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

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

Jun 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1239r&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 50, Issue 6, pp.1239-1247

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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 13 C 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 13 C 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 13 C 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 CO 2 / CH 4 ratios. Parental fluids are produced by devolatilization during metamorphism. Water-rich fluids with CH 4 > CO 2 and low 13 C are derived from pelites that contained organic matter; whereas fluids with CO 2 > CH 4 and high 13 C come from siliceous carbonates.

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