Geoscience and Exploration

Geochemistry of the Lucky Lake W-Zn-Pb skarn deposit, NWT, Canada: Epithermal overprinting of magmatic hydrothermal systems

Wednesday, November 21, 2018 - 12:20 to 15:59 Lobby - Capitol Theatre


G.S. Webb
University of Missouri

K.L. Shelton
University of Missouri

J.D. Schiffbauer
University of Missouri

S. Smith
University of Missouri

H. Falck (Presenting)
NWT Geological Survey

Lucky Lake is a granite-related tungsten-zinc-lead skarn deposit located ~ 60 km southeast of Cantung, one of the largest tungsten deposits in the world. Lucky Lake is unusual due to its base-metal enrichment relative to other tungsten deposits of the region. There is also the potential for gold and silver in its ores, as can be suspected by the deposit’s location adjacent to the Hyland gold camp and the overlap of the Cretaceous tungsten belt and the Tintina gold-silver-bismuth province, which stretches across Alaska into northwestern Canada.

The goal of this research is to document the relative timing and pressure-temperature-geochemical conditions of tungsten and base-metal deposition. This study employs a variety of geochemical techniques to decipher the nature of the ores, including: petrographic, cathodoluminescent and ore microscopy of thin and polished sections; SEM analysis of ore and gangue mineralogy; and fluid inclusion microthermometry of ore-related minerals. Detailed SEM analysis of the ore minerals document the trace and minor element character of the Zn-Pb event and will permit us to test the idea of involvement of gold-bearing fluids in the hydrothermal system.

Reflected light and cathodoluminescence (CL) microscopy indicate that pyrite (FeS2) and scheelite (CaWO4) were deposited first, followed by pyrrhotite (Fe1-xS) ± chalcopyrite (CuFeS2), and ultimately by sphalerite (ZnS) ± galena (PbS). Pyrite grain boundaries have ~ 120° interfacial angles characteristic of annealing during metamorphism driven by the adjacent granite. The early ore minerals (pyrite, scheelite, chalcopyrite and pyrrhotite) are associated with iron-rich carbonate gangue that exhibits a dull red-brown color in CL. In contrast, later sulfides (sphalerite and galena) are associated with bright yellow-orange CL, open-space-filling calcite cements, suggesting that Pb-Zn mineralization represents a distinct hydrothermal event from that which deposited tungsten and copper mineralization.

The tungsten ± copper mineralization was deposited from magmatic-related fluids similar to those at Cantung (~ 430 to 590°C). Fluid inclusions in later quartz (Th = 337 to 185°C) and sphalerite (Th = 153 to 155°C) suggest that zinc mineralization (followed closely by lead sulfide deposition) was the result of a later event(s) at lower temperatures, from fluids with salinities between 0.6 and 7.4 wt. % equivalent NaCl.

The Lucky Lake deposit’s zinc and lead enrichments appear to be related to a later hydrothermal overprint not described in the world-class Cantung mine’s ores. Recognition of this late-stage epithermal event may have implications for understanding the emplacement conditions of the tungsten skarns and regional exploration for zinc-lead ores elsewhere in the Mackenzie Mountains.