Evolution of the Hydrothermal System Associated with the ABM Replacement-Style Volcanogenic Massive Sulfide Deposit, Finlayson Lake District, Yukon, Canada

Abstract

The ABM deposit, Finlayson Lake district, Yukon, Canada, is a bimodal-felsic, replacement-style volcanogenic massive sulfide (VMS) deposit (19.1 Mt @ 6.6 wt % Zn, 0.9 wt % Cu, 2.0 wt % Pb, 1.4 g/t Au, and 148 g/t Ag) hosted by Late Devonian continental back-arc–related volcanosedimentary rocks of the Kudz Ze Kayah formation. The VMS-related hydrothermal alteration associated with the deposit extends >1 km beyond the mineralization. Zones of pervasive sericite and chlorite alteration occur proximal to the massive sulfide lenses (<50 m) both in the hanging wall and the footwall, and zones of pervasive sericite and moderate sericite ± chlorite alteration extend laterally from the mineralization and into the hanging wall and footwall for hundreds to thousands of meters. Geochemical data and petrographic observations indicate that feldspar destruction and formation of white mica and chlorite were the main alteration processes. In both the hanging wall and footwall to the mineralization, base (e.g., Zn, Cu, Pb) and trace metals (e.g., Sb, Tl, Mo) form halos with elevated values up to 100 and 200 m, respectively. The paragenesis and the formation conditions of the hydrothermal alteration were determined through petrography of hydrothermal alteration assemblages and their crosscutting relationships, electron microprobe analyses of the compositions of white mica, chlorite, and carbonate, and illite-chlorite geothermometry. These data suggest that the sericite ± chlorite assemblage was the earliest and most extensive phase of the hydrothermal alteration that formed at temperatures around 215° ± 30°C. Overprinting the sericite-chlorite assemblage is the pervasive sericite assemblage that formed at temperatures around 250° ± 15°C. The pervasive chlorite assemblage formed at temperatures around 320° ± 10°C and overprints the preceding sericite-rich assemblages. Microprobe analyses of white mica and chlorite generally show that Mg-rich varieties are more common proximal to mineralization and formed earlier in the deposit paragenesis than Fe-rich varieties. Mineralogy derived from short-wave infrared data for mica and chlorite, however, shows no clear spatial trends across the deposit due to complex overprinting relationships between alteration minerals.

Mineralogy and mineral chemistry of the ABM replacement-style volcanogenic massive sulfide deposit, Finlayson Lake district, Yukon, Canada

“…On the micro-scale, however, textures reflective of metamorphism and deformation are more pronounced and have features consistent with greenschist facies metamorphism and arsenopyrite geothermometry outlined above (Murphy et al 2006 ; Denisová and Piercey 2023 ). For example, pyrite commonly has fractures infilled by chalcopyrite, galena, or sphalerite (Fig.…”

“…The average pyrrhotite composition (59.80 ± 0.29 wt. % Fe, n = 77; ESM 2 ) matches that of monoclinic pyrrhotite (Kissin and Scott 1982 ), and while monoclinic pyrrhotite can form by direct precipitation from hydrothermal fluids, it commonly occurs at temperatures < 258 °C (Lianxing and Vokes 1996 ), which is at the lower end of the temperature range suggested by the chlorite thermometer (Denisová and Piercey 2023 ) and other sulfides common in assemblages 2 and 3. Locally, pyrrhotite displays annealed textures (Fig.…”

“…5 ). Formation temperatures calculated for chlorite in both mineralized and barren pervasive chlorite assemblages are ~ 275–345 °C (Denisová and Piercey 2023 ). Pyrrhotite and chalcopyrite commonly occur in this assemblage, and minor sphalerite has relatively high Fe content (~ 9–13 wt.…”

“…The grains likely precipitated before the hydrothermal fluid became more reduced and/or rich in H 2 S, possibly during the formation of early hydrothermal alteration assemblages. This was followed by continuous and extensive infiltration of hydrothermal fluids that were moderate temperature (200–270 °C), rich in H 2 S, acidic and reduced, into the subsurface along porous and permeable units, where the hydrothermal fluids interacted with infiltrated seawater (Denisová and Piercey 2022 , 2023 ) and precipitated assemblage 1. As the system heated up (> 270 °C), higher-temperature hydrothermal fluids ascended along the synvolcanic faults, permeated assemblage 1, and, through zone refining processes, dissolved existing Zn- and Pb-rich phases and precipitated chalcopyrite in their stead, leading to the deposition of assemblage 2 in the centers of the massive sulfide lenses, and with the Zn- and Pb-rich fluids derived from dissolution being reprecipitated on the margins of the existing sulfide lenses.…”

“…The rocks in the ABM deposit footprint are hydrothermally altered (~ 1 km radius around the deposit), particularly the felsic lithofacies (Denisová and Piercey 2023 ). Proximal to mineralization, high-temperature pervasive chlorite assemblages (~ 315 °C) overprint lower-temperature pervasive sericite assemblages (~ 250 °C) or moderate sericite ± chlorite assemblages (~ 215 °C; Denisová and Piercey 2023 ). Mineralization overprints all types of pervasive alteration, although locally massive sulfides are contemporaneous with pervasive chlorite alteration.…”