Gold and base metal mineralization in the Nippers Harbour ophiolite, Newfoundland

Hudson, Karen A. (1988) Gold and base metal mineralization in the Nippers Harbour ophiolite, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Nippers Harbour Ophiolite is a southwesterly extension of the lower Ordovician Betts Cove Ophiolite. Extensive sheeted dyke and gabbro members, as well as minor ultramafic components, characterize the Nippers Harbour Ophiolite. The ophiolite is unconformably overlain by the Silurian Cape St. John Group, which consists of subaerial conglomerates, cross-bedded sandstones, basic pyroclastics and andesitic to rhyolitic ash-flow tuffs. The ophiolite also is intruded by the Late Silurian to Early Devonian Cape Brule quartz-feldspar Porphyry. The mafic members of the Nippers Harbour Ophiolite show similarities to Betts Cove boninite-type lavas in that they contain unusually low TiO₂ and high SiO₂, MgO, Cr and Ni contents. -- Mineralization is located commonly in shear or fault zones in mafic ophiolitic rocks. The Hill showing has characteristics resembling those of massive sulphide stockwork zones, with pyrite-chalcopyrite-quartz-chlorite breccia and sheared rock, surrounded by fragments of hydrothermally altered quartz-chlorite-albite rock. Relatively unaltered diabase dykes intrude these assemblages. The altered rocks have been enriched in FeOt, Cu and Zn, and depleted in Na₂O, CaO, Sr and LREE. Mineralization is believed to have been formed by the mixing of upwelling, hot, Fe-, Cu-, Zn-enriched seawater-derived hydrothermal fluids with cold seawater at the diabase-pillow basalt interface. -- Anomalous gold contents have been documented at Burtons Pond, Gull Pond and Showing Number 2. The gold is spatially related to sulphides, mainly chalcopyrite and arsenopyrite, which have precipitated with quartz-calcite, quartz-albite and quartz in veins in altered host rocks. Three alteration assemblages are recognized: (1) quartz-chlorite+/-albite, (2) quartz-chlorite-sericite and (3) quartz-sericite-calcite. Only the Burtons Pond showing displays all three assemblages, whilst the others are associated with assemblage (2). The alteration is expressed chemically by the addition of FeOt, S, K₂O, CO₂, Au, Ag, As, Ba, Co, Cu, Se and Zn, and variable depletions of CaO, Sr and Na₂O. Fluid inclusion and sulphur isotope data (ranging from 4.5 to 6.7 per mil) suggest that the fluids were seawater-derived and had the following characteristics: temperature ~ 250°C, pH 5, total reduced sulphur < 10⁻³M, aO₂ < 10⁻⁴² and aS₂ < 10⁻¹³.⁵. Gold probably was carried predominantly as a thio-complex, but it also may have been a chloride- or thio-arsenide complex. Mineralization is believed to have formed by movement of seawater-derived fluids along shallow thrust planes, depositing sulphides and gold with associated chlorite and sericite, in splays. -- Other sulphide showings include Pb-Zn and Cu-bearing quartz veins at Welshs Bight and Rogues Harbour respectively. Field evidence and sulphur and lead isotope data (-0.4 to 2.9 per mil, and ²⁰⁶Pb/²⁰⁴Pb = 17.659, ²⁰⁷Pb/²⁰⁴Pb = 15.464, ²⁰⁸Pb/²⁰⁴Pb = 37.562 respectively) suggest that these showings are related to the intrusion of the Cape Brule Porphyry.

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