Oxidation State of Iron in Alteration Minerals Associated with Sandstone-Hosted Unconformity-Related Uranium Deposits and Apparently Barren Alteration Systems in the Athabasca Basin, Canada - Implications for Exploration - 2013

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Ng and five co-authors reported in 2013 that sandstone-hosted unconformity-related U deposits in the Athabasca Basin at McArthur River Zone 4 and Maurice Bay as well as barren alteration systems at Wheeler River Zone “K” and Spring Point are surrounded by an outer, peripheral zone of illite and an inner, central zone of chlorite alteration.

In the figure above is the longitudinal cross-sections of McArthur River Zone 4 showing the spatial distribution and Fe³⁺/ΣFe ratios of alteration minerals. Drill hole MAC-255 is projected onto section B. Abbreviation for pre-ore minerals: C1 = Mg-rich sudoite, T1 = magnesiofoitite, K1 = dickite (dkt), kaolinite (kln), I1 = illite, I1bsmt = illite in basement, C1bsmt = clinochlore in basement, C2 = Mg–Fe-rich sudoite, C2bsmt = Mg-rich sudoite in basement, ICML = illite–chlorite mixed layer clay; syn-ore minerals: C3 = Fe–Mg-rich sudoite; post-ore minerals: K2 = post-ore kaolinite.

The oxidation state of Fe determined by ⁵⁷Fe Mössbauer spectroscopy of illite, sudoite, and clinochlore from these systems indicates that pre-ore illite contains mostly Fe³⁺ (Fe³⁺/ΣFe = 0.77–1.00), reflecting formation from a predominantly oxidizing basinal fluid. Lower Fe³⁺/ΣFe ratios are recorded by late pre-ore sudoite and clinochlore occurring near faulted unconformities, consistent with the contribution to the chlorites from reducing basement-derived
fluids.

Sudoite and clinochlore in the alteration halo of barren systems are more reduced (Fe³⁺/ΣFe median = 0.3–0.4) than sudoite from mineralized systems (Fe³⁺/ΣFe median = 0.45–0.55). Sudoite from both systems becomes broadly more oxidized with increasing distance from the fault zone. Variations
in the Fe³⁺/ΣFe ratios of sudoite and clinochlore support a mechanism in which Fe²⁺ transported by reducing basement-derived fluids is partially oxidized to Fe³⁺ by basinal fluids prior to incorporation into the chlorite.

Thus, scarcity of oxidizing U-bearing basinal fluids available for interaction with reducing basement fluids is a critical factor precluding U mineralization in barren systems. Trends of decreasing Fe³⁺/ΣFe ratios in sudoite coupled with decreasing ²⁰⁷Pb/²⁰⁶Pb ratios of leachable Pb can be used as geochemical vectors to the sites of potential U mineralization.