U-Th-Pb dating of hydrothermal ore deposits from electron probe micro-analysis of uraninite: the cases of Cogne, Val Vedello and Novazza (Italian Alps)

Uraninite (UO2) is an uncommon accessory mineral in some magmatic, metamorphic and sedimentary rocks. Chemical analyses of uraninite performed by electron probe micro-analyzer (EPMA) can be profitably used for U-Th-Pb dating. We employed this technique to constrain the age of formation of two types of hydrothermal deposits from the Italian Alps, namely an ophiolite-hosted magnetite deposit (Cogne, western Alps) and two uranium deposits (Novazza and Val Vedello, Orobic Alps). In the first case, we aimed to establish whether the Cogne magnetite deposit, a relatively uncommon type of ophiolite-hosted deposit, was Alpine or pre-Alpine. In the second case, we used uraninite to date the hydrothermal events responsible for widespread siderite occurrences in the Southalpine domain. At Cogne uraninite occurs as rare inclusions in magnetite and forms anhedral to euhedral crystals, ranging in size from 1 to 40 μm. Textural evidence suggests that uraninite formed contemporaneously with magnetite, thus uraninite was used to date the magnetite mineralization. Based on all the radiometric age determinations (n = 53), we calculated a weighted average age of 152.8 ± 1.3 Ma (mean square weighted deviation, MSWD = 1.3), which is consistent with a pre-Alpine genesis of the ore, related to an oceanic seafloor hydrothermal system. At the two uranium deposits of Novazza and Val Vedello (Orobic Alps), abundant carbonate minerals (mainly siderite and Fe-rich dolomite) are associated with sulfides (dominant sphalerite, pyrite and galena) and uraninite, and form disseminations, veins and impregnations in the volcanoclastic facies of the Early Permian Collio Formation. At both sites, uraninite shows textural equilibrium with siderite and Fe-rich dolomite, but widespread alteration and recrystallization complicate the scenario. Therefore, we only analyzed apparently unaltered uraninite grains, which are a few-micron large at Novazza and submicron- to micron-sized at Val Vedello. Radiometric ages for the Novazza deposit range from ~ 290 to ~ 130 Ma (n = 20). The uraninite ages for the Val Vedello deposit (n = 62) are scattered between ~270 and ~ 50 Ma, but one large (20 μm) uraninite grain has a plateau age of 169 ± 3 Ma (MSWD = 1.4; n =13). By averaging the oldest ages obtained for Novazza and Val Vedello, we obtained a best-estimate formation age for siderite-uraninite mineralization of 275 ± 13 Ma (MSWD = 3.2; n = 6). Younger ages are interpreted to reflect protracted interaction with hydrothermal fluids and/or deformation/recrystallization during Jurassic rifting and successive Alpine orogenesis. Our examples confirm that uraninite EPMA chemical U-Th-Pb dating can be a valuable tool in solving some geological issues, having also the advantage of being more affordable and rapid, compared to other analytical techniques. Moreover, the improvement of EPMA resolution (spot size << 1 μm) in modern instruments may enlarge the range of potentially analyzable samples.