SEDIMENTARY PROTOLITHS OF ADIRONDACK GRAPHITE DEPOSITS

Mesoproterozoic metasedimentary rocks of the Adirondack Highlands (Grenville Province, New York) are host to a historic graphite district that was active in the late 19th and early 20th centuries (26 mined deposits and showings). Carbon-rich sedimentary protoliths deposited in the trans-Adirondack basin on the margin of Laurentia and were metamorphosed to the granulite facies during the Grenville orogenic cycle, producing crystalline flake graphite deposits (av. 0.4 to 14.6% gr) hosted in a variety of rock types. This study focuses on the protoliths and depositional environments of this ore district.

Analyzed samples were collected from graphite deposits, waste rock, and country rock of 16 different graphite deposits near Ticonderoga. Of these samples, the majority are from the 1917 NY State Museum graphite survey; the rest were collected in 2021. Samples were examined petrographically, analyzed for major elements by XRF, and graphite was analyzed for carbon isotope ratio. The classification diagram of Herron (J Sed Res 1988) was used to determine pre-metamorphic sedimentary protoliths. Samples from the Dixon schist, which hosts most deposits, fall into two distinct affinities: an Fe-shale group (δ¹³C= -8 to -28‰, n=5) and a lith-arenite group (δ¹³C= -18 to -23‰, n=4). Three other schist units were also sampled: the Bear Pond schist plots as a shale (δ¹³C= -22 to -23‰, n=2), the Rock Pond schist plots as an arkose (δ¹³C= -20 to -27‰, n=3), as does the Catamount schist (δ¹³C= -22‰, n=1). All of these groups’ carbon isotope ratios are consistent with organic sources, varying ranges of which could indicate a difference in the isotope ratios of the sources and/or alteration after deposition. Preliminary data show a negative correlation between % graphite and δ¹³C for each unit, which might be consistent with metamorphic devolatilization.