Boron- and magnesium-rich marine brines at the origin of giant unconformity-related uranium deposits: δ11B evidence from Mg-tourmalines

In giant unconformity-related U deposits from the Athabasca Basin (Canada), the origin of the ore fluids and the source of elements typically associated with U ore, such as Mg and B, are highly controversial. This paper presents a petrographic and boron isotopic study of Mg-tourmalines (dravites) coeval with U oxides and of primary tourmalines from basement rocks. The heavy B isotopic compositions of dravites (delta B-11 = 19.6 parts per thousand-36.5 parts per thousand) contrast with the light composition of basement tourmalines (delta B-11 = -8.1 parts per thousand-3.3 parts per thousand) and clearly show that most of the B was brought to the ore system by basinal brines of marine origin. Because the latter are also typically Mg-rich, a similar origin for Mg is proposed. These results support the idea that marine-derived brines were the only fluids involved in the formation of the deposits, and therefore challenge the previous models in which the B and Mg were leached from sedimentary and/or basement rocks, or carried by a basement-derived fluid.