GEOLOGY AND STABLE CARBON ISOTOPES OF THE RUBY GRAPHITE DEPOSIT, BEAVERHEAD COUNTY, MONTANA

The Ruby Graphite deposit, formerly known as the Crystal Graphite Mine, produced 2400 tons of high-quality crystalline graphite between 1902 and 1950, making it one of the larger historic graphite producers in the United States. The deposit, located in the southern Ruby Mountains of southwest Montana, is hosted by > 2.1 Ga metasedimentary rocks that experienced sillimanite-grade regional metamorphism. The graphite occurs as disseminations in calcitic marble (“flake” graphite), and as veins and replacements (“lump” or “vein” graphite) cutting granite pegmatite, marble, calc-silicate gneiss, and biotite gneiss. At one location, pods and veins of graphite occur within a skarn assemblage of coarse grained actinolite-clinopyroxene-clinozoisite. This skarn is limited in extent, and its relationship with the surrounding gneissic country rock is undetermined. Using a cavity ring-down spectrometer, the d¹³C values of 12 lump graphite and 2 flake graphite samples are -8.2 to -5.6‰ (avg. -6.5‰) and -6.7 to -5.3‰ (avg. -6.0‰), respectively. As yet, no statistical differences can be found in d¹³C for graphite from different modes of occurrence. The d¹³C of calcite marble cropping out near the graphite occurrences is -1.3 to 0.0 ‰ (n = 4), values that are typical of marine limestone. If we assume that the graphite was in carbon-isotopic equilibrium with the marbles, then this amount of isotopic separation corresponds to a temperature of 440 to 540°C, depending on the choice of experimental fractionation data. This temperature range is 120 to 300°C lower than the estimated peak temperature of metamorphism in the Ruby Mountains, which suggests that the graphite likely formed during a younger, retrograde event. There are few clues remaining as to the nature of the fluids involved in this event, however, since the veins are almost pure graphite, and there is little evidence of hydrothermal alteration, apart from the one isolated skarn deposit. Examination of hand samples and thin sections shows that some graphite preferentially replaces precursor minerals that contain ferrous iron, such as biotite and garnet. Locally, deposition of graphite could have been a redox reaction: 4Fe²⁺ + 4H⁺ + CO₂ = C + 4Fe³⁺ + 2H₂O. Other mechanisms are possible and are being evaluated.