Carbon isotope geochemistry of graphite vein deposits from New Hampshire, U.S.A.

Abstract

Graphite veins of hydrothermal origin occur throughout central New Hampshire. Veins truncate sillimanite-grade, metasedimentary rocks of Early Devonian-Silurian age and range in size from microscopic to meters in thickness. In addition to graphite, veins may contain quartz, tourmaline, ilmenite, rutile, sillimanite, muscovite or chlorite. Vein mineralogy is generally compatible with wall rock mineral assemblages. Mineralization structures include wall-rock alteration zones, coxcomb graphite crystals on vein walls, and botryoidal, concentrically layered graphite-silicate nodules.The δ13C values of graphite in 14 deposits studied range from − 28%. (PDB) to − 9%. Veins whose textures give evidence of a single stage of mineralization have a narrow range of δ13C values (± 0.2%.). Other veins record successive episodes of graphite precipitation and have ranges of 3–6%. In one sample, adjacent layers of graphite differ by 3%.The wide range of δ13C may be explained by mixing carbon from two crustal reservoirs: biogenic, reduced carbon and carbonate. Precipitation of graphite results from mixing two or more aqueous fluids with different CO2CH4 ratios. Parental fluids are produced by devolatilization during metamorphism. Water-rich fluids with CH4 >CO2 and low δ13C are derived from pelites that contained organic matter; whereas fluids with CO2 >CH4 and high δ13C come from siliceous carbonates.