Gold mineralization in the Paleozoic Appalachian Orogen; constraints from geologic, U/Pb, and stable isotope studies of the Hammer Down Prospect, Newfoundland

Memorial University, Department of Earth Sciences, St. John's, NF, Canada

The Hammer Down gold prospect occurs in an accretionary tectonic setting in the Paleozoic Appalachian orogen. It has many geologic and geochemical features in common with other shear-hosted, mesothermal gold deposits. The gold prospect is located about 25 km to the east of an exposed continental-oceanic suture zone and is near a major fault (the Green Bay fault) of regional extent. The main mineralized zones consist of structurally controlled quartz veins with abundant pyrite, chalcopyrite, sphalerite, and carbonate, hosted by volcanic and sedimentary rocks of the Ordovician Catchers Pond Group. Gold-bearing veins postdate a regional subgreenschist metamorphism and an early phase of deformation. Mineralization occurred synchronously with a locally intense S ₂ fabric within narrow brittle-ductile high strain zones in, and adjacent to, Early Silurian felsic porphyry dikes. Carbonate and chlorite alteration is widespread and, in part, predates the mineralization; however, there is increasing chloritization near the gold-bearing zones and the mineralized veins contain abundant carbonate. Strong sericite-pyrite-rutile hydrothermal alteration is restricted to narrow zones adjacent to veins. Isotopic compositions of quartz (delta ¹⁸ O =11.4 + or - 0.7%) and carbonate (delta ¹³ C =-6.2 + or - 0.7%) from gold-bearing veins at Hammer Down are typical of shear-hosted, mesothermal gold deposits of Archean and younger age. Oxygen isotope thermometry using quartz, chlorite, and muscovite yields temperatures of vein formation between 245 degrees and 320 degrees C. The ore-forming fluids have calculated delta ¹⁸ O and delta D values between 1.1 and 5.8 and -36 and -54 per mil, respectively, and are interpreted to be of mixed origin, dominantly from a metamorphic reservoir with a small component of low ¹⁸ O meteoric water. In the Hammer Down area, felsic tuff in the host arc-related sequence, the Catchers Pond Group, has an igneous U-Pb zircon age of 480 (super +4) (sub -3) Ma. This allochthonous sequence is intruded by felsic dikes which are correlative with granodiorite of the Burlington Intrusive Suite, a major regional unit of the eastern Baie Verte...

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