For mation). The banks developed on paleotopographic highs underlain by lo wer Paleozoic metaturbidites of the Meguma Group, but locally there is an intervening basal breccia unit which contains fragments (centimete r to meter scale) of Meguma Group lithologies within a mineralized (Pb greater-than-or-equal-to Zn) dolostone matrix. Petrographic examinati on of fragments in the breccia unit indicate traces of hydrothermal al teration which is considered to have been coincident with mineralizati on at Gays River. Maximum temperatures associated with the alteration ranged from ca. 300-degrees (fluid inclusions) to ca. 350-degrees-C (c hlorite geothermometry). Five Ar-40/Ar-39 step-heating experiments of metasedimentary clasts in the basal breccia give nearly identical age spectra profiles, with low-temperature gas fractions indicating appare nt ages of ca. 300 Ma followed by plateau ages of ca. 380 to 400 Ma fo r high-temperature gas fractions. Whereas the latter ages are consiste nt with previously determined whole-rock Ar-40/Ar-39 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 r ecent paleomagnetic determinations suggestive of a mid-Mississippian t o late Pennsylvanian age for mineralization. Model calculations of the age spectra, assuming degassing of micaceous phases and volume diffus ion, suggest that the most plausible interpretation is a reheating eve nt 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 c onjunction with the presence in Maritime Canada of Late Devonian-Carbo niferous mafic-felsic magmatism and the widespread resetting of radiom etric systems in the Meguma terrane at ca. 300 Ma, suggest that the Ga ys River mineralization may be related to the same broad-scale tectoni c processes that formed the Martimes basin and involved regional-scale structures and probably magma generation.