U/PB AGE CONSTRAINTS ON HOST ROCK DEPOSITION AND HIGH-T METAMORPHISM AT THE GRAPHITE CREEK FLAKE GRAPHITE DEPOSIT, SEWARD PENINSULA, ALASKA

The Graphite Creek deposit of Alaska’s Seward Peninsula may prove to be a domestic source of flake graphite, a commodity utilized in Li-ion batteries and other green technologies. Understanding the mineralizing processes and regional distribution of flake graphite concentrations requires determination of the depositional and metamorphic ages of the graphite-bearing host rocks. We present preliminary detrital zircon (DZ), igneous zircon, and metamorphic titanite LA-ICP-MS ages that constrain deposition and peak high-T metamorphism associated with graphite enrichment. The Graphite Creek deposit is hosted in upper amphibolite- to granulite-facies paragneiss of the Nome Complex on the northern margin of the Kigluaik gneiss dome. Massive lenses or veins 0.1 – 0.5 m in thickness are composed almost entirely of pure flake graphite and lesser quartz ± tourmaline. Inclusions of garnet porphyroblasts in the massive graphite lenses/veins suggest syn- to post-metamorphic formation of flake graphite.

Regional DZ populations from the paragneiss have Neoproterozoic to Devonian minimum ages, but rocks proximal to the deposit have not been dated prior to this study. A DZ sample (n = 110) from quartz-plagioclase-biotite paragneiss yielded an unexpectedly young population (n=6) of ~275 Ma, suggesting that the Nome Complex may be as young as Permian. This new potential stratigraphic component of protoliths to the graphite host rocks has important regional implications for tectonic setting and exploration strategies and requires further investigation. Highly-strained sillimanite orthogneiss dikes (<2 m thick) range from ~100 to 93 Ma and overlap with a metamorphic titanite age of ~93 Ma. Cross-cutting granites with recrystallized contacts, but no penetrative fabric, were previously dated to ~90–92 Ma. Collectively, these ages bracket most high-T metamorphism, and associated graphite mobilization and concentration, to ~100–93 Ma. Our results suggest that upper Paleozoic rocks in the Nome Complex that experienced at least one high-T metamorphic event may have potential to host ore grade flake graphite on the Seward Peninsula and in regionally correlative rocks.