Reconstruction of volcanic and hydrothermal settings and mineralization of the ABM deposit, Finlayson Lake district, Yukon, Canada

Denisová, Nikola (2023) Reconstruction of volcanic and hydrothermal settings and mineralization of the ABM deposit, Finlayson Lake district, Yukon, Canada. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The ABM deposit is a bimodal-felsic, replacement-style volcanogenic massive sulfi de (VMS) deposit located in the Finlayson Lake district, Yukon, Canada. In this dissertation, detailed core mapping, petrography, lithogeochemistry, and microanalytical methods are used to reconstruct the tectonostratigraphic framework for the deposit, the hydrothermal footprint, mineral chemical composition of the mineralization, and to integrate these to understand the genesis of the ABM deposit. The deposit is hosted by Late Devonian continental back-arc-related volcano-sedimentary rocks of the Kudz Ze Kayah formation. The distribution and character of coherent felsic and mafi c rocks suggest that the rocks were deposited in a back-arc basin proximal to a volcanic center, and the chemostratigraphy shows three sequences with distinct geochemical signatures. Reconstruction of the basin architecture has identified two sets of synvolcanic faults, and an argillite lens at the contact between the sequence hosting the mineralization and the hanging wall sequence that documents a period of volcanic inactivity during which the hydrothermal system was active. Hydrothermal alteration assemblages extend for 100s-1000s of meters laterally and into the footwall and hanging wall. The main alteration processes are feldspar destruction and formation of white mica and chlorite. The earliest and most extensive alteration assemblage is moderate white mica ± chlorite that formed at ~215±30 °C. It is overprinted by a pervasive white mica assemblage that formed at ~250±15 °C. A pervasive chlorite assemblage formed at ~320±10 °C (temperatures were determined by illite and chlorite thermometry) and overprints the white micarich assemblages. The massive sulfi de mineralization occurs as a series of stacked and stratabound lenses subparallel to the volcanic stratigraphy, overprinting the pervasive alteration assemblages. Three major mineralization assemblages occur: (1) a pyrite-sphalerite assemblage enriched in Zn- Pb-As-Sb-Ag-Au that formed at temperatures ~200-270 °C; (2) a pyrite-chalcopyrite-magnetitepyrrhotite assemblage enriched in Cu-Bi-Se-Co that formed at temperatures ~300-350 °C and occurs at the center of the mineralization lenses; and (3) a minor chalcopyrite-pyrrhotite-pyrite stringer assemblage that occurs at the margins of the lenses and formed at temperatures >300 °C. The mineralization formed by mixing of seawater infi ltrated in the subsurface with hot reduced acidic hydrothermal fl uids and consequent zone refi ning.

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