Lithogeochemistry and hydrothermal alteration of the Pine Cove orogenic gold deposit, Baie Verte Peninsula, Newfoundland, Canada

Ybarra, Samuel (2020) Lithogeochemistry and hydrothermal alteration of the Pine Cove orogenic gold deposit, Baie Verte Peninsula, Newfoundland, Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Pine Cove gold mine is hosted within Ordovician gabbro sills and volcano-sedimentary rocks of the Venam’s Bight and Bobby Cove formations, Snooks Arm Group, Baie Verte Peninsula, Newfoundland Appalachians, Canada. The deposit is located approximately 50 to 200 m above the Scrape thrust fault, a brittle-ductile structure associated with Silurian transpression. Gold mineralization occurs as microscopic native Au inclusions in pyrite disseminated in the selvages of quartz-calcite-chlorite±albite breccia-veins. Multiple generations of overlapping veins are enveloped by sericite-rutile-calcite-pyrite alteration selvages. Mineralized quartz breccia veins are most abundant at the contacts of gabbro sills and volcano-sedimentary strata. Ore-grade mineralization predominantly occurs within Fe-rich gabbros and basalts of the Venam’s Bight Formation. These relationships suggest that there were two fundamental controls on mineralization: 1) hydrofracturing assisted by anisotropic deformation of rigid gabbro sills within more ductile country rocks; and 2) sulfidation of the Fe-rich gabbros and basalts. Comparatively, the volcaniclastic rocks of the Bobby Cove Formation lack abundant gabbro intrusions and are relatively Fe-poor, and were thus a less receptive host rock to gold mineralization at Pine Cove. Three distinct alteration assemblages are identified at Pine Cove: proximal sericite-rutile-chlorite-calcite, intermediate carbonate-chlorite, and distal epidote-titanite-chlorite-calcite. Mass balance calculations for altered wall rock show enrichments in CO₂, K₂O, S, Rb, W, In, Pb, Bi, Te, Se, Cs, and Ba, and depletions in As, Sb, and Na₂O. REE, HFSE, Al₂O₃, and most transition metals were generally immobile during alteration. There are distal to proximal elemental variations in relation to gold mineralization, and the generalized sequence of geochemical variation includes (from distal to proximal - 50m to 1m): CO₂ enrichment + As-Sb depletion Æ K-Rb enrichment Æ Na depletion Æ W±Pb±Ba enrichment Æ Au. The alkali-carbonate index K₂O + CO₂ / K₂O + CO₂ + Na₂O + Al₂O₃ coupled with As-Sb depletion is considered the most consistent metric of ore-related alteration at Pine Cove. Combined short wave infrared (SWIR) spectrometry and electron microprobe analysis (EMPA) of white micas show that Tschermak substitution (AlVI + AlIV l [Fe²⁺,Mg]VI + SiIV) and K/Na content correlate with AlOH absorption wavelengths. White mica compositions progress from phengite Æ paragonitic muscovite Æ muscovite approaching mineralization. A new potential SWIR index for orogenic gold exploration in greenschist facies rocks is the quantity Fe²⁺ slope/MgOH-depth extracted from reflectance spectra of altered rock. At Pine Cove, this index measures the relative proportion of (Fe-carbonate + sericite)/chlorite, and correlates with the alkali-carbonate alteration index K₂O + CO₂/K₂O + CO₂ + Na₂O + Al₂O₃ and Au mineralization. Although the physico-chemical conditions during mineralization in orogenic gold deposits may vary from deposit to deposit, the collective visual, geochemical, and SWIR indices identified at Pine Cove represent potential vectoring tools for the discovery of similar deposits in orogenic Au camps globally.

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