Late Hercynian U-vein mineralization in the Alps: fluid inclusion and C, O, H isotopic evidence for mixing between two externally derived fluids

Abstract

Late Hercynian U-bearing carbonate veins within the metamorphic complex of La Lauzière are characterized by two parageneses. The first is dominated by dolomite or ankerite and the second by calcite and pitchblende. Fluids trapped in the dolomites and ankerites at 350–400° C are saline waters (20 to 15 wt % eq. NaCl) with δD∼ −34 to −49‰. In the calcite they are less saline (17 to 8 wt % eq. NaCl) and trapped at 300–350° C with δD∼ −50 to −65‰. All fluids contain trace N₂, CO₂ and probably CH₄. The carbonates have Δ ¹³C∼ −8 to −14‰. and derived their carbon from organic matter. Evolution of the physico-chemical conditions from dolomite (ankerite) to calcite deposition was progressive.

H and O-isotope studies indicate the involvement of two externally derived fluids during vein development. A D-rich (∼ −35‰) low fO₂, saline fluid is interpreted to have come from underlying sediments and entered the hotter overlying metamorphic slab and mixed with more oxidizing and less saline U bearing meteoric waters during regional uplift. This evidence for a sedimentary formation water source for the deep fluid implies that the metamorphic complex overthrusted sedimentary formations during the Late-Hercynian.