Mesoproterozoic graphite deposits, New Jersey Highlands: geologic and stable isotopic evidence for possible algal origins
deposits within metaquartzite yielded δ 13 C values of ‐26 ± 2‰ (1σ), and graphite from massive deposits within biotite‐quartz‐feldspar gneiss yielded δ 13 C values of ‐23 ± 4‰. Disseminated graphite from biotite‐quartz‐feldspar gneiss country rock was ‐22 ± 3‰, indistinguishable from the massive deposits hosted by the same lithology. Anatectic pegmatite is graphitic only where generated from graphite-bearing host rocks; one sample gave a δ 13 C value of ‐15‰. The δ 34 S values of trace pyrrhotite are uniform within individual deposits, but vary from 0 to 9‰ from one deposit to another. Apart from pegmatitic occurrences, evidence is lacking for long-range mobilization of carbon during Grenvillian orogenesis or post-Grenvillian tectonism. The field, petrographic, and isotope data suggest that massive graphite was formed by granulite-facies metamorphism of Proterozoic accumulations of sedimentary organic matter, possibly algal mats. Preservation of these accumulations in the sedimentary environment requires anoxic basin waters or rapid burial. Anoxia would also favour the accumulation of dissolved ferrous iron in basin waters, which may explain some of the metasediment-hosted massive magnetite deposits in the New Jersey Highlands.