Paleoproterozoic vein graphite mineralization caused by decarbonation in the Ruby Range, Montana, USA

Hydrothermal graphite veins are a possible source for modern battery materials and require better understanding of their carbon source(s) and absolute timing to develop mapable criteria for exploration models. We present new observations of graphite vein and alteration paragenesis and U-Pb LA-ICP-MS titanite age data from the Ruby prospect, Montana, USA, that constrain mineralization timing and source. The graphite veins cut high-temperature metamorphic rocks of the lower Christensen Range suite  and are associated with intense diopside (Di0.69Hd0.27Jo0.04) alteration of marble. The oldest titanite ages in fresh marble and partially altered calc-silicate gneiss are ca. 2,500 – 2,450 Ma, show elevated REE values, and Eu/Eu* 2; suggesting a low fO₂ fluid generated from metamorphism during the Big Sky orogeny. Our paragenetic observations and titanite ages indicate graphite vein formation via skarnoid decarbonation reactions in marble late in regional orogenesis. Granulite-facies, carbonate-bearing, supracrustal rock terranes are thus favorable for hydrothermal graphite vein deposits.