Geological and geochemical studies of a metaturbidite-hosted lode gold deposit; the Beaver Dam Deposit, Nova Scotia; II, Isotopic studies

Abstract

The Beaver Dam gold deposit is located within metaturbidites of the lower Paleozoic Megurea Group of southern Nova Scotia, part of the Meguma terrane of the Canadian Appalachians which records a mid-Devonian metamorphic and deformational history. The deposit, one of several hosted by the Meguma Group, is dominated by bedding-concordant veins, with lesser discordant or fissure veins, that record a two-stage paragenesis: stage 1--quartz-calcic plagioclase-biotite [Mg/(Mg + Fe (super 2+) ) or Mg. no. = 0.50]-tourmaline + or - amphibole (Mg. no. = 0.54) + or - garnet + or - epidote + or - apatite + or - ilmenite; and stage 2--quartz-calcite-chlorite (Mg. no. = 0.50)-albite-scheelite. Muscovite and sulfides (Cu, Fe, Zn, Pb) transcend the two stages and gold is late parDgenetically. Mineralization formed from a low-salinity, H 2 O-CO 2 (X (sub CO 2 ) = 0.15) fluid and vein formationis constrained geochronologically at 370 Ma from 40 Ar/ 39 Ar dating of hydrothermal amphibole, muscovite, and biotite.Isotopic studies (O, C, H, Sr) of mineral phases (quartz, amphibole, feldspar, mica, carbonate, tourmaline) indicate the following:1. The veins were emplaced over a temperature interval of ca. 500 degrees to 300 degrees C based on oxygen isotope geothermometry of silicate phases. This range is comparable to temperatures inferred from silicate-sulfide geothermometry (Kontak and Smith, 1993).2. The stable isotope composition of the parental fluid is delta D = -40 + or - 6 and delta 18 O = 6 to 12 per mil based on mean values of analyzed phases and inferred temperatures of formation. The delta D water values calculated from mica phases concur with extracts from vein quartz samples dominated by a single generation of primary fluid inclusions. These results demonstrate that a delta D signature of the primary fluid can be obtained via crushing of bulk samples where the potential influence of secondary inclusions has been eliminated.3. Interaction of the ore-forming fluid with wall-rock lithologies (i.e., Meguma Group) at the site of deposition has ;reatly influenced the S and C isotope signatures of the carbonates (delta 13 C = -21.1 + or - 1.1ppm) and sulfides (delta 34 S = 10 + or - 1ppm; Kontak and Smith, 1989).4. 87 Sr/ 86 Sr measured ratios of calcite (0.70787-0.71358, n = 8) and tourmaline (0.713055) indicate modification of the primary signature of the fluid (ca. < or =<0.708) due to contamination, probably by Meguma Group lithologies.Collectively, the isotopic data indicate that the ore-forming fluids were not generated within the Meguma Group (Sr; also see Pb isotope data of Chatterjee et al., 1992), but that subsequent interaction with the Meguma Group has modified some isotopic signatures (C, S, Sr). The O and H isotope data are equivocal with regard to the Meguma Group and do not permit a definitive distinction between magmatic and metamorphic reservoirs, although the latter are preferred on the basis of geologic grounds and analogies with other similar deposit types.We conclude that the isotopic data for the Beaver Dam deposit are most consistent with a model invoking generation of fluids from metamorphic devolatilization reactions deep within the crust and structural focusing of such fluids. In contrast, a magmatic source is incompatible with the isotopic data unless such a fluid has been highly modified by fluid-rock interaction. Finally, there is no support for a meteoric component to the ore fluid.