Geology Seminar Series - Yumeng Wang - Geochemical modeling of unconformity-related uranium mineralization in the Athabasca Basin, Saskatchewan, Canada

Wed., Mar. 2, 2022 4:00 p.m. - Wed., Mar. 2, 2022 5:00 p.m.

Location: https://uregina-ca.zoom.us/j/95666431738?pwd=NXBjL081ZHYzMk5CSUx1OUJxcW1oZz09

Presenter: Yumeng Wang (University of Regina, PhD Candidate)

Topic: Geochemical modeling of unconformity-related uranium mineralization in the Athabasca Basin, Saskatchewan, Canada

Date & Time: Wednesday March 2nd, 2022 at 4:00 pm CST

Zoom link: https://uregina-ca.zoom.us/j/95666431738?pwd=NXBjL081ZHYzMk5CSUx1OUJxcW1oZz09

Abstract: The unconformity-related uranium deposits in the Athabasca Basin can be broadly divided into two sub-types: sandstone-hosted, typically polymetallic U-Ni-Co deposits, and basement-hosted, typically monometallic U deposits. It has been widely accepted that contrasting physiochemical processes associated with “egress”- and “ingress”-style of fluid flow under “diagenetic-hydrothermal” conditions are responsible for their distinct mineralogical composition and ore location. However, the detailed fluid-fluid and fluid-rock interactions and physiochemical conditions (pH, fO₂, temperature, fluid composition) governing the metal assemblages remain unclear. Thermodynamic calculation of U-Ni-Co mineral stability with updated thermodynamic data compiled from recent publications indicates that Ni-Co minerals, including nickeline (NiAs), cobaltite (CoAsS), gersdorffite (NiAsS), rammelsbergite (NiAs₂), skutterudite (CoAs₃), safflorite (CoAs₂), millerite (NiS) and NiS₂ (vaesite), tend to precipitate at a much more reducing (lower fO₂) condition than uraninite (UO₂). A series of geochemical reaction path modeling were carried out on binary or ternary mixing between three distinct fluids: (1) an oxidized, U-rich, NaCl-dominated brine, (2) a moderately reducing, Ni-Co-As-Fe²⁺-rich, CaCl₂-dominated brine and (3) a highly reducing, metal-poor, CH₄-Fe²⁺-rich, CaCl₂-dominated brine. The mixing results show that uraninite and hematite precipitate together without any Ni-Co minerals when the fluid-1 mixes with fluid-2; Ni-Co minerals precipitate when the fluid-2 mixes with fluid-3; Uraninite precipitates with hematite and then the produced hematite is dissolved when the fluid-1 mixes with fluid 3; Ternary mixing of the fluid-1, fluid-2 and fluid-3 results in the precipitation of a U-Ni-Co metal assemblage. The results indicate that the CH4 is an essential in the formation of polymetallic U-Ni-Co deposits, whereas simple uraninite precipitation does not necessarily require the involvement of CH4. This is consistent with the understanding that the CH4 is a stronger reducing agent than Fe²⁺, and has the capacity to reduce arsenate and sulfate as manifested by the calculated pH-fO₂ diagrams. Based on these new findings on geochemical reaction paths and previous studies on fluid flow, it is proposed that episodic fault-valve behaviors of reverse basement faults control the permeabilities of hydrofracture networks in the basement, periodically releasing Ni-Co-As-enriched brines and CH₄-bearing metal-poor brines, and ternary mixing with oxidized uraniferous brines at fault-unconformity intersection results in polymetallic U-Ni-Co mineralization. In contrast, the monometallic U mineralization in basement is interpreted to be controlled by episodic faulting suction-pump behavior, whereby the oxidized uraniferous brines flow downward along the reactivated faults and react with a limited amounts of reducing basement fluid and silicate minerals of the basement, e.g., biotite, K-feldspar, anorthite, which change the oxygen fugacity and pH of mineralizing fluid and lead to the precipitation of uraninite.