Geology, U-Pb geochronology and stable isotope geochemistry of the Hammer Down Gold Prospect, Green Bay District, Newfoundland

Ritcey, David Henry (1993) Geology, U-Pb geochronology and stable isotope geochemistry of the Hammer Down Gold Prospect, Green Bay District, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Hammer Down gold prospect is an important example of the numerous mesothermal gold occurrences associated with structural zones of regional extent in the Newfoundland Dunnage Zone. -- The main mineralized zones in the Hammer Down prospect consist of structurally controlled quartz veins with abundant pyrite, chalcopyrite, and sphalerite, hosted by volcanic and sedimentary rocks of the Ordovician (Arenig) Catchers Pond Group, and by Early Silurian felsic porphyry dykes. Gold-bearing veins postdate sub- greenschist metamorphism and an early phase of deformation, and were produced synchronously with a locally intense S₂ fabric related to folding. Veins were preferentially emplaced in narrow brittle-ductile high strain zones within and adjacent to felsic dykes. The gold-bearing veins and Sj fabric are locally folded, and mineralized zones are truncated by a late brittle fault. Barren, paragenetically late veins, characterized by fluorite and low contents of sulphide minerals, are abundant in and near the gold-bearing zones. -- Gold is present within pyrite and along sulphide grain boundaries in veins and in locally sericitized, carbonatized, and pyritized wall rock. Hydrothermal alteration adjacent to veins is restricted to narrow zones, or is absent. Alteration of mafic rocks is characterized by muscovite, calcite, pyrite, quartz, and rutile, and is marked by enrichment in the lithophile elements K and Rb, and by sporadically elevated levels of Cu, Zn, and S. -- Felsic tuff from the Hammer Down prospect has an igneous U-Pb zircon age of 480 + 4/-3 Ma, and records arc volcanism in the Dunnage Zone. Felsic dykes from the prospect, which have a strong spatial association with gold, are petrographically and chemically distinct from the nearby King's Point volcanic-plutonic complex, but they are correlative with the Burlington Granodiorite, a regional plutonic unit of the eastern Baie Verte Peninsula. Zircon from a felsic dyke cut by a gold-bearing vein has a high degree of Proterozoic inheritance. Acicular zircon grains from this dyke have a ²⁰⁷Pb/²⁰⁶Pb age of 437 +/-3 Ma, which is considered the age of intrusion, and is a maximum age limit for gold deposition. -- Isotopic compositions of quartz (δ¹⁸O = 11.4±0.7) and calcite and ferroan dolomite from gold-bearing veins (δ¹³C = -6.2±0.7) are typical of mesothermal vein systems elsewhere. Oxygen isotope thermometry using quartz, chlorite, and muscovite indicates mineralization temperatures between 240 and 320°C. The ore- forming solutions had calculated δ¹⁸O values between 2.2 and 5.2 %o and δD values between -27 and -42 0/00, which are compatible with a predominantly metamorphic source having some meteoric contribution. Age constraints and regional considerations place the gold mineralizing event wthin a period of crustal extension and widespread magmatism that were part of an important Silurian orogenic phase in central Newfoundland. A meteoric component in the vein fluid system is atuibuted to the extensional setting, which promoted influx of surface water to deeper reservoirs.

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