Petrogenesis of extra-large flake graphite at the Bissett Creek deposit, Canada

Abstract

The petrogenesis of extra-large flake graphite is enigmatic. The Bissett Creek graphite deposit, consisting of flake graphite hosted in upper-amphibolite facies quartzofeldspathic gneisses and rare aluminous gneisses, provides an analogue for graphite exploration. In the Bissett Creek gneisses, graphite is homogeneously distributed and composes 2–10 vol. % of the rocks. Disseminated graphite flakes (~ 1 to 6 mm in size) are interleaved with biotite and are petrologically associated with upper-amphibolite facies metamorphic mineral assemblages. Thermobarometry and phase equilibrium modeling yield peak temperatures of > 760 °C at 0.5–0.9 GPa. Whole-rock samples with abundant graphite yield δ13CVPDB from − 28 to − 14‰. δ34SVCDT values of sulfide-bearing samples vary from 10 to 15‰. Sulfur and carbon isotope values are compatible with a biogenic origin, flake graphite probably formed from metamorphism of in situ organic material. However, the variability of δ13C values from the deposit along with graphite microstructures suggest that carbon-bearing metamorphic fluid (or melt) generated during metamorphism may have remobilized carbon resulting in anomalously large to extra-large flake sizes. This may be a common mechanism globally to explain large graphite flake sizes where graphite formed through in situ metamorphism of organic matter is coarsened due to remobilization of CO2-rich fluids (or melt) during high-temperature metamorphism.