Ambient Redox and Hydrothermal Environment of the Wolverine Volcanogenic Massive Sulfide Deposit, Yukon: Insights from Lithofacies and Lithogeochemistry of Mississippian Host Shales

Abstract

The Wolverine volcanogenic massive sulfide (VMS) deposit is a polymetallic, felsic-siliciclastic deposit hosted by ~352 to 347 Ma volcanic and sedimentary rocks of the Yukon-Tanana terrane, Yukon, Canada. Shales are located at various stratigraphic levels and in various mineralized zones within the Wolverine deposit area (e.g., Fisher, Puck, and Sable zones). Shales present along the mineralized horizon near the immediate hanging wall and footwall were deposited under anoxic conditions (e.g., low Mn, anoxic V-Cr-Mo-U systematics), whereas in the stratigraphically deeper footwall and uppermost hanging wall, the redox signatures imply deposition under suboxic to oxic conditions. The Mo-U systematics of the shales suggest that during the time of sulfide mineralization, the ambient basin was periodically euxinic with aqueous H2S present in the water column that this H2S contributed to the sulfur budget of the deposit. Furthermore, the Mo-U and Corg-Ni systematics favor deposition in a restricted basin (i.e., nutrient trap) where restriction of the water column led to H2S formation via sulfate reduction associated with excess organic carbon preservation. There is also evidence of a progressive shift upward in the stratigraphy to more oxygenated conditions in the uppermost hanging wall. The shift from euxinic to oxic-suboxic conditions is consistent with regional tectonic models that indicate a change from rifting during Wolverine deposit formation, where the basin was partially restricted with minimal circulation (i.e., restricted depocenter), to an incipient back-arc basin accompanied by extension and likely ingress of oxygenated seawater. The rare earth element and Y (REY) systematics in Wolverine shales illustrate that proximal to the mineralized horizon, shales have higher Y/Ho (>27), and Ce/Ce*<<1 and negative Ce anomalies indicative of oxygenated seawater. The Ce/Ce* values in the Wolverine shales have an inverse correlation with P2O5 content and suggest partial control by detrital apatite. It is envisioned that apatite formed in the upper, oxygenated portion of the water column, inherited the REE signature of oxygenated seawater (i.e., Ce/Ce*<<1), and was subsequently deposited into deeper waters as detrital grains. The Ce/Ce* and Y/Ho also correlate with CO2 and carbonate content of the shales. Moreover, the shales that have the strongest REY signature of oxygenated seawater also coincide with the strongest euxinic signatures. This paradox can be reconciled by enhanced deposition of apatite coincident with deposit formation, coupled with a late hydrothermal overprint on the shales from low-temperature, CO2-rich (oxygenated?) hydrothermal fluids (i.e., high Y/Ho and Ce/Ce*<<1) in a vent-proximal environment. This model is consistent with the geology, stratigraphy, and hydrothermal alteration in the immediate footwall and hanging wall of the deposit. Prospective shales in the Wolverine basin, and similar sediment-rich hydrothermal basins globally, should exhibit evidence for deposition under anoxic conditions (i.e., Mn <1,000 ppm; high V-Mo-U, UEF, MoEF), hydrothermal Fe-Al-Mn systematics (i.e., high Fe/Al), and evidence for hydrothermal alteration (i.e., high CIW values, high molar sericite and chlorite). Identification of samples having these features is useful in targeting prospective VMS environments in shale-dominated successions and potential targets within shale-rich basins.