Origin of boron-rich fluids in the formation of unconformity associated deposits of the Sibley Basin

Abstract

Hydrothermal magnesio-foitite or dravite (tourmaline) is common within the ore zones of unconformity associated uranium deposits globally. Magnesium-rich tourmaline ranging in composition from dravite to magnesio-foitite is identified in two uranium showings within the Sibley Basin. Hydrothermal dravite from the Malborne Lake showing (delta-11B = -22.5 permille to -20.2 permille) is significantly isotopically lighter than dravite and magnesio-foitite analyzed from the Innis Lake showing (delta-11B = 16.5-20.0 permille). The isotopically light delta-11B values of tourmaline from Malborne Lake is consistent with local metamorphic tourmaline, indicating that boron is primarily basement-derived. This contrasts with the isotopically heavy compositions of hydrothermal tourmaline from the Innis Lake showing that are comparable to values reported from the Athabasca Basin. A positive delta-11B value indicates that boron was sourced from, or potentially equilibrated with, the overlying marine evaporites, such as the chemical sediments of the Sibley Basin. The delta-11B values of hydrothermal tourmaline from the two occurrences are markedly different and suggest that vein-hosted mineralization in the basement rocks of the Sibley Basin did not form via a unique mechanism.