THE GRAPHITE CREEK FLAKE GRAPHITE DEPOSIT, SEWARD PENINSULA, ALASKA: A PRODUCT OF IN SITU METAMORPHISM AND/OR HYDROTHERMAL FLUIDS?

Graphite is considered a critical mineral by the US Government for its high demand in battery and electronic technologies. The Graphite Creek deposit (10.95 Mt at 7.8% Cg; measured and indicated), Seward Peninsula, Alaska, is the largest flake graphite deposit in the United States, yet its genesis remains unclear. New drill core observations, petrography, and carbon (C) isotope data provide insights into the mineralizing processes and possible carbon source(s) in the system.

The Graphite Creek deposit is hosted in granulite-facies metamorphic rocks on the fault-bounded northern edge of the Kigluaik gneiss dome. Mineralization is sub-continuous along strike for 18 km. Graphite ores are hosted in strongly-deformed sillimanite-quartz-feldspar-biotite-graphite-garnet gneiss of uncertain age. Graphite occurs most commonly as foliated flake disseminations and mm-scale layers, but also forms 0.1 – 0.5 m-thick massive lenses (veins?) composed almost entirely of pure flake graphite and lesser quartz that have inclusions of foliated biotite and porphyroblastic garnet. The massive lenses are generally concordant with macrofolds but are locally discordant with the dominant gneissic foliation. Intercalated with the graphite-rich sillimanite gneiss unit is a sillimanite-absent quartz-plagioclase-biotite gneiss unit with minor disseminated flake graphite and pyrrhotite. Both gneisses contain migmatitic textures and lack carbonate or significant calc-silicate assemblages that might invoke calcareous protoliths. Tectonically-late quartz – feldspar ± tourmaline ± graphite pegmatite dikes and late Cretaceous granite stocks that have recrystallized contacts cut the gneisses. Preliminary ¹³C_PDB values for graphite in the sillimanite gneiss cluster tightly around -21 ‰, while graphite in the quartz-biotite gneiss is heavier at -15 ‰; both suggest an organic carbon source.

Collectively, these observations are difficult to reconcile with genetic models for graphite systems that invoke metamorphic processes (i.e. in situ graphitization or localized decarbonation) or hydrothermal activity separately. Our evidence suggests a combination of metamorphism and hydrothermal activity. Understanding the nature of this deposit will provide insights for graphite ore genesis and aid in prospectivity analysis.